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A formalism for the calculation of the optical cross section of a deep impurity level in a covalently bonded semiconductor is developed. This formalism, which is based upon quantum mechanical time dependent perturbation theory, assumes a tight binding representation for the bandstructure of the host semiconductor and models the deep level and its associated potential using the Hjalmarson theory of deep levels. The optical cross sections for the deep levels associated with several impurities in selected semiconductors are calculated numerically using a semiempirical tight binding bandstructure model as input into the formalism. The chemical trends which emerge from these calculations are discussed.

Beginning with the theory of deep traps due to vacancy-impurity pairs, we have developed a theory of deep levels associated with vacancy-impurity complexes in zincblende hosts. We consider triplet complexes which consist of a vacancy that is a nearest-neighbor to two identical, substitutional impurities. Such defects are thought to play an important role in recombination processes in device materials. On the basis of this theory, the chemical trends in such deep levels are predicted for 12 semiconductors and for the semiconductor alloy GaAsi-xPx* These predictions show that a triplet complex may have deep levels which are very different from those of either the isolated impurity or the vacancy-impurity pair. For example, complexing with a second impurity can produce deep levels at energies where neither the pair nor the isolated impurity produce any. Examples of such effects are discussed for GaP and GaAs1-xPx.