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

• 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 materials are the fundamental building block for a wide variety of devices such as Josephson junctions, magnetic field probes, and electromagnetic radiation detectors. Moreover, they form a platform for quantum computing as well as neuromorphic circuit architectures. To utilize the full potential of superconducting thin films and take advantage of their versatile functionalities, fabrication processes suitable for integration on different platforms are required. For instance, superconducting strip photon detectors1 (SSPDs; nomenclature according to the International Standard IEC is used,2 whereas in the literature, these devices are also referred to as superconducting nanowire single-photon detectors) have been demonstrated with different thin film systems on multiple substrate materials and have evolved into the leading technology for single-photon detection.3,4 They offer a wide wavelength sensitivity range,5 high detection efficiency, low dark count rate, and high time resolution6–9 and can be integrated on waveguides in photonic integrated circuits.10 However, integration of SSPDs is often complicated by application-specific restrictions and dedicated growth processes using high temperatures or intermediate buffer layers. While amorphous materials such as WSi are associated with high detection efficiencies and a forgiving fabrication process resulting in a good detector fabrication yield,11 it is challenging to achieve low timing jitter12 and detector operation typically requires sub-Kelvin temperatures. On the other hand, the nitride-based superconductors NbN and NbTiN excel in time resolution but are less forgiving in terms of fabrication yield due to their nanocrystalline structure, often requiring deposition at elevated temperatures.In this Letter, we show the integration of NbTiN-based SSPD devices on photonic and monolayer two-dimensional materials using a universal reactive co-sputtering process at room temperature. Six substrate materials were studied: silicon dioxide (SiO2), silicon nitride (Si3N4), gallium arsenide (GaAs), lithium niobate (LiNbO3), aluminum nitride (AlN), and hexagonal boron nitride (hBN). SiO2 is commonly used for the fabrication of free-space or fiber-coupled SSPDs due to the refractive index difference between SiO2 and the Si substrate underneath forming a weak optical cavity.13 Si3N4 is a CMOS-compatible material that offers a wide transparency window from the visible to the mid-infrared and is suitable for efficient photonic waveguiding. SSPDs can be integrated either before Si3N4 growth as embedded detectors14 or on top of the photonic circuit.15 AlN is used as a piezo-electric material, for instance, in resonators, transducers, and actuators. Superconducting detectors were also fabricated using a pick and place technique16 and by NbN deposition at high temperatures.17 LiNbO3 as an optically non-linear material with a large transparency window and electro-optical properties allows for second-harmonic generation and electro-optic modulation. SSPDs were demonstrated on planar substrates,18,19 whereas superconducting transition-edge sensors were realized on titanium in-diffused waveguides.20 GaAs is a common photonic platform that also allows for the fabrication of quantum dot-based non-classical light sources. The integration of NbN SSPDs requires precise control of deposition temperature to preserve the substrate integrity21–23 or the use of a buffer layer.24 Finally, two-dimensional crystals and van der Waals heterostructures have emerged as optoelectronic platforms with unique characteristics.25 hBN, in particular, is an important building block that is used as a dielectric, for passivation or for its optical properties in the ultraviolet range.26 However, SSPDs realized on two-dimensional crystals as substrate material have remained unexplored so far.We realized NbTiN thin films by reactive co-sputtering from separate Nb and Ti targets at room temperature. We developed a universal recipe for the deposition of 10 nm NbTiN on all six material platforms without substrate-dependent adaptation. The deposition rate and nominal film thickness were monitored in situ using a rate monitor calibrated for SiO2/Si substrates (uncertainty 5%). The magnetron sources were operated at a DC bias of 120 W and a RF bias of 240 W for Nb and Ti, respectively, using an Ar/N2 ratio of 10 and a sputtering pressure of 3 mTorr. These deposition conditions result in polycrystalline films with a Nb/Ti ratio around 60% suitable for high-efficiency SSPDs with a sub-20 ps timing jitter, as reported previously for SiO2/Si substrates.27 The following samples were used: thin film SiO2 on Si (150 nm thermal oxide), thin film Si3N4 on SiO2/Si (250 nm low pressure chemical vapor deposition; Rogue Valley Microdevices), bulk GaAs wafer (Wafer Technology); bulk LiNbO3 wafer (x-cut; CasTech), thin film AlN on Si (200 nm plasma vapor deposition; Kyma Technologies), and monolayer hBN on SiO2/Si (chemical vapor deposition growth and PMMA transfer, oxide thickness 285 nm; Graphene Supermarket). Measurements of NbTiN step heights by atomic force microscopy in tapping mode suggested well-comparable thicknesses for films on SiO2 compared to Si3N4, GaAs, LiNbO3, and AlN with relative differences below 4% (hBN was excluded from the step height analysis due to surface irregularities resulting from the transfer process). Furthermore, the surface morphology of all substrates was assessed (Fig. 1), characterizing areas with NbTiN as well as bare substrate areas covered during the deposition. The root mean square surface roughness Rq was extracted and is summarized for all the cases in Table I. Sputtering of 10 nm NbTiN at room temperature had a negligible influence on the surface roughness, confirming the homogeneity of film deposition. Low Rq values of 0.3–0.6 nm were found for SiO2, Si3N4, and GaAs, whereas larger values were measured for LiNbO3 (0.9 nm), hBN (1.0 nm), and AlN (1.2 nm). In the latter case of AlN, the surface roughness was determined by its distinct grain morphology [Fig. 1(e)]. Note that for the monolayer hBN substrate, circular surface irregularities were present, which were excluded from the roughness analysis.

Ämnesord

NATURVETENSKAP  -- Fysik -- Den kondenserade materiens fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Condensed Matter Physics (hsv//eng)

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