Abdelaziz Zouari

Effect of the Oxygen Deficiency on the Structural, Magnetic and Electrical Properties in Perovskite-Like La0.6Sr0.4MnO3–δ

Structural, magnetic and electrical properties of oxygen-deficient La 0.6 Sr 0.4 MnO 3-δ with 0 ≤ δ ≤ 0.20 are presented. X-ray diffraction reveals a two-phase region separating a rhombohedral phase stable over 0 < δ < 0.127 and an orthorhombic phase stable in the range 0.15 < δ ≤ 0.20. Transport and magnetic studies indicate that the magnetic coupling and the electronic properties are strongly dependent on the oxygen deficiency δ. Rhombohedral samples are ferromagnetic with Curie temperature T C varying with δ; T C reached a maximum of 367 K for δ = 0.025 which corresponds to a 54% Mn 4+ /Mn 3+ ratio. Orthorhombic samples present an antiferromagnetic order. The magnetic interactions between manganese ions are estimated; their sign and magnitude confirms the obtained magnetic properties. The study of the electrical properties shows a semiconductor to metal transition as the temperature is lowered from 300 to 80 K for samples with oxygen vacancy 0 < δ ≤ 0.10. A semiconductor behavior for La 0.6 Sr 0.4 MnO 2.873 and an insulator one for 0.15 < δ < 0.20 are observed throughout the total temperature range studied. The electronic phase diagram in the plane of the temperature vs. oxygen deficiency δ has been deduced from the magnetic and electrical measurements.

Effect of the interface states on the cell parameters of a thin film quasi-monocrystalline porous silicon as an active layer

Unlike crystalline silicon, quasi-monocrystalline porous silicon (QMPS) layers have a top surface with small voids in the body. What is more pertinent to the present study is the fact that, at a given wavelength of interest for solar cells, these layers are often reported, in the literature, to have a higher absorption coefficient than crystalline silicon. The present study builds on existing literature, suggesting an analytical model that simulates the performance of an elementary thin QMPS (as an active layer) solar cell. Accordingly, the effects that the interface states located at the void-silicon interface and that the porosity of this material have on the cell parameters are investigated. Furthermore, the effects of the optimum base doping, QMPS thickness, and porosity on the photovoltaic parameters were taken into consideration. The results show that the optimum base doping depends on the QMPS thickness and porosity. For an 8@mm thickness, the film QMPS layer gives a 35.4mA/cm^2 for short-circuit current density, 15% for conversion efficiency, and 527mV for open-circuit voltage when the value of the interface states is about 10^1^2cm^-^2 and the base doping is about 2x10^1^8cm^-^3 under AM 1.5 conditions.

Analytical Model and Current Gain Enhancement of Polysilicon-Emitter Contact Bipolar Transistors

This paper presents an analytical model that simulates the current gain improvement of polysilicon-emitter bipolar transistors based on the effective recombination velocity method. The theoretical framework developed in this paper incorporates the 2-D structure effect of the polysilicon layer with columnar grain boundary, tunneling processes of holes through polysilicon/silicon interface oxide layer, and nonuniform doping concentration and bulk recombination effects in the single-crystal emitter. The study goes on to derive an analytical expression for the base current density from which the analytical expression of the current gain was then derived. The effect of oxide breakup, at the polysilicon/silicon interface, on current gain was also considered. The dependence of the current gain on temperature was analyzed numerically; the results are in good agreement with experimental data. The approach outlined in this paper allows one to avoid many of the unnecessary simplifications inherent in previous works and to clearly assess the relevance of each physical mechanism.