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.
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.
Abstract We demonstrate and characterize superconducting nanowire single-photon detectors made from atomic layer-deposited (ALD) NbN layers. To assess the suitability of these films as a detector material, transport properties of bare films and bridges of different dimensions and thicknesses are investigated. Similar ratios of the measured critical current to the depairing current are obtained for micro-bridges made from ALD and sputtered NbN films. Furthermore, we characterized the single-photon response for 5 and 10 nm thick nanowire detectors. A 100 nm wide straight nanowire with a length of 5 μ m exhibits saturated count rate dependencies on bias current and a cut-off wavelength in the near-infrared range. The ALD technique could open up the possibility to fabricate NbN-based detectors on the wafer scale and to conformally cover also non-planar surfaces for novel device concepts.
Abstract The advancement of superconducting nanowire single-photon detectors (SNSPDs or SSPDs) is intrinsically linked to breakthroughs in superconducting materials engineering. These detectors are prized for their excellent sensitivity and performance across a wide wavelength range, from ultra-violet to mid-infrared (mid-IR). Plasma-enhanced atomic layer deposition (ALD) is an advanced deposition technique that enables exceptional film uniformity across large wafers, making it ideal for a variety of applications. In this study, ALD NbTiN films are fabricated into SNSPDs that exhibit single-photon response across near- to mid-IR wavelengths. A 5 nm thick NbTiN film was patterned into devices consisting of ∼100 nm wide meandered nanowires, and the nanowire cross-section and composition were analysed by transmission electron microscopy. The devices were tested in a closed-cycle cryostat with a base temperature of 0.9 K. Photon detection rate and timing jitter were measured across the wavelength range 1.5–4.4 µ m under low dark count conditions, confirming single-photon sensitivity across the measured wavelength bands. This work introduces the ALD-grown NbTiN films as a new platform for SNSPD detectors, achieving mid-IR single-photon sensitivity. The intact film quality after fabrication demonstrates the potential of ALD processes for scalable infrared detector technologies.
Abstract 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 H 2 /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.
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.
At the present time, ultrahigh performance superconducting nanowire single-photon detectors are the key elements in a variety of devices from biological research to quantum communications and computing. Accurate tuning of superconducting material properties is a powerful resource for fabricating single-photon detectors with desired properties. Here, we report on the major theoretical relations between ultrathin niobium nitride (NbN) film properties and superconducting nanowire single-photon detector characteristics, as well as the dependence of ultrathin NbN film properties on reactive magnetron sputtering recipes. Based on this study, we formulate the exact requirements for ultrathin NbN films for ultrahigh performance superconducting nanowire single-photon detectors. Then, we experimentally studied the properties of ultrathin NbN films (morphology, crystalline structure, critical temperature, and sheet resistance) on silicon, sapphire, silicon dioxide, and silicon nitride substrates sputtered with various recipes. We demonstrate ultrathin NbN films (obtained with more than 100 films deposition) with a wide range of critical temperature from 2.5 to 12.1 K and sheet resistance from 285 to 2000 Ω/sq and report a sheet resistance evolution of more than 40% within two years. Finally, we found out that one should use ultrathin NbN films with a specific critical temperature near 9.5 K and a sheet resistance of about 350 Ω/sq for ultrahigh performance state-of-the-art superconducting nanowire single-photon detectors at 1550 nm wavelength.
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.