64 research outputs found
Growth Route Toward III-V Multispectral Solar Cells on Silicon
To date, high efficiency multijunction solar cells have been developed on Ge
or GaAs substrates for space applications, and terrestrial applications are
hampered by high fabrication costs. In order to reduce this cost, we propose a
breakthrough technique of III-V compound heteroepitaxy on Si substrates without
generation of defects critical to PV applications. With this technique we
expect to achieve perfect integration of heterogeneous Ga1-xInxAs
micro-crystals on Si substrates. In this paper, we show that this is the case
for x=0. GaAs crystals were grown by Epitaxial Lateral Overgrowth on Si (100)
wafers covered with a thin SiO2 nanostructured layer. The cristallographic
structure of these crystals is analysed by MEB and TEM imaging. Micro-Raman and
Micro-Photomuminescence spectra of GaAs crystals grown with different
conditions are compared with those of a reference GaAs wafer in order to have
more insight on eventual local strains and their cristallinity. This work aims
at developping building blocks to further develop a GaAs/Si tandem demonstrator
with a potential conversion efficiency of 29.6% under AM1.5G spectrum without
concentration, as inferred from our realistic modeling. This paper shows that
Epitaxial Lateral Overgrowth has a very interesting potential to develop
multijunction solar cells on silicon approaching the today 30.3% world record
of a GaInP/GaAs tandem cell under the same illumination conditions, but on a
costlier substrate than silicon.Comment: Preprint of the 28th EUPVSEC proceedings, September 2013, Paris,
France. (5 pages
Anharmonicity in Raman-active phonon modes in atomically thin MoS
Phonon-phonon anharmonic effects have a strong influence on the phonon
spectrum; most prominent manifestation of these effects are the softening
(shift in frequency) and broadening (change in FWHM) of the phonon modes at
finite temperature. Using Raman spectroscopy, we studied the temperature
dependence of the FWHM and Raman shift of and
modes for single-layer and natural bilayer MoS over a
broad range of temperatures (T K). Both the Raman shift and FWHM
of these modes show linear temperature dependence for K, whereas they
become independent of temperature for K. Using first-principles
calculations, we show that three-phonon anharmonic effects intrinsic to the
material can account for the observed temperature-dependence of the line-width
of both the modes. It also plays an important role in determining the
temperature-dependence of the frequency of the Raman modes. The observed
evolution of the line-width of the A mode suggests that electron-phonon
processes are additionally involved. From the analysis of the
temperature-dependent Raman spectra of MoS on two different substrates --
SiO and hexagonal boron nitride, we disentangle the contributions of
external stress and internal impurities to these phonon-related processes. We
find that the renormalization of the phonon mode frequencies on different
substrates is governed by strain and intrinsic doping. Our work establishes the
role of intrinsic phonon anharmonic effects in deciding the Raman shift in
MoS irrespective of substrate and layer number
In-plane magnetic domains and N\'eel-like domain walls in thin flakes of the room temperature CrTe van der Waals ferromagnet
The recent discovery of magnetic van der Waals materials has triggered a
wealth of investigations in materials science, and now offers genuinely new
prospects for both fundamental and applied research. Although the catalogue of
van der Waals ferromagnets is rapidly expanding, most of them have a Curie
temperature below 300 K, a notable disadvantage for potential applications.
Combining element-selective x-ray magnetic imaging and magnetic force
microscopy, we resolve at room temperature the magnetic domains and domains
walls in micron-sized flakes of the CrTe van der Waals ferromagnet.
Flux-closure magnetic patterns suggesting in-plane six-fold symmetry are
observed. Upon annealing the material above its Curie point (315 K), the
magnetic domains disappear. By cooling back down the sample, a different
magnetic domain distribution is obtained, indicating material stability and
lack of magnetic memory upon thermal cycling. The domain walls presumably have
N\'eel texture, are preferentially oriented along directions separated by 120
degrees, and have a width of several tens of nanometers. Besides microscopic
mapping of magnetic domains and domain walls, the coercivity of the material is
found to be of a few mT only, showing that the CrTe compound is
magnetically soft. The coercivity is found to increase as the volume of the
material decreases
Hybrid Organotin and Tin Oxide-based Thin Films Processed from Alkynylorganotins: Synthesis, Characterization, and Gas Sensing Properties.
Hydrolysisâcondensation of bis(triprop-1-ynylstannyl)butylene led to nanostructured bridged polystannoxane films yielding tin dioxide thin layers upon UV-treatment or annealing in air. According to Fourier transform infrared (FTIR) spectroscopy, contact angle measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM) data, the films were composed of a network of aggregated âpseudo-particlesâ, as calcination at 600 °C is required to form cassiterite nanocrystalline SnO2 particles. In the presence of reductive gases such as H2 and CO, these films gave rise to highly sensitive, reversible, and reproducible responses. The best selectivity toward H2 was reached at 150 °C with the hybrid thin films that do not show any response to CO at 20â200 °C. On the other hand, the SnO2 films prepared at 600 °C are more sensitive to H2 than to CO with best operating temperature in the 300â350 °C range. This organometallic approach provides an entirely new class of gas-sensing materials based on a class II organicâinorganic hybrid layer, along with a new way to include organic functionality in gas sensing metal oxides
Thermal conductivity reduction measurement on Ge and Si NWs: heterostructuration and diameter size effect.
Thermal conductivity reduction measurement on Ge and Si NWs: heterostructuration and diameter size effect.
International audienc
Thermal conductivity reduction measurement on Ge and Si NWs: heterostructuration and diameter size effect.
International audienc
Thermal conductivity reduction measurement on Ge and Si NWs: heterostructuration and diameter size effect.
Morphology of GaAs crystals heterogeneously integrated on nominal (001) Si by epitaxial lateral overgrowth on tunnel oxide via Ge nano-seeding
cited By 0International audienceIn this study, GaAs crystals were grown by chemical beam epitaxy on nominal (001) Si substrate over Ge nano-seeds placed within nano-holes opened through a 0.6 nm silica layer. GaAs crystal morphology and atomic organization at the interface between Ge and GaAs were studied by using complementary Scanning Transmission Electron Microscopy, energy dispersive X-ray spectrometry and dark-field electron holography. Fourfold symmetry GaAs crystals were obtained and found to be completely relaxed and twin free. Thus, the use of Ge nano-seeds to initiate the growth of GaAs results in the suppression of twins previously observed for direct GaAs growth on nominal (001) Si. Nevertheless, anti-phase domains were detected. A simple atomistic model is proposed which explains how Anti-Phase Boundaries develop at the junctions between Ge {113} facets, and {113} and {111} facets for As-stabilized GaAs growth
InteÌgration de microcristaux de GaAs sur Si(100) sans domaines d'antiphase ni dislocation
National audienc
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