Homologous self-assembled superlattices: What causes their periodic polarity switching? Review, model, and experimental test

TL;DR

This paper reviews the phenomenon of homologous superlattices in binary oxides, proposes an electrostatic growth model to explain their periodic polarity switching, and tests this model against experimental data.

Contribution

It offers a comprehensive review of homologous superlattices and introduces a new electrostatic growth model supported by experimental testing.

Findings

Electrostatic interactions drive superlattice formation.

Polarity inversion occurs due to electrostatic stability.

Model successfully explains experimental polarity switching.

Abstract

Quantum semiconductor structures are commonly achieved by bandgap engineering that relies on the ability to switch from one semiconductor to another during their growth. Growth of a superlattice is typically demanding technologically. In contrast, accumulated evidence points to a tendency among a certain class of multiple-cation binary oxides to self-assemble spontaneously as superlattice structures. This class has been dubbed the homologous superlattices. For a famous example, when a mixture of indium and zinc is oxidized, the phases of In-O and ZnO separate in an orderly periodic manner, along the ZnO polar axis, with polarity inversion taking place between consecutive ZnO sections. As we review here, the same structure has been observed when the indium was replaced with other metals, and perhaps even in ZnO alone. This peculiar self-assembled structure has been attracting research over the past decade. The purpose of this study is to gain understanding of the physics underlying the formation of this unique structure. Here, we first provide an extensive review of the accumulated literature on these spontaneously-formed structures and then propose an explanation for the long-standing mystery of this intriguing self-assembly in the form of an electrostatic growth phenomenon and test the proposed model on experimental data.