Hiroyuki Kishihara

Preparation and characterization of polycrystalline CdZnTe films for large-area, high-sensitivity X-ray detectors

This article reviews the preparation and characterization of polycrystalline CdTe and CdZnTe films to be used in large-area, high-sensitivity X-ray panel detectors intended for medical diagnostics. The films, deposited by closed-spaced sublimation, are expected to exhibit excellent efficiency at low X-ray doses because of their high sensitivity. The detectors constructed using these films incorporated a novel hybrid technique, in which zinc-doped CdTe was pre-deposited onto a ceramic substrate and then connected to a TFT circuit substrate. The sensor substrate material was specially selected to avoid both incident X-ray attenuation in the substrate and micro-cracks in the film. Zinc doping, which was used to grow the CdTe film, also served to form the heterojunction diode structure that suppressed leakage current. Moreover, the quality of a polycrystalline CdZnTe film deposited on a 9″×9″ substrate was characterized, revealing its applicability to large-area X-ray detectors. Further investigation and improvements are in progress.

Close-Spaced Sublimation Growth and Characterization of Polycrystalline Cd1-xZnxTe Thick Films for Flat-Panel X-ray Detectors

Polycrystalline Cd1-xZnxTe thick films (x ~0.05) with thicknesses above 400 µm were prepared by the close-spaced sublimation (CSS) as a conversion layer for next-generation highly efficient flat-panel X-ray detectors. The effects of the substrate temperature on the properties of the Cd1-xZnxTe layer were investigated, and it was found that the surface morphology and preferred crystallographic orientation depended on the substrate temperature. Furthermore, the control of the initial stage of deposition of Cd1-xZnxTe films was attempted, and high-quality Cd1-xZnxTe thick films were obtained by deposition after removing the surface layer of the sintered CdTe and ZnTe powder source.

Grain growth and characteristics of chlorine doped thick polycrystalline CdZnTe films

Chlorine (CI)-doped polycrystalline CdZnTe is expected to be used as photoconductor material for Flat Panel Xray detectors (FPDs) due to its high detection efficiency and environmental resistance. However, polycrystalline film causes great variability among pixels in the X-ray image of FPDs. To reduce the variability among pixels, chlorine doping methods and their effects were investigated. By combining the two methods, the addition of CdCl2 to the source and the introduction of vapor chloroform during deposition, we were able to reduce the variability among pixels.