Ellipsometric studies of low-temperature metalorganic chemical vapor deposited ZnS thin films

Abstract

The most common techniques used for the determination of thin film thickness are profilometry, spectrophotometry, and ellipsometry. Spectrophotometers require the refractive index of the film (generally assumed to be transparent) as an input parameter. Profilometers yield nonreproducible and often inaccurate film thickness measurements when the films exhibit rough surface structure. Ellipsometric analyses of polycrystalline metalorganic chemical vapor deposition (MOCVD) ZnS films which assume that the thin films are single, uniform, transparent layers do not produce reliable or self-consistent optical parameters or thickness values for the films. We employed multiple wavelength ellipsometry to analyze thin, polycrystalline MOCVD ZnS films. A two-layer model incorporating absorption and surface roughness was used to determine the thicknesses of the two layers self-consistently at wavelengths of 4050, 5461, and 6328 Å. The optical constants of the two layers are related through the Bruggeman effective medium approximation. The outer layer represents surface roughness and is characterized by a volume void fraction. The thickness of the rough outer layer was assumed proportional to the total film thickness based on scanning electron micrographs of the films. Results for MOCVD ZnS films deposited on HgCdTe and Si substrates indicate that the nucleation period differs for the two materials.