Piezoelectric effect and polarization switching in Al$_{1-x}$Sc$_x$N

TL;DR

This study uses first-principles calculations to analyze polarization and piezoelectric properties of Al$_{1-x}$Sc$_x$N alloys, revealing how scandium enhances ferroelectricity and piezoelectricity, and providing insights into switching behavior.

Contribution

It provides a detailed first-principles analysis of polarization, piezoelectric enhancement, and ferroelectric switching in Al$_{1-x}$Sc$_x$N alloys, including phase stability and microscopic mechanisms.

Findings

Scandium increases the internal displacement parameter u.

The alloy approaches a layered hexagonal structure with more Sc.

The ferroelectric switching barrier depends on Sc concentration and temperature.

Abstract

Aluminum nitride is piezoelectric and exhibits spontaneous polarization along the $c$-axis, but the polarization cannot be switched by applying an electric field. Adding Sc to AlN enhances the piezoelectric properties, and can make the alloy ferroelectric. We perform a detailed first-principles analysis of spontaneous and piezoelectric polarization. Comparisons between explicit supercell calculations show that the virtual crystal approximation produces accurate results for polarization, but falls short in describing the phase stability of the alloy. We relate the behavior of the piezoelectric constant $e_{33}$ to the microscopic behavior of the internal displacement parameter $u$, finding that the internal strain contribution dominates in the Sc-induced enhancement. The value of $u$ increases with scandium concentration, bringing the alloy locally closer to a layered hexagonal structure. Our approach allows us to calculate the ferroelectric switching barrier, which we analyze as a function of Sc concentration and temperature based on Ginzburg-Landau theory.