Effect of surface topology of amorphous substrates on the growth mechanism and grain size of APCVD grown silicon for solar cells

In order to improve the crystallization of APCVD silicon on amorphous substrates we have investigated different techniques to grow films with sufficient average grain size. An experimental study of the influence of substrate surface topology on nucleation and grain size of APCVD grown silicon is reported here. Oxidized silicon wafers with a striation pattern and pyramidal structure are used as substrates for deposition. The size of the striation pattern varies from 8 to 128 μm in period and from 40 nm to 36 μm in height. The pyramids and inverted pyramids (2–40 μm high) have been made using selective etching of (100) oriented wafers. A 20–50 μm thick silicon film has been deposited on all substrate types at high temperature (1100–1200°C) and SiH2Cl2 partial pressure of 500 Pa. The structure of the deposited film strongly depends on the surface topology. To study the mechanism of this effect we have determined the nucleation densities on textured surfaces for a deposition time of 2 s. A value of 6–10 μm−2 was found. Although the free-energy change is lowered at the edges (E.I. Givargizov, Oriented Crystallization on Amorphous Substrates, Plenum, New York, 1991) there was no significant increase of nucleation density in these areas. Thus the effect observed is supposed to occur due to the change of geometry at competing grain growth. The described effect seems to be applicable to grow silicon films appropriate for solar cell production.