  Wurtzite ZnO can be substituted with up to ~30% MgO to form a metastable
ZnMgO alloy while still retaining the wurtzite structure. Because this alloy
has a larger band gap than pure ZnO, ZnMgO/ZnO quantum wells and superlattices
are of interest as candidates for applications in optoelectronic and electronic
devices. Here, we report the results of an ab-initio study of the spontaneous
polarization of ZnMgO alloys as a function of their composition. We perform
calculations of the crystal structure based on density-functional theory in the
local-density approximation, and the polarization is calculated using the
Berry-phase approach. We decompose the changes in polarization into purely
electronic, lattice-displacement mediated, and strain mediated components, and
quantify the relative importance of these contributions. We consider both
free-stress and epitaxialstrain elastic boundary conditions, and show that our
results can be fairly well reproduced by a simple model in which the
piezoelectric response of pure ZnO is used to estimate the polarization change
of the ZnMgO alloy induced by epitaxial strain.
