Characterization of an a-Si:H/c-Si interface by admittance spectroscopy

The capabilities of admittance spectroscopy for the investigation of a-Si:H/c-Si heterojunctions are presented. The simulation and experimental results, which compare very well, show that the admittance technique is sensitive to the parameters of both the a-Si:H layer and the a-Si:H/c-Si interface quality. In particular, the curves showing capacitance versus temperature have two steps, accompanied by two bumps in the temperature dependence of the conductance. The first step, occurring in the low temperature range (100–200 K), is related to the transport and response of gap states in the a-Si:H layer. The second step, occurring at higher temperatures (>200 K), is caused by a carrier exchange with interface states and appears when the interface defect density exceeds 5 × 1012 cm−2. Then, the interface defects affect band bending, and, thus, the activation energy of de-trapping, which favors exchange with electrons from a-Si:H and holes from c-Si, respectively, for an increasing defect density.