Relaxation phenomena of image sensors made from a-Si:H

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Abstract
Image sensors made from amorphous silicon ( a‐Si:H ) are under development. Their elements consist of back‐to‐back Schottky diodes. For practical operation, long‐term stability is of great importance. We investigated dark conductivity and photoconductivity, capacitance‐voltage characteristics, and response behavior after switching off illumination. Even after light soaking for many hours, no change in photocurrent occurred, whereas dark current, capacitance, and response time increased. These changes are metastable and can be reversed by annealing above 200 °C. Contrary to the Staebler–Wronski effect, [Appl. Phys. Lett. 3 1, 292 (1977)], the dark‐current increase disappears at room temperature after several hours. We investigated the time dependence of this relaxation and calculated the energetic depth of the states involved. The contact between a‐Si:H and indium‐tin‐oxide is described as a Schottky–Bardeen‐metal‐insulator‐semiconductor junction. Its properties are strongly dependent on interface states, in particular on the position of the neutrality energy of the interface states with respect to the Fermi energy. We show that besides the well‐known Staebler–Wronski effect, a new degradation process is observed. We suggest a model where holes are trapped in interface states about 1.0–1.4 eV above the valence band. Their thermal emission governs the relaxation behavior of the dark current.