Abstract
SiO_2 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. GeO_2 can also be crystallized into the alpha-quartz structure and it has better piezoelectric properties, with higher piezoelectric coefficient and electromechanical coupling coefficients, than SiO_2. Experiments on bulk crystals and theoretical studies have shown that these properties can be tuned by varying the Si/Ge ratio in the Si_xGe_{1-x}O_2 solid solution. However, to the best of our knowledge, thin films of Si_xGe_{1-x}O_2 quartz have never been reported. Here we present the successful crystallization of Si_xGe_{1-x}O_2 thin films in the alpha-quartz phase on quartz substrates (SiO_2) with x up to 0.75. Generally, the films grow semi-epitaxially, with the same orientation as the substrates. Interestingly, the Si_{0.75}Ge_{0.25}O_2 composition grows fully strained by the quartz substrates and this leads to the formation of circular quartz domains with an ordered Dauphiné twin structure. These studies represent a first step towards the optimization of piezoelectric quartz thin films for high frequency (> 5 GHz) applications.
Highlights
Α-quartz, SiO2, crystals are currently used as clocks in basically all integrated circuits and as frequency stabilizers in radio transmitters and receivers
This is in agreement with the surface morphology obtained by Atomic Force Microscopy (AFM), showing that the film is composed of grains of about 30 nm in size
The thicknesses of the films are estimated from X-ray reflectivity (XRR) scans (Fig. S3 of the supplementary material) to be 46 nm and 47 nm for Z-cut and Y-cut substrates, respectively
Summary
Α-quartz, SiO2 , crystals are currently used as clocks in basically all integrated circuits and as frequency stabilizers in radio transmitters and receivers. Lignie et al measured GeO2 single crystals by Differential Scanning Calorimetry, showing that α-quartz is the only phase present until melting at 1116 °C16. This contradiction may arise from size or surface effects: Lignie et al measured GeO2 single crystals while Sarver and Hummel presumably used powders. A study has shown that nano-crystals of quartz (S iO2 ) display a continuous α − β phase transition, in contrast to the first-order transition in macroscopic quartz ( SiO2)[22]. The α-quartz phase of GeO2 is more stable than that of SiO2 and a study shows that in the Si0.76Ge0.24O2 composition, the α-β transition temperature is increased to about 1026 °C23. To the best of our knowledge, crystalline thin films of the SixGe1−xO2 solid solution have not been reported
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