M. Maul

Growth of high quality YBa2Cu3O7 films on various substrate materials and influence of Zn-doping on superconductivity

Abstract By DC-sputtering from a single stoichiometric target we prepared high quality YBa2Cu3O7 thin films on SrTiO3, LaGaO3, ZrO2, MgO and NdAlCaO4. All films sputtered on these substrates (except NdAlCaO4) showed narrow inductive transitions (ΔTc 86 K), but only on LaGaO3 the transition was as sharp (width dT c dx = −10.5 K (1% Zn) as observed in bulk samples.

Epitaxial CeO2 buffer layers for YBa2Cu3O7−δ films on sapphire

Epitaxial CeO2 buffer layers and YBa2Cu3O7−δ thin films have been grown in situ on (1102) sapphire by electron beam evaporation. Buffer layers of only 20 nm thickness inhibit interdiffusion between YBa2Cu3O7−δ and Al2O3 as determined by depth profiling using x‐ray photoelectron spectroscopy. The layers show smooth surfaces and narrow interfaces. High lattice perfection of the CeO2 layer has been shown by x‐ray diffraction. Laue oscillations up to ninth order have been observed in thin CeO2 buffer layers on sapphire. We found only one epitaxial orientation with YBa2Cu3O7−δ (001) ∥ CeO2 (001) ∥ Al2O3 (1102) and YBa2Cu3O7−δ [110] ∥ CeO2 [100] ∥ Al2O3 [1120]. YBa2Cu3O7−δ films grown on these buffer layers reveal Tc=88±0.5 K, ρ(300 K)=380 μΩ cm, and jc(77 K, 0 T)=1.3×106 A/cm2.

Carrier gas‐free chemical vapor deposition technique for in situ preparation of high quality YBa2Cu3O7−δ thin films

A new carrier gas‐free metalorganic chemical vapor deposition technique was developed yielding a high deposition rate. The β‐dicetonate precursor compounds were synthesized with reproducible water content which is of great importance for their vapor pressure. With the technique described above c‐axis oriented YBa2Cu3O7−δ thin films were prepared with Tc(R=0)=91.8 K and jc(77 K)=1.3×106 A/cm2 on (100)SrTiO3 substrates and Tc(R=0)=91.1 K and jc(77 K)=3×105 A/cm2 on (100)MgO substrates at B=0 T. The influence of the metal composition on the formation of precipitation was studied by SEM investigations.

Compositional effects in YxBayCuzO7−δ thin films prepared by metalorganic chemical vapor deposition

We report on effects of composition onto superconducting and structural properties of Y‐Ba‐Cu‐O thin films, prepared by a carrier‐gas free metalorganic chemical vapor deposition technique. Thin films with critical temperatures Tc≥90 K, transition widths ΔTc≤1 K, and critical current densities jc (T=77 K, B=0 T)≥106 A/cm2 can be prepared by this method. Here we report on a strong dependence of the Tc on the ratio of Ba/Cu. For Ba/Cu<2 the surfaces show CuO precipitates, increasing in quantity and size with increasing concentration of Cu. An excess of Y leads to oriented inclusions of cubic Y2O3. A deficit of Y and Cu causes an expansion of the c‐lattice parameter. To investigate the influence of Y2O3 and CuO precipitates on the pinning potential, resistive transitions in magnetic fields up to 12 T were measured and interpreted by a thermal activated flux‐creep model. These measurements indicate the inefficiency of both, inclusions and precipitates, on the enhancement of the pinning potential.

Twinning of YBa2Cu3O7 thin films on different substrates and modification by irradiation

Abstract Bragg reflections of YBa 2 Cu 3 O 7 (YBCO) films with c -axis orientation on (001) SrTiO 3 , (001) MgO and (110) NdGaO 3 substrates prepared by MO-CVD, sputtering techniques and laser ablation were examined with Co Kα and synchroton radiation (λ=0.56 A, HASYLAB, Hamburg) using the q -scan technique at four circle diffractometers. One of the specimens was doped with Ni. The evaluation of positions and relative intensities of the splitted reflections shows a strong correlation between the structure of the substrate surface and the twinning geometry of the YBCO layer. Irradiation of the films on SrTiO 3 substrate by heavy ions (1.2 GeV 209 Bi, 1.4 GeV 238 U, UNILAC, GSI Darmstadt) leads to a gradual parallel alignment of a , b s and a , b f (subscripts (s) denoting substrate, and (f) film) which is completed for a dose between 10 11 and 10 12 particles/cm 2 .

Critical current density and upper critical field of YBa2Cu3O7 thin films

Epitaxial YBa2Cu4O7-thin films with thec-axis oriented perpendicular to the film plane were prepared bydc-sputtering from a single stoichiometric target on (100) SrTiO3-substrates. Typical values of the inductively measured superconducting transitions were about 90 K with a width less than 0.5 K. Critical current densities were measured on 5 to 10 μm wide strips as function of magnetic field and temperature. The temperature dependences ofjc follow a universal functionjc(B,T)=jc*(T=0,B)·(1−T/Tc(B))α with α=1.5±0.1. ForB=0 andT=77 K we obtainedjc=4·106 A/cm2. The field dependence of the resistive transitions was measured with the magnetic field parallel to thec-axis. The slope of the upper critical fieldBc2 (T) was determined for different criteria. The carrier concentration evaluated from Hall-effect measurements was found to decrease linearly from one per unit cell at 240 K with decreasing temperature extrapolating nearly through zero forT=0. Highly resolved angular dependent measurements of the critical current density withB perpendicular to the current but tilted from thec-axis show a very strong and sharp enhancement ofjc for the magnetic field parallel to the (CuO2)-layers (B⊥c). Additionally to this phenomenon, which is caused by an intrinsic pinning mechanism due to the layered structure of high-Tc-superconductors the influence of the anisotropy of the upper critical field onjc(B, T, φ) is evident nearTc.

Thin YBa2Cu3O7−δ films by electron‐beam coevaporation— Growth and in situ characterization

We report on the in situ preparation and characterization of thin YBa2Cu3O7−δ films on various substrates. The films were grown by electron‐beam coevaporation of the pure metals Y, Ba, and Cu. A pressure stage is used to obtain high partial O2 pressure close to the substrate and low pressure in the main vacuum system. In situ resistance measurements were performed to investigate growth dynamics during film formation, and temperature dependence during the cool down from T≊1000 K to T≤Tc. The films were analyzed by R(T), ac susceptibility, x‐ray diffraction, and critical transport current measurements.

Thin films of YBa2Cu3O7−δ prepared by a new MO-CVD technique

Abstract We report a new metalorganic chemical vapour-deposition technique (MO-CVD) for the preparation of thin films of YBa 2 Cu 3 O 7−δ (YBCO). The new technique directs the precursors from three individual sources without carrier gases to the substrate. Oxygen is supplied close to the substrate. The films grow epitaxially with high deposition rates (1–10μm/h) on single crystal (100) SrTiO 3 and (100) MgO. At temperatures T =800° C a high degree of c -axis orientation and excellent critical data are obtained.

External modification of twinning in YBCO thin films on different substrates

Abstract Bragg reflections of YBa 2 Cu 3 O 7 (YBCO) films with c-axis orientation on (001) SrTiO 3 , (001) MgO and (110) NdGaO 3 substrates prepared by different techniques (MO-CVD, sputtering and laser ablation) were examined with CoK α- and synchroton radiation ( λ=0.56 A , HASYLAB, Hamburg) using the q-scan technique at four circle diffractometers. The evaluation of positions and relative intensities of the splitted reflections shows a strong correlation between the structure of the substrate surface and the twinning geometry of the YBCO layer. The influence of Fe doping on Cu(1) and changes of the oxygen content on the structure in the YBCO basalplane and the substrate/film orientation is analysed. Irradiation of YBCO films on SrTiO 3 , MgO, NdGaO 3 substrate by heavy ions (1.2GeV 209 Bi, 1.4GeV 238 U, 0.95 (2.5)GeV 197 Au UNILAC, GSI Darmstadt) produce detwinning and leads to a gradual parallel alignment of a , b s and a , b f (subscripts: s:substrate, f:film) which is completed for a dose between 10 11 and 10 12 particles/cm 2 .