Real time spectroscopic ellipsometry on ultrathin (<50Å) hydrogenated amorphous silicon films on Si(100) and GaAs(100)

Abstract

Real time spectroscopic ellipsometry was used to determine the time evolution of the dielectric function, bulk thickness, and surface roughness during hot-wire chemical vapor deposition of hydrogenated amorphous silicon (a-Si:H). The amorphous silicon films were deposited on native-oxide-covered c-Si(100) and GaAs(100) substrates at temperatures in the range from 70to350°C. Data analysis by a three layer optical model, consisting of substrate, bulk, and surface roughness layer, revealed that the dielectric function of the a-Si:H film changes in the initial growth regime (d<50Å), which can be attributed to a higher optical band gap for films with a smaller thickness. It is argued that the origin of this higher band gap lies most likely in quantum confinement effects of the electron wave function in the ultrathin film, with possibly a small contribution of a higher hydrogen content in the ultrathin film. In addition, we show that the trends in surface roughness and bulk thickness are only marginally affected, regardless of whether the change in dielectric function with film thickness is incorporated in the data analysis.