The Influence of W Filament Alloying on the Electronic Properties of Hwcvd Deposited a-Si:H Films

Abstract

In depositing a-Si:H by HWCVD using W filaments, one issue common to this technique is that of filament lifetime. When using undiluted silane as the source gas, a buildup of silicon at the colder ends of the filament is routinely observed (thickening), and it is here that filament breakage usually occurs. Less well understood is the effect of filament alloy formation on a-Si:H electronic properties. In this work we combine ambipolar diffusion length (SSPG) measurements on consecutively deposited a-Si:H films with SEM surface topography and sputter (SP) Auger depth profiling of the filament Si/(W+Si) composition to track film electronic properties as a function of the Si buildup on short filaments entirely exposed to the growing film surface. We find that with increasing exposure time of the filament to silane, appreciable Si exists everywhere along the filament length, even in the non-thickened central regions. We discuss the effect of this alloying on the film deposition rate and electronic properties, and suggest that the nature of the filament surface must be carefully considered when optimizing a- Si:H film electronic properties. Finally, we discuss possible ways to minimize this alloying by post deposition treatments, which include different filament ‘run’ temperatures.