X-ray Induced Effects and Charge Collection Efficiency in Stabilized Amorphous Selenium Based X-ray Photoconductors

Abstract

Stabilized amorphous selenium (a-Se) photoconductive layers are currently used in most modern flat panel digital x-ray image detectors for mammography. The performance of these detectors depends, in part, on the electronic transport properties of the a-Se photoconductive layer. The transport properties consist of the charge carrier drift mobility μ and deep trapping time (lifetime) τ, of both electrons and holes in the photoconductive layer, which determine the x-ray sensitivity of the x-ray image detector through the charge collection efficiency. The product of a carrier’s drift mobility, lifetime, and the applied electric field μτF is the carrier schubweg, which refers to the average distance a charge carrier can travel in the photoconductive layer before it becomes trapped. Trapped carriers are not collected and cause a decrease in the charge collection efficiency and the x-ray sensitivity. The time-of-flight (TOF) and the interrupted-field time-of-flight (IFTOF) experimental techniques are used to measure the transport properties of both holes and electrons in a-Se layers. The TOF transient photoconductivity technique measures the transient photocurrent response of photoinjected charge carriers as they travel through a highly resistive sample under the influence of an electric field. The time width of the TOF pulse is directly related to the drift mobility of the carrier. The IFTOF technique interrupts the drift of the charge carriers through the sample by temporarily removing the electric field, which allows carriers to interact with deep traps in the bulk. The number of free carriers in the bulk decreases over the interruption time. When the electric field is reapplied the remaining free carriers resume drifting through the sample. The ratio of the recovered charge carriers before and after the interruption is related to the charge carrier deep trapping time (lifetime). The effects of high dose x-ray radiation of a-Se on the transport properties is examined. Stabilized amorphous selenium films, with a nominal composition of a-Se: 0.3% As + 10 ppm Cl, were fabricated through evaporation techniques. X-ray radiation was provided by an Al-filtered tungsten target x-ray tube. The absorbed dose rate was varied from 0.12 Gy/s - 2.5 Gy/s, the x-ray energy was varied from 50 kVp - 90 kVp (corresponding to a mean photon energy from 31.9 keV – 44.7 keV), and the applied electric field was varied from 0 V/μm - 10 V/μm. X-ray irradiation had no effect on the drift mobility of either holes or electrons. In the absence of an applied electric field it was found, for both electrons and holes, that reduction in the electron and hole lifetimes depended only on the total or accumulated dose D, absorbed in a-Se, and not on the rate of dose delivery or on the x-ray energy over the ranges examined. This allows the reduction in the carrier lifetimes to be simply modeled by τo/τ = 1 + AD, where τo is the lifetime before x-ray exposure (equilibrium lifetime), τ is the lifetime after exposure, D is the total absorbed dose, and A is a constant, which is 0.203 (± 0.021) 1/Gy for hole lifetime and 0.0620 (± 0.0090) 1/Gy for electron lifetime. In the presence of an applied field, the reduction in the hole lifetime due to absorbed dose in a-Se depends on the total energy absorbed as well as the applied electric field during exposure, and not on the x-ray intensity or the x-ray photon energy. The x-ray induced drop in the hole lifetime is modeled with the empirical equation τho/τh = 1 + B(1 − exp(−D/C)), where τho is the hole lifetime before x-ray irradiation, τh is the hole lifetime after irradiation, D is the total accumulated dose absorbed in the sample (Gy), B is a constant measured to be 4.21, and C is a parameter that depends on the applied field during irradiation, related to the applied field F empirically through a stretched exponential expression of the type C = C1exp[−(F/C2)β] + C3 where the constants have been experimentally determined to be C1 ≈ 19.9 Gy, C2 ≈ 4.44 V/ μm, C3 ≈ 0.06 Gy and β ≈ 2.49. This equation reduces to τho/τh = 1 + BD/C = 1 + AD (where A = B/C, and is a constant) for sufficiently low doses, which is a linear approximation of the x-ray induced effects. The latter accurately maps the reduction in the hole lifetime in the case with no applied field. The implications of these findings can be seen through the calculation of the charge collection efficiency. The results show that the effects of x-ray irradiation on a-Se detectors in the absence of an electric field are small, but in the presence of an electric field are considerable. These effects are minimized when the operating field is high (corresponding to a high collection efficiency) and the a-Se is of high quality electronic grade material.