Light Induced AC Loss in Amorphous Silicon

Condensed matter physics

Anderson, Michael James

University:University of Glasgow

关键词: Condensed matter physics;   

Others  :  http://theses.gla.ac.uk/77847/1/10999380.pdf

来源: University of Glasgow

【 摘 要 】

Light-induced AC losses have been observed in sputtered a-Si, a-Si:H and a-Ge. The aim of this work was to investigate the effect of light on the ac loss in sputtered a-Si and glow discharge a-Si:H and compare the results with previous research. The glow discharge a-Si:H showed an optical response which was intensity dependent. At intensity greater than ~1muWcm-2 the response was rapid and reached a saturation value within minutes. At lower illuminating intensities an integrating response was observed in which saturation values may not be reached for many hours. The intensity dependence of the optically induced loss is similar to that observed in sputtered a-Si, although the illuminating intensities used to create a similar magnitude of response are around 2 orders of magnitude loss in sputtered material. A power law relation is found in which Deltaepsilon alpha IA where A = 0.2 in glow discharge samples and A - 0.25 in sputtered a-Si. The induced loss in sputtered a-Si has been attributed to optically excited carriers becoming trapped in defect states within the band gap where they respond to the ac measuring field. Evidence is found from the frequency and temperature dependences of the dark and optically induced losses to show that band-tail states are responsible for trapping carriers and hence for the induced ac loss, in glow discharge a-Si:H. The mechanism postulated to account for the light induced ac loss involves the excitation of carriers by the applied light. Carriers thermalise rapidly through the extended states and subsequently become trapped in the band-tails. They can now recombine by tunnelling through the band-tails until they recombine with an excess hole via defect states. The dark decay towards equilibrium is accounted for by considering trapped carriers recombining, by tunnelling, leading to a greater spacing between the remaining carriers. This then leads to a reduced response which decays slowly with time. The temperature dependence of these decays was also investigated and it was found that the decays were faster as the temperature was increased from 4.2K to 60K. A model involving the physical principle described above was used to obtain parameters to quantify the rate of decay.