Transport properties of a-Si: H alloys prepared by r.f. sputtering I

Abstract

We present conductivity and thermoelectric power data as a function of temperature for amorphous silicon–hydrogen films prepared by reactive r.f. sputtering. The results are used to examine the effect of the deposition parameters, hydrogen partial pressure, argon partial pressure and substrate temperature on the efficiency of dangling-bond compensation by hydrogen. Two additional effects of hydrogen incorporation are discussed: (i) removal of states from the vicinity of the valence-band edge, resulting in an increase in optical band gap and (ii) creation of structural defects which would not exist in the hydrogen-free material, and which may give rise to gap states. To account for the transport data on films containing ≳20 at.% hydrogen, we suggest that such material may be structurally inhomogeneous, consisting of two distinct phases, one resembling pure a-Si in which the dangling bonds have been compensated, the other a hydrogen-rich phase in which multiply bonded hydrogen groups may predominate. In low-hydrogen-content films, in which the hydrogenated a-Si phase is dominant, the transport data show great consistency over a wide range of deposition conditions. Conduction-band transport therefore appears to be affected little by hydrogen incorporation, and the implications of the present data for any model for transport in a-Si: H are discussed.