Effects of the host conduction band energy on the electronic band structure of ZnCdTeO dilute oxide alloys

Interband optical transitions in highly mismatched ZnTe_1-xO_x and Zn_1-yCd_yTe_1-xO_x alloys with Cd content y = 0.1 and 0.32 and oxygen content x < 0.016 grown on ZnTe substrates were studied by photoreflectance (PR) and photoluminescence (PL) in a broad temperature range. The incorporation of oxygen into a Zn(Cd)Te matrix results in a splitting of the conduction band (CB) into two E_- and E₊ subbands forming a semiconductor with an intermediate band. In ZnTeO, only the E_- band could be probed by PR and there was no PL signal. An addition of Cd atoms to form a ZnCdTeO quaternary alloy significantly improves the optical quality as evidenced by an emergence of an E₊ related transition in the PR spectra and the appearance of a PL emission related to the E_- band visible up to 260 K. Moreover, for Cd content above 25%, a change in the E_- band character is observed from localized O-like to CB-like. The analysis of a PR signal shows a strong reduction of the temperature dependence of the energy gap of Zn(Cd)TeO alloys compared to ZnTe. The temperature related reduction of the bandgap shift with increasing O content is well explained by the band anticrossing interaction between the temperature dependent conduction band of the host Zn(Cd)Te matrix and the temperature independent energy of highly localized O states.