Semiconductor nanostructures towards electronic and opto-electronic device applications - VII

Resume : When the interaction between an excitonic transition and a confined photonic mode is larger than losses, the optical response of the system is dominated by the physics of exciton-polaritons. These hybrid light-matter quasi-particles have shown to be promising candidates for photonic applications (low-threshold laser, all-optical switching) and fundamental research (correlated many-body physics) but are mostly limited to cryogenic temperatures. Here we address the strong coupling regime of atomically-thin transition metal dichalcogenides and single-crystals 2D perovskites, showing that their remarkable excitonic properties make these structures appealing for room-temperature polariton devices. We focus on the optical nonlinearities inherited by the strong coupling regime and measurable as the energy shift of the polariton energy, finding that single crystals of 2D perovskite share notable similarities with the ideal case of GaAs quantum wells at cryogenic temperatures and, in particular, the presence of significative spin-dependent polariton interactions.

Resume : Recently, artificial honeycomb structures have attracted wide attention for fundamental research due to the tunable interplay between topology and quasiparticle interactions. In fact they offer a fascinating platform for studying Dirac physics especially thanks to the possibility to vary the physical parameters in regimes which are not accessible in graphene or in other 2D materials. A natural way to fabricate such materials is by modulating the potential seen by a two-dimensional electron gas (2DEG) found in conventional III-V semiconductor heterostructures. The idea is to create a periodic array of cylindrical holes in the active layer in order to form potential barriers for the electrons. So that, guided by previous works and predictive atomistic tight-binding calculations, we are working on the nanoperforation of InGaAs quantum wells (QWs) epitaxially grown on InP substrates using high-resolution e-beam lithography and highly plasma based dry etching. The goal is to push the patterning to its limit in order to reveal Dirac fermions and non-trivial band structures predicted in these artificial 2D materials. Here, we present the work done to obtain triangular antidot lattices with periodicities of the pores down to 40 nm giving an effective honeycomb lattice constant of 23 nm. Based on initial tunneling spectroscopic measurements, we show that InGaAs/InP heterostuctures are appropriate materials to engineer band structure from the nanoperforation of the InGaAs QW to be able to observe Dirac cones in the conduction band of the QW. Furthermore, we report the calculations that show the possibility to measure Dirac physics in these type of samples.

Resume : Exciton-polaron interaction is one of the major luminescence quenching mechanisms in organic optoelectronic devices and this is a key challenge in the realization of electrically pumped organic lasers. In this work, we show that excitons dissociate into free charge-carriers in the presence of polarons, which leads to luminescence quenching. We perform phase sensitive photocurrent measurements to detect photo-carrier generation processes. Measurements are done on a bottom-gate bottom contact field effect transistor (FET) with pentacene as the active material. We choose ohmic source and drain contacts so that the charge density required to terminate the electric field lines in the channel, due to the applied gate bias, is satisfied. The low source to drain electric field, combined with the screening effect of gate field lines by accumulated polarons, turns off exciton-dissociation due to external electric-field. We investigate the effect of channel charge densities on the photocurrent spectral response and the photocurrent-voltage characteristics. From a systematic analysis of the external quantum efficiency measurements, with the support of optical simulations which account for interference effects in the device, we show that excitons dissociate in the presence of injected polarons in the channel. This is an efficient pathway for free charge carrier generation in organic transistors with pristine semiconductor material. From gate-bias dependent photocurrent and mobility measurements we show that photo-generation saturates at relatively low gate biases. We simulate the exciton dynamics from the exciton interaction with the injected charge concentration. The simulation suggests a similar saturation trend for gate bias-modulation of steady-state exciton density. These results indicate that exciton-polaron interaction is a pathway for exciton dissociation into free charge carriers as compared to non-radiative exciton quenching reported in earlier studies [1, 2]. [1]W. A. Koopman, M. Natali, G. P. Donati, M. Muccini, and S. Toffanin, “Charge-Exciton Interaction Rate in Organic Field-Effect Transistors by means of Transient Photoluminescence Electro modulated Spectroscopy,” 2016. [2] J. M. Hodgkiss, S. Albert-seifried, A. Rao, A. J. Barker, A. R. Campbell, R. A. Marsh, and R. H. Friend, “Exciton-Charge Annihilation in Organic Semiconductor Films,” pp. 1567–1577, 2012.

Resume : Near zero dielectric permittivity is an uncommon property typical of plasmonic systems. The associated optical response, called Epsilon-Near-Zero (ENZ) leads to several exciting phenomena. Unfortunately, obtaining such a response in the visible is challenging. Here we demonstrate how to obtain a customizable ENZ response in the entire visible range by means of Metal/Insulator/Metal nano-cavities. The quantum modeling we propose elucidates the resonant tunneling nature of the ENZ response occurring at the cavity modes, bringing usually classically treated MIM systems under the quantum domain. As two remarkable applications, we implement a Superabsorber and a Refractive Index Sensor based on such nano-cavities. We also describe the hybridization of the ENZ modes occurring between two strongly coupled MIMs, demonstrating a mode splitting of more than six times the linewidth of the resonances.[1] In the end, we show how the proposed strongly coupled double ENZ nano-resonators can be used to enhance the photophysical properties of a CsPbBr3 perovskite emitter. Indeed, a noticeable Purcell effect takes place thanks to the Surface Plasmon Coupled Emission (SPCE) and Surface Plasmon Enhanced Absorbtion (SPEA) phenomena. References: [1] Caligiuri, V.; Palei, M.; Imran, M.; Manna, L.; Krahne, R. Planar Double-Epsilon-Near-Zero Cavities for Spontaneous Emission and Purcell Effect Enhancement. ACS Photonics 2018, 5, 2287–2294.

Resume : Usual way to manipulate light consists in using bulky and heavy refractive optical components. Since recent years, new components, known as metasurfaces have been proposed. Instead of using the propagation of light inside the media, metasurfaces work by utilizing the resonant properties of nano-antennas disposed along interface to deflect or focus the light in arbitrary manner. Nevertheless, these devices are limited since their functionalities are fixed by design. New degrees of freedom could be obtain by designing tunable metasurfaces. In this work we present metasurfaces based on Gallium Nitride material. We propose a fabrication method able to preserve the active optical properties of semiconductors, thus paving the way for the realization of tunable metadevices. Here, we present two nanofabrication processes to realize semiconductor based meta-optics. We choose Gallium Nitride material as it is thoroughly used in the semiconductor industry, thus having potential for real world applications. Beyond its widespread applications in optoelectronics and electronics applications, this material present various advantages in terms of refractive index, chemical and thermal stability, wideband transparency in the visible. One of these nanofabrication process, used with semiconductor would offer new perspectives for the realization of next generation of light emitting meta-devices.

Resume : Abstract. In this contribution, we report on negative refraction effect occurring in layered semiconductors. The origin of this effect is attributed to the presence of intersubband plasmon resonance induced by the electronic confinement of the electrons in thick quantum wells. Relying on the effective medium theory, we analyse both the in plane and the out of plane effective optical properties of highly doped ZnO/ZnMgO semiconductor material. We theoretically show and experimentally demonstrate that thicknesses and doping levels of each layer can be carefully chosen to feature strong intersubband transitions, leading to type 1 and type 2 hyperbolic response. Understanding the optical properties of intersubband materials in the frame of hyperbolic materials would shed new light on which physical mechanisms are controlling the radiative decay of intersubband plasmon excitations. This approach could be further utilized to designing efficient mid-IR sources. 1. Introduction Metamaterials are artificial materials that feature uncommon physical properties. In optics the field of metamaterials has received tremendous interest over the last few years, with the demonstration of various unexpected and intriguing effects such as ultra-high refractive index and extraordinary optical activity. However optical metamaterials are mainly known owing to the fascinating possibility of realizing devices based on negative index of refraction, also called negative index materials (NIM). Material with negative index of refraction was firstly been considered in a theoretical paper by Veselago in 1968. In this latter paper, it is shown that negative index can be obtained in a material presenting simultaneous negative dielectric permittivity and magnetic permeability, in brief by adjusting two and matching electric and magnetic resonances in the material. It implies that in such systems the absorption is quite relevant. In order to mitigate optical losses, lots of efforts have been made to reduce the absorption losses, considering for example gain material. A more efficient approach has been proposed in recent years. It consists on exploiting hyperbolic dispersion, i.e. opposite signs of permittivity along orthogonal in- and out- of plane directions. Affecting the optical response only along one direction, hyperbolic dispersion can be realized in anisotropic medium using a single optical resonance. In such a system, also dubbed hyperbolic metamaterial (HMM), one of the electric or magnetic resonance is replaced by leveraging on the material anisotropy. The main idea behind HMM is to make use of an alternating composition of both resonant and non-resonant materials. Peculiar attention has been given to semiconductor HMM [1-3] because of their functional and fabrication advantages, substituting the metallic resonant part by a highly doped semiconductor. In this paper we present theoretical and experimental results confirming negative refraction in highly doped layered semiconductor materials and highlight the fact that this behaviour is essentially driven by the intersubband transitions (ISBT) in the quantum wells (QW) [4]. 2. Samples: materials and geometry The physical system proposed in this paper is a stack of alternated layer of doped ZnO and undoped MgZnO forming a quantum wells along z direction grown on native ZnO substrate to reduce the dislocation density for sharper ISBT. The choice of ZnO is mainly related to the possibility of reaching very high doping level and because of its non-polar growth possibility, therefore avoiding the detrimental quantum confined Stark effect [5]. 3. Model The problem was mathematically treated following Maxwell-Garnett theory, in the presence of both the QW and the barrier to obtain realistic material permittivity. It provides optical properties ranging from pure metal to pure dielectric passing through HMM type 2 and 1. 3. Experimental results The theoretical prediction were tested experimentally with a samples 35 nm thick of ZnO quantum wells. The sample is composed of 15 couples of QW-barrier having total thickness equal to 590 nm. The hyperbolic type 1 behaviour has been detected by using a blade to stop either positive or negative refraction depending on the sample orientation and polarization. We show that for s polarization there is not any negative refraction effect while for p polarization its presence is highlighted by the sample orientation. 4. Conclusion In conclusion we have theoretically and experimentally demonstrated the possibility of reaching negative refraction due to intersubband transition in a layered semiconductor system. Although it is clear that intersubband designs are composed of several subwavelength layer of Drude-like materials, this work connects for the first time the concepts of intersubband plasmons with the photonic response of hyperbolic metamaterials. Due to the advanced photonic properties of HMM, we believe that these results could initiate new design for efficient mid-IR light emitting sources and quantum well detectors. 5. References [1] Gururaj V. Naik, Jingjing Liu, Alexander V. Kildishev, Vladimir M. Shalaev and Alexandra Boltasseva , Demonstration of Al:ZnO as a plasmonic component for near-infrared metamaterials, PNAS June 5, 2012. 109 (23) 8834-8838. [2] Anthony J. Hoffman, , Aishwarya Sridhar, Phillip X. Braun, Leonid Alekseyev, Scott S. Howard, Kale J. Franz, Liwei Cheng, Fow-Sen Choa, Deborah L. Sivco, Viktor A. Podolskiy, Evgenii E. Narimanov, and Claire Gmachl, Midinfrared semiconductor optical metamaterials, Journal of Applied Physics 105, 122411 (2009); [3] Anthony J. Hoffman, Leonid Alekseyev, Scott S. Howard, Kale J. Franz, Dan Wasserman, Viktor A. Podolskiy, Evgenii E. Narimanov, Deborah L. Sivco and Claire Gmachl, Negative refraction in semiconductor, Nature Materials volume 6, pages 946–950 (2007); [4] Angela Vasanelli, Yanko Todorov, Carlo Sirtori, Ultra-strong light–matter coupling and superradiance using dense electron gases, C. R. Physique 17 (2016) 861–873; [5] M. Montes Bajo, J. Tamayo-Arriola, M. Hugues, J. M. Ulloa, N. Le Biavan, R. Peretti, F. H. Julien, J. Faist, J. M. Chauveau, A. Hierro, Intersubband polarons in oxides, arXiv:1703.07743.

Resume : Over the past decade, plasmonic structures and metamaterials have been intensively investigated for the control of electromagnetic radiation. The spectral response of such structures has a high dependence on the combination of materials used and their structural dimensions. Despite the promise of metamaterials and plasmonics for miniaturisation and on-chip integration, for many optoelectronic applications (e.g. optical modulators, switches, filters and polarizers), the inability to tune optical response post-fabrication presents serious limitations. To overcome this, we consider tunable plasmonic elements formed by coupling plasmonic components with a thin film consisting of vanadium dioxide (VO2), a phase change material. VO2 is attractive as a phase change material due its fast and reversible semiconductor-metal transition at a critical temperature of 68°C and a threshold electric field of the order 106 V/m, lower than other material options such as GST or AIST. This work focuses on the changes in spectral response in the visible regime, something previously unexploited. Thin films of VO2 are grown by pulsed laser deposition (PLD). The change in scattering properties of plasmonic structures through the phase change of VO2 was investigated. Fluorescent spectra and time resolved measurements were taken to probe the change in fluorophore emission. All experimental results are complemented by FDTD simulations. Our results demonstrate that the hybrid structures incorporating VO2 are a promising route to dynamic tuning.

Resume : Nanowires are filamentary crystals with a tailored diameter ranging from few to ~100 nm. The reduced dimensions and longitudinal morophology of these nanowires results in interesting optical and electrical properties and provides a great potential for many applications, including sensing, quantum computing and all those that involve photonics. In this talk we will first review two different strategies to obtain III-V nanowires in an organized fashion on both Si and GaAs substrates. We will include nanostructures in the form of nanowires, nanoneedles and nanoscale membranes[1,2,3,4]. Then, we will review how these properties can be used to improve photonic applications such as lasers and solar cells[5,6]. Finally we will propose and substantiate the use of alternative compound semiconductors that have the potential to substitute III-V in the solar cell arena due to their higher availability in the earth crust: II3V5 compounds. References: [1] J. Vukajlovic-Plestina et al, Nano Lett. 17, 4101 (2017) [2] W. Kim et al, Nano Lett. 18, 49 (2018) [3] G. Tütüncüoglu et al, Nanoscale 7, 19453 (2015) [4] L. Güniat et al, in review (2018) [5] P. Krosgstrup et al, Nature Photon 7, 306 (2013) [6] E. Bermudez-Ureña et al, Nano Lett. 17, 747 (2017) [7] M. Friedl et al Nano Lett. (2018) ; DOI : 10.1021/acs.nanolett.8b00554

Resume : III-V semiconductor NWs have become widely studied nanostructures for nanophotonics, optoelectronics and advanced nanoelectronics. For InAs NWs very strong and tunable quantum confinement effects are expected to occur for diameters well below 30 nm [1, 2] – which has hitherto been difficult to reach. We present InAs NWs grown along the [111] direction on SiO2-masked Si (111) using a completely catalyst-free vapour-solid (VS) growth mechanism [3, 4] via selective-area molecular beam epitaxy. In a first approach we show how by direct bottom-up epitaxy the NW dimensions can be controlled by tuning the growth parameters to finally obtain sub-25 nm diameter InAs NWs. In a second approach, we explore a so-called reverse reaction growth mechanism to intentionally thin as-grown NWs by in situ thermal annealing [1]. Starting from > 1.5 µm long NWs with diameters of > 90 nm we performed various different thermal annealing experiments by varying in situ vacuum conditions, As-overpressure, annealing temperature and time. We thereby realized microstructure conserving InAs NWs (resolved by TEM analysis) with diameters < 15 nm. First preliminary low-temperature PL spectra of a NW-array with sub-30 nm diameter show a substantial blue-shifted peak emission compared to conventional unconfined NWs in good agreement with simulation data. 1 Loitsch et al Adv. Mat., 27 (2015), 2195 2 Koblmüller et al APL, 101 (2012), 053103 3 Becker et al ACS Nano, 12 (2018), 1603 4 Sonner et al APL, 112 (2018), 091904

Resume : Nanowires (NWres) experience stress by their environment. In return, the environment of NWres experiences a stress response which may lead to propagated strain and change in shape and size of nanowire cross sections [1]. Recently, we introduced analytic number series to zb-nanocrystals to describe their structure down to the individual atom/bond [2]. Here, we extend these geometrical number series to zinc-blende(zb) NWres of diameter d_Wre, obtaining the number of nanowire atoms N_Wre(d_Wre), bonds between nanowire atoms N_bnd(d_Wre) and interface bonds N_IF(d_Wre) for 6 high symmetry cross sections frequently occurring in nanowire growth [3]. Along with these parameters, we present specific length of interface facets, cross section widths/heights as well as the cross section area [3]. Our fundamental insights into NWre structures offer a universal gauge and thus enable major advancements in data interpretation and understanding of zb NWres [3]. We underpin this statement with results from the literature on cross-section images from III-V core-shell NWres [4]. As outlook, we introduce the concept of morphing nanowire cross sections by number sub-series starting from nominal cross section shapes. With these, we can describe nearly arbitrary cross sections as encountered in experiment. [1] DOI: 10.1088/1361-6633/aa56f0 [2] DOI: 10.1063/1.4960994 [3] https://arxiv.org/pdf/1901.00935v3.pdf ; D. König and S. Smith, Submitted to Acta Cryst. B (2019) [4] DOI: 10.1021/nl061692d

Resume : Radial-junction (RJ) solar cells based on silicon nanowires (SiNWs) are currently being investigated and improved by several research groups around the world [1, 2]. Recently, SiNW RJ solar mini-modules with area of 10 cm2 have been demonstrated [3]. To increase the performance, tandem modules with μc-Si:H bottom cell and a-Si:H top cell are being developed with the main challenge of achieving a high open circuit-voltage (Voc) and a high energy conversion efficiency. We have succeeded to deposit commercially available tin dioxide (SnO2) nanoparticles (NPs) onto the ZnO coated substrates with a controlled density of 10^6 cm−2 needed for tandem RJ devices [4]. Different particle size distributions have been achieved by centrifugation and dilution processes, after which SnO2 NPs have been reduced to metallic Sn droplets by the hydrogen plasma treatment to serve as catalysts for the plasma-assisted vapor-liquid-solid growth of SiNWs. In our study we have explored a wide range of deposition parameters of intrinsic μc-Si:H material and different doped window materials (a-Si:H, μc-Si:H and μc-SiOx:H) to improve solar cell performance. Moreover, we have explored the performance of NIP configuration in addition to previously used PIN [2]. We have achieved the Voc of over 0.4 V during the optimization of the bottom μc-Si:H solar cells. [1] M. Adachi et al., Scientific reports, vol. 3, p. 1546, 2013. [2] S. Misra et al., IEEE Journal of Photovoltaics, vol. 5, no. 1, pp. 40–45, 2015. [3] M. Al-Ghzaiwat et al., physica status solidi (RRL)–Rapid Research Letters, vol. 12, p. 1800402, 2018. [4] L. Dai et al., Nanotechnology, vol. 29, no. 43, p. 435301, 2018.

Resume : We explore for the first time the direct growth of high quality InAs nanowires (NWs) on flexible plastic substrates by metalorganic vapor phase epitaxy (MOVPE). We have recently reported on self-catalyzed growth of InAs NWs using a new isolated low-temperature growth regime, where NWs were obtained at temperatures as low as 280°C. We showed that low-temperature growth beneficial for a variety process flows and growth substrates where excessive heat is detrimental, particularly for plastic substrates. In this work, we synthesize InAs nanowires in-situ on flexible PI substrates without any need for transfer techniques. Presently semiconductor nanowires are routinely grown on high-priced crystalline substrates as it is extremely challenging to grow directly on plastics and flexible substrates due to high temperature requirements and substrate preparation. At the same time plastic substrates can offer many advantages such as extremely low price, light weight, mechanical flexibility, shock and thermal resistance and biocompatibility. Hence there is a high demand for direct growth of semiconductor nanowires on flexible plastics using epitaxial techniques. We show that the fabricated NWs are optically and electrically active with strong light emission in the infrared range. Overall, we demonstrate that light-emitting nanowires can be synthesized directly on flexible plastic substrates inside a MOVPE reactor, and we believe that our results will further advance the development of the nanowires-based flexible electronic devices.

Resume : In the last decade, metal oxide core-shell nanowires have focused the attention from both scientific and technologic point of views. Due to their unique properties such as high aspect ratio, quantum confinement and large interface area which can enhance the generation of electron-hole pairs leading to new functionalities, these nanostructures have applications in photodetectors, solar cells, photocatalysis, sensors etc. From the metal oxides, ZnO is an n-type semiconductor with a wide direct band gap of 3.3 eV, while CuO is an p-type semiconductor with a narrow indirect band gap of 1.2 eV. Thus, by combining ZnO and CuO into radial core-shell nanowires, a type II heterojunction featured by a good control of the charge carrier generation at the interface between the two semiconductors can be obtained. In this context, ZnO-CuO core-shell radial heterojunction nanowire arrays were prepared using thermal oxidation in air and radiofrequency magnetron sputtering. Morphological, structural, optical, compositional and surface chemistry properties of the obtained nanowire arrays were evaluated. Further, individual ZnO-CuO core-shell nanowires were contacted using photolithography, electron beam lithography and thin film deposition techniques in order to investigate their electrical and photoelectrical properties for applications in optoelectronic devices. The architecture was found to be a promising one for a wide field of applications ranging from photodetectors to biosensors.

Resume : The increasing interest in area, such as pollution control, of detection of hazardous gases has lent prominence to gas sensing devices. Solid state gas sensors based on tin oxide thin films have been broadly investigated due to their manufacturing simplicity. For the thin films prepared by chemical vapor depostion technique, the reports on the presence of the Sn (0) and/or Sn(II) phases are quite conflicting. In present study the growth of tin oxide shell around the 1D Si core formed by Metal Assisted Wet Chemical Etching has been investigated. Synchrotron XANES, XPS and PEEM techniques were applied using synchrotron radiation facilities at Helmholtz Zentrum (BESSY II) and NRC "Kurchatov institute" storage rings. For the first time, in-situ mechanical treatment of the grown SiNWs array which allows non-destructive surface composition estimation, atomic and electronic structure profiling have been performed. Tin oxide MOCVD shell is continuously covered through whole Si nanowires and according to the SEM studies has 30 nm thickness of pebble-like crystallites. The decomposition of SnO2 phase was observed at the temperature higher than 600 C with phases transformation from SnO2-x and SnO to metallic tin. Composition and electronic structure reconstruction were confirmed by applying surface sensitive methods like, microspot PEEM or general XANES/XPS studies and independently by XPS depth profiling. The study was supported by Russian Science Foundation (Project 17-72-10287).

Resume : This talk will cover our work on micro- and nano-scale lasers. We demonstrated integration of dye-based whispering-gallery micro-lasers into live cells and showed that this provides a novel way of tagging and tracking individual cells in large cell populations over extended periods of time. We are currently developing this approach further to perform in vivo intracellular sensing and cell tracking, e.g. in the heart of zebrafish. In very recent work, we also demonstrated miniaturization of our intracellular lasers to sub-micrometer dimensions which we achieved by making use of the large optical gain and high refractive index of inorganic semiconductor quantum well materials. In other work, we used semiconducting polymers and soft lithography to develop ultra-thin and flexible membranes with integrated distributed feedback lasers that can serve as “emissive holograms” for counterfeiting applications.

Resume : Nanostructured materials have the potential to significantly enhance the performance of electronic devices as recently demonstrated for chemical sensors, batteries, and photodetectors. This has resulted in a gold rush toward novel applications ranging from flexible electronics to wearable nanogenerators. Despite these achievements, integration of nanomaterials in devices is challenging, and their assembly in suboptimal structures, lacking of hierarchical design, drastically limit the final performance. Here, we will present the fabrication of highly performing optical gas sensors by the multi-scale engineering of ultraporous semiconductor nanoparticle networks on Au metasurfaces. We will showcase the use of scalable and low cost synthesis approaches for the wafer-level fabrication of tailored and well-reproducible 3D morphologies of multi-functional nanoclusters. We will conclude with recent achievements in the nanofabrication of semiconductor-plasmonic nanoparticle structures for localized surface plasmon resonance and chemical sensing [1]. 1. Fusco, Z.; Rahmani, M.; Bo, R.; Verre, R.; Motta, N.; Käll, M.; Neshev, D.; Tricoli, A., Adv. Mater. 2018, 30 (30), 1800931.

Resume : The use of magnetic nanoparticles (NPs) to convert electromagnetic energy into heat is known to be a key strategy for numerous biomedical applications but is also an approach of growing interest in the field of catalysis. We have shown that, associated to catalytic metals (Ni, Ru), iron carbide NPs submitted to magnetic excitation very efficiently catalyze CO2 hydrogenation in a dedicated continuous-flow reactor.[1] Since a huge gradient of temperature between the self-heated particles and the “cold” environment is expected, nanoscale measurements need developments to correlate the result of catalysis reaction to NPs temperatures, but also to control the process to keep constant the heating power or the NP surface temperature. Thermometric probes have been described at low temperatures with different luminescent species.[2] However, using phosphors to determine high temperatures under harsh experimental conditions remains largely unexplored and highly challenging.[3]. At the nanoscale, quantum dots (QDs) optical properties offer a rare opportunity to build an innovative complex nano-object integrating heating capacities, catalytic ability and thermal reporting property. The temperature-dependent emission of different QDs (InP@ZnS and CdSe@ CdS) have been investigated in this purpose. Peak emission wavelength, intensity, and full width at half maximum were completely characterized as a function of temperature. We will present here the use of QDs as local temperature probes during the magnetic heating of iron carbides NPs in catalysis conditions. We evidence that the global measured temperature of the catalytic solid is drastically lower than the local temperature assessed by the QDs at the nanoscale. [1] A. Bordet, et al. Angew. Chem. Int. Ed., 2016, 55, 1.
 [2] M. Quintanilla, et al. NanoToday, 2018, 19, 126. [3] X.D. Wang, et al. Chem. Soc. Rev., 2013, 42, 7834.

Resume : In the last decade, silicon nanowires (NWs) presenting ground-breaking quantum effects are of great impact for their implementation into industrial nanodevices. We report the synthesis of 2D random fractal arrays of vertically aligned Si NWs by using the metal-assisted chemical etching, a low-cost and facile synthesis that allows fine control over the NWs structural parameters, such as their length, doping, and density. In particular, varying the thickness of the metal catalyst it is possible to control their diameter to obtain quantum confined Si NWs. Indeed, a remarkable visible luminescence is attested from Si NWs at room temperature, whose emission wavelength can be varied as a function of the NWs diameter, in agreement with quantum confinement effect 1. A light emitting diode was obtained based on Si NWs with intense EL at room temperature is also reported. NWs optical properties can be further controlled by tuning their fractal arrangement, indeed a strong multiple scattering and light trapping is attested within the NW arrays for strategic applications in photonics and photovoltaics 2. The novel optical features of Si nanowires combined with their high aspect ratio allow fabricating innovative label-free optical biosensors at low-cost by exploiting the PL quenching upon the selective capture of a specific target. Selective NW sensors with sensitivity of a few fM were realized for the specific detection of C-reactive protein with extended concentration range for non-invasive analysis in saliva 3. 4. Si NWs open the route towards new optical label-free sensors with low cost and a full industrially compatible approach for the primary health care diagnosis. 1 Light Sci. Appl. 5, 4, 2016 2 Nat Photonics 11, 170, 2017 3 ACS Photonics 5, 2, 2018 4 ACS Sensors 3, 9 2018

Resume : Titanium dioxide (TiO2), a promising semiconductor with mature morphology control methods, has attracted tremendous applications such as photocatalytic degradation, solar cells and water splitting, due to, e.g. its excellent photoelectric property. Although the plasmonic properties of TiO2 have been studied extensively, there are only a few studies that employed TiO2 in the plasmonic applications of label-free biology sensing within the visible wavelength. In this work, TiO2 columnar thin film, synthesized by anodization method with a titanium thin film on BK7 glass substrate, was applied on top with self-assembly gold nanoislands (AuNIs). Based on a common-path optical sensing system, this structure was found to provide plasmonic resonance from near-infrared to visible range. The optimized sensor chip exhibited strong plasmonic effect in the middle of visible region, i.e. around 591 nm, with competitive refractive index sensitivity of 1.49 × 10−7 RIU for NaCl sensing. As TiO2 coupled with AuNIs improved the excitation of photons by cooperation between metal and semiconductor, the visible‐light activity in plasmonic sensing performance was enhanced. Furthermore, its biosensing capability will be reported via the sensing of human IgG from serum. The structure of TiO2 columns would localize the plasmonic field and achieve better sensitivity toward single biomolecule detection. Thus, this device is predicted to have great potential in plasmonic biodetection applications.

Resume : Two-dimensional materials, such as transition metal dichalcogenides (TMD), graphene or boron nitride compose a unique toolkit of atomically-thin crystals with remarkable properties. Monolayer TMDs feature bright direct bandgap emission, strong excitonic effects and spin-valley locked properties. As a result, using circularly polarized light, so-called valley excitons can be selectively formed at well-defined locations in reciprocal space (namely the K and K’ valleys). Conversely, linearly polarized light makes it possible to form coherent superpositions of valley excitons. Controlling the valley pseudospin is the essence of opto-valleytronics, a very active field of research. The observation of significant valley polarization and coherence in bare TMD monolayers requires stringent experimental conditions such as low temperatures and quasi-resonant optical excitation. Indeed exciton-exchange and other depolarization and dephasing mechanisms are prone to suppress valley-contrasting properties on (sub-)ps timescales. Efforts are currently being made to find new schemes, where the valley pseudospin remains addressable under less demanding conditions. Here, we present two complementary approaches to preserve valley contrasting properties up to room temperature. Our general strategy is to optimally couple TMD monolayers to either i) other partner 2D materials (including, e.g., graphene and boron nitride) in the form of van der Waals heterostructures or ii) 2D plasmonic fields.

Resume : Iron germanides (FeGex) is a fascinating class of material that provides several phases with attractive and, in some cases, exotic properties including ferromagnetism (Fe5Ge3, Fe3Ge2…), antiferromagnetism (FeGe2…), or helimagnetism (B20-FeGe). This latter phase has become the focus of intense interest due to the chiral magnetic ordering at RT which makes them highly relevant for the next generation of magnetic information storage.[1] Despite the attractiveness of nanoscale FeGex structures, there exist only two synthetic approaches for their preparation: chemical vapor transport process to Fe1.3Ge at 650°C,[2] and solution phase thermolysis of precursors at high T (>260°C).[3] This latter example provides a proof-of-concept of the relevancy of solution-based strategies. However, the size, the shape or the phase purity are poorly controlled. We will present herein a novel approach for the preparation of iron-germanium nanocrystals, which relies on the design of single source organometallic precursors that display special features: i) a preformed iron germanium bond, ii) labile substituents to facilitate their removal and iii) low coordinate metal to provide an easy access to FeGex NCs. We will show i) how the design of novel organometallic single source precursors allows the synthesis of nanocrystals at the lower temperature ever reported using thermolytic approach, ii) the dramatic influence of the substitution on the germanium atom to control the NCs formation and their magnetic properties. The comparison with the double source approach will also be provided. [1] N. Nagaosa et al., Nat. Nanotechnol., 2013, 8, 899. F. Zheng et al., Nat. Nanotechnol., 2018, 13, 451. [2] K. Chang et al., J. Am. Chem. Soc., 2010, 132, 17447. [3] R. Schaak et al., Chem. Mater, 2013, 25, 4396.

Resume : Carbon nanodots (CDs) are a new class of strongly fluorescent nanomaterials consisting in ≤10 nm-sized, surface-passivated nanoparticles, composed by carbon, oxygen, nitrogen and hydrogen only.1 Their structure strongly depends on the synthesis procedure as the specific optical characteristics but, in the literature, most samples gather the same properties: bright fluorescence in the visible spectral range, high water solubility, high sensitivity to local environment and non-toxicity.1 All these features make CDs extremely appealing for many electronic and optoelectronic applications, motivating an intense ongoing research in the field. Nevertheless, the optical performance of CDs and the concrete possibility of applying them in several fields are often limited by problems which stem from the synthesis procedure and which are due to the large heterogeneity of most CDs, which reflects in low absorption strengths and excitation-dependent emission. Here, we developed a synthesis method capable of producing 6-nm sized -C3N4 nanodots which show quasi-perfect crystalline structure and a very narrow size distribution (fhwm2 nm).2 Moreover, they display an extremely high fluorescence quantum yield (73%) and absorption strength (>3·106 M(dot)-1cm-1). These characteristics clearly have indicated the pronounced homogeneity of these nanodots, and have allowed to their use as active medium in a laser cavity.2 1. C, 2018, 4(4), 67 2. Chem. Mater., 2018, 30, 1695

Resume : Heat management is critical for the performance of most modern electronic devices. It was recently proposed that heat transfer could be increased in hyperbolic metamaterials, i.e., materials having a hyperbolic electromagnetic dispersion relation. [1] However, artificial metamaterials can be difficult to fabricate and integrate. Fortunately, most of Van der Waals materials are natural hyperbolic metamaterials due to their anisotropic layered structure. In these materials, the strong light-matter coupling can give rise to a 100-10⁵ increase in the e.m. local density of states. As a consequence, the radiative heat transfer is turned from negligible to dominant. Moreover, the radiative character of heat exchange allows quasi-ballistic transport of heat on micrometric scales. Finally, as Van der Waals materials are atomically flat, the impact on electronic mobility on nearby conducting channels is limited. We have studied heat transfer in graphene on hexagonal Boron Nitride: a natural 2D hyperbolic metamaterial. In our devices, graphene is used as a versatile heat source or as hyperbolic phonon-polariton emitter, depending on the driving regime, and as a sensitive noise thermometer. [2] Using mono-, bi and trilayer graphene, we demonstrate a strong increase of the electron gas cooling resulting in a large temperature drop most probably due to an extreme case of hyperbolic cooling: the broadband Purcell effect. Bibliography [1] S. Biehs, et al "Hyperbolic metamaterials as an analog of a blackbody in the near field." Phys. Rev. Lett. 109, 104301 (2012) [2] W. Yang, et al "A Graphene Zener–Klein Transistor Cooled by a Hyperbolic Substrate." Nat. Nanotech. 13, 47 (2018)

Resume : Van der Waals heterostructures have recently emerged as promising materials for a new generation of optoelectronic devices. Due to dangling-bond-free surface of 2D materials, the constraints of lattice matching are lifted giving unprecedent flexibility for the device fabrication. Van der Waals heterostructures comprising semiconducting 2D layers are of particular importance, since monolayer materials (for example MoS2 and WS2) can be direct-bandgap semiconductors in contrast to their bulk counterparts [1]. Various van der Waals heterostructures have been explored for electrically induced light emission [2], where proof-of-principle devices were typically made of exfoliated materials drastically limiting reproducibility and scalability. Consequently, for practical device applications, large scale materials grown by chemical vapour deposition (CVD) technique are crucial. In this work, we study electrically induced light emission in vertical van der Waals heterostructures fully produced from CVD materials. Vertical graphene contacts are employed to facilitate efficient injection of both the electrons and holes in the active materials such as WS2. Using hBN tunnelling barriers between graphene contacts and active semiconductor materials, the current leakage is minimised for efficient electron-to-photon conversion. Next, the van der Waals heterostructures are combined with plasmonic nanocavities to enhance light emission efficiency and control emission spectrum. References: [1] F. Xia, H. Wang, D. Xiao, M. Dubey, A. Ramasubramaniam, Nat. Photonics. 8, 899, (2014). [2] F. Withers et al., Nat. Mater. 14, 301, (2015).

Resume : Near-infrared (NIR) organic photodiodes (OPDs) consisting of organic layers sandwiched between two electrodes detect NIR light by converting absorbed incident photons into electrical signals. Especially in biorecognition area, NIR OPDs become more important in recent years, as the electromagnetic waves carry information of vein patterns, thus facilitating the identification. Until now, there have been many research interests in fabricating highly efficient NIR OPDs, by using mainly solution processes. However, solution methods have various drawbacks, such as difficulties in process integration and high density array fabrication. Here, a high-performance small molecular NIR OPD including an indium(III) phthalocyanine chloride (CllnPC) and C60 bulk heterojunction used as a photoactive layer is fabricated by utilizing only vacuum processes that are highly compatible with common manufacturing processes used in the photodiode industry. A maximum noise-current-based detectivity (D*) of 3.3 × 1012 Jones, a low noise equivalent power of 4.11 × 10-10 W/cm2, wide linear dynamic range of 77 dB, and fast enough temporal response with a -3dB frequency of 2.85 kHz are obtained, that can be ascribed to the excellent NIR absorbing-ability of ClInPc:C60, the efficient charge separation, and the effectively suppressed dark current.

Resume : The excellent optoelectronic properties of perovskite nanocrystals (NCs) has stimulated a widespread investigation of this class of semiconductors. Very recently, it has been demonstrated that CsPbBr3 NCs can reach near-unity PLQY in solution. Yet, retaining the PLQY in film is not trivial. Here, a room temperature synthesis of perovskite NCs displaying near-unity PLQY in solid state films is presented. Spin-coated films of the obtained CsPbBr3 NCs show PLQY values approaching unity (>95%). The as-obtained NCs show PLQY = 80% in spin-coated films which further enhancement via addition of PbBr2. Following the synthesis, the obtained NCs were employed in optoelectronic devices. Efficient solar cells based on mixed-halide (CsPbBrI2) NCs obtained via anion exchange reactions under ambient conditions were fabricated. Solar cell devices were fabricated in air with two different deposition methods: single step (SP) and layer-by-layer (LbL). The solar cells display a photoconversion efficiency of 5.3%, independently of the active-layer fabrication method, and open circuit voltage (Voc) up to 1.31 V, among the highest reported for perovskite-based solar cells with bandgap below 2 eV. The potential of material is further tested in light-emitting diodes (LEDs) employing an inverted structure comprising a ZnO nanoparticles electron-transport layer and a conjugated polymer hole-transport layer. The LEDs demonstrate a maximum external-quantum-efficiency of 6.04%, with luminance of 12998 Cd/m2 and low efficiency droop (around 10%). These results show the versatility of our synthetic protocol while the material quality is pointed out by the high performance of the optoelectronic devices.

Resume : There are intensive ongoing research efforts on for short-wave infrared (SWIR) photodetection in particular for cost-effective alternatives to current systems, which mostly requiring costly epitaxial growth and/or highly toxic elements. In this work, we propose an innovative ultra-fast photodetector via tactfully combining a Pt microwire with solution-processed colloidal plasmonic gold (Au) NRs to form a hybrid photodetector sensitive to photons with a wavelength of 1.54 micron. Such hybrid devices demonstrate an ultra-fast response time (rise time = 130 μs, decay time = 190 μs) and a 5 times responsivity enhancement compared to the control device with a Pt microwire alone. They benefit from the efficient and ultra-fast hot carrier excitation and relaxation in plasmonic metallic nanoparticles, an important process leading to the plasmonic-induced photothermal effect. Aspects including Au NR synthesis, device fabrication, hybrid device optimization, and device characterizations will be discussed in detail. The performance of these Au-NR/Pt photodetectors are competitive comparing to current SWIR photodetection technologies owning to their solution-processed fabrication (without toxic heavy elements) and their fast response time.

Resume : Optical sensing in the mid and long-wave infrared (MWIR, LWIR) is of paramount importance for a large spectrum of applications including environmental monitoring, gas sensing, hazard detection, food and product manufacturing inspection, etc. Yet, such applications to date are served by costly and complex epitaxially grown HgCdTe, quantum-well and quantum-dot infrared photodetectors. The possibility of exploiting low-energy intraband transitions make colloidal quantum dots (CQD) an attractive low-cost alternative to expensive low bandgap materials for infrared applications. Unfortunately, fabrication of quantum dots exhibiting intraband absorption is technologically constrained by the requirement of controlled heavy doping, which has limited, so far, MWIR and LWIR CQD detectors to mercury-based materials. We have developed a robust doping strategy for PbS quantum dot solid-state films that allows harvesting of mid- and long-wave infrared radiation, well beyond the reach of PbS even in its bulk form. Our doping method leads to simultaneous interband bleach and rise of intraband absorption. We show doping to be stable under ambient conditions allowing, for the first time, to realize intraband PbS CQD photodetectors for energies below the bulk bandgap, in the 5‒9 µm range. We have developed a quantum transport model to analyse the performance of PbS intraband photodetectors. We show that complete filling of the conduction band is required for achieving highly sensitive devices.

Resume : Nano-structured Sb2Se3 film fabricated by low temperature gains high sensitivity in photo-response at room temperature. Sb2Se3 film employed as photo-detector performs approximately 100 times enhancement in resistance variation. A flexible TFT as photo-response amplifier was realized using laser-buffer-layer (LBL), which directly enables crystallization and dopant activation of poly-Si on the polyimide by visible pulsed laser. As-fabricated flexible poly-Si TFT shows Ion/Ioff of 105-106 and subthreshold swing of 200 mV/dec. The integration of flexible Sb2Se3 photo-detector and poly-Si TFT intensifies the photo-response and identify the light wavelength for future electronic-skin application.

Resume : Detection and imaging of near-infrared (NIR) light is central to science and technology, including information technology, robotics or machine vision. All-organic upconversion devices (OUCs) recently emerged as candidates for next-generation NIR light imaging. OUCs consist of a monolithic semiconductor nanostructure stack of an organic NIR photodetector (OPD) and an organic light-emitting component. OUCs convert NIR directly to visible light which can be detected with a conventional camera. Here, we report on the fabrication of all-solution processed OUCs based on a nanostructured NIR squaraine dye OPD [1] and a visible-emitting polymer. The device consists of five coated functional layers and converts NIR light at 1 μm to visible light with efficiency close to the expected maximum. To eliminate the vacuum step to deposit the Ca injection layer we added a salt to the emitter, thereby transforming the emitting diode into a light-emitting electrochemical cell (OLEC) [2]. Due to the presence of mobile ions, OUCs comprising an OLEC are dynamic devices. We find that an appropriate pre-conditioning voltage step results in a lower turn-on voltage and higher efficiency than the corresponding OUC-OLED. Nanostructured OUC-OLEC devices can now be realized entirely from solution and promise novel NIR light sensing applications in various fields such as medical imaging. [1] Strassel, K. et al, ACS Appl. Mater. Interfaces 2018,10,11063. [2] Diethelm, M. et al, Adv. Opt. Mater. 2018,1801278.

Resume : Colloid quantum dots (CQDs) solar cells received great attention in recent years due to its solution processability and strong light harvesting capability in infrared region. The efficiency of CQD solar cells has been improving quickly in recent years, with certified power conversion efficiency up to 11.3%. The use of quantum dots surface engineering effectively reduced defects and improve carriers transport. In addition, device structure engineering significantly prompted the carriers transporting and inhibited interfacial recombination.1 Unlike most other solar cells, both normal structure and inverted structure exhibit similar high efficiency, the efficiency of CQD solar cells with inverted structure is generally poor (below 5%). In this work, we fabricated lead sulfide CQD solar cells with inverted structure by using nickel oxide as bottom hole transporting layer. The best device show short circuit current density of 27.6 mA/cm2, open circuit voltage of 0.53 V, fill factor of 65.7% and an overall power conversion efficiency (PCE) of 9.70% under standard 1.5G solar illumination2. This indicate that inverted structure, with appropriate material and interface design, could be another opportunity for more efficient CQD-based photovoltaics. In addition to solar cells, our recent work of using surface and interface engineering to improve the efficiency of QDs or nanosheet based photocatalysis will be presented. A nanocomposite based on the combination of QD and 2D MoS2 nanosheets give a record quantum efficiency over 40.8% (hydrogen generation speed of 40.1 mmol·h-1·g-1 under standard visible light illumination), which is record for Cd-free QDs based photocatalysis. Reference: 1. Ruili Wang, Yuequn Shang, Pongsakorn Kanjanaboos, Wenjia Zhou, Zhijun Ning*, and Edward H. Sargent*, Energy Environ. Sci., 2016, 9, 1130-1143. 2. Highly Efficient Inverted Structural Quantum Dot Solar Cells, Ruili Wang, Xun Wu, Kaimin Xu, Wenjia Zhou, Yuequn Shang, Haoying Tang, Hao Chen, and Zhijun Ning*, Adv. Mater. 2018, 1704882. 3. Quaternary Two Dimensional Zn-Ag-In-S Nanosheets for Highly Efficient Photocatalytic Hydrogen Generation, Hao Chen, Xiao-Yuan Liu, Shizhuo Wang, Xu Wang, Qi Wei, Xianyuan Jiang, Fei Wang, Kaimin Xu, Jianxi Ke, Qiong Zhang, Qian Gao, Youqi Ke*, Yi-Tao Long* and Zhijun Ning*, Journal of Materials Chemistry A, 2018, 6, 11670-11675 4. Efficient defect-controlled photocatalytic hydrogen generation based on near-infrared Cu-In-Zn-S quantum dots, Xiao-Yuan Liu, Guozhen Zhang, Hao Chen, Haowen Li, Jun Jiang, Yi-Tao Long, and Zhijun Ning*. Nano Research, 2018, 11, 1379–1388. 5. 0D–2D Quantum Dot: Metal Dichalcogenide Nanocomposite Photocatalyst Achieves Efficient Hydrogen Generation, Xiao-Yuan Liu, Hao Chen, Ruili Wang, Yuequn Shang, Qiong Zhang, Wei Li, Guozhen Zhang, Juan Su, Cao Thang Dinh, F. Pelayo García de Arquer, Jie Li, Jun Jiang, Qixi Mi, Rui Si, Xiaopeng Li, Yuhan Sun, Yi-Tao Long,* He Tian, Edward H. Sargent, and Zhijun Ning*. Adv. Mater., 2017,29, 1605646.

Resume : For many years the semiconductor industry has been driven by decreasing the structural dimensions thus increasing the device density and boosting computing and memory performance. Today, true nanoscale dimensions are reached with, e.g., transistor fin-width of smaller than 10 nm in the current technology node. The semiconductor industry research agenda is no longer driven by exclusively scaling dimensions but by integrating new materials and devices offering additional functionality. In this regard, the integration of nanoscale, high quality crystalline materials with precise control of dimension and location on the silicon platform is crucial. We have developed the Template-Assisted Selective Epitaxy (TASE) approach to monolithically integrate a broad range of III-V compounds on silicon for the applications in nanoelectronics and nanophotonics. In TASE the III-V semiconductor is grown within the confined space of an oxide template and results in an III-V-on-insulator structure which can be used for fabricating devices. High mobility semiconductors like InAs, GaAs and GaSb were grown by TASE and the structural and electronic properties were investigated in detail. Our investigations of the TASE materials as well as the fabricated nanoscale electronic and optical devices demonstrate the attractiveness of TASE to integrate nanoscale materials on silicon.

Resume : In vapor-liquid-solid GaAs NWs, doping control has proven to be difficult due to strong amphoteric behavior of Si dopants in the presence of a liquid droplet catalyst [1,2]. We present for the first time clear evidence of n-type doping in direct bottom-up growth of GaAs:Si NWs by catalyst-free selective area molecular beam epitaxy. The NWs are grown in a high arsenic pressure regime to facilitate the predominant formation of Si(Ga) substitutional donors. This growth scheme on silicon substrates is enabled by applying a high temperature surface treatment to SiO2-masked nanoscale apertures before growth, which prepares a 1x1-As terminated surface reconstruction that is essential for efficient nucleation. In TEM, our NWs show a high density of twinned segments, indicating a twinning induced growth mechanism where the twin induced rotation of the crystal structure exchanges fast and slowly growing facets, leading to strongly enhanced growth rates in the axial direction of the NW crystal. We further investigate the Raman spectra of GaAs:Si NWs with varied doping levels. By analyzing the signals of local vibrational modes caused by substitutional defects, we can study the site distribution of the amphoteric Si dopants and determine a predominant occupation of Ga sites. In addition, we probe the carrier density and material quality by photoluminescence spectroscopy. [1] Dimakis, Ramsteiner, et al. Nano Res. 5, 796-804 (2012). [2] Dufouleur, et al., Nano Lett. 10(5), 1734-1740 (2010).

Resume : Engineering of electronic and optical devices based on first-principles predictions has been one of the biggest dreams of materials scientists and engineers. However, the presence of non-periodically placed interfaces in macroscopic electronic and optical devices prevents the use of reciprocal space, while directly solving 10^23 coupled Schrödinger equations in the real space would require unavailable yottabytes (YB) of memory just for recording the spatial position of 10^23 particles. In this work, we report a hybrid quantum-classical design of Fabry-Perot multilayer resonator. Such design starts from an ab initio calculation of the dielectric function for each semiconducting layer with a specific atomic structure, followed by a study of wave scattering through the device using the transfer matrix method within the classical electromagnetic theory. This photonic resonator consists of two multilayer reflectors separated by a single impurity layer, which is tuned to exhibit a resonant peak at the center of reflection band. The validation of this multiscale design was carried out on a freestanding nanostructured porous silicon multilayer film fabricated by electrochemical etching of a highly-doped p-type [100]-oriented crystalline Si wafer alternating two anodic current densities and finishing with a high current to separate the multilayer from the substrate. The measured infrared transmittance spectra are compared with those predicted from the multiscale design.

Resume : Germanium-Tin (GeSn) has recently emerged as a potential Si-compatible light source thanks to its direct band gap for high Sn contents. Lasing was investigated in various types of GeSn optical cavities, with rapid performance improvements – namely a reduction of lasing threshold and an increase of maximum lasing temperature – in recent years. The first GeSn 16% Photonic Crystal (PhC) laser – thanks to the band-edge mode effect – was reported in 2018, with a relatively high threshold at 15 K (227 kW/cm2) and a rather low lasing temperature (60 K at most). We have used here a thicker GeSn 16% active layer (800 nm) grown on a GeSn step-graded buffer, itself on a Ge-buffered Si substrate (to minimize defect density). A definite increase of the maximum lasing temperature (from 60K up to 180 K) and of the light extraction of the GeSn PhC laser was achieved. We indeed obtained a significant increase of the output signal, by more than 80 times compared to the first GeSn PhC laser in [1] (same experimental setup). We explain the higher lasing temperature by a better overlap between the gain distribution and the optical mode, equipped further with a better optical confinement. The enhanced light extraction is attributed to the band-edge mode found outside the light line, which is well known for its ability to couple with radiative modes in the continuum. [1] Q. M. Thai et al., 2D hexagonal photonic crystal GeSn laser with 16% Sn content, APL 113, 051104 (2018)

Resume : Small electrically driven and CMOS compatible infrared light sources are important building blocks for numerous lighting and sensing devices that are of high interest for on-chip optical communication. Due to the indirect bandgap of silicon, efficiencies of silicon-based light sources are still quite low and cannot compete with efficiencies of hetero-integrated III-V based devices, which already are available commercially. However, photonic design principles of light sources permit to enhance the efficiencies of group IV devices by amplifying the local density of optical states and therefore Si - based emitters deserve a closer investigation. In the present study, we integrate a p-n junction into an ellipsoidal silicon nanophotonic resonator and drive it electrically as a light emitting diode (LED), using a tungsten atomic force microscope (AFM) probe as a top contact. The electroluminescence emission at room temperature is centered around the silicon band to band emission (1100nm), has a width of about 300nm, and is up to three times enhanced in narrow photonic modes hosted by the ellipsoidal device geometry. Overall emission intensity as well as the intensity of individual modes rise linearly with power over about two orders of magnitude and saturate together with the device current for high applied powers. Furthermore, the devices exhibit a reproducible I-V characteristic and performance even after the application of very high power densities of up to ~10^5A/cm^2.

Resume : III-nitride compound, in particular GaN, have attracted considerable attention for high power electronic and optoelectronic devices like LED and high electron mobility transistors (HEMTs). Monolithic integration of III-N nanostructures with Si substrates could provide a further significant development of nanoelectronics. However, the growth of GaN on Si substrate is an important technological challenge due to the large lattice mismatch and difference in thermal expansion coefficients. Here we explore a new approach for growth of GaN on Si substrates, which consist of a combination of ultra-thin GaN nucleation layer formation by low-temperature (380°C) plasma enhanced atomic layer deposition (PE-ALD) and subsequent growth of GaN layers by molecular beam epitaxy (MBE). Choose of appropriate nucleation conditions is a key issue for successful crystal growth. A set of PE-ALD modifications including continuous hydrogen plasma mode; pulsed plasma at nitrogen step; argon plasma surface activation was tested. Also, an effect of rapid thermal annealing in nitrogen ambient on properties of nucleation layer was studied. The structural and optical properties of GaN layers grown on Si and sapphire substrates with buffer layer grown at different conditions and annealed at different temperatures were studied using SEM, TEM, AFM, UV-vis-IR, PL and Raman spectroscopy.

Resume : A Visible Light Communication LED-assisted navigation system for large indoor environments is presented. White RGB-LEDs, whose original function is providing illumination, are used as transmitters due to the ability of each individual chip to switch quickly enough to transfer data. This functionality is used for communication where the multiplex data can be encoded in the emitting light. The receiver is implemented using a double p-i-n/pin SiC photodetector with light filtering properties. An OOK modulation scheme is used to modulate the light. Optoelectronic characterization of the transmitter and receiver are performed. A detailed analysis of the component’s characteristics within the VLC system, such as multiplexing techniques, visible light sensing, indoor localization and navigation recognition are discussed. A VLC scenario is established and the communication protocol presented. The SiC optical sensor receive the modulated signals from the LEDs containing the information to broadcast and geographical position of the emitters, demultiplex and decode the data and locate the receiver in the environment. Bi-directional communication between the infrastructure and the mobile receiver is tested. Two cellular networks are tested and compared: the square and the hexagonal. A 2D localization design, demonstrated by a prototype implementation, is presented. The key differences between both topologies are discussed. The results show that the LED-aided VLC navigation system make possible to determine the position of a mobile target inside the network, to infer the travel direction along the time and to interact with information received.

Resume : ZnO is the most promising transparent conductive oxide for UV optoelectronic applications such as photodetectors, light emitting diodes, and solar cells, due to its large direct band gap (3.37 eV) and large exciton binding energy (60 meV) at room temperature. However, being difficult to p-dope ZnO, it is necessary to rely on heterojunctions with other materials, most commonly Si. We developed photodetectors based on ZnO/Si heterojunctions with laser-microstructured Si substrates in order to increase the absorption and the active area of the heterojunction. This way, we achieve an increased heterojunction area without the need for intermediate insulating materials between ZnO and Si. Arrays of Si microcones were developed by nanosecond laser irradiation of a p-Si wafer in SF6. ZnO was conformally deposited by ALD on the micro-Si substrate, forming a p-n heterojunction. Aluminum was thermally deposited for top and back electrodes. For reference, a heterojunction was also formed on a flat Si substrate. The morphology and conformal coating of the samples were characterized by SEM and EDX measurements. Dark and illuminated I-V characteristics show a rectifying behavior with a 10-fold increase in the current values for the microstructured device. The devices show photoconductivity with a broader spectral response than a Si p-n homojunction, due to the contribution of the wide band gap of ZnO. The photoconductivity shows three distinctive spectral regions due to Si, electronic states within the band gap of ZnO, and band-to-band transitions in ZnO. More importantly, a considerable enhancement in photoresponsivity was observed in the case of microstructured ZnO/Si (x10-x20), which correlates well with the increase in the surface area.

Resume : A reduction of the band gap in GaAsN alloy due to the substitution of small amounts of nitrogen at anion sites in GaAs was observed. Ion implantation is a simple and efficient method for the introduction of N into GaAs beyond its equilibrium solubility. However, N+ implantation into a blank GaAs surface and subsequent high temperature furnace annealing are not suitable because GaAs tends to decompose and dilute N ions tend to evaporate. We supposed to use GaAs with AlN capping layer. This paper presents results confirming an effectiveness of such technique. 70 nm AlN capping layer was deposited by PECVD on a GaAs surface (AlN/GaAs structures) before the implantation. Multiple N+ implants with energies in the 30–310 keV range and doses of (1.3-10)х1015 cm-2 were carried out to create 370 nm thick layers with a uniform N concentration of 3.3x1020 cm-3. Furnace annealing was carried out in a flowing Ar ambient in the temperature range of 650-750°C for 10-120 min. The properties of GaAsN layers were studied by c-RBS, X-ray diffraction, TEM and photoluminescence techniques. Presence of AlN capping layer confirmed an effectiveness using the furnace annealing for AlN/GaAs:N structures to achieve a high value of fraction of ‘‘active’’ substitutional N on As sublattice. The light-emitting structures exhibit a high photoluminescence intensity in the near infrared range at room temperature. This work was supported by the Russian Science Foundation (Project No. 17-19-01200).

Resume : The La0.8Ba0.1Ca0.1Mn0.85Co0.15O3 sample was prepared by polymerization complex sol-gel method. The crystallographic study indicates that our sample crystallizes in the rhombohedral structure with the R-3c space group, at room temperature. The dielectric properties of this material were studied,in the frequency range of 500-106 Hz and a bias voltage range of 0- 2 V, using the impedance spectroscopy analyzer, at room temperature. The imaginary part of the complex impedance frequency dependence revealed one relaxation peak. The Nyquist plots were modelledwith grain and grain boundary resistance by introducing properac equivalent circuit. The impedance loss plot showed that the relaxation peak shifts towards the higher frequency, with increasing bias voltages. This implies the possibility of a charge hopping between the localized charge states. High frequency dielectric constant of our sample are found to be in the order of 105.The dielectric loss at different bias voltages shows the possibility of hopping mechanism for electrical transport processes in the system.Our results are interesting and indicate that this material can be a potentiel candidate in some applications.

Resume : As an alternative to liquid crystal display (LCD), micro-LEDs based on organic light-emitting diode (OLED) or inorganic LEDs has attracted increasing interest to create thinner, lighter and flexible display. Especially, GaN/InGaN-based LEDs possess advantages over OLEDs for high-efficiency and long-term stability. However, implement of red-green-blue (RGB) colors on a single chip substrate as well as emission quenching through surface nonradiative recombination sites remain a challenge for GaN/InGaN-based micro-LEDs. Here we have explored bottom-up based epitaxial growth of InGaN/GaN crystals with precisely-controlled position and variable size on same substrate. To achieve our target structure, we established a low-temperature solution-phase synthesis of 3D architectures constructed with well-regulated, single crystalline ZnO crystals, and subsequent heteroepitaxial MOCVD growth of GaN/InGaN multi quantum wells with following strategies. First, we study systematically the axial and lateral growth rate in correlations with the diameter and interspacing of nanocrystals, as well as the concentration of additional ion additives. Second, we prevent collapse of the ZnO crystals during GaN growth by employing graphene sheets as a protection/mask layer with initial GaN growth at a relatively low temperature under hydrogen-depleted conditions. Third, we investigated the temperature-dependent growth behaviors of GaN and GaN/InGaN MQWs on diverse types of ZnO templates, which can facilitate the control of hexagonal crystal shapes as well as In composition and QW width change. We further demonstrated a LED array through incorporating high-quality n-type and p-type GaN layers. This achievement represents additional opportunities to impose new functions and applications in LEDs, such as full-color flexible display, optogenetics and so on.

Resume : Recently, aluminum nitride (AlN) buffer layer has been actively studied to fabricate high quality gallium nitride (GaN) template for high efficiency Light Emitting Diode (LED) production. We confirmed that AlN deposition after N2 plasma treatment on substrate has positive influence on GaN epitaxial growth. In this study, N2 plasma treatment was conducted on commercial patterned sapphire substrate by using RF magnetron sputtering equipment. GaN epitaxial layer was grown by metal organic chemical vapor deposition (MOCVD). Surface with N2 plasma treatment was analyzed to binding energy by x-ray photoelectron spectroscopy (XPS). As a result of XPS, the surface was changed from Al2O3 to AlN and AlON, and we confirmed that thickness of pretreated layer was approximately 1nm by high resolution transmission electron microscope (HR-TEM). The AlN buffer layer deposited on the grown pretreated layer had lower crystallinity than as-treatment on PSS. Therefore, effect of surface N2 plasma treatment on PSS has lower crystallinity of AlN buffer layer, and is a factor to increase epitaxial growth quality of GaN template.

Resume : We investigated the effects of different growth conditions and surface passivation on the growth of CdSe QDs. Moreover, we studied the formation and optical properties of CdSe/ZnS/CdSe/ZnS quantum dot quantum well (QDQW) heterostructures. The synthesis of CdSe QDs by pyrolysis of organometallic reagents was performed by using the hot-matrix method. Synthesized CdSe QDs and fabricated QLED were studied to evaluate the structural and optical properties by using tunneling electron microscopy (TEM), photoluminescence (PL), and electro-luminescence (EL) measurement.

Resume : Vertically-aligned carbon nanotubes (VA-CNTs) can have the ability to act as compliant small-scale springs or as shock resistance micro-contactors, however it is crucial to understand how to engineer their electromechanical properties. This work investigates the performance of VA-CNTs as micro-contactors in electromechanical testing applications for probing Si wafers at wafer-level chip-scale-packaging (WLCSP) and wafer-level-packaging (WLP). Fabricated on ohmic substrates via photo-thermal chemical vapour deposition (PTCVD), where the optical energy efficiently coupled with the growth front (the catalyst) whilst the substrate is water cooled and protected with a use of ohmic thermal barrier layer (TiN) allowing high quality, highly dense CNT deposition (ID/IG < 0.4, 1 x 10^10 /cm^2 CNT areal density) at CMOS compatible temperatures (< 400 degrees C). 500-µm-tall CNT-metal (CNT-Au and CNT-Pd) composite contact structures are electromechanically characterized. The probe design and architecture are scalable, allowing for the assembly of thousands of probes in short manufacturing times, with easy pitch control. The effect of the metallization morphology and thickness on the compliance and electromechanical response of the metal-CNT composite contacts is discussed. Pd-metallised CNT contactors show up to 25 μm of compliance, with contact resistance as low as 460 mΩ (3.6 kΩ/µm) and network resistivity of 1.8 x 10^-5 Ω cm, after 50,000 touchdowns, with 50 μm of over-travel, displaying reproducible and repeatable contacts with less than 5% contact resistance degradation. Failure mechanisms are studied in-situ and after cyclic testing show that, for fully-metalized contacts, the CNT-metal shell provides stiffness to the probe structure in the elastic region, whilst reducing the contact resistance. In the plastic region, micro-crack induced tearing occurs along the edges of the probe, causing lateral expansion of the shell walls. Tip-only metallised Au-CNT contacts on the other hand show “foam-like” buckling under compression without lateral expansion due to the highly porous structure. Additionally, only metallizing the tip of the CNT probe did not provide extra compliance. The stable low resistance with high current carrying capacity achieved, the high repeatability and endurance of the manufactured probes make CNT micro-contacts a viable candidate for compliant micro-probing applications at small pitches for Si wafer testing before packaging in which current solid metal contactors have fundamental drawbacks in terms of the shortest pitch that can be achieved without compromising elastic properties, cost and efficiency. References: Tas M.O., Baker M.A., Bentz J., Boxshall K., Stolojan V., 2018. CNT-Based Microprobe Design for Electromechanical Probing Applications at Wafer Level Chip Scale and Wafer Level Packaging. SWTW 2018, 4 June, San Diego, USA. * awarded the most-inspiring presentation award.

Resume : Graphene produces an atomically thin, flat and uniform Schottky contact with the semiconductor, enabling a large, well-defined photoactive area. Especially, the graphene on n-type GaN (Gr-GaN) Schottky Junction photodetectors (SJPDs) are of great interest for filter-free visible-blind UV detection due to their high responsivity and fast response. At the same time, graphene transparent electrodes enable the build-up of optically active built-in potential over regions just beneath the electrode, without substantial loss of light absorption by the metal grid. Nevertheless, a systematic analysis of graphene and GaN junction characteristics has not yet been carried out. Here, we report the photoresponse characteristics of graphene-on-GaN Schottky junction photodiodes (Gr−GaN SJPDs), showing a typical rectifying behavior and distinct photovoltaic and photoelectric responses. This allowed us to create a high-performance SJPD showing distinct photovoltaic and photoelectric responses, with Voc of 0.57 V and maximum responsivity of ~110 mA/W at Va = −1 V. Time-dependent analysis of the UV response of the photocurrent (Ip) allows a direct observation of the photo-carrier dynamics under various applied bias (Va). Importantly, this study reveals distinctly different responses of Ip as a function of Va. The underlying mechanism of the anomalous response is elucidated by considering the interplay between UV irradiation-induced electrostatic molecular interactions over the graphene sheet and trapped charges at the defect state in the GaN thin film.

Resume : Inductively Coupled Plasma-CVD (ICP-CVD) can provide dense and homogeneous layer deposition thanks to its high plasma densities during the deposition process. More interestingly, ICP-CVD can enable low temperature deposition which could offer great potentials in the increasing domains of interests for flexible electronic applications. The deposition of silicon dioxide (SiO2) and amorphous silicon (a-Si) layers has been optimized. Different combinations of parameters such as radio frequency (RF) power, low frequency (LF) power, process temperature (20˚C-120˚C) and pressure have been evaluated. Capacitance-Voltage measurements of metal-oxide-semiconductor (MOS) capacitors were carried out to characterize SiO2 films. It has been confirmed that the presence of RF power and lower chamber pressure would decrease the flatband voltage. In addition, moderate LF power of 500W and process temperature of 20˚C would not only benefit the flatband voltage but increase the breakdown field. Nano-crystalline silicon nanowires fabrication by spacer method has also been investigated. SiO2 layer is first deposited and plasma etched to form vertical sidewalls. Then a-Si layer is deposited and plasma etched to form nanowires along sidewalls. Thin film transistors (TFTs) were fabricated with maximum process temperature of 220˚C and show promising electrical properties using ICP-CVD technique as a potential alternative for electronics compatible with low-cost flexible substrates.

Resume : Terahertz (THz) imaging triggers are of great interest for applications in security, surveillance, non-destructive control, biomedicine, etc. However, high-sensitive detectors that enable passive imaging are still missing. In this study, we investigate the performance of ultra-thin Silicon On Insulator (SOI) lateral PiN-like diodes at low temperature for cooled THz microbolometer detectors. Several PiN diodes and Schottky diodes have been designed and simulated, showing temperature coefficients of current (TCC) above 20%/K for operating current and temperature of 1 nA and 77 K respectively. In addition, some PiN and Metal/Semiconductor Schottky lateral diode prototypes have been fabricated on 4” silicon wafers with different geometries. PtSi metallization has been chosen to form both ohmic and Schottky contacts, depending on the region doping level. The top dielectric layer is made of Al2O3 deposited by Atomic Layer Deposition (ALD). I-V and C-V electrical measurements are on-going to provide diode parameters (ideality factor, leakage current and series resistance) as well as physical parameters such as silicon/oxide interface states and trap levels. Noise measurements will be presented as well in order to derive the electrical noise-equivalent power (NEP) of the future bolometer. Finally, experimental results are used to implement an accurate TCAD model suitable for a reliable design of the final diode to be integrated in the THz microbolometer pixel.

Resume : A safe smart vehicle lighting system that combines the functions of illumination, signaling, communications, and positioning is presented. The bidirectional communication between the infrastructures and the vehicles (I2V), between vehicles (V2V) and from the vehicles to the infrastructures (V2I) is performed through Visible Light Communication (VLC) using the street lamps and the traffic signaling LEDs to broadcast the information. Vehicle headlamps are used to transmit data to other vehicles or infrastructures (traffic lights) allowing digital safety and data privacy. As receivers and decoders, pin/pin SiC Wavelength Division Multiplex (WDM) photodetectors, with light filtering properties, are being used. Optoelectronic characterization of the transmitter and receiver are performed. A detailed analysis of the component’s characteristics within the VLC system, such as multiplexing techniques, visible light sensing, indoor localization and navigation recognition are discussed. We propose the use of white polychromatic-LEDs to implement the WDM. This allows modulating separate data streams on four colors which together multiplex to white light. When a probe vehicle enters the infrastructure’s capture range, the receivers respond to light signal and the infrastructure ID and traffic message are assigned. They perform simultaneously the V2V distance measurement and data transmission functions and, using the headlamps, resend the data to the other vehicles or to the traffic signals (V2I). A I2V2V2I traffic scenario is stablished. A phasing traffic flow is developed as a Proof of Concept (PoC). The experimental results confirm that the cooperative vehicular VLC architecture is a promising approach concerning communications between road infrastructures and cars, fulfilling data privacy.

Resume : We investigated the effect of In composition in InGaN quantum wells (QWs) on the on ultraviolet sensitivity of InGaN-based photodetectors (PDs). The InGaN-based QWs structure as the active layer improves the photoreactivity. In particular, it is known that the In composition of the InGaN active layer is an important factor for evaluating the photoreaction characteristics. Therefore, we controlled the growth temperature of InGaN/GaN QWs and changed the In composition of InGaN active layer from 8 to 15%. The EL wavelengths of these three p-i-n InGaN-based PDs were 400, 427 and 450nm, and their In compositions were measured at 8, 12, 15%, respectively. When irradiated with the 405nm laser, UV PDs with 12 and 15% In contents exhibited a photoreaction of several μA. However, the ultraviolet sensor with 8% InGaN QWs hardly shows the photoreaction. We believed that a very small photoreaction was obtained because the 405nm excitation laser was transmitted through the active layer with 8% InGaN QWs. The InGaN-based PDs with 12% and 15% In contents showed the same photocurrent of ~ 1.6 μA at -1.0 V. However, as the reverse bias increased, the photocurrent of InGaN-based PD with 12% InGaN active layer increased more than the 15% InGaN active layer. We believe that the higher In content in InGaN quantum well can increase the band offset between InGaN well and GaN barrier, and deteriorate the crystallinity of InGaN active region due to In localization-induced crystal defects.

Resume : We investigated the effect of Ni thickness on optical and electrical properties of ZnO-based oxide-metal-oxide photodetectors (PDs). We grew 30nm-thick ZnO thin film on a glass substrate using atomic layer deposition. In order to improve the electrical characteristics of the ZnO thin film, Al: ZnO (AZO) films were achieved by feeding the Al precursor. AZO-Ni-AZO was formed by varying the thickness of Ni from 2 to 8 nm using a thermal evaporator. As-grown ZnO film exhibited strong PL band edge emission of 3.3 eV, while the band edge emission of AZO film was not detected by room temperature PL measurement. However, the PL spectrum of AZO film showed the deep level emissions of 2.4 eV and 1.78 eV. It is believed that the oxygen vacancy and oxygen interstitial impurities are greatly increased by the Al dopant of the AZO film. We fabricated metal-semiconductor-metal PDs by depositing Al/Au electrodes on AZO-Ni-AZO transparent films to find the photo-reactivity of the ultraviolet and visible regions. The photocurrent of AZO-based MSM PDs was not measured by 365 nm excitation because there is no band edge emission of AZO film. However, a relatively high photocurrent of AZO-based MSM PDs was obtained by a white light source. It is believed that the deep levels of AZO films can generate photo-carrier and increase the photocurrent in the visible wavelength region. In addition, the highest photocurrent was obtained by a 2nm-thick Ni embedded AZO-Ni-AZO structured PDs.

Resume : Optically pumped GeSn lasers with high tin contents ([Sn]> 12 %) were demonstrated by several research groups, with a maximum lasing temperature of 230K [1]. However, lasing thresholds are still high and have to be reduced to go towards CW laser emission. In this study, we exploit tensile engineering using silicon nitride stressor layer on laser performances of low tin concentration alloys ([Sn] <10 %). Thick GeSn layers with 6, 8 and 10% of Sn were grown at around 350°C on Ge-buffered Si substrates to (i) minimize defects and (ii) target the indirect-to-direct bandgap transition in the optically active films. A specific process flow was developed to transfer the GeSn layers disk on a metallic post, allowing to strongly reduce detrimental heating effects on the material gain under optical pumping [2]. A photoluminescence red shift have been observed thanks to the SiN stressor, and Raman spectroscopy has been used to estimate the biaxial strain value in the GeSn microdisk. An optical pumping were then performed on several SiN -encapsulated GeSn microdisks showing laser effects such as Ge0.92Sn0.08 with a threshold of 46 kW/cm2 at low temperature (25K). The impact of (i) microdisk diameter, (ii) tin content and (iii) tensile strain injected by the silicon nitride stressor on the lasing performances of quasi-suspended GeSn micro-disks will be thoroughly discussed. [1] Q. M. Thai et al., Opt. Expr. 26, 32500 (2018). [2] A. Elbaz et. al.,Appl. Photonics 3, 106102 (2018).

Resume : GeSn-based heterostructures with high Sn content have already shown optically pumped lasing in the mid infra-red up to 230K [1]. The next steps would be lasing (i) at higher temperature, (ii) with reduced lasing threshold and (iii) under electrical pumping. Different approaches are considered to progress in those directions. One of them is to control the strain of the active GeSn layer. Another is to improve carrier confinement by using GeSn/SiGeSn heterostructures. Our work focuses on the optimization of SiGeSn barriers added to a GeSn emitting layer. Previously, we have developed an 8-band k.p model of GeSn conduction and valence bands [2]. Here we extend it to SiGeSn with certain assumptions on band-offset values between GeSn and SiGeSn layers. Following the modelling, we grew several pseudomorphic SiGeSn layers with Si and Sn contents ranging from few percent up to 15% using a low temperature (below 350°C) 200 mm RPCVD tool . The SiGeSn layers act as carrier-confining barriers for GeSn emitting layers with Sn content from 6 to 13%. With the grown structures, we fabricated GeSn light-emitting diodes. Optoelectrical characterizations of LEDs with and without carrier-confining barriers highlighted the light emission improvement given by the barriers, supporting the idea that those barriers are a promising way towards future electrically pumped GeSn lasers. [1] Q. M. Thai et al., Opt. Express 26, 32500-32508 (2018) [2] M. Bertrand et al., Annalen der Physik, accepted.

Resume : Solid-state lighting materials such as quantum dots (QDs) have been great interest the researchers due to their superior emitting properties in terms of broad and accurate color expression. Despite the considerable progress in this area, the light extraction efficiency of the QD based display is a critical issue, which is suppressed by numerous numbers of protective layers for preventing QDs degradation caused by moisture and oxygen penetration. In this study, the light extraction efficiency of the QD-Photoluminescence (PL) structure is significantly improved using a unique top-down lithography technique. With one-step low energy plasma bombardment, three-dimensional (3D) and vertical Ag nanowall structures are perfectly hybridized on each wall structures between QD regions. Because of high-aspect-ratio (>20) and high light reflectance (98.3% @ 550nm) of Ag nanowall structure, light extraction efficiency of the QD-PL structure increased to a meaningful level for especially high-resolution pixel structures. More importantly, this process is an innovative and efficient process in the formation of high aspect ratio metal structure by completely excluding conventional methods like the photolithography and wet etch processes. This concept can be considered to be the best practical and most easily applicable alternative to the improvement of light extraction efficiency in all display concepts that use materials with the omnidirectional emitting property regardless of PL or electroluminescence (EL).

Resume : Heterostructures based on polar GaN have the strong intrinsic electric field caused by piezoelectric and spontaneous polarizations due to mismatch of the crystal cells of adjacent heterostructure layers. In the present report the internal electric fields in active region of LEDs structures based on InGaN/GaN QWs were investigated by electrotransmission (ET) spectroscopy. The samples under investigation were green LED heterostructures with 1, 2, 3 or 5 QWs in the active area grown along [0001] direction on the sapphire substrate by MOCVD technology. ET spectra were obtained at room temperature in the range of 2.3-3.5 eV with DC bias voltages in the range of -21.. 4 V so that there wasn’t radiative recombination in pn-junction. The energies (2.8-3.2eV) of interband transitions from the QW to barriers in active area were determined from ET spectra. ET spectra show that increasing the number of QWs leads to increasing the number of heterointerface interband transitions. This is connected with a different degree of the indium incorporation in QW of active area. The dependence of ground energy transitions on the bias voltage was approximated by solving Schrödinger equation with strained confined quantum well with electric field as the parameter. Obtained electric field is decreased from 3.55 to 3.05 MV/cm as the number of QW increased from 1 to 5.

Resume : LED heterostructures based on wurtzite GaN with quantum wells InGaN/GaN in active area of pn-junction can be a model object for study absorption and photoexcitation properties of optoelectronic devices, e.g. photodiodes or solar cells. Composition of layers, doping concentrations and inner electric fields determine a height of potential barriers for carriers affecting on the direction of the current through the active area. In the present report the photoexcitation properties of the InGaN/GaN LED heterostructure with different number of quantum wells were investigated by photocurrent spectroscopy. The samples under investigation were green LED heterostructures with 1, 2, 3 or 5 QWs in the active area grown along [0001] direction on the sapphire substrate by MOCVD technology. PC spectra were obtained at room temperature in the range of 2.5-3.5 eV with DC bias voltages in the range of -1..+4 V so that there was not radiative recombination in pn-junction. Photocurrent spectra have showed that direction of the photoexcitation current depends on the wavelength of the incident light at the certain bias voltage. Presumably, the forward current is created by electrons excited in the acceptor level in p-GaN, while the reverse current is produced by barrier electrons. The threshold bias voltage for this photoreversible current effect increases with the number of quantum wells.

Resume : The development of radiation resistant semiconductors for radiation detectors and electronic circuits is very important for applications in space, accelerators or nuclear facilities. Group-III-nitrides are wide bandgap semiconductors considered remarkably robust (thermally and chemically) and radiation resistant. The present work aims to understand the response of nanostructures based on GaN and related alloys to different types of radiation. In particular xenon (Xe) swift heavy ions (SHI) interaction with InGaN/GaN multi-quantum wells (MQWs) in order to achieve higher efficiency of green LEDs. In this work, InGaN/GaN MQWs (5 QWs, period: 13.7 nm with 11 nm barrier) grown on sapphire, were irradiated using 129Xe swift heavy ions with the fluence of 2×1012 ions/cm2 at different energies. GaN layers submitted to the same irradiation conditions were used as reference. Optical techniques such as transmission, micro-Raman, photoluminescence (PL), micro-PL and PL excitation are used. Xe SHI irradiation generated crystal damage and the activation of GaN phonon DOS for the MQWs and the layers, independently of the used energy.In addition, the transparency of the samples decreases with the irradiation energy, followed by a red-shift of the near band edge. Furthermore, the behaviour of the MQWs green emission is discussed as a function of SHI irradiation energy.

Resume : Recently, the micro-LED research spotlighted as the next generation of optical devices being actively conducted. Micro LED requires substrate separation. However, when the Laser Lift-Off(LLO), which is widely used recently, is applied, it is difficult to produce due to chip damage. Therefore, it is necessary to study the Chemical Lift-Off(CLO) which does not damage the chip. Our research team studied the substrate structure for efficient peeling of the sapphire substrate. In this study, we introduce the process of manufacturing an air tunnel in order to improve the efficiency of chemical lift-off. In the chemical stripping method, the sacrificial layer is inserted into the interface between the substrate and the film, and the substrate and the film can be peeled off by removing the sacrificial layer. When the etching solution for removing the sacrificial layer is transported using the inner air tunnel, the effect of the peeling time reduction can be expected. We performed the following process to fabricate the air tunnel structure. A sapphire substrate with a mesa shape pattern is fabricated. PR is coated on the substrate with the same mesa pattern height and the sacrificial layer is deposited. Then, PR is removed to make the sacrificial layer floated on the sapphire mesa pattern. A GaN film was grown on the fabricated structure by MOCVD. Through our air tunnel manufacturing process, we were able to manufacture a tunnel with a height of 1.2 μm and a width of 0.55 to 2 μm. We also report on the characterization of GaN films grown on air tunnel structures.

Resume : Blue and green light from III-N light emitting diodes can be obtained based on InGaN bandgap engineering. By incorporating a higher indium content, the emission wavelength can be shifted towards red, but the crystal quality deteriorates. Therefore, intense research activity is dedicated to achieve efficient red emitters. In order to provide additional solutions for red emission from III-N, this work studies the implantation of europium followed by high temperature and high pressure (HTHP) annealing of AlGaN/GaN superlattice-based diode structures. Non-destructive optical techniques were used to access the Eu3+ population mechanisms, as well as its emission lines and their temperature dependence. We have demonstrated that the implanted Eu ions reached the first quantum wells and that the HTHP partly removed the implantation defects, recovering some of the as-grown luminescence and optically activating the Eu3+ in the diode structure. A model was built based on the different excitation bands in the diode structure, demonstrating that an energy transfer between the AlGaN/GaN superlattice excitons and the Eu3+ ions occurs. This enhances the ion’s red luminescence. In addition, Eu3+ luminescence was observed not only with above but also with below GaN bandgap excitation. Furthermore, we found that at least three non-equivalent active sites are created by the Eu implantation in the diode structure, namely: Eu1, Eu2 and Eu-Mg defect in its both configurations Eu0 and Eu1(Mg).

Resume : This paper describe the characteristics of dual-gated FET using silicon nanowire (Si-NW) which is fabricated standard microelectromechanical system (MEMS) processes. The Si-NW FETs detecting the change of transient current induced by the binding biological molecules have attracted much attention because of the label-free biosensing application with high sensitivity. In this works, Si-NW FET includes the dual-gate (i.e. top and back gate) structure to amplify the signal by several times through a capacitive-coupling effect. The Back gate accumulates or inverts the entire channel and a top gate locally controls the energy-band profile of the channel [1]. Furthermore, the Si-NW with 70 nm in wide and 10 µm long is easily fabricated by the combination process of LOCOS and anisotropic wet etching. The process could reduce the cost in batch fabrication compared with the conventional top-down nano-patterning method such as electron beam lithography. The source and drain was implanted with 5X1015cm−2 of Arsenic ions at 30keV to form an ohmic contact. As a results, we observed electrical characteristics of Si-NW FET including a threshold voltage of 0.8V, steeper subthreshold swing of 162 mV/dec, on/off current ratio of 6.3x104. In dual-gate operation, the capacitive-coupling ratios of 10.1 was experimentally analyzed from the shift of threshold voltage while the top gate bias is changed from -300 mV to 300 mV in steps of 50 mV. For further works, we will investigate the pH solution and bio molecules sensing characteristics of Si-NW FET.

Resume : Optical communications operating in the visible range, either indoor or outdoor, are labelled as Visible Light Communication (VLC) and use white LEDs or single-color LEDs to code and transmit the information data. This technology is nested on the ubiquitous use of LEDs for lighting solutions and to the high energy efficiency of LEDs lighting when compared to conventional lamps. Indoor navigation based on VLC are attractive solutions for Indoor Positioning Systems (IPS), as Global Positioning Systems (GPS) signals are strongly absorbed by the buildings and other wireless solutions need to be used. In this work it is proposed an indoor navigation system based on the use of VLC technology for assisting warehouse management with autonomous vehicles. The system is designed to establish bidirectional communication between a static infrastructure and the mobile picking robot. Data transmission uses three different communication channels ensured by three different wavelength emitters of white tri-chromatic LEDs. A dedicated photodetector with selective spectral sensitivity in the visible spectrum is used. Different codification schemes are proposed to establish both uplink and downlink communication between LED lamps and mobile robots. The code schemes were designed to ensure synchronization between frames, to shield the decoding process from errors and to minimize flickering effects. The localization algorithm makes use of the Fourier transform to identify the frequencies present in the photocurrent signal and the wavelength filtering properties of the sensor under front and back optical bias to detect and identify the transmitted optical signals and make the adequate spatial position correspondence.

Resume : Germanium-Tin (GeSn) has been shown to behave as a direct gap material at relatively high Sn concentrations. It has prompted several studies on the lasing performances – lasing threshold and maximum lasing temperature - of GeSn-based optical cavities and on the intrinsic properties of the material itself. Here, we simulate the optical gain of GeSn 16% using as an input the band structure calculated with the local empirical pseudopotential method (EPM). The calculated gain, together with other inputs from experimental data (non-radiative carrier lifetime, absorption at pump wavelength …) were then introduced into the laser rate equation to simulate the lasing threshold. Modelling results showed that a transparency threshold was reached when the injected density carrier was found between 5.10^17 cm-3 and 6.10^17 cm-3 at low temperature. Simulations definitely underestimated lasing thresholds compared to experimental values found in [1,2]. There were indeed 3 to 12 times lower, in line with earlier optical gain calculations obtained using the k.p method [3]. The impact of the input parameters, as well as the difference in energy between the direct and indirect gaps of GeSn on the simulation outputs will be discussed. [1] V. Reboud et al., APL 111, 092101 (2017) [2] Q. M. Thai et al., Opt. Express 26(25), 32500 (2018) [3] R. Geiger, Thesis: Direct Band Gap Germanium for Si-compatible Lasing (2016)

Resume : With the rapid development of portable electronics, the dimension of the electronic devices continuously decrease and the degree of the device integration dramatically increase. Consequently, an reliable and low-temperature bonding (die-attachment) technique become very critical for the current opto-electronics industry. Au-Sn solder layer has been successfully used for the die-attachment for opto-electronics industry. Yet, the high melting (over 300 C) of the Au-Sn solder is the major limitation for many advanced applications with temperature sensitive components. In this work, we used the Sn layer as the bonding layer for the die-attachment. Yet, an X interlayer was designed in the middle of the Sn bonding layer. The electron beam evaporator is used to deposit the Sn/X/Sn sandwich solder structure. The thickness of the bottom and the capping Sn layer is 450 nm. The function of the X interlayer is to modify (1) the melting point of the Sn/X/Sn sandwich solder structure and (2) the outer Sn capping layer. The main X interlayer is either single Bi layer (400 nm) or Zn/Bi (Bi/Zn) bilayer. The thin Zn (26 nm) layer can further modify the melting point of the Sn/Bi/Sn sandwich solder structure. Also, the thin Zn (26 nm) layer can modify the surface of the bottom Sn layer and morphology of the Bi layer, which eventually affects the morphology of the Sn capping layer. For the Sn/Bi/Sn sandwich solder structure, we found that the Sn grain size of the Sn capping layer is larger and the grain size of the Bi is moderate. Interestingly, with the Zn surface modification layer on the bottom Sn layer, we found that the Sn grain size of the Sn capping layer was smaller and the grain size of Bi was larger and smaller in the Zn/Bi and Bi/Zn bilayer, respectively. Indeed, the morphology of the Sn capping layer and Bi grain size would be affected by the X interlayer. Also, the results of the bonding strength showed that the larger Sn and Bi grains on the top view surface had the lower bonding strength. The detailed mechanism will be discussed in the talk.

Resume : Various types of sensors find an application in almost all the fields of technology ranging from domestic front to space programme. Recent growth in industrialization, urbanization and increase in automobiles has led to manifold increase in air pollution which causes serious concerns in scientific community of all over the world. Moreover, the most challenging task is to monitor the quality of air. Hence a continuous monitoring of ambient air quality is required for suitably controlling the levels of pollutant gases. In the present scenario, there is no individual gas sensor that can sense multiple gases and hence array of gas sensor in the form of electronic nose is preferred which can sense different chemical inputs at a time. In this work, we present the performance comparison of SnO2 based laboratory fabricated electronic nose (EN-1) against electronic nose (EN-2) fabricated via commercially available gas sensors from FIGARO INC, Japan. The sensor materials were synthesized by co-precipitation method and characterized by XRD, FESEM, TEM and XPS. In discrepancy to the earlier cited work, stating non-selective nature of metal oxide based gas sensors, we have examined the effect of doping, change in work function and operating temperature for enhanced selectivity of fabricated electronic nose (EN-1). We have found that commercially available based sensors electronic nose (EN-2) shows cross sensitivity for different chemical inputs while laboratory fabricated electronic nose (EN-1) has minimized the cross sensitivity factor by four for the different chemical inputs.

Resume : We propose an insertion of a GaAs/Alloy multi-quantum well (mqw) GaAs/AlGaAs thin layer between the oxide and the n-GaAs layers to improve cell performance, by opening a wider reception window for incident solar photons through an AlGaAs/GaAs layer (1.80eV/1.42eV). Solar photons generate transport of mobile photo-excited carriers in all three major regions of the hetero-structured cell (graphene layer, depletion region and the bulk semiconductor). This transport includes (a) electrons thermionically escaping from the graphene side to the depletion region through the oxide layer (b) electron-hole pairs as photo-generated in individual quantum wells with subsequent thermionic escape for the wells and (c) holes generated in the bulk n-GaAs (1.42eV) and diffusing to the junction. The existence of lattice-matched AlGaAs/GaAs quantum wells (with a wider optical gap 1.59 eV) ensures electron hole separation due to the strong existence of the junction electrostatic field. Photoelectrons can overcome the junction barrier and by tunneling through the oxide layer, they can reach the n-GaAs side. We calculate two types of thermionic currents at the junction (a) thermionic emission current (TE1) from graphene to the semiconductor and thermionic emission- current (TE2) from the quantum wells in the depletion region. In both cases these currents strongly depend on temperature (T^3/2) due to both quantum well geometry (2D transport) and graphene’s density of states respectively. Simultaneously, minority photo-holes from the bulk and the mqw region, reach the junction by forming hole current. Note also that the oxide layer and the quantum wells reduce recombination at the junction of the Schottky device. Under illumination, the built-in field at the junction forces minority photo-holes, while electrons from the graphene side surmount the junction barrier. The device will generate three major current components: TE currents as mentioned above, photo-generated minority holes from the depletion region and holes reaching the junction by diffusion. The proposed device design (mqw layer in the depletion region) offers a wider band gap for enhanced photon absorption (1.80 eV), wider optical gap for further absorption (1.59 eV GaAs-AlGaAs interfaces) with subsequent photo-carrier escape to the conduction band. We predict higher total current due to thermionic emission from the graphene and the mqw regions and from photo-holes. Under AM 1.5 conditions we predict open circuit voltage in excess of 1.12 V, short circuit current density near 18 mA/cm^2, fill factors in the neighborhood of 75% leading to collection efficiency levels in excess of 15%.

Resume : While silicon (Si) and germanium (Ge) are the mainstay of contemporary microelectronics, their indirect band gaps render them inefficient emitters of light. Despite the development of CMOS-compatible passive photonic components, there remains a capability gap due to the unavailability of direct-gap materials to realise CMOS-compatible active photonic components. Interest in the group-IV alloys Ge(C,Sn) has surged: it has been suggested that incorporating ~1% carbon or ~10% tin in Ge generates a direct band gap. The demonstration in 2015 of the first GeSn-based laser has verified the feasibility of this approach. However, there has been relatively little detailed analysis of the fundamental nature of the alloy electronic structure, which is critical to determine its implications for technologically relevant material properties. We present a multi-scale theoretical analysis of Ge(C,Sn) alloys, combining first principles and semi-empirical approaches. Using hybrid density functional theory (HDFT) we identify and quantify the mechanisms responsible for, and the nature of, the indirect- to direct-gap transition in Ge(C,Sn) alloys. Our calculations highlight the critical roles played by alloy disorder, band mixing and carrier localisation in determining the evolution of the alloy electronic structure. We parametrise a semi-empirical tight-binding (TB) model, enabling large-scale atomistic analysis of realistic, disordered Ge(C,Sn) alloys. Using TB alloy supercell calculations we also derive and parametrise model Hamiltonians describing the impact of C- and Sn-induced band mixing in determining the nature of the perturbed Ge(C,Sn) conduction band structure. On the basis of our calculations we identify important consequences of the unusual Ge(C,Sn) alloy electronic structure for practical applications. Our analysis elucidates the mechanism driving, and nature of, the indirect- to direct-gap transition in Ge(C,Sn). For GeSn we demonstrate the continuous evolution of a direct band gap with increasing Sn composition - driven by Sn-induced band mixing - and describe the consequences of the hybridised conduction band for applications in light-emitting devices and tunneling field-effect transistors. Our results confirm the promise of GeSn alloys for practical applications, but provide significant new information regarding key material properties. For dilute GeC we describe the emergence of a "quasi-direct" band gap - driven by localised impurity-like behaviour - and, contrary to the existing literature, describe how this limits practical applications of GeC.

Resume : Catalysis-assisted vapor-liquid-solid nanowire (NW) growth offers opportunities to prepare versatile, axial and radial III-V homo- and heterostructures, which combine multiple scientific and economic benefits including application in innovative solar energy conversion [1]. To achieve high efficiencies, suitable doping profiles as well as well-defined junctions are required. In this work, multi-probe electrical characterizations were conducted on GaAs-NWs with axial pn-junctions. By the utilization of a multi-tip scanning tunneling microscope (MT-STM) [2], which is equipped with a scanning electron microscope (SEM), four tips can be controlled independently via nanopositioners at the nanoscale. With this setup it is possible to perform four-point probe measurement on single freestanding NWs. Besides the non-linear IV-characteristic, we detected a threshold voltage, which correlates to the forward bias of the GaAs-pn-junction. Local ideality factors of the diode can be extracted from the IV-curves, which enable a classification of the quality of the diode. By performing four-point probe measurements axial resistance profiles were recorded, which allow extraction of the axial doping profile of single NWs. The doping concentration of the p- and n-doped region was determined, as well as the position and width of the charge separating contact. The latter could also be visualized with electron beam induced current (EBIC) images [3]. [1]: Krogstrup, P. et al., Nat. Photonics 2013, 7 (March), 1–5. [2]: Cherepanov, V. et al., B. Rev. Sci. Instrum. 2012, 83 (3). [3]: Nägelein, A. et al., J. Photovoltaics 2019, in print

Resume : Four different sizes (~0.3, 1.0, 2.0 and 4.0 μm) of nearly monodispersed hollow hierarchical Co3O4 nanocages consisting of nanosheets were prepared by controlled precipitation of ZIF-67 (Zeolitic imidazolate framework-67) rhombic dodecahedra, solvothermal reaction, and thermal annealing. The sensors with the size of ~1.0 μm of Co3O4 nanocages showed extremely high responses (resistance ratios) toward 5 ppm of p-xylene (78.6) and toluene (43.8), while the responses toward interference gases such as ethanol, benzene, HCHO and CO were negligibly low. The significantly high methylbenzene selectivity emanated from the catalytic activity of Co3O4, while high gas responses were attributed to high gas-accessibility through the thin shell of hollow hierarchical morphology, high surface area per unit volume, and abundant mesopores. The key parameters to determine the gas sensing characteristics such as size, shell thickness, mesopores, and hollow/hierarchical morphology of the nanocages could be easily controlled by tunning the precipitation of the ZIF-67 rhombic dodecahedra and solvothermal reaction. Co3O4 hollow hierarchical nanostructures with tunable size and morphology can be used to design extremely high-performance methylbenzene sensors for indoor air quality monitoring.

Resume : All inorganic perovskites with good excellent physical properties received a considerable attention in optoelectronic applications. Despite the high performance photoelectric properties of all inorganic cesium lead iodide (CsPbI3) perovskites retaining the black α-phase, the phase instability in the ambient condition and the high annealing temperature process inhibit their further development. To improve the availability and long-term stability of CsPbI3, multi-anion structures have been studied in the ABX3 structure of CsPbI3. Herein, we suggest inorganic heterojunction with CsPbIxBr3-x and In-Ga-Zn-O (IGZO) semiconductor which applied bi-anion IxBr3-x instead of I3 in CsPbI3. Using this hetero-junction, we demonstrate a new phototransistor with a structure of p Si / SiO2 / IGZO / CsPbIxBr3-x / Ti-Al-Ti where IGZO is a charge transfort layer and CsPbIxBr3-x is a light absorption layer. The material stable properties of CsPbIxBr3-x were confirmed by changing the ratio of I : Br from 10: 0 to 0:10. When the ratio of CsPbIxBr3-x perovskite I: Br is 8: 2, the inorganic perovskite maintains the black α phase in the ambient condition. Phototransistor based on CsPbIxBr3-x perovskite/IGZO heterojunction shows excellent responsivities both ultraviolet region and entire visible light region, unlike the single-IGZO phototransistor reacting only in the ultraviolet region. Furthermore, long term reliability was confirmed by maintaining the α-phase of the IGZO/ CsPbIxBr3-x heterojunction layer well up to 300 hours.

Resume : The development of core-shell nanowires based on zinc oxide and copper oxide represents an interesting and cost-effective path for obtaining efficient water stable photocatalysts. Zinc oxide is a n-type wide band semiconductor (3.3 eV) while copper oxide is a p-type narrow band semiconductor (1.74 eV), these two metal oxides forming a p-n staggered gap radial heterojunction with zinc oxide as core and copper oxide as shell. The use of copper oxide as shell has two major advantages: to harvest a higher range of solar energy for pollutants removal and to overcome the zinc oxide nanowires’ dissolution in water based solutions. The zinc oxide pristine nanowires and the core-shell nanowires based on zinc oxide and copper oxide were investigated from the structural, optical, morphological and electrical point of view in order to evaluate the degradation of methylene blue in their presence. Degradation mechanisms were proposed taking into consideration the dissolution in water of the zinc oxide nanowires and an optimum copper oxide shell thickness where the radial heterojunction improves the separation of the photocharges when illuminating with UV light resulting in the degradation of methylene blue.

Resume : In recent years, amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistors (TFTs) are widely employed for various technological applications; however, they still have the problem of relatively low mobility due to their non-crystalline structure. In this study, we fabricated InGaZn4O7 nanowires (IGZO NWs) with different concentration of Ga by ambient-pressure chemical vapor deposition (CVD) method which can lead to the production of NWs with highly periodic superlattice structure. By fabricating the IGZO NWs based field-effect transistors (FETs), we realized that the concentration of Ga has a great effect on the electrical performance of the device. By increasing the concentration of Ga, the electrical performance can be improved as a result of the reduced amount of oxygen vacancies in the NWs. Further increasing the amount of Ga atoms can reduce the electrical properties, which can be caused by the deteriorated lattice structure because of excessive amount of Ga in the NWs. With the optimal Ga amount (30 at.%), the NW device on-off ratio can be as high as 6.9×105 and mobility ~110 cm2V-1s-1. Furthermore, the optimized In1.8Ga1.8Zn2.4O7 NWs are employed to fabricate the highly ordered NW parallel array FETs to demonstrate the potency for utilizing these high-performance NWs for the fabrication of future integrated circuits.

Resume : Herein, we describe the design and fabrication of the three-dimensional (3D) orderly superhydrophobic surface-enhanced Raman scattering (SERS) substrates for environmental monitoring in real condition. The Ag dendrites and ZnO nanorod arrays were employed as the trunk of 3D superhydrophobic SERS substrates respectively. The silicon nanoneedles were grafted on the surface of trunk, and a subsequent in-situ reduction was used for the deposition of Ag nanoparticles. By virtue of the interaction and decomposition reaction between silane and Au nanoparticles under the plasma condition, the bottleneck of grafting process and the subsequent in-situ deposition could be overcome and the desired high-active SERS substrates could be obtained. The Raman enhancement characteristics, growth mechanism, and their structural evolution process of the superhydrophobic 3D interfaces were further clarified. Although these effects and their interactions have been demonstrated, some main technical specifications valuable for industrial applications were also investigated by optimizing the superhydrophobic SERS substrate materials, micro-structure and compositions, as well as the deposition processes of Ag nanoparticles. Some typical harmful components in environment water or food were systematically discussed, and the quantitative analysis results were obtained.

Resume : We show that a stacking defect or a shift has a striking effect on carrier transport of bilayer graphene. Charge transport through a ballistic n–neutral–n junction of shifted bilayer graphene may result in minimal conductivity and shot noise anomalies which are found to be sensitive to the shift defect. Minimum conductivity and shot noise in shifted bilayer graphene exhibit an anisotropic beahavior as a function of the orientation of the electrodes. The minimum conductivity could be suppressed for somme specific value of twist defect while the shot noise takes the unit value. Our results provide a way to control transport properties in an undoped graphene bilayer structure by adjusting the layer stacking.

Resume : Detection of UV emission is important in multiple commercial applications ranging from health sciences to missile detection. Over recent years, there has been a great effort in power independent (self-biased) functioning of UV photodetectors due to the capability of operation in remote areas without an external power source. Self-biased photodetectors active in UV region are typically fabricated using 1D metal oxide nanostructures, which offers high surface-to-volume ratio, carrier confinement, and excellent electric transport property. Selectively ordered vertically arranged metal oxide such as ZnO (1D) nanostructures has drawn much attention for achieving light localization, waveguiding, and light retardation within the structures. In this article, we report the fabrication of light trapping ordered ZnO nanorods over ITO/glass substrate using a transfer printable PS colloidal template. Positioning soft lithographically patterned PEDOT:PSS/ITO over the ordered 1D ZnO/ITO substrate in sandwich configuration exhibited Schottky-like rectifying behaviour in current-voltage (I-V) characteristics and an increased photo to dark ratio (~3320) at zero bias condition under UV light illumination (Wavelength 325 nm, Power 45 mW) in comparison to non-patterned (control) photodetectors (photo to dark ratio ~600). The enhanced performance in ordered photodetectors mainly attributed to the combination of both enhanced interface and piezo-phototronic effect. The self-biased photodetection has been observed to be reduced in control devices due to the negligible piezoelectric effect. The present study provides a novel strategy to enhance the optoelectronic performance of self-biased piezo-phototronic enhanced UV photodetectors introducing a cost-effective technique to engineer heterojunction interfaces.

Resume : The rapid development of nanoscience and nanotechnology has greatly pushed the scientific community and industrial companies to explore new features of both typical and novel materials at the nanoscale level. Nanomaterials and one-dimensional nanostructured metal oxides are addressing the challenge, attracting new possibilities to approach a new physics and also to realize devices operating on original principles. In the present work 1-D Zinc Oxide nanostructures were produced, characterized and their sensing properties were established. To get a new class of sensor devices, the present work could furnish vital information about the several deposition conditions and one dimensional (1-D) characteristics that could permit the precise control over the shape, density, size and form. Indeed, the way for controlling these, was explained by two effects, catalysts nature and temperature. We report a simple VLS process typified by its simplicity, low cost process, reproducibility and its feasibility for large area deposition. The structural, optical, morphological and electrical analyses were carried out and discussed. Pure ZnO phase, high quality, good morphologies and high aspect ratio were established using XRD, FESEM/EDS and TEM. Different chemical compounds (Ethanol, Acetone, CO and H2) were tested, results show a high response, good selectivity toward hydrogen and good speed sensor proofing the fabrication of high-performance chemical sensor with low cost production methods.

Resume : Self-healing electronic materials can substantially enhance the life-time of a device as they can self-repair mechanical damages, thereby recovering their initial electronic performance similar to human skin. Despite the development of various self-healing electronic components such as electrodes and semiconducting carrier transport layers, self-healing electroluminescence (EL) layers suitable for deformable displays, which require both high stretchability and self-recovery function, have been rarely demonstrated. Here, shape-deformable and self-healing EL displays (SSELD) are presented. Light emitting materials are developed to realize viscoelastic properties of polymer composites containing light-emitting Cu-doped ZnS microparticles by the addition of Trion X-100, a surfactant that efficiently plasticizes elastomeric and amorphous poly(urethane). A capacitive SSELD exhibits frequency-dependent field-induced light emission under alternating current (AC). Color mixing and tuning of EL is conveniently achieved by mechanically mixing two or more Cu-doped ZnS microparticles with different EL characteristics. More importantly, a SSELD self-recovers its EL within few minutes of electrical failure. Further, the AC EL device endures more than 100 cycles of failure-recovery operations. By combining with a shape-deformable ionic liquid, a novel fiber display which exhibits excellent shape-deformable and self-healing EL performance, is demonstrated.

Resume : The characteristics of a vertical InGaN/GaN light-emitting diode (LED) with a special designed electron-blocking layer (EBL) are simulated and analyzed numerically. The special designed EBL is immersed in the p-GaN layer. It has the same size with the n-pad layer instead of the traditional EBL, which is covered the entire layer of the p-GaN layer. The three-dimensional (3D) non-linear Poisson and drift-diffusion equations is employed to calculate the carrier transport of the LED. The energy band diagram, the electron and hole concentrations, the electron current density, the radiative and non-radiative recombination rates, the current-voltage (I-V) curve, and the local and lumped internal quantum efficiency (IQE) are investigated. The 3D results show that current crowding always occurs in the region under the n-pad. The special design EBL successfully mitigates the electron current crowding in the region under the n-pad at high injection levels, which leads to a more uniform distribution of local carrier density, radiative recombination and IQE compared to the structure with the traditional EBL. The simulated results show that the LED with the special designed EBL has better lumped IQE than the LED with the traditional EBL. The effect of the thickness of EBL and the composition in AlxGa1-xN EBL is considered.

Resume : We report a new high T1 hole type host material with a new donor moiety for deep blue thermally activated delayed fluorescent organic light-emitting diodes (OLEDs). This host has a sufficiently short conjugated structure, which could result in high bandgap energy and high T1 energy level. Our new host (KHU-host) shows a 3.28 eV T1 value with proper highest occupied molecular orbital (HOMO) level of 5.70 eV. This host has hole type characteristic, which could shift exciton recombination zone to ETL side and show a deeper blue color. To examine the effectiveness of this host, newly synthesized TADF dopant material (KHU-TD) is employed together for the blue device. Fabricated blue device with these host and dopant exhibited high external quantum efficiency (EQE) of 28.4% with deep blue color of (0.14, 0.20), and well-known blue host, DBFPO based device indicated much higher EQE of 37.4% with sky blue color of (0.16, 0.34). Moreover, the efficiency roll-off ratio in KHU-host and DBFPO devices were also very low as 0.86 and 0.81, respectively. Acknowledgement This work was supported by Grant No. NRF-2016R1A6A3A11930666.

Resume : Over the past decades, continued scaling down of metal-oxide-semiconductor field-effect transistors (MOSFETs) has been driving modern electronics into a broadening spectrum of various applications [1]. Nevertheless, the scaling down of the transistor has reached physical limitations, because the subthreshold swing (SS) limitation of 60 mV/decade in MOSFETs increases rapidly the power consumption due to high off-current characteristics. To overcome this physical limitation, various devices have been proposed, such as tunneling FET (TFETs) [2], impaction ionization MOSFETs (IMOSs) [3] and feedback FETs (FBFETs) [4-6]. Among them, FBFETs have been considered the most promising devices due to their high performance and low power consumption. Here, we demonstrate FBFET devices with steep switching and memory characteristics. In our work, a modeling and optimization method of FBFETs is proposed to achieve latch-up behaviors with high current gain. The latch-up mechanism is examined by an equivalent circuit analysis. The band diagram, I-V characteristics, memory window, subthreshold swing and on/off current ratio are investigated using a commercial device simulator. Our proposed FBFETs exhibit wider memory windows than 3.0 V, less subthreshold swings than 0.1 mV/decade, on/off current ratios of approximately 1010, and on-currents of approximately 10-5A at room temperature. Based on the superior device characteristics and controllable memory window, we demonstrate the promising possibility of the FBFET as next-generation electronic devices. [1] T. Sakurai, IEICE Trans. Electron. E87-C (2004) 429-436. [2] S. Woo, M. Kim and S. Kim, Microelectronic Engineering. 191 (2018) 66-71. [3] E.-H. Toh, G. H. Wang, G.-Q. Lo, L. Chan, G. Samudra, Y.-C. Yeo, Jpn. J. Appl. Phys. 47 (2008) 3077-3080. [4] M. Kim, Y. Kim, D. Lim, S. Woo, K. Cho and S. Kim, Nanotechnology. 28 (2017) 055205 [5] J. Wan, C. Le Royer, A. Zaslavsky, and S. Cristoloveanu, Solid State Electron. 76 (2013) 109-111. [6] Y. Jeon, M. Kim, D. Lim and S. Kim, Nano Lett. 15 (2015) 4905-4913.

Resume : ZnO nanoparticles (NPs) of 4–5 nm, widely adopted as an electron transport layer (ETL) in quantum dot light emitting diodes (QD-LEDs), were synthesized using the solution-precipitation process. It is notable that synthesized ZnO NPs are highly degenerate intrinsic semiconductors and their donor concentration can be increased up to ND=6.9×10e21/cm3 by annealing at 140 °C in air. An optical bandgap increase of as large as 0.16-0.33 eV by degeneracy is explained well by the Burstein-Moss shift. In order to investigate the influence of intrinsic defects of ZnO NP ETLs on the performance of QD-LED devices without a combined annealing temperature between ZnO NP ETLs and the emissive QD layer, pre-annealed ZnO NPs at 60, 90, 140, and 180 °C were spin-coated on the annealed QD layer without further post-annealing. As the annealing temperature increases from 60 to 180 °C, the defect density related to oxygen vacancy (VO) in ZnO NPs is reduced from 34.4 % to 17.8 %, whereas the defect density of interstitial Zn (Zni) is increased. Increased Zni reduces the width (W) of the depletion region from 0.21 nm to 0.12 nm and lowers the Schottky barrier (Фb) between ZnO NPs and the Al electrode from 1.19 eV to 0.98 eV. We reveal for the first time that carrier conduction between ZnO NP ETLs and the Al electrode is largely affected by the concentration of Zni above the conduction band minimum (CBM), and effectively described by space charge limited current (SCLC) and trap charge limited current (TCLC) models.

Resume : Inserting an interlayer between barrier and well layers in a superlattice (SL) decreases the accumulated strain and drastically improves the interface quality. Moreover, the higher energy states than the pre-existing states are able to be formed in the SL without changing the thickness of the well layer. For evaluating the carrier transport properties, the carrier escaping process through the upper levels must be considered together with the material properties of the interlayer. In this study, we revealed the effect of interlayer on the carrier recombination and escaping processes by discussing the temperature dependence of photoluminescence (PL) intensities from 4 to 300 K. The SL solar cell structure samples with and without the interlayer were prepared. The temperature dependence of the integrated PL intensities due to the transition between the relevant quantum levels was explained by the carrier relaxation models consisting of (i) radiative and (ii) nonradiative recombination in the SL, (iii) thermionic emission to the barrier, and (iv) tunneling after thermal excitation from the lower to the upper quantized levels. We found that the activation energy for the carrier thermal escaping process was considerably reduced by the interlayer insertion, and the tunneling process after the thermal excitation became dominant in all processes at 300 K. These results indicate that the interlayer insertion is an important key factor to achieve the highly efficient SL solar cells.

Resume : We have succeeded in obtaining high quality warm w-LEDs by adopting hybrid two-dimensional (2D) structure of SiNx photonic crystal layer (PCL) assisted cyan-emitting ceramic-plate thiosilicate SrLa2Si2S8:Ce3+ with red-emitting film SrLiAl3N4:Eu2+ phosphor on a 430 nm blue LED chip at 350 mA. 2D SiNx PCL was capped with thiosilicate is because it can enhance the luminous efficacy and maintain the low correlated color temperature (CCT) and high color-rendering index (CRI). High luminous efficacy (82.3 lm/W), high special CRI (R9 = 75) as well as the low CCT (5431 K) of the optimal w-LED was obtained due to the assistances of 2D SiNx PCL and narrow-band red-emitting phosphor with the doping percentage at 10 wt%. The synthesis processes, structural analysis, optical properties and LED device performances were detailed investigated to find out the relationship between the optimum composition and good optical properties. Based on intriguing luminescence properties by the 2D SiNx PCL and red-emitting film phosphor introducing, we proclaim this method could also have high potential application in other phosphor-converted w-LEDs.

Resume : III-nitrides are promising materials application in light emitting diodes (LEDs). However, there are still some demands for high performance and high reliability LEDs. Reverse leakage and reverse breakdown characteristics are major parameters for evaluating electrical and optical properties. It is known that crystal imperfections have a strong influence on a high reverse leakage current. However, many researches have not been focused on the reverse leakage current and breakdown. In this study, we systematically studied the effect of InGaN active layer structure on the reverse bias-induced local breakdown of GaN-based LEDs. The GaN-based LEDs were grown on c-plane sapphire by using MOCVD. The epitaxial layer of LEDs have the same structure except the thickness and In composition of quantum wells (QWs) and the number of superlattice under the QWs. After growing LED epitaxial layers, LEDs were fabricated by a conventional mesa fabrication process with lateral electrode structure. As the number of superlattice pairs under QWs and the thickness of quantum well increase, breakdown voltage increases. However, it found that the breakdown voltage was not affected by In content of InGaN QWs. When local breakdown occurred, the shape of local breakdown changed from anisotropic to isotropic spot in the thick well and low In composition. Based on these results, we believed that the local breakdown may be significantly affected by the stress state of the InGaN QWs region.

Resume : Organic light-emitting transistors are pivotal components for emerging opto- and nano-electronics applications, such as logic circuitries and smart displays. Within this technology sector, the integration of multiple functionalities in a single electronic device remains the key challenge. Here we show optically switchable organic light-emitting transistors–fabricated through a judicious combination of light-emitting semiconductors and photochromic molecules. The irradiation of the solution-processed films at selected wavelengths enables the efficient and reversible tuning of charge transport and electroluminescence simultaneously, with a high degree of modulation (on/off ratios up to 500) in the three primary colors. Different emitting patterns can be written and erased, through a non-invasive and mask-free process, on a length scale of few microns in a single device, thereby rendering this technology potentially promising for optically gated highly-integrated full-color displays and active optical memory.

Resume : Light trapping in the spectral range of visible to the near infrared is important for a plethora of energy-related photonic devices. The study numerically examines light trapping in arrays of subwavelength silicon light funnels (LF arrays) realized on silicon-on-insulator (SOI) wafers. We demonstrate the possibility of light trapping beyond the Yablonovitch limit and ~5% enhancement beyond the limit is shown. The SOI wafers are used for two reasons: firstly, SOI wafers introduce two bottom interfaces which allow efficient optical coupling between the LF arrays and the underlying substrates and, secondly, the potential for the realization of energy harvesting photonic devices realized on SOI wafers. Strong light trapping and high absorption in LF array-substrate complexes is shown for relatively short LF arrays. The strong absorption peaks correspond to a high optical intensity in the arrays which is 3–4 orders of magnitude higher than in ambient. Subsequently, these absorption peaks conclude strong excitations in the substrates. We show that the overall transmission is low on account of the two bottom interfaces of the SOI geometry. Specifically, the strong absorption peaks are due to low transmission coupled with low reflectivity which suggests forward scattering by the arrays into the substrates. Next, light trapping in LF array-substrate complexes is examined for higher LF arrays, and the dependency of the LF bottom diameter on the overall absorption of the complex is studied. We show that for small bottom diameter the excitation of the substrate is poor possibly due to lack of photonic states at the LF bottom. The excitation of the substrates increases as the LF bottom diameter is increased, however dramatic decrease in absorption is recorded for a bottom diameter that reflects the geometry of a nanopillar array. We show that the substrate excitation by a LF array is more efficient than the excitation by a nanopillar array as LFs provide a homogenous power spread in the substrate, whereas the substrate excitation by nanopillars is governed by an anisotropic power spread.

Resume : Neuromorphic computing system which imitates our brain’s efficient data processing and storage has caught the attention because of the limitation of big data processing of von Neuman computing system. Artificial synaptic device is the indispensable technology to realize neuromorphic system. 3-point-probe synaptic transistor researches have been conducted actively due to their small power consumption, good reproducibility of post synaptic current characteristics. The hysteresis characteristic which the SWCNT memory device on the high-k dielectrics has is the significant content to realize the synaptic properties. Therefore, it is noted that implementation of the stable hysteresis behavior with low-power consumption on an operating voltage is the important preliminary study. In this research, by using high-k dielectrics(Al-doped ZrHfO2, Al2O3) which can induce hysteresis characteristics, we demonstrate the transistor devices operating stably on lower driving voltage. Explaining fabrication process of the device, 30nm the high-k dielectrics was deposited on a heavily-doped Si substrate by the atomic layer deposition(ALD). SWCNTs network was formed by a spin-coating method onto each high-k dielectric and patterned by a customized mask. Al electrodes defined as 50μm channel length, 500μm width were deposited by the thermal evaporator as a source-drain electrode. Finally, back-gating type SWCNTs transistor was fabricated. Not only a few nanometer single wall CNTs but also tens of

Resume : The resistive random access memory is promising to replace the traditional memory technology [1]. The switching mechanism of metal doped chalcogenide thin film involves the redox of ions, leading to the formation and dissolution of metal filaments. Such memory cell is referred as programmable metallization cell (PMC) or conductive bridging RAM (CBRAM) [2]. The chalcogenide based resistive cell owns excellent storage performance, for instance, low operative voltage, low operative current, fast respond and long time data storage [3]. In this presentation, a switching cell based on Ag or Cu doped Ge-Se electrolyte is introduced. And its relevant property and potential application are discussed. Reference [1] Zhang B, Prokop V, Strizik L, Vitezslav Z. ,et al. THE FUNCTION OF BUFFER LAYER IN RESISTIVE SWITCHING DEVICE. Chalcogenide Lett,14,291–5 (2017). [2] Waser R, Aono M. Nanoionics-based resistive switching memories. Nat Mater,6,833–40 (2007). [3] Schindler C, Guo X, Besmehn A, Waser R. Resistive Switching in Ge0.3Se0.7 Films by Means of Copper Ion Migration. Z Phys Chem, 221,1469–78 (2007). Acknowledgments This work was supported by the Technology Agency of the Czech Republic (TA03010994) and project GAMA.

Resume : Colloidally prepared, quantum-confined, semiconductor nanocrystals offer tunable energy gaps, strong photoluminescence, and, in some cases desirable properties such as optical gain and lasing. The role of thermal energy deposition in these nanoscopic structures has not been substantively probed. This presentation will consider ultrafast optical pump, X-ray diffraction probe experiments performed at Argonne National Lab’s Advanced Photon Source on semiconductor nanocrystal (NC) colloidal dispersions as functions of particle size, polytype, and pump intensity to examine lattice response. Shifts of diffraction peaks relate lattice heating and peak amplitude reduction conveys transient lattice disordering (or melting). Intraband and Auger-derived heating is clearly observed for low fluences, and disordering was observed upon absorption of larger numbers of photons excitations per NC on average. Diffraction intensity recovery kinetics, attributable to recrystallization, occur over hundreds of picoseconds with slower recoveries for larger particles. NCs studied revert to initial structures following intense photoexcitation. These findings suggest a need to take into account nanomaterial physical stability and transient electronic structure for high intensity excitation applications such as lasing and solid-state lighting. Additional, emerging experimental routes to probing and controlling thermal energy in nanocrystals will also be presented.

Resume : Although solvent−ligand interactions play a major role in nanocrystal synthesis, dispersion formulation, and assembly, there is currently no direct method to study this. Here,[1] we examine the broadening of solution 1H NMR resonances associated with bound ligands and turn this poorly understood descriptor into a tool to assess solvent−ligand interactions. We show that the line broadening has a minor homogeneous and a major heterogeneous component. The former is nanocrystal-size dependent, and the latter results from solvent−ligand interactions. Our model is experimentally supported by diffusion filtered NMR spectra of NCs capped with the ligand 2-[2-(2-methoxyethoxy)ethoxy]acetic acid in various solvents. We also present theoretical evidence that correlates broad NMR lines with poor ligand solvation. This correlation is found across a wide range of solvents, extending from water to hexane, for both hydrophobic and hydrophilic ligand types, and for a multitude of metal oxide, sulfide, and selenide semiconducting nanocrystals. In conclusion, the solvation of the ligand shell is the main contributor to heterogeneous line broadening. Given the ubiquity of solvent-ligand effects, we thus expect the NMR line width to become a widely used, quantitative tool in nanocolloid research, with applications ranging from size-tuning during synthesis and rational ligand exchange methods to the formation of NC coatings and NC superstructures. [1] De Roo, J. et al. Chem. Mater. 2018, 30, 5485

Resume : Similar to atoms in classical crystals, nanoparticles self-assemble into ordered superstructures with symmetrical shapes and defined facets and edges.[1] These supercrystals are macroscopic objects which preserve the unique properties of the nanoscale building units and allow the integration of nanoparticles into electronic and opto-electronic devices. In order to investigate the superstructure formation, we employed a gas-phase destabilization method[2] for the self-assembly of metal and semiconductor nanoparticles, yielding supercrystals of various materials, sizes and morphologies. The choice of the formation conditions allows to tune the shape and size of the supercrystals. The fcc structure of the nanoparticle assembly has been revealed by HRSEM and SAXS measurements. In order to gain some insight into the formation process, the geometry of the most common trigonal supercrystal shape was examined revealing a two-step growth mechanism, which has been confirmed by DLS measurements.[3] Literature: [1] Reichhelm, A., Haubold, D. and Eychmüller, A., Adv. Funct. Mater. 2017, 27, 1700361. [2] Simon, P., Rosseeva, E. et al., Angew. Chem. Int. Ed. 2012, 51, 10776. [3] Haubold, D., Reichhelm, A. et al., Adv. Funct. Mater. 2016, 26, 4890.

Resume : Semiconducting polymer nanoparticles (SPNs), also called semiconducting polymer dots (PDs), have attracted notable attention for being promising emissive nanoprobes in the field of biomedical imaging and analysis, owing to their excellent photophysical properties: large extinction coefficients, high single-particle brightness, and exceptional photostability. Especially, the tunable optical and surface properties, as well as good biocompatibility, make PDs a better candidate than their inorganic analogues, i.e. quantum dots, which are considered to be less photostable and biocompatible. Near-infrared (NIR)-emissive fluorescence probes, in particular, are highly desirable for biological imaging applications considering the NIR window of biological tissue imaging. Building upon our recent work that reported a novel post-polymerisation functionalisation approach, a new type of NIR-emissive SPNs developed which can be readily functionalised after polymerisation will be reported. The preparation of SPNs, along with the facile tuning of their photophysical properties and surface chemistry by the direct backbone functionalisation technique, will be demonstrated.

Resume : Colloidal synthesis of halide perovskite nanocrystals like CsPbBr3 in different shapes like nanocubes (NCs), quantum dots, nanorods, nanowires, nanoplatelets is extensively investigated due to their excellent optoelectronic applications in solar cells, light emitting diodes, lasers, and photodetectors. Nanocubes out of all other geometries having six equivalent (100) facets would be an ideal choice for making ordered close-packed assemblies like artificial photonic crystals or ‘metamaterials’, which exhibit collective electronic and optical behaviors for applications in nanoelectronics and nanophotonics. Here in this work, we have synthesized highly monodisperse CsPbBr3 perovskite NCs with the size of 7 - 8 nm. After the synthesis, NCs are capped with hydrophobic organic chains which hinder the subsequently controlled self-assembly due to the strong hydrophobic interactions between these NCs. Taking advantage of the surface chemistry engineering, we have successfully modified the surface hydrophobicity of the CsPbBr3 NCs by treating them with ammonium thiocyanate salt, which is confirmed by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) analysis. Next, we successfully fabricated large area (up to several millimeters) monolayer assembly of thiocyanate ligand treated NCs by controlled droplet evaporation of their solution. We have also patterned these perovskite NCs on a silicon substrate using the capillary assembly on pre-patterned trenches (lines, grids, and circles) on silicon fabricated by e-beam lithography. Detailed investigation of the facets modifications in the ripening stage after nucleation, treatment of thiocyanate to modify the surface ligands and surface atoms for controlling the NCs-Si substrate interactions is still in progress. Our results shed the new light on the device fabrication through a controlled bottom-up assembly.

Resume : Colloidal nanocrystals are an interesting platform for the design of low cost optoelectronic devices.[1] This is especially true in the infrared range where current technologies remain driven by expensive devices based on epitaxially grown semiconductors. Mercury chalcogenide nanocrystals have generated a large interest thanks to their broad spectral absorption in the infrared. In the mid-infrared range (3-5 µm), these materials exhibit two types of narrow energy transition, interband transition in the case of HgTe[2] and intraband transition for HgSe and HgS.[3,4] As doping level sets the absorbance properties and is largely responsible for the magnitude of the signal to noise ratio, controlling and tuning its magnitude is of utmost importance. To do so, different strategies can be adopted. For example, applying a dipole on HgSe nanocrystal surfaces can tune the doping level by up to one order of magnitude (typically in the 0,1 to 1 electron per nanocrystal range).[5] Approaches based on oxydo-reduction of nanocrystals by polyoxometalates can also be suited as a strategy to massively tune the doping magnitude. To do so, polyoxometalates are used as ligands for HgSe nanocrystals, and a shift from doped to quasi intrinsic behavior can be obtained.[6] Finally, I will show how these doped nanocrystals can be used to expand the range of wavelength reachable up to the THz range.[7] [1] G. Konstantatos and E. Sargent, Cambridge, University Press, 2013 [2] E. Lhuillier et al., Nanotechnology 2012, 23, 175705 [3] A. Jagtap et al., Opt. Mater. Express 2018, 8, 5, 1174-1183 [4] J. Kim et al., Chem. Commun. 2018, 54, 8435-8445 [5] A. Robin et al., ACS Appl. Mater. Interfaces 2016, 8, 40, 27122-27128 [6] B. Martinez et al., J. Phys. Chem. C 2018, 122, 46, 26680-26685 [7] N. Goubet et al., J. Am. Chem. Soc. 2018, 140, 5033-5036

Resume : Recently, the synthesis of 2D colloidal nanocrystals was demonstrated. This new class of nanomaterials involves both nanocrystals of 3D materials such as CdSe with a 2D morphology and nanosheets of van der Waals crystals such as MoS2 that are intrinsically two dimensional. Here, we use CdSe nanoplatelets and MoS2 nanosheets as model systems to discuss the surface chemistry of colloidal 2D materials. By means of NMR spectroscopy, we demonstrate that CdSe nanoplatelets are stabilized by cadmium carboxylates. Using ligand displacement, we demonstrate that the (100) surfaces offer a heterogeneous set of binding sites with different adsorption energies. This finding concurs with DFT calculations, which enables us to localize the weaker binding sites at the platelet edges and the stronger sites on the facets. In the case of MoS2, NMR analysis indicates that possible ligands such as amines or carboxylic acids exhibit a weak, dynamic interaction with the nanosheet surface. We demonstrate that the colloidal stability can be ensured by choosing a solvent that minimizes the effective inter-sheet interaction energy. Alternatively, we demonstrate that Z-type ligands, such as metal carboxylates, can bind to the sulfur-rich top and bottom facets and stabilize MoS2 nanocolloids in a wider range of solvents. We conclude that in the case of CdSe nanoplatelets, surface termination can be understood within the framework that was derived from the study of 0D CdSe nanocrystals. MoS2 nanosheets, on the other hand, behave differently. In line with their being 2D van der Waals solids, ligands show little interaction with the nanosheet surface; a behavior that calls for a new surface termination approach with these materials.

Resume : Halide perovskite nanocrystals are prepared via several synthetic routes to obtain dimension-controlled nanocubes, nanoplatelets or nanowires. We investigate the effect of size and dimensionality on the optical and electronic properties of the nanocrystals, focusing on quantum-confinement and carrier dynamics. With binding energies of up to 300 meV, 2D nanoplatelets exhibit many properties reminiscent of epitaxially grown quantum wells but at room temperature. However, due to their unique geometry and organic ligand surrounding, they exhibit vastly different carrier relaxation dynamics, with a strong dependence on the thickness of the samples. These nanoplatelets are shown to be excellent emitters in the blue spectral region, where perovskite nanocrystals typically perform poorly. Inorganic Cs-based nanocubes, which have previously displayed extremely high quantum yields, are used as precursors to create larger structures. We explore the energetic and electronic coupling between the NCs and explore the resulting optical properties, especially for light harvesting and light emitting applications.

Resume : Due to their anisotropic shape and their precise atomic scale thickness, quasi-two dimensional semiconductor nanoplatelets show excellent optical properties, such as spectrally narrow, directional light emission. In order to make use of their anisotropic optical properties in the solid state, colloidal nanoplatelets have to be organized into oriented films. Existing methods use additives to control their self-assembly, which could hinder the application of nanoplatelet assemblies in electronics. Instead, we developed a kinetically driven method to control the orientation of nanoplatelets in a film without using additives, exploiting solely the metastable character of nanoplatelets dispersions. Films with nanoplatelets collectively standing up (edge up) or lying down (face down) can be produced by using a liquid interface assembly process. We find that switching between the face down and the edge up configuration can be achieved by controlling the evaporation rate of the solvent of the nanoplatelets, either by the choice of solvent or the temperature. Hence, our method can easily organize colloidal nanoplatelets into oriented self-assembled films without the use of additives that could hamper their application in electronics.

Resume : Two-dimensional (2D) materials have received much attention in the past years for a wide variety of photonic applications due to their pronounced excitonic features leading to unique properties in terms of light emission. However, only a few studies focus on the use of these materials for light amplification or net optical gain development and the ensuing high carrier density photo-physics. The beneficial nature of the strong excitonic effects on optical gain remain hence unquantified and , despite the large binding energies, it remains unclear what the involvement of is at the concomitant high carrier densities. Here, we use colloidal 2D CdSe nanoplatelets as a model system and show, using a quantitative and combinatory approach to ultrafast spectroscopy, that several distinct and carrier density-dependent optical gain regimes exist for these materials. At low density, optical gain is found to originate from excitonic molecules delivering large material gains up to 20.000 cm-1, yet with an Auger limited lifetime of few hundred picoseconds. At increasing pair density, we observe a surprising transition to a combined regime of blue-shifted and disruptively large optical gain, combined with the typical exciton mediated gain. We show that this peculiar situation originates from a carrier cooling bottleneck at high density. Surprisingly, the insulating (multi-)exciton gas is found to co-exist with the conductive phase in a density regime nearly one order of magnitude beyond the expected Mott transition. The ensuing exciton ground state absorption even counter-acts the development of net optical gain in certain spectral regions. Our results shed a new light on the disruptive photo-physics of high binding energy excitons in strongly excited 2D materials and pave the way for the development of more efficient broadband optical gain media and/or high density excitonic devices such as polariton lasers.

Resume : Controllable interface characteristics are demonstrated at the nanoscale, using two types of semiconducting nanoparticles with nearly identical optical band gaps, CsPbBr3 nanocrystals, and CdSe nanoplatelets, capped with molecular cross-linkers. By exploiting chemical recognition of the capping molecules, the two types of nanoparticles are brought into mutual contact, thus initiating spontaneous charge transfer and the formation of a strong junction field. Depending on the choice of capping molecules, the magnitude of the latter field is shown to vary in a broad range, corresponding to an interface potential step of 0.1-1.1 eV. The band diagram of the system, as well as the emergence of photo-induced charge transfer processes across the interface of this heterojunction are studied here by means of optical and photoelectron based spectroscopies. Our results propose an interesting template for generating and harnessing internal built-in fields in heterogeneous nanocrystal solids.

Resume : Recent experimental evidences show that two-photon absorption (TPA) in colloidal CdSe nanoplatelets is unusually large, with record high cross sections over 10^7 GM which appoint platelet-like nanostructures as optimal candidates for two-photon imaging and nonlinear opto-electronics.[1] In addition, TPA into the continuum is found to be anisotropic, in clear contrast with one-photon absorption.[2,3] This makes such platelets attractive for applications like directional photon converters. In this work, we provide theoretical interpretation for both properties. We show that the anisotropic absorption follows from the symmetry of the conduction and valence band-edges in zinc-blende, together with the quasi-2D confinement of the envelope wave function.[3] The large TPA cross-section, in turn, results from the strong exciton interaction in a quasi-2D space, which gives rise to a quadratic dependence on the platelet area -as opposed to the linear dependence observed in quantum dots-.[4] Our findings can be extrapolated to different materials and geometries. We propose that exciton interaction in semiconductor nanostructures with at least one weakly confined dimension is a powerful knob to tune the radiative recombination rate of interband and intraband optical transitions. In the former case, it leads to the so-called "Giant Oscillator Strength". In the latter one, it leads to an opposite behavior, a "Dwarf Oscillator Strength". [1] Nano Letters 15, (2015) 4985. [2] Nature Nanotechnol. 12, (2017) 1155. [3] Nano Letters 17, (2017) 6321. [4] ACS Photonics 5, (2018) 3680.

Resume : Colloidal Quantum Dots (CQDs) are semiconductor nanoparticles with broadly tunable optical feature from UV to THz. [1] During the past 10 years, a lot of efforts have been made to develop a new class of photodetectors based on nanocrystals, especially in the infrared range of wavelengths. Among narrow-bandgap nanocrystals used to address infrared, mercury telluride (HgTe) is particularly interesting. Its bulk bandgap is null, allowing for the design of a material with arbitrarily low energy absorption band edge. Photoconductive and photovoltaic devices can then be fabricated from this material. [2,3]. In spite of this success, the electronic structure of these HgTe CQD remains poorly known, in particular the carrier relaxation dynamics needs to be better revealed. This requires the development of new experimental method to measure the carrier relaxation in this narrow band gap semiconductor. We started by investigating transport dynamics in HgTe nanoplatelets with optical features in the Near-Infrared (NIR). We demonstrated that we can control the majority carrier in this material by switching the capping ligand, and using time-resolved photoemission , we were able to measure independently the relaxation dynamics of majority and minority photocarriers in HgTe nanoplatelets. [4] [5] Alternatively, we have developed transient photocurrent setup in the mid-infrared to resonantly probe the photocarrier relaxation of mid-infrared HgTe CQDs while excited at their infrared band edge [6]. We demonstrated that > 50 MHz photoresponse can be achieved in the mid infrared and that significant trap-state contribution arises only under illumination far above the band-edge of the material. Finally, we also show that multi-exciton generation (MEG) effects in HgTe nanocrystals can be detected using transport measurements, and measured a threshold around 3 times the bandgap energy. [6] [1] N. Goubet et al. J. Am. Chem. Soc. 140, 5033 (2018). [2] S. Keuleyan et al. Nat. Photonics 5, 489 (2011). [3] A. Jagtap, et al. J. Phys. Chem. C 122, 14979 (2018). [4] E. Izquierdo, et al. J. Am. Chem. Soc. 138, 10496 (2016). [5] C. Livache, et al. Nano Lett. 17, 4067 (2017). [6] C. Livache, et al. ACS Appl. Mater. Interfaces 10, 11880 (2018).

Resume : This presentation will be about doped Halide perovskite nanocrystals that hold promise for printable optoelectronic and photonic applications. Doping enhances their functionalities and is being investigated for substituting lead with environmentally friendlier elements. The most investigated dopant is Mn2+ that acts as a color center sensitized by the host excitons. The sensitization mechanism is far from understood and no comprehensive picture of the energy-transfer process has been proposed. Similarly, the role of shallow states, particularly abundant in defect tolerant materials, is still unknown. Here, we address this problem via spectroscopic studies at controlled excitation density and temperature on Mn:CsPbCl3 nanocrystals. Our results indicate a two-step process involving exciton localization in a shallow metastable state that mediates the thermally assisted sensitization of the Mn2+ emission, which is completely quenched for T < 200 K. At T ≤ 60 K, however, such emission surprisingly reappears, suggesting direct energy transfer from band-edge states. Electron spin resonance supports this picture, revealing the signatures of conformational rearrangements below 70 K, possibly removing the potential barrier for sensitization. Our results demystify anomalous behaviors of the exciton-to-Mn2+ energy-transfer mechanism and highlight the role of shallow defects in the photophysics of doped perovskite nanostructures.

Resume : The memory market, which is worth $177bn per annum, is completely dominated by DRAM and Flash. DRAM is fast and has low switching energy, but is volatile and destructively read, so needs constant refreshing. Flash is non-volatile and cheap, but requires large switching voltages (~20 V), which make it slow and limit lifetime (endurance). The ‘holy grail’ of memories would be to combine the best features of both, i.e. a fast, non-volatile memory with low switching energy. However, a memory with states that are both robust and easily changed is widely considered unfeasible. In this talk, I report room-temperature demonstration of novel compound-semiconductor floating-gate memory cells with non-destructive read that take advantage of the extraordinary 2.1 eV conduction band offset of InAs and AlSb to achieve non-volatility, but exploit resonant tunnelling to allow low switching voltages (2.5 V) and a switching energy (per unit area) that is 100 times lower than DRAM. Detailed simulations of the physical properties of these devices using nextnano, combined with device modelling in SPICE, have led to the conception of a compact (4F^2) NVRAM architecture with low disturb, and predicted device switching speeds of ≤1 ns at the 20 nm node. Very recent work to fabricate 2x2 arrays of cells and to implement devices on Si will also be presented. Acknowledgements: This work was supported by EPSRC, IQE plc and the Joy Welch Educational Charitable Trust.

Resume : One among the major challenges in today’s electronics consists in increasing the storage capability of non-volatile memory devices. So far, the most popular approaches have relied on a continuous scaling down to enable the integration of an increasing number of memory cells per unit area. These strategies have shown their limits, as scaling down is hampered by photolithography, and fabrication complexity has increased dramatically over the years.(1) An alternative method is based on the development of memory cells with increased storage capabilities, that is, multilevel memories. Transistor devices based on a blend of a p-type semiconductor, poly(3-hexylthiophene) (P3HT), and a photochromic diarylethene (DAE) were reported to feature over 256 (8 bit storage) distinct current levels.(2) A practical way to further increase the functionality of a single device relies on the combination of different organic components, each one responding to a distinct external stimulus. Here, we report on an integrated solution which exploits a ferroelectric polymer (PVDF-TrFE) film as gate insulator layer in transistors based on DAE/P3HT blend. In these devices, the information can be written by means of UV/vis light irradiations or by voltage cycles. Such technology enables a bi-stimuli modulated multi-bit data storage with stable retention and endurance in transistor memories. 1. Liu, X. et al. Chinese Sci. Bull. 56, 3178–3190 (2011) 2. Leydecker, T. et al. Nat. Nanotechnol. 11,769-75 (2016)

Resume : The prospect of large-area and potentially low-cost fabrication of innovative technologies such as flexible displays or wireless communication devices has driven the research on the deposition of organic materials from solution. For various semiconducting materials it has been shown that the solution-processed films can exhibit superior properties as compared to the ones prepared by vacuum processes. Thus, the realization of solution-based devices is highly desirable. Here, we optimize the high-mobility semiconductor 2,7-dioctyl [1]benzothieno[3,2-b]benzothiophene (C8-BTBT) film deposition using the solution-shearing method. The introduced structural changes by the varied processing conditions are studied and their impact on the electrical performance of organic field-effect transistors (OFETs) is reported. The optimized conditions result in gate-voltage independent and record device characteristics with charge carrier mobilities as high as 12 cm²/Vs.[1] We further show how these high-mobility films can be utilized in devices that operate at voltages as low as -1V, while the superior device characteristics are maintained. Finally we demonstrate that the solution-shearing technique can be used to coat high quality gate dielectric films with low leakage currents and high capacitances, an important step to the realization of high-performance OFETs by large-area deposition techniques. [1] K. Haase, C. Teixeira da Rocha, C. Hauenstein, Y. Zheng, M. Hambsch, S. C. B. Mannsfeld, Adv. Electron. Mater. 2018, 4, 1800076.

Resume : Most of the current realizations in flexible printed electronics and circuits make use of a composite process by employing non-printing processes such as optical lithography, sputtering, thermal evaporation and others to configure at least one of their integral parts such as the semiconductor, the dielectric or the passive structures. Solution process-able passive structures as well as intra-cell wiring would accomplish the demand for fully printable, large area electronics with all the advantages of direct writing processes. In our work we target solution-processed, electrolyte-gated field-effect transistors (EGFETs) and logics based on In2O3 channel material and printed graphene passive components. The EGFETs were initially prepared on glass substrates using a chemical precursor route to print polycrystalline In2O3 as the active material, graphene ink for the passive structures and a composite solid polymer electrolyte (LiClO4/PC/PVA/DMSO) as the gating material. The In2O3 precursor and the electrolyte were ink-jet printed while for the graphene electrodes, an ultrasonics-controlled fluid dispensing microplotter was used. The fully printed transistors are analyzed by cyclic voltammetry to measure the capacitances and reactions of the individual components or the combinations thereof. Especially the interface between graphene source/drain electrodes and the In2O3 channel was investigated regarding the electrical contact behavior. PEDOT:PSS was printed when a top-gate is required. The prepared fully printed transistors were characterized and the performance analyzed using a parameter analyzer. We observed that they exhibited relatively high output current values for a fully printed transistor. The on/off ratio from the transfer curve was measured to be in the range of 105-107. A fully printed transistor-resistor based inverter is then fabricated by printing a pull-up resistor of 60kΩ in series to the pull-down transistor. This all-printed inverter can now be printed on a high temperature stable polyimide substrate to check for its mechanical flexibility. Additionally, the geometry and device architecture was optimized to further improve the characteristics of the logics. Bullet Points • Fully printed field-effect transistors and inverters • Graphene/Inorganic hybrid electronics • Electrical characterization of electrolyte-gated hybrid systems References [1] Subho Dasgupta, Robert Kruk, Norman Mechau & Horst Hahn, Inkjet Printed, High Mobility Inorganic-Oxide Field Effect Transistors Processed at Room Temperature, ACS Nano 2011, 5 (12), 9628-9638. [2] Ethan B. Secor, Jeremy Smith, Tobin J. Marks & Mark C. Hersam. High-Performance Inkjet-Printed Indium-Gallium-Zinc-Oxide Transistors Enabled by Embedded, Chemically Stable Graphene Electrodes. ACS Appl. Mater. Interfaces, 2016, 8, 17428-17434. [3] Donghoon Song, Ankit Mahajan, Ethan B. Secor, Mark C Hersam, Lorraine F. Francis & C. Daniel Frisbie. High Resolution Transfer Printing of Graphene Lines for Fully-Printed, Flexible Electronics. ACS Nano, 2017, 11(7), 7431-7439.

Resume : Metal oxide memristors gained great attention during the last decade in using them for nonvolatile data storage, as well as for artificial synapses. The key reason for changing the resistance state of a metal oxide material is the possibility to form conductive filaments due to oxygen vacancies. The known mechanisms for building such filaments are unipolar, bipolar and complementary switiching, which are based on the electromigration of oxygen ions by applying an electric field and thermophoresis caused by Joule heating. The investigated hydrothermally grown TiO2 nanorod arrays (NRAs) show a combination of these effects, whereas the dependence of the growth temperature on the memrisitive behavior of the NRAs will be demonstrated.

Resume : The introduction of nanoscale device structures such as the fin field effect transistor (finFET) has challenged the implementation of fast, in-line metrology aimed at the electrical characterization of such features. Indeed, the electrical properties of these structures, which can be impacted by their nanoscale dimensions via e.g. size dependent dopant activation/diffusion, can only be measured by using large area measurements, where such size effects cannot be probed, or on fully processed devices using dedicated test structures (at the expense of learning speed). Consequently, little is known about the impact of the increased surface/volume ratio of nanoscale structures on their electrical properties. In this paper, we use the fully automatic in-line micro four-point probe (µ4PP) technique, which has recently shown able to provide direct sheet resistance measurements in fins, to evaluate size effects on the electrical properties of both ex- and in-situ doped Si and Ge fins. First, we measure the sheet resistance of B implanted and (laser-)annealed Si fins with a width down to ~20 nm, where we observe that the sheet resistance increases for decreasing fin width. Using a correlative study including TEM and SSRM, we show that the measured increase in sheet resistance can be explained by a size dependent dopant activation in the fins. Finally, we extend our study by discussing size effects on the dopant activation in epitaxially grown doped Ge fins.

Resume : High performance photodetectors play important roles in the development of innovative technologies in many fields, including display and imaging, optical computing, environment monitoring, and industrial processing control. Graphene, two-dimensional material, has demonstrated promising applications in various types of photodetectors from terahertz to ultraviolet, due to its ultrahigh carrier mobility and light absorption in broad wavelength range. Graphene field effect transistors are recognized as a type of excellent transducers for photodetection thanks to the inherent amplification function of the transistors, the feasibility of miniaturization and the unique properties of graphene. The performance of graphene-based photodetectors, especially with regard to responsivity, is still lower than expected. Asymmetric electrodes have been receiving intensive attention due to their technological importance for the next-generation electronic/photonic applications. Hence, in addition to functioning solely as photodetectors with high performance, it is easier for these chemisorbed asymmetric electrode metals to combine each other’s advantages by forming heterostructures through van der Waals interactions. Photodetectors with high performance, such as broadband, ultra-fast, can be realized based on these layered materials. In this research, we will introduce the applications of graphene with asymetric electrodes in hybrid photodetectors in different wavelength ranges including terahertz, infrared, visible, and ultraviolet, focusing on the device design, physics and photosensitive performance. Since the device properties are closely related to the quality of graphene, the devices based on graphene prepared with different methods will be addressed separately with a view to demonstrating more clearly their advantages and shortcomings in practical applications. Constructing hybrid structures in conjunction with different low-dimensional materials may be an appealing way to realize the simplification and high performance of photodetectors at the nanoscale. graphene heterostructures display remarkable dual optoelectronic functionality, including highly sensitive photodetection and gate-tunable persistent photoconductivity. The responsivity of the hybrids was found to be nearly 5mA/W at room temperature, making them the sensitive graphene-based photodetectors. It is expected that highly sensitive photodetectors based on graphene transistors will find important applications in many emerging areas of high performance optical computing. We report a novel approach to the controlled synthesis of thin metal electrodes at predefined locations on chip by sputtering and have demonstrated their application as fast photodetectors with photocurrent response time ∼5 μs. But responsivity is low, 5mA/W. So we hybrid 2D materials with graphene to make the high speed photodetectors. This opens a pathway for the largescale production of layered 2D semiconductor devices, important for applications in integrated nanoelectronic/photonic systems.

Resume : For long-reach fiber based optical communication, the application wavelength has to be in the 1.5 µm wavelength range related to the minimum attenuation of the fiber. This is valid for classical high-performance data links as well as for future quantum communication networks. Low-dimensional optically active materials, such as semiconductor quantum dot materials, have numerous advantages in comparison to their quantum well material counterparts. However, most of them are emitting at shorter wavelengths not useful for fiber-based technologies. This is true for classical device applications as well as for quantum emitters. In this talk, the recent progress in the development of InP-based quantum dot materials dedicated for the emission at the 1.5 µm wavelength range will be reported and application in classical optoelectronic devices, such as high-speed direct-modulated lasers, optical amplifiers and narrow-linewidth lasers as well as single-photon emitters for quantum communication.

Resume : From the Shockley-Queisser limit, the efficiency of a single band-gap solar cell is limited by the band gap of the absorber. Hence overcoming this limit is one of the challenges for achieving higher efficiencies in the next generation of solar cells. One way to overcome this limit is the concept of “down-shifting”. In a solar cell, “down-shifting” can improve the harvesting of the sun spectrum by converting photons from the not accessible UV range to photons with energies in the high quantum efficiency region of the solar cell. One possibility for this approach is the use of quantum dots with high luminescence yield. In this study, we investigate the impact of InP/ZnS quantum dots on the performance of thin film solar cells. InP/ZnS quantum dots are used as an alternative to the established Cd based quantum dots, which are non-environmental friendly and highly toxic. The incorporation of these InP based quantum dots into the device structure enhances the external quantum efficiency of the investigated solar cells from 0% to 25% in the 300–400 nm spectral range. Although the application of the quantum dot layer leads to an improvement in the harvesting of high energy photons, the overall efficiency under standard test conditions is not improved. This lack of improvement in the efficiency has previously been observed in similar systems and can be attributed to reflections additionally induced by the quantum dots layer. In this study, we propose another overall strategy by investigating the effect of the applied QD layer on solar cells illuminated at non-ideal angles and at varied temperatures of the solar cell and discuss the potential of this approach for enhancement of the overall energy harvesting yield of solar cells under non-standard conditions.

Resume : Environmentally safe, high power and portable AlGaN ultraviolet light-emitting diodes (UVB-LEDs) and ultraviolet laser diodes (UV-B LDs) with emission wavelengths of 290-320nm are inevitable for several real world applications, such as those in immunotherapy (psoriasis, vitiligo), vulgaris treatment, plant growth under UVB lightning (310nm), the production of vitamin D3 in the human body(293nm-304nm) and production of phytochemicals in the green leaves of vegteable (310nm)[1-2]. The distributions of vitamin D3 and the prevalence of adequate levels in a population living above 638 N0 (Europ) due to less sunny days, were investigated by Ramnemark et al. and it was found that only 0.7% of the population were vitamin D3 deficient but 23.1% of men and 17.1% of women had insufficient levels of vitamin D3. Torii et al., confirmed that 310nm UVB light irradiation, induces the secretion of high-mobility group box-1 (HMGB1; nuclear protein), without inducing the death of skin cells. Thefore, safe UVB light sources are inevitable for both medical and agricultural applications shown in Fig.1. Recently, Susilo et al., from TU-Berlin, reported about the p-GaN contact layer in AlGaN UVB LEDs on AlN template, where the output power of 0.83mW under 20mA at 302nm emission were demonstrated. LG Innotek also announced about high output power of 100mW under 350mA at 305nm emissiom from UVB LED shown on website, but their device structure is still unknown in the literature. We also demonstrated to AlGaN 294nm-UVB LED with EQE of 4.4 % at 30mA with output power of 13 mW on bare-wafer level using Ni/Mg p-electrode [3-4]. Previously, we successfully suppressed the edge type, threading dislocation densities, TDDs up to~4.5×108cm-2, in the n-AlGaN CSL grown on the overlayer of n-AlGaN BL in 310nm-UVB LED, which were confirmed by HAADF-STEM observation [4]. Subsequently EQE of 1.5 %, with output power of 11mW under 180mA, at 310nm-emissiom were achived, but we encountered with current injection issue in MQWs due to both thciker quantium well barriers (QWBs) and quantium wells (QWs)[2]. When the QWBs was reduced from 20 nm to 10 nm in the MQWs of next 310nm-AlGaN UVB LED structure, the EQE were improved from 1.5 to 1.8 % at 80 mA under CW-operation at RT, with output power of 6 mW [in preparation]. But we still did not optmize to the quantium wells (QWs) strcuture as well as p-AlGaN (hole injection layer) [Future direction]. Finally we attempted to grow and fabricate 290-304nm-Band UVB LEDs devices, which consist of sample-I, -II, -III and -IV respectively. All these samples were grown under the same growth conditions, except the thickness variation of QWs. In this work slightly high Al-contents (28sccm) in the undoped AlGaN FB was introduced. All the samples consisted of an approximately 4µm-thick AlN template on sapphire substrate, a 2µm-thick two-stack of n-Al0.55-0.61Ga0.45-0.39N BL including n-Al0.47-0.52Ga0.53-0.48N CSL. Except the 3-fold MQWs, consisting of approximately 3nm-thick (sample-I), or < 1nm-thick (sample-II), or 2nm-thick (sample-III) and 5nm-thick (sample-IV) Al0.38Ga0.72N wells and 7-8nm-thick Al0.47Ga0.53N barrier layers respectively. The remaining structure were kept the same. We demonstrated high internal quantum efficiency (IQE) of 47 % for an AlGaN MQW at 294 nm emission measured by the excitation power density and temperature dependencies of PL with QWs thickness of 2 nm (sample-III) [4]. When thickness of QWs were changed approximately from 2nm (sample-III) to 3nm (sample-I) respectively, the EQE remained the same around 6.2-6%, with emission wavelength of 295nm and 304nm respectively at 20 mA on bare-wafer level under CW-operation at RT (submitted). But the maxium output power were enhanced from 18 mW at 294nm (sample-III) to a recod 23 mW at 304nm (sample-I), which has a great potential for the development of UVB laser diodes (LDs) as well as for medical applications, including Vitamin D3 production in human body. This enhacement is attributed to both QWBs and QWs thickness respectively. Most importantly the undoped AlGaN FB having relatively high Al-composition were found to be most effective, to behave both as an electron blocking (to stop overshooting of low energy electron toward p-AlGaN side) as well as Mg-blocking (difussion spression) from p-AlGaN side toward MQWs. Future Direction: We will struggle in the near future to improve the IQE from the MQWs to be grown both on the c-plane on nano-PSS as well as on the semi-polar plane of the AlN template. A high improvement in the LEE is expected by combining a highly transparent p-AlGaN contact layer with a PhC, as well as using highly reflective Ni/Al or Rh p-electrode. The IQE can be enhanced up to 70 % by further suppression of TDDs in the AlN templates to be grown on nano-PSS, and so we expect a very large improvement in the EQEs of UVA LED devices in the future. Recently polarization doping was realized by North Carolina University (USA) and Prof. Iwaya from Meijo University of Japan for Deep UVC applications. We will also attempt for UV-B LED deivices in the near future. "This work was supported in part by the New Energy and Industrial Technology Development Organization (NEDO). We would like to thank the SPDR office of RIKEN for their continual financial support to make this research possible." References [1] M. Kneissl, J. Rass (Eds), III-Nitride Ultraviolet Emitters-Technology and Applications, (Springer, Cham, 2016) Springer Series in Material Science, Vol. 227, Ch. 1. [2] Khan et al., Ext. Abstr. Int. Symp. On Growth of III-Nitrides (ISGN-7, Poland), 2018, p-Mo4.4. [3] Khan et al., Jpn. J. Appl. Phys, SAAF01 (2019). [4] Khan, N.Maeda, M. Jo, Y. Yuki Akamatsu, R. Ryohei Tanabe, Y. Yamada and H. Hirayama, J. Mater. Chem. C, 2018, DOI: 10.1039/C8TC03825B.

Resume : Spinorbitronics in III-V semiconductors, e.g. involving the GaMnAs ferromagnetic semiconductors, uses the properties of spin-orbit coupling (SOC) to generate currents of angular momentum [1-2]. Thoses are now essential to control the magnetization state of a magnet [3], or moving a domain wall [4] via the generalized spin-Hall effect of III-V possibly involving Rashba and Dresselhaus terms [4]. The interplay between particle spin and orbital motion is also at the basis of new families of effects played e. g. by the Anomalous Tunnel Hall effect described by the appearance of a lateral charge current transverse to a tunneling spin-current [5–7] ; or the spin-galvanic effects [8]. The ensemble of those complex phenomena requires a clear description of the spin-currents anatomy with advance calculation tools. In this work, as an extension to previous contributions [5], we study unconventional quantum effects resulting in a giant transport asymmetry of carriers and spin-to-charge conversion in semiconductor interfaces, tunnel barriers or quantum wells. Those are composed of ferromagnets and strong spin-orbit materials, e. g. III-V compounds with magnetizations of opposite direction (AP) or in the geometry of spin-injection devices. The symmetry of the structure allows a difference of transmission upon respective positive or negative incidence vs. the reflection plane defined by the magnetization and the surface normal. We will restrict ourself to the effect of bulk Dresselhaus and Rashba terms by using the simplest form of the quantum boundary conditions. We will first detail the robustness of our advanced 30-band and 40 band tunneling codes free of spurious states effects and involving the higher electronic bands involving the relevant spin-orbit contributions. We will demonstrate that refined boundary conditions involving surface potentials, like Rashba terms, arising from the symmetry breaking at interfaces may lead to equivalent effects by their own. In a second part, we emphasize on the perturbation calculation techniques needed to understand this phenomena and to the case of the core SOI in the valence band (VB). References: [1] Tomasz Dietl and Hideo Ohno, Rev. Mod. Phys. 86, 187 (2014). [2] T. Jungwirth et al., Rev. Mod. Phys. 86, 855 (2014). [3] M. Elsen et al., Phys. Rev. B 73, 035303 [4] L. Thevenard et al., Phys. Rev. B 95, 054422 (2017). [5] T. Huong Dang, H. Jaffrès, T. L. Hoai Nguyen, and H.-J. Drouhin, Phys. Rev. B 92, 060403(R) (2015). [6] A. Matos-Abiague and J. Fabian, Phys. Rev. Lett. 115, 056602 (2015). [7] M. Jamet, A. Barski, T. Devillers, V. Poydenot et al., Nat. Mat. 5, 653-659 (2006). [10] S. D. Ganichev et al., Spinpolarization by current, ""Handbook of spin-transport & magnetism"", (Chapman and Hall), 2016.

Resume : Quantum dot (QD) laser diodes have been studied because they have advantages of low threshold current density and high thermal stability. The photoluminescence (PL) of S-K mode grown QDs typically shows a red-shift that deviated from the expected Varshni's law around 100 K. This behavior was explained by the steady-state model supposing the QD-size distribution as well as a presence of wetting layer level (WL) acting the carrier sink in the emission mechanism. Recently, we observed a blue-shift in the higher temperature range above 150 K followed by the reported red-shift in the GaAs QDs in Al0.3Ga0.7As layer grown by a DE method. In this study, the origin of this anomalous temperature-shift of PL peak energy is discussed by using a rate equation involving size distribution of the QDs. The PL peak line shape was theoretically calculated by using the size distribution observed from an atomic force microscopy. Although its anomalous temperature dependence could be explained by above model, present GaAs QDs was grown without any WL. We, then, concluded that the anomalous temperature-shift of PL peak energy in GaAs QDs were caused by not the presence of the WL but the size distribution of QDs, or additional intermediate level acting the carrier sink such as WL in S-K mode grown QDs.

Resume : Semiconductor nanowire (NW) lasers are unique Fabry-Perot type nanolaser sources, which exhibit single-mode low-threshold lasing characteristics, and properties suitable for monolithic integration onto silicon (Si) photonic circuits [1]. Here, we present our recent progress on monolithically integrated GaAs-based vertical-cavity NW lasers on silicon and silicon-on-insulator (SOI) platform. Individual GaAs NW-lasers integrated by site-selective molecular beam epitaxy on Si ridge waveguides (WG) show low lasing threshold down to ~20 µJ/cm² under optical pumping [2]. With numerical simulations we further explore how the alternating refractive index of the SiO2-masked Si WG influences the modal reflectivity and in-coupling efficiency of the NW laser as a function of NW and WG dimensions [3]. Consequently, we demonstrated direct coupling of lasing emission into Si WG and mode propagation over 60 µm at a wavelength of ~820 nm, limited by absorption losses of the bulk GaAs gain material [2]. To surpass this, we replace the GaAs gain material by coaxial InGaAs/AlGaAs multiple quantum wells (MQW) and tune the emission wavelengths towards the Si transparency window [4]. We also show how the NW-MQW performance critically depends on QW/barrier design and growth conditions. [1] G. Koblmüller, et al., Semicond. Sci. Technol. 32, 053001 (2017) [2] T. Stettner, et al., ACS Photon. 4, 2537 (2017) [3] J. Bissinger, et al., under review (2019) [4] T. Stettner, et al., Nano Lett. 18, 6292 (2018)

Resume : Control of impurity doping is crucial for functionalization of nanowires (NWs) used in optoelectronics and electronics. Doping evaluation in NWs is challenging with traditional electrical measurements due to Ohmic contact requirements. Atom probe tomography can provide atomic-ppm detection limits (DLs) and sub-nanometer resolution but needs extensive sample preparation; electron beam induced energy dispersive X-ray spectroscopy has excellent spatial resolution, but a DL of around 0,1%, insufficient for quantification. Here we demonstrate that X-ray fluorescence with a nanofocused synchrotron beam is a valuable tool to evaluate quantitative Zn doping profiles of in situ doped III-V NWs, with a 50 nm lateral resolution and a DL of around 7 ppm. Three distinct sets of III-V NWs were grown via metal organic vapor phase epitaxy, with in situ Zn doping via diethylzinc (DEZn). We investigated a GaInP NW grown under a constant flux of DEZn and we found that the Zn concentration was in direct correlation with the Ga concentration. We then studied InP NWs used in solar cells, with a p-i-n axial structure, observing an unexpected non-intentional (“background”) Zn doping in the middle segment. Finally, we studied a multi-segment InP NW, with an axial sequence of non-intentionally doped alternated by increasingly p-doped sections. The goal was studying switching dynamics of DEZn during growth and we observed non-steep gradients, suggesting complex Zn dopant incorporation mechanism.

Resume : Group III-nitride semiconductors based on AlxGa1-xN alloys span a wide range of bandgap energies for color tunable laser diodes. However, one of the major issues is related to their low efficiency in the green/red spectral region, which hinders further development of powerful monolithic white LEDs and full-color displays. In order to achieve laser diodes at the nanometer scale, or nano-emitters, AlxGa1-xN nanowires (NWs) are explored. The studied AlxGa1-xN NWs were grown by plasma-assisted molecular beam epitaxy on Si (111) substrate, implanted with europium (Eu) ions at the same fluence of 1×1014 Eu/cm2, and submitted to rapid thermal annealing (RTA) treatments in nitrogen for 30 s, at 1000°C and 1200°C. A detailed spectroscopic analysis based on: micro-Raman, temperature-dependent steady-state photoluminescence (PL), and time-resolved photoluminescence is presented. For all the AlxGa1-xN NW samples, a recovery of the original crystal structure was achieved after RTA annealing. PL measurements revealed the red Eu3+ intra-4f 6 luminescence for all samples with the most intense emission assigned to the 5D0 - 7F2 transition, indicating that such implantation and annealing conditions successfully activated the Eu ions. The same transition is found to shift by 2 nm towards longer wavelengths (from 622 to 624 nm) by increasing AlN molar fraction (from x=0 to x=1), in good agreement with results obtained previously for AlxGa1-xN:Eu layers. For AlN NWs, two well-resolved Eu optically active centers, Eu1 and Eu2, were observed, with a predominance of the Eu1 center at RT, independently of the annealing temperature. In addition, the 5D0 - 7F2 intensity exhibited a lower thermal quenching for AlN NWs annealed at the highest temperature.

Resume : Nanometer-sized 1D materials are promising for gate all-around architectures that are attractive for future low-power field effect transistors (FETs). Top-down approaches for nanowires (NWs) are well established in research and industry, but suffer from several limitations such as high costs, undesirable surface modifications, and many process steps. This is especially true for sensitive or high-priced materials such as germanium. For this reason, a reliable bottom-up method without material waste is required. To achieve this goal, we utilize molecular beam epitaxy with gold as catalyst, a method based on the vapour-liquid-solid mechanism. Nevertheless, it is a challenging task to grow germanium NWs with high crystal quality on oxide materials such as silicon dioxide due to the unpredictable coalescence of metal atoms on oxide surfaces. Therefore, we apply a nano-patterned Si(100)/SiO2 substrate where nanometer-sized silicon tips cresting the oxide surface. The interaction of gold nano-droplets with the silicon/silicon oxide surface has been investigated in detail to effectively improve the NW growth. The wanted exclusive nucleation of gold droplets on the silicon tips and the following efficient growth of in-plane germanium NWs along <110> over the oxide takes place at temperatures of 500 °C – 550 °C. Substrate temperature below 550 °C leads to undesired germanium clusters on silicon oxide, but an increase in temperature above 600 °C reduces the NW growth rate significantly.

Resume : Semiconductor nanowires (NW) are often described as being defect free due to their ability to expel mobile defects with long-range strain fields. Most defects can be detrimental to electronic properties and so the device performance. Less than perfect tip regions are formed during the droplet consumption phase of self-catalysed vapour-liquid-solid grown NWs [1]. The type and occurrence of defects in the tip region of NWs have been identified using aberration corrected scanning transmission electron microscopy (STEM). The stability of the defects found has been probed by exposing the NWs to high temperatures in-situ whilst examining them at atomic scale with STEM. It is found that the type and specific configuration of defect dictate if and how the defect moves, with a range of velocities observed for different configurations and temperatures. The defect motion is found to be dependent on size, position, and surrounding environment of the defect, with the forces behind the motion being relatively large. The geometry of the NW is seen to be an important factor in how defects move. Examples of defect behaviour are given and range from defects seen to be completely removed from the system, to some getting trapped from interacting with other defects, to some which do not move at all. An upper limit to activation energy for the motion of the Σ=3 {112} twin boundary type defects is found to be around 2eV in GaAsP NWs. [1] A.M. Sanchez , J. A. Gott et al., Nano. Lett. 18, 3038 (2018)

Resume : Semiconductor nanowires (NW) can solve the problem of inefficient conversion of waste heat to electricity. The large surface-to-volume ratio results in reduced thermal transport due to phonon scattering while the electrical performance can be improved by pushing the transport into ballistic regime. For our studies we employ Si-delta doped high-mobility GaAs/AlGaAs core-shell NW heterostructures, which hold the potential for high-performance steep-slope NW-field effect transistors (NWFET) [1] and for in-depth investigations of low-temperature quantum transport [2]. Top-gated NW-FETs were used to study the quantum transport at low temperature (4-7 K). During pinch-off we observe clear plateau-like signatures, consistent with the depopulation of quasi-1D subbands as confirmed by correlated simulations [2]. Measurements of the Seebeck voltage as a function of applied gate voltage show distinct spikes, which correspond to the 1D-like plateaus in conductivity [3]. Furthermore, thermal conductivity measurements were carried out on suspended NWs using Raman spectroscopy [4] to probe the effect of the surrounding AlGaAs barrier on phonon scattering at room temperature showing reduced thermal conductivity with respect to their uncapped pure GaAs NW counterpart [3]. References [1] S. Morkötter, et al., Nano Lett. 15 (5), 32953302 (2015) [2] D. Irber, et al., Nano Lett. 17, 4886-4893 (2017) [3] S. Fust, et al., in preparation (2019) [4] M. Soini, et al., Appl. Phys. Lett. 97, 263107 (2010)