First-principles study of polarization in ZnMgO

TL;DR

This study uses first-principles calculations to analyze how the spontaneous polarization of ZnMgO alloys varies with composition, considering strain effects and decomposing polarization contributions, relevant for optoelectronic applications.

Contribution

It provides a detailed ab-initio analysis of polarization in ZnMgO alloys, including strain effects and component decomposition, which was not previously comprehensively studied.

Findings

Polarization increases with MgO content in ZnMgO alloys.

Epitaxial strain significantly influences the polarization.

A simple piezoelectric model effectively predicts polarization changes under strain.

Abstract

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.