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The changes in defect density of hydrogenated amorphous silicon (a-Si:H) in thin film transistor (TFT) structures by prolonged bias stress or light soaking have been studied through constant photocurrent method (CPM) measurements. The CPM absorptions due to defect absorptions do not change after positive or negative bias-stress. On the other hand, the CPM absorptions due to dangling bond defects increase remarkably after light soaking. These experimental results demonstrate that the bulk defect density of a-Si:H in TFT structure is not changed after long time bias stress even though the characteristics of the TFT degrade significantly.

The capatiance–voltage characteristics associated with the hole accumulation in hydrogenated amorphous silicon metal–insulator–semiconductor structures were investigated. The capacitance was measured by using an ac voltage and a quasistatic method. In the ac measurements, we observed the partial response of the hole capacitance, while the full response of the hole capacitance was confirmed by quasi-static measurements. The effect of illumination on the hole capacitance was also investigated. One possible mechanism accounting for the frequency and temperature dependencies of the hole capacitance is proposed.

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Abstract:In this investigation, we added some new findings and interpretations on the behaviors of both electrical conductivities and optical absorption spectra upon adsorption of atmospheric molecules in microcrystalline silicon films (µc-Si:H). The photo conductivity followed similar decay-recover processes upon air exposure-thermal anneal cycles as did the dark conductivity. The dark and photo conductivities decay according to a stretched-exponential function with time upon air exposure. The absorption spectra showed a reduced defect absorption for the films at air exposed state compared with those of at thermal annealed state. There is a one-to-one correspondence between atmospheric adsorption in µc-Si:H and bias-stress induced instabilities in a-Si:H thin film transistors(TFTs).