Superconducting NbN thin films on various (X/Y/Z-cut) lithium niobate substrates

Abstract

Lithium niobate (LN) exhibits outstanding properties in various application of photonics, electronics, and optoelectronics, showing potentials in integration. Due to the directional dependence of LN tensor properties, optical elements made up by LN favor the type of LN substrate. To introduce high-performance superconducting nanowire single-photon detectors to LN-integrated photonics chips, superconducting NbN thin films with thicknesses from 3 to 50 nm were deposited on X-cut, Y-cut, and Z-cut LN substrates using magnetron sputtering at room temperature. The different thickness dependencies of Tc, δTc, and residual resistance ratios are observed in NbN thin films on different LN substrates. NbN thin films on X-cut and Y-cut LN substrates are polycrystalline with a transition temperature (Tc) of ~6 K for a 6-nm-thick film. While NbN thin films are epitaxially textured on Z-cut LN substrates with Tc of 11.5 K for a 6-nm-thick film. NbN-SNSPD on X-cut LN substrates shows a weak saturation trend of its system detection efficiency; however, the performance of NbN-SNSPD on Z-cut LN substrates is limited. We evaluated the selection of cuts and concluded that X-cut and Y-cut LN are more suitable to be a platform of integrated LN photonic chips from the aspect of NbN-SNSPD. This study helps fabricate high-performance SNSPDs on fully integrated photonics chips on LN substrates.

Superconducting nanowire single photon detector on 4H-SiC substrates with saturated quantum efficiency

“…Based on our research on NbN thin film on LN substrate, epitaxial NbN thin films contribute less to the high performance of SNSPD than polycrystalline NbN thin films. 41 The results of our device are consistent with this conclusion. Here, the device performance is more similar to and comparable with the results of NbN-SNSPDs on Si substrates, indicating high device detection efficiency of on-chip waveguide integrated NbN-SNSPD on 4H-SiC photonics chips once the evanescent field absorption efficiency be enhanced.…”

“…Superconducting nanowire single photon detectors (SNSPDs), normally based on silicon substrates, offer high detection efficiency, 34,35 low dark count rate, low timing jitter, 36 and wide bandwidth from x-ray 37 to mid-infrared range. 38,39 Currently, SNSPDs can be integrated on various substrate materials, such as X-cut lithium niobate (LN), 40,41 silicon, 42 and silicon nitride 43,44 with intrinsic detection efficiency near 100% at 1550 nm and shorter wavelength. On silicon carbides, SNSPDs have been fabricated on 3C-SiC.…”

“…Previous research suggests that polycrystalline thin films are more likely to induce better performance in SNSPDs than epitaxial films. 41 In this research, the natural oxide layer of 4H-SiC was retained to prevent NbN thin films from epitaxial growth on 4H-SiC substrates. Following the popular fabrication processes of NbN-SNSPD on the Si substrate, we obtained an NbN-SNSPD on 4H-SiC with broadband saturated detection.…”

“…Figure 3(c) compares the dependence of T c values on NbN film thickness on various substrates, including MgO, X-cut, Z-cut, and 4H-SiC substrates. 41 Previous research has shown that NbN thin films are single crystalline, epitaxial with dense twins, and polycrystalline on MgO, Z-cut LN, and X-cut LN substrates, respectively. 41,47 The T c -thickness dependence of NbN thin films on 4H-SiC substrates is more similar to that of polycrystalline NbN thin films, which were grown on Si and X-cut LN substrates, meanwhile different from those of epitaxial NbN thin films on MgO and Z-cut LN substrates.…”