Defect-induced luminescence in high-resistivity high-purity undoped CdTe crystals

This work focuses on the influence of stoichiometry deviation (Cd/Te excess in the crystal) on the type and density of crystalline defects. In particular, we report a study on the evaluation of extended defects and precipitates, carried out by specific preferential etching, high-resolution x-ray diffraction, double-crystal x-ray topography and cathodoluminescence (CL) analyses. A remarkable difference between the crystals grown from the vapour phase and from the melt has been found. Dislocations were found to arrange in cellular structures 2-300 mum in diameter while precipitates were homogeneously distributed on the growth plane. The CL spectra show an intense near-band-edge emission through the whole range of temperatures and two large emission bands centred at 1.2 and 1.4 eV. It has been observed that the luminescence intensity of the 1.4 eV band increases close to crystal defects and simultaneously the near-band-edge intensity decreases. The disappearance of this emission both in the CL spectra and in the CL images on increasing the temperature up to 300 K suggests a donor-acceptor pair nature for the transition involved.