Synthesis and properties of highly mismatched II-O-VI alloys

Ternary and quaternary dilute II-VI oxides were synthesised using a highly non-equilibrium method: the combination of O ion implantation and pulsed-laser melting. CdO_xTe_1-x thin films have been produced with x up to 0.015 and with the energy gap reduced by 0.15 eV. Optical transitions corresponding to both the lower (E_-) and upper (E₊) conduction sub-bands, resulting from the anticrossing interaction between the localised O states and the extended conduction states of the matrix, are clearly observed in quaternary Cd_0.6Mn_0.4O_xTe_1-x and Zn_0.88Mn_0.12O_xTe_1-x layers. These results have important implications for the existing theoretical models of the electronic structure of the highly mismatched alloys. In Zn_1-xMn_xTe, where the O level lies below the conduction band edge, it was demonstrated that incorporation of a small amount of oxygen leads to the formation of a narrow, oxygen-derived band of extended states located well below the conduction band edge of the ZnMnTe matrix. The three absorption edges of this material (∼0.73, 1.83 and 2.56 eV) cover the entire solar spectrum providing a material envisioned for multiband, single-junction, high-efficiency photovoltaic devices.