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arXiv:2501.14965 · APL Quantum

Origin of performance enhancement of superconducting nanowire single-photon detectors by He-ion irradiation

Stefan Strohauer, Fabian Wietschorke, Markus Döblinger, Christian Schmid, Stefanie Grotowski, Lucio Zugliani, Björn Jonas, Kai Müller, Jonathan James Finley

Published Jun 1, 2025• 2 citations
arXivHTMLPDF

Abstract

Superconducting nanowire single-photon detectors (SNSPDs) are indispensable in fields such as quantum science and technology, astronomy, and biomedical imaging, where high detection efficiency, low dark count rates, and high timing accuracy are required. Recently, helium (He) ion irradiation was shown to be a promising method to enhance SNSPD performance. Here, we study how changes in the underlying superconducting NbTiN film and the SiO2/Si substrate affect device performance. While irradiated and unirradiated NbTiN films show similar crystallinity, we observe He bubble formation below the SiO2/Si interface and an amorphization of the Si substrate. Both reduce the thermal conductance between the superconducting thin film and the substrate from 210 to 70 W m−2 K−4 after irradiation with 2000 ions nm−2. This effect, combined with the lateral straggle of He ions in the substrate, allows the modification of the superconductor-to-substrate thermal conductance of an SNSPD by selectively irradiating only the regions around the nanowire. With this approach, we achieved a broader bias current range (9.8 μA vs 3.7 µA) in which the detector operates at its maximum detection efficiency, which is beneficial for reducing dark counts while maintaining high sensitivity. Moreover, the photon-assisted critical current remained similar to that of the unirradiated reference device (59.0 µA vs 60.1 µA), while full irradiation reduced it to 22.4 µA. Our results suggest that the irradiation-induced reduction of the thermal conductance significantly enhances SNSPD sensitivity, offering a novel approach to locally engineer substrate properties for improved detector performance.

DOI
10.1063/5.0259935

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Superconducting nanowire single-photon detectors (SNSPDs) are indispensable in fields such as quantum science and technology, astronomy, and biomedical imaging, where high detection efficiency, low dark count rates, and high timing accuracy are required. Recently, helium (He) ion irradiation was shown to be a promising method to enhance SNSPD performance. Here, we study how changes in the underlying superconducting NbTiN film and the SiO2/Si substrate affect device performance. While irradiated and unirradiated NbTiN films show similar crystallinity, we observe He bubble formation below the SiO2/Si interface and an amorphization of the Si substrate. Both reduce the thermal conductance between the superconducting thin film and the substrate from 210 to 70 W m−2 K−4 after irradiation with 2000 ions nm−2. This effect, combined with the lateral straggle of He ions in the substrate, allows the modification of the superconductor-to-substrate thermal conductance of an SNSPD by selectively irradiating only the regions around the nanowire. With this approach, we achieved a broader bias current range (9.8 μA vs 3.7 µA) in which the detector operates at its maximum detection efficiency, which is beneficial for reducing dark counts while maintaining high sensitivity. Moreover, the photon-assisted critical current remained similar to that of the unirradiated reference device (59.0 µA vs 60.1 µA), while full irradiation reduced it to 22.4 µA. Our results suggest that the irradiation-induced reduction of the thermal conductance significantly enhances SNSPD sensitivity, offering a novel approach to locally engineer substrate properties for improved detector performance.
Abstract Superconducting nanowire single-photon detectors (SNSPDs) offer high system detection efficiency (SDE), low dark counts, and excellent timing resolution, making them indispensable for applications from quantum optics to deep-space optical communications. Although near-unity SDE and intrinsic detection efficiency (IDE) have been achieved at near-infrared wavelengths through advanced fabrication and optical design, further improving IDE at the material and device levels remains essential for achieving broader bandwidths, higher operating temperatures, and simplified processing. Here, we explored helium ion irradiation as a post-fabrication method to modify the thermal properties of NbN SNSPDs without altering their geometry or material stack. With increasing fluence (0–1.1 × 10 17 ions cm −2 ), the thermal boundary conductance decreases by 57% from 127 W m −2 K −4 to 54 W m −2 K −4 , and saturates near 9 × 10 16 ions cm −2 . At this fluence, the minimum hotspot relaxation time increases by 41%, rising from 17 ps to 24 ps, while the electron–phonon interaction time decreases, with the magnitude of change depending on temperature and sample batch. Transmission electron microscopy reveals interfacial disorder near the NbN/SiO₂ interface (6–8 nm) and localized defects in the SiO₂ layer (30–260 nm), correlating with thermal transport suppression. These thermal modifications have a significant impact on relaxation dynamics and contribute to IDE enhancement. Our findings establish ion irradiation as an effective strategy for thermal tuning in SNSPDs, providing new insights into defect engineering for superconducting optoelectronic devices.
Detecting single photons is essential for applications such as dark matter detection, quantum science and technology, and biomedical imaging. Superconducting nanowire single-photon detectors (SNSPDs) excel in this task due to their near-unity detection efficiency, subhertz dark count rates, and picosecond timing jitter. However, a local increase of current density (current crowding) in the bends of meander-shaped SNSPDs limits these performance metrics. By locally irradiating the SNSPD’s straight segments with helium ions while leaving the bends unirradiated, we realize current crowding–free SNSPDs with simultaneously enhanced sensitivity: After irradiation with 800 ions nm −2 , locally irradiated SNSPDs showed a relative saturation plateau width of 37%, while fully irradiated SNSPDs reached only 10%. This larger relative plateau width allows operation at lower relative bias currents, thereby reducing the dark count rate while still detecting single photons efficiently. We achieve an internal detection efficiency of 94% with 7 mHz dark count rate near the onset of saturating detection efficiency for a wavelength of 780 nm.
Abstract Achieving homogeneous performance metrics between nominally identical pixels is challenging for the operation of arrays of superconducting nanowire single‐photon detectors (SNSPDs). Here, local helium ion irradiation is utilized to post‐process and tune single‐photon detection efficiency, switching current, and critical temperature of individual devices on the same chip. For 12 nm thick highly absorptive SNSPDs, which are barely sensitive to single photons with a wavelength of 780 nm prior to He ion irradiation, an increase of the system detection efficiency from <0.05% to (55.3 1.1)% is observed following irradiation. Moreover, the internal detection efficiency saturates at a temperature of 4.5 K after irradiation with 1800 ions nm −2 . Compared to 8 nm SNSPDs of similar detection efficiency, a doubling of the switching current (to 20 µA) is observed for irradiated 10 nm thick detectors, increasing the amplitude of detection voltage pulses. Investigations of the scaling of superconducting thin film properties with irradiation up to a fluence of 2600 ions nm −2 revealed an increase of sheet resistance and a decrease of critical temperature towards high fluences. A physical model accounting for defect generation and sputtering during helium ion irradiation is presented and shows good qualitative agreement with experiments.
Superconducting nanowire single-photon detectors (SNSPDs) have emerged as a highly promising infrared single-photon detector technology. Next-generation devices are being developed with enhanced detection efficiency (DE) at key technological wavelengths via the use of optical cavities. Furthermore, new materials and substrates are being explored for improved fabrication versatility, higher DE, and lower dark counts. We report on the practical performance of packaged NbTiN SNSPDs fabricated on oxidized silicon substrates in the wavelength range from 830 to 1700 nm. We exploit constructive interference from the SiO2/Si interface in order to achieve enhanced front-side fiber-coupled DE of 23.2 % at 1310 nm, at 1 kHz dark count rate, with 60 ps full width half maximum timing jitter.
Superconducting single-photon detectors (SSPDs) are the leading detectors in terms of high-speed single-photon counting and high detection efficiency (DE). One factor that limits the DE is the critical current Ic, which is the maximum current before the superconductor switches to the normal state. Increasing device’s bias current towards the Ic can improve the DE. However, the device’s Ic is reduced due to constriction and current crowding at the edges of the wire. Typically, this is caused by fabrication defects. Locally suppressing superconductivity at these defects can potentially lessen the occurrence of current crowding. In this thesis, we used the beam from the helium ion microscope (HIM) and measured the Ic to observe the effects of locally irradiating specific areas on a SSPD wire. Due to the HIM’s small spot size and high collimation, we can control the superconducting gap precisely at the center and edges of the wire. Suppressing the edges can potentially reduce current crowding and increase the device’s critical current while suppressing the center can improve detection sensitivity for photons incident at that location. Our results showed that the irradiated devices had reduced Ic compared to unirradiated devices for both cases. We then extend this method of local suppression of superconductivity to explore an alternative method of fabricating SSPDs by directly writing the device on the superconducting thin film. This can enable the fabrication of devices without the use of lithography resist. In our experiment, we fabricated a 3 μm wire using optical lithography that was disconnected at the center and connected it by writing a single 1 μm wire with the He+ ion beam. We measured the Ic for samples with and without the 1 μm wire pattern and observed that the Ic decreased as we increased the ion dose. Overall, this work aims to contribute to the continuing investigation of the detection mechanism for SSPDs and the improvement of nanofabrication methods using the HIM.
Superconducting nanowire single-photon detectors (SNSPDs) are a key building block for photonic quantum technologies, as they provide a means for efficient readout of optical circuits with low timing jitter, dark counts and dead time. In this contribution we give a general overview of the working principle of these devices and look into several engineering aspects. We discuss the influence of the stoichiometry of NbTiN thin-films on detector performance and find that a ratio of 65:35 Nb:Ti results in the highest saturation of SNSPDs fabricated from these films. Furthermore, we study the impact of helium ion irradiation on the efficiency of SNSPDs. We show that with this technique the efficiency of an SNSPD can be improved from below 1% to 42%. Finally, we discuss the impact of embedding the SNSPDs into an optical cavity on the system detection efficiency. By employing a half-cavity under the detector we are able to boost the efficiency from 30% to 50% for a detector with a fill factor of 0.33.