Optical properties and structural phase transitions in Mg_xZn_1−xO under hydrostatic pressure

Abstract

This article reports on the optical properties under pressure of Mg _x Zn_1−x O (x < 0.13 ± 0.2) thin films deposited on mica and fluorite substrates by pulsed-laser deposition. The absorption edge of the semiconductor alloy is measured in both the ambient pressure wurtzite phase and the high-pressure rock-salt phase for several Mg contents. In wurtzite Mg _x Zn_1−x O, a larger value of the band-gap is clearly correlated to a larger pressure coefficient of the band-gap. This effect is shown to be consistent with the decreasing contribution of cation d-levels to the valence band maximum states as the Mg content increases. The wurtzite-to-rock-salt transition pressure is observed to decrease from 9.5 ± 0.2 (pure ZnO) to 7.0 ± 0.2 GPa (for x = 0.13), with an almost linear dependence on the Mg content. The same linear dependence on x, with virtually the same slope, is also found for the rock-salt-to-wurtzite reverse transition in the pressure down-stroke. For x > 0.13, the rock-salt phase is observed to be metastable at room pressure, after a pressure cycle up to 15 GPa.

Keywords:

• Optical properties
• High pressure
• Phase transition
• Oxides
• MgZnO alloys
• Metastable phases

Acknowledgements

This paper was supported through Spanish Government MCYT grant MAT2002-04539-C02-01. One of the authors (A. S.) wish to thank the Spanish MCYT FPI fellowship program.