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High-temperature growth of ultra thin NbTiN films on lithium niobate for integrated single photon detection

Sjoerd Telkamp, Odiel Hooybergs, Myriam Rihani, Giovanni Finco, Tummas Napoleon Arge, Robin N. Dürr, Victor Mougel, Scheffler, Daniel, Filip Křížek, Rachel Grange, Robert J. Chapman, W. Wegscheider

Published Sep 9, 2025• 0 citations
Publisher

Abstract

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.

DOI
10.3929/ethz-c-000792756

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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.
We demonstrate an amorphous molybdenum–rhenium (MoRe) film on a thin-film lithium niobate substrate as an efficient superconducting nanowire single-photon detector (SNSPD) operating at T = 2.5 K. MoRe films were deposited by DC magnetron sputtering with a composite target of 56 at. % Mo and 44 at. % Re on lithium niobate on insulator substrates at room temperature. We investigated the spectral and temporal performance of MoRe SNSPDs as well as the detection regimes of one and multiple photons. The fabricated detector with a nanowire width of about 150 nm shows an internal detection efficiency of approximately 98% and 73.5% for the most crucial wavelengths for integration on the chip, 780 and 1550 nm, respectively. Our results show great potential for the use of MoRe for scalable and cryogenic tunable quantum photonic integrated circuits.
Niobium nitride (NbN) is widely used in the production of superconducting nanowire single-photon detectors (SNSPDs) due to its high superconducting transition temperature and suitable energy gap. The processing parameters used for the preparation of NbN films and the subsequent processing of nanowires have a significant effect on the performance of the SNSPD. In this review, we will present various thin film growth methods, including magnetron sputtering, atomic layer deposition (ALD), and chemical vapor deposition (CVD). The relationships between the superconducting performance of each thin film and the corresponding deposition process will be discussed. Subsequently, NbN nanowire fabrication methods and microstructures based on thin film etching will be summarized, and their impact on the qualities of the finished SNSPDs will be systematically analyzed. Finally, we will provide an outlook for the future development of preparation for SNSPD.
Lithium Niobate On Insulator (LNOI) has recently emerged as a promising platform for the realization of electro-optically reconfigurable waveguides combining small footprint and ultra-low propagation loss. Integration of these components with superconducting nanowire single photon detectors (SNSPDs) opens up the possibility to perform on-chip manipulation and detection of quantum states of light in a physically stable and scalable architecture. Here we report recent theoretical and experimental progress towards the realization of fast electro-optic modulators compatible with cryogenic temperatures and high-performance NbTiN superconducting nanowires single-photon detectors (SNSPDs) monolithically integrated in LNOI waveguides.
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.