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Pulsed-laser deposition of smooth high-Tc superconducting films almost free from droplets and precipitates with the use of velocity filtration of plasma particles is reported. We have removed droplets from laser-induced plasma by using a shutter technique; a reduction of the droplet density by a factor of 105 has been achieved. We have applied the technique to the preparation of high quality YBa2Cu3O7−δ films on (100)-oriented SrTiO3, MgO, Y2O3-stabilized ZrO2 (YSZ) substrates and, furthermore, on (11̄02)-oriented sapphire covered with (100) sublayers of Si and (100) YSZ buffer layers.

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...

We report on broad-band coupling of THz radiation to an YBa 2 Cu 3 O 7−δ superconducting hot-electron bolometer. The bolometer-a micro-bridge of an YBa 2 Cu 3 O 7−δ film with a PrBa 2 Cu 3 O 7−δ protection layer-was built in a hybrid quasioptical antenna system consisting of a planar self-complementary spiral antenna on a dielectric substrate and an extended hyperhemispherical lens. The planar antenna was integrated into a coplanar transmission line for broad-band intermediate-frequency matching. Measurements performed in the direct detection regime between 2.5 THz and 4.2 THz delivered a system coupling efficiency for the radiation in the fundamental Gaussian mode of about 0.1 and a beam width of the radiation pattern of less than one degree. The pattern was consistent with the far-field radiation pattern that we calculated with a ray-tracing technique. At an intermediate frequency of 1.5 GHz we measured an output noise temperature of ≈160 K for the bolometer driven in the resistive state by both dc and THz current and estimated, for the heterodyne regime, a system noise temperature of ≈3 × 10 5 K. We studied the effect of a protection layer on the rate of phonon escape from the micro-bridge and, thus, on the output frequency bandwidth of the device. We discuss possibilities of a significant improvement of the device performance.