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Applied Physics Letters

NbTiN thin films for superconducting photon detectors on photonic and two-dimensional materials

Stephan Steinhauer, Lily Yang, Samuel Gyger, Thomas Lettner, Carlos Errando-Herranz, Klaus D. Jöns, Mohammad Amin Baghban, Katia Gallo, Julien Zichi, Val Zwiller

Published Apr 27, 2020• 43 citations
Publisher

Abstract

Integration of superconducting devices on photonic platforms opens up a wide range of functionalities and applications. We report on NbTiN thin films deposited on SiO2, Si3N4, GaAs, LiNbO3, and AlN as well as on a monolayer of hexagonal boron nitride, using a universal reactive co-sputtering recipe. The morphology and the superconducting properties of the NbTiN thin films with a thickness of 10 nm were characterized by atomic force microscopy and electrical transport measurements. Superconducting strip photon detectors were fabricated using a design suitable for waveguide integration and compared in terms of their internal quantum efficiency and detection pulse kinetics. Our results show well-comparable performances for detectors integrated on different platforms, while also demonstrating that reactive co-sputter deposition of NbTiN at room temperature provides a robust method for realizing superconducting devices on various materials.

DOI
10.1063/1.5143986

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Integration of superconducting devices on photonic platforms opens up a wide range of functionalities and applications. We report on NbTiN thin films deposited on SiO2, Si3N4, GaAs, LiNbO3, and AlN as well as on a monolayer of hexagonal boron nitride, using a universal reactive co-sputtering recipe. The morphology and the superconducting properties of the NbTiN thin films with a thickness of 10 nm were characterized by atomic force microscopy and electrical transport measurements. Superconducting strip photon detectors were fabricated using a design suitable for waveguide integration and compared in terms of their internal quantum efficiency and detection pulse kinetics. Our results show well-comparable performances for detectors integrated on different platforms, while also demonstrating that reactive co-sputter deposition of NbTiN at room temperature provides a robust method for realizing superconducting devices on various materials.
Superconducting single-photon detectors (SSPDs) have developed into a mature device technology and excel due outstanding performance metrics, in particular high detection efficiency combined with high time resolution and low dark count rate for a wide wavelength range from the visible to the mid-infrared. In addition to commercially available systems with devices coupled to optical fibers, SSPDs can be integrated with photonic circuits using scalable nanofabrication technologies. Here, we will present recent progress on SSPDs based on NbTiN thin films and their integration on different photonic material platforms. Our process for NbTiN growth at room temperature will be described, using magnetron reactive co-sputtering to achieve high-quality superconducting layers down to thicknesses of few nanometres. Optimized SSPD devices are realized by tuning the superconducting properties of NbTiN thin films, adjusting the material composition and nanocrystalline structure. The realization of different types of detectors and geometries will be shown, including nanofabrication techniques for achieving fully suspended nanowire structures. Furthermore, we will discuss challenges and prospects for scaling-up SSPD device technology as well as detector systems. Multiplexing schemes such as dispersion engineering of superconducting transmission lines will be highlighted as powerful approach to address multiple detectors and reduce the number of required feedthroughs and electrical lines in the cryostat. Eventually, exemplary applications of SSPDs for photon counting in quantum optics and light detection and ranging (LIDAR) will be outlined.
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.
Lithium niobate-on-insulator (LNOI) is an emerging photonic platform with high potential for scalable quantum information processing due to its strong second-order nonlinearity. However, little progress has been made in developing on-chip single-photon detectors on LNOI. Niobium tita nium nitride (NbTiN) superconducting nanowire single-photon detectors (SNSPDs) are a promising candidate for this application. In this work, we use DC reactive magnetron sputtering to grow high quality NbTiN thin films using an ultra-high vacuum deposition system with a base pressure lower than 2×10‾¹⁰ mbar. Enabled by the low concentration of background impurities in this system, we investigate the impact of substrate temperature during NbTiN growth. We achieve four nm thick superconducting films with a critical temperature (Tc) of 12.3 K grown at a substrate temperature of 825 K. We find that the NbTiN films grow in the (111) orientation and evolve from a porous pillar structure when grown at low temperatures to densely packed fibrous grains at higher temper atures. Furthermore, we demonstrate that the increased substrate temperature reduces the oxygen concentration in our films and improves the overall stoichiometry. In addition, we integrate these films with the LNOI platform and investigate the obtained interface quality. Lastly, we fabricate SNSPDs from the NbTiN film on LNOI and characterize the detector performance.
Lithium niobate-on-insulator (LNOI) is an emerging photonic platform with high potential for scalable quantum information processing due to its strong second-order nonlinearity. However, little progress has been made in developing on-chip single-photon detectors on LNOI. Niobium titanium nitride (NbTiN) superconducting nanowire single-photon detectors (SNSPDs) are a promising candidate for this application. In this work, we use DC reactive magnetron sputtering to grow high-quality NbTiN thin films using an ultrahigh vacuum deposition system with a base pressure lower than 2 × 10 − 10 mbar. Enabled by the low concentration of background impurities in this system, we investigate the impact of substrate temperature during NbTiN growth. We achieve four nm thick superconducting films with a critical temperature ( T c ) of 12.3 K grown at a substrate temperature of 825 K. We find that the NbTiN films grow in the (111) orientation and evolve from a porous pillar structure when grown at low temperatures to densely packed fibrous grains at higher temperatures. Furthermore, we demonstrate that the increased substrate temperature reduces the oxygen concentration in our films and improves the overall stoichiometry. In addition, we integrate these films with the LNOI platform and investigate the obtained interface quality. Lastly, we fabricate SNSPDs from the NbTiN film on LNOI and characterize the detector performance.
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.
Abstract In this paper, we investigate the superconducting critical temperature, morphology and structural properties of NbN ultra-thin films (5–7 nm) deposited by reactive sputtering on top of silicon nitride (SiN) over 200 mm silicon wafers. We demonstrate the effectiveness of a 10 nm thick spacer layer of AlN in raising the superconducting critical temperature from 5.0 K to 7.2 K and from 7.3 K to 9.3 K for 5 nm and 7 nm NbN thickness respectively. By combining investigations with x-ray diffraction and transmission electron microscopy, we highlight significant modifications of the crystalline properties of the NbN film on top of the AlN spacer layer. When directly deposited on SiN, the grains of the NbN polycrystalline layer are randomly oriented. In contrast, when deposited on top of the AlN (0001) spacer layer, the NbN layer is textured along the (111) direction, leading to increased critical temperatures. As AlN is a CMOS-compatible material, these findings are particularly relevant in view of the future integration of high-performance superconducting nanowire single photon detectors on large-scale quantum photonic chips.