Two Carrier Sensitization as a Spectroscopic Tool for a-Si:H
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ABSTRACT In a recent letter we have reported the first observation of the phenomenon of minority carrier-lifetime sensitization in hydrogenated amorphous silicon (a-Si:H). We find now that combining the study of this phenomenon with the study of the well-known phenomenon of majority carrier lifetime sensitization, in this material, can provide direct information on its density of states (DOS) distribution. This finding is important in view of the limitations associated with other methods designed for the same purpose. We have carried out then an experimental study of the effect of light soaking on the phototransport in a-SilH. We found that the increase of the dangling bond concentration with light soaking affects the sensitization and thermal quenching of the majority carriers lifetime. Using computer simulations, we further show that the details of the observations associated with the sensitization effect yield semiquantitative information on the concentration and character of the recombination centers in a-Si:H. INTRODUCTION The phenomenon of thermal quenching of the photoconductivity in intrinsic a-Si:H is known for many years [1]. The interpretation of this phenomenon is two-fold. First, it requires that the capture coefficients of the electrons is much smaller for the bandtail states (BTS) than for the dangling bonds (DB), and second, it requires that with increasing temperature (T) the number of BTS available for recombination, Pv, decreases until at some T it equals the concentration of the dangling bonds ND. While this basic picture is in accord with the results of numerical simulations [2,3], we do not know of a direct experimental proof that establishes this model and that ties this phenomenon with the phenomenon of sensitization. The latter phenomenon is that the addition of excess recombination centers can prolong the lifetime of the charge carriers in a given photoconductor [4]. In this paper we study this effect by applying the tool of light soaking (LS) in order to change the concentration of desensitizing recombination centers in a-Si:H. We do that here in the "reverse mode" to the one used classically [4], i.e. by keeping the total concentration of the sensitizing centers nearly constant and changing the concentration of the desensitizing recombination centers. After establishing experimentally the above basic picture of the sensitization, we have turned to numerical simulations that enabled us to evaluate the relation between the manifestations of the phenomenon of the sensitization and various parameters of the concentration and capture coefficients of the recombination centers in the mobility gap of a-Si:H. As we show below, these features give further semiquantitative spectroscopic information on the concentration and character of the recombination centers in the material.
439 Mat. Res. Soc. Symp. Proc. Vol. 557 ©1999 Materials Research Society
EXPERIMENTAL STUDIES The samples used in the present experimental study were device quality a-Si:H films which were deposited [5] on a Coming 7059 g
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