Degenerate impurity states in hopping conduction

A review of studies on nearest-neighbour hopping (NNH) between impurities, whose ground state (GS) is degenerate or split by the application of a unia xial stress X or a magnetic field B is presented. The case of acceptors in Ge and Si, whose GS components have different symmetry properties. is discu ssed in detail. A solution of the percolation problem involving all the pos sible transitions between the components of the acceptor GS is provided. Fo r Ge it was preceded by a construction of the acceptor ground state wavefun ctions, possessing both a correct symmetry and a correct asymptote behaviou r, indispensable for evaluating probabilities of phonon-assisted NNH. For S i the energy diagrams of the accepter GS for various configurations of X an d B were obtained and subsequently used in a model calculation. In both Ge and Si the calculation accounted qualitatively for the data.