Improvement of the critical temperature of superconducting NbTiN and NbN thin films using the AlN buffer layer

Abstract

Thin superconducting NbTiN and NbN films with a few nm thickness are used for various device applications including hot electron bolometer mixers. Such thin films have lower critical temperature (T c) and higher resistivity than corresponding bulk materials. To improve them, we have investigated an effect of the AlN buffer layer between the film and the substrate (quartz or soda lime glass). The AlN film is deposited by DC magnetron sputtering, and the process condition is optimized so as that the X-ray diffraction intensity from the 002 surface of Wurtzite AlN becomes highest. By use of this well-characterized buffer layer, T c and resistivity of the NbTiN film with a few nm thickness are remarkably increased and decreased, respectively, in comparison with those without the buffer layer. More importantly, the AlN buffer layer is found to be effective for NbN. With the AlN buffer layer, T c is increased from 7.3 K to 10.5 K for the 8 nm NbN film. The improvement of T c and resistivity originates Improvement of T c of NbTiN and NbN Thin Films Using the AlN Buffer Layer 2 from the good lattice matching between the 002 surface of AlN and the 111 surface of NbTiN or NbN, which makes better crystallization of the NbTiN or NbN film. This is further confirmed by the X-ray diffraction measurement.

Improvement of the critical temperature of NbTiN films on III-nitride substrates

“…To identify the crystalline orientation of the NbTiN film and its epitaxial relationship with the substrates, structural studies were performed by HRTEM along both [11][12][13][14][15][16][17][18][19][20] and zone axis of the III-nitride substrates. Figure 2 Note that these values are higher than the surface roughness of the substrates (shown in [23], who showed a decrease of Tc for increasing Al mole fraction.…”

Ultrathin NbTiN Films with High Ti Composition for Superconducting Nanowire Single Photon Detectors

“…N IOBIUM titanium nitride (NbTiN), like nitride niobium (NbN), is one of the most widely studied and valued nitride superconductors [1]- [7]. With high superconducting transition temperature (T C ), large superconducting energy gap and low resistivity, it is broadly used in RF cavities for particle accelerators [8], superconductor-insulator-superconductor (SIS) junctions [9]- [11], hot electron bolometer (HEB) [12]- [15] mixers, coplanar waveguide superconducting resonators [16], [17] and superconducting nanowire single photon detectors (SNSPD) [18]- [23].…”

Epitaxial NbN/AlN/NbN tunnel junctions on Si substrates with TiN buffer layers

“…Various thin films such as MgO, AlN, and TiN have been used as the buffer layers for the epitaxial growth of NbN films. [20][21][22][23] Among these, TiN is a promising material to be used as the buffer layer because TiN and NbN have the same crystal structure with a lattice mismatch of only 3.5%. Recently, we fabricated (200)-oriented TiN films on Si (100) substrates using DC magnetron sputtering and showed that TiN films deposited at high temperature exhibit highly crystalline structures and good superconducting properties.…”