The optimum conditions for the vacuum deposition of silicon on sapphire

Abstract

The conditions under which good quality single-crystal silicon films can be vacuum deposited on to sapphire substrates have been established. The critical deposition parameters, which are shown to be interdependent, are the silicon impingement rate and the sapphire substrate temperature. For any given silicon impingement rate there exists an etching-deposition boundary temperature TE above which the silicon will etch away the sapphire and below which silicon is deposited. The etching-deposition boundary has been determined for the (011bar above 2) and (0001) sapphire orientations for silicon impingement rates in the range 0·1 to 1·5 μm min−1. The silicon film quality, in terms of its electrical mobility, is presented for the two orientations as a function of the parameter TD/TE, where TD is the substrate temperature and TE is the etching-deposition boundary temperature corresponding to the impingement rate used. There is an optimum value of TD/TE; under optimum conditions (111) silicon films deposited on (0001) sapphire have mobilities up to 75% of bulk values but are not very reproducible, while (001) silicon films deposited on (011bar above 2) sapphire have mobilities about 45% of bulk values and are reproducible. Whilst the macroscopic surface finish of the sapphire substrates is important, the crystalline perfection is not and the limited mobilities are ascribed to the high fault density of the films and the high compressive stress resulting from the thermal expansion mismatch.Although ultra-high vacuum techniques have been used throughout the work, evidence is presented which shows that the vacuum environment is not critical.