Publications

The effect of tin impurities on optical spectra of thermally deposited AsSe films doped with Sn (1 to 10 at%) was studied in a wide energy interval from 0.8 to 6.2 eV by combination of reflection, absorption, photoresponse and photocapacitance spectroscopies. Most changes have been detected at the fundamental absorption edge over which a correlation between the band tail width and optical gap is demonstrated for various tin concentrations. The tin induced absorption band associated with a localized energy level at about 1.6 eV in the gap was revealed. Distinct variations in the reflectivity spectra of the fundamental absorption region suggest a tin assistance in the formation of the valence band states of the material.

We present the application of the inverse scattering method to the design of semiconductor heterostructures having a preset dependence of the (conduction) electrons' reflectance on the energy. The electron dynamics are described by either the effective mass Schrodinger equation or by the (variable mass) BenDaniel and Duke equations. The problem of phase (re)construction for the complex transmission and reflection coefficients is solved by a combination of Pade approximation techniques, obtaining reference solutions with simple analytic properties. Reflectance-preserving transformations allow bound state and reflection resonance management. The inverse scattering problem for the Schrodinger equation is solved using an algebraic approach due to Sabatier. This solution can be mapped unitarily onto a family of BenDaniel and Duke type equations. The boundary value problem for the nonlinear equation which determines the mapping is discussed in some detail. The chemical concentration profile of heterostructures whose self consistent potential yields the desired reflectance is solved completely in the case of Schrodinger dynamics and approximately for BenDaniel and Duke dynamics. The Appendix contains a brief digest of results from the scattering and inverse scattering theory for the one-dimensional Schrodinger equation which is used in the article. (C) 1999 Elsevier Science Ltd. All rights reserved.

Nitrate powder with cation composition Bi:Pb:Sr:Ba:Ca:Cu = 1.8: 0.4:1.8:0.2:1.2:2.0 was obtained by spray-frozen, freeze-drying technique. Samples of the nitrate precursor powder were placed in a heated furnace (heating rate similar to 100 degrees C/min) and extracted in air when temperature of the powdered samples attained values of 439, 495, 550, 600, 640, 647, 717, 766, 814, and 850 degrees C. Samples have been investigated by X-ray diffraction analysis. The obtained data allow us to propose and discuss phase formation and decomposition processes and reactions that occur in non-isothermal conditions at different temperatures during thermal decomposition of the nitrate powder.

A new model is proposed for conduction in thick-film SnO2 gas sensors, which offers a good explanation for the ageing effect. in this model, electrical resistance is governed by necks and it results from three interdependent resistances of three different regions of the sample, connected in parallel: a surface resistance which is that of a gas-sensitive, predominantly n-type semiconductor, a resistance, belonging to a different region of the sample, which, due to its interaction with water vapour, behaves like a mixed n-type and p-type semiconductor, and the n-type bulk resistance of the neck, slightly affected by gases and/or water vapour. (C) 1999 Elsevier Science S.A. All rights reserved.

A microstructural investigation of porous silicon films, related with a study of carriers transport. is presented. Scanning electron microscopy and conventional transmission electron microscopy with selected area electron diffraction were performed. The films show a double scale porosity: the macroporosity forms an alveolar structure at a micrometric scale and the nanoporosity forms a silicon nanowires network in the alveolar walls and at their surface. Current-voltage characteristics in the -5 to +5 V range and the temperature dependence of dark current in the 150-330 K interval at low voltage were measured. The transport properties are explained by means of a simplified quantum confinement model. Two symmetries of the nanowires, cylindrical and square, are investigated and the differences between them are shown to be insignificant. (C) 1999 The Electrochemical Society. S0013-4651(98)12-109-2. All rights reserved.

gamma-Fe2O3 nanoparticles dispersed in a polymer matrix have been characterized by x-ray diffraction (XRD) and transmission electron microscopy (TEM). The electron paramagnetic resonance (EPR) in 64 Angstrom particles of gamma-Fe2O3 dispersed in a polymer matrix has been investigated in the range 136-286 K.

Iron carbides and iron composites preseal a particular interest due to their magnetic and catalytic properties which could be highly enhanced if particles in the nanometric size range were used. We present results on the synthesis and characterization of iron-carbide nanoparticles by sensitized CO2 laser pyrolysis or gas mixtures containing iron pentacarbonyl (vapors) and ethylene/acetylene precursors. The influence of the nature of hydrocarbon on the structural properties of the nanopowders are revealed by different analytical techniques (IR spectrophotometry, X-ray diffraction, transmission electron microscopy and Mossbauer spectroscopy).

Carbon nitride (CNx) this films were produced by CO2 laser (lambda=10.6 mu m) irradiation of mixtures containing C2H2/N2O/NH3, in a flow reactor, on Si substrates. The experimental parameters (partial concentrations of the reactants, gas flows. total pressure) were chosen in order to maximize the nitrogen incorporation in films. Chemical composition and bonding structure of the deposited films were investigated by; X-ray photoelectron spectroscopy (XPS). Thus, it was found that nitrogen is chemically bonded to C in sp(2) or sp(3) configurations, the NIC ratio (considering only the N atoms bonded to carbon) being similar to 20%. Scanning electron microscopy (SEM) shows a specific growth morphology, while the transmission electron diffraction (TED) and X-ray diffraction (XRD) analysis revealed that the CN, films were crystalline, with diffraction lines that marches rather well with those of the predicted beta-C3N4 form.

We compare the quantum and the classical description of the two-dimensional motion of electrons subjected to a perpendicular magnetic field and a one-dimensional lateral superlattice defined by spatially periodic magnetic and electric fields of large amplitudes. We explain in detail the complicated energy spectra, consisting of superimposed branches of strong and of weak dispersion, by the correspondence between the respective eigenstates and the ''channeled'' and ''drifting'' orbits of the classical description. [S0163-1829(99)04732-3].

Hall effect, magnetoresistance, and electrical conductivity measurements, carried out on ZnO surface wells created by a large variety of methods, are analyzed in the frame of the weak-localization theory. The ZnO surface wells have some unique features that allow the investigation of the weak-localization effects: ZnO has a single valley conduction band; the Thouless length is much larger than the elastic mean-free path even at room temperature; the well accumulates the largest surface electron concentration obtained up to now in a surface quantum well; there are a large variety of preparation methods, some of them making it possible to modify independently both the width and the depth of the surface wells. These features allowed us to investigate: the presence of the weak-localization effect in the largest range of temperatures(1.6-300 K) reported up to now for a quantum well; the influence on the transport properties of the increase in the number of subbands in the well; the effect of the presence of more inelastic scattering mechanisms and their weights in the entire scattering process; and the passage from a quasi-two-dimensional system to a three-dimensional one. [S0163-1829(99)01932-3].