First-principles study of polarization in Zn1-xMgxO

Wurtzite ZnO can be substituted with up to similar to 30% MgO to form a metastable Zn1-xMgxO alloy while still retaining the wurtzite structure. Because this alloy has a larger band gap than pure ZnO, Zn1-xMgxO/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 Zn1-xMgxO 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 epitaxial-strain 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 Zn1-xMgxO alloy induced by epitaxial strain.