Raman scattering studies in glow discharge deposited a-Si:H films: effect of argon dilution

Abstract

The nature of structural changes and short range order in a-Si:H films has been studied using Raman scattering. The films were prepared by glow-discharge dissociation of silane diluted in argon and using high rf powers Thicker films deposited using 20% or more of silane yielded Raman spectra similar to those of hydrogen-rich amorphous silicon alloys. Films grown from a low concentration, C[SiH₄] of silane exhibit a thickness dependent improvement in the short range order in the amorphous phase. An amorphous to microcrystalline phase transition occurs at C[SiH₄] =7% whereas the transition disappears for C[SiH₄] < 5%. The TO-bandwidth in the Raman spectra of the amorphous phase decreases rapidly with increasing thickness and then becomes constant at a threshold thickness, (t_S). With decreasing silane concentration, the t_S value is first found to decrease, reaches a minimum at C[SiH₄] =7% and then abruptly increases nearC[SiH₄] =3.5%. These results are explained in terms of variation of compressive stress in the film as a function of silane dilution.