It was recalled that heavily Ga-doped bulk n-type Cd1-xMnxTe crystals were known to exhibit a strong persistent photoconductivity at temperatures below about 100K. In order to identify the origin of the photoconductivity, an investigation was made of the electronic properties of the deep-level defects in the material by means of transient spectroscopic and thermally stimulated capacitance measurements. Crystals with an x-value of 0.03, and a Ga doping level of 1019/cm3, were used. The deep-level transient spectroscopic measurements revealed the presence of 2 deep levels, with thermal activation energies for electron emission of 0.25 and 0.43eV. The electron capture process of the 0.25eV level was found to be thermally activated, with a thermal energy barrier of 0.11eV. Deep-level transient spectroscopic measurements which were carried out after sub-bandgap illumination at low temperatures established that, in addition to the persistent photoconductivity effect, the illumination caused a sharp increase in the concentration of the 0.25eV states. This persisted to temperatures of up to 200K. This indicated that, in addition to the metastability of conduction electrons with respect to the 0.25eV states (which resulted in the persistent photoconductivity below 100K), the 0.25eV states were also metastable with respect to other (ground) states from which they were created by photo-excitation. The thermally stimulated capacitance measurements also detected 2 transitions to 2 different deep levels. The first of these levels was donor-like, with an activation energy of 0.21eV. The second level was acceptor-like, and the electron transition to this level required a thermal energy of several hundred meV. Because the concentrations of the 0.25eV and 0.43eV states which were found by deep-level transient spectroscopy appeared to be very close to each other, they were attributed to a single defect with 2 charge states. The 0.43eV level was attributed to an acceptor-like 2-electron ground state of the defect, and the 0.25eV level was attributed to an excited donor-like 1-electron state. According to this model, the defect possessed a positive Hubbard correlation energy (positive-U system).

N.G.Semaltianos, O.Karczewski, B.Hu, T.Wojtowicz, J.K.Furdyna: Physical Review B, 1995, 51[24], 17499-505