Defects in CdTe single crystals for photon-counting imaging

Photon counting imaging represents a revolutionary breakthrough, surpassing current scintillator-based imaging technology and holding significant promise for the field of medical computed tomography. CdTe is a representative example of a high-performance photon-counting imaging material. However, defects within it, particularly point defects, act as carrier trapping centers and are a key factor limiting its count rate capability. In this paper, the size distribution and density of Te inclusions in CdTe single crystal were first analyzed using an infrared transmission microscope. Subsequently, point defects were studied both theoretically and experimentally using modern research methods. Specifically, a point defect complex model was first established using the first-principles method based on density functional theory plus the Coulomb interaction potential (U). The calculated results show that electrons are localized around the point defect complex and its surrounding region, resulting in an increased electron density around it. This provides a theoretical basis for the reduced positron lifetime (100.0 ps) for the point defect states. Based on this value, the concentration of the defect complex was calculated to be 2.17 × 10¹⁴ cm^-3. Finally, X-ray absorption fine structure spectroscopy revealed best-fit bond lengths of 2.79 Å for Te-Cd₁ (CdTe), and 2.81 Å (Te-Te₁) and 3.47 Å (Te-Te₂) for Te inclusions. These results primarily arise from fitting using three paths, based on a realistic cubic zinc-blende structure model together with an elemental Te model.