Thin Films of α-Quartz GeO 2 on TiO 2 -Buffered Quartz Substrates.

α-Quartz (SiO 2 ) is one of the most widely used piezoelectric materials. However, the challenges associated with the control of the crystallization and the growth process limit its production to the hydrothermal growth of bulk crystals. GeO 2 can also crystallize into the α-quartz phase, with a higher piezoelectric response and better thermal stability than SiO 2 . In a previous study, we have found that GeO 2 crystallization on nonquartz substrates shows a tendency to form spherulites with a randomized orientation; while epitaxial growth of crystalline GeO 2 thin films can take place on quartz (SiO 2 ) substrates. However, in the latter case, the α-β phase transition that takes place in both substrates and thin films during heating deteriorates the long-range order and, thus, the piezoelectric properties. Here, we report the ousting of spherulitic growth by using a buffer layer. Using TiO 2 as a buffer layer, the epitaxial strain of the substrates can be transferred to the growing films, leading to the oriented crystallization of GeO 2 in the α-quartz phase. Moreover, since the TiO 2 separates the substrates and the thin films, the thermal stability of the GeO 2 is kept across the substrate's phase transitions. Our findings reveal the complexity of the crystallization process of quartz thin films and present a way to eliminate the tendency for spherulitic growth of quartz thin films by epitaxial strain.