# Josephson supercurrent in Nb/InN-nanowire/Nb junctions 

R. Frielinghaus, 1 I. E. Batov, 2 M. Weides, 3, ∗ H. Kohlstedt, 3, † R. Calarco, 1 and Th. Sch¨ apers 1, ‡ 

$^{1}$Institute of Bio- and Nanosystems (IBN-1) and JARA-Fundamentals of Future Information Technology, Forschungszentrum J¨ ulich, 52425 J¨ ulich, Germany 

$^{2}$Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow district, Institutskaya 2, 142432 Russia 

$^{3}$Institute of Solid State Research (IFF) and JARA-Fundamentals of Future Information Technology, Forschungszentrum J¨ ulich, 52425 J¨ ulich, Germany (Dated: November 12, 2018) 

We experimentally studied the Josephson supercurrent in Nb/InN-nanowire/Nb junctions. Large critical currents up to 5.7 µ A have been achieved, which proves the good coupling of the nanowire to the superconductor. The effect of a magnetic field perpendicular to the plane of the Josephson junction on the critical current has been studied. The observed monotonous decrease of the critical current with magnetic field is explained by the magnetic pair-breaking effect in planar Josephson junctions of ultra-narrow width [J. C. Cuevas and F. S. Bergeret, Phys. Rev. Lett. 99, 217002 (2007)] 

Superconductor/normal-conductor/superconductor (SNS) junctions with a semiconductor employed as the N-weak link material offer the great advantage that here the Josephson supercurrent can be con- trolled by means of the field effect. 1,2 Gate-controlled superconductor/semiconductor hybrid devices such as superconducting field effect transistors 3 or split-gate structures 4 have been fabricated which find no coun- terpart in conventional SNS structures. In addition, the high carrier mobility attainable in semiconduc- tors in combination with the phase-coherent Andreev reflection leads to novel unique phenomena in the magnetotransport. 5–7 Usually for these devices the semiconductor is patterned by conventional lithography. As an elegant alternative one can also directly create semiconductor nanostructures, i.e. nanowires, by epi- taxial growth. 8 By using InAs nanowires connected to superconducting electrodes tunable Josephson super- currents, supercurrent reversal, and Kondo-enhanced Andreev tunneling have been realized. 9–11 

Among the various materials used for semiconductor nanowires InN is of particular interest for semiconduc- tor/superconductor hybrid structures, since the surface accumulation layer in InN can provide a sufficiently low resistive contact to superconducting electrodes. 12–14 Due to almost ideal crystalline properties of InN nanowires electronic transport along the wires, contacted by nor- mal metal electrodes, shows quantization phenomena, i.e. flux periodic magnetoconductance oscillations. 15 Furthermore, the carrier concentration in the surface electron gas is of the order of 10 13 cm − 2 and thus about a factor of ten larger than in InAs. Consequently when combined with superconducting electrodes one can ex- pect low resistive SNS junctions. 

Here, we report on transport studies of Nb/InN- nanowire/Nb junctions. We succeeded in observing a pronounced Josephson supercurrent and a relatively large I c R N product of up to 0.44 mV. The latter factor, the critical current times the normal resistance, is an impor- tant figure of merit for Josephson junctions. We devoted 

special attention to the dependence of the critical current I c on an external magnetic field B , where a monotonous decrease of I c with B was found. This experimental find- ing is interpreted in the framework of a recent theoretical model for the proximity effect in narrow-width junctions with dimensions comparable or smaller than the mag- netic length ξ B = √ Φ$_{0}$ /B , where Φ$_{0}$ = h/ 2 e is the flux quantum. 16,17 

The InN nanowires used for the normal conducting part of our junctions were grown without catalyst on a Si (111) substrate by plasma-assisted molecular beam epitaxy. 18 The wires had a typical length of 1 µ m. The nanowires were contacted by a pair of 100-nm-thick Nb electrodes. Before the Nb sputter deposition the con- tact area was cleaned by Ar + milling. The supercon- ducting transition temperature T c of the Nb layers was 8.5 K. The InN nanowire of the first junction (sample A) had a diameter d = 120 nm and a Nb electrode separa- tion L = 105 nm [cf. Fig. 1 (inset)], while for the sec- ond junction (sample B) the corresponding dimensions were 85 nm and 130 nm, respectively. From measure- ments on back-gate transistor structures performed on nanowires prepared in the same epitaxial run a typical electron concentration of 1 × 10 19 cm − 3 was determined. From measurements on nanowires contacted with nor- mal contacts with various distances a specific resistance of ρ = 4 . 2 × 10 − 4 Ωcm was estimated. 19 Using these val- ues we calculated a diffusion constant of D = 110 cm $^{2}$/s. 

The transport measurements were conducted in a He-3 cryostat in a temperature range from 0.7 K to 10 K. The magnetic field was applied perpendicularly to the plane of the Nb electrodes. The differential resistance was measured with a lock-in amplifier by superimposing a small 17 Hz ac signal of 50 nA to the junction bias current. 

The current-voltage ( I − V ) characteristics of sample A for various temperatures is shown in Fig. 1. As can be seen here, a clear Josephson supercurrent is observed at temperatures up to 3.5 K. At 0.8 K a critical current of 

