Recombination and transport in hydrogenated amorphous silicon
This dissertation is the study of properties of a-Si:H using photoluminescence (PL) and photoconductivity (PC) measurements to shed light on understanding of the recombination and transport process of photogenerated carriers. Based on the field dependent PL measurement, we developed a model to explain the field quenching in PL at low temperatures (below 100 K). Our model showed that most radiative recombination occurs between geminate pairs. We determined the distribution of separations of geminate pairs from results of the field quenching in PL. The distribution does not change much with excitation energy. Results of simultaneous PC measurement also indicated the geminate pair recombination, even though it is not so apparent due to, probably, the existence of a fast non-radiative recombination process. Our results imply that there is no significant change in the distance between an electron and a hole in a geminate pair in bandtail states due to thermalization, which is contradictory to a generally accepted model. Field dependent PC measurements at various temperatures confirmed the validity of the concept of effective temperature' invented for explaining field dependent PC in terms of temperature. Results from the measurements showed the existence of safe hole traps (SHTs), which might relate to light degradation. The intensity dependent PC measurements supported this conclusion. We also investigated phosphorous doped a-Si:H with above gap and sub gap excitation in order to study the effect of defects in a-Si:H. The results allow us to develop a possible recombination mechanism for defect related PL.