Nonequilibrium steady-state transport in hydragenated amorphous silicon

Abstract

Steady-state photoconductivity ($\sigma_{th}$) in hydrogenated amorphous silicon(a-Si:H) has been studied theorically and experimentally in order to understand the recombination processes of carriers and to obtain the information on the localized states in the mobility gap. Although the experimental data-published by many groups show somewhat different features, it is commonly observed that the $\sigma_{th}$ shows activated behavior in the temperature range between about 200 and 100 K. The activation energy depends on the doping concentration and incident light intensity, but it lies between 0.1 and 0.2 eV. The exponent $\gamma$ obtained from the intensity dependence of $\sigma_{ph}(\sigma_{ph}\,\propto\,I^\gamma$, here I is incident light intensity) is 0.5 for n-type a-Si:H films. In order to explain the temperature and intensity dependence of $\sigma_{ph}$,we developed a numerical model in which the recombination of free carriers through the exponentially distributed band-tail states and through the Gaussian-distributed dangling bonds were calculated in terms of Shockley-Read theory and the occupation statistics of correlated dangling bonds. From the comparison between calculated $\sigma_{ph}$ and observed $\sigma_{ph}$, it is concluded that the recombination of free carriers in band edge through the dangling bonds are dominant at room temperature. In the temperature range between 200 and 100 K, the recombination of carriers in band-tail states through the dangling bonds explain the observed experimental results of activated from and bimolecular behavior in $\sigma$ph.