Characterization of X- and Gamma- ray CdTe radiation detectors

CdTe crystals grown by the Traveling Heater Method (THM) often show a pronounced non-uniformity along the ingots due to the thermal irregularities, the Te-excess growth conditions resulting from the retrograde slope of the solidus line of the phase diagram, and to the introduced impurities. In addition, structural defects can be present that affect the electrical and optical properties of the crystals. X-ray and alpha particle spectroscopy measurements have been performed on differently prepared CdTe samples (as grown or annealed, different surface treatments, etc.) with Pt electrodes deposited by electroless technique in order to extract the typical figures of merit of the material and the detectors. Moreover, Rutherford Backscattering Spectrometry (RBS) using 8 MeV ⁷Li⁺⁺⁺ ions and X-Ray Fluorescence (XRF) using a Pd-anode X-Ray generator were performed to characterize the crystal surface as well as the semiconductor-electrode interface. In addition, contactless resistivity and mutau product mapping technique, infrared microscopy imaging and electrical and spectroscopy measurements have been used for material characterisation. The aim of this work is to determine in a routine way the best suited method to fabricate high-quality X- and Gamma-ray detectors to be used both as large size simple detectors and as elements of imaging systems in medical or industrial applications as well as to understand and improve the structure of the material-electrode interface. The thickness, the stoichiometry and the concentration profiles of platinum, cadmium and tellurium present at the surface layers were determined. The distribution of Cd deficiency at the interface layers was profiled using simulations and showed complex profiles in the samples, which can greatly affect the electrical quality of the detectors