The advantage of excitonic laser gain has been demonstrated for ZnSe-based quantum wells ( Ding et al. The ZnO exciton has a binding energy of 60 meV, which is significantly larger than the effective thermal energy at room temperature thus excitonic gain mechanisms could be expected to play a significant role in ZnO-based devices. Thus crystal field splitting and spin-orbit coupling give rise to three two-fold degenerate valence bands: A(Γ 9), B(Γ 7), and C(Γ 7) from the valence band maximum. Because of the lower symmetry of the wurtzite structure, the crystal field further lifts the valence band degeneracy. Spin–orbit splitting leads to a partial lifting of the valence band degeneracy: the six-fold degenerate valence band splits into a four-fold ( j=3/2) and a two-fold band ( j=1/2). The conduction band of ZnO is predominantly s-like, while the valence band is p-like. ZnO has a direct bandgap with the conduction band minimum and the valence band maximum located at k=0 in the Brillouin zone. Although the ideal wurtzite structure has four-fold coordination with a hexagonal unit cell having two lattice parameters of a and c ( c/ a= 8/3=1.633), the actual lattice of ZnO deviates from the ideal lattice with c/ a=1.602. A phase transition to the rock-salt structure occurs under high pressure. The most stable phase under thermal equilibrium is the wurtzite structure.
ZnO can crystallize in the wurtzite, zinc blende, and rock-salt structures. The chemical bonding in ZnO is predominantly covalent but with a significant contribution from ionic bonding. Yao, in Encyclopedia of Materials: Science and Technology, 2001 1 Material Properties