Characterization of Low Dark-Current Lateral Amorphous-Selenium Metal-Semiconductor-Metal Photodetectors

We report a lateral amorphous-selenium (a-Se) metal-semiconductor-metal photodetector with a blocking contact. The blocking contact, a polyimide layer, is shown to significantly reduce the dark current even at high applied biases that result in high photo-to-dark-current ratios, thus leading to wide dynamic range and high signal-to-noise ratio. The use of the polyimide blocking contact prevents the injection of both holes and electrons and improves considerably upon the dark current of previously reported lateral a-Se detectors. The presence of charge trapping at the polyimide/a-Se interface is found to be negligible through the use of pulsed light experiments. The effects of electrode spacing and electrode width on device performance are investigated through experiment and simulation for device optimization. From the devices that are fabricated, it is found that the dark current is strongly dependent on the comb fingers density while the same trend is not observed for the photocurrent. It is found that the device with 10- $\mu{\rm m}$ electrode spacing and 10- $\mu{\rm m}$ electrode width has the best performance in terms of photo and dark current. This paper demonstrates the promise of low-cost lateral a-Se devices for use in indirect conversion large area digital medical X-ray imaging applications, such as chest radiography, real-time fluoroscopy, and cone beam computed tomography.