Structure, band gap, and Mn-related mid-gap states in epitaxial single crystal (Zn_1-xMg_x)_1-yMn_yO thin films

Epitaxial (Zn_1-xMg_x)_1-yMn_yO thin films were grown on c-Al₂O₃ substrates by radio frequency oxygen plasma assisted molecular beam epitaxy. Single crystal structure of the (Zn_1-xMg_x)_1-yMn_yO films was revealed by reflection high energy electron diffraction and X-ray diffraction. The band gap of the films can be tuned dramatically with increasing the Mg concentration, while the onset energy of Mn-related mid-gap absorption band only shows a small blue shift. Photoconductivity measurements indicate the Mn-related mid-gap states in (Zn_1-xMg_x) _1-yMn_yO films can create free carriers and contribute to charge transfer transitions. The conduction band offset ΔE_C 0.13 eV and valence band offset ΔE_V 0.1 eV were obtained for ZnO/Zn_0.8Mg_0.2O heterostructures, which increase to ΔE_C 0.21 eV and ΔE_V 0.14 eV for ZnO/Zn _0.7Mg_0.3O heterostructures.