Amorphous molybdenum silicon superconducting thin films

Abstract

Amorphous superconductors have become attractive candidate materials for superconducting nanowire single-photon detectors due to their ease of growth, homogeneity and competitive superconducting properties. To date the majority of devices have been fabricated using WxSi1−x, though other amorphous superconductors such as molybdenum silicide (MoxSi1−x) offer increased transition temperature. This study focuses on the properties of MoSi thin films grown by magnetron sputtering. We examine how the composition and growth conditions affect film properties. For 100 nm film thickness, we report that the superconducting transition temperature (Tc) reaches a maximum of 7.6 K at a composition of Mo83Si17. The transition temperature and amorphous character can be improved by cooling of the substrate during growth which inhibits formation of a crystalline phase. X-ray diffraction and transmission electron microscopy studies confirm the absence of long range order. We observe that for a range of 6 common substrates (silicon, thermally oxidized silicon, R- and C-plane sapphire, x-plane lithium niobate and quartz), there is no variation in superconducting transition temperature, making MoSi an excellent candidate material for SNSPDs.

Superconducting MoSi nanowires

“…We have chosen a value that is close to the normal state resistance but still well resolved experimentally, so that it would be adequately applicable for the fluctuation affected wires, i.e., neglect the resistive transition tails. Previous experiments indicate that disorder increases the critical temperature from the value T c = 1 K of bulk crystalline MoSi up to T c = 7.6 K for amorphous MoSi films [50,13]. Thus it would be plausible that the grain growth at the surface of the nanowires is inhibited and more ordered crystalline structures are formed only within the MoSimatrix.…”

Superconducting Nanowires for Single‐Photon Detection: Progress, Challenges, and Opportunities

“…The effects of substrate on NbN film properties were surveyed also by the Glasgow group in 2012, in which other integration‐relevant substrates, such as LiNbO 3 and GaAs were demonstrated useful for lower temperature deposition. Likewise, the Glasgow group proposed another method to overcome the effects of substrate—using amorphous materials (Mo 1− x Si x ), in which there are no constrictions due to granularity or lattice matching. This approach can explain the previously reported relatively high detection efficiency in SNSPDs made of other amorphous materials, such as tungsten silicide .…”

Enhancement of Superconductivity by Amorphizing Molybdenum Silicide Films Using a Focused Ion Beam

“…The proportion of molybdenum to silicon affects the critical temperature of the compound [5]. The lower the silicon content, the higher the critical temperature [5].…”

“…A typical way to make amorphous thin films is by sputtering on cooled surfaces [5]. Amorphous films have also been made with other techniques including e.g., liquid quenching [6], neutron bombardment [7] and ion-mixing [8].…”

“…The superconducting properties of this material are strongly dependent on its fabrication process [11,12]. Its applications range from CMOS processes for quantum circuits [13] to SNSPD and QPS operation [5,14,15]. In addition, silicides have been shown to exhibit excellent microwave performance when fabricated with diffusion barriers [16,17].…”