Semiconductor nanostructures towards electronic & opto-electronic device applications V

Resume : The interaction of light with graphene and related 2d materials embodies a wide variety of physical processes such as photoconversion, energy transfer, nanoscale control of optical fields, etc., with strong potential for disruptive opto-electronic technologies [1]. In this talk, several examples of the opto-electronic and nano-photonic opportunities of novel 2d material heterostructures are being addressed. First, strong interactions between graphene and nanoscale light-emitters are controlled and detected. Because graphene is gapless with tunable carrier density, it can effectively behave as a semiconductor, a dielectric, or a metal. We exploit this to electrically control of optical emitter relaxation pathways. Specifically, we control whether emitter excitations are converted into either photons, electron-hole pairs, or plasmons with confinement to the graphene sheet below 15 nm [4,5]. Second, nano-structured sandwiches of graphene with boron nitride have resulted in high quality plasmonic systems for infrared light. We discuss new configurations of these electrically tunable metamaterials and plasmonic circuits with in-situ tunable control of light at length scales more than a factor 150 below the free-space wavelength. We report strong improvements of the graphene plasmon lifetime and propagation lengths and we assess the intrinsic loss mechanisms [2,3]. Finally, we show how scanning near-field photocurrent microscopy is a promising new technique to gain insight into different device properties of standard graphene devices with a nanometre-scale resolution. References: [1] F. H. L. Koppens, T. Mueller, Ph. Avouris, A. C. Ferrari, M. S. Vitiello, M. Polini, Nature Nanotechnol. 9, 780-793 (2014) [2] A. Woessner, M. B. Lundeberg, Y. Gao, A. Principi, P. Alonso-González, M. Carrega, K. Watanabe, T. Taniguchi, G. Vignale, M. Polini, J. Hone, R. Hillenbrand, F. H. L. Koppens, Nature Materials [online DOI: 10.1038/NMAT4169] (2014) [3] Alessandro Principi, Matteo Carrega, Mark Lundeberg, Achim Woessner, Frank H.L. Koppens, Giovanni Vignale, Marco Polini, Phys. Rev. B 90, 165408 (2014) [4] K.J. Tielrooij, L. Orona, A. Ferrier, M. Badioli, G. Navickaite, S. Coop, S. Nanot, B. Kalinic, T. Cesca, L. Gaudreau, Q. Ma, A. Centeno, A. Pesquera, A. Zurutuza, H. de Riedmatten, P. Goldner, F.J. García de Abajo, P. Jarillo-Herrero, F.H.L. Koppens, Nature Physics [online DOI: 10.1038/nphys3204] (2015) [5] Andreas Brenneis, Louis Gaudreau, Max Seifert, Helmut Karl, Martin S. Brandt, Hans Huebl, Jose A. Garrido , Frank H.L. Koppens, Alexander W. Holleitner, Nature Nanotechnoly [online DOI: 10.1038/nnano.2014.276] (2014)

Resume : The recent discovery of localized surface plasmon resonances in inorganic nanotubes and fullerene-like nanoparticles of WS2 and MoS2 opened up new interest in their optical properties. The coexistence of plasmons (with a broad spectral line) and excitons (a narrow discrete resonance) in these systems leads to destructive and constructive interference that can be described with Fano resonance. Addition of metallic nanoparticles with their own inherent plasmon resonance to the WS2 nanotubes can influence the transient absorption behavior. The morphology of the new system obtained by decorating WS2 nanotubes with gold nanoparticles is studied, as well as their optical properties. For the latter, techniques such as total extinction, decoupled and transient absorption are used.

Resume : Although GaSb NWs have been widely explored as promising active channel materials for various technological applications, the required performance improvement still lags behind the expectation, largely suffering from the uncontrolled radial VS growth during the NW synthesis. In this work, we report the use of sulfur surfactant in chemical vapour deposition to achieve very thin and uniform GaSb nanowires with diameters down to 20 nm. In contrast to surfactant effects typically employed in the liquid phase and thin-film technologies, the sulfur atoms contribute to form stable S-Sb bonds on the as-grown nanowire surface, effectively stabilizing sidewalls and minimizing unintentional radial nanowire growth. When configured into transistors, these devices exhibit impressive electrical properties with the peak hole mobility of ~200 cm2V-1s-1 , better than any mobility value reported for a GaSb nanowire device to date. Moreover, this surfactant-assisted CVD scheme might also be employed as a growth platform for other NW material systems, such as other typical III–V semiconductors of GaAs. These facts demonstrates that the use of sulfur surfactant enables the growth of uniform, well-controlled, small-diameter GaSb NWs, with impressive electrical properties and could open up the possibility for many practical applications.

Resume : ZnO is a semiconductor with a direct band gap (3.37eV) and a large exciton binding energy (60meV) at room temperature. ZnO nanowires have shown advantages due to their very large surface-to-volume ratio, superior stability owing to high crystallinity and simple preparation methods. Hydrothermal synthesis is known for being a very powerful and versatile technique due to its low cost, flexibility, facility and environmentally benign processes. ZnO is a polar crystal with polar and non-polar surfaces; some polymers can selectively passivate specific facets by chemical or physical absorption, leading to tailor the aspect ratio of nanowires without changing their morphology. In particular, Polyethylene glycol (PEG) acts as a shape-selective template to control the nucleation and growth along a specific direction by adjusting the rate of growth among the nuclei, resulting in long and thin nanowires when PEG’s concentration is appropriate. Here, we characterize and optimize such hydrothermal synthesis leading to ZnO nanowires of ~3µm with an aspect ratio of ~10. Furthermore, we introduce a new concept for the growth of ZnO nanowires by cycling the injection of precursors, resulting in very long nanowires with lengths of ~5µm and aspect ratio of ~20, results that to our knowledge are never been yet reported by this method. Electrical transport properties are studied specifically depending on the nanowire’s length (longer than 3µm) as a function of temperature.

Resume : White lighting emitting diodes (WLEDs) have been considered as promising candidates for next generation illumination because of low energy consumption, mercury free composition, long life time, high reliability and durable properties. It is very important to generate the white light. Many studies have been carried out, in which a primary method is to increase output power of LEDs. This makes it possible to increase luminous flux considerably per one LED, however, this also increase the chip temperature of the blue LED for the excitation. This may cause a deterioration of the resin, which is used to fix the powder phosphors onto the blue LED chip, decreasing the luminous efficacy and lifetime. We aimed at producing a novel durable phosphor without using resin. We fabricated phosphor in glass (PiG) and discuss the WLEDs. PiG is a simple mixture with phosphor and glass powder, such as frits, and glass powders can be formed into a stable matrix for the phosphors through the viscous sintering process. The sintering temperature can be considerably lower than those for phosphor ceramic sintering processes. We examined the effectiveness of luminous efficiency PiG for white LEDs. The emission characteristics of PiG s fabricated sample were examined by power X-ray diffraction (XRD) and photoluminescence (PL).

Resume : Observations of random laser have been reported in many semiconductors such as ZnO, GaAsN, GaAs, SnO2, and ZnSe . More recently random lasing action have been observed in GaN self–organized nano-columns grown on Si substrate. We report here the first random lasing action by optical pumping on a 10 μm thick GaN film grown on a sapphire substrate. The film was grown using MOCVD at T=1150° C. A quadruped NdYAG laser at 266 nm excited the photoluminescence of GaN film. The results are presented vs laser energy. The spectra exhibit a strong luminescence below 400 nm. At low pump energy the spectra show a broad band of around 20 nm wide (FWHM) centred at 363 nm. Above a threshold pumping, sharper peak emerges from the spectra at 373 nm. Composition, microstructure and morphology of the film were determined and electrical properties (resistivity, concentration and mobility of carriers) were measured and tentatively correlated to the PL properties.

Resume : In this paper we present the AC impedance analysis of Metal/porous silicon contacts in order to investigate their conduction mechanisms. The porous silicon layer was obtained by electrochemical etching of the p-Si wafer. The measurements were made between 5 Hz and 13 MHz, at room temperature and in the DC range from 0 to 10 V. An electrical equivalent circuit was used to fit the experimental data. The voltage dependent of the fitting.

Resume : LEDs brought a revolution in the field of lightings and displays with their potentials of longer life times, non-hazardous and above all energy efficiency. To generate white light from LEDs, blue InGaN chip is covered with a single yellow phosphor or a combination of red and green emitting phosphors. For applications in LEDS, rare-earth (mainly Eu2+) doped (Ca,Sr,Ba)2SiO4 (orthosilicates) based phosphors are highly investigated in recent years and have matured over a period of time. These offer a broad emission range from blue to red depending upon the compositions. The primary advantages associated with these phosphors are no decay in brightness over time for 1000 hours. Conventionally these phosphors are prepared through solid state reaction method which poses the limitations of high synthesis temperatures, longer reaction times and non-uniform distribution of activator ion. In this study, we prepared binary and ternary orthosilcate phosphors with emission ranging from blue to red through a cellulose assisted liquid phase precursor method. The method introduced employed, allows the synthesis at normal pressures and lower temperatures.The process not only eases the reaction procedure but also allows a multiple metal salts to be used as precursors. The investigations revealed that the as prepared phosphors exhibited higher emission intensities (even higher than the commercial available YAG phosphor). To further enhance the luminescence, experiment was further extended and different cellulose templates were investigated.

Resume : First principles calculations within the density functional theory (DFT) framework were carried out to calculate the structural and electronic properties of the AlN1-xBix ternary alloy. Firstly, we have performed the calculation of the structural and electronic properties of binary compounds AlN and AlBi of zinc-blend (ZB) structure. For AlN1-xBix ternary alloy we have studied ordered structures. We have used three arrangements for x = 0.5: CuAu, CuPt and chalcopyrite ones. It is observed that the structures are highly deformed for CuAu and CuPt arrangement. For x = 0.25 and x = 0.75, we have studied two arrangements: famatinite (Al3Ti-type) and luzonite (Cu3Au-type). It is observed that the structure is distorted when the bismuth (Bi) and nitrogen (N) is within the same plane.

Resume : GaN is one of the most studied semiconductors for applications in solid-state light emission. Low dimensional structures, such as GaN nanowires (NWs), constitute potential nanoscale building blocks for future optoelectronic devices. These NWs exhibit high compatibility with different substrates (like silicon) and are known to be almost free of threading dislocations due to their dimensions and efficient strain relaxation. Another advantage of these structures is the improvement of the light-extraction efficiency owing to their non-planar geometry. Trivalent europium (Eu3+) ions are known to be efficient red activators in several wide band gap hosts namely GaN. Ion implantation appears as an alternative approach to dope nanostructures since it allows the introduction of impurities in a controlled way and without solubility limits. In this work, GaN NWs implanted with Eu3+ were analysed by photoluminescence and Raman spectroscopy. Their luminescence properties were compared with GaN layers implanted in the same conditions of the NWs. The red 5D0->7FJ, with J = 1, 2, 3 and 4, luminescence transitions of the Eu3+ (4f 6) ions were optically activated after the recovery of the lattice damage by thermal annealing treatments. The peak position and spectral shape of the ion related luminescence are in good agreement with those observed in GaN layers. All the samples exhibit the sharp 5D0->7F2 transition at RT and in the analysed NWs the emission can be observed with the unaided eye.

Resume : GaN-based light-emitting diodes (LEDs) are attractive for many solid state lighting applications. For instance, white light can be obtained by color mixing of a commercial InGaN blue emitting LED coupled to a Ce-doped YAG phosphor: blue light (BL) from the LED excites the phosphor to emit yellow light (YL). The mixture of BL and YL is perceived as white by the human eye. Other techniques are used to obtain white light, in particular mixing the light of three LEDs emitting in the primary colors (RGB). In this work we demonstrate white light emission from a single monolithic InGaN/GaN multi quantum well (MQW) LED structure after heat treatment. High temperature and high pressure (HTHP) thermal annealing was performed on InGaN/GaN MQW LED structure, at different temperatures: 1000 ºC, 1100 ºC and 1200 ºC. The samples are analyzed using photoluminescence (PL), PL excitation (PLE) and decay times. At room temperature (RT), the as-grown structure showed two main emission bands, under laser excitation of 3,8 eV: a yellow band (YB) peaked at 2.15 eV and a blue band (BB) peaked at 2.8 eV. After annealing at 1000 ºC, the two bands persist, however, the intensity ratio of YB/BB increases with respect to the as-grown structure providing white light. These studies reveal that the annealing significantly changes the optical properties and that the best HTHP thermal treatment conditions to obtain RT white light emission from a monolithic InGaN/GaN MQW LED structure were achieved.

Resume : In this work the influence of strain on the energetics and electronic properties of nanowires (NWs) consisting of a GaN base followed by a superlattice of successive InxGa(1-x)N nanodisks (NDs) (In content ranging from 0 to 0.25) separated by GaN spacers is examined. The supercells are modelled with the LAMMPS Molecular Dynamics (MD) and the AIMPRO Density Functional Theory (DFT) codes. It is found by both simulation approaches that among three possible types of strain (biaxial, hydrostatic and uniaxial) and for all In contents, the biaxially strained NW superlattice is the one with the lowest excess energy. However, the energy difference between biaxially and hydrostatically strained states is small for an In content lower than 0.10 in the NDs, leading to the conclusion that up to that point, the biaxial and hydrostatic strain components should coexist in the embedded NDs. Concerning the optoelectronic properties, the strained InGaN NDs do not induce states in the bandgap of the NWs. However, a bowing parameter of 1.82 should be taken into account in the quadratic Vegard’s equation for the bandgaps for the various concentrations x in the InxGa(1-x)N NDs.

Resume : Oblique-angle deposition (OAD) method has recently been explored to develop better transparent conductive electrodes (TCEs) toward higher light extraction efficiency of GaN-based light-emitting diodes (LEDs), since it can form porous rod structures with an intended inclined angle. Indeed, the application of well-designed indium-tin-oxide (ITO) electrodes deposited by OAD method showed an improved optical output power due to an increased light extraction through a graded refractive-index media. However, the ITO films was deposited by an OAD method has some problems of poor adhesion and structural instability, preventing it from the practical use in the industry. Meanwhile, the oxidized Ni/Au electrode, which has been one of the representative TCEs in LEDs, is expected to show better adhesion and structural stability than the ITO due to its metallic nature though the films are grown with an OAD method. Furthermore, the deposition of thin Ni/Au films (~10 nm) is cheaper and faster than that of the relatively thick ITO films, it would be promising for practical application. In this regard, we attempted to maximize the properties of Ni/Au TCEs by using an OAD method. The Ni/Au films deposited at an incidence angle of 45-degrree followed by thermal annealing in oxygen ambient was best in terms of optical transmittance (~90 % at 450 nm), sheet resistance (13.6 ohm/sq), and the contact resistance (9.2x10^-5 ohm-cm2), resulting in a ~10 % improved light extraction of LEDs.

Resume : Owing to their widely tuneable optical properties and strong light-matter interaction, colloidal quantum dots (QDs) are considered for next-generation photonic devices such as all-optical wavelength conversion. Using QDs for this application is limited by either slow interband dynamics or energy consuming multi-exciton dynamics. Here we show, using white light pump-probe spectroscopy, that the interplay between two intrinsic material properties of QDs, intraband absorption and interband bleach, can lead to a very strong pump-induced modulation of near-infrared light on an ultrafast, picosecond, timescale with no background. The normalized absorption change (ΔA/A0) is chosen as a figure-of-merit, reaching up to 23 for a single exciton population, both in solution and film. We deduce the strength of the absorption burst as 5200 cm-1. To shown the potential for high speed conversion, a pump pulse sequence of up to 4 pulses, separated by 2.2 and/or 4.4 ps, is converted to a probe wavelength while preserving the intrinsic strength, speed and zero background of the single pulse case, showing the ability for handling 450 and 225 Gb/s datastreams. Combining the QDs with photonic devices is shown viable with conversion energies as low as a few femtojoule per bit. Moreover, due to the strong light-matter interaction of QDs, the integrated device footprints of a few hundred micron required are much smaller than existing approaches.

Resume : In present work properties and the direct one-step relief formation with the use of amorphous chalcogenide multilayers of composition based on As40S60:Mn?Se were studied. Amorphous As40S60:Mn?Se nanomultilayers were prepared by cyclic thermal vacuum deposition from two isolated boats with As40S60:Mn and Se chalcogenides on constantly rotated glass substrate at room temperature in one vacuum deposition cycle with chalcogenide thickness of 11nm and Se - 10nm. The total number of nanolayers was 180. Optical transmission was measured in 200-900 nm optical range in order to determine the refractive index, thickness and optical band-gap energy of As40S60:Mn and Se layers and As40S60:Mn?Se nanomultilayers. Diffraction gratings were recorded by two laser beams using different polarizations of light (DPSS green laser, λ=532nm and power 100mW) with synchronous diffraction efficiency measurement in first diffraction order using red laser s-polarization 650nm wavelength. Process of surface relief formation depended on the polarization of recording light beams. Diffraction efficiency of recorded gratings was ~ 7% in absolute value. AFM and Raman data allow to propose the mechanism of solid-state structure conformations in As40S60:Mn?Se multilayer system. Due to the changes in transmission, reflection, and in thickness under the influence of laser irradiation, As40S60:Mn?Se nanomultilayer structures may be used for effective amplitude-phase optical information recording, for the product

Resume : All commercial forms of electronic technologies use planar, rigid, brittle substrates. But this conventional rigid electronic systems have many limitations. For overcoming this limitations stretchable electronics such as flexible displays, smart skins, soft and human friendly devices have been considered as a promising technology. As a one strategy for this promising electronic circuits that maintain proper functions under a large amount tensile strain, the interconnects have been either produced as metal lines having stretchable configurations, or made from conductive elastomeric composites. Another method for metal interconnects, a supporting elastomeric substrates is first stretched, then metal interconnects are deposited or transferred onto the pre-strained substrates, But this method is less practical because it is precisely control the structure of wavy profiles over a very large substrates area. The non-uniform distribution of pre-strain also makes it difficult to obtain high reliability. Here we fabricated a stretchable wavy Al interconnect by introducing stress-free elastomeric substrates. In this process, the wavy structure is first produced within a rigid Si mold by conventional photolithography, dry etching, sacrificial layer coating process, and Al is produced for interconnect and then transferred on to an elastomeric polydimethylsiloxane(PDMS). Additionally, Parylene can be deposited for stretchability before metal producing. This stretchable wavy metal interconnect demonstrates high reliability and provide good stretchability (~20% resistance increase at 30% tensile strain) without any pre-stretching step.

Resume : The study of size dependence of thermoelectric properties of Au, Pt, Ag and Pd nanoclusters deposited onto highly oriented pyrolytic graphite HOPG(0001) by pulsed laser deposition is presented. The determination of the thermoelectric power value of nanoclusters was based on analysis of differential tunnel volt-ampere characteristics obtained by scanning tunneling spectroscopy. It was found out that thermoelectric power of Au, Pt, Ag and Pd nanoclusters changes dramatically for nanoclusters with lateral sizes less than 1 nm. The changes might be caused by loss of metal properties of nanoparticles. The results obtained might be used for development of nanoscale devices, for example, development of nanocoolers and temperature sensors of integrated circuits.

Resume : The magnetoconductance properties of InAs/GaAs core-shell nanowires (NWs) under axial magnetic fields have recently received noticeable attention [Gül et al., PRB 89, 045417 (2014) and NL 14, 6269 (2014)]. However, despite the NW hexagonal cross-section, the experimental observations have been exclusively analyzed approximating the device (NW + back-gate) as a cylindrical system and, therefore, a proper symmetry preserving calculation capable to reproduce the experimental regime is lacking. In this contribution we go beyond simple cylindrical and non-interacting models and perform a spin density functional theory calculation in a hexagonal domain that preserves the discrete symmetry of the system. Our calculations explain why Aharonov-Bohm magnetoconductance oscillations are observed in experimental devices with back-gates, in spite of the asymmetric voltage potential, and predict symmetry induced vanishings of the oscillations under specific gate-all-around voltages. The electronic system is shown to undergo different direct Coulomb and localization regimes as the magnetic field is increased up to the complete depletion limit. Abrupt transitions between the different Coulomb regimes occur at discrete magnetic fields when the system gets, first, spin-polarized and, second, exclusively formed of electrons from the first Landau band. These transitions show up as kinks in the subband spectrum, total electron density and free energy; and step-like magnetoconductance drops.

Resume : Self-assembled InAs quantum dots (QDs) were grown by molecular beam epitaxy (MBE) on GaAs (211)B substrate and then were buried under a GaAs layer. QDs tend to acquire an anisotropic pyramidal shape elongated along the <111> direction, while their length and height do not exceed 10 nm and 4 nm, respectively. Geometrical phase analysis (GPA) of high-resolution transmission electron microscopy (HRTEM) images along the in-plane direction showed a full in-plane registration of the two lattices. In view of the absence of any dislocations associated with the QDs, this suggests fully strained nanostructures. However, along the growth direction GPA showed a variation in the distribution of strain among various QDs with lattice strain values ranging from 14% to 5%, using the GaAs lattice as reference. Assuming the validity of Vegard’s law and a plane stress condition, these values correspond to QDs with a variety of indium mole fractions ranging from 1 (pure InAs) to less than 0.5 (stoichiometric InGaAs alloy). Moreover, the persistent trend of a gradual increase of In from the interface towards the apex of the QDs, implies that Ga segregation mainly occurred in the initial stages of QD growth. Acknowledgement Research co-financed by the European Union (European Social Fund–ESF) and Greek national funds - Research Funding Program: THALES, project NANOPHOS.

Resume : The aim of this study is to investigate the potential of organic polymer-derived carbon films for absorbing and reflecting the electromagnetic (EM) waves in the sub-terahertz frequency range (0.2-0.5 THz). The carbon films are easily produced by thermal degradation of commercial available organic polymers. The progress on the thermal conversion of polymeric precursor, as well as, structural and morphological features of heated samples have been carefully characterized by several techniques (Thermogravimetry coupled with mass spectrometry, Raman spectroscopy, and Scanning Electron Microscope). The carbon residue film obtained at 1200°C in flowing inert gas shows 55% carbon yield, 20% linear shrinkage, and 320 S.cm-1 D.C conductivity. Raman spectroscopy measurements on the pyrolyzed materials exhibited the two characteristic bands, D (1358cm-1) and G (1593 cm-1), indicating the presence of turbostratic carbon in which layer plans are misaligned. By using free-space S-parameters THz measurements, such materials demonstrate a gradual reflection and absorption depending on pyrolysis temperature and thereby the degree of organization of carbon layer planes.

Resume : We report the growth of high Mg content, high quality, wurtzite structure MgxZn1-xO epitaxial film using pulsed metal organic chemical vapor deposition (MOCVD) method. Series of MgxZn1-xO epitaxial films with variable Mg concentration were systematically deposited on bare and AlN coated sapphire substrates. The substrates were covered by low temperature grown thin (~30 nm) ZnO buffer layer which acts as a template for the growth of high quality crystalline film by reducing the lattice mismatch between the substrate and the film. The optical band gap for the epitaxial films estimated using UV-visible transmission spectra ranges from 3.24 eV to 4.50 eV, corresponding to the fraction of Mg between x=0.0 to x=0.5. Besides, the transmission spectrum for each epitaxial film has shown a sharp band edge, suggesting no evidence of phase segregation even with the highest Mg content (x=0.5). A linear blueshift of the MgxZn1-xO spectral peak position with increasing Mg content is observed in a room temperature cathodoluminescence (CL) spectroscopy. The absence of band splitting in the CL spectra further evidences that the grown films are homogeneous (single phase) materials. To the best of our knowledge, the current result shows the highest Mg content (x=0.5) wurtzite MgxZn1-xO epitaxial film ever grown by MOCVD without phase segregation.

Resume : Quantum dots (QDs) are called artificial atoms because they have discrete energy levels analogous to those of natural atoms. By controlling the spatial proximity and orientation of multiple QDs (e.g. InAs in GaAs), complexes of QDs can be created. These complexes are called quantum dot molecules (QDMs) because coherent tunnel coupling between pairs of QDs can lead to the formation of molecular states analogous to those in diatomic molecules. Many well-established concepts of physical chemistry and atomic physics can be used to understand the formation of molecular orbital states of both electrons and holes. However, unlike ‘natural’ molecular states, the parameters in these artificial molecules can be varied both during growth and in situ. As a result, QDMs present a unique opportunity to use molecular engineering principles to design new components for optoelectronic quantum device technologies that promise fundamentally secure information transmission, new paradigms for computation and new, exquisitely sensitive, platforms for sensing technologies. We will present and compare the properties of a variety of different QDM structures, including vertically-stacked and laterally-oriented diatomic pairs. We will describe how our understanding of these unique structures has advanced both theoretically and experimentally and present our initial progress in “molecular engineering” of new nanostructures with properties optimized for optoelectronic quantum device applications.

Resume : III-nitride core/shell nanowires (NWs) grown along the polar direction comprise defect free non-polar interfaces, even for greatly lattice mismatched materials, offering significant improvements in the light-extraction efficiency compared with their planar counterparts. In this work GaN/AlN and InN/GaN core/shell NWs are investigated with the LAMMPS Molecular Dynamics and the AIMPRO DFT codes. Structural models of several thousands of atoms with multiple shell-to-NW rations are examined. The variations of the a lattice constants along [10-10] though the middle and through the edge and [11-20] are calculated and the results show an variation which is dependent on the direction. The a lattice constant remains unchanged through the core but it decreases with the shell-to-NW ratio, leading to a conclusion that as the shell gets thicker the core is increasingly compressed. The average stress calculations along [11-20] and [10-10] show that the shells are under tensile stress at the sides and the inner edges while being almost relaxed at the outer edges. The cores are under compressive stress, which is larger at the edges, and are almost relaxed in the center. The density of states is examined for each case and the bandgaps of the core/shell NWs are found to be free of deep states. The bandgaps increase exponentially with the shell-to-NW ratio and reach a maximum value for approximately 0.5.

Resume : Multi-shell semiconductor nanowires (NWs) are attracting much interest due to their possible application as light harvesting devices, nanophotonic sources, and nanoscale FETs with novel geometries. Their potential partly relies on the ability to host an axially symmetric high-mobility electron gas (EG), similar to traditional planar 2DEGs formed in modulation doped GaAs/AlGaAs heterojunctions, but wrapped around the central core of the NW. However, a satisfactory demonstration of carriers-dopants separation and mobility enhancement of the EG has only been reported recently. In this talk, I will describe how the analysis of inelastic light scattering (ILS) spectra allows one to determine the localization pattern of the EG. We compute the collective electronic excitations observed in ILS experiments on modulation doped core-multi-shell GaAs/AlGaAs NWs and show that the additional features observed in the spectra of doped NWs, compared with reference undoped samples, can be assigned to collective excitations of a free EG formed at the heterointerface, away from the dopants [S. Funk, M. Royo, et al., Nano Lett. 13 (2013) 6189]. The EG exhibits a mixed dimensionality with 1D/2D states mostly localized in the corners/facets. Our modeling is based on a self-consistent solver exploiting symmetry compliant grids and a full description of material and doping profiles. Theoretical simulations of the ILS spectra have been performed within LDA and TDLDA multi-subband 3D models.

Resume : III-V compound semiconductor nanopillars for optical device application have potential to provide new electrical and optical properties, as well as more efficient performances. It is well known that fabricating quantum dots from a top-down approach is difficult because of the induced plasma damages, especially due to ultra-violet irradiation during plasma process. To overcome these problems, we have developed bio-template and neutral beam etching (NBE) processes to realize III-V nanopillars structures that enable bandgap-engineering. The nanopillars etched by the NBE are embedded GaAs/AlGaAs stacked structure. After the etching, 10-nm in diameter and 100-nm in depth nanopillars arrays with high density of 1E+11 cm-2 were fabricated including GaAs nanodisks (NDs). PL measurements were carried out by using an Ar ion laser as an excitation source, a photo-multiplier as a detector at a temperature of 4K. PL peak at 1.63 eV were measured. We have performed the PL measurements for multiple quantum wells (MQWs) and NDs samples for quantum level comparison. As a result, we have concluded that the 1.56 eV peak is due to the longitudinally confined quantum levels as detected in MQWs sample, and the 1.63 eV is due to GaAs NDs quantum levels in nanopillars. This means damage-less sidewalls were realized. Therefore, we succeeded in observing the PL emission for ND structures in nanopillars.

Resume : Semiconductor quantum dots (QDs) have attracted increasing interest, mainly because of the possibility to emit non-classical light states. However, bottom-up growth techniques that give best results in term of QD optical features cannot control the QD's position or shape, which are both crucial for the fabrication of optoelectronic devices. Thanks to a novel technique, based on H irradiation of a GaAs/Ga(AsN)/GaAs quantum well, it is possible to obtain site-controlled nanostructures. In Ga(AsN) and other III-V dilute nitrides, stable N-2H-H complexes form and neutralize all the effects N incorporation has on the host matrix, including a large reduction of the band-gap energy. Therefore, deposition of H-opaque masks on the sample surface and subsequent sample hydrogenation allows for the realization of site-controlled nanostructures with arbitrary shape and size. In this work, we studied the optical properties of Ga(AsN) QDs grown by this novel technique. Five arrays of dots with different sizes were prepared and many ensembles of dots for each array were characterized by several kinds of photoluminescence measurements in order to clarify different aspects of the growth protocol, with a particular attention for the best dots of each array. Quantum confinement effects, antibunching in single-exciton recombination, and sharp and bright emissions on the nanosecond timescale were observed, thus demonstrating that a deterministic control of quantum emitters location is possible.

Resume : The InGaN alloy has been recently shown to be an excellent candidate for full-solar-spectrum photovoltaic applications owing to its direct wide band gap, from 0.7 eV to 3.4 eV. High-efficiency solar cells (SCs) that could exhibit efficiencies higher than 50% are possible using InGaN sub-cells with an In content ranging from around 20 to 100%. We report a comprehensive comparison of structural investigations of high quality / high indium content InGaN epilayers in forms of bulk and multiple quantum wells (MQWs) to be used as active regions in solar cells. SC designs have active regions that include 200 nm (bulk) In0.26Ga0.74N and InxGa1-xN/GaN (x = 0.10, 0.19, 0.24 and 0.28) MQWs including 25 or 35 pairs grown on sapphire (0001) substrate by metal-organic chemical vapor deposition (MOCVD). This study also presents a comparison of structural characterization of InxGa1-xN/GaN (x=0.15) MQWs (25 pairs) grown on sapphire (0001) and bulk GaN (0001) substrates. The surface characterization, crystalline quality and the morphological evolution of the high In content MQW epilayers were investigated by AFM, SEM and High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM). Besides, compositional data is provided by EDX and HRXRD in addition to the optical characterization obtained by Photoluminescence Spectroscopy. The study includes also the photovoltaic characterization of the SCs InxGa1-xN/GaN MQWs (x from 0.10 to 0.24) on sapphire and bulk GaN substrates.

Resume : The quest for the perfect quantum dot (QD) is a drive for both chemists and physicists. Since the landmark synthesis of colloidal semiconductor nanocrystals1, many studies have tried to understand and limit QD emission blinking in time2 due to the presence of an excess charge, in or in close proximity to the nanocrystal3 that can recombine non radiatively with the exciton through Auger processes. We present here a new generation of nanoshell resonator that consists of a single quantum dot encapsulated in a silica shell coated with a continuous gold nanoshell.4 It provides a blinking-free system with a stable and Poissonian emission at room temperature that is preserved regardless of drastic changes in the local environment. This novel hybrid quantum dot/silica/gold structure behaves as a plasmonic resonator with a strong Purcell factor, reducing the radiative lifetime and overcoming Auger recombinations. The gold nanoshell also acts as a shield that protects the quantum dot fluorescence and enhances its resistance to high-power photoexcitation or high-energy electron beams. (1) Murray, C. B.; Norris, D.; Bawendi, M. G. Synthesis and Characterization of Nearly Monodisperse CdE (E= S, Se, Te) Semiconductor Nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706–8715. (2) Nirmal, M.; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J. J.; Trautman, J. K.; Harris, T. D.; Brus, L. E. Fluorescence Intermittency in Single Cadmium Selenide Nanocrystals. Nature 1996, 383, 802–804. (3) Kuno, M.; Fromm, D. P.; Hamann, H. F.; Gallagher, A.; Nesbitt, D. J. “On”/“off” Fluorescence Intermittency of Single Semiconductor Quantum Dots. J. Chem. Phys. 2001, 115, 1028. (4) Ji, B.; Giovanelli, E.; Habert, B.; Spinicelli, P.; Nasilowski, M.; Xu, X.; Lequeux, N.; Hugonin, J.-P.; Marquier, F.; Greffet, J.-J.; et al. Non-Blinking Quantum Dot with a Plasmonic Nanoshell Resonator. Nat Nano 2015, 10, 170–175.

Resume : In the last years the nonlinear response of plasmonic nanostructures have received an increasing attention due to the pronounced nonlinearity shown by metallic media. One of the disadvantages of the metallic nanoparticles is that the plasmon resonance can be tuned in the spectral range only, and in a very narrow window by playing with the shape of the nanoparticle. Beside noble metals, a novel class of synthetic plasmonic media is based on heavily-doped semiconductors. Cu2-xSe and Cu3-xP are among these novel materials, where the free carriers are the holes in the valence band generated by copper vacancies (self-doping mechanism) due to the sub-stoichiometry of the compound. With carrier densities of around 1021cm-3 the plasmon resonances are located in the near infrared. Notably, in Cu2-xSe for example the free-carrier density can be chemically controlled by changing x, allowing the plasma frequency to be tuned in a broad wavelength range in the near infrared. We report on a detailed experimental investigation of the ultra-fast nonlinear optical response of Cu2-xSe and Cu3-xP nanocrystals in the visible and near-infrared under excitation with fs laser beams. The plasmon dynamics is spectrally resolved by fs pump-probe spectroscopy with broad-band probe pulses. [1] Scotognella et al. 2011 Nano Lett. 11 4711 [2] De Trizio et al. 2013 Chem. Mater. 27 1120

Resume : Surface plasmons are expected as carrier waves in nano- and micro-scale transmission systems. In these systems, optical field having signals is tightly bounded to collective oscillation of electrons and confined within a nano-scale area on devices. To realize the integration of plasmonic and electronic devices, we have developed a surface plasmon detectors, waveguides, a MOSFET having a surface plasmon detector [1-4]. This paper discusses suitable structures and materials for the monolithic integration of these devices. These surface plasmon devices were fabricated on a silicon substrate using thin metal films such as Au. A coherent light beam was incident to the gratings built on a waveguide composed of a thin Au film and converted to surface plasmon. The waveguide could simultaneously transmit the surface plasmon signals as well as electric signals. The transmitted signals carried by surface plasmons were converted to electric signals with the surface plasmon detector having a Schottky barrier. The converted electric signals were biased on the gate electrode of a MOSFET and drove it. These devices were simple structures and connected with electronic devices in nano- and micro-scale on a silicon substrate. The electrical outputs of the MOSFET were easily connected to electronic circuit. These performances indicate such photonic integration devices will be promising candidates as key components in future information systems. [1] T. Aihara, et al., Appl. Phys. Lett., vol.99, 043111, 2011. [2] T. Aihara, et al., IEEE Photonics J., vol. 5, 6800609, 2013. [3] M. Fukuhara, et al., Appl. Phys. Lett., vol. 104, 081111, 2014. [4] M. Fukuhara, et al., J. Lightwave Tech., vol. 32, p.3888, 2014.

Resume : Interplay between carrier localization and nonradiative recombination is the main factor determining the efficiency of light emission of the AlGaN-based multiple quantum wells (MQWs) deep-UV light-emitting diodes (LEDs) and laser diodes. Strong carrier localization results in higher spontaneous emission efficiency but increases the threshold for stimulated emission and decreases the onset of the efficiency droop in LEDs. We report on a detailed photoluminescence (PL) spectroscopy study of this interplay in the AlGaN MQWs grown by migration-enhanced metalorganic chemical vapor deposition on sapphire substrates. The measurements were performed in two configurations, front-surface and edge, under quasi-steady-state conditions in a wide range of excitations at the temperatures ranging from 8 to 300 K. The results show that the carrier localization is important for carrier dynamics up to the carrier densities sufficient for stimulated transitions to occur. Furthermore, the ratio of the thermal energy to the dispersion parameter of band gap fluctuations is the parameter indicating the predominant origin of PL droop in AlGaN-based MQWs: for low band gap fluctuations at high temperatures, the droop is caused by stimulated emission, whereas in the samples with strong localization at low temperatures, the droop occurs due to the enhancement of nonradiative recombination as the localized states are populated and an increasing fraction of carriers becomes free at elevated excitations.

Resume : Colloidal hybrid nanostructures provide the opportunity to optimize material properties for advanced applications, for example (opto-)electronics or photocatalysis [Banin et al. Chem. Mater. 2014, 26, 97–110.] . Semiconductor nanoparticles with grown-on metallic domains exhibit an increased conductivity compared to pure semiconductors in single particle devices [Steinberg et al. Nano Lett. 2009, 9, 3671-3675.]. In solution, nanoparticles consisting of Pt and CdSe were shown to efficiently separate photo-generated charge carriers, as Pt enables quick transfer of electrons away from the semiconductor [Yu et al. J. Phys. Chem. Lett. 2013, 4, 3596-3601.]. In the current work, we examined the modulation of the electrical transport through CdSe nanoparticle monolayers by the deposition of Pt domains with different sizes. For this, Pt-CdSe hybrid nanoparticles with different Pt contents were prepared by colloidal synthesis and assembled to monolayers onto Si substrates with e-beam defined Au electrodes by the Langmuir-Blodgett technique. It will be shown that already in the dark the conductivity of Pt-decorated CdSe can be up to nine orders of magnitude higher in comparison to bare CdSe samples while maintaining photoconductivity. Apart from reporting on the changing electrical transport properties of dark as well as illuminated samples, related transport mechanisms will be discussed.

Resume : Luminescent solar concentrators are cost-effective complements to semiconductor photovoltaics that can boost the output of solar cells and allow for the integration of photovoltaic-active architectural elements into buildings (for example, photovoltaic windows). Colloidal quantum dots are attractive for use in luminescent solar concentrators, but their small Stokes shift results in reabsorption losses that hinder the realization of large-area devices. Here, we use ‘Stokes-shift-engineered’ CdSe/CdS quantum dots with giant shells (giant quantum dots) to realize luminescent solar concentrators without reabsorption losses for device dimensions up to tens of centimetres. Monte-Carlo simulations show a 100-fold increase in efficiency using giant quantum dots compared with core-only nanocrystals. We demonstrate the feasibility of this approach by using high-optical-quality quantum dot–polymethylmethacrylate nanocomposites fabricated using a modified industrial method that preserves the light-emitting properties of giant quantum dots upon incorporation into the polymer. Study of these luminescent solar concentrators yields optical efficiencies >10% and an effective concentration factor of 4.4. These results demonstrate the significant promise of Stokes-shift-engineered quantum dots for large-area luminescent solar concentrators. Meinardi, F. et al . Nature Photonics 2014, 8, 392.

Resume : The photocurrent of oxide semiconductor thin film transistors (TFTs) can be observed when the device is exposed to a ultra-violet light, because oxide semiconductor is a wide band gap semiconducting material. Therefore, we decorated cadmium selenide (CdSe) quantum-dots (QDs) on the surface of oxide semiconductor to increase the photocurrent for low-energy light, i.e., visible light. A thin film of oxide semiconductor was deposited on the SiO2/Si substrate by a radio frequency sputter system. Also, we prepared CdSe QDs with sizes of ~6.3 nm, which can absorb red visible light. QDs were spin-coated onto the oxide semiconductor film, and post-annealing was done to provide cross-linking between QDs. The prepared devices showed a 231 % increase in photocurrent when exposed to 650 nm light due to the QDs on the surface of oxide semiconductor. Measurements to construct an energy level diagram were made using ultraviolet photoelectron spectroscopy to determine the origin of the photocurrent, and we found that the small band gap of CdSe QDs enables the increase in photocurrent in the oxide semiconductor TFTs. The device characteristics and origin of the improvement will be presented in detail. This result is relevant for developing highly transparent photosensors based on oxide semiconductors and QDs.

Resume : In this paper, 1D single-crystalline MnO2 nanowires have been synthesized using facile hydrothermal growth method using KMnO4 and Na2S2O8 as starting reaction reagents. The morphology, phase structure and composition of the as-prepared nanomaterial were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) with selected area electron diffraction (SAED) and energy dispersive x-ray spectroscopy (EDX). FESEM and TEM analysis shows that the as-prepared MnO2 nanowires have diameters of 25-35 nm. The structural features of as-synthesized MnO2 nanowires are studied to analyse the near-neighbour environment of oxygen coordination around manganese cations using Raman scattering (RS) spectroscopy. Photoluminescence Spectrophotometer was employed to study the optical properties of the synthesized material. The Photoluminescence (PL) spectra of the as-prepared MnO2 nanowires at room temperature exhibits prominent emission bands located in green-violet spectral region.

Resume : Recently, AlGaInP-based light emitting diodes (LEDs) have experienced an impressive evolution in both device performance and market volume. However, development of new applications is required in order to realize their full potential in areas such as use as a light source for auto focusing in digital cameras, special illumination for particular functions in agriculture, and in full color displays. To enlarge their utility in these applications, it is necessary to fabricate and understand a new structure capable of emitting longer wavelengths of around 700 nm. In particular, AlGaInP heterostructure LEDs are lattice-matched with respect to the GaAs substrate, which limits the emitting spectrum to around 650 nm at the longer peak wavelength side. To fabricate an LED structure capable of emitting at a 700 nm peak wavelength, the composition (x) of GaxIn1-xP material in the active layer requires a compressive strain of larger than 1 %. This large lattice mismatch, however, causes significant problems in terms of both growth and device properties due to the formation of defects. To overcome these problems, it is necessary to relieve the well strain via the formation of islands, referred to as a Stranski-Krastanow (S-K) growth mode, in order to prevent the generation of dislocations [1,2]. However, in AlGaInP-based LEDs emitting at a 700 nm peak wavelength, the effects of well strain on the epitaxial growth and the realization of device performance has yet to be extensively studied. In this study, we investigate the behaviors of morphological and optical characteristics on the composition of Ga0.33In0.67P material and demonstrate the performance of a device emitting at around 700 nm using quantum dot (QD)-based LEDs.

Resume : The effect of a uniform electric field across multibarrier systems (GaAs/AlxGa1-xAs) is exhaustively explored by a computational model using exact Airy function formalism and the transfer-matrix technique. In the case of biased DFHBSL structure a strong reduction in transmission properties was observed and the width of the miniband structure linearly decreases with the increase of the applied bias. This is due to the confinement of the states in the miniband structure, which becomes increasingly important (Wannier-Stark effect).

Resume : The growth, morphology, composition and structure of PbSe nanostructures grown on the atomically smooth, nanoporous and oxidized van der Waals (VDW) (0001) surfaces of layered crystals GaSe were studied by atomic force microscopy, X-ray diffractometry, photoelectron spectroscopy and Raman spectroscopy. Semiconductor heterostructures were grown by the hot-wall technique in vacuum. Nanoporous GaSe substrates were fabricated by thermal annealing in H2 atmosphere. The irradiation of GaSe(0001) surface by UV radiation was used to fabricate thin Ga2O3 layers (thickness <2 nm). It was found that PbSe shows a tendency to form clusters with a square or rectangular symmetry onto the clean low energy (0001) GaSe surface and (001)-oriented growth of PbSe films takes place on this surface. Using this growth technique it is possible to grow PbSe nanostructures with different morphologies: continuous epitaxial layers with the thickness <10 nm on the uncontaminated p-GaSe(0001) surfaces, homogeneous arrays of quantum dots with a high lateral density (>10^11 cm^-2) on the oxidized VDW surfaces, faceted square pillar-like nanostructures with a low lateral density (10^8 cm^-2) on the nanoporous GaSe substrates. The flexoelectric effect in the composite nanostructures consisting of layered nanoporous GaSe matrix (with a low dielectric permittivity) and PbSe nanopillars (with a large dielectric permittivity) formed in nanoscale pyramidal cavities on the VDW surfaces of GaSe layers was observed.

Resume : Multi-Gate Junctionless MOSFETs are promising devices to overcome the undesired short channel effects for low cost nanoelectronic applications. However, the use of uniformly doped channel, source and drain regions presents the well-known problem of the high series resistance associated to the extensions, which degrades the electrical performance of the device. Therefore, in order to obtain a global view of Double-Gate Junctionless (DGJ) MOSFET performance under critical conditions, new designs and models of nanoscale DGJ MOSFET including analog performance are important for the comprehension of the fundamentals of such device characteristics. Based on numerical investigation of a nanoscale DGJ MOSFET, in the present paper a numerical investigation for I-V and small signal characteristics by including the highly doped extension regions is presented. The proposed approach, which is a technologically feasible technique by introducing only one ion implantation step, provides a good solution to improve the drain current and small signal parameters at high gate and drain voltages for analog applications. In this context, I-V and analog characteristics of the proposed design are investigated by 2-D numerical modeling and compared with conventional DGJ MOSFET characteristics.

Resume : We employed Au@SiO2/QDs system to study localized surface plasmon-controlled Förster resonance energy transfer of QDs on single plasmonic metal NPs. The donor and acceptor QDs were attached to SiO2 NPs or Au@SiO2 core-shell NPs indiscriminately and randomly. When attached to SiO2 NPs, the PL intensity of 575QDs was enhanced due to intra-ensemble energy transfer. As to the case of 550QDs the lifetime increased as well，but the PL intensity reduced. When both of the QDs attach to SiO2 QDs, we observed a 2.30-fold emission enhancement of the acceptor and 0.91-fold of the donor. The PL lifetime of 550QDs decreased and that of 575QDs increased compared to the ones attached to SiO2 NPs individually, which is an indication of typical FRET. When Au NPs was introduced into the system by replacing SiO2 NP with Au@SiO2 core-shell structure, the PL of the donor 550QDs was thoroughly suppressed. On the other hand the PL intensity was enhanced when 550QDs was alone absorbed on Au@SiO2. This drastic reduction is a convincing evidence of localized surface plasmon-controlled Förster resonance energy transfer which was further confirmed by the PL decay curve displaying a reduction of donor decay time. The system could be optimized by changing the thickness of spacer layer, the size or shape of metal NPs, the feature of QDs absorbed and so on to pursue stronger PL intensity and FRET rate or efficiency.

Resume : We have successfully fabricated metal-oxide-nanoparticles-decorated ZnO-branched nanostructures, by means of growing ZnO branched nanostructures on the as-synthesized SnO2 nanowires. Subsequently, outer metal layers were sputtered on the branched nanowires. ZnO is a wide bandgap (3.37 eV) semiconductor, being one of the most widely studied materials, resulting from its excellent optoelectronic and piezoelectric properties. It is well known that ZnO as a variety of forms, including nanowires, nanobelts, nanoribbons, and nanotubes, have their potential use in gas sensing. We have selected the SnO2 nanowires as a stem/materials, because the SnO2 nanowires can be simply fabricated at a low-enough cost. The annealing induced the oxidation of metal phase. The surface roughness of the core-shell nanowires resulted from the generation of nanoparticles. XRD, SAED, and lattice-resolve TEM images coincidentally reveal that the nanoparticles are comprised of metal oxide particles. It is noteworthy that the sensor performances were significantly enhanced by employing the particle-decorated branched nanostructures. The enhancement of the gas response is attributed not only to efficient agglomeration of metal oxide nanoparticles, but also to exposure of the ZnO branch surface, with the spillover effect being increased. We expect that both chemical and electronic sensitization effects will play a role in particle-induced functionalization for sensor enhancement. The present study on three-composite-nanostructures will be a significant contribution to both the academic field and industrial applications, which will be useful in exploring new areas of multiple-component nanosystems.

Resume : In this paper, we exploit the nonlinear property of SiC multilayer devices under UV irradiation to design an optical processor for error detection and correction, that enable reliable delivery of spectral data of four-wave mixing over unreliable communication channels. The SiC optical processor for error detection and correction is realized by using a SiC pin/pin photodetector with UV biased optical gating elements. The operational principle of is discussed. Simulation results confirming the described method are presented and compared with experimental results. The relationship between the optical inputs and the corresponding digital output levels is established. Data shows that the optical bias act as selector that pick one or more states by splitting portions of the input multi optical signals across the front and back photodiodes. Boolean operations such as exclusive OR and three bit addition are demonstrated optically with a combination of such switching devices, showing that when one or all of the inputs are present the output will be amplified, the system will behave as an XOR gate representing the SUM. When two or three inputs are on, the system acts as AND gate indicating the present of the CARRY bit. The design of an optical full-adder is presented. Additional parity logic operations are performed and checked for errors together. As an example we describe an all optical processor for error detection and correction and then, provide an experimental demonstration of this fault tolerant reversible system. An intuitive representation with a 4 bit original string color message and the transmitted 7 bit string, the parity matrix and the encoding and decoding processes are presented.

Resume : We have studied the evolution of strain relaxation in (0001) InGaN thin interlayers layers grown on MOVPE (0001) GaN/Al2O3 templates by plasma-assisted molecular beam epitaxy (PAMBE) under a low temperature regime (at 515oC). High resolution transmission electron microscopy (HRTEM) was employed for this purpose. HRTEM strain measurements were extracted by the geometrical phase technique. We have observed that above the critical layer thickness, strain relaxation took place through gradual defect introduction along the thickness of the layers, i.e. misfit dislocations and basal stacking faults. Residual elastic strain was significantly reduced in layers thicker than 10 nm. The indium incorporation efficiency was strain-dependent in elastically strained layers, and saturated at InN molar fraction below 40%. Upon further reduction of the growth temperature to 475oC, elastic strain accommodation took place through the formation interfacial undulations, leading to InGaN troughs with higher indium content than in the case of flat layers. This concept was utilized for InGaN/GaN multi-quantum well growth with improved carrier confinement, as attested by photoluminescence spectroscopy. Acknowledgement: Research co-financed by the European Union (European Social Fund – ESF) and Greek national funds - Research Funding Program: THALES, project NitPhoto.

Resume : Electronic, optical properties and spontaneous polarization of cubic perovskite BaHfO3 have been investigated using the Full Potential Linear Augmented Plane Wave (FP-LAPW) method as implemented in the WIEN2K code, in connection with the Generalized Gradient Approximation (GGA) and the new potential approximation known as the Tran–Blaha modified Becke–Johnson exchange potential approximation (TB-mBJ). The calculation of band structure and density of states shows that BaHfO3 is direct gap by GGA and indirect gap by TB-mBJ approximation. The obtained gap values with the TB-mBJ (5.60 eV) is close to the experimental one (6.01 eV), however the GGA under estimate the band gap about 3.9 eV. The absorption coefficient () and the complex dielectric function () are also investigated. Furthermore, it is found that the spontaneous polarization for perovskite BaHfO3 is 0.40 C/m2, this value is acceptable compared to other perovskite.

Resume : The energy relaxation of hot carriers in semiconductors via electron-phonon interaction has been investigated extensively, both experimentally and theoretically in bulk and two-dimensional 2D structures. The determination of the temperature of electrons, under electric-field heating conditions in the steady state, provides useful information about the electron-phonon interactions involved in the energy relaxation process. Molecular Beam Epitaxy (MBE) growth InxGa1-xN/GaN samples with indium fractions ranging between x=0.44 and 1.00 have been studied by the pulsed current voltage (I-V) measurements at 1.7 K. The drift velocity, electron mobility and electric-field dependent power loss per electron were determined from the anal-ysis of the data. The drift velocity increased linearly and electron mobility was also remained constant with increasing electric field. The power balance equations were used to obtain power loss per electron as a function of applied electric-field. The results showed that the drift velocity, electron mobility and power loss per electron increased in the range x=0.44-0.66, and then slowly decreased in the range x=0.66-1.00.

Resume : Silicon nanoclusters (Si-ncs) and Rare Earth (RE) embedded in dielectric matrix (SiO2) have recently attracted much interest as potential candidate for many applications in photonics. Light emission properties exhibited by Si-ncs and RE are directly linked to the distribution of the dopants in the host materials and clustering characteristics (size, distribution, composition, interface nature with surrounding matrix…). Therefore, an accurate control of these parameters is essential in order to improve these systems. Conventional spectroscopy and microscopy techniques often encounter some limits to give quantitative information about Si-ncs and RE. Atom probe tomography (APT) provides a 3D chemical mapping of the analysed material at atomic scale (crystalline or not). Since the implementation of laser to APT, it becomes possible to analyse semi-conductors and oxides. In this work, RE-doped silica containing Si-ncs have been fabricated and characterized by APT. The microstructure such as (phase compositions, clustering and precipitation kinetics) has been investigated. Accessing this level of accuracy will help to understand and control the Si-ncs and RE characteristics which govern the properties of these materials.

Resume : It’s well known that photoluminescence spectra obtained by UV pulsed laser pumping reveals random lasing indicative of self-forming laser cavities [1-3]. We already reported [4] random lasing at 400nm in ZnO thin films with thickness of around 100 nm, whereas most of reported works by other groups have been obtained on rather thick films. In our works, ZnO thin films were grown on c-sapphire substrates using pulsed laser deposition (PLD). Subwavelength propagation has to take place in the dielectric slab for this closed loop cavities can exist. We propose here an experimental study of waveguiding cavities in an asymmetric slab configuration (air/ZnO/sapphire) versus film thickness ranging from 20 to several hundred nanometers. Calculation of the number of guided TE/TM modes, the propagation conditions, and the cut-off wavelengths are also linked to the experimental data. [1] P.H. Dupont, C. Couteau, D.J. Rogers, F. Hosseini Teherani, G. Lerondel, Appl. Phys. Lett. 97 (2010) 261109 [2] H. Cao, Y.G. Zhao, S.T. Ho, E.W. Seelig, Q.H. Wang, R.P.H. Chang, Phys. Rev. Lett. 82 (1999) 2278-2281 [3] Y. Tian, X. ma, L. Xiang, M.V. Ryzhkov, A.A. Borodkin, S.I. Rumyantsev, D. Yang, Optics Comm. 285 (2012) 5323-5326 [4] C. Cachoncinlle , C. Hebert, J. Perrière, M. Nistor, A. Petit, E. Millon. In press: Appl. Surf. Sci. doi:10.1016/j.apsusc.2014.09.186

Resume : Semiconductor nanowires are of high interest due to the possibility to tune their electrical and optical properties for applications such as in solar cells, sensors, and nanoelectronics. For such devices, the control of doping is a relevant key issue. In this work, we focus on the effect of Si doping in GaAs NWs grown on GaAs (111)B by molecular beam epitaxy, using Au nanoparticles as a catalyst. Different Si doping levels (1x1016-5x1018 cm-3) are investigated by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction and photoluminescence (PL). The NWs grown in high density groups of bunches are mostly with vertical orientation, as shown by SEM and cross-section SEM. The NWs have typical diameters of 100-200 nm and lengths of a few tens of μm. TEM characterization of the investigated NWs reveal large wurtzite structure segments interlaced with thin small zinc-blende segments. PL measurements at low temperature exhibit sharp lines in the energy range 1.41-1.48 eV, for the sample with the lowest Si doping level. However, as the Si doping level increases, the PL intensity increases and is accompanied by a clear broadening of the observed lines. In addition, the temperature at which the complete thermal quenching occurs also increases. The influence of excitation power and temperature is discussed in order to evaluate the nature of the radiative and non-radiative de-excitation channels.

Resume : InGaN-based nanocolumns (NCs) have the dislocation-free nature as well as the effective strain relaxation due to the free-edge effect. We have developed the fabrication of uniform regularly-arrayed InGaN-based NCs by the selective area growth using Ti-mask [1]. In this study, the detailed emission mechanisms in the regularly-arrayed InGaN-based NCs were assessed by the temperature dependence of PL and TRPL measurements. 3-period InGaN/GaN quantum wells on GaN NCs were fabricated. The period and diameter of NCs were 240 and 300 nm, respectively. PL spectra with double peaks were observed at any temperatures, but a high energy peak drastically decreased with increasing temperature up to 300 K. The double-peak emission was evinced to be originated from in-plane potential distribution by CL mapping [2]. To investigate these results in detail, temperature dependence of TRPL was evaluated. The PL lifetime at the high energy side of double-peak emission dramatically decreased with increasing temperature, because of increased surface recombination and carrier drift-diffusion to lower energy side. At the same time, dependence of PL lifetime at the low energy side on temperature was smaller, because of quantum dot-like structure of active layer [3]. The quantitative analyses of lifetimes will also be presented. [1] H. Sekiguchi et al., Appl. Phys. Express. 1, 124002 (2008). [2] T. Oto et al., ISSLED 2014, Mo-O42 (2014). [3] H. Sekiguchi et al., Phys. Stat. Soli. C 7, 2374 (2010).

Resume : Photon absorption by a semiconductor with an energy exceeding the band gap results in the formation of hot electron-hole pairs that quickly dissipate their excess free energy, resulting in quasi-thermalized conduction band electrons and valence band holes. Using colloidal quantum dots (QDs), it is possible to harvest this excess energy either by hot carrier transfer or the generation of multiple excitons (MEG), processes involving truly hot carriers with energies far above the band edge states. Consequently, the relevant rates are those of hot electron-hole pairs with energies far above the band edge transition. We analyzed the carrier cooling in PbS(e) QDs after photo-excitation with high energy photons where, by means of white light pump-probe spectroscopy, transitions throughout the entire Brillouin zone are probed. PbS(e) has a number of critical points at high energy, e.g. along the Σ- directions. We observe a transient accumulation of charge carriers at the quantized states around Σ and link this 'cooling bottleneck' via tight binding calculations to the level structure around Σ: a specific set of optical phonons is required to scatter the hot carriers away from Σ to the band edge. We show that cooling via Σ is the dominant pathway for hot carrier relaxation and extract cooling rates. The impact of our findings on understanding the high efficiency of MEG in PbS(e) QDs is discussed and implications for both theoretical and experimental work on MEG are discussed.

Resume : Nitrides have a wide range of applications in energy generation, transport and storage due to their fascinating and tunable optical properties. Whereas bulk nitride systems have been studied extensively both experimentally and theoretically, little attention has been given to their nanostructural form. M3N4 nanosystems are difficult to prepare and therefore their structure and properties are not well known, but are of high interest. To address this problem we perform a search on the DFT energy landscape of Si3N4 using the structure prediction capability of the Knowledge-Led Master Code (KLMC) program suite, with the best structures data-mined and refined for nitrides of C, Ge, Sn, Pb, Ti, Zr, Hf and Ce. The DMol3 program has been used as the DFT driver code for this work as it provides sufficient chemical accuracy at an acceptable computational cost. DMol3 uses numeric atom-centred basis sets, thus it is ideal for nanostructures. We will report compound stabilities along with their optical response, which is aimed at finding suitable candidates for new optoelectronic, solar-cell and photovoltaic applications.

Resume : In this work, we report a comparative study of both InxGa1-xN single layer (SL) and InxGa1-xN/GaN multiple quantum well (MQW) PIN photodiodes (PDs) with an indium content varying from 10% to 30%. Both structures are grown on (0001) sapphire substrates using metal organic chemical vapor deposition (MOCVD). The microstructural investigation has been achieved by means of X-Ray Diffraction (XRD), Atom Force Microscopy (AFM) and Scanning Transmission Electron Microscopy (STEM). We focus here on the design and the fabrication of photodiodes operating in visible wavelength range. Different configurations for photodiodes have been fabricated (size ranging from 5 to 200?m) and the technological processes are optimized including dry etching processes for patterning GaN/InGaN layers and p and n type contact optimization. Photodiodes fabricated on both structures have been electrically characterized. This comparison demonstrates the benefit of high-efficiency of InGaN/GaN heterostructure by decreasing the defect density and improving the quality of materials. A multiple quantum well structure offers the prospect to increase the indium composition and the intrinsic active region.

Resume : Owing to silicon’s large wafer size, low cost, and good thermal and electrical conductivities, it would be very interesting to use silicon substrates for the realization of InGaN-based devices. However, due to the large lattice mismatch between Si substrates and InGaN, the growth of InGaN-based devices has major issues. A possible solution to overcome this issue and prevent strain-related degradation of InGaN material is nano selective area growth (NSAG), which exploits 3D relaxation effects to release strain without creation of dislocations and reduce the piezoelectric effect thanks to the growth along nonpolar or semipolar directions. In this paper we study the NSAG of thick In-rich InGaN nanorods on thin AlN-buffered Si(111) templates. SEM analysis and cathodoluminescence measurements at LT (77k) in spot mode were done. A perfectly selective InGaN nanorods with excellent hexagonal shapes and smooth semipolar facets, and an increase of nanorods sizes according to the masks width were observed. An emission peak centered at =560nm with two components was observed in differents masks with a redshift when we increase the mask width. These two components can be attributed to growth along c-axis and r-plane facets or two different r-plane facets. The nanorod size enhancement and the variation of the indium content in InGaN with mask width are interesting to optimize the design of the mask for next generation devices. Further structural characterization will be presented.

Resume : Three-scale model for the calculation of 2DEG mobility in AlGaN/GaN heterostructures developed by our group earlier was used for the investigation of the dependence of 2DEG concentration and mobility on the Al concentration in AlGaN layer. The model allows to calculate following 2DEG properties: energy levels, corresponding wavefunctions, potential energy distribution, charge carriers concentration distribution over the heterostructure. It is also possible to calculate electron mobility in 2DEG taking into account various scattering mechanisms. In the framework of this model the values od 2DEG concentration and mobility were calculated for various Al concentrations x in AlxGa1-xN barrier (0,25≤x≤1). It is also taken into consideration that maximum barrier width should decrease with increasing Al concentration in order to allow pseudomorphic growth of barrier layer.

Resume : In this work we report the effect of aluminum doping on the structural and optical properties of ZnS thin films. Our samples have been synthesized by chemical bath deposition (CBD) from aqueous solutions containing ZnSO4 and SC(NH2)2at 80°C for 90min. X-Ray diffraction analysis revealed that the films were monocrystalline and showed (111) preferred orientation for all the doping concentration. Doping ZnS thin films shows significant changes in the transmittance characteristics in the visible range.The refractive index dispersion and extinction coefficient are adequately described by the Wemple-Di Domenico model. The value of oscillator energy E0, dispersion energy Ed, the high-frequency dielectric constant ε∞ and the ratio of the carrier concentration to the effective mass (N/m*)were estimated according to the models of Wemple-Di Domenico and Spitzer Fan. Photoluminescence behaviour of Al-doped ZnS thin filmswas also studied.

Resume : We have studied structural, electrical and optical properties of epitaxial GaN thin films annealed at different temperatures (600C-850C). The GaN layers were grown on c-sapphire substrates using low temperature GaN intermediate layer by plasma assisted molecular beam epitaxy (PA-MBE). GaN layers were investigated by high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), Hall and photoluminescence (PL) measurements. It has been found that there is a significant improvement in the quality of the GaN films annealed at 725C with regard to the electron mobility, sheet carrier density and full width at half maximum of GaN (0002)  scan. Dislocation density obtained from the FWHM of GaN (0002) - scan and etch pitch density estimated from AFM image are 1.4109cm-2 and 0.4 108 cm-2 respectively. PL measurement shows reduction of FWHM (~30 meV) for the layer annealed at 725C. Further, SEM images show that annealing improves the surface roughness as compare to the as grown GaN thin film. The best root mean square (RMS) roughness of the sample annealed at 725C, obtained from AFM image was estimated to be 1.14 nm. Similar studies on AlGaN/GaN heterostructure are under progress and will be reported later.

Resume : Group III-nitride semiconductors are attractive materials for optoelectonic and power electronic devices because of their wide band-gap and good thermal-mechanical stabilities. In particular, GaN has been considered as the most important semiconductor next to silicon because of its applications in solid-state lighting (SSL). One of current technical issues on SSL is the manufacturing cost. An LED is responsible for 30-40% of a SSL bulb which the price is more than 5 times higher than that of conventional light bulb. In order to reduce the manufacturing cost of GaN-based light-emitting diodes (LEDs), GaN-on-Si approach has collected more and more industry`s attention because of its cost-effectiveness and scalability. In spite of these excellent advantages, silicon has not been widely used as a substrate material for GaN growth owing to several structural and mechanical problems. The large lattice mismatch (~17%) between GaN and silicon leads to a relatively high dislocation density for the GaN-on-Si epiwafer, limiting the performance of LED structures. The large mismatch between GaN and Si in thermal expansion coefficient (~56%) induces a large tensile stress during the cooling process, which often results in cracked layers that are harmful for device applications. As the substrate size is scaled up, the requirement of wafer flatness becomes stricter due to the limited tolerance in wafer bow for processing a full wafer. For example, even with same curvature, the bow of 8-inch wafer is 16 times as much as that of 2-inch wafer. In addition, optical loss of the absorbing Si substrate is another barrier for realizing high brightness LEDs compared with those on sapphire or SiC substrates. Si is typically opaque, decreasing the LED efficiency due to the absorption of downward light by the substrate. In this study, a crak-free, uniform InGaN/GaN LED structure with strain-engineered buffer layer was grown on an 8-inch diameter Si(111) substrate. The full width at half maximum (FWHM) of (002) and (102) ω-scan is 280 and 420 arcsec, respectively. For LED on 8-inch Si, multiple quantum well (MQW) photoluminescence (PL) wavelength uniformity of 0.55% (2.4 nm) has been achieved by using proper curvature engineered wafer carrier. We demonstrated high brightness 1 x 1 mm2 LED devies utilizing vertical chip process then evaluated their device properties. The electo-optical characteristics of the fabricated vertical LED (VLED) shows around 1 W light output power at 1 A injection current with operating voltage of 4.0 V.

Resume : we investigate the structural, electronic and magnetic properties of the diluted magnetic semiconductors (DMSs) CdCoTe and CdMnTe in the zinc blende phase with 12.5 % of Co and Mn. The calculations are performed by the recent ab initio full potential augmented plane waves (FP_L/APW) method within the spin polarized density functional theory (DFT) and the generalized gradient approximation GGA. Structural properties are determined from the total energy calculations and we found that these compounds are stable in the ferromagnetic phase. We discuss the electronic structures, total and partial densities of states and total magnetic moments. The calculated densities of states presented in this study identify the half-metallic of CdCoTe and CdMnTe.

Resume : Development of highly integrated optoelectronic devices requires the use of optical components, whose size is comparable or less than the wavelength of light. In case of optical excitation of defects, implanted ions and clusters in thin-film structures one should take into account the effects of excitation light interference, which can affect the functional parameters. Previously it was shown that the effect of interference on the photoluminescence spectra, and can be considered and mostly neutralized. Similar effects in the spectral dependences of low-energy optically stimulated electron emission (OSEE) can significantly affect the physical interpretation of the results. The primary aim of this work is the development of appropriate techniques to compensate interference fringes the experimental results. The second task is to study the point defects formed in silica matrix by ion implantation. As a model system the 854 nm SiO2 film, grown on a Si substrate and implanted with Li, Na and K ions, was chosen. Such objects may serve as quasi-2D counterparts for alkali silicate glasses, which are conventionally used in laser and waveguide devices, integrated and power optics, microelectronics and other fields. Numerical analysis of the experimental OSEE curves according to original technique allowed eliminating interference effects and recovering initial spectra shape. Valuable energy structure parameters were determined for emission-active defect centers.

Resume : Enhancing electroluminescence (EL) efficiency might be a never ending pursuit from displays to lightings where OLEDs are being used as sources. For past years, phosphorescent OLEDs (PhOLEDs) based on heavy metal complexes have been considered as an only solution to realize high efficiencies by harvesting both singlet and triplet excitons as light. However, recent progress utilizing delayed fluorescence challenges the conventional idea of achieving high EL efficiencies and further has even proved its competency by achieving 100% internal quantum efficiency (IQE). There are two different mechanisms reported that drive delayed fluorescence which are triplet-triplet annihilation (TTA) and thermally activated delayed fluorescence (TADF). Fluorescent material showing TADF phenomenon enables additional harvest of triplet excitons as well as singlet excitons via reverse intersystem crossing (RISC) from triplet (T1) to singlet (S1) state due to thermal activation and small energy gap between the two excited states. Replacing phosphorescent material to fluorescent material will eventually lower cost, clear stability issues and help manufacture environment friendly products. Therefore, there has been tremendous need to synthesize fluorescent dye showing TADF phenomenon. Especially, among the three primary colors, blue dye has been considered as the most crucial due to its importance role on creating white light with purity and longer lifetime than phosphorescent material, however still technically challanging to synthesize. Therefore, commercially, blue fluorescent materials are being used for OLEDs in spite of their deficient IQE of ~25%. In order to enhance EL efficiency, TADF phenomenon must be involved in to take advantage of triplet manifold where excitons are rich. As significant as it is, numbers of studies about blue TADF materials and their devices have been demonstrated previously including the highest reported EQE of 19.5%. In this article, a new blue fluorescent dye showing strong TADF phemenon through RISC is introduced. By using mixed co-host system, a device structure with solid charge balance has been developed leading to high EL efficiency. The transient data is a clue to such high EL efficiency is an actual result from TADF phenomenon. In addition, the orientation of the transition dipole moment of the fluorescent dye was further discussed.

Resume : In this study, a mixture of commercially available green emitting polymers; PFO known as F8: Poly(9,9-di-n-octylfluorenyl-2,7-diyl) and F8BT known as PFBT: (Poly(9,9-dioctylfluorene-alt-benzothiadiazole) have been used for the fabrication of OLED devices on PET/ITO flexible substrates. The devices were prepared by a gravure printing process supported on a lab scale proofer. Initially, the PET/ITO substrates were patterned using a strong acidic solution. In a next step, the ITO remaining areas were cleaned sequentially with distilled water (d-H2O), isopropanol (IPA), acetone, ethanol and d-H2O, respectively. Then, after a short time of oxygen plasma activation treatment, a PEDOT:PSS solution was printed by gravure. Afterwards, the active emissive layer (EML) of F8:F8BT (F8:F8BT 5/95 wt.%) in o-xylene was printed using again gravure. Different grades of PEDOT:PSS have been used and printed with different speeds to check their effect on the device operation and performance. In addition, the active layer has been printed also by different speeds to obtain the optimum thickness for the device performance. The F8:F8BT OLED devices with an active area of approximately 1 mm2 have shown excellent green emission characteristics at relatively low operational voltages (Voc=4). Moreover, the devices exhibited relatively good stability when kept under inert atmosphere, and our first trials of encapsulating the flexible deices have shown quite promising results.

Resume : High resolution dynamic tactile/pressure display is indispensable to the comprehensive perception of force/mechanical stimulations such as smart skin, biomechanical imaging/analysis, or personalized signatures. Here, we present a new type of dynamic pressure sensor array based on pressure/strain tuned photoluminescence imaging without the need of electricity. Each sensor is a nanopillar that consists of InGaN/GaN multiple quantum wells. Its photoluminescence intensity can be modulated dramatically and linearly by small strain(0~0.15%). The sensor array has a high pixel density of 6350 dpi and exceptional small standard deviation of photoluminescence. High quality tactile/pressure sensing distribution can be real-time recorded by massively parallel photoluminescence imaging without any crosstalk. The sensor array can be inexpensively fabricated over large areas by semiconductor product lines. The proposed dynamic all-optical pressure imaging with excellent resolution, high sensitivity, good uniformity and ultrafast response time offers a new way for smart sensing, micro/nano-opto-electro-mechanical systems.

Resume : Recently, quantum-dot light emitting diodes (QLEDs) are considered as a next-generation display due to the superior luminescence properties, photo-stability and its solution processes. Therefore, various approaches have been proposed to enhance the performance of QLEDs. In this presentation, we adopted plasmonic nanostructure at the interface of the device to enhance the electroluminescent properties. Plasmon-enhanced QLEDs were fabricated by inserting gold (Au) nanorods at the interface of the QLEDs. The length of the nanorods was 60 nm, which corresponds to the plasmonic absorption of wavelengths in the range of 630 to 670 nm. CdSe/ZnS quantum-dots (QDs) were used as emission layers with additional hole injection, transport, and electron transport layers. The maximum emission was observed at 630 nm, which is in the range of the plasmon resonance of Au nanorods. The QLEDs with Au nanorods showed enhanced electroluminescent properties compared to the devices without the plasmonic nano-structure. A 172% increase in electroluminescent intensity was observed due to the plasmon coupling effect. The device characteristics and origin of the improvement will be presented in detail. The results demonstrate a promising method to develop high-performance QLEDs.