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The design, fabrication and operation of a hydrogenated amorphous silicon (a-Si:H) metal-semiconductor-metal (MSM) photoconductor for short wavelength imaging applications is presented. The design of the photoconductor sensor is simpler than that of pin photodiode sensors used in conventional active-matrix arrays owing to the similarity of the photoconductor and thin film transistor fabrication processes and by eliminating the need for a p+ contact layer commonly used in p-i-n diode fabrication. The reported devices exhibit three times better responsivity for ultraviolet wavelengths as compared to state-of-the-art pin photodiodes. Experimental results for different a-Si film thicknesses, bias voltages, and electrode gaps are presented and the integration of these devices with TFT fabrication processes is discussed.

Amorphous silicon photodiodes are increasingly being used as fundamental components in digital diagnostic medical imaging system including large area chest radiography, mammography and real time fluoroscopy. The intrinsic a-Si:H material (i-a-Si:H), commonly deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD), is well known to suffer from both light and bias stress induced instabilities over time that can result in an increase in dark current and a decrease in photoconductivity. In contrast, research in Hot-Wire Chemical Vapor Deposition (HWCVD) indicates that aSi:H films grown by HWCVD can have superior physical and electronic properties to those grown by PECVD. In this research, we report on the material properties and stability of i-a-Si:H material by comparing the photoconductivity degradation of the HWCVD and PECVD films over time. Then, we discuss the p-i-n diode fabrication process and examine the leakage and photo-current degradation in the HWCVD and PECVD photodiode structures over time via bias and time stress measurements. Also, we investigate the quantum efficiency degradation over time in a-Si:H p-i-n detectors grown by PECVD.

P-type hydrogenated microcrystalline silicon (µc-Si:H) thin films (~ 100 nm) were deposited using plasma enhanced chemical vapor deposition (PECVD) at a substrate temperature of 150 °C. RF power density and pressure were varied among films. These films reach a dark conductivity (σ d ) of 10 -1 S/cm, activation energy (E a ) of 10 -2 eV and crystalline volume fraction (X c ) of > 50 %. The structure of these films is composed of nano-sized crystallites embedded in an amorphous matrix, resulting in wide optical bandgap energies (E opt ). Using the Scherrer's formula, grain sizes were estimated to be < 20 nm.