Superconducting transition metal nitride films for THz SIS mixers

Abstract

Abstract-The development of sensitive THz SIS mixers requires iI low-loss supercodacting sttipline material with a transition temlwmtnre above 15 K. In thk puper we exuinine the propeilies of (Nb;ri)N, Nblv, and (Nb,Zr)X thin films sputtered at ambient substrate binperature (in glas wafers. The best properties are observed for the (Nb,'L'i)N films, spntteiwl fi3m an h%Ti alloy target with 30 at. ' YO Ti. A sinfir picture is observed for the epitaxial fdins depositwl on (lie MgO wafers. We have also examined the homogeneity of the (Nb,Ti)N films veniis tIlm thickness und in plane since this factor is clearly itnportant for the nicro-wave behavior of the stripline. We observe that (NbJQN finis deposited on silicon, sappliire, und ghss wafen have milch worse homogeneity compnml to the films deposited on the MgO wafets of BI type superconductor. For this application we need reproducible films with the lowest possible resistivity, the highest possible transition temperature (TJ, and intrinsic stress that does not cause the peeling of the f i l m layeis or photoresist Different authors have reported a broad range of soiiwtiines coiitradictory iesults regarding the proprlies of hose f i i s . These considerations lead us to the conclusion that a systematic experiniental study is needed. properties. we have used fits11 targets for all expeiimeiits. The depth 01' erosion mcks is less than 10 ' XI of the 6 nim thick targets. In order to niaximize film uniformity, the substrate-target distance is set to tlie niaxitiiuni for our sputtering system 8 cni. AU films are sputtered with 300 W dc power, resulling in a deposition rate ol' 80340 indinin. l i e substrates are fised to a copper chuck with diflbion punip oil, which is niaintaiiied al 10" C to stabilk& the tliemiodyimics of govvth. Since our sputtering source is located above the substiate holder no clamping is used additioimlly. Tlie mount of oil used between the substrate and the chuck is carefully monitored to avoid possible contamination of the growing lih. Our SIS devices are being routinely produced in this way w i t h excellent reproducibility. The stress in the films is evaluated by measuring the deflection of h e wfer before and after film deposition with a profiloiiieter. The stress value is calcrilated with the help of Stoney's equalion 191, using a Young's modulus and Poisson's ratio for the glass wakrs Uhen h m [IO]. T, is evaluated from the dependence of film resistivity 011 temperature. The width oftlie transition to the sliperconducting stale is studied by measuring the ac magnetic iield susceptibility versus teniperature I 1 I 1. Film resistivity is measured at 20 K, since this parameter is representative of rf-losses in a niiuowave strip-line and also more informative from the perspecbve of the concentration of the quenched-in crystal point defects. RRR value is measured as a ratio of the filii1 resistivities at room temperature and 20 K. X-Ray diffraction (XRD) data is collected using a Rigdm D:"ax-Rc diifiactorneter equipped with a thin-film difiacto...

Electron/phonon coupling in group-IV transition-metal and rare-earth nitrides

“…One of the earliest conventional superconductors discovered to have a relatively high transition temperature (T c ) is the group-V NaCl-structure transition-metal (TM) nitride NbN with T c ¼ 16 K. 1 Since then, conventional superconductors have found a broad range of device applications including microresonator detectors, 2,3 waveguide resonators, 4 accelerator cavities, 5 coil magnets, 6 qubit computing, 7 Josephson junctions in quantum interference devices, 8 hot electron bolometers, 9 single-photon detectors, 10 and THz mixers. 11 Recently, there has been increased interest in group-IV TM nitrides. [12][13][14][15] Here, we present a systematic investigation of the transport Eliashberg coupling function a tr 2 F( hx), an energy-resolved measure of the electron/phonon coupling strength a tr 2 ( hx) and the phonon density-of-states F( hx), for the epitaxial TM nitrides TiN, ZrN, and HfN, as well as the group-IV rare-earth (RE) nitride CeN.…”

NbTiN/SiO/sub 2//Al tuning circuits for low-noise 1 THz SIS mixers

“…The source of this discrepancy needs to be confirmed. However, one likely explanation is the nonhomogeneity in the electrical properties of NbTiN films deposited at room temperature [25]. (Grains with particularly low values of T, may introduce rf losses below the Fgap of the bulk film, but may not be seen in a resistive measurement of T,.)…”

Texture-related roughness of (Nb,Ti)N sputter-deposited films

“…Many researchers have observed strong dependence of texture and film roughness on growth conditions for this class of materials [3]- [9]. However, only a few manuscripts are dedicated to a detailed analysis of texture and/or roughness formation.…”

“…T HE INTEREST in the analysis of texture formation in (Nb,Ti)N films is motivated by a need to develop a reliable production line for THz SIS mixers [1]- [3]. Many researchers have observed strong dependence of texture and film roughness on growth conditions for this class of materials [3]- [9].…”