Abstract
We report on the evolution of three groups of gap states ( Dh ,Dz , and De) in the charge deep-level transient spectroscopy ~Q-DLTS! spectra during light soaking of undoped hydrogenated amorphous silicon (a-Si:H!. Recently, correlation between Q-DLTS and electron-spin resonance showed that neutral (Dz) states correspond to neutral dangling-bond defects. The present Q-DLTS spectra demonstrate the annihilation of positively charged (Dh) states above midgap different from dangling-bonds in the early stage of light soaking. The initial decrease of the Dh states is not accompanied by an increase in the Dz or negatively charged ( De) states below midgap. Further, we do not observe a direct correlation in light induced changes of all three groups of defect states. Combining our results with findings from recent nuclear magnetic resonance experiments at low temperature and computer simulations of a-Si:H network, microscopic configurations introducing charged gap states are proposed. Positively charged states are related to a complex formed by a hydrogen molecule and a Si dangling bond. Negatively charged states are attributed to the floating bonds. A comprehensive model for the Staebler-Wronski effect ~SWE! is presented, which overcomes a weak point of previous models regarding the origin of mobile hydrogen and provides new insight into complexity of the SWE.
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