Preparation, Optimization And Characterization Of Niobium-based Superconducting Ultrathin Films

Superconducting hot electron bolometer (HEB) and superconducting nanowire single-photon detector (SNSPD) are both based on hot electron effect of superconducting nanowires and respectively used for terahertz(THz) wave and infrared to visible light detection. Superconducting ultrathin films are critical for device performance of these high sensitivity superconducting hot electron detectors, which are already widely used in radio astronomy, remote sensing, imaging, quantum communication and so on.This article describes the preparation, optimization and characterization of several niobium-based superconducting ultrathin films, which are utilized for HEB and SNSPD. We have fabricated high performance3-10nm thick NbN, NbTiN, NbTi and doped Nb (Nb*) films on MgO, Si, SiOx/Si and sapphire substrates with magnetron sputtering systems for improvement of device performance and practical applications. The films’surface and interface morphology, chemistry component and structural characterization have been studied by atomic force microscopy (AFM), X-ray photo electronic spectroscopy (XPS), Auger electron spectroscopy (AES), transmission electron microscopy (TEM) and X-ray diffraction (XRD) and provide guidance to improve properties of the films. These high performance films are already utilized for fabrications of HEB and SNSPD devices. Specifically, this study has yielded the following results:1. It is found that the superconducting properties of NbN film on Si will be significantly improved by an Nb5N6buffer layer. With30nm thick Nb5N6, the zero resistance superconducting transition temperature (Tco) of a6nm thick NbN film on Si is up to13.5K and the critical current density (Jc) of the film is more than107A/cm2. This is the best reported result and lays the foundation for the further performance improvement in HEB and SNSPD.2. It is studied by TEM and XRD that the Nb5N6buffer layer plays a role not only in reducing the lattice mismatch between the NbN film and Si substrate, but also in strain redistribution. This can account for the improvement of the films’ superconducting properties.3. A method for preparing superconducting thin films on thickness controllable and flexible substrates has been developed, which is spin-coating polyimide on Si substrates, sputtering NbN films and peeling-off substrates. The superconducting NbN film with flexible polyimide substrate has been studied by various means. TC0of this10nm thick film is8.3K and TC0of30nm thick film in9tesla magnetic field is still7K. This film can be utilized for multilayer terahertz metamaterial device, magnetic cloak and magnetic shielding.4. An alloy with47wt.%Ti and53wt.%Nb as the target material is employed. Using DC magnetron sputtering, NbTiN thin films with thicknesses ranging from5nm to20nm are grown onto MgO(100), Si (100) and SiOx/Si substrates. The TCo of about8.7K and Jc of about7.5x106A/cm2at4.2K have been obtained for the NbTiN film of8nm thick on MgO. With more Ti content, the lattice mismatch and film quality have been further improved. It is possible for preparing high-speed, low dark count SNSPD based on this film.5. Using a doped sputtering target, high performance Nb*ultrathin films are grown on high resistivity silicon (Si), MgO, and sapphire substrates, optimized by grown1-nm-thick aluminum nitride (AlN) films on the top. TC0of about7.5K and JC of about8.2X106A/cm2at4.2K have been obtained for the Nb film of6.5nm thickness on MgO substrates. The superconducting characteristics of this film are much higher than those of pure Nb film of same thickness.6. Based on NbN and Nb*films on Si substrates, superconducting HEB devices are fabricated, and resistance-temperature (R-T) curves and current-voltage (I-V) curves are studied. The results of THz response of the device are presented.