Structure, conductivity and hall effect of electron bombardment evaporated silicon films on sapphire

Abstract

Silicon films about 1-μ thick were deposited on sapphire substrates in a vacuum of about 7 × 10 −7 mm Hg by electron bombardment evaporation. The sapphire substrates were also heated by electron bombardment. With a substrate temperature of 950°C, films deposited on sapphire cut perpendicularly to the optic axis gave essentially single crystal electron diffraction patterns. The epitaxy was less good with sapphire cut perpendicularly to the sapphire growth axis (i.e. at 60° to the optic axis.) Because of re-evaporation, no film was deposited if the substrate temperature was above about 1100°C. (The limiting temperature will depend to some extent on the rate of supply of silicon atoms to the surface.) At 850°C the films were composed of small, almost randomly oriented grains. Measurement of conductivity and Hall effect over a range of temperature (100–550°K) showed that, as with radiation damaged silicon, crystallographic defects provided both donor and acceptor centres, with acceptors predominating. With a deposition temperature of 850°C, the doping of the source was ineffective compared to the “doping” provided by crystallographic defects. At 950°C, the source doping had appreciable effect, but films made from n-type source material were still p-type. At 1050°C, aluminium doping from the substrate gave p-type films. The Hall mobility was less the higher the temperature of deposition. This apparent anomaly may be explained in terms of the grain structure, using a model treated by Volger. The optical band edge absorption was more or less spread out according to the degree of crystallographic imperfection.