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

TL;DR

This study reveals how helium-ion irradiation modifies substrate thermal conductance and enhances superconducting nanowire single-photon detector performance by selectively irradiating regions around the nanowire.

Contribution

It demonstrates a novel method to locally engineer substrate properties via helium-ion irradiation to improve SNSPD detection efficiency and sensitivity.

Findings

Increased plateau width of detection efficiency from 3.7 to 9.8 uA.

Thermal conductance between superconductor and substrate reduced from 210 to 70 W/m^2/K^4.

Critical current remained similar after partial irradiation.

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 W/m^2/K^4 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 the regions around the nanowire. With this approach, we achieved an increased plateau width of saturating intrinsic detection efficiency of 9.8 uA compared to 3.7 uA after full irradiation. Moreover, the critical current remained similar to that of the unirradiated reference device (59 uA versus 60.1 uA), while full irradiation reduced it to 22.4 uA. 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.