Thickness dependence of the mechanical properties of piezoelectric high-$Q_m$ nanomechanical resonators made from aluminium nitride

TL;DR

This study investigates the thickness-dependent mechanical properties of tensile-strained aluminium nitride nanomechanical resonators, demonstrating high quality factors and dissipation dilution effects relevant for quantum technologies.

Contribution

It provides comprehensive characterization of AlN thin films and demonstrates how thickness influences the quality factor and dissipation dilution in nanomechanical resonators.

Findings

Resonators below 200 nm thickness achieve Qf~10^12 Hz.

AlN films show high crystal quality and piezoelectric response.

Dissipation dilution enhances quality factors in tensile-strained AlN resonators.

Abstract

Nanomechanical resonators with high quality factors (\Qm{}) enable mechanics-based quantum technologies, in particular quantum sensing and quantum transduction. High-\Qm{} nanomechanical resonators in the kHz to MHz frequency range can be realized in tensile-strained thin films that allow the use of dissipation dilution techniques to drastically increase \Qm{}. In our work, we study the material properties of tensile-strained piezoelectric films made from aluminium nitride (AlN). We characterize crystalline AlN films with a thickness ranging from \SI{45}{\nano\meter} to \SI{295}{\nano\meter}, which are directly grown on Si(111) by metal-organic vapour-phase epitaxy. We report on the crystal quality and surface roughness, the piezoelectric response, and the residual and released stress of the AlN thin films. Importantly, we determine the intrinsic quality factor of the films at room temperature in high vacuum. We fabricate and characterize AlN nanomechanical resonators that exploit dissipation dilution to enhance the intrinsic quality factor by utilizing the tensile strain in the film. We find that AlN nanomechanical resonators below \SI{200}{\nano\meter} thickness exhibit the highest \Qf{}-product, on the order of $10^{12}$\,Hz. We discuss possible strategies to optimize the material growth that should lead to devices that reach even higher \Qf{}-products. This will pave the way for future advancements of optoelectromechanical quantum devices made from tensile-strained piezoelectric AlN.