Ionic Bonds Control Ferroelectric Behavior in Wurtzite Nitrides

TL;DR

This study reveals that the ionic character of bonds, rather than structural distortion, governs ferroelectricity in wurtzite Al1-xScxN, enabling targeted property control through ionicity engineering.

Contribution

It demonstrates that bond ionicity controls ferroelectricity in wurtzite nitrides, introducing ionicity engineering as a novel method to tune ferroelectric properties.

Findings

Bond ionicity influences ferroelectric response.

Increasing Sc content reduces coercive field and polarization.

Invariance of c/a ratio despite compositional changes.

Abstract

Ferroelectricity enables key integrated technologies from non-volatile memory to precision ultrasound. Wurtzite ferroelectric Al1-xScxN has recently attracted attention because of its robust ferroelectricity and Si process compatibility in addition to being the first known ferroelectric wurtzite. However, the origin and control of ferroelectricity in wurtzite materials is not yet fully understood. Here we show that the local bond ionicity, rather than simply the change in tetrahedral distortion, is key to controlling the macroscopic ferroelectric response, according to our coupled experimental and computational results. Across the composition gradient in Sc < 0.35 range and 140-260 nm thickness in combinatorial thin films of Al1-xScxN, the pure wurtzite phase exhibits a similar c/a ratio regardless of the Sc content, due to elastic interaction with neighboring crystals. The coercive field and spontaneous polarization significantly decrease with increasing Sc content despite this invariant c/a ratio, due to the more ionic bonding nature of Sc-N relative to the more covalent Al-N bonds, supported by DFT calculations. Based on these insights, ionicity engineering is introduced as an approach to reduce coercive field of Al1-xScxN for memory and other applications and to control ferroelectric properties in other wurtzites.