Characterization of intrinsic a-Si:H in p-i-n devices by capacitance measurements: Theory and experiments

A technique has been developed for characterizing the intrinsic layer in a‐Si:H p‐i‐n structures. The method is based on the measurement of the differential capacitance under forward bias, i.e., with injection of free carriers in the intrinsic layer. In these conditions, capacitance is extremely dependent on charges trapped in the band‐gap defects. Measurements were performed on several p‐i‐n samples, in a wide range of frequencies and voltages. All the samples showed the same trend: Capacitance diminished with increasing signal frequency and increased with forward applied voltage. An analytical model explains the obtained behavior. In particular, the model shows that the band‐tail contribution to capacitance decreases slowly with frequency, while deeper defects are effective only below 100 Hz. At higher frequencies, trapping phenomena play a lesser and lesser role in the measurement while depletion charge becomes relevant and the measured capacitance tends to its asymptotic junction value (i.e., the rati...