Z. Wilamowski

Parameters of substrates-single crystals of ZnTe and Cd1-xZnxTe (x < 0.25), obtained by physical vapor transport technique (PVT)

Abstract It is shown that very high quality (in particular – very good real crystal structure) of the single crystals of ZnTe and Cd 1− x Zn x Te, intended to be substrates for MBE and other techniques of epitaxy, can be reached by an optimized physical vapor transport (PVT) technique. The obtained crystals are twin-free, the narrowness of the rocking curve nearly reaches the theoretical limit and the density of dislocations is low. The results of characterization of the crystals are presented.

Properties of metal-insulator transition and electron spin relaxation in GaN:Si

We investigate properties of doping-induced metal-insulator transition in GaN:Si by means of electron spin resonance and Hall effect. While increasing the doping concentration, Si-related bands are formed below the bottom of the GaN conduction band. The D0 band of single-occupied Si donor sites is centered 27 meV below the bottom of the GaN conduction band, the D- band of double-occupied Si states at 2.7 meV below the bottom of the GaN conduction band. Strong damping of the magnetic moment occurs due to filling of the D- states at Si concentrations approaching the metal-insulator transition. Simultaneously, shortening of electron spin relaxation time due to limited electron lifetime in the single-occupied D0 band is observed. The metal-insulator transition occurs at the critical concentration of uncompensated donors equal to about 1.6 * 10^18 cm^-3. Electronic states in metallic samples beyond the metal-insulator transition demonstrate non-magnetic character of double-occupied states.

Pressure dependence of electron concentration and mobility in GaAs:Si-effects of on-site and inter-site interactions within a system of DX centres

A novel special experimental procedure, where a helium gas pressure cell is employed to vary the hydrostatic pressure at low temperatures, is used to study the freeze-out of carriers on the metastable states of Si donors in GaAs. This technique separates the modifications of the mobility originating from alterations in the band structure from those related to the electron transfer to DX centres. The procedure allows a more precise evaluation of the ability of high-pressure experiments to distinguish between models of positive and negative U for the DX centre. The mobility increase with pressure is theoretically shown to occur for both repulsive (positive U) and attractive (negative U) on-site electron-electron interactions, provided that the inter-site Coulomb interactions are taken into account.

Correlation of donor electrons in diluted magnetic semiconductors with iron

Substitutional iron in HgSe forms a resonant donor state whose energy is superimposed on the conduction band continuum. Above a certain doping level the system of Fe donors is only partially occupied by electrons, i.e. two charge states, Fe3+ and Fe2+, coexist, corresponding to the inhomogeneous mixed valence regime. Under these conditions the Coulomb repulsion between the donor electrons tends to keep them apart, leading to a correlation of their positions. The existence of the correlation results in a dramatic reduction of the rate of scattering by ionised impurity potentials. The same Coulomb interactions are responsible for the formation of the Coulomb gap in the one-particle density of impurity states, which in turn suppresses (otherwise very efficient) resonant scattering. As a result, the mobility and the Dingle temperature in HgSe:Fe can exceed those in HgSe doped with Ga to similar doping levels. A review is given of relevant experimental results obtained in HgSe:Fe, Hg1-xMnxSe:Fe and HgSe1-xTex:Fe by means of the Shubnikov-de Haas effect, resistivity and Hall voltage studies. Experimental findings are compared with results of numerical simulations and with a simple model calculation which accounts for short-range correlations.

DX centres and Coulomb potential fluctuations

DX centres, as well as other charged impurities with random location in the host crystal, give rise to fluctuations in the local potential. These fluctuations are comparable in amplitude to the level splittings of the DX centres due to different numbers of Al neighbours in AlxGa1-xAs:Si and also to the variation in the quasi-Fermi level in experiments involving the kinetics of electron capture and emission processes. Thus, to describe the situation in AlxGa1-xAs:Si in a quantitative way the authors develop a self-consistent model of the fluctuations. The resulting broadening of the DX level is taken into account in evaluating transport experiments. The proposed model yields the ground state energies and the barrier height for the four types of Si DX centre as functions of alloy composition and hydrostatic pressure. They explain also the non-exponential behaviour of the capture kinetics as well as the mobility in terms of the impurity level broadening and self-screening which results from a minimization of the Coulomb energy of interacting charges localized on donors.

Electron paramagnetic resonance and Coulomb gap in HgSe:Fe

The temperature and concentration dependence of the EPR absorption due to the Fe3+ state in HgSe is shown to be consistent with a Coulomb gap in the defect density of states. The latter is a consequence of inter-ion Coulomb interaction which renders a correlated ground state with short-range order of the partially ionised resonant donors. Possible mechanisms for a decrease of both the mobility and the EPR intensity for very high iron concentrations in this ordered, glass-like state are discussed.

Coupling of Mn2+ spins with a 2DEG in quantum Hall regime

Abstract An experimental study of electronic and magnetic properties of an n-type modulation doped CdMnTe quantum well structure has been performed in order to investigate the s–d exchange interaction and its description within the mean field theory. The results are differently interpreted at low and high magnetic fields. At low magnetic fields, the system is found to behave as two paramagnetic subsystems coupled via s–d exchange interaction. At high magnetic fields, the observed Knight shift is too large to be explained by the simple mean field approach.

Monte Carlo simulations of spatial correlation effects of charged centres in delta -doping layers

This paper presents results of Monte Carlo simulations of a system of charged donors in delta -doping layers, aiming at elucidating the effects related to a spatial correlation of donor charges in a 2D configuration. The simulations yielded several quantities describing the system, among them the pair correlation functions, gain in the energy of the system due to correlation of charge locations and density of impurity states, all as functions of temperature. The calculations have direct application in analysis of the behaviour of the DX centres in GaAs planarly doped with Si.

Appearance and destruction of spatial correlation of DX charges in GaAs

Measurements of the Hall coefficient and the conductivity in n-GaAs heavily doped with silicon were performed at 4.2 K under hydrostatic pressure applied at an elevated temperature. The procedure ensures that a metastable occupation of the DX centres by electrons is induced. As shown earlier, electric charges on DX centres form, under these conditions, a spatially correlated system of charges. The correlation marks its presence as an enhancement of the electron mobility. Here the authors study the destruction of the spatial correlation of donor charges by illumination with an LED. A calculation within the short-range correlation model accounts well for the decrease of the mobility seen after illumination.

Investigation of DX centers in AlxGa1−xAs by space charge spectroscopy

We present a critical analysis of both deep level transient spectroscopy and transient microwave absorption spectroscopy (MAS) for the case of DX centers in AlGaAs. We show that, even for a single level, a strongly nonexponential time dependence of the transients occurs. Our MAS experiments on Si‐DX centers in Al0.3Ga0.7As extend the available data for the emission rates by more than three orders of magnitude. They are successfully interpreted by a single emission time constant using our model whereas at least three decay time constants are needed to explain the data by a pure exponential. Observed recapture processes in the neutral region of the sample underline the complexity of space charge spectroscopy in the case of the DX center.