Kinetic Roughening During Hot-Wire Chemical Vapor Deposition of Hydrogenated Amorphous Silicon

Abstract

Despite the widespread use of hydrogenated amorphous silicon (a-Si:H), the fundamental surface processes during film growth are not well understood. One approach to studying these mechanisms is to analyze the surface morphology that results from their action. In this dissertation, hot-wire chemical vapor deposition is used to deposit a-Si:H thin films. Both post-deposition atomic force microscopy (AFM) and in situ spectroscopic ellipsometry (SE) are used to characterize the surface morphology and its dynamics. Results of this work indicate that the surface morphology is shaped by geometric shadowing of growth particles and thermally-activated smoothening mechanisms. For films grown at low temperature, the local slope of the surface is found to exhibit power law scaling with time that is consistent with anomalous roughening behavior also observed in models that include shadowing. A temperature-dependent transition in roughening behavior is observed, and an activation energy is extracted that agrees with previous estimates for SiH3 surface diffusion. Additionally, a-Si:H grown on rough substrates is examined. Smoothening at short lateral length scales is observed simultaneously with global roughening. Behavior is found to generally agree with deterministic models in the literature. This work also explores the difference between SE and AFM in how roughness is measured. Rayleigh-Rice theory (vector perturbation theory) is used to calculate ellipsometric data that is subsequently compared to the usual method of using an effective medium layer to approximate roughness. SE measurements are found to critically depend on both the vertical extent of roughness and the root-mean-squared slope of the surface.