Photo-electronic investigation of CdZnTe spectral detectors

Using a focussed and pulsed beam of 850 nm laser radiation, it is possible to assess a number of bulk properties of CdZnTe. The sub-bandgap pulsed radiation (pulse width 80 ns) excites a detectable signal from defects near the band edges in CdZnTe. This is detected using a conventional charge sensitive preamplifier. Further information is obtained by observing the transmitted optical beam simultaneously with the detected photo-induced current from the CdZnTe detector. The intensity of the laser beam is adjusted such that an average of 157 keV is deposited in the CdZnTe detector per optical pulse. Using optical polarisers, electric field distribution has been measured without the space-charge induced distortion of electric field normally experienced when using higher intensity d.c. light sources, and the results correlated with charge collection efficiency measurements. By varying the pulse repetition frequency, information is obtained about the lifetime of shallow defects at room temperature.