Superconducting properties of lifted-off Niobium nanowires

Abstract

Hybrid superconductor/semiconductor devices play a crucial role in advancing quantum science and technology by merging the properties of superconductors and semiconductors. To operate these devices at high temperature, Niobium could substitute the widespread aluminum as superconducting element. Niobium devices show the best superconducting properties when shaped by etching, but this technique is often incompatible with semiconductors and two-dimensional materials. Our work investigates the influence of oxygen diffusion on the superconducting transition of Nb nanowires fabricated by lift-off technique. To this scope, we fabricate and measure Nb devices of different width (W) and thickness (t). By using the Berezinskii-Kosterlitz-Thouless (BKT) model for charge transport, we demonstrate that our nanowires behave as two-dimensional superconductors regardless of W and t. While the normal-state transition temperature (TN) remains constant with decreasing W, the temperature of the fully superconducting state (TS) decreases. Thus, the superconducting transition width (δTC) increases as W shrinks, due to oxygen diffusion from the lithography resist occurring during deposition. These insights provide essential knowledge for optimizing Nb-based hybrid quantum devices, paving the way for operating temperatures above 2 K and contributing to the development of next-generation quantum technologies.

Figures (5)

• Figure 1: (a) False-color scanning electron micrograph of a typical Nb nanowire. The devices are AC current-biased (amplitude $I_{ac}$) and the voltage drop across the wire ($V_{out}$) is measured via a voltage pre-amplifier connected to a lock-in amplifier. $R_L=1$ M$\Omega$ serves as load resistor. Inset: blow-up of the core of the device showing the Nb nanowire of length $L$ and width $W$. (b) Resistance ($R$) versus temperature ($T$) characteristics of Nb nanowires of length $L=1.89\;\mu$m and thickness $t=49$ nm and widths ($W$) 332 nm (blue), 624 nm (orange) and 875 nm (red), respectively, sputtered with an initial chamber pressure $p=5\times 10^{-8}$ mbar. Inset: schematic $R$ vs $T$ characteristic where the normal-state resistance ($R_N$), and the temperatures of fully superconducting ($T_S$) and fully normal ($T_N$) states are indicated. (c) Logarithmic-scale representation of the experimental $R$ vs $T$ characteristics from panel (b) [dots, same of panel (b)] along with the $BKT$ model for dirty 2D superconductors (lines). The applied parameters are $R_{0}=80,\; 27,\; 15.1\; \Omega$, $b=0.12,\; 0.095,\; 0.09$ and $T_{BKT}=6.26,\; 7.4,\;7.87$ K for $W=332,\;624,\;875$ nm, respectively.
• Figure 2: (a) $T_N$ versus $W$ measured for samples of different thickness ($t$). Dots and squares indicate devices sputtered at a base pressure ($p$) $5\times 10^{-8}$ mbar and $8.8\times 10^{-8}$ mbar, respectively. The dotted lines show the average value of $T_N$ for each film thickness. (b) $T_S/T_N$ versus $W$ measured for different values of $t$ and $p$. (c) Normal-state resistivity ($\rho_N$) measured at 9 K versus $W$ for samples of different $t$ and $p=5\times 10^{-8}$ mbar. Inset: $\rho_N$ vs $W$ for a device of $t=81.6$ nm and $p=8.8\times 10^{-8}$ mbar.
• Figure 3: (a) Critical current ($I_C$) versus normalized temperature ($T/T_S$) measured in a Nb device of $t=50$ nm, $W=800$ nm sputtered at a base pressure $p=5\times 10^{-8}$ mbar. The blue dashed line is the fit with the Bardeen curve providing $I_{C,0}\simeq1.39$ mA and $T_S=6.85$ K. (b) Spread of the measured values of critical current ($\delta I_C$) versus $T/T_S$ for the data in panel (a). (c) $T_S$ (top, green squares) and $\rho_N$ (bottom, purple squares) versus $W$ measured for an Al/Cu bilayer with $t_{Al}=15$ nm and $t_{Cu}=15$ nm. (d) Critical current ($I_C$) versus normalized temperature ($T/T_S$) measured in a Al/Cu device of $t_{Al}=15$ nm, $t_{Cu}=15$ nm and $W=1\;\mu$m. The purple dashed line is the fit with the Bardeen curve providing $I_{C,0}\simeq131\;\mu$A and $T_S=895$ mK.
• Figure 4: (a) Normalized concentration of oxygen in the Nb film ($C_{ox}/C_{ox,max}$) versus the transverse position ($y$) calculated at $T=375$ K and different time periods ($\tau$) for device widths of 1 $\mu$m (left) and 300 nm (right). The vertical lines represent the positions corresponding to $C_{ox}=0$. (b) Width of the superconducting transition ($\delta T_C=T_N-T_S$) versus $W$ measured for samples of different thickness ($t$) sputtered at a base pressure ($p$) $5\times 10^{-8}$ mbar. Inset: $\delta T_C$ vs $W$ for a device of $t=81.6$ nm and $p=8.8\times 10^{-8}$ mbar.
• Figure 5: $T_S$ versus $\rho_N$ for all the devices sputtered at a chamber base pressure $p=5\times 10^{-8}$ mbar. The dotted lines depict the linear fit of the data.