DX CENTERS 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 we 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. We 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.