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.