Study on Instability Phenomena in CdTe Diode-like Detectors

Despite the enhanced spectroscopic performance offered by the blocking contact configuration, diode-like CdTe detectors suffer from instability problems when working at room temperature: a progressive degradation of energy resolution and a shift of the photopeak are observed after applying the bias voltage. This effect, known since several years as polarization, has been attributed to deep levels inside the crystal, but it does not affect detectors equipped with standard ohmic contacts, though realized starting from crystals nominally equal. In a previous work, by means of Pockels effect measurements performed on diode-like CdTe detectors at room temperature, we were able to map the distribution of the internal electric field and to follow its time evolution correlating it to the degradation of spectroscopic performance. We found that, under operative bias, it occurs a progressive reduction of the depletion region, which becomes confined under the anode. At the present stage, we will investigate this phenomenon in more detail, looking for correlation between the electric field distribution and i) the current transients likely associated with a lowering of the Schottky barrier; ii) the charge ransients induced by laser pulses which, laterally impinging on the detector side, allow us to probe the dynamics of the carriers with a good spatial resolution between cathode and anode. The study, carried out at different temperatures, up to 55°C, points out the presence of a mechanism strongly temperature dependent, with an activation energy surprisingly close to the CdTe energy gap. Finally, the effect of the applied voltage on the detector stability will be shown, and the results will be compared with those obtained on standard detectors, fabricated starting from nominally the same bulk material, as diode -like ones, but equipped with ohmic contacts.