  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.
