Abstract
Zinc oxide under biaxial inplane tensile strains is studied theoretically by first-principles density functional calculations. Different material properties (including structural response of cell shape, chemical bonding, total-energy curvature, electrical polarization, Born effective charge, electronic band dispersion, optical interband transitions, and effective masses) are examined. We found that (1) the ratio decreases in a rather linear fashion with the increasing tensile strain when the inplane lattice constant (denoted as ) of ZnO is varied below a critical transition value of ( is the equilibrium inplane cell length). However, at , ZnO exhibits a pronounced structural discontinuity in ratio, as well as in cell volume. (2) The structural discontinuity results from the existence of two energy minima (labeled as A and B), both being metastable. Minimum A is energetically favorable when is below , while minimum B is more stable when exceeds . (3) As the inplane lattice constant approaches from below, ZnO becomes markedly soft along the polar axis, promising a large electromechanical response. (4) At , spontaneous polarization in ZnO collapses, leading to a polar-nonpolar phase transformation. (5) Despite that the spontaneous polarization vanishes when , Born effective charge of Zn atom nevertheless increases, demonstrating an interesting anticorrelation. (6) Above , covalent overlapping charge largely disappears between those polar Zn-O bonds collinear with the axis, indicating that the bonds are predominantly ionic. (7) The polar-nonpolar structural transformation simultaneously gives rise to a direct-indirect band gap transition. When is above , the valence band maximum is no longer at zone center but at zone-edge point. Occurrence of indirect band gap originates from the fact that the orbital energy of the top valence state at shows a sensitive dependence on the inplane strain. (8) At zone center , optical excitations from the top two valence states to the lowest conduction state remain strongly allowed after the structural phase transformation. (9) Accompanying the occurrence of the indirect band gap transition is a considerable reduction of the hole effective mass, and hence a large increase of hole mobility.
2 More- Received 2 August 2007
DOI:https://doi.org/10.1103/PhysRevB.77.045213
©2008 American Physical Society