Energy level associated with theDXcenter inGa1−xAlxAs

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Abstract
We examine in detail the variation of the DX-associated energy-level position versus alloy composition. It is found to be located at a constant energy from the L band whatever the nature of the impurity and the lattice site it occupies. Using the enhancement of the electron emission rate from the DX center with electric field, which happens to be characteristic of a Poole-Frenkel effect, we deduce that this energy level corresponds to a single donor state. The existence in the gap of two single donor levels, one shallow and one deep associated with the DX center, implies that these levels must arise from two effective-mass states associated with two different conduction bands: the shallow one with the lowest conduction band, and the deep one with the L band. We develop the statistics of occupation corresponding to this unusual situation. The identification of the DX level with the L effective-mass state deepened by intervalley mixing is in agreement with the Coulombic nature of its potential.