Optical properties of wurtzite and rock-salt ZnO under pressure

This paper reports on the pressure dependence of the optical absorption edge of ZnO in the wurtzite and rock-salt phase, up to 14 GPa. Both vapor-phase monocrystals and pulsed-laser-deposition thin films have been investigated. In both types of samples the wurtzite to rock-salt transition is observed at 9.7±0.2 GPa. The absorption tail of the fundamental gap, as measured in monocrystals, exhibits a pressure coefficient of 24.5±2 meV/GPa. The evolution under pressure of the full absorption edge of the wurtzite phase is studied with thin film samples, yielding a slightly lower pressure coefficient (23.0±0.5 meV/GPa for the A-B exciton). Rock-salt ZnO is shown to be an indirect semiconductor with a bandgap of 2.7±0.2 eV. At higher photon energy a direct transition (Egd-4.5 eV) can be also identified in thin films transited to the rock-salt phase. Results on the high-pressure phase are interpreted on the basis of density-functional-theory (DFT) electronic structure calculations.