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We present a systematic synchrotron radiation photoemission spectroscopy study of the interaction of O2 with defective La0.8Sr0.2Co0.2Fe0.8O3 − δ(100) surfaces at low temperatures. First, the surface chemical evolution during low energy Ar+ ion sputtering is investigated and is found to create oxygen vacancies. Subsequently, the interaction of the O2 molecules with La0.8Sr0.2Co0.2Fe0.8O3 − δ(100) surface is observed to create an insulating phase. We performed a detailed core-level peak-fitting analysis to identify the chemical nature of this phase and to probe the role of vacancies in the formation of this phase.

Indium induced surface reconstructions of Si(1 1 1)-7 × 7 are used as templates to grow high quality InN. We grow InN on Si(1 1 1)-7 × 7, Si(1 1 1)-4 × 1-In and Si(1 1 1)-1 × 1-In reconstructed surfaces and study the quality of the films formed using complementary characterization tools. InN grown on Si(1 1 1)-1 × 1-In reconstruction shows superior film quality with lowest band-edge emission having a narrow full width at half maximum, intense and narrow 0 0 0 2 X-ray diffraction, low surface roughness and carrier concentration an order lower than other samples. We attribute the high quality of the film formed at 300 °C to the integral matching of InN and super lattice dimensions, we also study the reasons for the band gap variation of InN in the literature. Present study demonstrates the proposed Superlattice Matched Epitaxy can be a general approach to grow good quality InN at much lower growth temperature on compatible In induced reconstructions of the Si surface.

The structural modifications of an Si(111)- 7x7 reconstructed surface and the evolution of growth induced by Ga adsorption in the submonolayer regime at various substrate temperatures ranging from room temperature (RT) to 600 °C, with a low Ga flux rate of 0.1 ML/min (1 ML∼6.8×1014 atoms/cm2) have been studied in-situ in Ultra High Vacuum (UHV) using Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED) and Electron Energy Loss Spectroscopy (EELS) as characterization probes. Ga grows in the Stranski–Krastanov (SK) growth mode for temperatures ranging from RT to 350 °C, where 3D-islands form after one and two flat monolayers of Ga adsorption, while for higher temperatures ranging from 450 to 550 °C, Ga grows in the Volmer–Weber (VW) growth mode. A comprehensive 2D-phase diagram for this Ga/Si(111) system for adsorption, which provides pathways to attain the observed superstructural phases, viz., √3x√3-R30°, 6.3x6.3, 6.3√3x6.3√3-R30° and 11x11, has been investigated. The characteristic EELS spectrum for each superstructural phase is also reported in this study.

Using the morphological differences of low and high index surfaces as templates for metal growth, several low dimensional overlayer structures with novel structural and electronic properties can be formed. We present here a first report on submonolayer adsorption and residual thermal desorption studies of In adatoms on reconstructed high index Si (5 5 12)−2 × 1 surface and compare it with the observations on planar Si (111)−7 × 7 surface. The study is done by using in-situ Ultra High Vacuum surface sensitive probes like Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). These conventional wide area techniques provide an understanding of atomistic issues involved in the evolution of the interface. We have observed an anomalous growth mode during adsorption at room temperature (RT) above 2ML, which includes adatom layering and clustering on Si (111) surface. This is also manifested during the desorption experiments on both surfaces, and the subtle differences on the two surfaces are discussed. The observation of LEED pattern during the adsorption process shows formation of different superstructural phases on Si (111)−7 × 7 surface. On Si (5 5 12) 2 × 1 surface we observe the sequential 2× (225), 2× (337) and 2× (113) facet formation during adsorption/desorption, which include quasi 1D-nanowire/chain structures. A combination of lattice strain effects, presence of step-edge barrier and quantum size effects are employed to speculate the differences in adsorption and desorption.

We report hard x-ray photoemission spectroscopy measurements of the electronic structure of the prototypical correlated oxide ${\mathrm{Sr}}_{x}{\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{VO}}_{3}$. By comparing spectra recorded at different excitation energies, we show that 2.2 keV photoelectrons contain a substantial surface component, whereas 4.2 keV photoelectrons originate essentially from the bulk of the sample. Bulk-sensitive measurements of the O $2p$ valence band are found to be in good agreement with ab initio calculations of the electronic structure, with some modest adjustments to the orbital-dependent photoionization cross sections. The evolution of the O $2p$ electronic structure as a function of the Sr content is dominated by $A$-site hybridization. Near the Fermi level, the correlated V $3d$ Hubbard bands are found to evolve in both binding energy and spectral weight as a function of distance from the vacuum interface, revealing higher correlation at the surface than in the bulk.

International audience; We report on the association of Ultra High Vaccum Chemical Vapor Deposition (UHVCVD) and Molecular Beam Epitaxy (MBE) to achieve III-V (GaP) integration on Si/Si(100) substrates. We first demonstrate that a very good flatness (0.3 nm) can be obtained when growing directly GaP on a chemically prepared Si substrate. X-ray diffraction pole figure however demonstrates that a residual amount of micro-twins originating from the hetero-interface still remains. Silicon homoepitaxial buffer layer is then optimized in the UHVCVD chamber on different Si substrates misorientation (+/-0.15-6°-off) . A flat, clean and bistepped Si surface is achieved during the homoepitaxial growth on 6°-off silicon substrates. Samples are then transferred under UHV conditions to the MBE chamber to perform GaP overgrowth. Keeping the same III-V overgrowth conditions, influence of silicon homoepitaxial buffer layer on micro-twins generation is determined quantitatively using Synchrotron X-Ray Diffraction. We finally demonstrate that growing a flat, clean and bistepped silicon buffer layer on a 6°-off substrate, and transferring it under UHV to the MBE chamber for GaP overgrowth reduces significantly the amount of anisotropic defects generated in the GaP epilayers.

International audience; In this work, we study the nitrogen-phosphorus competition during the molecular beam epitaxial growth of GaPN alloy with N content ranging from 0 to 6%. N2 decomposition in the valved RF plasma cell is first optimized through the spectroscopic analysis of the plasma luminescence. Strain relaxation process and determination of N content in GaPN layers are then clarified using a reciprocal space mapping around the (224) Bragg reflection. Influence of growth temperature and phosphorus beam equivalent pressure (BEP) on the nitrogen incorporation is finally studied. It is demonstrated that nitrogen incorporation is mainly governed by temperature and phosphorus BEP, through the nitrogen-phosphorus competition. A good control of the nitrogen content is achieved when the temperature is low enough to avoid irreproducibility due to N or N2 desorption, and the phosphorus BEP high enough to limit the effects of nitrogen-phosphorus competition. Finally, a good accuracy on the nitrogen content control is achieved for a wide range of nitrogen composition, using effects of growth rate and valve opening.

We study the surface charge accumulation on InN thin films that strongly effects mobility of charge carriers. The films are formed by MBE in the temperature range (400–470 °C) yielding films with different morphology, crystallinity, and optical properties. The band-gap values determined by optical absorption and PL studies are found to depend on the Hall carrier concentration as per the Moss-Burstein relation. The magnitude of the near surface band bending is measured on InN thin films by knowing the surface and bulk Fermi level position with respect to the valence band maximum using X-ray photoelectron spectroscopy and optical absorption measurements, respectively. The studies show that the surface charge occurs due to the In adlayer on the film causing band-bending is up to 1 eV for low band gap single crystalline films and nearly a flat band for the highly degenerate polycrystalline films. The absence of the band bending is seen to occur when the bulk carrier concentration related to the crystalline quality of the InN films, tends to equate with the surface charge density.

The issue of ambiguous values of the band gap (0.6 to 2 eV) of InN thin film in literature has been addressed by a careful experiment. We have grown wurtzite InN films by PA-MBE simultaneously on differently modified c-plane sapphire substrates and characterized by complementary structural and chemical probes. Our studies discount Mie resonances caused by metallic In segregation at grain boundaries as the reason for low band gap values (≈ 0.6 eV) and also the formation of Indium oxides and oxynitrides as the cause for high band gap value (≈ 2.0 eV). It is observed that polycrystallinity arising from azimuthal miss-orientation of c-oriented wurtzite InN crystals increases the carrier concentration and the band gap values. We have reviewed the band gap, carrier concentration, and effective mass of InN in literature and our own measurements, which show that the Moss-Burstein relation with a non-parabolic conduction band accounts for the observed variation of band gap with carrier concentration.