Current instability and polarization phenomena in lead iodide crystalline detectors

The study of the evolution of the leakage current after suddenly reversing the bias has been used to investigate the origin of polarization phenomena in lead iodide single-crystal detectors. The presence of a peak directly related to the accumulated charge and the temperature in the current vs. time curves can be explained using a model previously proposed for mercuric iodide. This behaviour results from the presence of a trap level, in the bandgap whose energy has been estimated to be 0.41 eV from current vs. time and temperature measurements. Actually, Thermally Stimulated Current measurements indicated that this value corresponds only to an average and that several levels are present in the bandgap, the main ones being situated at 0.29, 0.45 and 0.50 eV. Correlation with the time behaviour of the detector characteristics (noise and pulse height) indicates that polarization is due to the modification of the charge of these levels with time.