Evaluating the Effects of Breaking Vacuum During the Fabrication of Amorphous Selenium Detectors.

Direct conversion X-ray detectors offer high spatial resolution and improved sensitivity over indirect conversion detectors. As interest increases in their applications, including utilization as the top layer in dual-layer detectors for polyenergetic X-ray detection, additional studies on fabrication techniques are required. Amorphous selenium is a well-studied high-Z semiconductor capable of high-resolution and high-sensitivity imaging in direct architectures. It is already commercially available for mammography, and much work has gone into developing it for higher-energy applications (>20 keV.) To fully attenuate energies required for the low energy (<35 keV) top layer in dual-layer detectors, a-Se thicknesses greater than 200 μm must be fabricated. However, evaporation crucibles have limited capacity and require reloading of crucible material to achieve higher thicknesses, which is performed by opening the chamber and breaking vacuum mid-fabrication. We investigate the effects of splitting the fabrication into two depositions - exposing the sample to air in between - on device performance. We find that there is no significant effect on the transport properties, and a small range of performance parameters can be found - suggesting small fluctuations between devices of ± 5%, independent of fabrication technique and thickness. This implies that as we increase our thicknesses to those required for the low-energy layer of the dual-layer detector, we can expect performance to be maintained.