Infrared Ellipsometry Investigation of Hydrogenated Amorphous Silicon

Hydrogenated amorphous silicon (a-Si:H) has been extensively investigated experimentally in the infrared spectral region via techniques such as Fourier Transform Infrared (FTIR) and Raman spectroscopy. Although spectroscopic ellipsometry has been proven to be an important tool for the determination of several parameters of a-Si:H films, including dielectric constant, surface roughness, doping concentration and layer thickness, the spectral range used in these studies has rarely covered the infrared region below 0.6 eV, and never over the complete spectral region of interest (0.04 - 0.3 eV), which contains atomic vibration frequencies. We have measured for the first time the dielectric function of a-Si:H films grown by the saddle field glow discharge technique by spectroscopic ellipsometry in the energy range from 0.04 eV to 6.5 eV, thus extending the analysis into the far infrared region. The a-Si:H films were deposited on germanium substrates for the ellipsometry studies, and on crystalline silicon substrates for the comparative FTIR analysis. Preparation parameters were chosen to obtain films with different hydrogen content. In this paper, we present the results of the ellipsometry analysis, evaluate different fitting techniques, and compare the results with the corresponding FTIR spectra. The similarities and differences between the spectra are discussed in terms of the a-Si:H properties.