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

TLDR

The study compares HgI₂ and PbI₂ as direct detector materials for large‑area matrix x‑ray image sensors, describing factors that determine their sensitivity and proposing a model to explain their differing properties. The authors aim to distinguish the x‑ray response of HgI₂ and PbI₂ by their depletion layer properties and to present a model explaining PbI₂’s sensitivity, image lag, and spatial resolution. The authors performed imaging studies on 512×512‑pixel test arrays, measuring x‑ray sensitivity, spatial resolution, charge collection, absorption, fill factor, image lag, and comparing the data to theoretical ionization energy to identify loss mechanisms. PbI₂ shows reduced spatial resolution with increased thickness and lower sensitivity mainly due to large image lag, whereas HgI₂ maintains good spatial resolution, high fill factor, and charge collection, with its sensitivity explained by only a few small loss factors.

Abstract

The factors determining the x-ray sensitivity of HgI2 and PbI2 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 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 PbI2 decreases with increasing film thickness, but this effect is not observed in HgI2. 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 HgI2 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. PbI2 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 PbI2.