Space-charge limited transport in CdTe-based X- and γ-ray detectors

The principle feature limiting spectrometric performance of CdTe and CdMnTe-based X/γ-ray detectors is the impossibility to increase the operating voltage to provide sufficient collection of charge carriers because of raising leakage current. We have carried out the detailed analysis of electrical characteristics of CdTe and CdMnTe-based detectors with two Ohmic contacts. It is shown that a rapid increase in leakage current at applying high bias voltage in the CdTe and CdMnTe crystals is caused by the current limited by space charge (SCLC). The same activation energy for the current of equilibrium holes and the current caused by nonequilibrium holes at high voltages confirms the fact that SCLC in the Ni/CdMnTe/Ni detector formed by injection of majority carriers (holes) due to lowering the barrier at an imperfect Ohmic contacts. This fact distinguishes Ni/CdMnTe/Ni detector from Pt/CdTe/Pt detector where SCLC formed by tunnel transitions of minority carriers from the Fermi level in the metal (or slightly below it) to the semiconductor through a thin insulating film between the crystal and metal contact. Comparison the measured and calculated current-voltage characteristics of CdMnTe crystal at different temperatures taking into account SCLC according to the Mott-Gurney theory allowed to determine the density of discrete trapping centers (1.4 × 1013 cm3) and energy of hole traps (0.39 eV) in the Ni/CdMnTe/Ni detector.