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Under the generic term ``Stranski-Krastanov'' are grouped two different growth mechanisms of SiGe quantum dots. They result from the self-organized Asaro-Tiller-Grinfel'd (ATG) instability at low strain, while at high strain, from a stochastic nucleation. While these regimes are well known, we elucidate here the origin of the transition between these two pathways thanks to a joint theoretical and experimental work. Nucleation is described within the master equation framework. By comparing the time scales for ATG instability development and three-dimensional (3D) nucleation onset, we demonstrate that the transition between these two regimes is simply explained by the crossover between their divergent evolutions. Nucleation exhibits a strong exponential deviation at low strain while ATG behaves only algebraically. The associated time scale varies with $exp(1/{x}^{4})$ for nucleation, while it only behaves as $1/{x}^{8}$ for the ATG instability. Consequently, at high (low) strain, nucleation (instability) occurs faster and inhibits the alternate evolution. It is then this different kinetic evolution which explains the transition from one regime to the other. Such a kinetic view of the transition between these two 3D growth regimes was not provided before. The crossover between nucleation and ATG instability is found to occur both experimentally and theoretically at a Ge composition around 50% in the experimental conditions used here. Varying the experimental conditions and/or the system parameters does not allow us to suppress the transition. This means that the SiGe quantum dots always grow via ATG instability at low strain and nucleation at high strain. This result is important for the self-organization of quantum dots.

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 (

Light emission from Si nanostructures has been of great interest for some time now owing to the need for silicon-based light sources for applications in silicon photonics. Both Si and Ge possess indirect band gaps, which makes them very inefficient light emitters. Band gap engineering has been proposed as one way to overcome this limitation and although light emission with greatly improved efficiency has been obtained at low temperatures the emission at room temperature is still very weak, because of exciton dissociation. Recently, through employing novel band gap engineering computations, entirely new Si/Ge [M. d’Avezac, J.-W. Luo, T. Chanier, and A. Zunger, Phys. Rev. Lett. 108, 027401 (2012)] and Si [H. J. Xiang, B. Huang, E. Kan, S.-H. Wei, and X. G. Gong, Phys. Rev. Lett. 110, 118702 (2013)] supercell structures possessing direct gaps have been proposed. According to d’Avezac et al., a SiGe2Si2Ge2SiGen superstructure should have a direct and dipole-allowed gap of 0.863 eV, which is ideally suited for optical fiber data transmission applications. Here we report on the growth of such a structure and its optical properties. Two similar samples were prepared by different growth methods: molecular beam epitaxy (MBE) and solid phase epitaxy (SPE). In both samples the superstructure (terminated with n = 12 monolayers of Ge) was grown on a 30-nm thick buffer layer of Si0.4Ge0.6 on a 5 µm thick relaxed buffer layer of Si0.4Ge0.6on a 750 µm thick (001) Si substrate. The photoluminescence (PL) spectra were measured using a Bomem DA3 FTIR spectrometer, with the samples excited at low temperatures with either 70 mW of 405 nm or 35 mW of 458 nm laser light. Photoluminescence (PL) spectra obtained at 6 K with excitation at 405 nm are shown in Fig. 1. Similar spectra were obtained with 458 nm excitation. No PL was detected from the two samples in the energy range 1000-1850 meV or at room temperature. The sharp drop at low energy near 700 meV is due to the cut-off in the instrumental response. A strong low-energy PL doublet is seen, with peaks near 780 and 820 meV, together with a much weaker peak at 872 meV. The ratio of intensities of the strong and weak peaks is the same in both samples. The intensities of all three PL peaks decrease with increasing temperature up to 25 K, but the weak peak decreases in intensity faster than that of the strong peaks. The weak peak at 872 meV is most likely the dipole-allowed direct-gap transition expected at 0.863 eV in the superstructure. The small difference in energy between theory and experiment could be the result of a difference in strain within the layer in the sample compared with the ideal (perfect) modeled structure or from assumptions in parameter values in the model. The strong peaks at 820 and 780 meV are assigned to the no-phonon and transverse-optic-phonon emission lines, respectively, of the Si0.4Ge0.6 buffer layer. The ~40 meV separation between the two strong peaks is characteristic of the phonon energies in SiGe alloys. The energies of the peaks, however, are much lower than that expected for a bulk Si0.4Ge0.6 alloy (~0.97 eV). The energies and general appearance of these peaks is reminiscent of what has been obtained from PL studies of SiGe nanostructures imbedded in Si. It is therefore likely that this PL arises predominately at the Si0.4Ge0.6/superstructure interface where there is type-II band alignment. In conclusion, we have obtained experimental evidence of the predicted direct-gap optically-allowed transition in a special supercell comprised of a number of ultrathin layers of Si and Ge.

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The extraordinary properties of graphene have spurred huge interest in the experimental realization of a two-dimensional honeycomb lattice of silicon, namely, silicene. However, its synthesis on supporting substrates remains a challenging issue. Recently, strong doubts against the possibility of synthesizing silicene on metallic substrates have been brought forward because of the non-negligible interaction between silicon and metal atoms. To solve the growth problems, we directly deposited silicon on a chemically inert graphite substrate at room temperature. Based on atomic force microscopy, scanning tunneling microscopy, and ab initio molecular dynamics simulations, we reveal the growth of silicon nanosheets where the substrate-silicon interaction is minimized. Scanning tunneling microscopy measurements clearly display the atomically resolved unit cell and the small buckling of the silicene honeycomb structure. Similar to the carbon atoms in graphene, each of the silicon atoms has three nearest and six second nearest neighbors, thus demonstrating its dominant sp

International audience; Thin film dewetting can be efficiently exploited for the implementation of functionalized surfaces over very large scales. Although the formation of sub-micrometer sized crystals via solid-state dewetting represents a viable method for the fabrication of quantum dots and optical meta-surfaces, there are several limitations related to the intrinsic features of dewetting in a crystalline medium. Disordered spatial organization, size, and shape fluctuations are relevant issues not properly addressed so far. This study reports on the deterministic nucleation and precise positioning of Si-and SiGe-based nanocrystals by templated solid-state dewetting of thin silicon films. The dewetting dynamics is guided by pattern size and shape taking full control over number, size, shape, and relative position of the particles (islands dimensions and relative distances are in the hundreds nm range and fluctuate approximate to 11% for the volumes and approximate to 5% for the positioning).

Large nanostructures of silicene have been successfully grown on graphene covered 6H-SiC (0001) substrates. The graphene plays an important role to obtain honeycomb structures of silicon in the sp2 configuration and acts as an ideal template for 2D materials. The scanning tunneling microscopy images showed nanosheets of silicene with a very small buckling among the Si atoms. Our scanning tunneling spectroscopy confirmed the metallic character of the deposited silicene in excellent agreement with band structure calculations that also exhibit the presence of Dirac cones.

Nanopatterns of gold clusters on a large surface of oriented Si(111) substrates, from the galvanic displacement of gold salt (via the spontaneous reduction of AuCl4 -), are demonstrated in this work. The Si substrate is patterned by Focused Ion Beam (FIB) prior to being dipped in a gold solution. Here, we show that these patterns lead to successful control of the position and size of gold clusters. Sequential patterning reveals a powerful maskless alternative to surface preparation prior to Si nanowire growth.

International audience; We compare the morphological and structural features of SiGe membranes fabricated by three different processes: direct deposition of Si0.5Ge0.5 on. Si(001) nominal substrate, direct deposition Of Si0.5Ge0.5 on silicon on insulator, and deposition of SiGe with low Ge concentration on silicon on insulator followed by Ge enrichment by condensation. We show that the formation of fully strained Ge-rich layers free of defects with a flat surface is possible only by the two-step epitaxy-condensation process. We demonstrate that the condensation based process enables the total inhibition of the morphological instability, together with the hindering of dislocations for critical thickness much greater than those commonly obtained by direct deposition. Those behaviors could be explained by the injection of self-interstitials in the Ge-rich layers during condensation. Such remarkable properties systems using a similar condensation process. could be generalized to many other systems using a similar condensation process.

International audience; We fabricated flat, two-dimensional germanium sheets showing a honeycomb lattice that matches that of germanene by depositing submonolayers of Ge on graphite at room temperature and subsequent annealing to 350 degrees C. Scanning tunneling microscopy shows that the germanene islands have a small buckling with no atomic reconstruction and does not give any hints for alloy formation and hybridization with the substrate. Our density functional theory calculations of the structural properties agree well with our experimental findings and indicate that the germanene sheet interacts only weakly with the substrate underneath. Our band structure calculations confirm that the Dirac cone of free-standing germanene is preserved for layers supported on graphite. The germanene islands show a small but characteristic charge transfer with the graphite substrate which is predicted by our ab initio simulations in excellent agreement with scanning tunneling spectroscopy measurements.

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; Correction for ‘Fabrication of poly-crystalline Si-based Mie resonators via amorphous Si on SiO2 dewetting’ by Meher Naffouti, et al., Nanoscale, 2016, 8, 2844–2849.

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.

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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.

International audience; We describe the fabrication of Ge nanowires during a single co-deposition step of Ge and Mn at high temperature. In these experimental conditions, a phase separation occurs and two different phases Ge and Ge1 − xMnx are formed with Ge1 − xMnx in the shape of small clusters distributed randomly in the Ge matrix. Because of the high deposition temperature, a new Ge1 − xMnx phase with low eutectic point is stabilized; this phase is different from the one (commonly Ge3Mn5) stabilized at lower temperature. During the growth process at 350 °C, the crystalline clusters remain solid but they are highly mobile and can float at the surface, serving as seeds to direct the growth of crystalline Ge nanowires from the vapor. The sketch steps of NWs formation are first the phase separation with formation of specific Ge1 − xMnx critical nuclei with low eutectic point and second the growth of Ge NWs directed by the Ge1 − xMnx solid cluster seeds. Ge NWs growth is forced along particular crystalline axis by the cluster seeds that lower the interfacial energy Ge/Ge1 − xMnx and the energy formation of the germanium crystal stabilizes the cluster position at the tip of the NWs. The density of NWs can be tuned by varying the nominal Mn concentration since this density is related to the number of clusters with the specific Ge1 − xMnx phase (with low eutectic point). The single step MBE process presented here has the main advantage to fully avoid any incorporation of unintentional impurity into Ge nanowires (apart from Mn atoms) and could be applied to several other systems. This work also provides new insights into the vapor–solid–solid growth mechanisms of Ge NWs.

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 show a two-step epitaxy/condensation process at low temperature to produce atomically flat Ge rich layer fully strained and free of defects. We demonstrate that the condensation based process enables the total inhibition of the classical ATG morphological instability, together with the hindering of dislocations for critical thickness much greater than those commonly obtained by direct deposition. Those behaviors could be explained by the injection of self-interstitials in the Ge-rich layers during condensation. An 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 is used. 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 non uniform composition, three-dimensional morphology, and broad core–shell interface. We specially focus on SiGe small 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 of these novel nanostructures as well as their wide potential for advanced device applications. Similar core-shell structures can be formed around small nanocrystals for photonic applications.

Self-assembled Ge nanodots were formed by in-situ thermal annealing of a thin amorphous Ge layer deposited by molecular beam epitaxy on a thin SiO2 or TiO2 layer on Si(001). The dot photoluminescence (PL) appeared primarily as a wide near-infrared band peaked near 800 meV. Using both the k•p and tight binding theoretical models, we have analyzed the PL spectrum in terms of the dot size distribution required to reproduce the observed asymmetric band shape. The peak energy of the PL band reflects the average dot size and its shape depends on the dot size distribution. The observed size distribution determined from transmission electron and atomic force microscopy allowed the determination of the nonlinear increase in the PL quantum efficiency with decreasing dot diameter. In addition, we show it is possible to evaluate the size distribution of Ge dots from their PL energy dependence.

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.

Mn0.06Ge0.94 samples have been grown by molecular-beam epitaxy on Ge(001)2 x 1. High-resolution transmission electron microscopy shows the coexistence of an ordered diluted Mn0.06Ge0.94 film and of nanoscopic crystallites, which were identified as Mn5Ge3 by electron diffraction. The magnetic properties of the Mn0.06Ge0.94 samples show a superposition of a paramagnetic behavior, due to the interaction of Mn atoms diluted in the Ge host, and a ferromagnetic behavior attributed to the Mn5Ge3 crystallites dispersed into the films. The Mn L-2.3 X-ray absorption spectra of the Mn0.06Ge0.94 films exhibit a lineshape typical of metallic Mn, with considerably reduced multiplet structure. (C) 2006 Elsevier B.V. All rights reserved.

Formation and ordering of Ge nanocrystals (NC) are studied on Si(0 0 1) and SiO2/Si(0 0 1) substrates patterned by focused ion beam (FIB). In both cases we use a three step process consisting of FIB milling of hole patterns with various periodicities, ex-situ substrate cleaning to remove Ga contamination and Ge NC growth by molecular beam epitaxy (MBE). We show that Ge NC can be ordered between or inside the holes on patterned Si(0 0 1) substrates and inside the holes on patterned SiO2/Si(0 0 1) substrates.

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; The fabrication of an ultrathin Ge-rich SiGe body on silicon on insulator (SOI) is highly challenging for the next generation of fully depleted complementary metal-oxide semiconductor devices that will be implemented in the near future. Ge-rich layers (GRLs) could be fabricated using a Ge enrichment process which takes place during dry thermal oxidation of SiGe thin films. While several studies make use of the GRL for many applications, the basic mechanism at work during the enrichment process is still unclear. In this study, we address the mechanism of formation of GRL and we determine the major driving forces of the enrichment process. We highlight the particular role played by the Si0.5Ge0.5 which is stabilized for various experimental conditions and strain levels. A systematic study demonstrates that the 50% Ge content is stabilized by a self-limited interdiffusion process regulated by the entropic term of the formation energy, which is a minimum at Si0.5Ge0.5 at the expense of the elasticity-driven interdiffusion. The process developed provides an easy and efficient way to produce planar GRLs free of dislocations with abrupt GRL/Si interfaces and tunable thickness. These GRLs could be fashioned for the heterogeneous integration of various systems on SOI.

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...

Structural properties of Mn x Ge 1-x films grown by molecular beam epitaxy (MBE) have been investigated by transmission electron microscopy (TEM). Films grown at T G = 160°C reveal the presence of Mn 5 Ge 3 nanoclusters embedded into the Ge crystalline host matrix doped by residual Mn atoms. Mn 5 Ge 3 nanoclusters show different features depending on the nominal Mn concentration in the Mn x Ge 1-x films. However, results evidence the existence of an Mn concentration threshold, around x = 0.01, below which crystallites formation is prevented. Our study evidences the relevant role of a suitable choice of the epitaxial growth parameters. These results make this material a promising candidate for spintronic applications.

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.

We review the study of nanocrystals or quantum dots using inelastic light scattering spectroscopies. In particular recent calculations of the phonon density of states and low frequency Raman spectra in Ge nanocrystals are presented for comparison with experimental results.

We have investigated the lattice diffusion of B and Sb by means of molecular beam epitaxy in Si1−xGex (x < 0.2) layers grown on Si(001) substrate. Using Si1−xGex relaxed buffers we were able to differentiate the chemical effect (change in the Ge composition) as opposite to the biaxial stress effect (due to the epitaxy on Si) on dopant diffusion. B diffusion follows a behavior opposite to Sb diffusion versus Ge composition and biaxial stress. These results are explained in view of the difference of diffusion mechanism between B (interstitials) and Sb (vacancies). We also show that dopant diffusion follows contrasting behaviors under biaxial pressure and hydrostatic pressure, and that the activation volume of dopant diffusion is of opposite sign for biaxial pressure and for hydrostatic pressure. This is explained using a formalism based on the extra work done by the system for diffusion under pressure, concluding that for biaxial stress the activation volume depends mainly on the relaxation volume linked to the defect formation.

International audience; The Stranski–Krastanov growth of Ge islands on Si(001) has been widely studied. The morphology changes of Ge islands during growth, from nucleation to hut/island formation and growth, followed by hut-to-dome island transformation and dislocation nucle-ation of domes, have been well described, even at the atomic scale, using techniques such as scanning tunneling microscopy and transmission electron microscopy. Although it is known that these islands do not consist of pure Ge (due to Si/Ge intermixing), the composition of the Ge islands is not precisely known. In the present work, atom probe tomography was used to study the composition of buried dome islands at the atomic scale, in the three-dimensional space. The core of the island was shown to contain about 55 atom % Ge, while the Ge composition surrounding this core decreases rapidly in all directions in the islands to reach a Ge concentration of about 15 atom %. The Ge distribution in the islands follows a cylindrical symmetry and Ge segregation is observed only in the {113} facets of the islands. The Ge composition of the wetting layer is not homogeneous, varying from 5 to 30 atom %.

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.

Subwavelength-sized dielectric Mie resonators have recently emerged as a promising photonic platform, as they combine the advantages of dielectric microstructures and metallic nanoparticles supporting surface plasmon polaritons. Here, we report the capabilities of a dewetting-based process, independent of the sample size, to fabricate Si-based resonators over large scales starting from commercial silicon-on-insulator (SOI) substrates. Spontaneous dewetting is shown to allow the production of monocrystalline Mie-resonators that feature two resonant modes in the visible spectrum, as observed in confocal scattering spectroscopy. Homogeneous scattering responses and improved spatial ordering of the Si-based resonators are observed when dewetting is assisted by electron beam lithography. Finally, exploiting different thermal agglomeration regimes, we highlight the versatility of this technique, which, when assisted by focused ion beam nanopatterning, produces monocrystalline nanocrystals with ad hoc size, position, and organization in complex multimers.

In this work, we have developed two original processes for the fabrication of 2-dimensional arrays of ordered Ge nanocrystals embedded in thin SiO2 layers for their use in nanocrystal floating gate memories. Ordering is achieved by a combination of focused ion beam nano-patterning and self assembly of Ge islands on the patterned SiO2 surface. In the first process the Ge islands are grown by selective chemical vapor deposition of Si/Ge/Si on Si holes fabricated by FIB patterning, while the second process uses solid phase epitaxy by MBE of amorphous Ge on SiO2 at room temperature, followed by annealing for crystallization. Highly ordered and highly dense (1.5 × 1011/cm2) ultra-small (~20 nm) Ge dots on SiO2 were achieved by both processes. This work has been carried out within the European IST project FORUM FIB: Fabrication, Organisation and Use of Memories obtained by Focused Ion Beams.

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 propose a two-step process, which is based on substrate nano-patterning by means of growth instabilities in a first step and self-assembling of Ge dots on the top of surface instabilities in a second step. We used the instabilities that develop during the growth of Si(Ge) layers on both nominal and vicinal Si (1 1 1) or (0 0 1) surfaces. Depending on the growth conditions (Ge concentration, growth temperature, thickness), various growth instability regimes were observed: pure kinetic regime, kinetically activated strain-induced regime and pure strain-driven regime. In the case of Si/Si growth, kinetic instabilities developed at different growth temperatures depending on the surface orientation. The critical exponents describing evolution with time have been determined: amplitude A∼tβ and wavelength L∼tα. Experimental results show that each instability regime appears for a given growth temperature range that critically depends on the concentration of Ge. Evolution with time also depends on the Ge concentration. But in all cases, we evidence discrepancies between the experimental critical exponents and those predicted by classical modelling. We also give some examples of Ge dots self-organization on substrates nano-patterned (periodically undulated) by means of the different growth instabilities described above. In all cases, we observe Ge dots ordering along the substrate undulations due to step and/or strain effects. On kinetic instabilities (Si/Si (0 0 1) vicinal), Ge islands preferentially nucleate on step bunches. On SiGe (0 0 1) template layers, Ge dots nucleate on top of the SiGe undulations. In that case, strain gradients improved island ordering. The best ordering was achieved using SiGe (0 0 1) 10° off misoriented template layers as a result of almost perfect anisotropic morphology.

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.

Extensive research activity has been devoted to self-assembly of very small coherent islands. However, while island formation is commonly described by a widely used S–K growth scheme, more complex mechanisms based on competitive effects of kinetics and thermodynamics take place during the epitaxy of Si1−xGex on Si(0 0 1). The aim of this paper is to explain the formation and the evolution of Si1−xGex islands on Si(0 0 1). The paper presents a comprehensive investigation of the different growth modes of Si1−xGex films (with x varying from 0 to 1) on Si(0 0 1) and Si(1 1 1). The results are presented in the form of kinetic morphological growth diagrams of as-grown samples. Two and four growth regimes are distinguished on (1 1 1) and (0 0 1) respectively. These growth regimes correspond to different levels of relaxation. In particular the four regimes observed on Si(0 0 1) correspond to (i) no relaxation in regime I (2D layer), (ii) 15–20% relaxation in regime II (“huts” islands with (1 0 5) facets), (iii) 20% and 50% relaxation in regime III (in “huts” and “domes” respectively) and (iv) 50% and 80% relaxation in regime IV (“domes” with bimodal size distribution). Every growth regime characteristic of as-grown sample is also associated with a specific equilibrium steady state morphology which is obtained after long-term annealing of the as-grown samples. In the two first regimes (no or small strain relaxation) the equilibrium morphology of highly strained Si1−xGex deposits corresponds to (1 0 5) faceted islands. We show that these islands are stabilised by the compressive stress. As soon as strain is released, (1 0 5) facets disappear at the expense of the (1 1 3) and (1 1 1) facets and first-order transition occurs between “huts” and “domes” islands.

Desorption of Sb from Si 1− x Ge x layers ( x =0, 0.05, 0.1, 0.2, 1) grown by molecular beam epitaxy (MBE) on Si(1 0 0) substrates is investigated using Auger electron spectroscopy thermodesorption (TD-AES). Sb desorption process on Si 1− x Ge x is well described by a first-order reaction. No extra TD-AES peaks are observed on Si 1− x Ge x compared to Si. For 1 ML of Sb coverage the TD-AES peak shifts to lower temperature when Ge bulk concentration increases. The Sb monolayer desorbs at 801, 752, 740, 715, and 706 °C for x =0, 0.05, 0.1, 0.2 and 1, respectively. We explain the non-linear decrease of the Sb desorption energy when x increases by the strong Ge surface segregation during the MBE growth of Si 1− x Ge x layers, resulting in an almost pure Ge surface even for low x .

Selective growth and self-organization of silicon-germanium (SiGe) nanowires (NWs) on focused ion beam (FIB) patterned Si(111) substrates is reported. In its first step, the process involves the selective synthesis of Au catalysts in SiO2-free areas; its second step involves the preferential nucleation and growth of SiGe NWs on the catalysts. The selective synthesis process is based on a simple, room-temperature reduction of gold salts (Au3+Cl?/4 ) in aqueous solution, which provides well-organized Au catalysts. By optimizing the reduction process, we are able to generate a bidimensional regular array of Au catalysts with self-limited sizes positioned in SiO2 -free windows opened in a SiO /Si2 (111) substrate by FIB patterning. Such Au catalysts subsequently serve as preferential nucleation and growth sites of well-organized NWs. Furthermore, these NWs with tunable position and size exhibit the relevant features and bright luminescence that would find several applications in optoelectronic nanodevices.

International audience; We report diffusion induced germanium nanowire growth and its dependence on the Ge evaporation flux. The wires show a growth rate (dL/dt) in agreement with the previously reported models, but detection of anomalies in the grown wires may indicate the prevalence of the direct Ge impinging effect on large diameter wires. Additionally, we demonstrate that change in deposition flux could directly affect the diffusion length of the Ge adatoms on the wire sidewalls. This in turn modifies the geometry of the grown wires by introducing a lateral growth starting from the base of the wire. A detailed understanding of the deposition flux effect on the growth and geometry of wires will result in improved knowledge of physical properties of wires.

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.

The authors present a study of the structure, compn. and optical properties of SiGe quantum dots, grown by gas-source mol. beam epitaxy on Si (001). At. force microscopy (AFM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy dispersive X-ray anal. (EDX), photoluminescence (PL) spectroscopy and decay time measurements of the quantum dots suggest that there are two distinct sizes of quantum dot, contributing two distinct emission bands in the PL spectra. [on SciFinder (R)]