We present the analysis of magnetotransport measurements of n-type GaAs samples grown by MBE technique. The samples cover a wide range of donor concentrations, from 4.0 x 10(14) cm(-3) to 3.0 x 10(16) cm(-3). Experiments were performed for a wide range of temperatures and magnetic fields. The apparition of a magnetic freeze-out for conduction electrons allows us to extract the binding energy of donors as a function of the magnetic field as well as the impurity concentration. The measured binding energies decrease as the doping level increases. This behavior is due to the overlap of the wavefunctions of neighbouring impurities which results in the formation of an impurity band. (c) 2005 WILEY-VCH Verlag GmbH W Co. KGaA, Weinheim.

We have calculated the density of states (DOS) of a two-dimensional electron gas in a perpendicular magnetic field, using a multiple scattering method, in the ultraquantum limit. We have considered doped and disordered 2D systems. The results of the scattering method are compared with direct simulations of disordered samples. Using the DOS, we have studied the metal-insulator transition and the magnetic freeze-out including a comparison with experimental results. (C) 2004 Elsevier B.V. All rights reserved.

Our study presents experimental measurements of the contact and longitudinal voltage drops in Hall bars, as a function of the current amplitude. We are interested in the heating phenomenon which takes place before the breakdown of the quantum Hall effect, i.e. the pre-breakdown regime. Two types of samples has been investigated, at low temperature (4.2 and 1.5 K) and high magnetic field (up to 13 T). The Hall bars have several different widths, and our observations clearly demonstrate that the size of the sample influences the heating phenomenon. By measuring the critical currents of both contact and longitudinal voltages, as a function of the filling factor (around i = 2), we highlight the presence of a high electric field domain near the source contact, which is observable only in samples whose width is smaller than 400 mum. (C) 2004 Elsevier B.V. All rights reserved.

We have used silicon impurities in an aluminum arsenide barrier to probe an adjacent two-dimensional electron gas (2DEG). A single impurity acts as a local spectrometer and scans the local density of states of the 2DEG. Magnetotransport experiments have been performed at low temperature with a magnetic field B applied along the direction of the current. Current-voltage characteristics strongly depend on B and reveal the formation of Landau levels (LLs).

In this paper we report on both experimental and theoretical studies of the voltage drop across the source contact in the quantum Hall effect regime at high currents. Our investigations have been performed on the plateau v = 2 using two Hall bars of different widths, processed on the same sample. A steep increase of the voltage drop across the source contact is observed experimentally well before the breakdown of the quantum Hall effect. This behaviour is analysed in the framework of inter and intra Landau level scatterings. The theoretical curves reproduce correctly the main aspects of the experimental data. Our work corroborates the results published recently by the group of Komiyama upon the Landau emission of two-dimensional electron gas.

Discrete states of conduction electrons bound to ionized acceptors are observed in intraband magneto-optical experiments on Be-doped GaAs/Ga0.67Al0.33As quantum wells. The electrons are bound to acceptors by a joint effect of the quantum well and an external magnetic field. The observed transition energies are successfully described using states of single magnetoacceptors. The energies show evidence for oscillatory screening of acceptor potentials. Also, two disorder modes of the cyclotron resonance related to acceptor potential fluctuations are observed at higher filling factors.

Binding energy of donors in high quality epitaxial GaAs is investigated as a function of the magnetic field between 0 and 12 T. Transverse magnetoresistance and the Hall effect are used as experimental tools. The samples are characterized using temperature dependence of free electron density and mobility, taking consistently into account the Hall scattering factor and the effective conduction depth of the structure. Our analysis of the data at the freeze-out regime of higher magnetic fields allows for the hopping conductivity over donor states. The determined magnetodonor energies are about 1 meV lower than the theoretical ones, which represents a very large improvement in comparison with previous studies.