Thin films of the $$\alpha$$-quartz $$Si_xGe_{1-x}O_2$$ 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.

Thin Films of α-Quartz GeO2 on TiO2-Buffered Quartz Substrates

“…Similarly, RSMs of films on the Y-cut also have an in-plane spread as on the X-cut, but with weak intensity, probably due to more spread of the film orientation. The out-of-plane lattice parameter is extracted from the 2θ–ω scan with d 10 1̅ 0 = 4.336 Å, which is comparable to GeO 2 directly grown on SiO 2 quartz substrates d 10 1̅ 0 = 4.331 Å . The out-of-plane expansion is induced by in-plane strain from the substrate.…”

“…The out-of-plane lattice parameter is extracted from the 2θ−ω scan with d 10 1 0 = 4.336 Å, which is comparable to GeO 2 directly grown on SiO 2 quartz substrates d 10 1 0 = 4.331 Å. 15 The out-of-plane expansion is induced by in-plane strain from the substrate.…”

“…On the contrary, as we have found in our previous work, when the GeO 2 is grown directly on SiO 2 quartz substrates, it will follow the α−β phase transition of the substrates, losing d 11 in the βphase at 573 °C. 15 Competition between Epitaxial Growth and Spherulitic Growth. However, in some cases, GeO 2 can crystallize into spherulites locally by bypassing the strain from the substrates.…”

“…The strain arises from the lattice mismatch, and it is calculated as ϵ = ( a s – a f )/ a f , where a s and a f are the bulk lattice parameters for the substrate and thin film, respectively. We have reported that, when the GeO 2 is deposited directly on quartz (SiO 2 ) substrates, due to the small lattice mismatch with the same crystal structure, the thin films grow epitaxially, with the same orientation as the substrates . However, when the sample is heated at 573 °C, the thin films transform into the β-quartz phase following the substrate, again leading to the disappearance of the longitudinal piezoelectric response associated with the d 11 component of the piezoelectric tensor.…”

α-Quartz Phase Stabilization, Surface Texturing, and Tunable Optical Properties of Nanocrystalline GeO2 Films Made by Pulsed-Laser Deposition: Implications for Optical and Optoelectronic Applications

“…This UWBG nature of α-quartz-type GeO 2 enables application into high-energy, DUV photodetection in the range of UVC (200–280 nm). Theoretical calculation of α-quartz GeO 2 also implies a higher electromechanical coupling coefficient value of K 2 = 0.34% compared to conventionally used SiO 2 ( K 2 = 0.16%) for enhanced piezoelectric applications. , These piezoelectric properties coupled with high thermal stability (1049 °C) and UWBG properties make α-quartz GeO 2 a versatile material system for application in power electronics, DUV detection, and communication industries.…”