Investigation of Single-Crystal Niobium for Microwave Kinetic Inductance Detectors
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The Advanced Technology Centre (ATC) of National Astronomical Observatory of Japan is developing microwave kinetic inductance detectors (MKIDs) for large-array pixel cameras for millimeter and sub-millimeter astronomy. We investigated single-crystal Nb thin layers to form superconducting microresonators. We compared the performances of MKIDs based on crystalline Nb structure and those based on polycrystalline Nb. We carried out the entire manufacture of the detectors in the ATC clean room. DC magnetron sputtering is used to grow single-crystal Nb films on r-plane sapphire substrates at an elevated temperature of 800 °C. The residual resistivity ratio (RRR) measured on these single-crystal Nb layers reached values ranging from 40 to 80. We made MKIDs with this crystalline Nb layer, and we measured internal quality factors of the detectors up to 106. The measurement of the noise power spectral density of these MKIDs gave a low value of − 95 dBc/Hz from 100 Hz to 100 kHz. The internal quality factor Qi and the fractional resonance frequency change δfr/fr of MKIDs with respect to the temperature variation are usually following the extended Mattis–Bardeen equations. However, we noticed a deviation from the theoretical prediction for temperature lower than 1 K (in our case). This deviation has already been observed on Al MKIDs and explained by a theory taking into account the Kondo effect and the kinetic inductance contribution. We demonstrated that our measurements on single-crystal Nb MKIDs are also in agreement with the same theory.