A model for the interpretation of measurements of photoionisation and capture cross sections associated with deep-level impurities

Abstract

A model of a deep-level impurity centre involving a highly localised wavefunction is suggested as a convenient basis on which to interpret experimental results for photoionisation cross section and multiphonon capture rate. The model is probably the simplest one which incorporates all the basic features of a deep-level centre; size and symmetry, charge, and degree of lattice coupling, without being committed to an effective-mass approximation. Its application to measurements of photoionisation allows the lattice coupling strength and the zero-phonon optical threshold to be obtained. It is pointed out that a substantial part of the observed temperature dependence of threshold is caused solely by thermal broadening and a method of estimating the magnitudes of the effect is given. Only measurements in the neighbourhood of the threshold are considered to be useful, since far above threshold the role of band structure is too great. The effect of charge on the dependence of photoionisation is shown to be relatively unimportant. Charge is important in determining the temperature dependence of capture rate at low temperatures. Explicit expressions are given for the thermally broadened photoionisation cross section and for the multiphonon process capture rate in this simple model.