Nanopatterning of Weak Links in Superconducting Oxide Interfaces

Abstract

The interface between two wide band-gap insulators, LaAlO3 and SrTiO3 (LAO/STO), hosts a quasi-two-dimensional electron gas (q2DEG), two-dimensional superconductivity, ferromagnetism, and giant Rashba spin-orbit coupling. The co-existence of two-dimensional superconductivity with gate-tunable spin-orbit coupling and multiband occupation is of particular interest for the realization of unconventional superconducting pairing. To investigate the symmetry of the superconducting order parameter, phase sensitive measurements of the Josephson effect are required. We describe an approach for the fabrication of artificial superconducting weak links at the LAO/STO interface using direct high-resolution electron beam lithography and low-energy argon ion beam irradiation. The method does not require lift-off steps or sacrificial layers. Therefore, resolution is only limited by the electron beam lithography and pattern transfer. We have realized superconducting weak links with a barrier thickness of 30-100 nm. The barrier transparency of the weak links can be controlled by the irradiation dose and further tuned by a gate voltage. Our results open up new possibilities for the realization of quantum devices in oxide interfaces.

Keywords: LaAlO3/SrTiO3 interface; nanopatterned materials; top-down lithography; two-dimensional superconductivity.

Figure 1

Two-step fabrication process for weak links in the LAO/STO interface. In the first step, narrow channels are created by electron beam lithography and low-energy ion irradiation. In the second step, the weak links are created by low dose Ar${}^{+}$ ion irradiation through a slit across the prefabricated channel using electron beam lithography and a positive e-beam resist.

Figure 2

Electrical resistance as a function of Ar${}^{+}$ ion beam irradiation time for three different LAO/STO samples (one 6 uc and two 10 uc thick) patterned using electron beam lithography and the negative e-beam resist ma-N2401. All samples were cleaned in RF oxygen plasma for 10 s prior to the irradiation process. Vertical bars indicate the time interval used in the second Ar${}^{+}$ ion irradiation of weak links. The electrical resistance was measured in situ during the ion irradiation process in a two-probe configuration. The Ar${}^{+}$ ion beam acceleration voltage was 150 V, and the ion current was 30 $\mathsf{\mu }$A/cm${}^2$.

Figure 3

(a) AFM topography 3D image of the negative resist template after first electron beam lithography. Resist thickness is 60 nm. (b) AFM topography image of the same structure after ion beam irradiation and resist removal. (c) AFM topography image of open slits with a width ranging between 20 and 140 nm which were fabricated in positive e-beam resist after dose optimization in the second electron beam lithography. (d) AFM image of the final device containing a weak link with barrier thickness 30 nm. (e) Enlarged region of the AFM image shown in (d) containing weak link structure. Darker regions correspond to conducting paths, whereas light regions are highly resistive and insulating areas.

Figure 4

Normal state properties. (a–c) Normal resistance of three devices with barrier thickness d${}_{WL}$∼30, 40, and 60 nm, as a function of gate voltage for weak links (green data point) and uniform channels (yellow data point) measured in two point configuration measured at T = 5 K. (d) Comparison of the evolution of resistance as a function of gate voltage measured across the weak link. The resistance is normalized to the values corresponding to the maximum gate voltage in all panels, R(V${}_{max}$).

Figure 5

Superconducting properties of weak link devices. (a) Color plot of a resistance of the weak link device with barrier width 30 nm as a function of temperature and gate voltage. The resistance is normalized to the resistance at 400 mK. (b) The evolution of average critical current ($I_c=\frac{I_c^{+}-I_c^{-}}{2}$) and average return current ($I_{cr}=\frac{I_{cr}^{+}-I_{cr}^{-}}{2}$) as a function of gate voltage for the weak link device measured in four-probe configuration. The I${}_c$ and I${}_{cr}$ are identified as switching current, in I–V curves, to normal state in forward bias, and returning current to superconducting state respectively. Inset shows the I–V curve of the device measured at 11 V. (c) The gate voltage dependent I${}_c$ and I${}_{cr}$ for the uniform device measured in two point configuration. Inset shows the I–V curve of the device at 11 V. The comparison of normalized average critical current (d) and normalized average returning current (e) as a function of gate voltage.