Thin films of SiO2-GeO2 for piezoelectric applications
Quartz crystals (SiO₂) are widely used in devices requiring sharp piezoelectric resonances, such as clocks, filters, and oscillators, due to their temperature stability and high Q-factor. Traditionally, high-quality quartz (SiO₂) is produced via hydrothermal growth followed by mechanical cutting and etching. However, this method limits the crystal size to tens of micrometers, restricting operating frequencies to hundreds of MHz, which does not align with the high-frequency and miniaturization demands of modern microelectronics.
In her thesis, Silang Zhou explores the growth of SiO₂-GeO₂ thin films with thicknesses ranging from 50-200 nm, using Pulsed Laser Deposition technique. While SiO₂ is stable but difficult to crystallize, GeO₂ can be crystallized into quartz structure but with challenging crystallinity control. For pure GeO₂, Zhou successfully crystallized high-quality single crystal as large as 1mm X 200 μm in size on sapphire substrate (Al2O3).
Zhou demonstrates that up to 75% Si can be crystallized into solid solution using a quartz (SiO₂) substrate. By growing thin films on different substrates and strain conditions, this study highlights the complexity of quartz crystallization and provides insights for future research.