Dispersive charge transport studies in hydrogenated amorphous silicon

Abstract

IN THIS THESIS we have investigated a dispersive charge transport in hydrogenated amorphous silicon. The employed experimental technique is time-resolved photocurrent measurement: time-of-flight technique. The time-of-flight photocurrent transient signals are very sensitive to temperature, applied electric field and gap state distribution. It was found that the dispersion parameters obtained from the photocurrent transient signals were temperature and field dependent for the both electron and hole. The electron current transient reveals a transition from non-dispersive to dispersive below room temperature. But the hole current transient maintains dispersive transport behaviours. The both transient signals exhibit more dispersion at lower electric field. These show that the dispersive charge transport is caused by the multiple trapping process. It was observed that the drift mobility of the electron revealed thermally activated behaviour with activation energy of 0.17 eV and was electric field-independent in the wide temperature range of 180-340 K. Time-resolved activation energy has been obtained from the temperature dependence of the transient photocurrent measured at fixed time after short light flash on sample. The observed activation energy was 0.17 eV and remained unchanged for different fixed measurement times. On the other hand the hole mobility activation energy was field-dependent and increased with decreasing electric field. These results are explained by the multiple trapping processes of the charge carriers with band tail states on the base of the assumption of the double exponential distributions. The effects of the band tail state distribution on the transient charge transport have been studied by the post-treatments of the sample such as annealing and light soaking. It was observed that the both treatments resulted in the distortion of the current transient signal and reduced the mobility-lifetime product. However it was found that the both was r...