Raman Scattering Studies in a-SiH Films Grown with Columnar Morphology

The nature of columnar growth and short range order in a-Si:H films has been studied as a function of thickness using Raman scattering. The films have been prepared by glow-discharge dissociation of silane diluted in argon and using high rf powers. The TO-bandwidth in the Raman spectra has been used as a measure of the order in the amorphous phase. Thicker films deposited using 20% or more of silane yielded Raman spectra similar to those of hydrogen-rich amorphous silicon alloys. Raman spectra of thinner films were similar in structure to those of rf sputter deposited pure a-Si films. Films grown under identical conditions exhibit thickness dependent improvement in the short range order in amorphous phase. The TO-bandwidth decreases rapidly with increasing thickness and then becomes constant at a threshold thickness, t s . With decreasing silane concentration (σ), the t s value is first found to decrease, reaches a minimum at 7% and then abruptly increases near σ=3.5%. These results are explained in terms of variation of compressive stress in the film as a function of silane dilution.