Characterisation of amorphous molybdenum silicide (MoSi) superconducting thin films and nanowires

We report on the optimisation of amorphous molybdenum silicide thin film growth for superconducting nanowire single photon detector (SNSPD/SSPD) applications. Molybdenum silicide was deposited via co-sputtering from Mo and Si targets in an Ar atmosphere. The superconducting transition temperature (Tc) and sheet resistance (Rs) were measured as a function of thickness and compared to several theoretical models for disordered superconducting films. Superconducting and optical properties of amorphous materials are very sensitive to short- (up to 1 nm) and medium-range order (~1-3 nm) in the atomic structure. Fluctuation electron microscopy (FEM) studies showed that the films assumed an A15-like medium-range order. Electron energy loss spectroscopy (EELS) indicates that the film stoichiometry was close to Mo83Si17, which is consistent with reports that many other A15 structures with the nominal formula A3B show a significant non-stoichiometry with A:B > 3:1. Optical properties from ultraviolet (270 nm) to infrared (2200 nm) wavelengths were measured via spectroscopic ellipsometry for 5 nm thick MoSi films indicating high long wavelength absorption. We also measured the current density as a function of temperature for nanowires patterned from a 10 nm thick MoSi film. The current density at 3.6 K is 3.6 x 105A/cm2 for the widest wire studied (2003 nm), falling to 2 x 105A/cm2 for the narrowest (173 nm). This investigation confirms the excellent suitability of MoSi for SNSPD applications and gives fresh insight into the properties of the underlying materials.