Photon counting performance of amorphous selenium and its dependence on detector structure

Abstract

Photon counting detectors (PCD) have the potential to improve x-ray imaging; however, they are still hindered by high costs and performance limitations. By using amorphous selenium (a-Se), the cost of PCDs can be significantly reduced compared with modern crystalline semiconductors, and enable large-area deposition. We are developing a direct conversion field-shaping multiwell avalanche detector (SWAD) to overcome the limitation of low carrier mobility and low charge conversion gain in a-Se. SWAD's dual-grid design creates separate nonavalanche interaction (bulk) and avalanche sensing (well) regions, achieving depth-independent avalanche gain. Unipolar time differential (UTD) charge sensing, combined with tunable avalanche gain in the well region allows for fast response and high charge gain. We developed a probability-based numerical simulation to investigate the impact of UTD charge sensing and avalanche gain on the photon counting performance of different a-Se detector configurations. Pulse height spectra (PHS) for 59.5 and 30 keV photons were simulated. We observed excellent agreement between our model and previously published PHS measurements for a planar detector. The energy resolution significantly improved from 33 keV for the planar detector to ∼7 keV for SWAD. SWAD was found to have a linear response approaching 200 kcps∕pixel.

Theoretical investigation of the count rate capabilities of in‐pixel amplifiers for photon counting arrays based on polycrystalline silicon TFTs

“…[19][20][21] Other efforts to develop converters for photon counting, based on materials which lend themselves to large-area deposition (i.e., a-Se and poly-perovskite), have been reported. [22][23][24] Given its demonstrated role in enabling the creation of large-area, monolithic AMFPI backplanes, a-Si:H would be a candidate for photon counting circuits. Unfortunately, a-Si:H has electron and hole mobilities that are simply too low (~1 and 10 À2 cm 2 /V-s, respectively) to allow creation of the complex circuitry required for photon counting circuits.…”

Recent Developments of Amorphous Selenium-Based X-Ray Detectors: A Review

“…As reported by previous studies [24], [25], the charge conversion efficiency of a-Se is not ideal, mostly because of its high ionization energy. The electron-hole pair creation energy of a-Se at an applied field of 10 V/μm is 45-50 eV [26], [27], while other semiconductor materials, such as HgI 2 and silicon, have electron-hole pair creation energy equal to or less than 5 eV under the same electric field [28].…”

“…One of the most undesirable properties of a-Se is low charge carrier mobility. The hole drift mobility is around 0.13-0.14 cm 2 /V.s, and the electron mobility is around 5-7 × 10 −3 cm 2 /V.s, which are orders of magnitude smaller than those of other common semiconductor materials like silicon (480 cm 2 /V.s and 1400 cm 2 /V.s for holes and electrons respectively) [1], [24]. Traditional a-Se X-ray design has a sandwich structure: two electrodes with an a-Se layer in between.…”

“…All these effects eventually lead to low time resolution and high depthdependent noise [21]. In practice, these limitations can hinder the complete charge collection of X-ray interactions, especially for breast imaging applications where a-Se is used widely because of its excellent spatial resolution [24].…”

“…Currently, the SWAD has not been manufactured. However, two successive studies [24], [25] have used simulations to evaluate the performance of the SWAD and revealed its potential of becoming a photon-counting detector. The energy resolution of the SWAD is expected to be comparable to current CdTe detectors [25], [74].…”