Isolating physical mechanisms for surface lying two-level system loss using surface acoustic wave resonators*

Identifying sources of two-level system (TLS) loss in mechanical devices is necessary for making long lifetime quantum acoustic devices. While it has been demonstrated that mechanical resonators such as phononic crystals have TLS dominated loss at low temperatures, this loss is not yet attributed to chemical or material properties. This study uses a series of 690MHz surface acoustic wave resonators fabricated on bulk Lithium Niobate with different surface treatments. Cryogenic temperature sweep measurements of the SAWs and surface analysis techniques including x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and transmission electron microscopy (TEM) of the substrate are compared to isolate sources of TLS loss. Surface treatments performed on the LN substrate include annealing, submerging in buffered oxide etchant or piranha, ion milling via gas cluster ion beam, and 5% magnesium oxide (MgO) co-doping. XPS studies suggest excess surface lying metal oxide increases TLS loss; ion milled substrates also have a large increase of TLS loss. SAW insensitivity to TLS loss improvement after MgO co-doping motivates future TLS studies using bulk acoustic wave resonators, which will also be described in detail.