The photoionisation cross section of deep-level impurities in semiconductors

Abstract

A simple analytic expression for the photoionisation cross section of deep-level impurities in semiconductors is obtained in a form that is explicitly related to the electron-scattering strength of the impurity. It also includes the dependence on size and charge, and the character of the localised-state Bloch functions. All of these quantities affect the spectral dependence immediately above threshold. The model introduces a simple billiard-ball wavefunction and the shape functions near threshold are derived, for neutral and charged (plus or minus) centres which are either donor-like or acceptor-like. The effect of phonon coupling is discussed in the context of a single-frequency model. At low temperatures the spectral shape near the shoulder of the curve is determined by the charge, and the effect of phonon coupling is to raise the apparent threshold.