Enhanced piezoelectric response of AlN via CrN alloying

TL;DR

This study demonstrates that alloying AlN with Cr significantly enhances its piezoelectric response, with experimental and theoretical evidence showing a fourfold increase in piezoelectric modulus at 30% Cr content, suggesting new applications.

Contribution

The paper introduces the first detailed theoretical and experimental investigation of Cr alloying in AlN, revealing a tunable, significantly improved piezoelectric response in Cr-AlN alloys.

Findings

Cr can be incorporated into AlN up to 30% without phase transition.

Piezoelectric modulus d33 is approximately four times larger in Cr-AlN than in pure AlN.

Cr-AlN alloys exhibit a highly sensitive piezoelectric response to Cr concentration.

Abstract

Since AlN has emerged as an important piezoelectric material for a wide variety of applications, efforts have been made to increase its piezoelectric response via alloying with transition metals that can substitute for Al in the wurtzite lattice. Herein, we report density functional theory calculations of structure and properties of the Cr-AlN system for Cr concentrations ranging past the wurtzite-rocksalt transition point. By studying the different contributions to the longitudinal piezoelectric coefficient, we propose that the physical origin of the enhanced piezoelectricity in Cr$_x$Al$_{1-x}$N alloys is the increase of the internal parameter $u$ of the wurtzite structure upon substitution of Al with the larger Cr ions. Among a set of wurtzite-structured materials, we have found that Cr-AlN has the most sensitive piezoelectric coefficient with respect to alloying concentration. Based on these results, we propose that Cr-AlN is a viable piezoelectric material whose properties can be tuned via Cr composition; we support this proposal by combinatorial synthesis experiments, which show that Cr can be incorporated in the AlN lattice up to 30\% before a detectable transition to rocksalt occurs. At this Cr content, the piezoelectric modulus $d_{33}$ is approximately four times larger than that of pure AlN. This finding, combined with the relative ease of synthesis, may propel Cr-AlN as the prime piezoelectric material for applications such as resonators and acoustic wave generators.