The Tail States as Sensitizing Recombination Centers for Holes Lifetime in a-Si:H
- PDF / 366,409 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 9 Downloads / 151 Views
ABSTRACT We have measured the dependence of the holes mobility-lifetime product on temperature under various light intensities in intrinsic a-Si:H. We find that this product exhibits thermal quenching which is accompanied by a superlinear light intensity dependence. Numerical calculations that we have carried out show that these results can be accounted for within the framework of the conventional recombination model. However, to yield such an agreement the capture coefficients for both charge carriers at the tail states must be smaller than the corresponding coefficients for the dangling bonds. Thus the sensitizing nature of the tail states is revealed. INTRODUCTION Despite extensive studies, the full understanding of the recombination processes in hydrogenated amorphous silicon, a-Si:H, has remained an elusive goal. It has been recognized, that apart from the dangling bonds there is a considerable contribution of the tail states to the determination of the charge carriers lifetimes [1]. However, the exact relation between the recombination rates through these two channels is under debate. This is because the models used to fit experimental results contain a high number of fitting parameters, each of which offers a range of possible values to be used. The study of the phototransport properties of the minority carrier (holes in the case of intrinsic a-Si:H) is most helpful in narrowing down the number of possible scenarios [2]. One of the most basic phenomena in the study of photoconductors is the sensitization effect [3]. This effect occurs when the recombination processes shift from states with a high capture cross section to states with a low capture cross section for one of the charge carriers. Thus the addition of recombination centers of the latter kind yields an increase in the lifetime of the charge carrier in question. Experimentally, sensitization can be manifested by an increase of the lifetime with increasing light intensity (a phenomenon which we call superlinearity for reasons to be explained below) and by thermal quenching (TQ) of the lifetime (i.e. a decrease of the lifetime with increasing temperature). In the first case the increase of the light intensity shifts the relevant quasi Fermi level through the sensitizing centers, thus making them recombination active. Due to the sensitization this causes an increase in the lifetime, yielding the superlinearity (SL). In the case of TQ, the increase of temperature deactivates the sensitizing centers (through the opposite shift of the quasi Fermi level), resulting in a decrease of the lifetime. For a-Si:H, measurements of the photoconductivity have been used to study the majority carrier (i.e. the electrons) lifetime, since the electrons control the photoconductivity in undoped aSi:H. Both TQ and SL behavior of this lifetime have been observed in numerous works [ 1,4]. It is thus well established (though rarely discussed in these terms) that the tail states are sesitizing centers for the majority carriers (i.e. the electrons) lifetime. On the other hand,
Data Loading...
