Bismuth-induced deep levels and carrier compensation in CdTe

First-principles calculations show that Bi on Cd site in CdTe can be either a donor Bi-Cd(+) or an acceptor Bi-Cd(-), depending on the Fermi level. The Bi-Cd(-) can bind a substitutional O (O-Te) with large binding energy of 1.40 eV. The calculated (0/-) transition level for Bi-Cd(-)-O-Te complex is in good agreement with experimentally observed deep hole trapping level. Bi can also substitute Te to form an acceptor. The amphoteric nature of Bi in CdTe results in the pinning of the Fermi level and the high resistivity. The transition of the CdTe samples from semi-insulating to p type at high Bi doping levels is explained by the formation of secondary phase that contains Bi and Te.