ITO/a-Si:H photodiode and thin film transistor for optical imaging

Abstract

This dissertation deals with an a-Si:H photodiode (ITO/a-Si:H) and an a-Si:H thin film transistor which are pixel components in a large area imaging array. A low temperature process to deposit polycrystalline ITO has been developed. ITO films were characterized in terms of their degree of crystallinity, transmittance, and resistivity. The ITO/a-Si:H interface integrity was studied by measurements of oxygen and indium distributions.

An ITO/a-Si:H Schottky photodiode was designed and fabricated with low leakage current and high stability. The photosensitivity was improved by optimizing the a-Si:H thickness and the geometry of collection electrode. The mechanism underlying the leakage current was studied.It was concluded that the increase in the leakage current was due to oxygen diffusion rather than indium if the ITO was annealed at high temperature (260oC). X-ray detection was achieved by coupling a phosphor film with the photodiode.

The stability and the leakage current of the a-Si:H TFT are two issues for large area imaging or display applications. In this dissertation, the effects of passivation a-SiNx:H on TFT performance was studied by the variation of a-SiNx:H composition, a-SiNx:H was deposited to yield either silicon- rich or nitrogen- rich film by varying the deposition gas ratio of NH3/SiH4. The measurements indicated that the shift in Vt and the leakage current were minimal with nitrogen- rich and a-SiNx:H films.