Effects of annealing on plasma-depositeda-Si:H films grown under optimal conditions

Abstract

A comprehensive study is presented of the effects of isochronal annealing on the optical properties, spin density, and hydrogen evolution of plasma-deposited $a$-Si:H films grown under optimal conditions. In contrast to studies covering a wide range of growth parameters, purely monotonic behavior is observed in most of the properties of these films for annealing temperatures from 300 to 600°C. While changes in the optical gap parallel the hydrogen content of the films, the quenching of the main luminescence band more closely reflects the spin density. Two quenching processes, occurring on very different time scales, are observed in time-resolved measurements of this band. Both depend on spin density, and are modeled as two different trapping mechanisms at dangling-bond defects. An additional luminescence band peaking at about 0.7 eV is studied, which is enhanced by annealing up to 525°C, and quenched above. The ratio of intensities in the two bands scales linearly with spin density, and we propose that the 0.7-eV band represents radiative recombination between an electron in a conduction-band-tail state and a hole trapped at a dangling bond. We present a simple quantitative model of the competing radiative and nonradiative processes that self-consistently accounts for the observed cw and time-resolved optical properties of both bands. These results are compared with a variety of other measurements in amorphous and microcrystalline Si.