Optical properties and electronic structure of rock-salt ZnO under pressure

This letter reports on the pressure dependence of the optical absorption edge of ZnO in the rock-salt phase, up to 20 GPa. Both vapor-phase monocrystals and pulsed-laser-deposition thin films on mica have been investigated. Rock-salt ZnO is shown to be an indirect semiconductor with a band gap of 2.45±0.15 eV, whose pressure coefficient is very small. At higher photon energies, a direct transition is observed (4.6 eV at 10 GPa), with a positive pressure coefficient (around 40±3 meV/GPa between 5 and 19 GPa). These results are interpreted on the basis of first-principles electronic band structure calculations.