First-principles study of polarization in ZnMgO

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.