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In this paper, it is shown for the first time that Ge nanocrystals (Ge NCs) obtained via solid-state dewetting of amorphous GOI can be used as the active absorbers embedded in a silicon dioxide matrix of metal–insulator–semiconductor photodetectors (MIS PD). The Ge NCs have been obtained by a combination of Ge deposition by molecular beam epitaxy (MBE) on tunnel thermal silicon oxide and solid-state dewetting processes. The structural and morphology characterizations performed using high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) show that the Ge NCs embedded in SiO2 of MIS PD are monocrystalline, homogeneous and have well-defined shape and high density suitable for optoelectronic applications. The I–V and photocurrent measurements performed on these innovative structures show that the Ge NCs contribute efficiently in the electrical transport by increasing the current density via creating an intermediate conduction step in the MIS structure and enhance the photocurrent via photogeneration of new carriers. We have observed that for the structure with Ge NCs, the photocurrent increases 10 times at reverse bias Vg = − 1 V when it is illuminated. These results indicate that the crystalline Ge NCs obtained via solid-state dewetting can be integrated with optoelectronics and photonics technologies to produce new high-performance optoelectronic devices fully compatible with complementary oxide metal (CMOS) technology.

We report a new strategy for the ordering of 2D arrays of Ge and Au nanodots on a silicon wafer using a patterned titanium oxide layer. In a rst step, a TiO 2 layer is prepared by block-copolymer-micelles-assisted sol-gel deposition on a full Si wafer, followed by a thermal annealing. The process leads to hexagonally positioned perforations of homogeneous size and spacing. In a second step, these TiO 2 Inorganic NanoPatterns (INPs) are used as templates for the organization of Ge and Au nanodots. Germanium adatoms deposited by Molecular Beam Epitaxy on INPs, diuse and self-assemble into nanodots, located within the INPs pores. They form homogeneous sub-20 nm Ge nanodots in epitaxy on the silicon substrate and regularly distributed with one dot per perforation. The same approach is used for the formation of Au nanodots. In this case, a gentle mechanical polishing is requiered to suppress the dots seating at 1 the top of the TiO 2 network. The process developed in this study paves the way to the large scale self-organisation of quantum dots that are highly interesting for various applications, such as opto-electronics, and microelectronics.

We report the high performances of Metal-Insulator-Semiconductor Photodetectors (MIS PD) made with crystalline Ge nanocrystals (Ge NCs) as the active absorbers embedded in a silicon dioxide matrix. The Ge NCs have been obtained by a combination of Ge deposition by Molecular Beam Epitaxy (MBE) on tunnel thermal silicon oxide and solid state dewetting processes. Ge NCs structure and morphology are characterized by High Resolution Transmission Electron Microscopy (HRTEM) and Scanning Electron Microscopy (SEM). The photocurrent generation is determined by I-V spectroscopy and Photocurrent spectroscopy. We evidence the role of high quality Ge NCs on photocurrent and explain the high sensitivity of MIS photodetector as a result of transport mechanisms via photoexcited Ge NCs.These results indicate that the crystalline Ge NCs obtained via solid state dewetting can be integrated with opto-electronics and photonics technologies to produce new high performance optoelectronic devices fully compatible with Complementary Oxide Metal (CMOS) technology.

International audience; We report a novel approach for engineering tensely strained Si layers on a relaxed silicon germanium on insulator (SGOI) film using a combination of condensation, annealing, and epitaxy in conditions specifically chosen from elastic simulations. The study shows the remarkable role of the SiO2 buried oxide layer (BOX) on the elastic behavior of the system. We show that tensely strained Si can be engineered by using alternatively rigidity (at low temperature) and viscoelasticity (at high temperature) of the SiO2 substrate. In these conditions, we get a Si strained layer perfectly flat and free of defects on top of relaxed Si1–xGex. We found very specific annealing conditions to relax SGOI while keeping a homogeneous Ge concentration and an excellent thickness uniformity resulting from the viscoelasticity of SiO2 at this temperature, which would allow layer-by-layer matter redistribution. Remarkably, the Si layer epitaxially grown on relaxed SGOI remains fully strained with −0.85% tensile strain. The absence of strain sharing (between Si1–xGex and Si) is explained by the rigidity of the Si1–xGex/BOX interface at low temperature. Elastic simulations of the real system show that, because of the very specific elastic characteristics of SiO2, there are unique experimental conditions that both relax Si1–xGex and keep Si strained. Various epitaxial processes could be revisited in light of these new results. The generic and simple process implemented here meets all the requirements of the microelectronics industry and should be rapidly integrated in the fabrication lines of large multifinger 2.5 V n-type MOSFET on SOI used for RF-switch applications and for many other applications.

Despite the large literature focused on the growth of graphene (Gr) on 6H-SiC(0001) by chemical vapour deposition (CVD), some important issues have not been solved and full wafer scale epitaxy of Gr remains challenging, hampering applications in microelectronics. With this study we shed light on the generic mechanism which produces the coexistence of two different types of Gr domains, whose proportion can be carefully controlled by tuning the H2 flow rate. For the first time, we show that the growth of Gr using CVD under H2/Ar flow rate proceeds in two stages. Firstly, the nucleation of free-standing epitaxial graphene on hydrogen (H-Gr) occurs, then H-atoms eventually desorb from either step edges or defects. This gives rise, for H2 flow rate below a critical value, to the formation of (6x6)Gr domains on 6H-SiC(0001). The front of H-desorption progresses proportionally to the reduction of H2. Using a robust and generic X-ray photoelectron spectroscopy (XPS) analysis, we realistically quantify the proportions of H-Gr and (6x6)Gr domains of a Gr film synthetized in any experimental conditions. Scanning tunnelling microscopy supports the XPS measurements. From these results we can deduce that the H-assisted CVD growth of Gr developed here is a unique method to grow fully free-standing H-Gr on the contrary to the method consisting of H-intercalation below epitaxial Gr on buffer layer. These results are of crucial importance for future applications of Gr/SiC(0001) in nanoelectronics, providing the groundwork for the use of Gr as an optimal template layer for Van der Waals homo-and hetero-epitaxy.

Large-scale, defect-free, micro- and nano-circuits with controlled inter-connections represent the nexus between electronic and photonic components. However, their fabrication over large scales often requires demanding procedures that are hardly scalable. Here we synthesize arrays of parallel ultra-long (up to 0.75 mm), monocrystalline, silicon-based nano-wires and complex, connected circuits exploiting low-resolution etching and annealing of thin silicon films on insulator. Phase field simulations reveal that crystal faceting and stabilization of the wires against breaking is due to surface energy anisotropy. Wires splitting, inter-connections and direction are independently managed by engineering the dewetting fronts and exploiting the spontaneous formation of kinks. Finally, we fabricate field-effect transistors with state-of-the-art trans-conductance and electron mobility. Beyond the first experimental evidence of controlled dewetting of patches featuring a record aspect ratio of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim$$\end{document}~1/60000 and self-assembled \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim$$\end{document}~mm long nano-wires, our method constitutes a distinct and promising approach for the deterministic implementation of atomically-smooth, mono-crystalline electronic and photonic circuits., Fabricating defect-free micro- and nano-circuits over large scales with controlled interconnections remains a challenge. Here, Bollani et al. show a dewetting strategy for engineering arrays of parallel Si-based nanowires up to 0.75 mm and complex interconnected circuits of monocrystalline Si on a chip.

In the present work, we report on the charge trapping properties of Germanium Nanocrystals (Ge NCs) self assembled on SiO2 thin layer for promising applications in next-generation non volatile memory by the means of Deep Level Transient Spectroscopy (DLTS) and high frequency C-V method. The Ge NCs were grown via dewetting phenomenon at solid state by Ultra-High Vacuum (UHV) annealing and passivated with silicon before SiO2 capping. The role of the surface passivation is to reduce the electrical defect density at the Ge NCs-SiO2 interface. The presence of the Ge NCs in the oxide of the MOS capacitors strongly affects the C-V characteristics and increases the accumulation capacitance, causes a negative flat band voltage (VFB) shift. The DLTS has been used to study the individual Ge NCs as a single point deep level defect in the oxide. DLTS reveals two main features: the first electron traps around 255 K could correspond to dangling bonds at the Si/SiO2 interface and the second, at high-temperature (>300 K) response, could be originated from minority carrier generation in Ge NCs.

International audience; We present the photoluminescence (PL) emission of Silicon and Germanium nanocrystals (NCs) of different sizes embedded in two different matrices. Formation of the NCs is achieved via solid-state dewetting during annealing in a molecular beam epitaxy ultra-high vacuum system of ultrathin amorphous Si and Ge layers deposited at room temperature on SiO2. During the dewetting process, the bi-dimensional amorphous layers transform into small pseudo-spherical islands whose mean size can be tuned directly with the deposited thickness. The nanocrystals are capped either ex situ by silicon dioxide or in situ by amorphous Silicon. The surface-state dependent emission (typically in the range 1.74 eV-1.79 eV) exhibited higher relative PL quantum yields compared to the emission originating from the band gap transition. This red-PL emission comes from the radiative transitions between a Si band and an interface level. It is mainly ascribed to the NCs and environment features deduced from morphological and structural analyses. Power dependent analysis of the photoluminescence intensity under continuous excitation reveals a conventional power law with an exponent close to 1, in agreement with the type II nature of the emission. We show that Ge-NCs exhibit much lower quantum efficiency than Si-NCs due to non-radiative interface states. Low quantum efficiency is also obtained when NCs have been exposed to air before capping, even if the exposure time is very short. Our results indicate that a reduction of the non-radiative surface states is a key strategy step in producing small NCs with increased PL emission for a variety of applications. The red-PL band is then an effective tool for the quality assessment of NCs based structures. (C) 2017 Elsevier B.V. All rights reserved.

The dual action of oxide-related defects in the visible and infrared emission of germanium (Ge) self-assembled quantum dots (QDs) is discussed. The Ge particles were fabricated by solid-state dewetting on a thin layer of $${\hbox {SiO}}_{2}$$ . Subsequent surface passivation by amorphous silicon was carried out for several samples. All samples were encapsulated by $${\hbox {SiO}}_{2}$$ . Atomic force microscopy analysis indicates a linear relationship between the size of QDs and the initial thickness of the amorphous Ge films. The crystallization of the QDs was evidenced by transmission electron microscopy and Raman spectroscopy. Photoluminescence measurements show that the main visible emission is blue-green centred around 520 nm. The luminescence attributed to the radiative recombination of quantum-confined excitons is only observed when the surface is in-situ passivated prior to the deposition of the oxide matrix. The results of this work are helpful for optimizing the performance of the optoelectronic devices based on the infrared emission of Ge nanocrystals.

This study set out to modulate and simulate a new quantum dot solar cell GaAs 1-x Bi x /GaAs. In this work, we have treated several phenomena such as: short-circuit current (J sc ), open-circuit voltage (V oc ), the Forme-Factor (FF), conversion Efficiency ($\eta $) and Quantum Efficiency (EQE) of GaAs 1-x Bi x /GaAs QDSC, for optimization. It has been found that 30 layers of GaAs 0.95 Bi 0.05 /GaAs QD, provides a relative improvements of 6.86 % and 12.05% of J sc and $\eta $ respectively. The absorption range edge of the pin GaAs solar cell extended from 1060 nm to 1200 nm.

In this paper, we present simulations and the optimization of electrical and optical properties of GaAs Quantum Dots (QD) in a GaAs1-xPx matrix. Our results showed that 25 GaAs/GaAs1-xPx QD layers provide a relative enhancement of 41.34% and 75.40% of the short-circuit current and efficiency, respectively. With the same number of the QD layers, the External Quantum Efficiency (EQE) measurement shows that the absorption range edge of low energy photons has been extended from 875 to 1200 ​nm (ΔEQE ​= ​25%). The temperature effect has been studied for a different number of QD inserted. The optimal conversion efficiency of 25 QD layers is degraded from 23.50% to 18.70% by increasing the temperature from 273 ​K to 350 ​K. Moreover, the electrical features obtained and EQE measurements for GaAs/GaAs1-xPx QDSC have been compared with those obtained for In0.47Ga0.53As/GaAs0.86P0.14 QDSC, In0.167Ga0.53As/GaAs and GaAs reference cell, in order to show the better structure.

All-dielectric photonics is a promptly developing field of optics and material science. The main interest at visible and near-infrared frequencies is light management using high-refractive-index Mie-resonant dielectric particles. Most work in this area of research was done exploiting Si-based particles. Here, we study monocrys- talline Mie-resonant particles made of Ge-rich SiGe alloys with refractive index higher than that of Si. These islands are formed via solid state dewetting of SiGe flat layers by using two different processes: i) dewetting of monocrystalline SiGe layers (60-80% Ge content) obtained via Ge condensation of SiGe on silicon on insulator, ii) dewetting of a SiGe layer deposited via molecular beam epitaxy on silicon on insulator and ex-situ Ge con- densation, forming a Ge-rich shell surrounding a SiGe-core. Using high-spatial-resolution Raman microscopy we monitor Ge content x and strain of flat layers and SiGe-islands. We observe strain relaxation associated with formation of trading dislocations in the SiGe islands compared to the starting SiGe layers, as confirmed by TEM images. For initial high Ge concentration in the flat layers, the corresponding Ge content in the dewetted islands is lower, owing to diffusion of Si atoms from Si or SiO 2 into SiGe islands. The Ge content also varies from particle to particle on the same sample. Size and shape of the dewetted particles depend on the fabrication process: thicker initial SiGe layers lead to larger particles. Samples with narrow island size distribution display rather sharp Mie resonances in the 1000-2500 nm spectral range. Larger islands display Mie resonances at longer wavelength. Positions of the resonances are in agreement with the theoretical calculations in the discrete dipole approximation.

International audience; In this work we report a modeling and optimization of a novel core (p-GaN)-multishell (i-InxGa1-xN/i-GaN/n-Al0.1Ga0.9N/n-GaN) coaxial NWs with a total length of about 3 μm and a radius of 0.19 μm. The variation of Indium composition in the i-InxGa1-xN active layer shows that x = 15% is the optimal composition that gives a good short circuit current and efficiency of about 2.24 mA/cm2 and 2.93%, respectively. Beyond x = 35% the structure starts to present deteriorations. Moreover, the optimal x composition founded provides a gain of about 12.9% of EQE in the mid-infrared part. The study of the variation of the total current density along the nanowire demonstrates that the maximum of this latter is located in AlGaN cladding shell layer and at the interfaces between the i-region and AlGaN cladding shell layer. In other hand, our results show that our structure presents an optimal efficiency of about 3.5% at a temperature of 275 K with the same AM1.5G solar spectrum.

International audience; Selective oxidation of the silicon element of silicon germanium (SiGe) alloys during thermal oxidation is a very important and technologically relevant mechanism used to fabricate a variety of microelectronic devices. We develop here a simple integrative approach involving vapor−liquid−solid (VLS) growth followed by selective oxidation steps to the construction of core−shell nanowires and higher-level ordered systems with scalable configurations. We examine the selective oxidation/ 16 condensation process under nonequilibrium conditions that gives rise to spontaneous formation of core−shell structures by germanium condensation. We contrast this strategy that uses reaction-diffusion-segregation mechanisms to produce coherently strained structures with highly configurable geometry and abrupt interfaces with growth-based processes which lead to low strained systems with nonuniform composition, three-dimensional morphology, and broad core−shell interface. We specially focus on SiGe core−shell nanowires and demonstrate that they can have up to 70% Ge-rich shell and 2% homogeneous strain with core diameter as small as 14 nm. Key elements of the building process associated with this approach are identified with regard to existing theoretical models. Moreover, starting from results of ab initio calculations, we discuss the electronic structure 23 of these novel nanostructures as well as their wide potential for advanced device applications.

Si-based nanoarchitectures are formed with unprecedented precision and reproducibility via templated dewetting of thin SOI., Dewetting is a ubiquitous phenomenon in nature; many different thin films of organic and inorganic substances (such as liquids, polymers, metals, and semiconductors) share this shape instability driven by surface tension and mass transport. Via templated solid-state dewetting, we frame complex nanoarchitectures of monocrystalline silicon on insulator with unprecedented precision and reproducibility over large scales. Phase-field simulations reveal the dominant role of surface diffusion as a driving force for dewetting and provide a predictive tool to further engineer this hybrid top-down/bottom-up self-assembly method. Our results demonstrate that patches of thin monocrystalline films of metals and semiconductors share the same dewetting dynamics. We also prove the potential of our method by fabricating nanotransfer molding of metal oxide xerogels on silicon and glass substrates. This method allows the novel possibility of transferring these Si-based patterns on different materials, which do not usually undergo dewetting, offering great potential also for microfluidic or sensing applications.

International audience; In this study, we demonstrate a completely novel synthesis route for producing magnetic porous silicon. The magnetic properties of this material are induced by manganese atoms. The Mn-doping in Si is achieved by ion implantation. A subsequent anodization of the substrate is done to turn it into porous silicon. Several characterization techniques, such as transmission electronic microscopy, atomic force microscopy and photoluminescence are combined to probe the structural and the optical properties of this material. Furthermore, temperature and magnetic field dependent magnetization is analyzed using superconducting quantum interference device. In addition to the well-reported structural and optical properties of the porous silicon, our Mn-doped porous silicon samples exhibit a magnetic behavior with a curie temperature (T-C) higher than room temperature. These results indicate that the magnetic porous silicon can be integrated with microelectronics and photonics technologies to produce new devices, such as magnetophotonic crystals and polarized emitting diodes.

International audience; Dielectric Mie resonators have attracted a great deal of attention over the past few years thanks to their remarkable capabilities in manipulating light propagation at the nanoscale. However, the practical implementation of technological products is still elusive. One of the important limits is the absence of a high-performing material and a fabrication method that can be easily integrated into modern microelectronic devices at affordable costs. Here, we provide theoretical and experimental evidence of an alternative semiconductor material, SiGe alloys, for dielectric Mie resonator applications. As a material compatible with the processing requirements of the semiconductor industry, it possesses comparable optical properties to its conventional Si-based counterpart at visible frequencies in spite of its higher optical losses. These dielectric resonant 18 particles can be obtained over very large surfaces on arbitrary silica substrates via spontaneous solid state dewetting of ultrathin (

International audience; We report on a novel method for the implementation of core-shell SiGe-based nanocrystals combining silicon on insulator dewetting in a molecular beam epitaxy reactor with an ex situ Ge condensation process. With an in situ two-step process (annealing and Ge deposition) we produce two families of islands on the same sample: Si-rich, formed during the first step and, all around them, Ge-rich formed after Ge deposition. By increasing the amount of Ge deposited on the annealed samples from 0 to 18 monolayers, the islands' shape in the Si-rich zones can be tuned from elongated and flat to more symmetric and with a larger vertical aspect ratio. At the same time, the spatial extension of the Ge-rich zones is progressively increased as well as the Ge content in the islands. Further processing by ex situ rapid thermal oxidation results in the formation of a core-shell composition profile in both Si and Ge-rich zones with atomically sharp heterointerfaces. The Ge condensation induces a Ge enrichment of the islands' shell of up to 50% while keeping a pure Si core in the Si-rich zones and a similar to 25% SiGe alloy in the Ge-rich ones. The large lattice mismatch between core and shell, the absence of dislocations and the islands' monocrystalline nature render this novel class of nanostructures a promising device platform for strain-based band-gap engineering. Finally, this method can be used for the implementation of ultralarge scale meta-surfaces with dielectric Mie resonators for light manipulation at the nanoscale.

International audience; Single crystal Silicon (Si) layers have been deposited by molecular beam epitaxy on double-layer porous silicon (PSi). We show that a top thin layer with a low porosity is used as a seed layer for epitaxial growth. While, the underlying higher porosity layer is used as an easily detectable etch stop layer. The morphology and structure of epitaxial Si layer grown on the double-layer PSi are investigated by high resolution X-ray diffraction and transmission electron microscopy. The results show that, an epitaxial Si layer with a low defect density can be grown. Epitaxial growth of thin crystalline layers on double-layer PSi can provide opportunities for silicon-on-insulator applications and Si-based solar cells provided that the epitaxial layer has a sufficient crystallographic quality.

International audience; We report the fabrication of Si-based dielectric Mie resonators via a low cost process based on solid-state dewetting of ultra-thin amorphous Si on SiO2. We investigate the dewetting dynamics of a few nanometer sized layers annealed at high temperature to form submicrometric Si-particles. Morphological and structural characterization reveal the polycrystalline nature of the semiconductor matrix as well as rather irregular morphologies of the dewetted islands. Optical dark field imaging and spectroscopy measurements of the single islands reveal pronounced resonant scattering at visible frequencies. The linewidth of the low-order modes can be ∼20 nm in full width at half maximum, leading to a quality factor Q exceeding 25. These values reach the state-of-the-art ones obtained for monocrystalline Mie resonators. The simplicity of the dewetting process and its cost-effectiveness opens the route to exploiting it over large scales for applications in silicon-based photonics.

We report on the generation of photocurrent in the visible and ultraviolet range from planar devices built from the Ge nanocrystals grown on a heavy n-doped Si(001) substrate covered with 5 nm thick thermally grown SiO2. These Ge nanostructures/SiO2/n(+)-Si devices are shown to generate photocurrent with an Incident-Photon-to-electron Conversion Efficiency (IPCE) spectral range depending on the Ge nanocrystals size. The increase of the IPCE value of our devices in the 350-600 nm range correlates well with the absorbance of Ge.

For Ge nanodots ∼20 nm in diameter grown by annealing a thin amorphous Ge layer deposited by molecular beam epitaxy on a mesoporous TiO2 layer on Si(001), photoluminescence (PL) was observed as a wide near-infrared band near 800 meV. Using a tight binding theoretical model, the energy-dependent PL spectrum was transformed into a dependence on dot size. The average dot size determined the peak energy of the PL band and its shape depended on the size distribution, including bandgap enlargement due to quantum confinement. Combining the dot sample PL with an established dependence of emission efficiency on dot diameter, it was possible to derive a dot size distribution and compare it with results obtained independently from atomic force microscopy.

The photocurrent generation of nanometric Ge droplets has been investigated by using electrochemical measurements. Dots have been grown by annealing Ge layers of different nominal thicknesses (ranging from 0.5 to 5 nm) deposited on clean SiO2 surface at room temperature. The photocurrent signals for the largest dots show features, which can be ascribed to the Ge nanocrystals direct electronic transitions. Only in the case of 0.5 nm Ge film a broad and intense feature at 2.4 eV has been observed while the peaks at higher energy result to be dramatically reduced. Since this last sample is characterized by (5±1) nm nanodots size (the smallest among the measured samples) the appearance of this broad band in the photocurrent active spectrum can be ascribed to quantum confinement effect.

Ge nanocrystals have been obtained from the dewetting process during thermal annealing of anamorphous Ge layer deposited by molecular beam epitaxy on a thin SiO 2 layer on Si(001). The Genanocrystals were then capped with a thin layer of amorphous Si. The mean nanocrystal size – 2.5–60nm – depends on the initial Ge layer thickness. Low-temperature photoluminescence (PL) measurementswere performed to investigate quantum confinement effects on the Ge nanocrystal energy gap and defectstates. For the present range of particle sizes, the nanoparticle PL emission appeared as a wide near-infraredband near 900 meV although a weak confined band was also observed for the smallest nanoparticles. Furtherthermal annealing of the samples increased the interband recombination by nearly two orders of magnitude.Crown Copyright c 2008 Published by Elsevier Ltd. All rights reserved. Keywords: Photoluminescence; Nanocrystals; Germanium; Silicon dioxide; Silicon 1. IntroductionAlthough achieving efficient light emission from group IV semiconducting materials, asubject of intense research activity, has proven elusive, carrier localization methods have led tosignificantly enhanced optical emission from indirect gap materials and at higher temperatures.For example, room-temperature visible photoluminescence (PL) has been observed from porousSi [1] and strong infrared PL from various island or quantum dot systems [2–9]. A particular

Ge nanocrystals (NCs) are produced by a dewetting process during annealing of an amorphous Ge layer deposited on an ultra thin SiO 2 layer. We have investigated the characteristics of the resulting NCs as a function of the nominal Ge layer thickness. Thanks to transmission electron microscopy images, we have extracted both the wetting angle and the NCs aspect ratio. We found that these characteristics remain constant whatever is the nominal thickness in the range of 1.5 to 10 nm. These results suggest that NCs have reached their equilibrium shape. We also experimentally determined the evolution of the NCs with the nominal thickness of the amorphous layer and found a linear relation. These results are in agreement with mass conservation and energetical considerations. Moreover a memory effect was evidenced in all the samples by C − V measurements. At last, we demonstrate that the use of a patterned SiO 2 surface improves considerably the ordering of NCs and reduces their size distribution. Such a process is promising for future integration of NCs in memory devices.

This work was devoted to the development of a Ge quantum dot memory structure of a MOSFET type with laterally ordered Ge quantum dots within the gate dielectric stack. Lateral ordering of the Ge dots was achieved by the combination of the following technological steps: (a) use of a focused ion beam (FIB) to create ordered two-dimensional arrays of regular holes on a field oxide on the silicon substrate, (b) chemical cleaning and restoring of the Si surface in the holes, (c) further oxidation to transfer the pattern from the field oxide to the silicon substrate, (d) removal of the field oxide and thermal re-oxidation of the sample in order to create a tunneling oxide of homogeneous thickness on the patterned silicon surface, and (e) self-assembly of the two-dimensional arrays of Ge dots on the patterned tunneling oxide. The charging properties of the obtained memory structure were characterized by electrical measurements. Charging of the Ge quantum dot layer by electrons injected from the substrate resulted in a large shift in the capacitance-voltage curves of the MOS structure. Charges were stored in deep traps in the charging layer, and consequently the erasing process was difficult, resulting in a limited memory window. The advantages of controlled positioning of the quantum dots in the charging layer will be discussed.

International audience; Synthesizing Au 0.8 Si 0.2 nanocatalysts that are homogeneous in size and have controlled position is becoming a challenging and crucial prequisite for the fabrication of ordered semiconductor nanowires. In this study, Au 0.8 Si 0.2 nanocatalysts are synthesized via dewetting of Au layers on Si(111) during thermal annealing in an ultra-high vacuum. In the first part of the paper, the mechanism of homogeneous dewetting is analyzed as a function of the Au-deposited thickness ( h Au ). We distinguish three different dewetting regimes: (I) for a low thickness ( ##IMG## [http://ej.iop.org/images/0957-4484/26/50/505602/nano514960ieqn1.gif] \h_\\rmAu\\leqslant 0.4\\;\\mathrmnm\ ), a submonolyer coverage of Au is stabilized and there is no dewetting. (II) For an intermediate thickness ( ##IMG## [http://ej.iop.org/images/0957-4484/26/50/505602/nano514960ieqn2.gif] \0.4\\;\\mathrmnm\\lt h_\\mathrmAu\\leqslant 5\\;\\mathrmnm\ ), there is both dewetting and Au 0.8 Si 0.2 phase formation. The size and density of the Au 0.8 Si 0.2 clusters are directly related to h Au . When cooling down to room temperature, the clusters decompose and reject the Si at the Au/Si substrate interface. (III) For a large thickness ( ##IMG## [http://ej.iop.org/images/0957-4484/26/50/505602/nano514960ieqn3.gif] \h_\\rmAu\\gt 5\\;\\mathrmnm\ ), only dewetting takes place, without forming AuSi clusters. In this regime, the dewetting is kinetically controlled by the self-diffusion of Au (activation energy ∼0.43 eV) without evidence of an Si-alloying effect. As a practical consequence, when relying solely on the homogeneous dewetting of Au/Si(111) to form the Au 0.8 Si 0.2 catalysts (without a supply of Si atoms from vapor), regime II should be used to obtain good size and density control. In the second part of the paper, a process for ordering the catalysts using focused ion beam-(FIB) assisted dewetting (heterogeneous dewetting) is developed. We show that no matter what the FIB milling conditions and the Au nominal thickness are, dewetting is promoted by ion beam irradiation and is accompanied by the formation of Au 0.8 Si 0.2 droplets. The droplets preferentially form on the patterned areas, while in similar annealing conditions, they do not form on the unpatterned areas. This behavior is attributed to the larger Au-Si interdiffusion in the patterned areas, which results from the Si amorphization induced by the FIB. A systematic analysis of the position of the nanodroplets shows their preferential nucleation inside the patterns, while thicker platelets of almost pure Au are observed between the patterns. The evolutions of the size homogeneity and the occupancy rate of the patterns are quantified as a function of the FIB dose and annealing temperature. Nice arrays of perfectly ordered AuSi catalysts are obtained after optimizing the FIB and dewetting conditions.

The aim of this work is to give quantitative guides for the fabrication of strain-engineered SiGe epitaxial nanomembranes using a compliant substrate. We theoretically determine the effect of the elastic properties (softness and strain level) of a compliant substrate on the morphological evolution of an epilayer. The experimental system under investigation is the SiGe on Si(001) model system, which develops an Asaro–Tiller–Grinfel’d (ATG) growth instability or misfit dislocations for large epitaxial stresses. The compliant substrate is a porous silicon layer whose softness and strain level can be adjusted by varying the density of pores and the annealing conditions. The softness and strain level of the compliant substrate are analyzed independently. We show that the softness of the compliant substrate produces a significant enhancement of the growth instability that normally develops in SiGe/Si systems. We rationalize the counterintuitive and commonly misunderstood instability enhancement by considering t...

We first investigate the spontaneous formation of nanometric and highly dense (∼3×10 12 cm −2 ) Ge droplets on SiO 2 /Si(0 0 1) by scanning tunnelling microscopy and spectroscopy. Ge dots are grown using a two-step process: first, Ge deposition on the clean SiO 2 surface at room temperature (RT) and second, thermal annealing at 500 °C. Ge dots appear free of germanium oxides. Then we show the ordering of Ge nanocrystals (NCs) on SiO 2 /Si(0 0 1) substrates patterned by focused ion beam (FIB). Lateral ordering of the ultra-dense Ge nanodots was achieved by the combination of the following technological steps: (a) use of a FIB to create ordered two-dimensional (2D) arrays of regular holes on a field oxide on the silicon substrate, (b) chemical cleaning and restoring of the Si surface in the holes, (c) further oxidation to transfer the pattern from the field oxide to the silicon substrate, (d) removal of the field oxide and thermal re-oxidation of the sample in order to create a tunnelling oxide of homogeneous thickness on the patterned silicon surface and (e) self-assembly of the 2D arrays of Ge dots on the patterned tunnelling oxide. We show that Ge NCs are ordered in the FIB holes whatever is their dimension/density.

In order to improve the electrical performance of the conventional amorphous SiGe thin-film solar cells, a new multi-trench technique has been proposed. In this method, the multi-trench is created in the intrinsic SiGe layer and filled with p-type doped Si. The p-type trenches in the intrinsic SiGe layer improve the electrical performance of the proposed design. Electrical characteristics of the proposed structure are analyzed and optimized by 2-D numerical modeling and compared with conventional amorphous SiGe thin-film solar cell characteristics. The extracted results show that the multi-trench-based technique has excellent effect on the fill factor and efficiency of the device.

In this work we present a method to fabricate Ge nanocrystals embedded in an oxide matrix by MBE deposition of amorphous Ge and Si layers combined with RTA and oxidation steps. A thin a-Ge layer was deposited on a SiO 2 thermal oxide on p-Si substrate. The structures are then capped with a-Si. A dry oxidation is used to transform the a-Ge into a GeO 2 layer. In order to precipitate the Ge in a SiO 2 matrix, the sample was annealed under N 2 . The control of the formation of Ge traps in SiO 2 has been optimized by the annealing time. For that purpose MOS capacitors have been characterized using C – V and I – V measurements. In addition, the electrical measurements allow the study of electron trapping in Ge. Those experiments have shown storage effects only for samples annealed for 30 s. Finally, we have optimized the process for the fabrication of Ge-based NVMs.

We present a study of the electrically active defects produced in p-type silicon crystals underneath a titanium layer deposited by sputtering to form a Schottky contact. The defects were investigated by Deep Level Transient Spectroscopy and free carrier profiles were determined by Capacitance–Voltage measurements. The sputtering deposition experiments were performed on silicon crystals grown either by Molecular Beam Epitaxy (MBE) or by the Czochralski technique (Cz). All the layers contained hydrogen as a result of normal cleaning and etching procedures. In MBE-grown silicon, four defects were observed [referred to as H(0.61), H(0.52), H(0.33), and H(0.23)]. H(0.23) has been identified as the donor level of the divacancy, while it has been shown that the three other defects were boron-related. In the Cz-grown silicon crystals only H(0.52) has been observed. This defect is hydrogen-related in addition to its correlation with boron. In both MBE and Cz grown crystals, the H(0.52) defect displayed a bell-shap...

The electronic behaviour of boron-doped single germanium layers grown in a silicon matrix by solid source MBE has been studied by deep level transient spectroscopy (DLTS) and photoluminescence (PL). The structures result from the Si-capping of nine monolayers of germanium on (100) silicon at different substrate temperatures (650, 550 and 450 °C). Due to the growth procedure, the buried germanium structures consist of a wetting layer (WL) (typical of the Stranski–Krastanov growth mode of germanium in silicon) plus the self-assembled structure under the silicon cap. The detailed morphology of the structure depends on the growth temperature. Emission and trapping behaviours of the layer have been studied by DLTS and PL measurements, while Transmission Electron Microscope (TEM) observations were used to characterise the actual film morphology. We first show that the major electrical signal arising from the structure comes from the WL. The corresponding ground state depth clearly follows the trend imposed by a typical quantum confinement: the thinner the well, the higher the level. We also show that the electrical signal is maintained in the low-temperature-grown sample even if light emission from the WL is non-existent. The growth procedure coupled with PL results also allowed us to confirm the high level of elastic relaxation inherent to dome islands nucleation. The hole trapping behaviour of the WLs have also been investigated. A surprising point-defect-like behaviour is observed, suggesting that the WL capture rate is charge state independent.

International audience; The growth of heteroepitaxial planar fully strained SiGe layers with high Ge concentration and large thickness enables tailoring electronic properties for enhanced transport properties and photoemission. We give here the first experimental and theoretical proof that high temperature flashed porous silicon layers (HT-PSi) perfectly accommodate the stress of SiGe layers and provide compliant substrates with unprecedented capabilities for the fabrication of planar SiGe nanomembranes. We show that the stress driven morphological evolution leading to self-organized quantum dots commonly observed on nominal Si (001) is fully inhibited when growing SiGe on such a HT-PSi substrate. The elastic behavior of HT-PSi results from two specific features: It is ten times softer than Si and tensily strained. Theoretical analysis proves that the compliant behavior of HT-PSi is due to the strain effect, while on the contrary its elastic softness favors the development of 3D growth. The inhibition due to the tensile strain produces atomically flat layers free of misfit dislocation.

International audience; Thick porous silicon (PS) buffer layers are used as sacrificial layers to epitaxially grow planar and fully relaxed Ge membranes. The single crystal Ge layers have been deposited by molecular beam epitaxy (MBE) on PS substrate. During deposition, the pore network of PS layers has been filled with Ge. We investigate the structure and morphology of PS as fabricated and after annealing at various temperatures. We show that the PS crystalline lattice is distorted and expanded in the direction perpendicular to the substrate plane due to the presence of chemisorbed –OH. An annealing at high temperature (> 500 °C), greatly changes the PS morphology and structure. This change is marked by an increase of the pore diameter while the lattice parameter becomes tensily strained in the plane (compressed in the direction perpendicular). The morphology and structure of Ge layers are investigated by transmission electron microscopy, high resolution X-ray diffraction and atomic force microscopy as a function of the deposition temperature and deposited thickness. The results show that the surface roughness, level of relaxation and Si-Ge intermixing (Ge content) depend on the growth temperature and deposited thickness. Two sub-layers are distinguished: the layer incorporated inside the PS pores (high level of intermixing) and the layer on top of the PS surface (low level of intermixing). When deposited at temperature > 500 °C, the Ge layers are fully relaxed with a top Si1 − xGex layer x = 0.74 and a very flat surface. Such layer can serve as fully relaxed ultra-thin SiGe pseudo-substrate with high Ge content. The epitaxy of Ge on sacrificial soft PS pseudo-substrate in the experimental conditions described here provides an easy way to fabricate fully relaxed SiGe pseudo-substrates. Moreover, Ge thin films epitaxially deposited by MBE on PS could be used as relaxed pseudo-substrate in conventional microelectronic technology.

International audience; Optical technologies and devices rely on the controlled manipulation of light propagation through a medium. This is generally governed by the inherent effective refractive index of the material as well as by its structure and dimensionality. Although a precise control over light propagation with subwavelength size objects is a crucial issue for a plethora of applications, the widely used fabrication methods remain cumbersome and expensive. Here, a sol-gel dip-coating method combined with nanoimprinting lithography on arbitrary glass and silicon substrates is implemented for the fabrication of TiO2-based dielectric Mie resonators. The technique allows obtaining submicrometric pillars featuring unprecedented vertical aspect ratios (> 1) with relatively high fidelity and precision. Spectroscopic characterization at visible and near-infrared frequencies demonstrate that the resonant properties of these dielectric pillar arrays allow for a drastic reduction of light transmission (cutting more than 50% on glass) and reduced reflection (reflecting less than 3% on glass and 16% on bulk silicon), accounting for an efficient light trapping. These results provide a guideline for the fabrication of Mie resonators using a fast, versatile, low-cost, low-temperature technique for efficient light manipulation at the nanoscale.

Ge nanodots, produced by solid phase crystallization of amorphous Ge layers deposited at room temperature on SiO 2 thermally grown on a Si(100) surface, generate a photocurrent in the visible and ultraviolet range. The photocurrent signal was detected by measuring the current flowing through the sample under irradiation either in planar or in up-down geometry. In both cases the quantum efficiency of the bare SiO 2 /Si(100) substrate was dramatically enhanced up to a factor of 10 3 . Current–voltage curves were acquired in the latter geometry, evidencing an increase in the short circuit current induced by the Ge nanodots presence.

This paper describes fabrication and optical properties of Ge nanostructures (dots or wires) grown on atomically controlled and self-patterned Si 1− x Ge x template layers. All the difficulty of the process lies in the realization of the pre-patterned layer which is based on the kinetic development of a step-bunching instability. A comprehensive study on the influence of growth kinetics on morphological features of the Si 1− x Ge x template layer is presented. It mainly concentrates on the quantification of correlation length ( L ), amplitude ( A ) and aspect ratio ( A / L ) of the profiles by atomic force microscopy (AFM) Fourier transform measurements. Results show the determinant role of annealing and interruption time on the dot stabilisation, aspect ratio and optical properties. Analysis of the surface profile evolution during the subsequent deposition of Ge on the template layer shows that Ge dots are fully located on the top of the Si 1− x Ge x undulations. Finally we show enhanced optical properties for self-organised Ge dots deposited on the top of a self-patterned Si 1− x Ge x template layer.

Photoluminescence (PL) measurements were carried out on Si/Ge( n )/Si 0.7 Ge 0.3 /Si structures ( n is varying from 1 to 7 ML) deposed by gas source molecular beam epitaxy (GS-MBE) on Si(100) surfaces and high index Si(118) vicinal surfaces. Ge nanostructures were confined on the top of the undulation of the Si 0.3 Ge 0.7 wetting layer, according to the Stranski–Krastanov growth mode. PL measurements reveal a correlation between the substrate orientation and the island morphology: square dots for (001) and wires for (118) surface orientation. The results suggest that the SiGe wetting layer is required to ensure a good dot size uniformity. The dependence of the luminescence on the excitation power and the PL decay time indicate that the luminescence transitions likely occur in a type-II band line up. Finally, the dot-related PL persists up to room temperature which is very promising for optoelectronic device applications.

In this paper, we present a photoluminescence (PL) study of Si/Ge/SiGe/Si structures grown by gas source molecular beam epitaxy on an (1 1 8) undulated surface with various Ge coverage. Nucleation and growth of Ge films is obtained by the Stranski–Krastanov mechanism. The influence of the substrate orientation on the changeover 2D–3D growth mode is investigated. Furthermore, we show the use of growing an SiGe wetting layer to control the uniformity of the Ge island size. The PL signal related to the Ge islands is found to be highly dependent of the power excitation and is observed up to room temperature.

Presently, most microelectronic devices are fabricated using top-down approaches. According to Moore’s law, with the predicted ultimate reduction in size over the next years, these processes will reach the limits of technological instrument resolution. In addition, a major bottleneck in top-down approaches is the prohibitive cost due to the large number of technological steps required to reduce the device size. Furthermore, a broad range of applications require ultrasmall, complex devices that cannot be produced using top-down methods. New processes building on the natural self-organization of matter should therefore be conceived and developed, along with adequate characterization methods in order to allow for their application in innovative devices. Such self-organization processes are already implemented in various materials such as biological materials, soft matter, metals and semiconductors. Self-assembly is a process that builds an ordered structure, brick-by-brick, starting from disordered building blocks, using simple key ingredients. Self-assembly is commonly controlled by certain intrinsic material parameters (e.g., composition, strain, thickness, phase transformation, structural changes) and results from the interaction between different factors (e.g., deposit/substrate, liquid/gas/solid phases, crystals). Besides these intrinsic parameters, a number of extrinsic factors, including thermal treatment, chemical and electrochemical reactions, mechanical stress, electric or magnetic fields, can strongly influence the self-assembled morphologies. Self-assembly processes are generally low cost, large-scale techniques, which can be suitable for various industrial environments. Identified as one of the key topics in nanoscience with potential to shape future scientific research, self-assembly is the most promising path to breakthroughs in nanoelectronics, optoelectronics, spintronics, molecular nanotechnology, biology, materials science, software, robotics, manufacturing, transportation, energy harvesting, infrastructure and construction. Research on self-assembled nanostructures encompasses fundamental issues in chemical synthesis, crystal growth and self-organization of 0D, 1D and 2D nanostructures, nanopatterning, lithographic techniques, nanocharacterization, scaling of materials properties down to molecular dimensions, quantum properties and applications of nanoscale assemblies to advanced devices. The main topics of interest involve 2D nanomaterials such as nanomembranes, graphene, silicene and ordered mesoporous oxides, 1D nanomaterials such as nanowires and nanotubes, and 0D nanomaterials such as quantum dots, nanocrystals and Q-bits. Focusing on this dynamic new field, self-assembly “the science of things that put themselves together” explores the physics of nanostructures, new synthesis approaches, in addition to size-, shape- and composition-dependent properties. The major obstacles concern the reproducibility and control of the basic mechanisms in order to predict and produce patterns with tunable size, periodicity and position and the new physical properties resulting from low dimensionality. The goal of this Thematic Series is to bring together studies in the broad field of research on self-assembled nanostructures, which may seem far away, but are expected to promote an extraordinary exchange of ideas appropriate to conceive multidisciplinary processes suitable for various materials with the aim to produce exceptional new nanomaterials with revolutionary properties. This Thematic Series covers the physics of nanostructures at the nanoscale including: large-scale patterning obtained by spontaneous structuring as well as local probe nanopatterning for nanostructure size and position control; theoretical and experimental efforts dedicated to a better understanding of the formation, evolution, and organization of nanoscale systems; fundamental and new issues in nucleation, crystal growth, surface and interface atomistic mechanisms; and new optical, electrical, magnetic, and mechanical properties of self-assembled systems. Isabelle Berbezier and Maurizio De Crescenzi Marseille and Rome, June 2015

International audience; Abstract In this work we study the influence of the major focused ion beam operating parameters: ion chemical species, beam current, lens voltage and ion dose on the ultimate nanopatterning resolution. We propose a two-step process based on first ion milling of a SiO2 sacrificial layer and second SiO2 chemical etching for the fabrication of nanopatterns with ultimate size/density and ad libitum shape. Examples of resulting patterns are presented.

International audience; We demonstrate that perfectly reproducible and homogeneous core-shell Si1−xGex/Ge nanowires can be produced by a two step nanofabrication process. The process makes use of a combination of Liquid Metal Alloy Ion Source–Focused Ion Beam (LMAIS-FIB) nanomilling and condensation. In a first step, we fabricate arrays of SiGe wires by LMAIS-FIB milling of fully relaxed Si1−xGex pseudo-substrates. The use of Ge2+ ions during this step avoids any metallic contamination of the nanowires. In a second step, we both reduce the diameter of the wires and form the core-shell configuration by oxido-reduction of the wires. Large arrays of core-shell nanowires with extended aspect ratio (length over diameter), small diameters and ultra-thin shell thickness are fabricated. Multilayer core-shell configurations with tunable arrangements could also be produced by repeated condensation cycles.

International audience; Aberration-corrected scanning transmission electronmicroscopy is employed to investigate the local chemistry in the vicinity of a Si0.8Ge0.2/Si interface grown bymolecular-beamepitaxy.Atomic-resolution high-angle annular dark field contrast reveals the presence of a nonuniform diffusion of Ge from the substrate into the strained Si thin film. On the basis of multislice calculations, a model is proposed to quantify the experimental contrast, showing that the Ge concentration in the thin film reaches about 4% at the interface and decreases monotonically on a typical length scale of 10 nm. Diffusion occurring during the growth process itself therefore appears as a major factor limiting the abruptness of interfaces in the Si-Ge system

The present study examines the kinetics of dry thermal oxidation of (111), (110), and (100) silicon-germanium (SiGe) thin epitaxial films and the redistribution of Ge near the oxidation interface with the aim of facilitating construction of single and multi-layered nano-structures. By employing a series of multiple and single step oxidations, it is shown that the paramount parameter controlling the Ge content at the oxidation interface is the oxidation temperature. The oxidation temperature may be set such that the Ge content at the oxidation interface is increased, kept static, or decreased. The Ge content at the oxidation interface is modeled by considering the balance between Si diffusion in SiGe and the flux of Si into the oxide by formation of SiO2. The diffusivity of Si in SiGe under oxidation is determined for the three principal crystal orientations by combining the proposed empirical model with data from X-ray diffraction and variable angle spectroscopic ellipsometry. The orientation dependence of the oxidation rate of SiGe was found to follow the order: (111)>(110)>(100). The role of crystal orientation, Ge content, and other factors in the oxidation kinetics of SiGe versus Si are analyzed and discussed in terms of relative oxidation rates. This research was originally published in the Journal of Applied Physics. © AIP Publishing

Under the proviso that the existing tight-binding (TB) and effective mass (EM) theoretical models provide a good description of the Ge dot energy gap versus dot diameter, this work investigates the effect of nanoparticle size and the size distribution on the near infrared PL spectrum obtained from self-assembled Ge dots grown on a thin layer of TiO2 or SiO2 on Si. For the as-grown samples, the dot PL emission occupies a wide near-infrared band between 0.8 and 1 eV. The PL efficiency versus dot size for four samples was obtained in three steps. Firstly, the PL spectrum was converted to an intensity plot versus dot diameter rather than energy by taking the PL emission from each dot to occur at the dot bandgap calculated using the TB or EM model. Secondly, a numerical form for the physical size distribution of that sample was obtained by performing a least-squares fit of a Gaussian to the dot size distribution measured by atomic force microscopy or transmission electron microscopy. Finally, the PL efficiency versus dot size was calculated using the fitted Gaussian dot size distribution to normalize the PL intensity distribution obtained in the first step. Although the absolute intensities of the PL from the samples vary, the calculated curves are all well-fitted by straight lines on a log-log plot with essentially the same slope for all samples, which indicates that under weak confinement there is a universal power-law increase in PL efficiency with decreasing dot size. © 2013 SPIE., Photonics North 2013, June 3-5, 2013, Ottawa, ON, Canada, Series: Proceedings of SPIE; no. 8915

Because its optical gap is close to 0.85 eV, the semiconducting orthorhombic disilicide β-FeSi 2 has been extensively studied since it might find applications in integrated optoelectronic devices. During the course of these investigations several unexpected pseudomorphic phases of ultra thin FeSi 2 layers have been observed on top of the Si substrates. The growth of these phases and their physicochemical properties have been investigated for a large variety of preparation methods and surface techniques. In this paper, particular attention will be paid to the mechanisms of stabilization of such metastable phases due to their epitaxy on Si substrates, and to their transition, both electronic and structural, towards the β-FeSi 2 phase. Recent findings concerning the growth at low temperature of the metallic α-FeSi 2 phase will also be discussed since they might shed light on the hierarchy of these phase transitions and offer a useful way to achieve, by a subsequent post-annealing, high quality semiconducting β-FeSi 2 grains.

International audience; Porous silicon (PSi) layers are used as templates to grow epitaxial planar and fully relaxed Ge pseudo-substrates. An annealing at 600 °C, dramatically changes the PSi morphology and produces compliant template layers which serve in a second step, as substrate for the epitaxy of fully relaxed SiGe layers with a Ge content between 50% and 94%. The SiGe pseudo-substrates produced by such process exhibit a remarkable planar surface resulting from the penetration of Ge inside the pores. They could be integrated into conventional microelectronic technology for the subsequent deposition of active layers such as tensily strained Si or relaxed Ge.

International audience; We report the observation of a structurally induced doping compensation mechanism in doped semiconductor nanowires that results from the reduced size geometry. This kind of compensation can significantly affect the electronic transport properties of the doped nanowires. We demonstrate that in a crystalline n-type doped Ge wire, compensated by the acceptor-like localized surface states, strong electron-electron interactions occur. Variable range hopping conduction detected in these nanowires is directly generated from strong interactions, exhibiting an unusual large Coulomb gap in the density of states of wires. Published by AIP Publishing.

The low temperature homoepitaxial growth of silicon has been probed in situ by Reflection High Energy Electron Diffraction (RHEED) in a Molecular Beam Epitaxy (MBE) chamber in the temperature range 250 o C-400 o C. The continuous change in the RHEED patterns during the growth of thick films (several hundred angstroms) shows the progressive appearance of a surface roughness during and after the decay of RHEED oscillations. This is a clear evidence for kinetic roughening in the case of silicon epitaxial growth at low temperatures on the Si(111) face. The surface width, σ, measured as the film thickness, h, is increased, can be described by: σ(h, T)∼Ah exp(E f /kT) with E f ∼0.65 eV. This is in marked difference with the kinetic roughening behavior measured during the growth of a metal like iron by means of the same RHEED technique (see Chevrier et al., Europhys. Lett. 8 (1991) 737). Furthermore, an extrapolation of this behavior to epitaxial growths at higher temperatures (T∼600 o C-800 o C) suggests an effective influence of kinetic roughening in the determination of growth temperatures generally used for silicon MBE (i.e. the empirical usual growth temperatures Tepi ∼ 650 o C). During growth at substrate temperatures between 250 o C and 400 o C, the nucleation of misoriented silicon islands takes place following the occurrence of kinetic roughening at the surface. In this range of temperatures, this suggests that the loss of epitaxy occurs on a rough surface through the proliferation of defects and of misoriented crystals