Low-Gap Hf-HfOx-Hf Josephson Junctions for meV-Scale Particle Detection

TL;DR

The study demonstrates hafnium-based Josephson junctions with a low superconducting gap suitable for meV-scale detection. Through double-angle shadow evaporation, they fabricate Hf–HfO_x–Hf JJs and perform extensive material characterization (SEM/AFM, XPS, XRD) alongside electrical testing from room temperature to millikelvin temperatures. They observe a stable oxide barrier (~4–5 nm), polycrystalline Hf with small grains, and clear Josephson behavior, yielding $I_c$ ≈ 6.2 nA and $Δ$ ≈ 17 μeV, albeit with subgap states that complicate exact gap extraction. The results indicate that low-gap $Hf$ junctions are promising for ultra-low-threshold THz photon and phonon detectors and could enable next-generation qubit architectures that leverage low gap energies.

Abstract

Superconducting qubits have motivated the exploration of Josephson-junction technologies beyond quantum computing, with emerging applications in low-energy photon and phonon detection for astrophysics and dark matter searches. Achieving sensitivity at the THz (meV) scale requires materials with smaller superconducting gaps than those of conventional aluminum or niobium-based devices. Here, we report the fabrication and characterization of hafnium (Hf)-based Josephson junctions (Hf-HfOx-Hf), demonstrating Hf as a promising low-Tc material platform for ultra-low threshold single THz photon and single-phonon detection. Structural and chemical analyses reveal crystalline films and well-defined oxide barriers, while electrical transport measurements at both room and cryogenic temperatures exhibit clear Josephson behavior, enabling extraction of key junction parameters such as critical current, superconducting gap and normal-state resistance. This work presents the first comprehensive study of Hf-based junctions and their potential for next-generation superconducting detectors and qubit architectures leveraging low superconducting gap energies.

Figures (6)

• Figure 1: (a) Top-view SEM image of the JJ overlap region. (b) XPS depth profile of a blanket Hf-HfO$_x$-Hf stack on a Si substrate, showing the evolution of relative atomic percentage with increasing etch time. The etch time corresponds to the composition from top to bottom as shown in schematic of the thin film stack.(c) Resistance measurements as a function of temperature to extract the $T_c$ of 30 nm and 60 nm Hf films. (d) Grazing incidence XRD patterns of 30 nm and 60 nm Hf films on Si.
• Figure 2: (a) Room temperature resistance measurements with varying dimensions. The plot shows the resistance of Hf JJ along with that of a standard Al JJ, with the Hf JJ resistance up to 2-5 times higher than Al JJ (b) $I–V$ characteristics of the Hf JJ measured at 13 mK. The plot shows the extracted $I_c$, $\Delta$ and $R_N$.
• Figure 3: (a) Temperature dependent $I-V$ characteristic of the JJ showing that the supercurrent reduces with increasing temperature. (b) The supercurrent resistance ($R_S$) and normal resistance ($R_N$) as a function of temperature. The resistance values of R$_S$ and R$_N$ are extracted from the slope of $I-V$ curves marked in (a). (c) Numerical differentiation of the temperature dependent $I-V$ curve. The dashed line corresponds to measured gap (2$\Delta$) region. (d) The extracted gap from (c) as a function of temperature. The fit is used to extract the gap at 0K ($\Delta_0$) and the JJ's critical temperature T$_c$.
• Figure S1: (a) SEM image of the Hf JJ fabricated using the Manhattan style geometry (b) SEM image showing the grain size of the Hf film. The grains sizes are below 10 nm. (c) AFM image of the Hf JJ.
• Figure S2: Hf$\,\mathrm{4f}$ XPS spectra from a blanket Hf-HfO$_x$-Hf stack on a Si substrate at various etch times.