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arXiv:2006.09643 · Applied Physics Letters

Epitaxial niobium nitride superconducting nanowire single-photon detectors

Risheng Cheng, John G. Wright, Huili Grace Xing, Debdeep Jena, Hong X. Tang

Published Sep 28, 2020• 44 citations
arXivHTMLPDF

Abstract

Superconducting nanowires used in single-photon detectors have been realized on amorphous or poly-crystalline films. Here, we report the first use of single-crystalline NbN thin films for superconducting nanowire single-photon detectors (SNSPDs). Grown by molecular beam epitaxy (MBE) at high temperature on nearly lattice-matched AlN-on-sapphire substrates, the NbN films exhibit a high degree of uniformity and homogeneity. Even with relatively thick films, the fabricated nanowire detectors show saturated internal efficiency at near-IR wavelengths, demonstrating the potential of MBE-grown NbN for realizing large arrays of on-chip SNSPDs and their integration with AlN-based χ(2) quantum photonic circuits.

DOI
10.1063/5.0018818

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Superconducting nanowires used in single-photon detectors have been realized on amorphous or poly-crystalline films. Here, we report the first use of single-crystalline NbN thin films for superconducting nanowire single-photon detectors (SNSPDs). Grown by molecular beam epitaxy (MBE) at high temperature on nearly lattice-matched AlN-on-sapphire substrates, the NbN films exhibit a high degree of uniformity and homogeneity. Even with relatively thick films, the fabricated nanowire detectors show saturated internal efficiency at near-IR wavelengths, demonstrating the potential of MBE-grown NbN for realizing large arrays of on-chip SNSPDs and their integration with AlN-based χ(2) quantum photonic circuits.
Superconducting nanowire single photon detectors (SNSPDs) made with thin NbN films can reach high performances. While sputtering has been the deposition method of choice, here, we show that ammonia-molecular beam epitaxy (NH3-MBE) can produce pertinent epitaxial cubic NbN thin films on silicon substrates using an AlN buffer. Despite granular morphology and a high density of grain boundaries as well as the presence of rotational twins, Tc = 12.7 K for a 5.6 nm thick film and saturation of internal detection efficiency up to 850 nm are achieved. Morphology and stoichiometry as well as strain have a strong impact on the detector properties, highlighting the importance of a precise control of the growth parameters. These results pave the way for high fabrication yield of SNSPDs on large-scale silicon wafers using epitaxial NbN thin films grown by MBE.
Abstract Superconducting nanowire single-photon detectors (SNSPDs) have become increasingly important for both classical and quantum applications requiring single-photon sensing. Most current SNSPDs based on superconducting niobium nitride (NbN) ultrathin films are fabricated utilizing polycrystalline or amorphous superconductor films, and film inhomogeneity and defectivity can cause significant issues. Here, we present an all-epitaxial approach for fabricating SNSPDs with integrated metal reflectors, semiconductor (dielectric) resonators, and superconductor nanowire detectors. A working example is realized by molecular-beam epitaxy growth of a tri-layered heterostructure, consisting of a titanium nitride (TiN) reflector with ~ 92% reflectivity at the telecommunication wavelength (1550 nm), an aluminum nitride (AlN) resonator with a broad optical transparency window, and a NbN nanowire on c -plane sapphire wafer. The all-epitaxial approach provides a manufacturable platform with superior material properties, as well as uniformity across the wafer, which manifest themselves in tunable material parameters of the superconducting nanowire, including normal-state resistance, superconducting critical temperature, superconducting critical current, and kinetic inductance. The optimized material properties can be utilized to achieve excellent SNSPD performance, including wafer-scale uniformity in device characteristics and broadband (~300 nm) system detection efficiency (>90%) centered at 1550 nm, suitable for large-scale detector arrays and wavelength-division multiplexing (WDM) applications.
We demonstrate efficient nanowire superconducting single photon detectors (SSPDs) based on NbN thin films grown on GaAs. NbN films ranging from 3 to 5 nm in thickness have been deposited by dc magnetron sputtering on GaAs substrates at 350 °C. These films show superconducting properties comparable to similar films grown on sapphire and MgO. In order to demonstrate the potential for monolithic integration, SSPDs were fabricated and measured on GaAs/AlAs Bragg mirrors, showing a clear cavity enhancement, with a peak quantum efficiency of 18.3% at λ=1300 nm and T=4.2 K.
Superconducting nanowire single photon detectors stand today as the best technology, due to their near-unity detection efficiency, low dark count rates, and low timing jitter. In this work, we demonstrate the improvement of the superconducting properties of NbN thin films on 8” silicon-on-insulator wafers by using an ultra-thin (10-15 nm) sputtered AlN buffer layer. The higher crystalline quality of NbN, leads to an increase of the superconducting critical temperature up to 10 K for 9-nm-thick NbN films. The material was validated for single photon detection using a fiber-coupled vertical SNSPD with half-cavity architecture. This results opens the way for the development of CMOS compatible waveguide-integrated detectors. The implementation of such guided devices is a keystone for the development of a fully integrated quantum photonics platform able to generate, manipulate and detect a large number of photonic qubits for secure communications and quantum computing applications.
We have developed a practical superconducting nanowire single photon detector (SNSPD) system with our superconducting thin films and devices fabrication, optical coupling packaging, and cryogenic technology. The SNSPD devices were fabricated from high-quality niobium nitride (NbN) ultra-thin films that were epitaxially grown on single-crystal MgO substrates, and optical cavity structure were applied for improvement of photo absorption efficiency. The SNSPD system was developed based on a compact Gifford-McMahon (GM) cryocooler, in which SNSPD can operate continuously at temperature of 2.9 K +/- 10 mK with low power consumption without any liquid cryogens. The system detection efficiencies at 1550 nm wavelength (at a 100 Hz dark count rate) were found to be 2.6% for device without optical cavity and 21% for device with optical cavity. The response speed of devices showed maximally 200 MHz with 5 × 5 mm square device, and timing jitter of 100 ps full width at a half maximum (FWHM) was observed.
High-quality ultrathin films of niobium nitride (NbN) are developed by the plasma-enhanced atomic layer deposition (PEALD) technique. Superconducting nanowire single-photon detectors patterned from this material exhibit high switching currents and saturated internal efficiencies over a broad bias range at a telecommunication wavelength of 1550 nm. Statistical analyses on hundreds of fabricated devices show a near-unity throughput yield due to the exceptional homogeneity of the films. The ALD-NbN material represents an ideal superconducting material for fabricating large single-photon detector arrays combining high efficiency, low jitter, and low dark counts.