Experimental studies of the light-induced effects in undoped hydrogenated amorphous silicon as a function of deposition conditions

Abstract

The magnitude of light-induced metastable effects in undoped hydrogenated amorphous silicon (a-Si:H) was studied as a function of the film deposition conditions using several complementary techniques. Junction capacitance-temperature ( C-T) measurements correlated with electrical conductivity, electron spin resonance (ESR), photothermal deflection spectroscopy (PDS) and IR spectroscopy measurements were carried out on samples deposited at three different substrate temperatures in both the full dark-annealed state (state A) and the saturated light-soaked state (state B). The C-T results clearly indicated that the three types of samples behave differently when going from state A to state B. While the Fermi level shifted towards the valence band, as is usually reported, for films deposited at low temperatures, it shifted in the opposite direction for film deposited at high temperatures. However, the largest change in the density of states at the Fermi level was observed for the film deposited at the lowest temperature (exhibiting the highest hydrogen content); this was also true for the integrated defect density obtained by PDS. In contrast, the change in neutral dangling bond density deduced from ESR was rather small and about the same for all samples. As for the IR spectra, they did not show any modification between state A and state B. Our experimental results, discussed in the context of the different models proposed to explain light-induced effects in a-Si:H, are more easy to understand if one assumes the existence of other types of defects, such as the T + 3−T - 3 centres which present a negative correlation energy, in addition to the neutral dangling bonds D 0.