Steady-state photoconductivity and recombination process in sputtered hydrogenated amorphous silicon

Steady-state photoconductivity has been studied on $a$-Si: H films prepared either by diode rf or triode dc sputtering in order to determine the predominant recombination process of free carriers acting in these materials. The experimental results have been compared with the predictions of the two currently used models proposed, respectively, by Spear et al. and by Rose. In all of the investigated samples, in agreement with Rose's model, the recombination process involves a continuum of localized states obeying Shockley-Read statistics. The photoconductivity has then been calculated using the formal approach of Taylor and Simmons. The theoretical equations allow one to obtain basic information about localized states around the Fermi level from the dependence of the photoconductivity on photon flux intensity. The characteristic parameters determined in this way have been used to calculate the variation of the photoconductivity as a function of the temperature: A very good agreement with the experimental variation has been observed. Although the recombination process is the same either for the diode-rf- or the triode-dc-sputtered samples, the behavior of the photoconductivity can be quite different for these two materials. An explanation involving the capture cross section of the traps is proposed.