Structural evolution of the elastic properties in nano-structured AlN film

TL;DR

This study investigates how the elastic properties of nano-structured AlN films vary with different morphologies using Brillouin spectroscopy, revealing structure-dependent phonon modes and sound velocities.

Contribution

It provides new insights into the relationship between nano-structure and elastic phonon behavior in AlN films, including the unexpected higher sound velocity in amorphous phases.

Findings

Nano-structure influences transverse phonon modes.

Amorphous AlN exhibits higher sound velocity than nano-crystalline.

Distinct phonon behaviors observed in different morphologies.

Abstract

A study of the transverse acoustic phonons on nano-structured AlN films has been carried out by using high-resolution micro-Brillouin spectroscopy. Dense films have been deposited by radio frequency (r.f.) magnetron sputtering under ultra high vacuum at room temperature. Films with different morphologies were prepared and investigated by transmission electron microscopy and Brillouin Spectroscopy (equiaxed nano-sized, nano-columnar grains and amorphous phase). Results show a dependence of the transverse modes on the nano-structure. The nano-columnar film exhibits two transversal modes as expected for the AlN w{\"u}rtzite while the equiaxed nano-sized and the amorphous films only exhibit one isotropic transverse mode as expected in amorphous materials. One important result is that the sound propagation velocity in the AlN amorphous phase is higher than the one in the non-textured nano-crystalline phase. This phenomenon has, however, already been observed in ferroelectric ceramics.