Effect of the thickness of CdTe crystals on electrical and detection properties of Cr/CdTe/Au Schottky-diode detectors

The electrical and detection properties of the X/gamma-ray detectors, fabricated using commercial detector-grade p-like CdTe single crystals grown by Acrorad Co., were studied. The detectors were developed as Cr/CdTe/Au Schottky diodes using CdTe wafers with an area of 5 x 5 mm(2) and different thicknesses (0.25, 0.5, 0.75, 1.0 and 2.0 mm). Both the Schottky and Ohmic contacts were formed on the opposite sides of semi-insulating CdTe(111) crystals after preliminary chemical and Ar-ion etchings using different parameters. The Cr/CdTe/Au Schottky diodes demonstrated steep rectification that made it possible to apply high reverse bias voltage up to V = 1500-2000 V at moderately low dark currents. Having the rectifying contact area of 10 mm(2), the diodes showed dark currents similar to 2-3 nA at V = 1000 V (T = 300 K). The dominant charge carrier transport mechanisms were analyzed using the I-V characteristics and determined as: generation-recombination in the space-charge region (SCR) at V = 1-100 V, the charge transport in the conditions when the SCR width exceeded the thickness of a semiconductor crystal at elevated voltages and, finally currents limited by space charge at even higher bias voltages. For radiation from a Cs-137 radioisotope, the Cr/CdTe/Au detectors fabricated on thin (0.5 mm) semiconductor crystals showed the highest energy resolution (FWHM = 0.5%@662 keV). The detection efficiency increased in 4 times with increasing the crystal thickness (up to 2 mm), although the energy resolution deteriorated from 0.5 % to 3 % that, however, was still acceptable for spectroscopic operation of the detectors.