Physical and Microwave Characterization of Superconducting Indium and Aluminium/Indium Thin Films*

In a superconducting quantum circuit, understanding the microwave losses in each implemented material is crucial to the optimization of device performance and ultimately the scalability of the system. Indium, a low temperature superconductor and common solder, has become a recent addition to the superconducting quantum computing toolbox. In this work, we characterize microwave loss in thin film indium and aluminium/indium by measuring superconducting microwave resonators and performing quality factor fitting at a variety of powers down to the sub-photon regime and a variety of temperatures down to 10 mK. We also fit the two level system (TLS) loss model to this data, as well as perform surface metrology. We compare films deposited by thermal evaporation, sputtering, and molecular beam epitaxy to capped and uncapped aluminium/indium films, as well as films processed in various ways. We find unprocessed thermally evaporated indium films to be the highest performing, with a high power quality factor of 3,600,000.