Tungsten Germanide Single-Photon Detectors with Saturated Internal Detection Efficiency at Wavelengths up to 29 μm

TL;DR

This work introduces tungsten germanide (WGe) superconducting nanowire single-photon detectors engineered for mid-infrared operation, addressing material and scalability limitations of existing detectors. By co-sputtering WGe to ~8 nm thickness, patterning large-area nanowire meanders, and encapsulating with Ge, the authors demonstrate saturated internal detection efficiency up to $29~\mathrm{μm}$ using 360 nm-wide wires and up to the same wavelength range with 200 nm-wide wires, across two cryogenic measurement setups. They provide detailed material characterization (WGe thickness ~7.7 nm, Ge cap ~5.1 nm, Tc ~0.9 K) and show that thicker films facilitate scalable fabrication while maintaining mid-IR sensitivity. The results indicate WGe SNSPDs as a viable path toward large-scale mid-IR single-photon cameras with competitive noise performance, opening avenues for remote sensing, molecular spectroscopy, and astronomy, and offering a potential alternative to HgCdTe and impurity-band detectors.

Abstract

Superconducting nanowire single-photon detectors (SNSPDs) are among the most sensitive single-photon detectors available and have the potential to transform fields ranging from infrared astrophysics to molecular spectroscopy. However, extending their performance into the mid-infrared spectral region - crucial for applications such as exoplanet transit spectroscopy and vibrational fingerprinting of molecules - has remained a major challenge, primarily due to material limitations and scalability constraints. Here, we report on the development of SNSPDs based on tungsten germanide, a novel material system that combines high infrared sensitivity with compatibility for large-scale fabrication. Our detectors exhibit saturated internal detection efficiency at wavelengths up to 29 $\mathrm{μm}$. This advance enables scalable, high-performance single-photon detection in a spectral region that was previously inaccessible, opening new frontiers in remote sensing, thermal imaging, environmental monitoring, molecular physics, and astronomy.

Figures (6)

• Figure 1: Four-probe room temperature sheet resistance of co-sputtered bulk WGe films as a function of tungsten target sputtering power.
• Figure 2: Superconducting transition temperature Tc as a function of sputtering power of the tungsten target during co-sputtering of the WGe. The germanium sputtering power was fixed at 60. Measurements were performed on bulk films with thickness of approximately 50. The insets show the tungsten (W) and germanium (Ge) film composition of some samples, which was determined by Rutherford backscattering spectrometry (RBS).
• Figure 3: (a) Scanning electron micrograph of an SNSPD meander with 200 wide nanowires on a 600 pitch. The size of the active area is 16 x 16. (b) High-angle annular dark-field scanning transmission electron micrograph (HAADF-STEM) of the layer stack. (c) HAADF-STEM micrograph of the WGe layer with the Ge capping layer in comparison to energy dispersive X-ray spectroscopy (EDS) analyses showing (d) the tungsten signals, (e) the germanium signals, and (f) the oxygen signals. The dashed orange lines serve as guides to the eye to illustrate the layer structure.
• Figure 4: Scanning grating monochromator cryostat system based upon a blazed diffractive grating. The thermal shields at the 40 stage and the 4 stage together with the vacuum chamber are removed in the model to show the inside of the system.
• Figure 5: Oscilloscope traces of SNSPD pulses for a bias current of (a) 0.6 and (b) 2.2 of a 360 wide and about 8 thick nanowire. The orange trace was averaged 1000times. The trigger level for the photon count rate measurements is shown as a red dashed line.