Thin-film silicon formation on foreign substrates by rapid thermal chemical vapour deposition for photovoltaic application

Abstract

Polycrystalline silicon thin-film layers were deposited on foreign substrates such as SiO2, alumina, mullite and graphite. The deposition studies were carried out in a single-wafer, horizontal, rapid thermal chemical vapour phase reactor at temperatures ranging from 900°C to 1250°C at atmospheric pressure. We employed the gas precursor trichlorosilane and the layers were doped with boron from the dopant source trichloroborine rarified in a hydrogen carrier gas. The surface structures and grain sizes of the thin films obtained were evaluated by Nomarski microscopy and scanning electron microscopy characterization methods. X-ray diffraction analyses were used to determine the preferential crystalline orientations at various operational parameters. Furthermore, electrical properties in terms of Hall mobility and lifetimes of the minority carriers were investigated by means of Van-der-Pauw and photoconductivity decay methods, respectively. Generally, it has been shown that at elevated deposition temperatures maximum grain sizes of 3–20 μm for 10-μm thick layers can be found, depending critically on the type of the substrate. For polycrystalline silicon deposited at 1100°C on silicon dioxide, alumina, and graphite substrates, a preferred crystallographic orientation of (220) was observed, implying columnar grain structures. © 1998 John Wiley & Sons, Ltd.