Performance of CdTe-based p-n junction-diode X/γ-ray detectors

Abstract

The electrical and spectroscopic properties of the CdTe-based p-n junction-diode X/γ-ray detectors have been studied by the measurements of I-V characteristics and emission spectra of 137Cs, 57Co, and 241Am radioisotopes. The In/CdTe/Au diodes were fabricated by the frontside laser irradiation doping technique. Detector-grade p-like CdTe(111) crystals, pre-coated with an In dopant (electrode) film, were irradiated with nanosecond KrF laser pulses in distilled water. Laser stimulated solid-phase doping was attributed to generation and propagation of stress and shock waves, barodiffusion of In dopant atoms into the thin CdTe surface region and thus, creation of high concentration donors. The second contact (quasi-ohmic) was formed on the opposite (Cd-terminated) crystal side by vacuum evaporation of Au. In addition, some diodes were created on CdTe crystals, which were preliminary annealed in vacuum prior to deposition of metal films. The I-V characteristics of the In/CdTe/Au p-n junction diodes evidenced that the reverse dark current in the initial part was described by the Sah-Noyce-Shockley theory. Creation of the favorable conditions for efficient collection of photogenerated charge carriers, decrease in the reverse dark current in the In/CdTe/Au detectors ensured obtaining the high-energy resolution spectra. Based on the electric field dependence of the 137Cs isotope spectrum, it was established that the applied bias V = 800 V was the optimal for the diodes formed on preliminary annealed CdTe crystals which ensured the energy resolution FWHM = 0.62 % at the 662 keV line