The theory and application of contactless microwave lifetime measurement

Abstract

The recombination lifetime of free carriers in silicon wafers can be determined by measuring the reflected microwave power from the sample following a pulsed optical excitation of free carriers. The total recombination process in semiconductor wafers consists of bulk and surface contributions, which are both in general non-linear processes. Thus the resulting process cannot be fitted to an exponential decay process with a constant value of τ. In order to describe this process a new definition of effective lifetime is introduced and different contributions to the process are discussed. The bulk contribution is modelled on the basis of the general form of the model of Shockley, Read and Hall, and the surface contribution is described on the basis of the assumption that the recombination centres at the surfaces are distributed in energy. It is shown that the contribution of the surfaces in this case is negligible at low injection levels.