Advances in silicon-nanoelectronics, -nanostructures and high-efficiency Si-photovoltaics

Resume : The advent of two-dimensional (2D) materials inspired new paradigms in the research and technology directions. The recent discoveries of silicene, germanene, and stanen [1] brings to the forefront thet class of two-dimensional (2D) Xenes with X currently including B, Ga, P, Sb, Bi, Se, and Te. Like graphene, these materials are arranged in a honeycomb lattice, but they are epitaxially grown with varying degrees of buckling.[2] I will present a general overview of the methods to synthesize Xenes by taking silicene as flagship material with focus on the application viability. I will classify two main device integration paths for the case of silicene, delamination from a cleavable substrate and substrate engineering. Within the former option is the fabrication of the silicene field effect transistor (both for the single-layer and for the multilayer regime) with a first-time proven graphene-like ambipolar transport character.[3] As a case in point for the latter option I will discuss the growth of 2D silicon on sapphire where by mean of THz optical absorption spectroscopy a Dirac-like electrodynamics was observed thus paving the way to unprecedented routes for silicene exploitation in photonics.[5] The extension to other Xenes will be also considered in this framework. [1] A. Molle, et al., Nature Mater.16, 163 (2017). [2] A. Molle, et al, Chem. Soc. Rev. 47, 7370 (2018). [3] L. Tao, et al., Nature Nanotech.10, 227 (2015). [4] C. Grazianetti, et al, Nano Lett. 18, 7124 (2018).

Resume : The integration of highly doped Ge on Si with a controlled amount of tensile strain is crucial for several applications in advanced nanoelectronic and photonic devices. However, obtaining n-type doping above 5x1019cm-3 and in-plane biaxial tensile strain above +0.2-0.25 % with conventional growth and annealing methods is highly challenging. Here we report on the combination of in‑situ doping of Ge-on-Si epilayers and pulsed laser melting (PLM) to improve the activation of phosphorous in germanium and increase the tensile strain. Secondary ion mass spectrometry measurements indicate that the box-like profile of as-deposited epilayers is preserved during PLM with minimal P out-diffusion. An activated n-doping concentration above 1e20 cm^-3 over 2-300 nm thick layers has been achieved, as measured by infrared reflectivity. High Resolution X-Ray Diffraction and Raman measurements show that, after PLM, the in-plane residual thermal strain reaches +0.6%. Concurrently, planar defects form as observed by Transmission Electron Microscopy. Such structural modifications are promoted by the extremely high thermal gradients achieved by PLM, as indicated by heat flow simulations. Thanks to the heavy doping and to the extremely high tensile strain, a significant Photoluminescence intensity increase is observed after PLM, with a clear evidence of the Fermi level being above the conduction band Gamma valley minimum.

Resume : Germanium is currently being incorporated into silicon-based systems as a channel material owing to its high electron and hole mobilities. Devices fabricated using high-mobility semiconductor materials such as germanium may improve device speed, lower power consumption and improve the overall performance of the device. An issue associated with germanium is the instability of its native oxide, GeO2. Unlike silicon’s native oxide, it is water soluble and the interface between the germanium and its native oxide is characterized by defects which result in charge trapping and thus poor overall device performance. To mitigate this issue, the native oxide must be replaced with a different, more reliable passivating material. In this study, self-assembled monolayers (SAMs) consisting of aliphatic thiols are investigated as a potential passivating layer for germanium so that it can be integrated into silicon-based systems. A novel approach for passivating germanium is explored whereby vapour-phase thiol-chemistry is employed rather than the rudimentary wet chemistry. A novel way to passivate germanium and prevent degradation by the ambient is outlined in an effort to bolster germanium’s position as an attractive channel material in silicon-based systems.

Resume : Defining the optoelectronic features of semiconductors quantum dots (QDs) by introducing a few impurity atoms is a novel way to tailor new functionalities towards the optoelectronic applications. In contrast to the great advancements in compound semiconductors, impurity-doping in Si QDs is still at the fundamental level despite its importance in optoelectronics and biophotonics applications. In this work, I present a comprehensive study on colloidal Si QDs codoped with boron (B) and phosphorus (P).[1,2] Codoped Si QDs are dispersible in polar solvents without organic ligands. We first show the detailed structural characterization including atom probe tomography analyses. Optical characterizations demonstrate that the codopants also modify the electronic structures of Si QDs and introduce donor and acceptor levels in the band gap of Si QDs, enabling the optical transition with the energy below bulk Si bandgap (~1.1 eV). The emission energy is tunable by size in the range of 0.9-1.8 eV[1] which is optimal for carrier multiplication-facilitated solar power conversion and bio-imaging applications. In addition to the properties above, we also present the development of size-purified doped QDs.[3] In the size-purified and selected QDs, we determine the degree of doping-induced shrinkage of the optical band gap over a wide size range. From the comparison of the experimental data with recent results on single QD analyses including scanning tunneling spectroscopy,[4] we discuss the size dependence of donor-acceptor (D-A) states in Si QDs. We also present the number of D-A pairs in a QD in a wide size range by the comparison with theoretical calculation. Finally, we will demonstrate the potential applications of codoped Si QDs in photonics and biomedical fields. [1] Hori, Kano, Sugimoto, Fujii, Nano Letters, 16, 2615 (2016) [2] Fujii, Sugimoto, Kano, Chem. Comm. 54, 4375 (2018). [3] Sugimoto, et al., Nano Lett., 18, 7282 (2018) [4] Ashkenazy, et al., Nanoscale, 9, 17884 (2017)

Resume : Doping of deep nanoscale (dns) silicon (Si) for current VLSI technology nodes reached a hard thermodynamic limit: Self-purification and out-diffusion in field effect transistors. Even if dopants manage to enter dns-Si, their ionization energy increases tremendously over values known from bulk Si; no free charge carriers can be provided [1-3]. Modulation doping of III-V semiconductors is used since the late 1970 [4] to achieve excellent electronic and optical material qualities as required for high power LEDs and semiconductor lasers. It took ca. 40 years before the concept was successfully applied to SiO2 as a Si-compatible wide bandgap material [5]. We will introduce criteria for acceptor candidates, showing in the process that a simple roll-over from knowledge about acceptors in Si is very misleading [6]. Since acceptor modulation doping of SiO2 is a fundamental principle, many new properties and applications can be accomplished: Hole-selective contacts for Si-based solar cells with excellent surface passivation [5] by a hitherto undiscovered passivation mechanism [6], very high fixed negative charge densities with good controllability [7], and applications as p-type background doping in VLSI [5]. [1] DOI: 10.1038/srep09702 [2] DOI: 10.1038/s41598-017-08814-0 [3] DOI: 10.3762/bjnano.9.141 [4] DOI: 10.1063/1.90457 [5] DOI: 10.1038/srep46703 [6] DOI: 10.1103/PhysRevApplied.10.054034 [7] DOI: 10.1063/1.5054703 [8] DOI: 10.1021/acsami.8b06098

Resume : N-type doping in Si by shallow impurities, such as P, As and Sb, exhibits an intrinsic limit due to the Fermi-level pinning via defect complexes at high doping concentrations. Here we demonstrate that doping Si with the chalcogen Te by non-equilibrium processing [1], a deep donor, can exceed this limit and yield higher electron concentrations [2]. In contrast to shallow impurities, the interstitial Te fraction decreases with increasing doping concentration and substitutional Te dimers become the dominant configuration as effective donors, leading to a non-saturating carrier concentration as well as to an insulator-to-metal transition. First-principle calculations reveal that the Te dimers possess lower formation energy than single substitutional Te and Te interstitials and donate two electrons per dimer to the conduction band. These results provide novel insight into physics of deep impurities and lead to a potential solution for the ultra-high electron concentration needed in today’s Si-based nanoelectronics. Reference: [1] M. Wang, et al. Phys. Rev. Appl. 10, 024054 (2018). [2] M. Wang, et al. Phys. Rev. Appl. revised (2019). (arXiv:1809.06055 [cond-mat.mtrl-sci])

Resume : The development of complex 3D-devices (like Finfets, TFET, nanowires) containing novel materials and extensive surface and interfacial interactions led to the observation that blanket experiments can no longer capture the relevant fundamental processes and created the need for metrology apt to deal with small volumes and atomic scale observations. In addition the small number of atoms involved (< few hundred dopants) requires extreme sensitivity or means to improve statistical relevance of the results. Within this presentation we will address recent solutions providing 3D-nm scale resolution on individual devices as well as concepts based on ensemble measurements thereby meeting the statistical relevance criterion with nevertheless applicability to small confined volumes. Among the obvious solutions we will discuss Atom probe tomography, electrical SPM when used in combination with the Scalpel method and hybrid metrology combining Scalpel SPM with TEM analysis. Statistical relevance can be obtained by ensemble measurements on arrays of devices whereby spatial resolution is provided by the device itself and not by the probing beam. Emerging concepts to probe composition and dopant incorporation in narrow FINs are based on micro-four point probe measurements, self focusing SIMS and Rutherford Backscattering Spectrometry.

Resume : Scanning Spreading Resistance Microscopy (SSRM) has been a preferred choice for two-dimensional carrier profiling of semiconductor devices due to its ability to provide quantitative information at sub-nm spatial resolution[1]–[3]. The key to high spatial resolution lies in the local phase transformation of the diamond Silicon (Si) to β-Sn underneath the tip[4]. This phase transformation lowers the contact resistance between the tip and sample making the technique directly sensitive to spreading resistance, consequently, allowing the quantification of carriers. However, continuous scaling of devices and introduction of non-planar architectures raise two important concerns pertaining to the Si phase transformation during SSRM measurements. First, the active layer thickness below the tip becomes extremely thin altering the effective mechanical properties of the layer. Secondly, the presence of underlying layers and interfaces further impact the mechanical response of the thin Si layer during nano-indentation. It can be expected that these effects will not only impact the formation and size of β-Sn pocket but may also affect the current spreading behavior, ultimately leading to faulty carrier quantification. This presentation addresses these issues in detail where we show simulations performed to track the phase transformation as a function of Si thickness. Because of the omnipresence of Si/SiO2 interface in today’s VLSI technology, our study also focuses on the role of Si/SiO2 interface on the formation of β-Sn pocket. We show that for film thicknesses below 10nm, mechanical properties of Si changes significantly and the number of atoms experiencing sufficient stress required for a phase transformation decreases monotonically, thereby making the size of metallic pocket smaller. Experimental evidence of the change of the metallic pocket is also shown using force-resistance curves obtained during in-situ SSRM. Finally, by calculating the electronic properties of these confined systems a one to one correlation with SSRM data was made to explain the observed differences in the electrical response of bulk and confined devices. [1] “Method for resistance measurements on a semiconductor element with controlled probe pressure,” Jul. 1991.[2] P. Eyben, M. Xu, N. Duhayon, T. Clarysse, S. Callewaert, and W. Vandervorst, “Scanning spreading resistance microscopy and spectroscopy for routine and quantitative two-dimensional carrier profiling,” J. Vac. Sci. Technol. B Microelectron. Nanom. Struct., vol. 20, no. 1, p. 471, Feb. 2002.[3] P. De Wolf, T. Clarysse, W. Vandervorst, J. Snauwaert, and L. Hellemans, “One- and two-dimensional carrier profiling in semiconductors by nanospreading resistance profiling,” J. Vac. Sci. Technol. B Microelectron. Nanom. Struct., vol. 14, no. 1, p. 380, Jan. 1996.[4] K. Mylvaganam, L. C. Zhang, P. Eyben, J. Mody, and W. Vandervorst, “Evolution of metastable phases in silicon during nanoindentation: mechanism analysis and experimental verification,” Nanotechnology, vol. 20, no. 30, p. 305705, Jul. 2009.

Resume : Scanning spreading resistance microscopy (SSRM) is a powerful technique to measure the doping of semiconductors on the nanoscale. Since the distribution of electrically active dopants mainly determines the behavior of functional nanoelectronic devices, SSRM should combine an accurate characterization of the doping level and dopant distribution. Calibration standards with staircase doping profiles characterized by secondary ion mass spectrometry (SIMS) are often used to quantify spreading resistance data. We present a calibration procedure that is based on well defined dopant diffusion profiles and offers a continuous comparison of dopant concentration and SSRM resistance with a concentration limit not determined by SIMS detection. The dopant profiles are characterized by SIMS analyses and diffusion-reaction-based simulations of the underlying diffusion processes in the considered semiconductor material. In comparison to experimental SSRM, numerical simulations using COMSOL multiphysics are performed to account for boundary and limited size effects in SSRM. The calibration procedure is demonstrated by SSRM analyses of dopant diffusion in silicon nano pillars. An accurate description of the doping profile is required to identify possible changes in dopant diffusion and in the properties of the defects involved that might occur in semiconductor structures with high surface-to-volume ratios.

Resume : THz and infrared spectroscopy has emerged as an important tool for the characterization of free charge carrier concentration and charge mobility in semiconductor nanostructures. As the wavelengths of THz radiation reach up towards 1 mm the potential of THz spectroscopy for the investigation of nanoscale structures is severely limited by the low spatial resolution that is given by the diffraction limit of light. Scattering-type near-field optical microscopy (s-SNOM) overcomes this limit. In this method an AFM tip is placed on the sample surface illuminated from the side with THz radiation or infrared light. The tip acts as an antenna and induces a nanofocus at its apex. The interaction between the sample and this nanofocus can be measured by analyzing the back-scattered light. Using broadband reflective optics, the microscope can be employed to do, for example, correlative monochromatic imaging in both the IR and THz region. This has been used to characterize the different doping concentrations via the free charge carrier concentrations in standard transistor structures with a spatial resolution of λ/3000. Applying a mid-IR broadband laser opens up also the possibility of FTIR-spectroscopy in the nanofocus that allows not only electronic, but also chemical mapping and imaging of structural material properties. In addition to static imaging, the second advantage of our microscope, the dual beam-path design, enables an optimal realization of pump-probe experiments in order to see fast excitation relaxation processes. In the future new developments, such as the recent integration of our neaSNOM into a cryostat will present even further opportunities for the investigation of low energy transitions.

Resume : With the transition from a planar to 3D device architecture and the continued increase in the density of the functional units, an alternative method for doping that is conformal and non-destructive will be required. Others and we, have shown that molecular layer doping (MLD) is a promising non-destructive alternative to conventional ion-implantation, specifically when carrier concentrations at the order of 1 x 1019 atoms/cm3 or higher are required. However, a uniform dopant profile in the radial (across the width) and axial (along the length) direction of the structures has been flagged as a potential issue with the MLD process. Moreover, extreme density of the structures e.g. sub-20 nm pitch at 10 nm width may present an additional challenge due to reduced access of the dopant-carrying molecules to the nanowire surface. Herein we present our first results on carrier dopant profiling of MLD doped Si nanowires fabricated with varying pitch and size using SOI wafers. To determine the radial uniformity of the carriers a room temperature chemical oxidation/etch is performed, consuming about 1 nm of Si per cycle, and the same device is tested electrically, while the Si removal is calibrated by x-TEM. The axial carriers profiling is obtained by developing on-wire TLM-type structures with minimum spacing between the contacts of about 40 nm.

Resume : Atom probe tomography (APT) is a powerful tool to study the 3-dimensional structure of materials with sub-nanometer spatial resolution. This allows us to study the location of atoms such as dopant positions in nanocrystals (NCs) with high accuracy. However, one of the limitations for the spatial resolution are the effects of local magnification when there are multiple elements with different evaporation rates in the atom probe specimen. For example, in the system of silicon (Si) NCs embedded in SiO2, the difference in the local evaporation field between Si NCs and SiO2 results in a non-uniform sequence of evaporation and this affects the accuracy of the 3-dimensional reconstruction of the atom probe experiment (i.e. over/under estimation of the number of doped atoms in the Si NCs). In this study, we use indium (In) as a dopant to investigate the local magnification effects. Due to the very low solubility of In-atoms in Si, the detected In-atoms inside of Si NCs can be attributed to the local magnification effects and we can quantitatively estimate the number of atoms which are projected inside of Si NCs due to the trajectory artefact. This approach provides a model system to quantify and correct local magnification effects which allows a more precise and advanced study of Si nanostructures.

Resume : Grain boundaries (GBs) in polycrystalline silicon (Si) ingots affect the electronic property via the segregation of impurity atoms. Three-dimensional distribution of impurity atoms at GBs can be determined by atom probe tomography (APT) with a spatial resolution of sub-nanometers. In general, a focused ion beam (FIB) processing is used to fabricate needle-shaped APT specimens, since there is no other effective technique to cut nanoneedles from bulk semiconductors, even though it induces damage in the specimens. In order to examine the effect of FIB-induced damage, we determine the distribution of impurity atoms at an artificial Si/Si boundary by using two kinds of specimens for cross-sectional transmission electron microscopy (XTEM) involving the boundary; “defective” specimens fabricated with FIB, and defect-free ones fabricated only by mechano-chemical polishing (MCP). The impurity distribution is modified by the conventional FIB processing operated at room temperature, presumably via the migration of impurity atoms assisted by point defects introduced by FIB irradiation. The migration length is the order of 1 nm. We show that the migration can be suppressed by reducing the operation temperature of FIB processing, and it is negligible at -150 degrees centigrade. We will discuss the effect of this low-temperature FIB processing on APT analysis.

Resume : As the area of the direct contact (DC) and its distance to the gate are scaled down, the contact resistance of the MOSFETs becomes more critical in DRAM devices. Furthermore, the contact resistance also rapidly increases when the devices operate at low temperature or low operating voltage (VDD). In this study, we propose a simple in-situ Si soft treatment technique right after DC etching to reduce the contact resistance. We first conducted the plasma induced surface damage analysis using TEM with different times and powers of the soft treatment. We found the damaged layer reduced by 19% with the time of 10 seconds, resulting in 16% reduction of the p+ contact resistance. The saturation current of the pMOSFET was increased by 3% at the same off-current and the propagation delay time (tPD) at the same standby current was decreased by 4%. The I-V characteristics and tPDs in the inverter circuits were analyzed from the temperature of -25℃ to 85℃. This simple in-situ technique not only removes byproducts and the damaged amorphous layer, but it also affects the effective implantation of dopants in subsequent plug processes. In addition, random dopant fluctuations are suppressed by improving Si surface roughness. It is also cost effective since the process time is short and the process step is not added. This will play an important role in low VDD operation of DRAM devices such as mobile and automotive application.

Resume : Further increasing in efficiency of silicon based solar cells requires new concepts and approaches. One of the possible realizations is usage of vertically aligned silicon nanostructures as a template for the further a-Si:H junction layers deposition [1]. This concept potentially provides an increasing of light absorption length in a-Si:H base layer with keeping low the distance between p- and n-layers and thus enhances the current matching in multi junction solar cell remaining high Voc value. Such approach requires an cryogenic plasma etching stage where the formation of high quantity of surface defects was observed [2,3]. For semiconductor devices such solar cells the quality of silicon surface and its bulk properties in general define the solar cells performance. This work presents last results for surface defects removing after cryogenic plasma etching with different techniques such as precise chemical treatment and thermal annealing. To estimate carrier’s lifetime in silicon following methods are used: transient photoconductance and 2D photoluminescence decay measurements. Defect properties are estimated by DLTS measurements. [1] A. Gudovskikh, et al. // Materials Today: Proceedings 4 (2017) 6797-6803 [2] Ngwe Zin // Solar Energy Materials and Solar Cells 172 (2017) 55–58 [3] G. Kumaravelu et al. // Solar Energy Materials & Solar Cells 87 (2005) 99–106

Resume : Spectrally narrow emission linewidth and high photoluminescence (PL) intensity of Si QD are significant for the efficient realization of different semiconductor quantum dot-based light sources. Here, we prepare Si quantum dots (QDs) on a nanoscale metal membrane. Reactive-ion etching of silicon-on-insulator (SOI) wafers and a subsequent high-temperature (1100 ℃) thermal oxidation was used to fabricate Si QDs in an oxide matrix. Aluminum was sputtered on the back side of the membrane, acting as a back-surface mirror and also changing density of optical modes and local excitation field. In this work, optical properties of such single Si QD were characterized by photoluminescence. The dots showed a broader emission range (650nm-950nm) and narrower homogenous PL emission linewidths (both at room temperature and at low temperature) than those fabricated at lower oxidation temperature (900 ℃). Besides that, under the same excitation power, the PL intensity of single Si QDs on the membrane was enhanced by approximately an order of magnitude compared to that of Si QD outside the membrane. These results indicate that advances in nanofabrication can substantially improve light emitting properties of silicon quantum dots.

Resume : The aim of this work is to investigate the performance of SiGe Double Gate TFET device including low doped drain region for Infra-red sensing applications. The electrical performance of the considered sensor is analysed numerically using ATLAS 2D simulator, where both reliability and optoelectronic performances are reported. In this context, we address the impact of the channel length, the drain doping parameters on the variation of some optcal Figures of Merit of the sensor such as: responsivety, detectivity and ION/IOFF ratio and commutation speed. The obtained results indicate the superior optoelectronic performance of the proposed design in comparison to the conventional DG FET-based infra-red sensor in terms of responsivety and commutation speed. Therefore, this contribution can provide more insights regarding the benefit of adopting tunneling Field-Effect Transistor design for future high performance and low-power nano-optoelectronic applications.

Resume : One of the ways to increase the efficiency of silicon solar cells can be a usage of core-shell structures with two junctions fully based on silicon. When the upper junction is formed on the top of vertically aligned silicon structures an increase of the light absorption in the active region could be achieved. These structures can be obtained by dry etching, which requires a mask. The mask for deep dry etching can be from by photolithography using photoresist or a hard mask, like metal or silicon oxide. Mask made by photolithography requires several additional technological steps being undesirable for PV mass production. There is also another way, which allows one to get ordered structures on the entire surface of the substrate namely latex sphere lithography. Unfortunately, classical latex nanosphere lithography does not allow one to obtain vertically aligned structures on silicon with a high aspect ratio due to etching of latex spheres during the process of dry etching. Here, we propose to use an additional layer of SiO2, which will play the role of a hard mask during the process of dry etching to obtain vertical structures on silicon without using additional steps of lithography or any hard metal masks within one dry etching process. We will present a study a latex sphere spin-coating on SiO2 layer deposited on Si. The effect of cryo-etching parameters on the vertically aligned silicon structures will also be demonstrated.

Resume : To date, the best conversion efficiencies have been obtained with multijunction solar cells on III-V substrates. However, maintaining the GaAs or germanium substrates to build these high-efficiency III–V solar cells is costly. We propose to explore tandem junctions associating single crystalline silicon bottom cell, with a bandgap of 1.12 eV and CIGS top cell, specially optimized for working in the blue/UV range (bandgap around 1.7 eV), with a new and disruptive approach based on using wide bandgap GaP intermediate layers. Our purpose is to grow wide band gap CIGS films under quasi-epitaxial conditions on GaP to improve the CIGS top cell efficiency, thanks to a reduction of the structural defects density detrimental for the cell performance. A first challenge is the formation of a low recombination contact with the GaP layer, taking advantage of the better structural and electronic matching than with the commonly-used Glass/Mo substrates, so that quasi-epitaxial CIGS-Si tandem solar cells can emerge as cost competitive for the next generation of PV modules. Epitaxial GaP quasi-lattice matched have been grown on Si(001) substrate by MBE, to realize III-V/Si dislocation-free pseudosubstrates. First results on the CIGS growth on a GaP/Si(001) pseudo-substrate are reported. In particular, x-ray diffraction evidences a strong crystalline texture of the CIGS, which illustrate the influence of the GaP(001) surface on the CIGS structural quality.

Resume : Si nanocrystals (SiNC) can be relatively efficient light emitters despite their indirect bandgap. A frequently studied way to boost their brightness even more lies in exploiting various kinds of surface passivation to improve the light emission efficiency. However, enhancing excitation efficiency, that is their absorption of light, is a second, not so well-studied channel towards improving their brightness. Here, we study diphenylantracene (DPA)-functionalized SiNCs, in which the DPA molecules function as molecular antennae in order to improve the absorption in the 350 nm-400 nm range while retaining the light emission properties of the underlying SiNCs. Time- and spectrally- resolved photoluminescence of DPA-SiNCs was measured using a system of a femtosecond laser combined with an optical parametric amplifier (150 fs, 200-2000 nm) and a streak camera with ps time resolution. SiNCs passivated with dodecene were used as a reference. Wavelength tunability of the excitation enabled us to study the above-mentioned energy transfer with great precision as we were able to preferentially excite the NCs or the DPA groups attached to them and quantify the difference between the simple and DPA-mediated modes of excitation.

Resume : Currently, an urgent task to create a nuclear frequency standard is to search for a low-lying nuclear state in the thorium-229 isotope. The goal of this work was to develop a method of the single photons energy measuring during the decaying of the isomeric thorium-229 nucleis implanted in a wide-gap dielectric SiO2 (Eg = 9 eV). The proposed method utilized thin silicon oxide layers obtained by thermal oxidization of the pure silica wafers, and allows us to measure the energy of photons in the ultraviolet range using an electronic spectrometer XSAM-800 designed for the X-ray Photoelectron Spectroscopy. In this paper we present the results of an experiment on the photoemission of thorium-229 atoms implanted in a silicon substrate. Excitation of atoms was provided by ultraviolet resonance radiation obtained by Kr and Xe discharged lamps. Energy spectra of photoelectrons were obtained. A qualitative evaluation of the energy spectrum of photoelectrons was carried out, and its agreement with the experimental data.

Resume : It is commonly observable the ongoing development and investigations of the structures with nanocrystals (NCs) embedded in dielectric layers due to their potential applications in the field of optoelectronics and photonics. The literature reports that the metal-insulator-semiconductor (MIS) structures with embedded nanocrystals attract significant attention because of their potential applications in several types of memory structures, especially in the Nonvolatile Semiconductor Memory (NVSM) and Resistive Random Access Memory (RRAM) devices. In this work, the technology of the metal-insulator-semiconductor (MIS) structures with silicon (Si) and silicon-carbide (SiC) nanocrystals embedded in dielectric layers will be presented. The results of optical, structural and electrical characterization of the fabricated test structures will be discussed. Comparative stress-and-sense measurements (I-t) for the MIS structures with and without NCs proved the essential difference resulting from the charging/discharging processes of the nanocrystals and significant improvement in retention time. Feasibility studies of the introduction of NCs into channel of Thin-Film Transistors (TFT) will be presented. Moreover, application of NCs in the RRAM structures will be also investigated. Acknowledgments This work has been supported by The National Centre for Research and Development (NCBiR) under grant No. V4-Jap/3/2016 (“NaMSeN”) in the course of “V4-Japan Advanced Materials Joint Call”.

Resume : Giant piezoelectric (PZE) effects in Si nanowires (NWs) are observed almost a decade ago and since then the Si NWs are seeking its application in routine life pressure sensing tools due to its superior physical and chemical properties, eco-friendly nature and compatibility with CMOS-chip. Herein, we investigated the PZE response of Si NW arrays towards the detection of ultra-low pressures. An efficient, large area, low-cost fabrication strategy is adopted to construct a highly sensitive pressure sensor by incorporating chemically etched Si NW arrays between two thin Au layers. The present piezoelectric sensor can quantitatively detect different pressures applied by various tools such as voice coil motor, mechanical load, precise flow of inert gases etc. A high sensitivity of 0.79 kPa-1 is achieved in the ultra-low range of pressure (0.4-1.2 Pa) enabling its utilization of monitoring human respiratory status. By precisely tracking the rate and depth of breathing, the Si NW-based sensor could offer its ability to detect different kinds of diseases related to human respiratory system. In addition, our pressure sensors resolve the acoustic vibration by distinguishing the modulation of vibration frequency and intensity. Simple, easy and inexpensive fabrication process, high sensitivity with ultra-low pressure detection, flexibility over high pressure (>200 kPa), excellent air stability (> several months) with huge life cycles (>1000 cycles), low power consumption (<400 microwatt for an operating voltage of 2 V) and the versatility of detecting pressures in different forms are advantageous key features of the present sensor, which enable it for various leading-edge applications including advanced healthcare devices.

Resume : Despite of great progress in development of third-generation PV cells (a non-univocal class including various materials: dye sensitized, multi-junction cells, quantum dots, perovskites etc), about 95% of the total PV market is still represented by crystalline silicon solar cells, which have rather low efficiency in the conventional configuration. Thus, a conceptually simpler and cheaper approach to exceed the theoretical efficiency limit of silicon cells is the addition of the special quantum cutting layer to these cells. It allows to fully exploit highly mature technologies of photovoltaics, and it theoretically can raise the solar cells efficiency from 31 to 37%. However, such result can be reached only after development of the proper quantum cutting material(s). We have demonstrated that Yb doped Scheelite like molybdate single crystals are promising quantum cutting materials. These crystals efficiently absorb UV Solar quanta and emit the sufficiently increased number of the secondary quanta near 1 micron, which are then absorbed by silicon cell with production of increased number of electron-hole pairs. In our contribution we present the growth of a several series of the crystals of Scheelite family, having various host compositions (CaMoO4, NaGd(MoO4)2, NaLa(MoO4)2, NaGd(1-x)Yx(MoO4)2, NaYMoWO8), doped with different concentrations of Yb, as well as crystallochemical and spectroscopic characterization of these crytals. This work was supported by RFBR (grant No 17-02-01245), and by the Presidium of RAS (Program I.7)

Resume : The formation of the electronic defects in crystalline silicon is studied during hydrogen plasma treatments and postannealing [1], based on in-situ photocurrent measurements and real-time spectroscopic ellipsometry [2]. From the experiments, we find the following results: (i) A hydrogen plasma treatment induces a reduction in the photocurrent, i.e., the generation of defects. The consecutive postannealing results in an increase in the photocurrent, i.e., the annihilation of defects. (ii) The generation and annihilation of these defects strongly depend on both the treatment time and the treatment temperature. (iii) The defects generated by long-time treatments remain in the silicon; the residual defects are created. The density of the residual defects is estimated to be of the order of 10^13 cm^-2. (iv) The residual defects are distributed mainly inside silicon, not on the silicon surface; those are the bulk defects. (v) The formation of these bulk defects may be associated with a higher-order transition of the lattice. The critical temperature for the transition is between 179C and 188C. (vi) The disordered (amorphized) surface layer is created when the silicon is treated with a sufficiently long-time treatment. The thickness of this layer is a few nm. The formation and growth of this layer do not cause an additional reduction in the photocurrent. It indicates that the photocurrent in silicon, i.e., the lateral electronic transport, is limited by the defects located underneath the disordered layer or the interface defects between the disordered layer and the silicon bulk. The authors are grateful to Prof. M. Shiratani (Kyushu Univ.) for fruitful discussions. This work was supported in part by JSPS KAKENHI (Grant Number 18K03603 and 15K04717) and New Energy and Industrial Technology Development Organization (NEDO). References : [1] S. Nunomura, I. Sakata, K. Matsubara, to be submitted. [2] S. Nunomura, I. Sakata, and K. Matsubara, Phys. Rev. Appl. 10, 054006 (2018)..

Resume : Polycrystalline silicon (poly-Si) thin films were fabricated by aluminum-induced crystallization of silicon suboxide (SiOx) via inverted aluminum-induced layer exchange (inverted-ALILE) mechanism by the first time. a-SiOx films (110 – 550 nm) were deposited by gas-jet electron beam plasma chemical vapor deposition method from SiH4, H2 and O2 gases on borosilicate Corning XG glass substrates. Next, a 220-nm-thick aluminum (Al) layers was sputtered on a-SiOx films. Thereafter, to provide the inverted-ALILE mechanism, glass/SiOx/Al bilayer samples with different SiOx/Al thickness ratio (0.5 – 2) were annealed in a tube furnace at 500 – 570 C during 2 – 20 hours in vacuum. Annealing process led to macroscopic exchange of SiOx and Al layers with formation of poly-Si material. The influence of the annealing parameters (temperature and time) on the morphology and crystalline properties of the poly-Si material (the crystallized fraction of Si grains, Si grain size, and their crystalline orientation) was investigated by transmission electron microscopy, optical microscopy, Raman spectroscopy, electron backscattering diffraction, and X-ray diffraction methods. This study was financially supported by the Russian Science Foundation, project # 17-79-10352.

Resume : The distance dependence of localized surface plasmon (LSP) coupled Förster resonance energy transfer (FRET) between Si quantum dots (QDs) is experimentally investigated utilizing core-shell nanostructures. The dependence of the energy transfer efficiency, rate, and characteristic distance, as well as the enhancement of the acceptor emission, on the shell thickness is examined. Compared to the structure without assistance of LSP from silver nanoparticles (Ag NPs), the FRET rate is enhanced by a factor of 15 and the Förster radius increases by 50% as a result of plasmon-coupling to the Si QDs and the FRET-assisted exciton transfer from the donors to the acceptors. The potential to tune the characteristic energy transfer distance has implications for applications in nanophotonic devices or sensors.

Resume : Recently, in solar energy society, several key technologies have been reported to meet a grid parity; cost efficient materials, simple process and designs. Hereby, Si based solar cells are fabricated and studied to enhance the performance. Different size of CdSe quantum dots (QDs) and gold nanostructures were incorporated on Si light absorbing layer acting as a luminescent down shifter. Since, pure QDs layer has low absorption coefficient, nano-anttena design using gold nanoparticles (Au NPs) were combined with QDs. The QDs layer absorbs high energy photons and re-emits lower energy photons in 627nm and 528nm of wavelength, respectively. Such a down shift layer can enhance the overall efficiency of Si solar cells due to poor intrinsic spectral response in UV region. For further enhancement in solar cell properties, average size and density of Au-NPs was varied. Moreover, an individual and combined effect of CdSe QDs with various Au-NPs has been investigated in depth. The mechanism of charge interaction between CdSe QDs and Au NPs has been analyzed regarding plasmon resonance energy transfer. The optical properties of Au NPs and CdSe QDs have been characterized using UV-visible spectroscopy and time resolved PL measurements. Upon increasing the size of Au, the redshift and enhancement of absorbance spectra has been observed. Such an enhancement in the absorption capability leads to increased quantum efficiency (QE) in UV and IR spectral range.

Resume : Recently it was suggested that Au doping in Si can be realized by ion implantation and pulsed laser melting. The sub-band-gap optoelectronic response is observed and increases with the implanted Au concentration [1]. In our work, Au implanted Si was fabricated by ion-implantation with three different fluences of 7×1014 cm-2, 1.4×1015 cm-2 and 2.1×1015 cm-2, followed by pulsed laser melting. The Raman spectrum results confirm the high-quality recrystallization of the Au implanted layer. And the Rutherford backscattering spectrometry / Channeling reveal that Au atoms diffused to the near surface region. In addition the detailed angular scans along Si [001] reveal that Au atoms are mostly in the interstitial lattice sites. From the transport measurements, a p-type conductivity and an increasing carrier concentration are observed in the implanted layer. Moreover, the transmission and reflection were measured using near infrared spectroscopy (NIR) to quantify the sub-band-gap absorptance in the hyperdoped silicon. In the Au implanted layer the spectral response extends to wavelengths as long as 3.2 μm. However, the sub-band-gap absorptance has no dependence on the Au fluence or the carrier concentration.

Resume : The fabrication of window layers more transparent than those based on amorphous silicon could be one of the key factors to enhance the efficiency of silicon-based heterojunction (HJ) solar cells. In recent years, Transition Metal Oxides layers such as MoOx and WOx have been shown to form efficient selective contacts on crystalline silicon (c-Si) based HJ solar cells thanks to their high work functions which establish the built-in voltage of the junction and ensure hole collection toward the electrode. Moreover, their high band gap energy values guarantee a high transparency at the lower wavelengths of the solar spectrum. However, such materials have always been used in combination with intrinsic amorphous silicon (a-Si:H) as passivation layer, which drawbacks are low transparency and the use of toxic gases during the deposition process. The combination of WOx and amorphous hydrogenated silicon oxide (a-SiOx:H) as passivating layer, both characterized by higher optical bandgaps than the amorphous silicon counterparts, could ensure a higher transparency of the front window layers with no need for toxic gases during the fabrication. In this work we describe the characteristics of WOx thin layers and we show their photovoltaic performances when used in HJ based on c-Si wafer passivated by a-SiOx:H. A comparison is made between WOx films grown by RF sputtering and by thermal evaporation, to evaluate the different effects of the two techniques on the hole-extraction mechanism.

Resume : The fabrication of highly doped and high quality Ge layers is a challenging and hot topic in nanoelectronics, photonics, aiming to integrate or substitute Si in its current technological monopoly. We deepen the use of pulsed laser melting (PLM) during diffusion annealing cycles of n-type dopant Sb previously sputtered on the surface of bulk Ge. A broad characterization has been performed, based on secondary ion mass spectrometry, channeling-Rutherford backscattering spectrometry, simple and differential Van der Pauw-Hall measurements, high resolution x-ray diffraction, infrared reflectivity, electron beam induced current. We show that PLM promotes an efficient diffusion of high Sb concentrations in the Ge melted subsurface layer, followed by an ultra-fast epitaxial regrowth. As a result, excellent surface morphology and crystalline quality is obtained, with record active Sb concentration well above 1e20 cm^-3 and ultralow resistivity below 1.5e-4 Ωcm over 100-150 nm. Key properties such as substitutional fraction, electron mobility, residual strain, infrared reflectivity and plasma frequency are also characterized and discussed. These results confirm Sb deposition followed by PLM as a simpler and cheaper doping method, and with lower thermal budget than the other methods commonly employed and, at the same time, able to achieve record activation levels with no residual damage and excellent electrical and optical properties, relevant for Ge based future advanced devices.

Resume : The photovoltaic industry is mainly dominated by crystalline silicon (c-Si) based solar cells where, the contact selectivity is usually achieved by doping the wafer surfaces with phosphorus and boron atoms. Several alternatives are used in order to avoid the high temperature, furnace-based, diffusion process. Examples include the well-known silicon heterojunction (HIT) using both intrinsic and doped amorphous silicon (a-Si:H) films, or the formation of p+ and n+ regions by laser-firing of doped dielectric films. Nevertheless, in both cases the use of toxic and flammable gases is required. Recently, the use of dopant-free materials based on transition metal oxides (TMOs) like MoOx, V2Ox and MgOx have shown excellent hole and electron selectivity [1-3]. The use of titanium oxide (TiO2) is an attractive option to form electron-selective contacts, due to its small conduction- and large valence-band offsets (ΔEc ~0.05 eV and ΔEv ~2.0 eV respectively), allowing an easy electrons transport through the c-Si/TiO2 interface while blocking the holes [4]. The introduction of a thermal dielectric SiO2 interlayer at the c-Si/TiO2 interface improves the quality of the selective contact and its thermal stability reaching efficiencies up to 21.6% [5]. The replacement of this high temperature SiO2 layer by other dielectric films deposited a low temperatures is an interesting objective. In this work we study the properties of atomic layer deposited (ALD) Al2O3/TiO2 stacks deposited at low temperatures as electron transport layers. The goal is to use optimized Al2O3/TiO2 layers as selective contacts in interdigitated back-contacted (IBC) c-Si(n) solar cells. Preliminary results confirm surface recombination velocities below 40 cm/s with implied open circuit voltage (iVoc) values of 675 mV in symmetrical Al2O3/TiO2 test samples. Specific contact resistance values below 3 mΩcm² are also measured on stacks properly covered with the metal capping electrode. These excellent results pave the way to use these stacks as electron selective contacts on IBC solar cells, in combination with V2Ox hole selective contacts. Experimental and technological details as well as first IBC solar cell results will be presented at the conference. References [1] L. G. Gerling, et al., Sol. Energy Mater. Sol. Cells, 2016, 145, 109 [2] G. Masmitjà, et al., J. Mater. Chem. A, 2017, 5, 9182 [3] Y. Wan, et al., Advanced Energy Materials, 2016, 1601863 [4] K. A. Nagamatsu, et al., Applied Physics Letters, 2015, 106, 123906 [5] X. Yang, et al., Advanced Materials 2016, 28, 5891

Resume : For highly efficient III-V-on-Si tandem solar cells it is crucial to avoid defects known as antiphase boundaries in the III-V material by preparing the Si(100) surface with double-layer (DL) steps. Here, we study the interaction of Si(100) surfaces with arsenic (As), which is present in most application-relevant III-V MOCVD reactors. Arsenic was supplied either directly via the precursor (TBAs) or indirectly as background As4. The entire process is controlled in situ by optical spectroscopy and the as-prepared surfaces are analyzed with STM, LEED and XPS after contamination-free transfer to UHV. In dependence on process routes, the optical in situ control allows us to monitor the changes in the line-shape of the spectra, which are directly correlated with changes on the Si surface. We show that specific process routes enable preparation of Si(100):As DL stepped surfaces with either prevalent (2×1) or (1×2) domain [1]. The low miscut Si(100):As surfaces exhibit atomically flat terraces with evenly spaced DL steps and exiguous presence of minority domain. The Si:As surface structure depends on the H and As chemical potentials and differs in dependence on preparation route and substrate miscut, as confirmed by DFT calculations. Our results indicate more complex Si:As surface structure than the commonly assumed monolayer of As-As dimers on top of Si. This work was supported by the GACR (no. 18-06970J) and DFG (no. HA3096/10-1). [1] A. Paszuk et al., Appl. Surf. Sci. 462 (2018) 1002

Resume : Nanoscale capacitors have caught great interest for the semiconductor industry by focusing on the vertical (3D) integration for a reduction in energy consumption. Current methods for reliable and repeatable capacitance measurements at nanoscale remain challenging. However, a non-destructive quantitative characterization tool, the SMM, has been developed to characterize nanodevices at microwave frequencies with nanometer resolution. It consists of an AFM combined with a vector network analyser (VNA), which sent a microwave signal over the sample via a conductive tip. This tip also serves as a receiver to capture the reflected microwave signal from the contact point. Then, the S11 reflection parameter is measured by the VNA and acquired simultaneously with the surface topography. The calibration procedure used here requires a structure composed of several MOS capacitors. S11 measurements carried out on 3 capacitors selected as references allow one to calibrate the capacitance of all capacitors of the structure. Here we present the results obtained when a random selection (162 draws) is made on a structure composed of 48 capacitors. Results show that the measured values from a single image agree with the theoretical values with a type A uncertainty less than 100 aF for C < 1fF and 190 aF for C > 1 fF. The recording of 13 successive images using a single draw allows one to estimate repeatability uncertainties less than 14 % for C < 1fF and 0.9 % for C > 1fF.

Resume : Semiconducting nanowires (NWs) hold promises for functional nanoscale devices [1]. Although several applications have been demonstrated in the areas of electronics, photonics and sensing, the doping of NWs remains challenging. Ion implantation is a standard doping method in top-down semiconductor industry, which offers precise control over the areal dose and depth profile as well as allows for the doping of all elements of the periodic table even beyond their equilibrium solid solubility [2]. Yet its major disadvantage is the concurrent material damage. A subsequent annealing process is commonly used for the healing of implant damage and the electrical activation of dopants. This step, however, might lead to the out-diffusion of dopants and eventually the degradation of NWs because of the low thermal stability caused by the large surface–area-to-volume ratio. In this work, we report on a method for cross-sectional doping of individual Si/SiO2 core/shell NWs using ion implantation followed by sub-second thermal annealing. P and B atoms are implanted at different depths in the Si core. The healing of the implantation-related damage together with the electrical activation of the dopants takes place via solid phase epitaxy driven by millisecond-range flash lamp annealing. Electrical measurements through a bevel formed along the NW enabled us to demonstrate the concurrent formation of n- and p-type regions in individual Si/SiO2 core/shell NWs [3]. These results might pave the way for ion beam doping of nanostructured semiconductors produced by using either top-down or bottom-up approaches. [1] Peidong Yang, Ruoxue Yan, and Melissa Fardy, Nano Lett. 2010, 10, 1529 [2] Michiro Sugitani, Rev. Sci. Instrum. 2014, 85, 02C315 [3] Y. Berencén, et al. Nanotechnology 2018, 29, 474001 a)Corresponding author: y.berencen@hzdr.de

Resume : The development of chemical sensors has been the subject of a strong research activity in the past decades, in particular for the selective detection of toxic gases. Among the numerous methods used to detect the chemical species, optical methods using luminescent probes are promising in terms of sensitivity, selectivity and feasibility. Rare-earth (RE3+) ions are good candidates to act as luminescent probe because they have sharp emission peaks, very large Stokes shifts and long luminescence lifetimes. In most of the strategies presented in the literature, a fluorescence quenching of the probe is observed in solution in presence of the analyte. However similar studies for the detection of species in the gas phase are very scarce. In this work, silicon wafers were functionalized with luminescent RE based complexes to investigate the potential of such hybrid materials for sensing applications in the gas phase. It is shown that the RE based complexes using Tb3+, Eu3+, Ce3+, Yb3+ or Nd3+ ions are all optically active. It has been demonstrated that, while the complexes using Yb and Nd elements show a very weak emission, those developed with Tb, Eu and Ce are strongly luminescent. In this presentation, the influence of the synthesis parameters on the optical properties of the materials will be shown. In addition the effect of the environment on the photoluminescence properties of these hybrid materials will be discussed in relation with potential applications for gas sensors.

Resume : Nowadays solar cell industry is in a constant search on individual film and interphase enhancement in order to achieve higher efficiencies even for commercial solar cells. Nevertheless, a reduction of the manufacturing cost and achieving good device quality is desirable for new solar cells development. Pm-Si:H films may be a good candidate due to their improved optoelectronic properties and stability compared to those of the conventional a-Si:H due to their nanocrystalline structure. Several studies revealed that the doping homogeneity of the p and n type films of a solar cell structure may affect the device efficiency, Voc and Isc. Furthermore, an improvement on the lifetimes of such photovoltaic devices is expected to be achieved increasing the film quality. In such terms, the present work focus on the study of the boron doping on p-type pm-Si:H thin films for solar cells deposited by PECVD. The later study is conducted by means of chemical mapping of the p-type pm-Si:H films at the nanoscale using HRTEM and EDS simultaneously. The obtained results show a variation on the boron concentration among the films is increased and within the nanocrystals. The significant variation observed may affect the device operation and a change on the deposition parameters showed a decrease. Nevertheless, varying the deposition parameters also affected the nanocrystals size and density, while the boron concentration was increased and almost homogenous within the film.

Resume : The majority of solar cells produced at present use multicrystalline silicon (mc-Si) wafers which are sawn from ingots grown by directional solidification. The wafer cutting process leaves saw damage on both faces of the wafer. The mechanically-damaged regions contain dislocation networks and cracks, and the first stage in most cell processes is a chemical etch to remove damage to avoid a detrimental impact on electronic properties later on. We have investigated whether the existence of saw damage can be exploited beneficially in a process to purify silicon wafers prior to cell production. The saw damage is used to trap harmful metallic impurities which diffuse to it from the bulk of the wafer (known as “saw damage gettering” (SDG)) at relatively low processing temperatures (≤ 700 °C). As impurities facilitate the recombination of photogenerated charge carriers, reducing their concentration improves the properties of the wafer. In this study, we have demonstrated that SDG at 500 to 700 °C can be used as part of a low-temperature processing strategy to improve minority carrier lifetime in as-grown mc-Si for solar cells. Lifetime improvements by a factor of 4.2 have been demonstrated by combining SDG with subsequent low-temperature internal gettering, with no process exceeding 600 °C. The simple SDG method has the potential to be a low thermal budget process for the improvement of low-lifetime “red zone” wafers, hence improving solar cell efficiencies and production yields.

Resume : Since the beginning of semiconductor industries, the contact between metal and semiconductor plays a crucial role. Few test structures like Cox and Strack, linear Transfer Line Method (TLM) or circular TLM (CTLM) were developed. In photovoltaic research the linear TLM is extensively used to determine the contact resistance between metal and silicon or similar systems. Due to the silver H-grid pattern the linear TLM is simply applicable by solar cell slicing. The emitter serves as thin, in good approximation one dimensional, conductor in accordance to TLM theory. In parallel, the pn-junction resistance works as vertical barrier to the underlying silicon bulk. The pn-junction resistance is determined by the reverse characteristic, i.e. parasitic current flows. Thus, it affects the TLM measurement which is not accounted by standard TLM theory. COMSOL Multiphysics simulations were used to evaluate the effect of reverse characteristics on TLM results, taking into account device geometry, material properties and pn-junction characteristics. The simulation revealed a potential of up to 5 % error for the resistance measurement with a consequence of up to 50 % discrepancy in the thereof calculated specific contact resistance. The presented model gives a brief overview on the physical correlations between used current density, emitter sheet resistance and sample geometry. Thus, sample design and measurement conditions for TLM analysis in regard to lower measurement error can be improved.

Resume : Moore's law has enjoyed steady progress for 50 years, since 1965 up to node 28nm. These were the years of planar bulk CMOS scaling. Planar bulk could not be used for further nodes because of short-channel effect problems. This prompted the industry to switch to FinFET or FDSOI. The development of these new device architectures is costly and ended Moore's law expressed in terms of transistor cost scaling. The FinFET architecture can be probably be used up to node 5 (gate length ≈ 15-18nm) and one may have to switch to even costlier gate-all-around nanowires or nanosheets structures to reach node 3 (gate length ≈ 10-12nm). Beyond the 10 nm mark, direct tunneling from source to drain prevents further scaling, no matter what semiconductor material or MOS transistor architecture is used. Actually, materials with heavier electron mass, such as silicon, are better than III-V materials for nanowire MOSFET fabrication because they have a higher density of states and lower direct S/D tunneling probability. Potential solutions to further increasing performance or complexity of CMOS circuits include 3D monolithic integration and subthermal subthreshold switching.

Resume : The concept of reconfigurable field effect transistors (RFET) was proposed more than 10 years ago as a possibility to enhance the functionality of conventional field effect transistors by adding the option to reversibly adjust the polarity of the transistor by an electrical signal at runtime [1]. Since then the technology has advanced from a viewpoint of device performance, technology and its exploitation in novel circuit topologies and applications. Most of the work shown in the last years involved silicon and germanium nanowires to form the transistor channels. However, also carbon nanotubes, 2D transition metal dichalcogenide materials and even more conventional technologies like fully depleted silicon on insulator have been shown to be a possible platform for realizing such device. Recently, the concept of non-volatility was added to the device functionality [2]. This can be exploited in multiple ways. First it adds an inherent multi-bit information storage capability to the device, second it has the potential to make the configuration state of the device itself non-volatile in order to save configuration resources. Third ? in a more general sense ? as it fuses memory and logic in a single device and circuit blocks It can allow novel computation paradigms. In this talk the basics of RFETs and nonvolatile RFETs will be described and then the current status of the technology and circuit possibilities will be reviewed. Finally the nonvolatile options will be discussed showing first results using both charge trapping and ferroelectric mechanisms to achieve the non-volatility. References [1] T. Mikolajick et al., The RFET?a reconfigurable nanowire transistor and its application to novel electronic circuits and systems, Semiconductor Science and Technology 32 (4), 043001 (2017) [2] S. J. Park et al., Reconfigurable Si Nanowire Nonvolatile Transistors, Advanced Electronic Materials 4 (1), 1700399 (2018)

Resume : Scaling of Si in modern nano-electronic fabrication is heading towards its ultimate atomic limit. Even before this limit is reached, where critical Si dimensions fall in the sub-10 nm range, dopant introduction and even achieving basic conduction present their own set of challenges. Furthermore, with the advent of 3D structures such as FinFET, nanowire, and gate-all-around devices, the need for gentler (crystal damage), more isotropic (better conformity to 3D structures) doping approaches becomes increasingly more important. In this work we investigate conduction in ultra-thin Si films in the range of 66 nm to 3 nm nm using a variety of non-conventional doping approaches. The doping methods used for this study were: liquid-phase monolayer deposition of phosphorus, vapour-phase monolayer deposition of phosphorus, gas-phase deposition of arsenic. Conventional ion implantation was also performed and served as a benchmark for the other doping processes. Each doping process was evaluated through electrical resistivity measurements using the circular transfer length method, and by determining dopant concentration using electrochemical capacitance voltage measurements. Each doping approach was found to be effective in doping Si films in the range of 10 nm to 66 nm, with resistivity between 103 and 105 Ω.nm. For Si films with 4.5 nm thickness, a stark increase in Si film resistivity was observed which was of the order of 106 - 1011 Ω.nm. This increase in resistivity indicates the difficulty in doping ultra-thin Si films, and/or the inherent difficulty in achieving electrical conduction in these films due to quantum and other effects.

Resume : The introduction of SiGe thin film for P-MOSFET technology plays a critical step in the improvement of RF performances. The integration of compressive strained SiGe layer, using Germanium condensation technique, strongly influence the hole mobility and therefore the device performances. As the thickness of the layer is below 10nm, each process step can change the mechanical deformation of the active region. For that sake, an extensive characterization technique at nanoscale dimension is required to determine the different parameters that influence the level of strain. This study presents experimental strain mapping performed on advanced 22 nm Ultra-Thin-Body and Buried-Oxide Fully-Depleted-Silicon-On-Insulator (UTBB-FDSOI) technology. The characterization technique uses transmission electron microscopy configured in nano beam diffraction setup which allows strain mapping with a spatial resolution of 2 nm and a precision of 0.1%. We demonstrate significant RF performances enhancement induced by layout optimizations for several samples. Indeed, experiments show that increasing contact poly pitch induces a higher deformation of the channel. Moreover, gate-to-contact distance also influences the strain in the channel region. The combination of FEM simulations, high resolution strain mapping and electrical characterizations allows us developing a broader calibrated geometrical model of device that will facilitate a better device layout and strain optimization for RF applications.

Resume : The silicon metal-oxide-semiconductor transistor is the workhorse of the microelectronics industry. By shrinking its size generation after generation the computational performance, memory capacity and information processing efficiency has increased relentlessly. However, the process of miniaturization is bound to reach its fundamental physical limits in the next decades. New computing paradigms are hence paramount to overcome the technical limitations of silicon technology. Quantum computing offers exponential speed-up over several classical algorithms [1-3]. However, finding the optimal physical system to process quantum information and scale it up to the large number of qubits necessary to run those algorithms remains a major challenge. Paradoxically, silicon technology itself could offer an optimal platform on which to fabricate scalable quantum circuits: Quantum computing with silicon transistors fully profits from the most established industrial technology to fabricate large scale integrated circuits while facilitating the integration with conventional electronics for fast data processing of the binary outputs of the quantum processor; all this offering long coherence times [4]. In this talk, I will present a series of results on fully depleted silicon-on-insulator (FD-SOI) transistors at miliKelvin temperatures that show this technology could provide a platform on to which implement electron-spin qubits [5-7]. Additionally, I will present a set of experiments that demonstrate the potential to scale FD-SOI technology and interface them naturally with conventional digital electronics [8-9]. References [1] P.W. Shor, SIAM Journal on Computing 26 (1997) 1484. [2] L. K. Grover, Phys. Rev. Lett 79 (1997) 325. [3] D. Poulin, Quantum Inf. & Comput. 15 (2015) 361. [4] M. Veldhorst, Nature, 526 (2015), 410. [5] A. C. Betz, M. F. Gonzalez-Zalba, Nano Lett. 15 (2015) 4622. [6] M. F. Gonzalez-Zalba, Nat. Commun. 6 (2015) 6084. [7] M. Urdampilleta, M. F. Gonzalez-Zalba, Phys. Rev. X 5 (2015) 031024. [8] A. C. Betz, M. F. Gonzalez-Zalba, App. Phys. Lett. 108 (2016) 203108. [9] S. Schaal, M. F. Gonzalez-Zalba, Phys. Rev. App. 9 054016 (2018), arXiv:1809.03894.

Resume : Deep nanoscale (dns) silicon (Si) nanoelectronic devices require an energy shift of electronic states for p- and n-conductivity. Self-purification and out-diffusion in field effect transistors cause doping to fail. Even if dopants manage to enter dns-Si, their ionization energy increases tremendously over values known from bulk Si; no free charge carriers can be provided [1-3]. We show that a massive energy offset of electronic states exists in dns-Si embedded/coated with silicon nitride (Si3N4) and silicon dioxide (SiO2) in analogy to doping, creating a dopant-free p/n junction [4]. Hybrid density functional theory (h-DFT), interface charge transfer (ICT), and experimental verifications arrive at the same size below which the dielectrics dominate the electronic properties of dns-Si [5]. The interface impact is described as nanoscopic field effect. We propose the energy offset as robust and controllable alternative to impurity doping of Si nanostructures and will elucidate its fundamental mechanism, providing a vista onto novel (undoped) p/n devices in dns-Si. [1] Sci. Rep., V. 5, 09702 (2015); DOI: 10.1038/srep09702 [2] Sci. Rep., V. 7, 8337 (2017); DOI: 10.1038/s41598-017-08814-0 [3] Beilstein J. Nanotech., V. 9, 1501 (2018); DOI: 10.3762/bjnano.9.141 [4] Adv. Mater. Interf., V. 1, 1400359 (2014); DOI: 10.1002/admi.201400359 [5] Phys. Rev. Appl., V. 10, 054034 (2018); DOI: 10.1103/PhysRevApplied.10.054034

Resume : HfO2-based high-k dielectrics have been commercially available in silicon-based complementary metal-oxide-semiconductor (CMOS) logic technology for more than a decade. Thus, the recently discovered HfO2-based ferroelectrics (FE) [1,2] are attractive candidate materials for future CMOS-based technologies such as negative capacitance low-power field effect transistor (FET) logic, FeRAM or FeFET memory, and FeFET- or ferroelectric tunnel junction (FTJ)-based neural network accelerators. The stabilization of the ferroelectric phase and identification of other phases present in such films are crucial for their successful implementation. However, although an orthorhombic ferroelectric phase with space group Pca21 is most commonly invoked, unequivocal phase identification by techniques such as X-ray diffraction has been elusive. In this study, we have used density functional theory (DFT)-assisted extended X-ray absorption fine-structure spectroscopy (EXAFS) to determine the structural symmetry of Al and Zr doped HfO2 thin films. The 8-nm thick HfO2-based films were grown by atomic layer deposition in a metal-insulator-metal (MIM) stack configuration with varying doping levels of Zr and Al and annealing temperatures. Grazing-incidence fluorescence-yield mode Hf L3 and Zr K absorption edge EXAFS experiments were performed at the 6-BM beamline at the National Synchrotron Light Source II of Brookhaven National Laboratory. We used DFT-based calculated references for various phases of Hf(Zr,Al)O2 and used EXAFS fitting to identify the crystal phases present in the HfO2-based thin films, with their corresponding ratios. The results of the EXAFS multiphase fitting will be discussed in conjunction with the electrical properties. In particular, we confirm that the orthorhombic Pca21 phase is present in ferroelectric HfZrO. [1] T. S. Böscke et al., Appl. Phys. Lett. 99, 102903 (2011) [2] M. H. Park et al., Adv. Mater. 27, 1811 (2015)

Resume : In the last decade, tailored visible electroluminescence (EL) emission from Si nanocrystals (NCs) has been achieved, due to quantum confinement, by means of NC size control through the multilayer (ML) approach, making this material a promising candidate for light emission applications. In addition, huge research efforts have been devoted to the field of memristors, whose ability for changing resistance (between high and low resistance states or through multiple states), the resistive switching (RS) phenomenon, is currently employed in non-volatile memories or neuromorphic circuits. Although Si suboxides have already stand out in this field due to their low operation voltages, the utilization of electroluminescent Si NCs as optically-readable RS material has not yet been explored, which would integrate electronics and photonics within the same device. In this work, we have investigated the EL and RS properties of Si NC/SiO2 MLs embedded into a ZnO/NCs/Si device. Stable RS behavior was ascribed to the creation of a conductive path through the MLs, whose different resistance states were found to influence the EL of the Si NCs. Additionally, defects in ZnO also yielded a characteristic emission when excited within the proper RS state. As a result, the reading of the RS state of the device can be directly identified through the EL emission of the distinct luminescent centers, thus paving the way to novel integrated optical memristors.

Resume : We report on the optical characterization of free charge carriers in silicon nanostructures prepared by wet chemical etching of crystalline Si wafers in hydrofluoric acid solutions. While the standard measurement of the electrical conductivity is required electrical contacts to the nanostructures and its interpretation is complicated due to unknown mobility of free charge carriers, we use contactless optical methods of the Fourier-transform infrared spectroscopy (FTIR) in attenuated total reflection (ATR) mode and Raman scattering to determine the charge carrier concentration in n- and p-type porous silicon (PSi) and silicon nanowires (SiNWs) in the range of 1019...1020 cm-3 . Data of the Raman spectroscopy were also analyzed to estimate a contribution of free charge carriers into the thermal conductivity of SiNWs arrays that is essential for thermoelectric applications of doped SiNWs.

Resume : Silicon nanowires have been widely pursued as a technology for electronic, photonic, and renewable energy devices. Here, we describe a bottom-up method to modulate nanowire shape and composition along the growth axis with sub-10 nanometer spatial resolution. The rapid modulation of phosphorus or boron incorporation during vapor-liquid-solid (VLS) growth can be used to encode a complex pattern of dopants. Subsequent solution processing creates high-resolution structures such as bow-ties, gratings, sawtooths, fractals, nanorods, sinusoids, and nanogaps. This synthetic capability enables the creation of new rectifying structures. First, we demonstrate a non-centrosymmetric nanowire that can rectify and ratchet the motion of ballistic electrons. By designing asymmetry on a length scale comparable to the mean free path of electrons, we create a geometric diode that funnels electrons in one direction and has a near-zero turn-on voltage. At GHz-THz frequencies, this structure ratchets electrons, enabling a new class of high-frequency diode. Second, we demonstrate the synthesis of p-i-n nanowire superlattices encoded with arbitrary numbers of junctions. Abrupt and degenerate doping levels ensure the creation of n-p tunnel junctions, enabling the realization of multijunction single-nanowire photovoltaic devices. Decuple-junction devices generate more than 2 V of photovoltage under 1-sun illumination. When functionalized with catalysts, these nanostructures can be used for single-particle water splitting in a particle suspension reactor that generates hydrogen and oxygen.

Resume : The global energy demand boosted the fabrication of novel materials with ground-breaking optoelectronic performances. In this framework, silicon nanowires arrays arose as a new platform, finding great impact in photovoltaics, sensing and optoelectronic applications. We report the synthesis of high-density arrays of vertically aligned silicon nanowires (NWs) with tunable aspect ratio by metal-assisted chemical etching using Au layers with fractal arrangement as catalyst 1. Different design of 2D random fractal arrays of Si NWs were realized without any mask or lithography, with a low cost and industrially compatible approach. The optical response of the NWs can be tuned by optimizing their fractal geometry. Indeed, the occurrence of a strong in-plane multiple scattering and efficient light trapping due to the fractal structure overall the visible and near IR range were demonstrated, remarking the promising potential of Si NWs for photovoltaic and photonics 2. The role of phase coherence in multiple scattering phenomena occurring in fractal NWs was exploited, demonstrating the first experimental observation of Raman coherent backscattering arising from the constructive interference of inelastically scattered Raman radiation in strongly diffusing Si NWs with an engineered disorder 3. Moreover, quantum confined Si NWs with fractal geometry show a remarkable room temperature luminescence tunable with NW size. Light emitting devices based on Si NWs showing an efficient room temperature EL emission at low voltage were also reported paving the route for their implementation in Si microphotonics. 1 Nano Lett. 19, 1, 342-352, 2019 2 Light: Sci. Appl. 5, 4, e16062, 2016 3 Nature Photonics 11, 170, 2017

Resume : Wearable devices for healthcare monitoring applications need reliable and miniaturized electrical power supplies. Human skin, involved in the regulation of the body temperature, can be exploited to harvest thermal energy in order to enable smart power sources. Converting heat into electricity requires a highly efficient thermoelectric generator (TEG) to facilitate adequate intake of the human skin thermal signature. For this purpose, silicon nanowires (SiNWs) have emerged as a promising material since nanofabrication allowed Si one-dimensional nanostructures to achieve good thermoelectric properties such as: high Seebeck coefficient, high electrical conductivity, and low thermal conductivity. A high thermal to electrical conversion efficiency can be achieved with SiNWs-based TEGs. Such TEGs require a thermoelectric couple based on n-type and p-type SiNWs that exploits the Seebeck effect. Here, we report on different techniques to improve the dimensionless figure of merit (zT) of SiNWs. For instance, zT can be altered by blending SiNWs with a polymer, for example, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), thus, lowering thermal conductivity. Measurements are sustained by numerical modeling and discussed within a general theoretical framework for SiNWs. Specifically, an array of SiNWs-based TEGs, interconnected according to electrical power levels required by electro-thermal drug delivery patches, is integrated on a human-skin compatible platform using polyethylene terephthalate as a substrate.

Resume : Bottom-up grown silicon nanowires (SiNWs) in transistor configuration were highlighted in the past as nanoscale transducers for label-free biosensing. Despite intense research, complex samples can still not be analyzed by single SiNW devices and also sensitive electrical readout is still challenging in a physiological environment. To address these challenges, we demonstrate here that SiNWs, grown by vapor-liquid-solid synthesis, can be used as nanoscale device platform carrying various plasmonic structures for miniaturized surface enhanced Raman spectroscopy (SERS). We decorated single-SiNWs with plasmonic structures, created for instance by metal thin film dewetting or metal nanoparticle attachment from solutions. Thus, nanoscale probes with optical read-out for label-free detection of organic molecules, such as methylene blue, were achieved. Supported by principal component analysis, discriminant function analysis and partial least squares regression for data analysis, SiNW-enabled SERS probes functionalized with para-mercapto benzoic acid were successfully employed for optical pH-measurements at the nanoscale. Furthermore, a highly localized decoration of single-SiNWs with only four, tightly packed gold nanoparticles (45 nm in diameter) by means of DNA-origami structures led to SERS probes with a measurement precision in principle beyond the lateral resolution of visible light. Despite miniaturization, these probes offered still a SERS enhancement factor in the range of 10^5.

Resume : Aiming for high-resolution biomedical pH- and ion-sensitive sensors, field-effect transistors made from single bottom-up grown silicon nanowires (SiNWs) resemble promising transducer building blocks. Even though examples of single-nanowire transducers with exceptional sensitivity were shown, reproducible SiNW device fabrication with suitability to operate in a physiological environment remains as a major challenge. For our studies, different kinds of kinked SiNWs, resembling a nanoscale probe, were synthesized in a bottom-up manner comprising SiNWs with p-doped, intrinsic and n-doped intersection at the kink. We screened numerous SiNW sensors to overcome at first the existing lack of statistical data on this matter. Optimizing the device assembly process allowed furthermore, to significantly increase the share of highly sensitive SiNWs, meaning below 6 mV (3σ) signal resolution, from 17 % to 29 % at 0.4 V gate voltage. Also the impact of SiNW post-treatments as well as so-called SiNW regrowth were evaluated. Device characterization was done in dry and liquid-gate configuration. A SiNW surface band structure reconstruction supported by electrical measurements proved also the central role of surface Fermi-level pinning. We discuss additionally, how the Fermi-level pinning can be adjusted by appropriate surface treatments considering also a physiological environment.

Resume : All Gate Around (AGA) silicon (Si) Nanowire Transistors (NWTs) have the ultimate electrostatic integrity and are considered as suitable candidates for 5nm CMOS technology and beyond [1]. Their operation is governed by strong quantum confinement effects and non-equilibrium quasi-ballistic transport. Therefore the Ensemble Monte Carlo transport simulations are the best vehicle for studying of their performance. Here we report a comprehensive EMC simulation study of NWTs suitable for 5nm CMOS technology generation. The quantum confinement effects are properly taken into account in the MC simulations using the effective quantum potential approach based both on the solution of the Poisson-Schrodinger (PS) equation and on the Density Gradient (DG) algorithm. The impact of the NWT cross sectional shape, channel orientation and strain are taken into consideration. The simulations are carried out with GARAND (Synopsys). Due to the heavy computational requirements only single NWT are simulated using the EMC approach. Multi-channel NWTs with complex contact arrangements are simulated using the Drift Diffusion (DD) approach meticulously calibrated to the EMC simulations. The study concludes with the optimal NWT design, meeting the requirements for the 5nm CMOS technology and beyond. The DD simulations are also used to evaluate the statistical variability in the multichannel NWTs.

Resume : Geometric and compositional modulations have been key parameters of control to engineer the band profile in germanium/silicon (Ge/Si) heteronanowires (NWs) to achieve artificially-designed electronic, photonic, and phononic properties and functionalities. This has been achieved mainly by alternating the feeding precursors during a uniaxial growth of Ge/Si NWs. In this work, a self-automated growth of Ge/Si hetero island-chain nanowires (hiNWs), consisting of wider c-Ge islands connected by thinner c-Si chains, has been accomplished via an indium (In) droplet mediated transformation of uniform amorphous a-Si/a-Ge bilayer thin films, via an in-plane solid-liquid-solid (IPSLS) growth mode developped in our previous works [1-3]. The surface-running In droplet enforces a circulative hydrodynamics in the nanoscale droplet, which can modulate the absorption depth into the amorphous bilayer and enable a single-run growth of a superlattice-like hiNWs, without the needs for any external manipulation. Meanwhile, the separation and accumulation of electrons and holes in the phase-modulated Ge/Si superlattice leads to a modulated surface potential profile that can be directly resolved by Kelvin probe force microscopy. This simple self-assembly growth and modulation dynamics can help to establish a powerful new concept or strategy to tailor and program the geometric and compositional profiles of more complex hetero nanowire structures, as promising building blocks to develop advanced nanoelectronics or optoelectronics. Finally, electric transport and optoelectronic applications based on the Ge/Si hiNWs will also be presented. References: 1. Yu, L.*, Alet, P.-J., Picardi, G. & Roca i Cabarrocas, P. An in-plane solid-liquid-solid growth mode for self-avoiding lateral silicon nanowires. Phys. Rev. Lett. 102, 125501 (2009). 2. Xue, Z., Yu, L.*, et al. Engineering island-chain silicon nanowires via a droplet mediated Plateau-Rayleigh transformation. Nature communications 7, 12836 (2016). 3. Zhao, Y., Yu, L.*, et al. Nanodroplet Hydrodynamic Transformation of Uniform Amorphous Bilayer into Highly Modulated Ge/Si Island-Chains. Nano Lett. 18, 6931–6940 (2018).

Resume : Group IV semiconductor heterojunction nanowires are potentially useful in electronic and thermoelectric applications. Growth of axial heterojunctions in nanowires is typically by switching gas precursors in the CVD growth process, no matter using the vapor-liquid-solid (VLS) or vapor-solid-solid (VSS) method. Here we present a new method to form heterojunctions in SiGe alloy nanowires by thermal oxidation. SiGe nanowires of 6 at.% Ge are grown in CVD. The nanowires are then oxidized in air at high temperatures, e.g. 700°C for several hours. Transmission electron microscopy analysis shows that not only an oxide layer is formed surrounding the nanowires, there is also an atomically abrupt axial heterojunction formed in nanowires. The concentration of Ge in the nanowires changes abruptly from 6% to 20% across the interface. Compositionally analysis shows that the oxide shell is composed of Si and O. It is therefore suggested that Ge atoms are ejected from the oxide shell and diffuse at the interface between the oxide shell and nanowire core. The AuGeSi eutectic liquid at the nanowire tip is a reservoir of the diffusing Ge atoms; as a result, the ratio of Ge to Si concentration in the AuGeSi eutectic liquid is increased. Once the concentration of the solute atoms reaches the solubility limit, a SiGe bilayer of a higher Ge concentration is precipitated at the liquid/solid interface. The two sections of the SiGe heterojunction nanowire have different thermal conductivities, so that such a compositionally abrupt heterojunction nanowire may be useful for thermal rectifying applications. We will also discuss the application of this growth mechanism for the formation of Ge-rich SiGe dots embedded in a SiGe thin film of a lower Ge content.

Resume : The creation of high-quality junctions in nanostructured semiconductor materials is the aim of several studies regarding physical and chemical doping methods. In particular, the surface functionalization with specific dopant atoms is arising as a promising way to induce formation of thin and conformal junctions. In this presentation, a gas phase antimony deposition on Ge (100) surface is studied, discovering for the first time an antimony self-limiting behavior to form a monolayer (ML). The process window for ML formation was characterized as a function of time and temperature. The ML structure was therefore investigated by synchrotron radiation angle Resolved X-Ray Photoelectron Spectroscopy showing the presence of oxidized Sb over a very thin layer of Ge oxide. Interestingly, native Ge oxide is reduced during the formation process without the need of strong acid pre-treatments. ML effectiveness as a dopant source was then demonstrated: by performing further thermal annealing in equilibrium conditions, Sb undergoes chemical reduction and it is released and diffused into the bulk forming the junction. Finally, the Sb monolayer was processed by a strongly out-equilibrium diffusion process i.e. Pulsed Laser Melting technique using the Sb ML as a dopant source: a junction with 2 10^20 cm^-3 Sb surface concentration and 100% electrically active dopant was obtained.

Resume : Owing to its high carrier mobility as well as compatibility with silicon-based technology, germanium recently attracted a renewed interest for its high performances in various technological area such as photonics, nano- and opto-electronics. Nevertheless, Ge-based devices often require both very high (>1e20 cm^-3) and ultra-shallow doping, which are challenging for most of dopants due to their low solubility and high diffusivity. For this purpose, pulsed laser melting (PLM) is the most promising technique, being able to promote ultra-fast liquid phase epitaxial regrowth allowing both enhancement of incorporation together with diffusion confinement, at the same time. Our latest experimental results on p- and n-type doping of Ge or Ge-on-Si as obtained by performing PLM in different conditions will be presented. In particular, the talk will be focused on possible issues (like contaminations, clustering and thermal stability) along with different strategies employed to improve electrical activation and to meet different applications: for example, PLM combined with ion-implantation or in-situ doping. Fundamental information about non-equilibrium diffusion or strain evolution will be discussed thanks to advanced chemical (1D and 3D), electrical and structural characterizations with nanometer resolution.

Resume : We create ambipolar quantum dots in planar silicon nanoscale transistors. We first investigate the conformity of Al, Ti and Pd nanoscale gates by means of transmission electron microscopy [1]. Next we define low-disorder electron quantum dots with Pd gates [2], and depletion-mode hole quantum dots in undoped silicon [3]. For the latter we use fixed charge in a SiO2/Al2O3 dielectric stack to induce a 2DHG at the Si/SiO2 interface. The depletion-mode design avoids complex multilayer architectures requiring precision alignment and allows directly adopting best practices already developed for depletion dots in other material systems. Finally, I will show ambipolar charge sensing: we have fabricated a single-electron transistor next to a single-hole transistor, and tuned both quantum dots to simultaneously sense charge transitions of the other quantum dot. Using active charge sensing the single-hole transistor can detect the few-charge regime in the electron quantum dot. [1] P. C. Spruijtenburg et al., Nanotechnology, (2018). [2] M. Brauns et al., Scientific Reports 8, 5690, (2018). [3] S. V. Amitonov et al., Applied Physics Letters 112, 023102 (2018).

Resume : During the last decade, the non-thermal plasma (NTP) techniques proved to be an efficient way to synthesize large amounts of silicon nanocrystals (Si-NCs) and also selectively terminate them, which is the key parameter related to Si-NCs PL properties. However, common NTP techniques require expensive equipment, which limits their mass exploitation. Here, we present results based on the treatment of Si-NCs in water by electric discharge in the regime of positive transient spark. In contrast to the traditional techniques, the transient spark (discharge – 0.5 mA) can be generated using a DC power source (6 kV) at atmospheric pressure, without the need for a high frequency source and a gas chamber. We showed that after 30 min of NTP treatment, the initial PL spectra of NCs increased its intensity about 3 times and red-shifted by about 25 nm with simultaneous proportional improvement of NCs quantum yield. The modified samples were stable for several weeks at minimum. A similar, albeit slightly weaker, improvement of Si-NCs PL properties was also detected when Si-NCs were treated only by NTP premodified water. FTIR characterization of surface chemistry showed that the plasma presence caused the passivation of dangling bonds and creation of surface nitrate-aqueous complexes. Thus, our results point to a simple yet effective way to enhance and stabilize the PL properties of Si-NCs in water-based media, which is an imperative for biological applications of this non-toxic material.

Resume : Si nanocrystals (NC) with tunable bandgap are a promising absorber material for an all crystalline Si tandem solar cell. Si NC in a SiC matrix are fabricated through plasma enhanced chemical vapor deposition of Si/SiC multilayers (ML) and a subsequent annealing step at 1100°C in which the crystalline phases are formed. As recombination of carriers is still dominating the electrical quality of the material, a new approach - the passivation of the NC? surface - was carried out by in situ oxidation. Additionally, the furnace anneal was replaced by a rapid thermal anneal step. The new process results in a significant quality improvement of the material towards its use as an absorber material for future all crystalline Si tandem solar cells. The successful oxidation in the form of Si-O bonds is demonstrated by FTIR and EDX spectroscopy yielding about 7 at% oxygen in the as-deposited as well as the annealed ML. The size of the nanocrystals is determined by GIXRD and yields average grain sizes in the range of the sublayer thicknesses (3-9 nm). SEM cross sections prove that the Si/SiC ML structure was preserved during the crystallization process at 1100°C. The ratio between crystalline Si and crystalline SiC in the oxidized Si/SiC ML is significantly increased for rapid annealing, which is explained by the kinetics of nucleation and crystal growth and by impeded diffusion due to oxygen incorporation. Finally we find a significantly enhanced photoluminescence signal around 630 nm.

Resume : The electrical and optical properties of semiconductor nanocrystals (NCs) can be controlled, in addition to size and shape, by doping. However, such a process is not trivial due to the high formation energy of dopants there and the tendency to expel any imperfections to NC's surface. Detecting effects of doping at the single NC level and relating it to macroscopic (e.g., optical) is an important, yet challenging, goal to achieve. In this talk I will present correlative local-probe (scanning tunneling spectroscopy and atomic force microscopy related techniques) and macroscopic (transport and optical spectroscopy) studies on single Cu2S and Si NCs and their ensembles. I will first present a new method for achieving p-type doping of Cu2S NCs, by which Cu vacancies are formed upon annealing at moderate temperatures, yielding free holes. This method enables patterned doping by applying focused laser illuminations. Next, I will discuss B and P co-doped Si NCs. Our tunneling spectra revealed two in-gap band-states, one close to the conduction band edge and the other to the valence band edge, attributed to the P and B dopant levels, respectively. The energy separation between these dopants states exhibit the quantum-confinement effect with NC size and correlate well with the photoluminescence (PL) peak energy. Finally, I will present evidence for Si NC surface doping by capping with certain types of ligands. Here we demonstrate that the PL is red-shifted via surface functionalization with alkynyl(aryl) surface groups, while the tunneling spectra on single NCs reveal the formation of new in-gap states adjacent to the conduction band edge of the Si NC. Our results from both Si NC systems indicate that PL takes place via the in-gap states. This research was dome in collaboration with: Prof. Isaac Balberg, Prof. Uri Banin and Dr. Doron Azulay (Hebrew University); Prof. Jonathan Veinot (University of Alberta, Canada); Prof. Bernhard Rieger and Dr. Arzu Angi (Technische Universität München, Germany)

Resume : In small (~2nm) Silicon nanocrystals (SiNCs), large fraction of the nanocrystal atoms resides on the surface (~50%), where they are exposed to either the environment or attached ligands. Si bonding chemistry is purely covalent and therefore all the bond-participating electrons contribute into the band-structure of the nanocrystal. In small NCs, the quantised k-vector is not a good quantum number defining the available states in the NC and hence also the band-structure is not anymore well defined as in bulk. Nevertheless, one can still study the so called "fuzzy" band-structures, coined in Ref.1 to study the role of ligands on opto-electronic properties of SiNCs. From our previous Tight Binding [2,3] and recent Density Functional Theory [4] approaches, we found that covalently bonded ligands do indeed strongly modify the band-gap, Fermi energy, oscillator strength/ radiative rate and the overall density of states in the "bands". In our study we focus on ligands that enhance emission and/or absorption of light - properties that are in high demand in Si-based materials. References: [1] P. Hapala et al., Phys. Rev. B 87 (2013) 195420. [2] A. N. Poddubny and K. Dohnalova, Phys. Rev B 90 (2014) 245439; [3] K. Dohnalova et al., Light: Science & Applications 2 (2013) e47. [4] K. Dohnalova, P. Hapala and I. Infante, manuscript in preparation 2019.

Resume : Light harvesting through excitonic energy transfer has inspired significant research efforts to realize and design energy transfer based light-harvesting systems for solar energy conversion, optoelectronic devices, and biomedical applications. Here, we demonstrate a promising structure and key factors to achieve high-efficiency hybrid heterojunction solar cells, which can facilitate development for potential industrial production. By combining nanocrystalline Si quantum dots (nc-Si QDs) to organic/Si nanostructure hybrid solar cells, we have achieved 14.06% efficiency in Si/PEDOT:PSS hybrid solar cells. The efficiency enhancement is mainly based on the energy transfer phenomenon of nc-Si QDs to generates excess electron-hole pairs in the Si nanostructure/PEDOT:PSS region for excellent carrier separation. The main reason is due to the energy transfer effect of nc-Si QDs, which absorb UV light and convert it to NRET (Nonradiative energy transfer) and RET (Radiative energy transfer). Additionally, we enhanced the NRET effect by changing the ligand passivation length of nc-Si QDs. We found that shorter ligand length has higher energy transfer rate, therefore, higher short-circuit current and higher efficiency was achieved. These hold the promise for developing energy transfer managing, low-cost and high-efficiency photovoltaic cells in the future.

Resume : The most characteristic luminescence band of Si-based nanostructures (called S-band, situated in the yellow to near-infrared spectral range) is known for its slow multiexponential decay kinetics (of the order of fraction of ms, even at room T). The decay shape is most often fitted by the Kohlrausch stretched exponential function. Many different theories were proposed during the three decades to explain the origin of this shape, including: exciton transfer, inhomogeneous size or shape distribution, non-radiative (NR) paths distribution etc. Here we summarize our recent work addressing the optimization of experimental properties [1], mathematical treatment [2] and extensive experimental studies [3]. Changes of onset and decay kinetics under variable excitation power can be used to determine absorption cross-section (ACS) [4,5]. Internal quantum yield (QY) can be determined by modification of local density of photonic states (Purcell effect), which influence the radiative rate but not the NR rate, therefore the decay kinetics can probe the changes of the nanoparticle environment, for example, an interaction with local plasmons of metalic nanostructures (spaced in a close proximity to the nanoparticle). Finally, a combination of knowledge of ACS, internal and external QYs, allows us to estimate a relative distribution of dark and bright nanocrystals. [1] Greben, Valenta: Rev. Sci. Inst. 87 (2017) 126101, [2] Greben et al. Appl. Spectr. Rev. 53 (2018), [3] Greben et al. J. Appl. Phys. 122 (2017) 034304, [4] Valenta et al. Appl. Phys. Lett. 108 (2016) 023102, [5] Greben et al. Beil. J. Nanotech. 8 (2017) 2315.

Resume : ‘Silicon nanocrystals (SiNCs)’ exhibit optoelectronic and chemical characteristics that make them particularly attractive as active materials in photovoltaic cells, luminescent solar concentrators, LEDs, battery anodes, as well as a variety of therapeutic and imaging modalities. Because SiNCs are biocompatible and toxic-metal-free, they also provide a convenient circumvention of legislation that limits the use of status quo metal-based quantum dots in consumer products. Not surprisingly, nanoparticle internal structure influences optical, chemical and material properties and it is essential to elucidate the NC core structure if they are to realize their full potential. By applying a diverse array of complementary techniques including 29Si solid-state NMR, FTIR, XPS, XRD, and TEM to the characterization of monodisperse hydride-terminated SiNCs (H-NCs; d = 3 to 64 nm) we have discovered they exhibit size-dependent layered structures consisting of surface, subsurface, and core silicon species. This presentation will include a detailed discussion of our investigation and outline the potential impact of these structural features on the widespread deployment of SiNCs in the applications noted above.

Resume : Silicon nanocrystals are now a recognized light emitter, chatacterized by photoluminescence characteristics improved by several orders of magnitude when compared to bulk silicon. In macroscopic measurements of ensembles, their photoluminescence is nearly universally reported to decay in time following the stretched-exponential function, as a result of the presence of fluctuations of photoluminescence lifetimes [1]. However, in single silicon nanocrystals, this decay was observed to be single-exponential [2]. Various scenarios have already been proposed to explain the origin of the stretched-exponential photoluminescence decay depending on the particular sample and mode of characterization [3] and in real-life situations, a mixture of these scenarios is very likely to occur. Here, we show that when applying appropriate experimental conditions and a simple fitting procedure, the photoluminescence decay of an ensemble of silicon nanocrystals can indeed be single-exponential, in complete contrast to the omnipresent stretched-exponential. Our approach confirms that silicon nanocrystals are a robust emitter, since the single-exponential ensemble decay implies the existence of a single, strongly preferential lifetime in the ensemble, which is, moreover, stable in time. Additionally, it allows us to access fundamental optical properties, such as the radiative rates. [1] Greben, M. et al. Applied Spectroscopy Reviews (2019), to be published. [2] Sangghaleh, F. et al. Nanotechnology (2013), 24(22), 225204. [3] Sangghaleh, F. et al. ACS Nano (2015), 9(7), 7097.

Resume : Bulk silicon (Si) is a well-known material used in photovoltaics and microelectronics for its abundance and adaptability in semiconductor industry. However, the drawback, indirect bandgap, leads to limits of its applicability as photo-emitters. In the past three decades, the raise of eco-awareness and search for sustainable development caused intensive research of Si photonics and Si nano-technology. Owing to the joint effects of quantum confinement and contribution from the surface states brought by the ligands, Si nanocrystals (NCs) can be a competitive light emitting candidate with advantages of non-toxicity, tunable band-gap, fast radiative rate, etc. Si-NCs with various sizes and surface ligands are investigated by micro-photoluminescence (PL). Si-NCs are excited by polarized laser beam and emit in visible spectral range. We investigate emission life-time and PL anisotropy by time-correlated single photon counting (TCSPC). We show that the character of the PL anisotropy suggests preference of the transition from single localized states on the surface of Si-NCs for both, organic and oxide passivations. The depolarization mechanisms could be ascribed to a degenerate transition, an energy trap or particle diffusion. The smaller Si-NCs with organic ligands have longer depolarization time due to discrete energy levels. For the size dependency with the same ligand, Si-NCs with larger core size tend to lose polarization faster that implies appearance of degenerated states.

Resume : Featuring narrow and stable emission at 1.5um, coincident with the absorption minimum of optical fibers, Er- doped materials are broadly explored for telecommunication applications. The weak absorption of Er, can greatly benefit from sensitizers. Past research has shown that excitation of Er3+ ions embedded in SiO2 solid-state matrix can be increased by up to three orders of magnitude by Si nanocrystals. The indirect excitation of Er3+ via Si nanocrystals can be achieved by different pathways. Here, we investigate the impact excitation mechanisms in detail by means of time-resolved photoluminescence spectroscopy. We explicitly demonstrate that the free carrier impact excitation mechanism is activated as soon as the free carriers attain sufficient excess energy. The “hot” carriers with the above-threshold energy can be created upon optical pumping in two ways: either upon absorption of (i) a single photon with an energy exceeding a certain threshold hν > Eth or (ii) following absorption of multiple photons of lower energy within the same nanocrystal, hν < Eth, followed by an Auger recombination of the generated multiple low-energy e-h pairs. We show that the impact excitation dynamics are identical in both cases.

Resume : The visible emission of silicon-nanocrystals (Si-NCs) is a milestone in the field of nanotechnologies and it is currently investigated for both fundamental and application aspects. The requirement of emission tunability, brightness and photostability is crucial to improve the performances of silicon-based devices (photovoltaic cells, sensors, light detectors) and stimulates, therefore, the development of convenient synthesis methods able to control the physical/chemical properties of materials [1]. To this aim, pulsed laser ablation (PLA) is a versatile top-down strategy that is normally adopted to realize a large variety of luminescent nanomaterials. In the present work, we use PLA in aqueous solution to produce Si-NCs surrounded by an amorphous SiO2 layer due to Si oxidation. These nanostructured systems emit a μs decaying band centered around 1.95 eV, that is associated with the radiative recombination of quantum-confined excitons [2]. The emission properties are strongly influenced by changing the pH of the aqueous solution: i) on decreasing the pH from 10 to 1, the quantum efficiency increases more than one order of magnitude; ii) in acid environment, the emission acquires stability over the investigated time range of about one week. On the basis of our results we propose that H+ ions remove non radiative defects, their structure (distorted Si- -Si bonds) being peculiar to the interlayer between Si-NC and the SiO2. Overall, our experiments demonstrate that PLA, in combination with the control of solvent properties, is a viable route towards higher emission efficiency and stability of Si-NCs. [1] D. Timmerman, J. Valenta, K. Dohnalova, W. D. A. M. de Boer, T. Gregorkiewicz, Nature Nanotech. 6, (2011), 710. [2] M. Cannas, P. Camarda, L. Vaccaro, F. Amato, F. Messina, T. Fiore, M. L. Vigni, Phys. Chem. Chem. Phys. 20, (2018), 10445.

Resume : Silicon microfabrication technologies, which has enabled the integration of billions of transistors on a single chip for use in computers or in mobile phones, are now reaching dimensions of only a few nanometers – almost the size of biomolecules. Nano-scaled devices therefore enable sensing of proteins or DNA at very low concentration in a liquid solution, sometimes down to the single molecule level. At the same time, integration of hundreds of such sensors on a single chip properly functionalized with different antibodies would allow a full palette of different proteins or DNA strands to be sensed targeting a particular medical situation or diagnosis. In this talk I will review our work on chip-based sensors which have been explored in various collaborative projects: (i) Nanowire (or nanoribbon) sensors. This is an electrical sensor working essentially as a MOS transistor with an open gate, onto which antibodies have been immobilized, sensing the charge of hybridizing proteins or DNA. (ii) Electro-kinetic capillary sensor. This uses a capillary with functionalized inner surface. By flowing the electrolyte solution through the capillary, an electro-kinetic potential is generated which changes upon binding of target molecules. Results and applications of these techniques towards detection of various proteins, DNA strands and allergens will be reviewed. Finally, very recent results towards exosome detection will be demonstrated addressing early detection and monitoring of cancer.

Resume : Biomolecule translocation through solid-state nanopores is a generic platform for various detection and sorting modalities. A notable example is DNA sequencing implemented by electrical detection of the ionic current when a molecule linearizes during the translocation. In this work we fabricated an array of nanopores in a silicon membrane and realized a parallel optical detection of translocation events. Moving from a single nanopore to an array is desirable for high throughput applications. Double-stranded DNAs were labeled with fluorophores and their presence in the nanopores while moving under external bias was recorded by photoluminescence traces with a high time resolution (1 ms). Pores are spaced a few micron apart, allowing for simultaneous imaging and extraction of translocation events for single molecules from many pores at once. Different length DNAs were investigated under varying external bias conditions and their capture and translocation statistics obtained. As a result a clear size-dependence of the capture rate is demonstrated, attributed to the thermophoretic effect [1]. Translocation for longer molecules can be selectively blocked, allowing size discrimination. We present analytical treatment of the molecule capture and translocation dynamics, where drift and diffusion processes lead to non-stationary transients [2]. Our results demonstrate that by combining a thermal and a potential gradient at the nanopores, such large nanopore arrays can potentially be used as a low-cost, high-throughput platform for molecule sensing and sorting. [1] M. Zhang, C. Ngampeerapong, D. Redin, A. Ahmadian, I. Sychugov, J. Linnros, Thermophoresis Controlled Size-Dependent DNA Translocation through an Array of Nanopores, ACS Nano 12, 4574-4582 (2018). [2] I. Sychugov, M. Zhang, J. Linnros, Non-stationary analysis of molecule capture and translocation in nanopore arrays, submitted (2019).

Resume : Porous materials are of scientific and technological importance due to the presence of controllable dimensions at nanometer scale. Research efforts in this field have been driven by the rapidly emerging applications such as biosensors, drug delivery, gas separation, energy storage and fuel cell technology. This research offers exciting new opportunities for developing new strategies and techniques for the synthesis and applications of these materials. Perfect control of the structure parameters of porous materials is of fundamental importance in order to tailor and verify their properties. Porous silicon, prepared by an electrochemical process , or electroless metal assisted etching , has also gained interest in research for many applications which have a demand for mechanical and chemical stability as well as the order of the pores. The structures are manufactured by applying a (photo-)electrochemical procedure of n- or p-doped silicon wafers. Depending on the lithographic prestructured surface and the doping concentration of the material different pore sizes and geometric arrangement can be obtained. The wafers are etched in aqueous fluoric acid-containing electrolytes while voltage and light intensity are controlling the pore diameter during the process. The current available pore diameters at SmartMembranes can differ from 15 nm up to 18 µm. Potential applications of porous silicon have been proposed in the fields of micro-processing, gas measurement, photonic crystals, biotechnology and many others. Examples are short-time optical filters, 2D and 3D photonic crystals and optical and capacitive sensors. The presence of microplastics in the environment and our food-chain is of growing concern. This has led to increased testing for the presence of microplastics in a variety of samples including bottled, ocean and fresh water, which has brought about tougher legislation to limit the amount of plastics entering the ecosystem. Fourier Transform Infrared (FTIR) and Raman spectroscopies have long been used for the analysis of polymers. It has been shown that porous silicon is suitable for both analysis methods, acting as a filter as well as the analysis substrate at the same time.

Resume : Silicon is one of the most attractive candidate for next-generation Li-ion battery anodes thanks to its high gravimetric capacity and safer operating potential. However the Li/Si alloying reaction induces a large volume change causing material pulverization and thus rapid capacity fading. Use nanostructured silicon is one of the key of success for improving Si-based negative electrode performance. Among main efforts made to produce efficient nanostructured silicon electrodes, good results are observed for porous structures. In this study, the aim is to further understand how the porous structure impacts the electrochemical performance of porous Si-based Li-ion negative electrodes. In order to create porous structure, the electrochemical etching method has been applied to p-type silicon wafer in presence of HF-based electrolyte. This method is easy to handle, quick and highly reproducible. The porosity of silicon relies on the nature and concentration of the dopant in the wafer as well as on its conductivity. In order to limit the dopant effect on the further electrochemical behavior, the chosen strategy was to post modify the porous membrane by flash annealing. This post treatment allows reaching highly open structures going from mesoporous to macroporous materials without changing the chemical nature nor the crystallographic orientation. The galvanostatic measurements show a positive impact of highly opened structures on capacity and coulombic efficiency likely due to better electrolyte accessibility to the electrode. This results are encouraging and allow us to imagine further optimization.

Resume : Silicon (Si) is a promising anode material of lithium (Li) ion batteries because of its exceptional specific capacity, which is ten times higher than commercial graphite materials. However, the Si material swells up to four times during Li insertion, and the severe volumetric expansion leads to mechanical fractures causing electrochemical degradation. Previous studies have been introduced nanostructured Si materials to prevent the mechanical fractures, but the large surface area occurs side reaction such as increased formation of solid electrolyte interphase (SEI) layer. Therefore, it is necessary to design robust Si structures with less surface area and to understand the mechanical behavior of Si anode during lithiation for the better design. In this study, we estimated induced stress of Li-Si alloy lithiated from crystalline Si (c-Si) surface using vertically aligned c-Si/Cu bimorph plate and its fracture resistance. Ex situ scanning electron microscopy (SEM) study and analytical and numerical modeling have found that the induced stress and its mechanical behavior. As the results of lithiation, the induced stress is estimated almost half value of the widely considered yield strength of lithiated Si, 1 GPa. Moreover, such low stress in the plate structures allows lithiation of ~2 μm thick plate structure without fracture. This dimension is six times larger than the critical dimensions of fracture reported in previous studies. The results presented not only contribute to understanding the fundamental behavior of lithiated Si anode, but also provide guidance for the optimal design of high capacity Si anodes without fracture.