Supercurrent and multiple Andreev reflections in Ge hut nanowire Josephson Junctions

Abstract

We report an experimental study of induced superconductivity in Ge hut nanowire Josephson junctions. The Ge hut nanowires are grown on prepatterned SiGe ridges via molecular beam epitaxy (MBE) and Josephson junction devices are fabricated by contacting the nanowires with Al electrodes. Low-temperature current-bias transport measurements of the Josephson junctions are performed and the measurements show that the devices exhibit gate-tunable supercurrent and excess current. The analysis of excess current indicates that the transparency of the Ge hut nanowire Josephson junctions is as high as 85%. Voltage-bias spectroscopy measurements of the devices show multiple Andreev reflections up to the fourth order. With magnetic field and temperature-dependent measurements of the multiple Andreev reflections, the critical field and the critical temperature of the induced superconductivity in the Josephson junctions are extracted to be ~0.12 T and ~1.4 K. The success in introducing superconductivity into Ge hut nanowires will stimulate their applications in building advanced quantum processors.

Figures (3)

• Figure 1: Basic characteristics of the Josephson junction device made from a Ge hut nanowire grown by MBE. (a) AFM image of a Ge hut nanowire on top of a SiGe ridge. The inset shows a linecut across the nanowire and the height of the entire structure is $\sim70\,\mathrm{nm}$ measured from the post growth substrate surface. (b) Cross-sectional schematic of a Ge hut nanowire. The layer structures are marked with different colors and the Ge hut nanowire has a triangular cross section. (c) TEM image of a Ge hut nanowire structure. Different contrasts correspond to the regions with different elements: Ge (black) and Si (white). The inset shows the corresponding FFT image. (d) False-color SEM image of device D1 and corresponding measurement circuit in current-bias measurements. Superconducting electrodes are made from Pd/Al (blue) and the gate is made from Ti/Au (green). The junction length of the device is about $100\,\mathrm{nm}$. (e) Measured voltage $V_$ across the junction as a function of current bias $I_\text{b}$ at $V_\text{g}$ = $-0.5\,\mathrm{V}$. Red (blue) curve is measured in upward (downward) scanning direction. Switching current $I_\text{sw}$ and retrapping current $I_\text{rt}$ of the upward scanning trace are marked.
• Figure 2: Current-bias measurement results of device D1 at $T_$ = $40\,\mathrm{mK}$ and $B_$ = 0. (a)$V_$-$I_\text{b}$ curve at $V_\text{g}$ = $-0.5\,\mathrm{V}$. The black dashed line is the fitting curve of the linear region. The slope of this line corresponds to the normal state resistance $R_\text{n}$ and the intersection point between the line and the $I_\text{b}$ axis signifies the excess current $I_\text{exc}$. (b) Differential resistance d$V_$/d$I_\text{b}$ versus $I_\text{b}$ and $V_\text{g}$. (c) Switching current $I_\text{sw}$ (red dots) and normal state resistance $R_\text{n}$ (blue dots) versus $V_\text{g}$. The data are extracted from figure (b). (d) Product $I_\text{exc}$$R_\text{n}$ as a function of $V_\text{g}$. The data of $I_\text{exc}$ and $R_\text{n}$ are extracted from figure (b). The value of the product ranges from 50 to $200\,\mathrm{\mu V}$.
• Figure 3: Voltage-bias measurements of device D1. (a) Differential conductance d$I_$/d$V_\text{b}$ as a function of voltage bias $V_\text{b}$ at $V_\text{g}$ = $0.828\,\mathrm{V}$ and $B_$ = 0. The conductance peaks marked at finite $V_\text{b}$ are due to MAR processes. The numbers n=$\pm{1}$, $\pm{2}$, $\pm{4}$ indicate the orders of the MARs. (b) Color map of d$I_$/d$V_\text{b}$ versus $V_\text{b}$ and $V_\text{g}$. High-order MAR peaks show up at lower $V_\text{g}$ as a result of more opened channels for hole transport. (c) Color map of d$I_$/d$V_\text{b}$ as a function of $V_\text{b}$ and magnetic field $B_$ at $V_\text{g}$ = $0.822\,\mathrm{V}$ and base temperature. The magnetic field is applied in the plane of the substrate, perpendicular to the axis of the Ge hut nanowire. The critical field $B_\text{c}$ is $\sim0.12\,\mathrm{T}$. (d) Temperature-dependent results of voltage-bias measurements at $V_\text{g}$ = $0.812\,\mathrm{V}$ and $B_$=0. The critical temperature $T_\text{c}$ is $\sim1.4\,\mathrm{K}$. The white dashed lines in (c) and (d) are fitting curves of the first-order MAR peaks according to the BCS theory.