Effects of excess tellurium on the properties of CdZnTe radiation detectors

Abstract

Room-temperature radiation detectors have been fabricated on high-resistivity, indium-doped Cd_0.90Zn_0.10Te crystals grown under different amounts of excess Te. The effects of the excess Te on the properties of the detectors are explained by a simple model using only three parameters: the density of Cd vacancies, the density of Te antisites (Te at Cd sites), and the deep level of doubly ionized Te antisites. The best detectors, which can resolve the low-energy Np-L and Te-K peaks as well as Cd and Te escape peaks of ²⁴¹Am, are produced from crystals grown with 1.5% excess Te. The detectors fabricated from crystals grown without excess Te are unable to resolve any characteristic-radiation peaks of ²⁴¹Am and ⁵⁷Co. This result is explained by a model of networked p-type domains in an n-type matrix or vice versa, which is caused by the lack of sufficient deep-level Te antisites. Such conduction-type inhomogeneity causes massive electron and hole trapping. As for the detectors fabricated from Cd_0.90Zn_0.10Te crystals grown with 2% and 3% excess Te, they are able to resolve the ²⁴¹Am 59.5-keV, ⁵⁷Co 122-keV, and ⁵⁷Co 136-keV radiation peaks. However, the full-width at half-maximum (FWHM) values of these peaks are broadened, especially the high-energy ⁵⁷Co peaks. These phenomena are attributed to the hole and, possibly, electron trapping by Cd vacancies and Te antisites, respectively. The result of the analysis indicates that sufficient Te antisites and a low density of carrier traps in Cd_0.90Zn_0.10Te are essential for producing high-quality radiation detectors. In the analysis, it was discovered that most of the excess Te, on the order of 1–2 × 10²⁰ cm⁻³, remain electrically inactive. A possible explanation for this phenomenon is that the excess Te atoms form neutral Te-antisite and Cd-vacancy complexes, such as Te_Cd·(V_Cd)², during the post-growth cooling process.