Evolution of the vibrational spectra of hydrogenated-amorphous-silicon thin films having columnar morphology

Columnar amorphous hydrogenated silicon (a-Si:H) has been prepared by microwave (2.45 GHz) discharge in Ar-${\mathrm{SiH}}_4$ mixtures using a large-volume microwave plasma apparatus. The films have been analyzed by Fourier-transform infrared spectroscopy, elastic recoil detection, and etching in both hydrofluoric and deuterofluoric acids. Time-dependent decreases in the intensities of the ${\mathrm{SiH}}_2$ vibrational modes have been shown to occur at the same rates as the incorporation of atmospheric contaminants into the films. Elastic recoil detection and etching experiments suggest that the films evolve hydrogen at room temperature as they absorb atmospheric contaminants. These findings are used to offer a model for the loss of ${\mathrm{SiH}}_2$ intensity.