Homologous Self-Assembled Superlattices: What Causes their Periodic Polarity Switching?

Quantum semiconductor structures are commonly achieved by bandgap engineering, which 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 is 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. The same structure is observed when the indium is replaced with other metals, and perhaps even in ZnO alone. This peculiar self-assembled structure is 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, an explanation is proposed for the long-standing mystery of this intriguing self-assembly in the form of an electrostatic growth phenomenon and a test of the proposed model is carried out on experimental data. While much scientific and technological effort is devoted today to bandgap engineering of complicated heterostructures, nature appears to be able to carry out this task seamlessly and effortlessly in the formation of homologous superlattices. However, the formation of the periodic polarity switching which underlies the self-assembly is a mystery. Here, an electrostatic phenomenon is shown to cause this self-assembly. image