Superconducting Niobium Nitride Research Articles

Superconducting single-photon detector integrated in DBR with optical microconnector for MM or SM fiber

This paper presents the development of a superconducting nanowire single-photon detector (SNSPD) integrated into a distributed Bragg reflector (DBR) with a design center wavelength of 830 nm and a bandwidth of 200 nm. This SNSPD is made of a superconducting niobium nitride (NbN) thin film that is produced using plasma-enhanced atomic layer deposition. The DBR is made of 15 alternating layers of silicon nitride and silicon oxide that are produced through plasma-enhanced chemical vapor deposition. The reflection efficiency of the mirror is 90% at a wavelength of 830 nm. For sufficient optical coupling, an optical micro-connector optimized for multimode or single-mode optical fibers with a diameter of 128 μm was formed using two-photon polymerization techniques. The niobium nitride film was deposited onto the DBR surface in-situ in two separate reactors connected by a vacuum transfer. The in-situ technique of deposition of a superconducting niobium nitride film and a DBR has allowed achieving a system detection efficiency of 90% at a wavelength of 830 nm and a dark count rate of 10 s−1 at a temperature of 2.5 K. Additionally, the detector jitter was 50 ps.

High-uniformity atomic layer deposition of superconducting niobium nitride thin films for quantum photonic integration

Atomic layer deposition (ALD) has been identified as a promising growth method for high-uniformity superconducting thin films for superconducting quantum photonic applications, offering superior uniformity, thickness control and conformality to techniques such as reactive sputtering. The potential scalability of ALD makes this method especially appealing for fabrication of superconducting nanowires and resonators across large areas. We report on the growth of highly uniform superconducting NbN thin films via plasma-enhanced atomic layer deposition (PEALD) with radio frequency substrate biasing, on a 200 mm (8 inch) Si wafer, specifically for superconducting nanowire single-photon detector applications. Niobium nitride films were grown using (tert-butylimido)-tris(diethylamido)-niobium(V) precursor and an H2/Ar plasma. The superconducting properties of a variable thickness series of films (5.9–29.8 nm) show critical temperature (T c) of 13.5 K approaching bulk thickness (28.8 nm) with low suppression down to the ultrathin regime (5.9 nm), with T c = 10.2 K. T c across the 200 mm wafer with 8 nm thick NbN, measured in 15 mm intervals, exhibits minimal variation (<7%). Microbridge structures fabricated on 8 nm thick NbN films also exhibit high critical current densities (J c), > 10 MA cm−2 at 2.6 K. PEALD could therefore be a pivotal technique in enabling large-scale fabrication of integrated quantum photonic devices across a variety of applications.

THz optical beat-note detection with a fast superconducting hot electron bolometer operating up to 31 GHz.

We study the performance of a hot-electron bolometer (HEB) operating at THz frequencies based on superconducting niobium nitride films. We report on the voltage response of the detector over a large electrical detection bandwidth carried out with different THz sources. We show that the impulse response of the fully packaged HEB at 7.5 K has a 3 dB cutoff around 2 GHz. Remarkably, detection capability is still observed above 30 GHz in an heterodyne beating experiment using a THz quantum cascade laser frequency comb. Additionally, the HEB sensitivity has been evaluated and an optical noise equivalent power NEP of 0.8 pW/√H z has been measured at 1 MHz.

High-speed detection of 1550 nm single photons with superconducting nanowire detectors

Superconducting nanowire single-photon detectors are a key technology for quantum information and science due to their high efficiency, low timing jitter, and low dark counts. In this work, we present a detector for single 1550 nm photons with up to 78% detection efficiency, timing jitter below 50 ps FWHM, 158 counts/s dark count rate, as well as a maximum count rate of 1.5 giga-counts/s at 3 dB compression. The PEACOQ detector (Performance-Enhanced Array for Counting Optical Quanta) comprises a linear array of 32 straight superconducting niobium nitride nanowires that span the mode of an optical fiber. This design supports high count rates with minimal penalties for detection efficiency and timing jitter. We show how these trade-offs can be mitigated by implementing independent readout for each nanowire and by using a temporal walk correction technique to reduce count-rate dependent timing jitter. These detectors make quantum communication practical on a 10 GHz clock.

Superconducting niobium nitride: a perspective from processing, microstructure, and superconducting property for single photon detectors

Superconducting niobium nitride (NbN) continues to be investigated decades on, largely in part to its advantageous superconducting properties and wide use in superconducting electronics. Particularly, NbN-based superconducting nanowire single-photon detectors (SNSPDs) have shown exceptional performance and NbN remains as the material of choice in developing future generation quantum devices. In this perspective, we describe the processing–structure–property relationships governing the superconducting properties of NbN films. We further discuss the complex interplay between the material properties, processing parameters, substrate materials, device architectures, and performance of SNSPDs. We also highlight the latest progress in optimizing SNSPD performance parameters.

Andreev Reflection in the Fractional Quantum Hall State

We construct high-quality graphene-based van der Waals devices with narrow superconducting niobium nitride (NbN) electrodes, in which superconductivity and robust fqH coexist. We find crossed Andreev reflection (CAR) across the superconductor separating two fqH edges. Our observed CAR probabilities in the particle-like fractional fillings are markedly higher than those in the integer and hole-conjugate fractional fillings and depend strongly on temperature and magnetic field unlike the other fillings. Further, we find a filling-independent CAR probability in integer fillings, which we attribute to spin-orbit coupling in NbN allowing for Andreev reflection between spin-polarized edges. These results provide a route to realize novel topological superconducting phases in fqH-superconductor hybrid devices based on graphene and NbN.

Analysis of Low-Temperature Magnetotransport Properties of NbN Thin Films Grown by Atomic Layer Deposition

Superconducting niobium nitride (NbN) films with nominal thicknesses of 4 nm, 5 nm, 7 nm, and 9 nm were grown on sapphire substrates using atomic layer deposition (ALD). We observed probed Hall resistance (HR) (Rxy) in external out-of-plane magnetic fields up to 6 T and magnetoresistance (MR) (Rxx) in external in-plane and out-of-plane magnetic fields up to 6 T on NbN thin films in Van der Pauw geometry. We also observed that positive MR dominated. Our study focused on the analysis of interaction and localisation effects on electronic disorder in NbN in the normal state in temperatures that ranged from 50 K down to the superconducting transition temperature. By modelling the temperature and magnetic field dependence of the MR data, we extracted the temperature-dependent Coulomb interaction constants, spin–orbit scattering lengths, localisation lengths, and valley degeneracy factors. The MR model allowed us to distinguish between interaction effects (positive MR) and localisation effects (negative MR) for in-plane and out-of-plane magnetic fields. We showed that anisotropic dephasing scattering due to lattice non-idealities in NbN could be neglected in the ALD-grown NbN thin films.

Nanoscale devices with superconducting electrodes to locally channel current in 3D Weyl semimetals

We report on the fabrication of nano-devices on the [1¯01] surface of a Weyl semimetal, a macroscopic crystal of TaAs, and low-temperature transport measurements. We can implement electron beam lithography by peeling off and transferring the resist for nanofabrication onto the irregular crystal. We fabricate the device electrodes with superconducting Niobium nitride (NbN) to control the current flow through the intended active area of the devices. Our device structure enables the reduction in the current jetting effect, and we demonstrate the negative magnetoresistance measurement as a function of angle. The high field magnetotransport shows three distinct oscillation frequencies corresponding to the three bands at the Fermi level. Resistance measured in the low magnetic field shows the usual weak anti-localization dip near the zero-field—a signature of a Weyl material. Our method of fabricating devices with superconducting electrodes provides a way to probe the electrical properties of macroscopic single crystals at the nanoscale. As we use conventional lithographic techniques for patterning, this method can be extended to a wide gamut of electrode materials and a large class of 3D quantum materials.

Wafer-level uniformity of atomic-layer-deposited niobium nitride thin films for quantum devices

Superconducting niobium nitride thin films are used for a variety of photon detectors, quantum devices, and superconducting electronics. Most of these applications require highly uniform films, for instance, when moving from single-pixel detectors to arrays with a large active area. Plasma-enhanced atomic layer deposition (ALD) of superconducting niobium nitride is a feasible option to produce high-quality, conformal thin films and has been demonstrated as a film deposition method to fabricate superconducting nanowire single-photon detectors before. Here, we explore the property spread of ALD-NbN across a 6-in. wafer area. Over the equivalent area of a 2-in. wafer, we measure a maximum deviation of 1% in critical temperature and 12% in switching current. Toward larger areas, structural characterizations indicate that changes in the crystal structure seem to be the limiting factor rather than film composition or impurities. The results show that ALD is suited to fabricate NbN thin films as a material for large-area detector arrays and for new detector designs and devices requiring uniform superconducting thin films with precise thickness control.

Niobium nitride plasmonic perfect absorbers for tunable infrared superconducting nanowire photodetection.

Quantum technologies such as quantum computing and quantum cryptography exhibit rapid progress. This requires the provision of high-quality photodetectors and the ability to efficiently detect single photons. Hence, conventional avalanche photodiodes for single photon detection are not the first choice anymore. A better alternative are superconducting nanowire single photon detectors, which use the superconducting to normal conductance phase transition. One big challenge is to reduce the product between recovery time and detection efficiency. To address this problem, we enhance the absorption using resonant plasmonic perfect absorber effects, to reach near-100% absorption over small areas. This is aided by the high resonant absorption cross section and the angle insensitivity of plasmonic resonances. In this work we present a superconducting niobium nitride plasmonic perfect absorber structure and use its tunable plasmonic resonance to create a polarization dependent photodetector with near-100% absorption efficiency in the infrared spectral range. Further we fabricated a detector and investigated its response to an external light source. We also demonstrate the resonant plasmonic behavior which manifests itself through a polarization dependence detector response.

Substrate dependent epitaxy of superconducting niobium nitride thin films grown by pulsed laser deposition

Niobium nitride (NbN) has suitable mechanical properties for application as protective coatings in mechanical engineering, and also its superconductivity can be utilized in thin film devices for sensorics or in combination with ferromagnet in spintronics. Long-range superconducting proximity effect at the heterostructures with a weak ferromagnet can be used for generation of spin-polarized current. For any operational heterostructure application the high quality NbN thin films need to be prepared. In this work we have investigated impact of the substrates on the structure and preferential orientation of niobium nitride thin films fabricated by pulsed laser deposition at 600 °C on Si (001), MgO (001), C-plane and R-plane Al2O3 substrates. Growth parameters have been tuned in order to obtain single superconducting δ-NbN phase. Microstructure was analyzed by X-ray diffraction and transmission electron microscopy. Si substrate does not induce the film preferential orientation. Films on MgO are epitaxial (001) oriented. Films on R-Al2O3 show (135) orientation with twinned crystallites on the lower symmetry substrate surface. The (111) epitaxial growth with the largest crystallites and their smallest tilting was achieved on C-Al2O3 substrate leading to the best superconducting properties.

Thermal transport in superconducting niobium nitride: A first-principles study

Superconducting metallic transition-metal nitrides, especially from the family of NbNx, are promising in various applications. Due to the fact that the lattice constants and the crystal structures are similar to those of GaN, GaN/NbN heterostructures have been grown to combine the benefits of superconductors and semiconductors, where the thermal transport property is of great significance. In this Letter, the thermal transport property of metallic hexagonal NbN is studied using a first-principles approach with the consideration of both electron and phonon scatterings. It is interesting to find that unlike most metals, phonons play a bigger role in hexagonal NbN compared to electrons, due to a relatively small electron density of states near the Fermi level. At room temperature, our calculated thermal conductivity is close to the experimental data. Our findings can provide a deeper understanding of how heat is transported in metallic transition-metal nitrides and may help design semiconductor/superconductor heterostructures.

Improving the Efficiency of the Integrated Optical Superconducting Single Photon Detector on a Lithium Niobate Substrate by Means of an Additional Cover Layer with a High Refractive Index

The mechanism of enhancing the interaction of optical radiation with a superconducting niobium nitride film on the surface of optical waveguides on a lithium niobate substrate using an additional cover layer with a high refractive index has been studied. It has been shown that adiabatic deformation of waveguide modes allows increasing the absorption of light in a superconducting film to the level of ∼1 dB/μm, which makes it possible to manufacture a single photon detector with a quantum efficiency of about 70% on lithium niobate.

Fabrication of superconducting niobium nitride nanowire with high aspect ratio for X-ray photon detection

The niobium nitride (NbN) nanowires fabricated with the high-quality ultra-thin NbN film with a thickness of 3 nm–6 nm were widely used for single photon detectors. These nanowires had a low aspect ratio, less than 1:20. However, increasing the thickness and the aspect ratio of highly-uniformed NbN nanowires without reducing the superconductivity is crucial for the device in detecting high-energy photons. In this paper, a high-quality superconducting nanowire with aspect ratio of 1:1 was fabricated with optimized process, which produced a superconducting critical current of 550 μA and a hysteresis of 36 μA at 2.2 K. With the optimization of the electron beam lithography process of AR-P6200.13 and the adjustion of the chamber pressure, the discharge power, as well as the auxiliary gas in the process of reactive ion etching (RIE), the meandered NbN nanowire structure with the minimum width of 80 nm, the duty cycle of 1:1 and the depth of 100 nm were finally obtained on the silicon nitride substrate. Simultaneously, the sidewall of nanowire was vertical and smooth, and the corresponding depth-width ratio was more than 1:1. The fabricated NbN nanowire will be applied to the detection of soft X-ray photon emitted from pulsars with a sub-10 ps time resolution.

Tunable and high quality factor Fano and toroidal dipole resonances in terahertz superconducting metamaterials

We experimentally studied a series of terahertz (THz) metamaterials with asymmetric resonator structures made from superconducting niobium nitride or gold films. Fano and toroidal dipole resonances are excited in superconducting metamaterials and their quality factors are higher in comparison with those of gold samples. Superconducting samples possess good tuning behaviors for both Fano and toroidal dipole resonance frequencies due to the change of kinetic inductance under different temperatures. The superconducting samples also exhibit good slow light effect due to the sharp dispersion in transmission spectra. These tunable and low loss metamaterials possess great significance to the development of THz nonlinear and slow light devices.

Optical and Tunneling Studies of Energy Gap in Superconducting Niobium Nitride Films

We have prepared an epitaxial niobium nitride (NbN) film and NbN/AlN/NbN tunnel junctions to investigate the energy gaps. By measuring the optical conductivity spectrum of a 41-nm-thick film with the critical temperature (TC) of about 14 K by terahertz time-domain spectroscopy, we obtained the gap frequency of about 1.2 THz. An ill-defined gap was suggested due to the broadening of the onset of absorption as the temperature increases. On the other hand, the current–voltage curve measurement of the tunnel junctions showed the current rise at the gap voltage of 5.6 mV (corresponding to 1.4 THz) with a smeared shape as the temperature increases. We found that both gap broadenings can be explained by introducing a temperature-dependent imaginary energy gap part into the superconducting energy gap, corresponding to a finite quasi-particle lifetime in the NbN films.

Повышение эффективности интегрально-оптического сверхпроводящего датчика одиночных фотонов на подложке ниобата лития за счет дополнительного покровного слоя с высоким показателем преломления

A mechanism of amplification of interaction between optical radiation and superconducting niobium nitride thin film placed on the surface of optical waveguides, which are fabricated on lithium niobate substrate, is investigated. This effect can be obtained with help of an additional covering layer with high refractive index. It was shown that adiabatic deformation of waveguide modes can increase absorption of light in a superconducting layer up to ~ 1 dB/µm, making it possible to fabricate a SSPD on lithium niobate with quantum efficiency of about 70%.

Optimization of the process of obtaining superconducting bilayer films based on niobium nitride and gold for the manufacture of a superconducting single-photon detector

We have manufactured a number of bilayer structures based on niobium nitride and gold with different deposition parameters for NbN-Au films (Table 1). We measured the characteristics of superconducting niobium nitride films both with the top layer of gold and without gold. On the basis of the measured characteristics of the films, the optimal parameters of the deposition of NbN-Au bilayer films were chosen for the further formation of a superconducting single-photon detector.

Ultra-broadband microfiber-coupled superconducting single-photon detector.

Broadband photon detectors are a key enabling technology for various applications such as spectrometers, light detection and ranging. In this work, we report on an ultra-broadband single-photon detector based on a microfiber (MF)-coupled superconducting nanowires structure operating in the spectral range from visible to near-infrared light. The MF-coupled superconducting nanowire single-photon detector (SNSPD) is formed by placing an MF on top of superconducting niobium nitride (NbN) nanowires, allowing ultra-broadband photon detection due to their nearly lossless transmission/absorption and nearly unity internal efficiency for ultra-broad waveband. The simulation results indicate that with optimal device structure, the optical absorption with efficiency > 90% can be realized over a wavelength range of 350 nm to 2150 nm. The fabricated MF-coupled SNSPD shows unparalleled broadband system detection efficiencies (SDEs) of more than 50% from 630 nm to 1500 nm. The SDEs reach 66% at 785 nm and 45% at 1550 nm. These results pave the way for ultra-broadband weak light detection with quantum-limit sensitivity.

Microwave probing of relaxation oscillations related to terahertz power detection in superconducting hot electron bolometers

Relaxation oscillations related to terahertz (THz) power detection in superconducting niobium nitride hot electron bolometers (HEBs) have been studied using microwaves as a probe. The reflected microwave signal, which is related to the impedance changes of the HEB irradiated by an incident THz signal, can be used to characterize relaxation oscillations. The frequency of relaxation oscillation switching between the superconducting and resistive states increases linearly with incident THz power but nonlinearly with bias voltage or bath temperature. The observed strong periodic pulse trains of relaxation oscillations have a maximum duty cycle of ∼50%. These strong switching pulses indicate that DC Joule heating, THz radiation or thermal energy is absorbed in the bi-stable region of the HEB intermittently. In addition to relaxation oscillations, we also see weaker oscillations when the device is in the resistive state, which we call intrinsic oscillations. These have a constant frequency at the order of 10 kHz which is independent of external stimuli. The weak periodic pulse trains at a constant frequency can be divided into mainly two types—one is the pulse train with a small duty cycle and the other is the quasi-square wave. Measurement of these oscillations in HEBs has some practical applications. Digital THz power measurement with frequencies below and above the gap frequency can be made based on the linearity between relaxation oscillation frequency and incident THz power. The relaxation oscillation frequency can be used to estimate the maximum speed of a THz direct detection system especially for a multi-pixel HEB array. In particular, extra noise induced by relaxation oscillations and intrinsic oscillations can be determined for the HEB operating as a THz direct detector.