Thin films of the alpha-quartz SixGe1-xO2 solid-solution

TL;DR

This paper reports the successful fabrication and characterization of alpha-quartz phase SixGe1-xO2 thin films on quartz substrates, revealing strain effects and domain structures, advancing piezoelectric applications at high frequencies.

Contribution

First demonstration of alpha-quartz phase SixGe1-xO2 thin films with controlled composition and strain on quartz substrates.

Findings

Films grow semi-epitaxially with substrate orientation

Full strain observed in Si0.75Ge0.25O2 films

Formation of Dauphine twin structures in strained films

Abstract

SiO2 with the alpha-quartz structure is one of the most popular piezoelectrics. It is widely used in crystal oscillators, bulk acoustic wave (BAW) devices, surface acoustic wave (SAW) devices, and so on. GeO2 can also be crystallized into the alpha-quartz structure and it has better piezoelectric properties, with higher piezoelectric coefficient and electromechanical coupling coefficients, than SiO2. Experiments on bulk crystals and theoretical studies have shown that these properties can be tuned by varying the Si/Ge ratio in the SixGe1-xO2 solid solution. However, to the best of our knowledge, thin films of SixGe1-xO2 quartz have never been reported. Here we present the successful crystallization of SixGe1-xO2 thin films in the alpha-quartz phase on quartz substrates (SiO2) with x up to 0.75. Generally, the films grow semi-epitaxially, with the same orientation as the substrates. Interestingly, the Si0.75Ge0.25O2 composition grows fully strained by the quartz substrates and this leads to the formation of circular quartz domains with an ordered Dauphine twin structure. These studies represent a first step towards the optimization of piezoelectric quartz thin films for high frequency (> 5 GHz) applications.