Search

This study investigated the physicochemical properties of a V2O5 thin film deposited on c-plane sapphire through thermal evaporation at a relatively high pressure. Using atomic force microscopy (AFM), X-ray diffraction (XRD) and a suite of X-ray spectroscopic techniques, it was shown that a high quality epitaxial V2O5 thin film was achieved. AFM step height analysis demonstrated that the film thickness was ~ 50 nm with a surface roughness of 1.5 A, as determined by root mean square roughness measurements. XRD analysis verified that the film was highly crystalline with a (0001) orientation on the substrate. Vanadium was predominantly in the 5 + oxidation state, with contributions from V4 + states at the surface, shown by X-ray photoemission spectroscopy analysis X-ray absorption spectroscopy further confirmed the predominant presence of V5 + in an octahedral crystal field. The existence with bulk V4 + states was shown through V L-edge X-ray emission spectroscopy which demonstrated the presence of d-d crystal field transitions in an otherwise d0 transition metal oxide. The data suggests that by increasing the partial pressure of oxygen in the vacuum chamber during growth, thermal evaporation can be used as a cheap and efficient way of growing stoichiometric V2O5 thin films.

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.

The ultra-high vacuum surface preparation of heteroepitaxial lanthanum strontium cobalt ferrite thin films has been studied using soft X-ray photoelectron spectroscopy. Specifically, the effect of annealing the films at low temperatures in low partial pressures of oxygen and argon has been investigated. We find that atmospheric surface carbon contamination of the films can be removed in select anneal temperature regimes in argon, but remains bound to the surface with oxygen annealing at any temperature. Irrespective of the gas used, an insulating phase transition occurs near 300 °C due to strontium segregation at the surface. The surface develops more insulating character if annealed with oxygen. Different species are proposed to be responsible for the discrepancy in insulating character.

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.

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.

original title in E-MRS Spring 2012 - Symposium W : "Synchrotron X-ray diffraction complementary peak analysis for quantitative defect evaluation in GaP/Si nanolayers"; International audience; In the context of III-V monolithic integration on silicon, synchrotron X-ray diffraction has been employed in this study using a bi-dimensional large area hybrid pixel detector (XPAD third generation) to characterize defects in the GaP layers. Despite a very coherent interface (low plastic relaxation) of GaP/Si, 2 types of defect are detected. Micro-twins contributions are evidenced and quantitatively evaluated from additional reflections analysis. Antiphase domains are evidenced using the Williamson-Hall-like plot method applied to transverse scans extracted directly from single XPAD images taken on specular GaP reflections.

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.