The capacitance measurements on a-Si:H Schottky diodes reported in the literature refer to large-area (some 10-mm2) specimens and were carried out in the dark at frequencies typically around 10 Hz. The main interest in these publications is the determination of the density of states in the band gap. For linear photodetector array applications, however, interest centers around the voltage and frequency characteristics of the capacitance of small-area back-to-back Schottky diodes under illumination and at frequencies typically around 1 kHz. But photoinduced changes in the capacitance are also of significance with a view to possible read-out circuits and their long-term stability. The capacitance was therefore measured over the frequency range 0.2-20 kHz and over the voltage range -5 to +5 V. Whereas the dark capacitance exhibited no changes due to photoinduced degradation, significant differences between degraded and nondegraded states were observed in the voltage and frequency characteristics of the photocapacitance. During degradation under high bias voltage (-5 V), the maximum of the capacitance shifted from -0.1 to +0.4 V indium tin oxide bias voltage and the capacitance showed a distinct increase. The photoinduced changes observed for the photocapacitance characteristics could be minimized by ensuring that the elements adjacent to the sensor were at equipotential with the sensor. Full reversibility of the photoinduced changes due to thermal annealing could be verified. The changes of the C-V and C-ω characteristics can be explained by the occurence of the well-known Staebler-Wronski effect.
ASJC Scopus subject areas
- Physics and Astronomy(all)