Capacitance temperature analysis of midgap states in hydrogenated amorphous silicon

Simultaneous low frequency capacitance-temperature and conductance-temperature measurements on Schottky diodes in the range 180–450 K enable us to deduce the real density of bulk midgap states N(E) of hydrogenated amorphous silicon in the zero frequency limit. The analysis of the results indicates that the interaction kinetics is controlled by the thermal release time of the midgap states. The response times are consistent with an electron capture cross section ranging from 2×10−16 to 5×10−15 cm2, very similar to the value reported for crystalline Si–SiO2 interface states. Furthermore, results show that thermal equilibrium is established directly with conduction band electrons at high temperature or through intermediate states at temperature lower than 300 K. The high accuracy of the method is demonstrated by detecting 15% variations of N(E) due to light-induced effects. The physical nature of midgap states is also discussed.