Enhancing Phononic Nonlinearities in Semiconducting-Piezoelectric Heterostructures and Suspended Lithium Niobate

The creation of efficient nonlinear phononic devices can enhance radio frequency processing. Preliminary work on studying nonlinearities in non-suspended lithium niobate structures with and without the inclusion of a semiconducting layer (indium gallium arsenide) was done prior. This work found that the semiconducting layer can enhance the strength of nonlinear interactions of piezoelectric phonons through a four wave mixing analysis. This thesis project also attempts to create surface acoustic wave delay lines that have a suspended lithium niobate membrane. Computer simulations were done through COMSOL to get a theoretical understanding on how the electromechanical coupling coefficient of suspended lithium niobate alters with and without a semiconducting layer. The electromechanical coupling coefficient was found to be greater (13% improvement) with suspended lithium niobate alone. The semiconducting layer of interest is a 2-dimensional electron gas heterostructure made of gallium arsenide and aluminum gallium arsenide. Multiple process flow iterations were done to optimize the fabrication process until a reliable process flow was found. The current process consists of creating etch pits into lithium niobate and depositing aluminum interdigitated transducers (IDT), then followed by removing an embedded oxide layer with a specially formulated form of hydrofluoric acid. To better understand how well the current process flow performs, the device’s scattering parameters were found and four wave mixing (FWM) and second harmonic generation (SHG) analysis was performed. From these experiments, the nonlinear interactions were lower than expected. From the scattering parameters, the resonances obtained were lower by about 7-12% than what was found in the computer simulations. Similarly, in the FWM and SHG studies the power conversion efficiency was also noticeably low, indicating that the devices will need to be optimized further. The process flow for creating a suspended lithium niobate membrane along with the IDT design will be completed and modified respectfully, such that a better comparison can be made with the devices with indium gallium arsenide and potentially see an improved nonlinear interaction.