Thin films of the α ‐ quartz Si x Ge 1 − x O 2 solid solution

α-quartz, SiO2 , crystals are currently used as clocks in basically all integrated circuits and as frequency stabilizers in radio transmitters and receivers. The availability of α-quartz films with sub-micron thickness and quality factor similar to those of the single crystals, would enable the new generation of devices for information and communication technology beyond 5 GHz. Efforts in this direction are still in their infancy guided by seminal work on thin films of SiO2 synthesised by chemical solution deposition methods1–4 The structure of α-quartz is a network of ordered corner-sharing tetrahedra5–8. It is believed that the piezoelectric properties are enhanced by the distortion of the structure, which can be represented by two interrelated angles: the bridging angle, θ , between the neighboring tetrahedra and the tilt angle, δ , which is the order parameter of the α-β quartz phase transition and corresponds to the tilt of the tetrahedra away from its position in β -phase, where δ = 09,10. The quartz family includes compounds with formulae TO2 (T = Si, Ge) and MXO4 (M = B, Al, Ga, Fe, Mn and X = P, As). Among them, GeO2 and GaPO4 have the most distorted structure, with GeO2 displaying a d11 = 6.2pC/N11,12 compared with the 2.31pC/N of SiO2. Theoretical calculations of a quartz-type GeO2 SAW device estimate an electromechanical coupling coefficient K that can reach a maximum value of 0.34%14, compared to the 0.16% of the well-known ST-cut SiO2 crystals15. In the SixGe1−xO2 solid solution, it is expected that its structure, and thus the piezoelecric properties, can be tuned continuously via varying the Si/Ge ratio9,16–18. Moreover, GeO2 quartz has better thermal stability than SiO2 quartz. The piezoelectric properties of SiO2 start to deteriorate about 300 °C19, due to the presence of the α-β phase transition at 573 °C20, above which the d11 of SiO2 vanishes. On the other hand, the α-β phase transition point of GeO2 is reported to be as high as 1049 °C by Sarver and Hummel21. However, 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. In addition, a study has shown that nano-crystals of quartz ( SiO2 ) display a continuous α − β phase transition, in contrast to the first-order transition in macroscopic quartz ( SiO2). In any case, the α-quartz phase of GeO2 is more stable than that of SiO2 and a study also shows that in the Si0.76Ge0.24O2 composition, the α-β transition temperature is increased to about 1026 °C23. Moreover, Brillouin spectroscopy measurements of elastic constants up to 1000 °C show preservation of piezoelectricity in GeO2 in the measured range24. Several efforts have been devoted to the growth of bulk single crystals of SixGe1−xO2 by hydrothermal synthesis25,26 and flux growth methods16–18. It was found that when GeO2 crystals are grown in aqueous media, hydroxyl groups catalyze it into the more stable rutile phase16. Experimental studies show that, at 1200 °C and 1 GPa, a maximum of about 40 mole% Ge can be dissolved into quartz in the bulk27. Recently, using an evaporative-recirculation method, Balitsky et al.28 have succeeded to grow 4–12 mm thick quartz-like GeO2 OPEN