Band offsets and polarization effects in wurtzite ZnO/Mg0.25Zn0.75O superlattices from first principles

Using first-principles calculations, we investigate the band offsets, built-in electric fields, and band gaps of (0001)-oriented wurtzite ZnO/Mg_0.25Zn_0.75O superlattices, including the dependence on superlattice geometry and strain. Significant built-in electric fields form inside the quantum-well region that are found to be tunable over the range 0.24 MV/cm ≤ E_w ≤ 0.63 MV/cm, and potentially up to 1 MV/cm by varying the relative width of the well and barrier regions. The valence band offset at the ZnO/Mg_0.25Zn_0.75O interface is calculated to be 0.25-0.26 eV which, in contrast to the "common anion rule," is a significant portion of the total band offset, and this is in support of recent experiment. Calculated values for the valence band offset were found to be insensitive to variations in superlattice geometry and strain. The band gap of the superlattice is determined by the competing effects of quantum confinement and the quantum-confined Stark effect, with the former being more dominant for the systems investigated. These findings will be useful in the design and optimization of ZnO/Mg_xZn_1-xO superlattices for electronics and optoelectronics applications.