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 c/a ratio decreases in a rather linear fashion with the increasing tensile strain when the inplane lattice constant (denoted as a) of ZnO is varied below a critical transition value of a(tr)=1.067a(0) (a(0) is the equilibrium inplane cell length). However, at a=a(tr), ZnO exhibits a pronounced structural discontinuity in c/a 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 a is below a(tr), while minimum B is more stable when a exceeds a(tr). (3) As the inplane lattice constant approaches a(tr) from below, ZnO becomes markedly soft along the polar c axis, promising a large electromechanical response. (4) At a=a(tr), spontaneous polarization in ZnO collapses, leading to a polar-nonpolar phase transformation. (5) Despite that the spontaneous polarization vanishes when a=a(tr), Born effective charge of Zn atom nevertheless increases, demonstrating an interesting anticorrelation. (6) Above a(tr), covalent overlapping charge largely disappears between those polar Zn-O bonds collinear with the c 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 a is above a(tr), the valence band maximum is no longer at zone center Gamma but at zone-edge H point. Occurrence of indirect band gap originates from the fact that the orbital energy of the top valence state at H shows a sensitive dependence on the inplane strain. (8) At zone center Gamma, 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.