Low temperature photoconductivity of hydrogenated amorphous silicon (a-Si:H) thin flims

Abstract

In this study low temperature photoconductivity of undoped hydrogenated amorphous silicon(a-Si:H) thin films have been studied to investigate the effect of native and Staebler-Wronski defects. The study covers undoped a-Si:H films prepared by various deposition techniques such as DC glow discharge, RF-PECVD with and without H-dilution, RF magnetron sputtering and hot-wire(HW) CVD.In the annealed state, the samples were characterized using temperature dependence of dark conductivity, steady-state photoconductivity, .ph, versus light intensity at room temperature and steady-state photoconductivity versus temperature down to 90 0K at three different intensities. Activation energy ,EF, of the samples changes from 0.60 eV to 1.0 eV. .ph shows a few orders of magnitude higher values from the dark conductivity and its magnitude is sample dependent due to differences in deposition conditions. The intensity dependence of .ph ,., (.ph . F.) is close to unity and varies between 0.70 to 0.90, indicating recombination kinetics through the midgap defect states in the bandgap of a-Si:H. Low temperature photoconductivity versus 1000/T spectrum shows three distinctly different regions. In Region I, .ph decreases with temperature until a transition temperature. Then Region II begins, where .ph begins to increase resulting a peak in spectrum or remains to be unchanged until a second transition temperature to Region III, where .ph continuously decreases with T. Transition temperatures and the degree of increase in .ph in Region II is sample dependent. These results indicate the presence of at least two different types of midgap defect states in the bandgap and exponential tail state present in the annealed state.In the light soaked state, Staebler-Wronski effect (SWE) was investigated after exposing the samples to white light illumination of a few suns intensity. The characterization involves dark conductivity and steady-state photoconductivity at room temperature and .ph versus temperature down to 90 0K for different intensities. Dark conductivity values decreased a certain factor indicating a slight shift in EF through midgap. .ph values decreased substantially from its annealed values due to creation of Steabler-Wronski defects in the bandgap. The intensity dependence of .ph become almost equal and close to unity for all the films even it shows slight variation in the annealed state. The shape of low temperature photoconductivity spectra becomes almost the same for all samples even drastic differences were observed in the annealed state. The spectrum is mainly dominated by only two regions.Region I dominates from room temperature down to 170 0K, where .ph decreases with a constant slope as T decreases. After that temperature, Region II sets in. .ph remains to be constant until temperature used in this study. Region III can only be detected at higher intensity and temperatures lower than 90 0K. Results indicate that more defects around the midgap are created by light, which decrease .ph and relatively less defects are created away from midgap and closer to band edge, which improve .ph instead of decreasing it as temperature decreases. The defect states in Region I responsible for decreasing .ph are more likely that they are neutral silicon dangling bond defects ,D0, and those in Region II responsible for increasing .ph are non-D0 defect states. They act as photosensitising defects with a very low capture cross-sections for electrons. They could be charged silicon dangling bonds ,D+ and D-, or floating bonds results in defect models proposed for a-Si:H.