Lattice strain and tilt mapping in stressed Ge microstructures using X-ray Laue micro-diffraction and rainbow-filtering

Micro-Laue diffraction and simultaneous rainbow-filtered micro-diffraction were used to measure accurately the full strain tensor and the lattice orientation distribution at the sub-micron scale in highly strained, suspended Ge micro-devices. A numerical approach to obtain the full strain tensor from the deviatoric strain measurement alone is also demonstrated and used for faster full strain mapping. We performed the measurements in a series of micro-devices under either uniaxial or biaxial stress and found an excellent agreement with numerical simulations. This shows the superior potential of Laue micro-diffraction for the investigation of highly strained micro-devices.Comment: 28 pages, 10 figure

Reversible Al Propagation in Si x Ge 1-x Nanowires

While reversibility is a fundamental concept in thermodynamics, most reactions are not readily reversible, especially in solid state physics. For example, thermal diffusion is a widely known concept, used among others to inject dopant atoms into the substitutional positions in the matrix and improve the device properties. Typically, such a diffusion process will create a concentration gradient extending over increasingly large regions, without possibility to reverse this effect. On the other hand, while the bottom up growth of semiconducting nanowires is interesting, it can still be difficult to fabricate axial heterostructures with high control. In this paper, we report a reversible thermal diffusion process occurring in the solid-state exchange reaction between an Al metal pad and a SixGe1-x alloy nanowire observed by in-situ transmission electron microscopy. The thermally assisted reaction results in the creation of a Si-rich region sandwiched between the reacted Al and unreacted SixGe1-x part, forming an axial Al/Si/SixGe1-x heterostructure. Upon heating or (slow) cooling, the Al metal can repeatably move in and out of the SixGe1-x alloy nanowire while maintaining the rod-like geometry and crystallinity, allowing to fabricate and contact nanowire heterostructures in a reversible way in a single process step, compatible with current Si based technology. This interesting system is promising for various applications, such as phase change memories in an all crystalline system with integrated contacts, as well as Si/SixGe1-x/Si heterostructures for near-infrared sensing applications

Investigation of Lasing in Highly Strained Germanium at the Crossover to Direct Band Gap

Efficient and cost-effective Si-compatible lasers are a long standing wish of the optoelectronic industry. In principle, there are two options. For many applications, lasers based on III-V compounds provide compelling solutions, even if the integration is complex and therefore costly. However, where low costs and also high integration density are crucial, group-IV-based lasers - made of Ge and GeSn, for example - could be an alternative, provided their performance can be improved. Such progresses will come with better materials but also with the development of a profounder understanding of their optical properties. In this work, we demonstrate, using Ge microbridges with strain up to 6.6%, a powerful method for determining the population inversion gain and the material and optical losses of group IV lasers. This is made by deriving the values for the injection carrier densities and the cavity losses from the measurement of the change of the refractive index and the mode linewidth, respectively. We observe a laser threshold consistent with optical gain and material loss values obtained from a tight binding calculation. Lasing in Ge - at steady-state - is found to be limited to low temperatures in a narrow regime of tensile strain at the crossover to the direct band gap bandstructure. We explain this observation by parasitic intervalence band absorption that increases rapidly with higher injection densities and temperature. N-doping seems to reduce the material loss at low excitation but does not extend the lasing regime. We also discuss the impact of the optically inactive carriers in the L-valley on the linewidth of group IV lasers.Comment: 29 pages, 70 references, 15 figure

Condensation des excitons dans les nanostructures de silicium

This work deals with the study of the different electron-hole phases created in silicon nanostructures at low temperature under ultra-violet illumination. Thanks to wavelength and time resolved photoluminescence techniques the exciton, electron-hole liquid and electron-hole plasma phases are clearly identified in such structures. The first two chapters give the necessary information to understand the physics of the condensation of excitons in bulk semiconductors. The experimental part developped in the following chapters concerns the spatial confinement of the liquid in Si/SiO$_2$ heterostructures obtained with Silicon On Insulator SOI (chapter three). The nucleation and coalescence of liquid droplets and a change in the condensation threshold are observed in these samples. The photoluminescence lineshape analysis gives the liquid equilibrium parameters (density, temperature). The chapter four shows that the three dimensional confinement obtained in sub-micronic boxes made on SOI involves an increase in the liquid-plasma transition temperature (pressure cooker effect). The chapter five deals with the influence of the electric field on the electron-hole liquid in SOI quantum wells. A redshift of the recombination line is observed due to the band curvature of the semiconductor in the Schottky junction. Finally, the chapter six deals with the study of the effect of quantum confinement on the liquid in thin wells. Data are analyzed with a model that takes account of the lowering of the dimensionality and the dielectric mismatch between the well and the barrier. The transition between ultra-thin SOI quantum wells and nanocrystals has been observed.Le travail présenté ici concerne l'étude des différentes phases de porteurs de charge générées sous excitation optique à basse température dans les nanostructures de silicium cristallin. Après avoir rappelé et décrit brièvement les mécanismes physiques responsables de l'apparition et de l'équilibre entre le gaz d'excitons, le plasma et le liquide électron-trou dans les semiconducteurs massifs, il est montré, en s'appuyant sur les techniques de photoluminescence résolues en longueur d'onde et en temps, que le seuil de condensation des excitons en liquide électron trou est abaissé dans les puits de silicium sur isolant (SOI) du fait du confinement spatial unidimensionnel. Cet effet permet également de mettre en évidence la nucléation et la coalescence des gouttelettes de liquide. Une augmentation de la température de transition liquide-plasma est observée dans les milieux confinés dans les trois directions de l'espace, obtenus à partir de puits de SOI. L'influence du champ électrique sur le liquide est examinée grâce à la fabrication de jonctions métal-oxyde-semiconducteur pouvant servir à localiser les gouttelettes sous les électrodes. Enfin, l'effet du confinement quantique sur le liquide est observé dans les puits fins de SOI et conduit à la création d'un liquide bidimensionnel. Les données sont analysées en s'aidant d'un modèle prenant en compte l'abaissement de la dimensionalité du silicium ainsi que l'apparition de charges image dans le matériau barrière. Pour les puits les plus fins, l'apparition de raies de luminescence caractéristiques de celles émises par des nanocristaux de silicium atteste de l'observation de la transition puits/boîte

Condensation des excitons dans les nanostructures de silicium

This work deals with the study of the different electron-hole phases created in silicon nanostructures at low temperature under ultra-violet illumination. Thanks to wavelength and time resolved photoluminescence techniques the exciton, electron-hole liquid and electron-hole plasma phases are clearly identified in such structures. The first two chapters give the necessary information to understand the physics of the condensation of excitons in bulk semiconductors. The experimental part developped in the following chapters concerns the spatial confinement of the liquid in Si/SiO$_2$ heterostructures obtained with Silicon On Insulator SOI (chapter three). The nucleation and coalescence of liquid droplets and a change in the condensation threshold are observed in these samples. The photoluminescence lineshape analysis gives the liquid equilibrium parameters (density, temperature). The chapter four shows that the three dimensional confinement obtained in sub-micronic boxes made on SOI involves an increase in the liquid-plasma transition temperature (pressure cooker effect). The chapter five deals with the influence of the electric field on the electron-hole liquid in SOI quantum wells. A redshift of the recombination line is observed due to the band curvature of the semiconductor in the Schottky junction. Finally, the chapter six deals with the study of the effect of quantum confinement on the liquid in thin wells. Data are analyzed with a model that takes account of the lowering of the dimensionality and the dielectric mismatch between the well and the barrier. The transition between ultra-thin SOI quantum wells and nanocrystals has been observed.Le travail présenté ici concerne l'étude des différentes phases de porteurs de charge générées sous excitation optique à basse température dans les nanostructures de silicium cristallin. Après avoir rappelé et décrit brièvement les mécanismes physiques responsables de l'apparition et de l'équilibre entre le gaz d'excitons, le plasma et le liquide électron-trou dans les semiconducteurs massifs, il est montré, en s'appuyant sur les techniques de photoluminescence résolues en longueur d'onde et en temps, que le seuil de condensation des excitons en liquide électron trou est abaissé dans les puits de silicium sur isolant (SOI) du fait du confinement spatial unidimensionnel. Cet effet permet également de mettre en évidence la nucléation et la coalescence des gouttelettes de liquide. Une augmentation de la température de transition liquide-plasma est observée dans les milieux confinés dans les trois directions de l'espace, obtenus à partir de puits de SOI. L'influence du champ électrique sur le liquide est examinée grâce à la fabrication de jonctions métal-oxyde-semiconducteur pouvant servir à localiser les gouttelettes sous les électrodes. Enfin, l'effet du confinement quantique sur le liquide est observé dans les puits fins de SOI et conduit à la création d'un liquide bidimensionnel. Les données sont analysées en s'aidant d'un modèle prenant en compte l'abaissement de la dimensionalité du silicium ainsi que l'apparition de charges image dans le matériau barrière. Pour les puits les plus fins, l'apparition de raies de luminescence caractéristiques de celles émises par des nanocristaux de silicium atteste de l'observation de la transition puits/boîte

Condensation d'excitons dans les nanostructures de silicium

Ce travail concerne l'étude par des méthodes de photoluminescence résolues en longueur d'onde et en temps des diverses formes d'excitons, liquide et plasma électron trou-prises par les porteurs de charge dans les nanostructures de silicium à basse température. Les deux premiers chapitres ont pour but d'expliquer les phénomènes gouvernant la physique des excitons et de leur phase liquide condensée dans les semiconducteurs massifs. La partie expérimentale développée dans les chapitres suivants aborde la question du confinement spatial du liquide dans des hétérostructures SI/SIO2 obtenus sur silicium sur isolant soi (chapitre trois). La nucléation et la coalescence des gouttelettes de liquide ainsi qu'une modification du seuil de condensation sont observés dans ces milieux. L'analyse des raies de photoluminescence permet de trouver les grandeurs d'équilibre du liquide (densité et température). Le chapitre quatre montre que le confinement dans les trois directions de l'espace obtenu dans des plots de taille sunmicronique réalisés à partir de soi ets à l'origine de l'augmentation de la temptéature de transition liquide-plasma (effet cocotte-minute). Le chapitre cinq traite de l'influence du champ électrique sur le liquide dans les puits de soi. Il est notamment vu un décalage vers le rouge des raies de recombinaison issu de la courbure des bandes du semiconducteur dans la jonction schottky. Enfin, le chapitre six étudie l'effet sur le liquide du confinement quantique qui est observé dans les puits de faible épaisseur. La transition entre les puits ultra fins de soi et les nanocristaux est mise en évidence.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Nanotubes array electrodes by Pt evaporation: Half-cell characterization and PEM fuel cell demonstration

International audienceA self-standing nanotubes (NTs) array electrode is produced by electron beam evaporation of Pt on porous alumina templates. NTs have a mean diameter ranging from 150 to 300 nm, a length of 150 nm and a wall thickness of 20 nm. The ultrathin electrode has a density ranging between 10(9) and 4 x 10(9) NTs CMgeo-2 with a corresponding catalyst loading of 100 mu g(Pt) cm(geo)(-2). The NTs are assembled on a Nafion (R) membrane to obtain a NTs Array based Membrane Electrode Assembly (NTA-MEA) for Polymer Electrolyte Membrane Fuel Cells (PEMFCs) application. Ex-situ half-cell tests are carried out on 0.5 cm(geo)(2) samples to characterize the electrochemical properties of the NTs array electrode. PEMFC tests are also performed on 17 cm(geo)(2) samples to demonstrate the great potential of this architecture as fuel cell cathode under real operating conditions. Comparisons with Pt/C dispersions are made to draw conclusions on the advantage of the NTs array on conventional electrodes in terms of catalyst utilization. The NTs array shows improved catalyst accessibility due to the absence of the porous carbon support. Double current density per catalyst unit surface over Pt/C dispersions is measured at 0.6V during PEMFC tests under dry O-2 and air with 30%RH humidification. (C) 2014 Elsevier BY. All rights reserved

Radial photovoltaic junction with single Si nanowire core-shell structure

International audienceA single silicon nanowire core-shell structure has been elaborated. Technological stages of the process are presented. The device results in a P-i-N radial junction: the core is a P-type silicon nanowire encapsulated in an intrinsic thin silicon layer and an N-type doped silicon layer. Scanning electron microscopy observations, as well as the electrical I(V) characterisation on single nanowires, are presented

Uniform phosphorus doping of untapered germanium nanowires

International audienceOne of the major challenges in the growth of vapor-liquid-solid (VLS) nanowires is the control of dopant incorporation in the structures. In this work, we study the n-type doping and morphology of nanowires grown by chemical vapor deposition when HCl is introduced. We obtain fully untapered nanowires with a growth temperature up to 410 degrees C and measure their resistivity using the 4-probe technique to be 2.0 m Omega cm. We perform energy dispersive x-ray measurements showing a concentration of dopants in the (5-7) x 10(18) cm(-3) range, being radially and axially uniform. The combination of these two measurements shows that the mobility is the same as for bulk germanium, demonstrating that the VLS mechanism has no detrimental effect for the electron transport in these nanowires