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 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 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 microstrip single photon detectors (SMSPDs) exhibit high signal-to-noise ratios and large active areas, demonstrating broad application potential in high-energy particle detection, wide-field imaging, LiDAR, and related fields. To further improve the detection efficiency and operating temperature of SMSPDs, it is essential to bias the device with a current as close as possible to the depairing current—that is, to achieve the constriction factor (the ratio of the switch current to the depairing current) approaching unity. Currently, the geometric current crowding effect in the meandering region of the microstrip is regarded as the primary factor limiting the current constriction factor. In this study, we systematically evaluated three strategies for mitigating this effect: thickening the superconducting film at meandering region, increasing the width of the meandering region and localized ion milling in the straight section of the microstrip. Through theoretical simulations and comparative measurements of the switching current in identically sized MoSi microstrips incorporating these three modified structures, we demonstrated that all approaches effectively suppress current crowding. Notably, the ion milling method increased the device’s constriction factor from 0.44 ± 0.02 to 0.69 ± 0.02. This technique not only reduced current crowding but also enhanced device sensitivity. By implementing this approach, we achieved near-saturated intrinsic detection efficiency at 1550 nm under 1.7 K, offering a practical strategy for optimizing the performance of large-area SMSPDs.
Superconducting devices are prone to reduced performance caused by impurities and defects along the edges of their wires, which can lead to local current crowding. In this study, we explored the use of helium ion irradiation to modify the lattice structure of the superconducting material to change its intrinsic properties. The process will allow us to directly pattern devices and potentially improve the quality of the nanowires. To achieve this, we used the ion beam from a scanning helium ion microscope (HIM) to localize damage on a superconducting material to create a nanowire. Two experiments were performed in this study. First, a range of helium ion doses was exposed on a niobium nitride (NbN) microwire to determine the estimated dose density to suppress superconductivity. Using the results of this first experiment, nanowires were patterned onto a microwire, and the current-voltage characteristics were measured for each sample. Our results showed that helium ion irradiation is an effective resistless fabrication method for superconducting nanowires.
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.
Nanowire superconducting single-photon detectors (SSPDs) are promising devices for single-photon detection applications at telecom wavelengths, in terms of high efficiency, low dark count rate and short dead time. They consist of a long, 100 nm wide, superconducting niobium nitride nanowire folded into a meander pattern and biased close to the critical current. Photons incident on the SSPD locally break the superconductivity which results in a resistive barrier across the nanowire. The voltage drop across this barrier is measured. The devices presented in this thesis are fabricated on a gallium arsenide substrate: a material system that allows integration of single-photon sources and detectors on a single chip. Such quantum photonic integrated circuits (QPICs) are of major importance for scalable quantum computing.Measurements on nominally identical SSPDs show large variations in critical current and efficiency of the devices . This is a huge problem for applications where arrays of SSPDs are needed such as linear optical quantum computing. The authors of [1] attribute the observations to geometrical constrictions: regions in the nanowire where the cross section is reduced. This reduces the critical current of the wire and the SSPD cannot be biased efficiently anymore. In this thesis, it is shown that the continuously distributed inhomogeneity of the nanowire dimension or the crystal structure, instead of localized constrictions, is the origin of the limited detection efficiency and poor uniformity of our SSPDs.To investigate the effect of inhomogeneities, SSPDs with lengths in the range from 100 nm to 15 µm are fabricated and characterized electrically and optically. The two main observations from the electrical characterization are a decrease of the critical current with increasing length and a decrease of the spread in critical current values with increasing length. These measurements also indicate that the length scale of the inhomogeneities is 100 nm or smaller. The main result of the optical characterization is a decrease of the detection efficiency with increasing wire length.Simple assumptions on how the inhomogeneities influence the critical current of the wires allowed us to create a numerical model that quantitatively reproduces the measured variations of the critical current. This result is used to model also the detection efficiency of the SSPDs. This model qualitatively reproduces the general trends in the efficiency measurements, although some deviations are observed for the longer wires. Nanowire superconducting single-photon detectors (SSPDs) are promising devices for single-photon detection applications at telecom wavelengths, in terms of high efficiency, low dark count rate and short dead time. They consist of a long, 100 nm wide, superconducting niobium nitride nanowire folded into a meander pattern and biased close to the critical current. Photons incident on the SSPD locally break the superconductivity which results in a resistive barrier across the nanowire. The voltage drop across this barrier is measured. The devices presented in this thesis are fabricated on a gallium arsenide substrate: a material system that allows integration of single-photon sources and detectors on a single chip. Such quantum photonic integrated circuits (QPICs) are of major importance for scalable quantum computing.Measurements on nominally identical SSPDs show large variations in critical current and efficiency of the devices . This is a huge problem for applications where arrays of SSPDs are needed such as linear optical quantum computing. The authors of [1] attribute the observations to geometrical constrictions: regions in the nanowire where the cross section is reduced. This reduces the critical current of the wire and the SSPD cannot be biased efficiently anymore. In this thesis, it is shown that the continuously distributed inhomogeneity of the nanowire dimension or the crystal structure, instead of localized constrictions, is the origin of the limited detection efficiency and poor uniformity of our SSPDs.To investigate the effect of inhomogeneities, SSPDs with lengths in the range from 100 nm to 15 µm are fabricated and characterized electrically and optically. The two main observations from the electrical characterization are a decrease of the critical current with increasing length and a decrease of the spread in critical current values with increasing length. These measurements also indicate that the length scale of the inhomogeneities is 100 nm or smaller. The main result of the optical characterization is a decrease of the detection efficiency with increasing wire length.Simple assumptions on how the inhomogeneities influence the critical current of the wires allowed us to create a numerical model that quantitatively reproduces the measured variations of the critical current. This result is used to model also the detection efficiency of the SSPDs. This model qualitatively reproduces the general trends in the efficiency measurements, although some deviations are observed for the longer wires.
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.