Comparison of PbI2 and HgI2 for direct detection active matrix x-ray image sensors

Abstract

The factors determining the x-ray sensitivity of ${\mathrm{HgI}}_{\mathrm{2}}$ and ${\mathrm{PbI}}_{\mathrm{2}}$ as direct detector materials for large area matrix addressed x-ray image sensors are described, along with a model to explain their different properties. The imaging studies are made on test arrays with $\text{512×512}$ pixels of size 100 μm. The x-ray sensitivity and spatial resolution are reported, along with measurements of the various mechanisms that influence the sensitivity, such as charge collection, x-ray absorption, fill factor, and image lag. The spatial resolution of ${\mathrm{PbI}}_{\mathrm{2}}$ decreases with increasing film thickness, but this effect is not observed in ${\mathrm{HgI}}_{\mathrm{2}}.$ The x-ray response data are used to compare the sensitivity to the theoretical values for the ionization energy and to identify the various loss mechanisms. We find that the sensitivity of ${\mathrm{HgI}}_{\mathrm{2}}$ can be explained by a few small and well characterized loss factors. This material exhibits good spatial resolution, high fill factor, and high charge collection. ${\mathrm{PbI}}_{\mathrm{2}}$ films exhibit lower sensitivity, principally attributable to a very large image lag. We propose that the x-ray response of the two materials is distinguished by their different depletion layer properties, and present a model that accounts for the sensitivity, image lag, and spatial resolution of ${\mathrm{PbI}}_{\mathrm{2}}.$