Abstract
We report on the optimisation of amorphous molybdenum silicide thin film growth for superconducting nanowire single-photon detector (SNSPD) 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 studies showed that the films assumed an A15-like medium-range order. Electron energy loss spectroscopy 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 × 105 A cm−2 for the widest wire studied (2003 nm), falling to 2 × 105 A cm−2 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.
Highlights
We have measured the transport properties including the temperature dependence of critical current density in nanowires patterned from these MoSi thin films via electron beam lithography (EBL) and reactive ion etching (RIE)
In order to better understand the atomic structure and composition of our MoSi films, we have employed a suite of advanced scanning transmission electron microscopy (STEM) techniques
To date we have reported a low temperature photoresponse map recorded at 350 mK from a waveguide integrated superconducting nanowire single-photon detector (SNSPD) fabricated on a 10 nm thick MoSi thin film deposited at the University of Cambridge with a similar composition (Mo83Si17) [69]
Summary
Polycrystalline NbN and NbTiN thin films have hitherto been the most widely used materials for SNSPD fabrication [21, 22], amorphous superconductors can offer various advantages [23] They do not have strict substrate requirements and have lower superconducting gap energies [24], giving a higher intrinsic single-photon detection efficiency at longer wavelengths. We have explored the growth and optimisation of amorphous MoSi thin films, in terms of the desirable superconducting properties for SNSPD fabrication. The complex refractive index of MoSi films has been measured using variable angle spectroscopic ellipsometry (VASE) This aids the simulation of optical absorption in SNSPDs. Critical current density is another important factor governing the performance of SNSPDs. We have measured the transport properties including the temperature dependence of critical current density in nanowires patterned from these MoSi thin films via electron beam lithography (EBL) and reactive ion etching (RIE)
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