New approach for high resistivity CdTe preparation

Cadmium telluride (CdTe) is one of the important members of II–VI semiconductor compounds. In various applications, for example, X-ray detectors, photo-refractive and electro-optical devices, semi-insulating (SI) materials are necessary. Though extensive studies have been devoted to the selection of a suitable doping element for obtaining SI crystals, only a few have dealt with intrinsic electronic properties. For this reason, a systematic exploration of the intrinsic properties of undoped CdTe with the aim of growing nominally pure CdTe crystals has been carried out. Results are reported on: (i) direct synthesis of 7N pure elements; (ii) stoichiometry correction procedure based on a specially devised post-synthesis heat treatment; (iii) evaluation of the stoichiometry deviation of CdTe by means of partial vapour pressure measurements; (iv) crystal growth carried out from vapour phase (sublimation) and from the melt (normal freezing); (v) electrical characterization. In particular it is found that in high-purity crystals the electrical resistivity critically depends on the stoichiometry deviation of the material source.