F. Fossard

Photooxidation and quantum confinement effects in exfoliated black phosphorus

Thin layers of black phosphorus have recently raised interest owing to their two-dimensional (2D) semiconducting properties, such as tunable direct bandgap and high carrier mobilities. This lamellar crystal of phosphorus atoms can be exfoliated down to monolayer 2D-phosphane (also called phosphorene) using procedures similar to those used for graphene. Probing the properties has, however, been challenged by a fast degradation of the thinnest layers on exposure to ambient conditions. Herein, we investigate this chemistry using in situ Raman and transmission electron spectroscopies. The results highlight a thickness-dependent photoassisted oxidation reaction with oxygen dissolved in adsorbed water. The oxidation kinetics is consistent with a phenomenological model involving electron transfer and quantum confinement as key parameters. A procedure carried out in a glove box is used to prepare mono-, bi- and multilayer 2D-phosphane in their pristine states for further studies on the effect of layer thickness on the Raman modes. Controlled experiments in ambient conditions are shown to lower the A(g)(1)/A(g)(2) intensity ratio for ultrathin layers, a signature of oxidation.

Molecular-beam epitaxial growth and characterization of quaternary III–nitride compounds

We report on the controlled growth and characterization of quaternary AlGaInN compounds by plasma-assisted molecular beam epitaxy. Two-dimensional growth is achieved with a monolayer of In segregating at the growth front. In incorporation is hindered by increasing growth temperature and Al mole fraction, which is explained by the lower binding energy of InN compared to GaN and AlN. The mosaicity of the layers is determined by the substrate quality, whereas the alloy disorder increases with the Al content, independent of the In mole fraction. Room temperature photoluminescence is dominated by a narrow band-edge emission, whose Stokes shift and activation energy increase with the In content. This behavior is interpreted in terms of carrier localization in self-formed alloy inhomogeneities. An In-related band bowing parameter of 2.5 eV has been estimated.

Growth kinetics of N-face polarity GaN by plasma-assisted molecular-beam epitaxy

We have studied the surface kinetics of N-face GaN during molecular-beam epitaxial growth by investigating the Ga wetting and the surface morphology. In the case of N-face GaN, it is not possible to establish the self-regulated Ga bilayer that is used as a surfactant for molecular-beam-epitaxy growth of Ga-face GaN. Indeed, to prevent the accumulation of Ga droplets, growth of the N-face GaN must be performed with less than one monolayer of excess Ga on the growing surface. Optimum surface morphology is achieved when growth is performed at the Ga accumulation limit.

Intraband absorptions in GaN/AlN quantum dots in the wavelength range of 1.27-2.4 μm

GaN/AlN quantum-dot superlattices grown by molecular-beam epitaxy on silicon (111) or sapphire (0001) substrate have been investigated using high-resolution transmission electron microscopy, photoluminescence, and photo-induced absorption spectroscopy. Under interband excitation at λ≈351 nm, three resonances are observed, respectively peaked at 2.1 μm (2.36 μm), 1.46 μm (1.69 μm), and 1.28 μm (1.27 μm) for the sample grown on silicon (sapphire) substrate. We show that the absorptions involve conduction-band interlevel transitions from the ground state to p-like or d-like states, and that their energy is governed mainly by the magnitude of the internal field in the GaN dots.

Effects of stacking on the structural and optical properties of self-organized GaN/AlN quantum dots

We report on the effect of vertical correlation on GaN/AlN quantum dots grown by plasma-assisted molecular-beam epitaxy using the modified Stranski–Krastanow growth mode. When increasing the number of GaN periods, we observe a homogenization of the island distribution and a redshift of the luminescence line. This redshift is attributed to an increase of the quantum Stark effect due to the increase of the piezoelectric contribution to the internal electric field.

Lateral epitaxial growth of germanium on silicon oxide

We have developed a method using local oxidation on silicon to create nanoscale silicon seeds for the lateral epitaxial overgrowth of germanium on silicon oxide. The germanium growth starts selectively from silicon seed lines, proceeds by wetting the SiO2 layer and coalesces without formation of grain boundary. Analysis by high resolution transmission electron microscopy have shown that Ge layers grown above silicon oxide are perfectly monocrystalline and are free of defect. The only detected defects are situated at the Ge∕Si interface. Geometrical phase analyses of the microscopy images have shown that the Ge layer is fully relaxed and homogeneous.

Dislocation and antiphase domain free microscale GaAs crystals grown on SiO2 from (001) Si nano-areas

The epitaxial lateral overgrowth of microscale GaAs crystals on a 0.6 nm thick SiO2 layer from nanoscale Si seeds is investigated in order to develop GaAs monolithic hetero-epitaxy onto (001) Si. The nucleation from small width openings enables to avoid the emission of misfit dislocations and the formation of antiphase domains. Consequently, the interface between the GaAs island and the SiO2 layer remains perfectly sharp and free of defects. The only defects found by transmission electron microscopy in each island are pairs of twins, and a simple model based on the anisotropy of zinc blende crystal is proposed to explain their formation. Micro-photoluminescence measurements performed at room temperature show that these twins are not detrimental for the quality of microscale GaAs crystals.

Gold anchoring on Si sawtooth faceted nanowires

This paper reports on the sawtooth faceting and the related gold coverage of silicon nanowires (NWs). 111-oriented Si NWs were grown on Si(111) substrates by ultra high vacuum chemical vapor deposition using the vapor-liquid-solid mechanism with a gold catalyst. We observed that gold nanoclusters are unequally distributed on the NW surface. They are mainly distributed on only three non-consecutive sidewalls corresponding to the (), () and () planes among the six available crystallographic {112} surfaces. In addition they are anchored on upward {111} facets. This original observation brings enhanced knowledge on the faceting mechanisms. The threefold symmetry of the facet formation and gold anchoring is supported by criteria of minimal Au/Si interfacial energy. Moreover results evidence that the selective presence of gold on the NW sidewalls affects the overall morphology due to an increased radial growth on the covered sidewalls.

Growth kinetics of Ge crystals on silicon oxide by nanoscale silicon seed induced lateral epitaxy

In this paper, we present our studies on the growth kinetics of Ge crystals on silicon oxide by nanoscale seed induced lateral epitaxy. We propose a simple and reliable method based on standard local oxidation of silicon technique for creating nanoscale silicon seeds at the edge of thermally grown silicon oxide stripes of desired thickness. The growth of Ge from germane is initiated in the two silicon seed lines and evolves toward a complete wetting of the SiO2 stripe after coalescence. The wetting mechanism of SiO2 by Ge is strongly dependent on the seed orientation and closely related to the development of {111} facets. The coalescence of adjacent Ge crystals results in an improvement in the organization of the initial material. As a result, no defect is visible in the inner part of the structure. The observed defects are arrays of misfit dislocations standing along the seed lines, while only few dislocations are visible through the Ge crystal. Geometric phase analysis of high resolution transmission el...

Angular resolved electron energy loss spectroscopy in hexagonal boron nitride

Core and low energy electronic excitations have been studied in hexagonal boron nitride. Electron energy loss spectra have been measured using an electron microscope equipped with a monochromator and an in-column filter. Energy filtered diffraction patterns have been recorded and provide us with a global view of anisotropic effects in reciprocal space. From combined angular and energy resolved measurements, we analyze the symmetry of the losses as a function of the energy transferred to the material. As an example we show how easily the core loss spectra at the K edges of boron and nitrogen can be measured and imaged. Low losses associated to interband and/or plasmon excitations are also measured and discussed from ab initio calculations. This electronic inelastic scattering technique is shown to provide results comparable to those obtained from non resonant X-ray inelastic scattering. Both techniques are complementary: X-ray scattering is more accurate at large q wave vector whereas electron spectroscopy is more accurate at low q and is an attractive tool to study excitonic effects, which are inaccessible by optical techniques. Furthermore the method is fast and local, at a nanoscale level, and is very promising for studying two dimensional materials and related heterostructures.