X-ray-induced changes in optical properties of doped and undoped amorphous selenium

Abstract

Amorphous selenium has been used as an important material for the flat-panel X-ray image detector. Its image quality is affected by a sensitivity which is reduced under high or long duration of X-ray exposure due to the generation of defects. Experiments were conducted to investigate the changes in optical properties of doped and undoped a-Se when the samples were exposed to two doses of X-rays, 900 Gy and 3200 Gy, at 30 and 70 kVp. The samples were: aSe, a-Se:0.5% As, a-Se:6%As, a-Se:6% As and a-Se:6% As: ppm of Cs. After the cessation of X-ray irradiation, the recovery of the optical properties towards pre-exposure equilibrium values was also investigated. In addition, the dependence of optical properties on the temperature, alloying and doping have been studied. The transmission spectra T(λ) of all films were scanned from 450 nm to 2000 nm, and the thickness d, wedge Δ d, absorption coefficient α(λ), optical band gaps EgT, EgU and refractive index n(λ) were extracted from Swanepoel method from T(λ). The result shows that optical properties were influenced by the absorption of X-rays, heat and doping. The change in n(λ) was related to densification by the Clausius-Mossotti equation. After the cessation of X-rays, the n(λ) of the films relaxes back to the original state, after a certain amount of time. The minimum time to relax to the equilibrium state after irradiation takes about 16 hours. The maximum changes in n(λ) and d were 0.27% and 0.27%. The changes in the optical band gaps, EgT and EgU, were within experimental errors ±0.04%. Changes in the optical constants induced by heating were measurably large. The changes in the optical constants by alloying with As and doping with ppm amounts of Cs have been also studied. Alloying with As increases n(λ) and decreases EgT, and further doping of Cs slightly increases n(λ)