Determination of deep trapping states of the hole blocking layer in multilayer amorphous selenium X-ray detectors using transient dark current analysis

Abstract

The density of defect states near the valence band of the hole blocking layer (commonly called the n-layer) is determined by analyzing the transient dark current behavior of multilayer amorphous selenium (a-Se) X-ray image detectors. The previous transient dark current model (Mahmood et al. Appl. Phys. Lett. 92, 223506 (2008)) is modified and compared with recently published experimental transient dark currents on commercial n-i-p and cold deposited n-i a-Se detector structures to determine the energy distributed deep defect densities in these two types of n-layer. The peak defect state exists at 0.75 and 0.78 eV from the valence band mobility edge in alkaline doped and cold deposited n-layers, respectively. The peak trap density in these n-layers varies in the range of 5 × 1016 – 5 × 1017 cm−3eV−1. The energy depths of the trap centers should be ∼(0.75–0.8) eV from the valence band mobility edge for a requirement of less transient time to reach a plateau. The shallower trap levels are unable to retain sufficient trapped charge to reduce the dark current, and the deeper trap centers create longer transient times to reach a steady level of dark current.