Amorphous silicon for photovoltaics produced by new microwave plasma-deposition techniques

Abstract

Amorphous hydrogenated silicon (a-Si :H) has been prepared by microwave (2.45 GHz) plasmas in Ar–SiH4 mixtures using two different deposition systems, a large-volume microwave plasma (LMP) apparatus, and a Surfatron system. Films of a-Si:H are characterized structurally (primarily by scanning electron microscopy), and by Fourier transform ir spectroscopy, as well as according to their electro-optical properties (dark and photoconductivity, I-V characteristics of Schottky-barrier diodes). Although microwave plasmas are thought to differ significantly from conventional lower frequency plasmas, results of the present characterizations show no evidence of this. Deposition in the Surfatron system gives rise to device-grade a-Si:H, as demonstrated by Schottky cell efficiences exceeding 3%. We have been unable to duplicate this in the LMP system in spite of nominally identical fabrication conditions; these films have gross columnar morphology, and they react with atmospheric constituents to give a-Si:(H, C, O) alloys. More pronounced ion bombardment of the substrate during deposition is thought to account for the better quality of Surfatron films.Finally, an ageing effect of Au/a-Si:H/Sb–Cr Schottky diodes is described, unlike any reported hitherto; in a typical device, conversion efficiency was observed to rise by roughly 60% after about 2 months, then it decreased slowly. Preliminary investigations of this effect using various surface analytical techniques suggest that Au acts as a catalyst for room-temperature chemical reactions between a-Si:H and atmospheric constituents, primarily oxygen and water vapor.