Tungsten Silicide Alloys for Microwave Kinetic Inductance Detectors

Microwave kinetic inductance detectors are used to detect photons over a large range of wavelengths from submillimeter to X-ray. The common material requirements for this application are: high internal quality factor (Q_i), high kinetic inductance fraction, long quasiparticle lifetime, and in the case of X-ray photons stopping power (i.e., dense, high atomic number materials). Superconducting tungsten silicide alloys have a tunable T_C with silicon content, a high normal state resistivity, and a high density. In this work, we investigate the properties of thin films of tungsten silicide made of two different stoichiometry: WSi₂ and W₅Si₃ with particular attention to their application to microwave kinetic inductance detectors. We present a study of the structural and transport properties of films deposited under different conditions for both stoichiometry. Quarter wavelength microwave coplanar waveguide resonators have been fabricated from films of both stoichiometry and we present measurements of the microwave properties of these films as well as quasiparticle lifetimes using X-ray photons.