Composition and Thermal Stability of Glow-Discharge a-Si:C:H and a-Si:N:H Alloys

Abstract

One impetus of amorphous semiconductor technology is its prospect of a cheap thin film semiconductor with a bandgap variable over a relatively wide range of energy. Indeed, it was demonstrated as early as 1977 by Anderson and Spear¹ that by adding hydrocarbon or ammonia gases to silane during the plasma deposition process amorphous Si:C:H and Si:N:H alloys with bandgaps between 1.6 and ~5 eV can be produced. Since then, a lot of work has been done in the field², and in fact a-Si:C:H films are nowadays produced industrially for the application as wide bandgap window layers of a-Si solar cells³. However, at present one still seems to be far from efficient solar cells or other electronic devices entirely based on a-Si:C:H or a-Si:N:H alloys, mainly due to basic material problems encountered. These are a strongly increasing electron spin resonance (ESR) signal^4,5, a decreasing photosensitivity³⁻⁶ and a disappearance of doping effects as the carbon or nitrogen concentration is increased to 20% or above. The general appearance of such material problems at various laboratories points to rather fundamental mechanisms and not merely to unfavorable deposition conditions. It is the aim of this paper to elucidate some of these adverse effects as far as film composition and structure is concerned. Reported are results of a relatively broad-scaled investigation of a-Si:C:H and a-Si:N:H films prepared both at Marburg and Jülich laboratories from a wide range of different gases.