Conventional Optics from Unconventional Electronics in ZnO Quantum Dots

We study the electronic and optical properties of ZnO quantum dots within the atomistic empirical pseudopotential framework. The highest occupied molecular orbital (HOMO) is found to be of orbital P character for structures larger than 2.6 nm in diameter. We identify the origin of this unconventional situation in the electronic character of the HOMO state, originating from an even mixture of the A- and B-bands of the Wurtzite band structure. This situation, however, does not lead to an orbitally dark exciton ground state, as one might expect. Coulomb interactions lower the bright (electron-S-hole-S) exciton below the orbitally forbidden (electron-S-hole-P) exciton to recover the conventional situation of an orbitally allowed, but spin-forbidden, exciton ground state and a Stoke's shift originating from electron-hole exchange interactions.