Recombination at mixed-valence impurity centers in PbTe(Ga) epitaxial layers

The photoconductivity kinetics in PbTe(Ga) epitaxial films prepared by the hot-wall method is studied. The recombination of nonequilibrium photoexcited electrons at low temperatures was found to proceed in two stages, with a period of relatively fast relaxation followed by delayed photoconductivity. The temperature at which delayed photoconductivity appears increases with decreasing film thickness. The relaxation rate over the period of fast relaxation depends on film thickness and is the lowest in the thinnest layers. In semi-insulating films, photoconductivity is always positive, whereas in samples with lower electrical resistivity positive and negative photoconductivities are observed to coexist. The data obtained are discussed in terms of a model in which the impurity gallium atom can be in more than one charged state.