Dual-beam photoconductivity modulation spectroscopy in a-Si: H

Abstract

Abstract D.c. photoconductivity and dual-beam photoconductivity modulation measurements on intrinsic a-Si: H are reported. The dual-beam technique is used to elucidate the recombination mechanisms which affect d.c. photoconductivity. In the dual-beam photoconductivity technique two light beams are used, a steady pump beam with hv > 1·5 eV and a chopped monochromatic beam with 0·6 eV hv < 2·0 eV. The pump beam creates free electrons and holes and thus changes the occupation of the semiconductor gap states through which recombination occurs. The second, monochromatic, beam is used to probe recombination processes by altering the occupation of selected gap states, thereby modulating the photoconductivity. The photoconductivity modulation spectrum, which is dependent on temperature and the intensity of the pump beam, is rich in information about gap states that act as recombination centres. Infrared quenching of photoconductivity is observed below T = 200 K. Analysis of this effect in terms of two classes of gap state allows the identification of a distribution of gap states with a small electron capture coefficient of ∼ 4 × 10−13 cm3 s−1 and with a low-energy cut-off at an optical energy of ∼ 0·6 eV above the valence band edge. Above 220 K, the infrared quenching signal disappears and a slow-response positive modulation effect appears which has a low hv cut-off of 0·8–0·9 eV and peaks at ∼ 1·1 eV. The origin of this effect is as yet unclear. Above 250 K a new signal appears with a higher hv threshold energy of ∼ 0·9–1·0 eV. The appearance and growth of this signal with increasing temperature may be due to a temperature-dependent majority-carrier capture coefficient for the gap state involved.