Mechanical Stress-Induced Defects in Thick a-PbO Layers

Amorphous lead oxide (a-PbO) X-ray photoconductors show potential for applications in direct conversion medical imaging detectors within the diagnostic energy range. a-PbO enables large-area deposition at low temperatures and exhibits no signal lag. Low dark current can be maintained through specialized blocking layers, similar to those used in multilayer amorphous selenium (a-Se) structures in commercial detectors. However, the current state of a-PbO technology faces challenges in thick layer deposition, leading to crystalline inclusions and cracks. Our proposed stress-induced crystallization model reveals that intrinsic stress in a-PbO layers amplifies with thickness, leading to crystallographic defects. These defects, which are associated with the stable phase of β-PbO, contribute to increased dark current and initiate layer cracking. We calculate the thermal expansion coefficient of a-PbO, indicating a thermomechanical mismatch between the photoconductor and the substrate as the primary source of stress. Furthermore, we demonstrate that layer deposition parameters significantly impact heat accumulation within the growing layer, thereby facilitating temperature-induced crystallization. Our study suggests that relieving stress in grown a-PbO layers by eliminating thermal expansion coefficient mismatches between different layers in a-PbO blocking structures, coupled with optimizing deposition parameters to prevent heat accumulation during layer growth, may inhibit or even prevent stress-induced crystallization and the emergence of structural defects in thick a-PbO layers.