Residual strain in nonpolar a-plane Zn1-xMgxO (0<x<0.55) and its effect on the band structure of (Zn,Mg)O/ZnO quantum wells

We investigate the dependence on Mg content of the lattice parameters and the surface morphology of nonpolar a-(1120) Zn1-xMgxO (x <= 0.55) grown by molecular beam epitaxy. The anisotropy of the lattice parameters gives rise to an unusual in-plane strain state in the ZnO QWs: tensile strain along [1100] and compressive strain along [0001]. For a Zn0.6Mg0.4O barrier, the strain in a ZnO QW reaches -1.3% along [0001] and +0.3% along [1100]. This induces a strong blueshift of the excitonic transitions, in addition to the confinement effects, which we observe in photoluminescence excitation experiments.