Origins of atmospheric contamination in amorphous silicon prepared by very high frequency (70 MHz) glow discharge

The authors have studied the effect of plasma power, reactor outgassing rates, and of silane purity on the oxygen, carbon, and nitrogen contents of amorphous silicon material prepared by the very high frequency (70 MHz) glow discharge technique. The silane purity could be optionally enhanced by the application of a getter-based silane gas purifier. It was found that oxygen incorporation was enhanced at lower deposition rates, whereas the nitrogen and carbon film contamination were unaffected. The deposition rate dependence of the incorporation is in excellent agreement with a proposed model. Apart from the effects of plasma power on the incorporation probability, the reactor outgassing rate and the purity of the silane gas itself were identified as the main contamination sources for the atmospheric contaminants in the deposited films. At the low outgassing rate, at least around one-half of the oxygen detected in the <i>a</i>-Si:H material originates from the silane gas. Due to the reduced outgassing rate and an enhanced purity of the silane gas used, the authors have deposited <i>a</i>-Si:H-material with the lowest concentrations of atmospheric contaminants reported to date. Furthermore, the present results exclude a contamination of the <i>a</i>-Si:H-material by a post- oxidation after air exposure.