Light-induced changes in the gap states above midgap of hydrogenated amorphous silicon

Abstract

The energy dependence of the capture coefficients and the density of states (DOS) above midgap of hydrogenated amorphous silicon (a-Si:H) in annealed and light exposed states are examined by modulated photocurrent experiments. In the annealed and light exposed states, the electrons are found to interact with two different kinds of gap states through trapping and thermal release. The densities of both gap state distributions at trap depths shallower than 0.67eV below the conduction band decrease rapidly during the initial stage of light degradation. The DOS of the annealed and light exposed states are found to cross at about 0.67eV. In addition, a parallel increase in the capture coefficients of the gap states at trap depths lower than 0.67eV is observed indicating a light-induced disorder. The above behavior is explained with the conversion of strained Si–Si and Si–H–Si bonds, which become highly strained during illumination, into dangling bonds near midgap.