Effects of x-ray irradiation on charge transport and charge collection efficiency in stabilized a-Se photoconductors

摘要

Stabilized amorphous selenium (a-Se) photoconductive layers are currently used in the majority of modern digital x-ray flat panel imaging detectors in mammography. We examine the effects of pre-exposure of a-Se to high-dose x-ray irradiation on both hole and electron lifetimes, τ_e and τ_h, respectively, without any field applied to the device. The x-ray irradiation was from an Al-filtered tungsten target x-ray tube. We show that reduction in τ_h and τ_e depends only on the total or accumulated dose, D, absorbed in a-Se, and not on the rate of dose delivery, dD/dt, over the range of 0.15Gy/s-2.5Gy/s or on the x-ray energy over 50-90 kVp, corresponding to a mean photon energy over 31.9keV-44.7keV. The x-ray induced effects on charge transport are independent of the x-ray intensity and x-ray photon energy but dependent on the total energy absorbed in a-Se. The latter finding allows x-ray induced drop in the carrier lifetimes to be simply and conveniently modeled by τ_o/τ= 1 + AD, where τ_o is the lifetime before x-ray exposure (equilibrium lifetime), x is the lifetime after exposure, D is the absorbed total dose, and A is a constant, which is 0.203 (±0.021) Gy~(-1) for the hole lifetime and 0.0620 (±0.0090) Gy~(-1) for the electron lifetime, a factor of three smaller than that for holes. X-ray irradiation had no effect on hole and electron drift mobilities. Reduction in carrier lifetimes with the total dose was examined at 10 °C, 23.5 °C, and 35.5 °C, close to the glass transition temperature, where x-ray induced effects are stronger. A is independent of hole and electron lifetimes but has a strong temperature dependence, increasing sharply with temperature. After the cessation of x-ray irradiation, carrier lifetimes relax (increase) to their pre-exposed equilibrium values over time scales that depend on temperature. Recovery has been interpreted and analyzed in terms of considering the kinetics of the rate at which x-ray induced capture centers are removed, as the structure restores the equilibrium concentration of deep traps. The annealing process of excess hole traps has a fast and a slow decay component, with time constants τ_(r1) and τ_(r2), respectively. The recovery processes associated with τ_(r1) and τ_(r2) exhibit activation energies larger than those typically involved in electronic transitions and are comparable to bond breaking energies in Se-Se and Se-As. Near 35.5 °C, close to the glass transition temperature, τ_(r2)-recovery has a large activation energy, pointing to structural relaxation phenomena. Fast annealing (～6 min) of excess hole traps at 35.5 °C is, in a general sense, in agreement with the disappearance of irreversible photoinduced effects and suppression of crystallization (strain relief) at the a-Se/ substrate interface, as observed previously. In the case of recovery of the electron lifetime, single exponential decay in excess electron traps and clear activation energy of 1.91 eV/atom point to a probable Se-Se bond breaking involved in returning excess electron traps to equilibrium concentration. Interpretations based on x-ray induced excess valence alternation pair (VAP) and intimate VAP type defects are also considered, including conversion from neutral defects to charged VAP defects. The implications of the present findings on x-ray sensitivity of a-Se detectors through the charge collection efficiency (CCE) are also examined and discussed. An effective carrier lifetime concept is used to describe the effect of x-ray irradiation on carrier lifetimes, which is then used to find CCE in a pre-exposed a-Se detector. The results indicate that x-ray induced effects are negligible for nearly all practical applications of a-Se mammographic detectors in use provided that the detector is operated at a sufficiently high field and a-Se has sufficiently long initial lifetimes, i.e., it is a high quality electronic grade material.