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 rocksalt transition is observed at 9.7G0.2 GPa. The absorption tail of the fundamental gap, as measured in monocrystals, exhibits a pressure coefficient of 24.5G2 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 (23.0G0.5 meV/GPa for the A–B exciton). Rock-salt ZnO is shown to be an indirect semiconductor with a bandgap of 2.7G0.2 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. q 2005 Elsevier Ltd. All rights reserved.