Surface photovoltage investigation of recombination at the a-Si/c-Si heterojunction

We investigate the use of time-resolved surface photovoltage (SPV) transients as a means to determine band bending and recombination properties at amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions. Experimentally, it is shown that for a-Si:H film thicknesses above ~ 6 nm, SPV transients do not depend on the film thickness anymore. On this basis, a simple numerical model is proposed that consists of a single rechargeable gap state on the c-Si wafer surface, into which the properties of the a-Si:H/c-Si interface and the adjacent a-Si:H are lumped. It is shown that this model can reproduce all principal features of high excitation SPV transients, i.e. an initial fast decay shown to be due to Auger recombination, a plateau region for high injection conditions and a fast decay when the sample returns into low injection and the defect states are recharged. Under sufficiently high excitation, the SPV saturates at a value that is determined by the a-Si:H/c-Si interface band bending in the dark. From the slope of the transient decay, defect parameters (density, energetic position) can be extracted.