Clarification of the Spontaneous Polarization Direction in Crystals with Wurtzite Structure

TL;DR

This paper clarifies the true direction of spontaneous polarization in wurtzite-structured crystals, resolving longstanding confusion and establishing that metal-polar units have an upward polarization, with implications for semiconductor device design.

Contribution

It provides a definitive resolution to the polarization direction ambiguity in wurtzite crystals, supported by analysis of experimental data and microscopy images, and generalizes findings beyond AlScN.

Findings

Metal-polar unit cell corresponds to upward polarization

Resolved polarization direction ambiguity in wurtzite structures

Implications for heterostructure interface properties

Abstract

The wurtzite structure is one of the most frequently found crystal structures in modern semiconductors and its inherent spontaneous polarization is a defining materials property. Despite this significance, confusion has been rampant in the literature with respect to the orientation of the spontaneous polarization inside the unit cell of the wurtzite structure, especially for the technologically very relevant III-N compounds (AlN, GaN, InN). In particular, the spontaneous polarization has been reported to either point up or down for the same unit cell orientation, depending on the literature source - with important implications for, e.g., the carrier type and density expected at interfaces of heterostructures involving materials with wurtzite-structure. This perspective aims to resolve this ambiguity by reviewing available reports on the direction of the energetically preferred polarization direction in the presence of external electric fields, as well as atomically resolved scanning transmission electron microscopy images. While we use ferroelectric wurtzite AlScN as a key example, our conclusions are generalizable to other compounds with the same crystal structure. We demonstrate that a metal-polar unit cell must be associated with an upward polarization vector - which is contrary to long-standing conventional wisdom.