Thin films of the $$\alpha$$ α -quartz $$Si_xGe_{1-x}O_2$$ S i x G e 1 - x O 2 solid solution

Abstract

Abstract $$SiO_2$$ S i O 2 with the $$\alpha$$ α -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. $$GeO_2$$ G e O 2 can also be crystallized into the $$\alpha$$ α -quartz structure and it has better piezoelectric properties, with higher piezoelectric coefficient and electromechanical coupling coefficients, than $$SiO_2$$ S i O 2 . Experiments on bulk crystals and theoretical studies have shown that these properties can be tuned by varying the Si/Ge ratio in the $$Si_xGe_{1-x}O_2$$ S i x G e 1 - x O 2 solid solution. However, to the best of our knowledge, thin films of $$Si_xGe_{1-x}O_2$$ S i x G e 1 - x O 2 quartz have never been reported. Here we present the successful crystallization of $$Si_xGe_{1-x}O_2$$ S i x G e 1 - x O 2 thin films in the $$\alpha$$ α -quartz phase on quartz substrates ( $$SiO_2$$ S i O 2 ) with x up to 0.75. Generally, the films grow semi-epitaxially, with the same orientation as the substrates. Interestingly, the $$Si_{0.75}Ge_{0.25}O_2$$ S i 0.75 G e 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.