Minority Carrier Diffusion Lengths And Photoconductivity In a-Si,N:H Deposited By Remote Pecvd

Abstract

We have deposited films of a-Si,N:H by remote PECVD from N₂ and SiH₄ for N-concentrations, [N], to about 12 atomic percent (at. %). Bonded-H concentrations were ~7-10 at. %, Mostly in Si-H groups. The films with [N] ≈ 9-12 at. % have εθ4 bandgaps of ~2.0 to 2.2 eV, which makes them potentially useful as wide bandgap photo-active materials in tandem PV cells. Several properties are of special interest for PV applications. First, like many other a-Si:H-based alloys, the photoconductivity relative to a-Si:H is degraded by alloying, but less than for a-Si,C:H alloys with the same bandgaps. Second, the ambipolar diffusion lengths (Ld) obtained with the Steady State Photocarrier Grating (SSPG) technique for films with [N] ≈ 10 at. % and εθ4 ≈ 2.1eV, are comparable to those of a-Si:H. For lightly-nitrided films to [N] ~5 at. %, Ld first decreases with respect to a-Si:H and then increases as [N] increases from ~7 at.% to 10-12 at. %. These trends follow the dark conductivity activation energy, E_a, which initially drops due to doping, and then increases into an alloy regime for [N] > 5 at. %. Films with [N1≈10 at. % have dark conductivities and E_a’s comparable to those of undoped a-Si:H. Third the magnitude of the Staebler-Wronski effect, as monitored by the photo- to dark conductivity ratio after a 1000 Minute lightsoak, was about the same as in a-Si:H. Finally, we contrast the properties of these films prepared from N₂ with a-Si,N:H alloys with the same [N] and E₀₄, but prepared from an ammonia N-atom source gas and attribute differences in their photoelectronic behavior such as a significantly enhanced Staebler-Wronski effect.to the presence of Si-NH bonding arrangements in the films grown from NH₃.