Electrical transport phenomena have been investigated in (Al,Ga)N/GaN heterostructures (without any intentional doping) with an Al content ranging from 10 to 32 %. Conductivity and Hall Effect measurements have been performed as a function of temperature up to 700 K. To analyse the temperature dependence of the two-dimensional electron gaz concentration (2DEG) n, a fully self-consistent calculation (coupled Schrodinger and Poisson equations), was developed for the conduction band edge of the heterostructures. The observed 2DEG concentration changes vs. temperature have been compared with the theoretical predictions taking into account the electric fields in the structure and the surface states. A calculation based on the relaxation time approximation has been developed to describe the temperature dependence of the mobility. The electrical performance of GaN/AlxGa1-xN heterostructures with x=0.32 % indicate that they could be good candidates for Hall sensor working at high temperature (up to 600 K). At room temperature, the magnetic field sensitivity is K K-H0= 60 V/A/T. Its temperature sensitivity S, is lower than +500 ppm/K up to 600K. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The effect of pressure on electrical transport phenomena has been studied in AlGaN/GaN heterostructures grown on Si substrates. The two-dimensional character of the conducting carriers has been verified with high magnetic field measurements (up to 13 T) at low temperature (T = 1.5 K). The two-dimensional electron gas (2DEG) concentration and mobility were measured as a function of hydrostatic pressure up to 1100 MPa and at different temperatures. The observed pressure changes of the free carrier concentration have been compared with theoretical predictions, taking into account the polarization change in the nitrides and using a fully self-consistent resolution of Schrodinger and Poisson equations.

The effect of an internal piezoelectric field on electrical transport phenomena has been studied in GaAlAs/GaInAs heterostructures grown on (001) and (111)B GaAs substrates. The two-dimensional electron gas (2DEG) in the structure was obtained by conventional modulation delta -doping in the barrier layer. The: conductivity and Nail effect were studied at room temperature as a function of hydrostatic pressures up to 1000 MPa. The observed pressure changes of the 2DEG concentration have been compared with theoretical predictions, taking into account the strain-induced electric fields in the barrier layer. The piezoresistive behaviour under uniaxial stress has also been determined. It was found that piezo-effects are more pronounced in the case of the material with the built-in piezoelectric field.

We have investigated the electron density of the delta -doped conventional AlGaAs/GaAs and pseudomorphic AlGaAs/InGaAs/GaAs heterojunctions. Shubnikov-de Hass (SdH) and quantum Hall effect measurements at 4.2 K have been carried out to investigate the existence of a quasi-two-dimensional gas (2DEG) in delta Si-doped AlGaAs/GaAs heterostructure. Cases have been also examined where the g-doping is placed in a narrow quantum well within the AlGaAs barrier with an Al concentration in the well (x(Al)(well)) less than in the barrier (x(Al)(Barr)). This is intended to increase the electron density. Secondary-ion mass spectroscopy (SIMS) measurements were also performed on one of these structures in order to demonstrate the spreading of the dopants. To improve the electron density, we have combined in only one structure the delta Si, in a narrow quantum well, and the InGaAs layer introduced at the heterointerface between the AlGaAs and the GaAs. A comparison between the theoretical and experimental results was performed. The variation trends of calculated electron density are in good agreement with the experimental ones. (C) 2001 Published by Elsevier Science B.V.

We report here on the study of the theoretical optical properties of Ga based nitrides systems. The dipolar matrix elements and hole effective masses are presented as a function of the reciprocal wavevector fur strained bulk GaN. We compute then the A. B and C exciton binding energies for strained GaN/Al0.2Ga0.8N quantum structures and present the calculated optical density for a 50 Angstrom wide well. (C) 1997 Elsevier Science S.A.

We have investigated theoretically the optical properties of strained (001) ZnSe-(Zn, Cd)Se single quantum wells with a low cadmium content (similar to 10%). Due to the band offset and strain energy, the electron-to-light-hole transitions are marginally type I. We compute the dispersion relations and joint density of states for a typical structure away from Gamma and show that the degeneracy call occur between lh(1) and hh(3) sub-bands in a large area of the Brillouin zone. This can cancel the selection rules for interband processes. (C) 1997 Elsevier Science S.A.