N. Tabet

On the existence of superstructure in TiNx thin films

Abstract Titanium nitride films deposited on NaCl single crystal substrates were prepared using reactive d.c. magnetron sputtering under deposition conditions yielding near stoichiometric films. Several large areas (φ≈20 nm) of superstructure were formed by annealing these films under vacuum at 873 K for 30 min. The existence of a superstructure was observed by high resolution transmission electron microscopy (HRTEM) in thin films which were annealed at 873 K. Only the normal structure was observed below this temperature.

Cathodoluminescence dependence upon irradiation time

Abstract The cathodoluminescence (CL) intensity varies with beam exposure time. In this work, the CL change as a function of irradiation time has been studied using various semiconducting materials: CdS single crystal, CdS evaporated thin films, ZnO ceramics and GaAs single crystal. A current density as low as 60 μm/cm 2 was used in an electron microprobe analyser. In the case of low excitation level, two stages of the CL variation have been generally observed, i.e. increasing and decreasing parts. In the case of a relatively high excitation, only a decreasing stage can be observed. It is believed that the CL time dependence is closely related to the adsorption-desorption process and the surface contamination which are stimulated by the electron beam excitation.

Calculation of the electron-beam-induced current (EBIC) at a Schottky contact and comparison with Au/n-Ge diodes

Abstract The electron-beam-induced current (EBIC) collection efficiency n of a Schottky contact perpendicular to the electron beam of a scanning electron microscope is calculated. The continuity equation is solved for a non-uniform generation function assuming a linear variation of the electric field within the depletion zone. The recombination of the carriers at the metal-semiconductor interface is considered. Majority carrier injection from the semiconductor into the metal is found to be non-negligible for low doping level and low beam energy. It is demonstrated that the assumption of 100% collection efficiency for the minority carriers generated within the depletion zone, and the related boundary condition, lead to an overestimation of the collected EBIC intensity, particularly for short minority diffusion lengths. The present model allows a more satisfactory analysis of the experimental data obtained on Au/n-Ge Schottky contacts.

KRXPS study of the oxidation of Ge(001) surface

Abstract The oxidation of the chemically etched Ge(001) surface has been investigated by means of kinetic resolved X-ray photoelectron spectroscopy (KRXPS). In situ oxidation treatments have been carried out at T =250°C, under 0.5 atm of pure oxygen atmosphere and for different durations. We propose a two step oxidation model. First, the bare portions of the surface are oxidized. Then, follows a lateral growth of the germanium oxide islands. The process is assisted by the decomposition of the overlayer that stems from the chemical etching and the air contamination. The model gives a consistent interpretation of the KRXPS measurements.

Characterization of polycrystalline germanium by EBIC

Abstract The electron-beam-induced-current (EBIC) mode of a scanning electron microscope has been used to characterize the electrical activity of the grain boundaries in germanium. The boundaries have also been characterized from a structural point of view using electron channelling patterns (ECP) and X-ray topography. The low-angle boundaries in Ge polycrystals show an EBIC contrast C which increases when the beam energy increases, while the Σ3 twins do not show a significant activity even when there is a deviation from the coincidence orientation. The collection efficiency η of Schottky contacts has been measured. Both C and η measurements cannot be quantitatively analysed using previously existing models. The basic ideas which should lead to a better modelling are discussed.

GROWTH OF ZnO NANOSTRUCTURES ON ZINC AND Pt SUBSTRATES

Nanocrystalline ZnO of various shapes was obtained without the use of catalysts by dry oxidation of metallic zinc. Oxidation treatments performed below the melting point of Zn(Tm = 419°C) led to the formation of flakes-type structure. At 500°C, long nanofibers of circular and uniform cross section covering the surface of the oxidized samples were observed. At 600°C, large density of cone-shape needles covered the oxidized surface. High quality nanocrystals of hexagonal shape were obtained by vapor deposition on Pt substrate. XRD results showed that the lattice parameters of the needles are 1% smaller than those of microcrystalline ZnO.

Electron beam induced current at a Schottky nanocontact

The electron beam induced current (EBIC) collection efficiency η of a circular nano Schottky contact of radius rc perpendicular to the electron beam was simulated using a Monte Carlo (MC) algorithm. The EBIC was obtained by simulating the random diffusion and collection of the minority carriers that are generated at point-like sources Si randomly distributed within the generation volume. The profile of the EBIC collection versus the distance to the nanocontact is simulated for two extreme values of the free surface recombination velocity vs (vs=0 and vs=∞). The dependence of the maximum value of the collection efficiency ηmax, obtained as the electron beam impinges the surface at the centre of the nanocontact, was simulated as a function of radius rc. In addition, the variation of ηmax versus the incident beam energy was obtained.

TiN?Fe nanocomposite thin films deposited by reactive magnetron sputtering

TiN–Fe films with various iron concentrations were deposited on Si and NaCl single-crystal substrates by direct current reactive magnetron sputtering. The structure and chemical composition of the films were examined by x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), energy-dispersive x-ray and x-ray photoelectron spectroscopy (XPS). The effects of Fe addition on the structural, mechanical and magnetic properties of TiN films were studied. XRD and HRTEM revealed for TiN–Fe a cubic B-1 structure while no sign of an iron phase was observed. It was found that the lattice parameter and the grain size decreased with an increase in the Fe/Ti atomic ratio. At Fe/Ti = 0.2, the XRD revealed a change in the preferential orientation from (1 1 1) to (2 0 0) with a tendency of line broadening as the iron concentration increased. The Fe 2p core level peak of XPS indicates that the greater part of Fe atoms in TiN–Fe films exists as free pure metallic iron while the lesser part was in the form of iron oxide. Film annealing at 500 and 600 °C led to iron precipitation of the α-Fe phase indicating that below 500 °C Fe–TiN, films can be considered as nanocomposites materials, as confirmed by nanoindentation measurements and HRTEM observations.

Pressure effect on the growth of oxide layers on germanium substrates

Abstract X-ray Photoelectron Spectroscopy (XPS) was used to investigate the growth of thin oxide layers obtained by dry oxidation on ( 011 ) germanium substrates. The heat treatments were carried out, in-situ, at T =380°C under various values of air pressure. The quantitative analysis of the XPS spectra suggests the growth of non uniform oxide layers. An apparent thickness of the oxide film was defined as function of the fraction of the oxidized surface and of the actual thickness of the oxide islands. The results show a quasi linear dependence of the apparent thickness versus the air pressure.

Synthesis and XRD/PL Studies of Pure and Sb2O3 Doped ZnO Nanophases

Pure and Sb2O3 (0 to 5% molar fraction) doped ZnO nanophases were synthesized using a sublimation‐condensation method in a solar furnace. The initial and final powders were characterized by X‐ray diffraction (XRD) and photoluminescence (PL) techniques. XRD results showed no significant change in the lattice parameters and the presence of a new phase Zn7O2Sb12 in the highly doped micropowders but not in the nanopowders. The photoluminescence spectra showed a strong donor‐acceptor pair (DAP) emission in the pure untreated ZnO micropowder which is drastically reduced in pure and doped nanopowders. The donor‐bound excitonic band (DX) includes three well resolved peaks in the PL spectra of the doped micropowders while the spectra of doped nanopowders showed a broader band. Furthermore, the free exciton emission was absent in all doped samples.