Characterization of superlattices based on amorphous silicon

Abstract

We have characterized the structure of superlattices (SL’s) based on amorphous silicon by optical interference microscopy and secondary electron micrographs. Differences in the etch rate of a-Si:H and a-SiNx:H in CP6 create a series of terraces and steps in a-Si:H/a-SiNx:H superlattices that show up clearly in interference contrast micrographs and allow an assessment of the quality of the SL. Secondary electron microscopy images of doping superlattices (npnp or nini) of a-Si:H reveal after plasma etching the layered nature of the samples. The necessary contrast is provided by differences in the etch rate of the two interfaces (i.e., n→p vs p→n). Evidence for structural differences of the interfaces of nini multilayers stems from secondary ion mass spectrometry and 15N depth profiles of hydrogen that show an extra amount of H of the order of 1015 cm−2 only at that interface where intrinsic a-Si:H is growing on top of the n-type material. We explain these results in terms of a growth model which entails a hydrogen enrichment over its bulk concentration that is not limited to the surface but extends into a subsurface region during the film deposition.