Measurement of polarization phenomena in CdTe radiation detector by optical laser pulses

To investigate mechanisms of the polarization phenomena in the CdTe radiation detectors, we used optical laser pulses for carrier generation and detected the electrical response from the CdTe detectors as a function of carrier-generation position in the detector. Temporal change of the internal electric field at defined position in the CdTe detector can be directly observed by the laser pulse measurements. Detectors measured in this study were Schottky-type CdTe detectors with a thickness of 0.5 mm and a size of 2 × 1 mm2. Laser light from laser diode with a wavelength of 850 nm were modulated to pulses by using pulse generator and were irradiated to the cleaved side surface of the CdTe detectors. The incident pulse power was 10 nW, which correspond to one photon energy of γ-lay with a energy of several tens of keV. Time traces of output voltage amplified by preamplifier were measured by oscilloscope as a function of time after applying bias voltage. Pulse height and rise-up time was obtained from the time trace of the output voltage. The pulse height was decreased and the rise-up time was increased as a function of the time. Rate of these changes are larger at cathode side than anode side. Depletion layer in the detector is assumed to be shrunk and internal electric field in cathode side is decreased with time. The change of internal electric field is considered to be caused by carrier accumulation to deep acceptor level.