Polycrystalline Silicon on Coated Steel Substrates

Abstract

Steel is the most economical substrate for the deposition of silicon. At temperatures used for the chemical vapor deposition of silicon, however, a barrier layer must be used to prevent the diffusion of iron from the substrate into the silicon layer. Tungsten was found to be ineffective as a diffusion barrier when silicon was deposited by the thermal decomposition of silane at 900°C and above. Borosilicate deposited by the oxidation of a silane‐diborane mixture was found to be an effective barrier at temperatures up to 1150°C. Silicon deposited on borosilicate/steel substrates is polycrystalline. The microstructure and crystallographic properties of silicon depend strongly on the substrate temperature, deposition rate, and extent of doping. Silicon layers deposited at low temperatures and high rates consist of small crystallites with a strong preferred {110} orientation, while those deposited at high temperatures and low rates consist of larger crystallites with more random orientation. Silicon p‐n junctions deposited on borosilicate/steel substrates show poor electrical characteristics because of the high concentration of grain boundaries, and solar cells have low conversion efficiencies.