Influence of deposition conditions on sputter-deposited amorphous silicon

Abstract

The electrical, optical, and photoelectronic properties of sputter‐deposited amorphous silicon films have been systematically related to deposition parameters for the case of pure Ar sputtering gas in an rf diode system. Variation of the sputtering gas pressure from 25 to 150 mTorr controllably and reproducibly increases the film resistivity 3 to 4 orders of magnitude and significantly decreases optical absorption for photon energies ≲1.6 eV. Similar film property improvements can be realized through the use of large substrate‐target spacings (∼10 cm) and substrate temperatures ≲500 °C. The film properties, however, are independent of deposition rate between 0.5 and 5 Å sec⁻¹. The influence of gas pressure and substrate‐target spacing has been linked to Ar bombardment of the growing film, which appears responsible for both high dangling‐bond densities and incorporated gas. The influence of substrate temperature could result from annealing of collision‐induced defects as the film grows, or from a decrease in the density of microvoids as reported by Paul et al. for amorphous germanium. The use of deposition conditions which minimize plasma interaction and maximize thermal reorganization results in remarkably good electrical and optical properties for nonhydrogenated amorphous silicon. Using the approach of Lewis et al. the addition of H₂ to the Ar sputtering gas produces film properties which are further improved and quite similar to those observed for films prepared by plasma‐assisted decomposition of silane.