Signal formation and decay in CdTe x-ray detectors under intense irradiation

The response of Cd(Zn)Te Schottky and resistive detectors to intense x-rays is investigated in a commercial computed tomography (CT) system to assess their potential for medical diagnostics. To describe their signal height, r esponsivity, signal-to-noise ratio (SNR), and detective quantum efficiency the devices are modeled as solid-state ionization chambers with spatially v arying electric field and charge collection efficiency. The thicknesses and pixel areas of the discrete detector elements are 0.5-2 mm and a few mm(2) , respectively. The incident spectrum extends from 26 to 120 keV and compri ses 10(10) quanta/s cm(2). It photogenerates a carrier concentration in the semiconductor that is two to three orders of magnitude above the intrinsic concentration, but remains to a similar extent below the charge densities on the device electrodes. Stable linear operation is achieved with the Scho ttky-type devices under high bias. Their behavior can be modeled well if ne gatively charged near-midgap bulk defects with a concentration of 10(11)-10 (13) cm(-3) are assumed. The bulk defects explain the amount and time const ant (about 100 ms) of the detrapping current measured after x-ray pulses (a fterglow). To avoid screening by the trapped space charge the bias voltage should exceed 100 (V)X[detector thickness/mm](2). Dark currents are of the order of the generation-recombination current, i.e., 300 pA/mm(3) detector volume. With proper device design the signal height approaches the theoreti cal maximum of 0.2 A/W. This high responsivity, however, is not exploited i n CT since the SNR is determined here by the incident quantum noise. As a c onsequence of the detrapping current, the response speed does not meet CT r equirements. A medium term effort for crystal growth appears necessary to a chieve the required reduction of the trap density by an order of magnitude. Scintillation based detectors an, therefore, still preferred in fast opera ting medical diagnostic systems. (C) 1999 American Association of Physicist s in Medicine.. [S0094-2405(99)00201-1].