We have previously shown3,6 that sputtered f ims of Bi2Sr2CaC1.120, (BSCCO) deposited onto single crystal MgO substrates held at substrate temperatures,T,, near 700OC and slowly cooled in oxygen, form High Temperature Superconducting (HTS) films with T,, between 50 and 70K. The conditions necessary to achieve superconductivity include having close to stoichiometric composition at the T, required to form the superconductor. Both T, and c-axis orientation increase with T, while the relative bismuth content decreases. Bismuth film content is affected by the type and amount of oxidant, total system pressure, target-to-substrate spacing, and substrate bias. We suggest oxygen resputtering of the film may play a role similar to that reported for Y B ~ ~ C U ~ @ ~ (YBCO). Oxygen partial pressure during and cooldown rate from T, to 2350°C also affects the superconducting properties.

A series of off-specular diffuse scattering measurements were performed by scanning the detector parallel to the sample surface during growth of platinum films on polished silicon substrates. During early stage of island growth, we observe a halo of diffuse scattering around the specular reflection which is similar in origin to Henzler's ring (P. Hahn, J. Clabes and M. Henzler, J. Appl. Phys. 51 (1980) 2079). A model of random islands was developed to explain the halo of diffuse scattering. During late growth, the film surface was consistent with a partially self-affine surface.

Using a two-zone thallium vapor transport furnace and a thick film ink process, we have successfully made superconducting films 10 to 50 /spl mu/m thick on a variety of substrates. Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ and Bi/sub 0.22/Sr/sub 1.6/Ba/sub 0.4/Ca/sub 2/Cu/sub 3/O/sub x/ precursor films with and without Ag additions were made by mixing powders in an organic vehicle, painting a substrate and burning the vehicle off. Films were converted to the superconducting phase by passing an O/sub 2/ carrier gas over a Tl/sub 2/O/sub 3/ source and then the sample. TBCCO 1223 films generally form over a narrow sample temperature range near 860/spl deg/C, whereas TBSBCCO films form 1213, 1223 or a mixture of these phases depending on sample annealing temperature and O/sub 2/ partial pressure. TBCCO T/sub c/'s average 104 K with zero-field J/sub c/ (77 K) /spl ap/3500 A/cm/sup 2/ while TBSBCCO T/sub c/'s are higher at 110 K with J/sub c//spl ap/6800 A/cm2. Both compounds show weak-link behavior in a magnetic field. >

We have measured the critical current density (J(C)) of two NbN films (500 angstrom and 1550 angstrom thick) as a function of temperature in magnetic fields up to 25 Tesla using transport measurements. In both films, the functional form of the volume pinning force F-P obeys the Fietz - Webb scaling law throughout the entire magnetic field and temperature range such thatF-P = J(C) x B = alpha.B-C2(m)(T)b(1/2).(1-b)(2) = alpha(1-T/T-C)(m)b(1/2).(1-b)(2)where alpha and alpha* are constants dependent on the film, B-C2(T) is the upper critical field, b = B/B-C2(T) is the reduced magnetic field, T-C is the critical temperature and we find m = 2.7 +/- 0.1. Over a limited range of magnetic fields close to B-C2(T), we can approximate this functional form by:F-P = beta.B-C2(M)(T)b.(1-b)(2) = beta*(1-T/T-C)(M)b.(1-b)(2)where beta and beta* are constants and we find M = 2.6 +/- 0.2. Values of J(c) derived from D.C. magnetisation data obtained using Bean's model show qualitative agreement with the transport measurements throughout the superconducting phase. Despite the marked granularity in the microstructure of these films, we interpret our results as evidence that a flux pinning mechanism determines the transport current density in NbN films in high magnetic fields.

We report values of critical current (Jc) for Tl2Ba2CaCu2Ox(Tl-2212) epitaxial thin films in magnetic fields up to 5 T and at temperatures of 5, 40, and 77 K determined from dc magnetization and using a standard transport technique. Although we verify the applicability of Bean’s model [Rev. Mod. Phys. 36, 31 (1964)], these values only agree at small applied magnetic fields and at low temperatures. Our results indicate that the differences observed in Jc’s determined from the two techniques may be understood by considering only the field-dependent, nonlinear E-J characteristic and the ‘‘effective’’ electric-field criterion used in determining Jc.

Using a specially designed off-axis faced magnetron sputtering chamber we have performed in situ x-ray diffraction studies of the growth of YBa2Cu3Ox films using a synchrotron light source. The orientation and rocking curve width were studied as a function of substrate temperature, O2/Ar partial pressures, and deposition rate. Growth rate was studied on SrTiO3, LaAlO3, and MgO.

The type and partial pressure of oxidant mixed with argon can affect the selective sputtering of Bi in composite-target, magnetron-sputtered Bi–Sr–Ca–Cu–O films. Comparative studies using oxygen and ozone show that ozone is a more potent oxidant, as well as a more potent source of resputterers than is oxygen. Severe resputtering from ozone is significantly reduced by a −40 V potential on the sample block. We suggest that oxygen causes resputtering by forming O+2, which interacts with the target to produce energetic O−. In contrast, ozone may form lower-energy O− by electron impact in the dark space. Negative oxygen ions from the target itself may be responsible for a background resputtering effect. Our results and those found for Y–Ba–Cu–O by others are comparable. Bi in Bi–Sr–Ca–Cu–O behaves as Ba in Y–Ba–Cu–O, with regard to selective resputtering; furthermore, the response of Sr, Ca, and Cu to oxygen in sputtered Bi–Sr–Ca–Cu–O is similar to what is observed for Cu in Y–Ba–Cu–O.

A number of recent theories predict scaling laws for the roughening of growing thin films. We present a powerful new experimental technique for studying the internal morphology of films exhibiting roughening, together with preliminary results first demonstrating some fundamental aspects of these models. Sputtered films of NbN are decorated periodically by AlN layers and examined by cross-section transmission electron microscopy to reveal scaling-law growth and parabolic-capped columns whose diameter and layer roughness (interface width) increase with time (i.e. film thickness).

The critical current density, Jc in granular NbN/AlN multilayer films shows a sharp crossover with reduced magnetic field, b≡H/Hc2 where Hc2 is the upper critical field, from the (1−b)2 dependence associated with bulk flux pinning at large b to a much weaker dependence at smaller b. We interpret this as coexistence of bulk flux depinning with phase slips in intergranular Josephson junctions, such that the one with the smaller Jc is observed. The dependence on b, pinning strength, Lorentz-force and neutron irradiation are all consistent with such a crossover.

ABSTRACTWe performed in-situ x-ray reflectivity measurements to study the growth dynamics of gold sputter deposited onto silicon using an x-ray scattering chamber equipped with a faced magnetron source where the substrate is held at a right angle relative to the sputtering guns. By operating the guns at low power (1 watt) and under 20 mTorr Ar, we could control the gold deposition rate to less than 1Å/sec. The observed x-ray reflectivity for gold deposited onto a silicon substrate at 300 K and 400 K is consistent with nucleated island growth for average gold particle sizes less than 50 Å. Above 50 Å, the reflectivity data indicates that the gold film uniformly covers the silicon surface, and that as the film thickness is increased the gold-vacuum interface gets progressively rougher. Detailed analysis of room temperature data is in progress, as is the temperature dependence on the roughness of the gold vacuum interface.

Four types of differently prepared NbN films were irradiated at 4.6 K with fast neutrons to a fluence of 5.3 × 1022 m−2 (E≳0.1 MeV). The critical current densities Jc were measured in magnetic fields up to 23 T prior to irradiation, following low-temperature irradiation, and again after an annealing cycle to room temperature. In all films, Jc was found to be completely unchanged by the radiation and annealing treatments in fields up to 15 T, but to increase at higher magnetic fields. At the same time, the upper critical fields Bc2 increased by about 0.5 T (∼2%). Replotting Jc versus reduced field B/Bc2 leads to identical field dependencies also in the high-field range. Hence, the observed increase of Jc is equantitatively explained as a Bc2 effect.

We present studies using a dc magnetron in an on-axis configuration to sputter Bi-Sr-Ca-Cu-O films from a composite target. These studies show that bismuth can be preferentially resputtered. The influence of ozone, molecular oxygen, and total pressure on the resputtering of bismuth is investigated and discussed. Ozone, in low concentrations, can dramatically affect the degree of resputtering. By comparing the effects of molecular oxygen and ozone, some insight is gained regarding the possible mechanisms of negative ion formation in the magnetron environment. Based on our results we suggest that molecular oxygen can bring about resputtering primarily by forming O2+, which collides with the target to produce energetic negative oxygen ions. In contrast, ozone may form negative ions by electron impact in the dark space above the target, giving rise to lower-energy negative ions, which can traverse the plasma unneutralized and can be stopped with an applied bias on the sample block. With no added oxidant, negative oxygen ions from the target oxygen may dominate the background resputtering. Similarity is found between our results and those for similar studies on Y-Ba-Cu-O by other workers. Bismuth in Bi-Sr-Ca-Cu-O behaves as barium in Y-Ba-Cu-O with regards to preferential resputtering; furthermore, the response of strontium, calcium, and copper to oxygen in sputtered Bi-Sr-Ca-Cu-O is similar to what is observed for copper in Y-Ba-Cu-O.

The conditions for deposition of Cu and Cu-oxide thin films by reactive dc sputtering have been investigated by characterizing the crystal structure of growing films both during and immediately following deposition using an in situ x-ray diffraction technique. The relationship between sputtering conditions and the phases deposited was established for a variety of conditions. At each temperature studied, increasing the oxygen pressure in the system resulted in a systematic change in the phases deposited. Of significant importance was the identification of temperature-pressure regimes in which CuO was stable during deposition but reverted to Cu2O after the sputtering plasma was extinguished, suggesting a shift in the oxidizing potential in the plasma environment. These results also suggest that the in situ analysis technique may be ideally suited for the investigation of phase relationships and phase diagrams in other systems.

The evolution of YBCO film growth with thickness at various oxygen pressures was observed by in situ synchrotron x-ray diffraction in real time. When the films were deposited at 2 Å/s and 730 °C under higher oxygen partial pressures (in an Ar/O2 mixture of 90 mTorr), the nucleation was observed to have c-axis orientation. After the films reached a critical thickness, the growth of the YBCO film changed from c axis to a axis and then propagated epitaxially. This provides evidence that a-axis epitaxial growth nucleates on a c-axis base. The critical thickness reflects the competition between the growth of the c and a axes, which is determined by the oxygen partial pressure in the process of thin-film formation. The a-axis oriented films showed a very sharp rocking curve (less than 0.1°) which indicates a very high structural quality. For very low oxygen partial pressures, the in situ growth process was very similar, but the initial nuclei involve a second phase mixed with a small amount of c-axis ‘‘123’’ phase. The nucleation and growth mechanisms of in situ YBCO films are discussed.

The authors report on the in-situ formation of superconducting films of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/ and Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ using composite-target sputtering with substrate temperatures, T/sub s/, significantly less than those required for postannealed films. Specifically, superconducting films of BSCCO 2212 and 2223 can be sputtered onto single-crystal MgO substrates by an in-situ processing technique with T/sub czero/'s as high as 64 K and a very pronounced c-axis orientation without the need for epitaxy. Bi loss in the films can be reduced by biasing the substrate negative or increasing the total system pressure. This should provide greater control over film properties and the possibility of sputter deposition at higher substrate temperatures to see if T/sub czero/'s close to bulk can be achieved. The microstructure generally reveals a smooth matrix, with submicron to micron-sized particles protruding from it. These particles increase in size and frequency with increasing T/sub s/ and appear to be a Bi-Sr-Ca-oxide mixture. It is suggested that oxygen resputtering of the film may play a role similar to that reported for YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO).

For H‖c-axis, the magnetic field induced broadening of the resistive transitions of high-Tc superconductors (HTS) is shown to depend strongly on the Cu-O layer spacing. For the highly anisotropic HTS, we show experimental evidence that flux motion results from a thermally activated crossover from three dimensional (3D) vortex lines to 2D independent pancake-like vortices in the Cu-O layers, which is intrinsic to the material and occurs when kBT exceeds the Josephson coupling energy of these layers. At low temperatures, however, thermally activated conventional depinning (which can be sample dependent) or melting in the uncoupled 2D Cu-O layers is also required for flux motion. For YBa2Cu3O7, this dimensional crossover does not occur belowHc2, presumably because the conducting Cu-O chains short-circuit the Josephson interlayer coupling, leading to better superconducting properties in a magnetic field. These results show that strong interlayer coupling is a key to finding good alternatives.

The absence of a Lorentz force dependence on dissipation in the highly anisotropic high-temperature superconductor, Tl2Ba2CaCu2O(x), has been measured over a wide range of current densities in broadened resistive transitions, current-voltage characteristics, magnetoresistances, and critical current densities, J(c). The magnetoresistances are very useful to find out the correct temperature and field dependences of the activation energy. As an alternative to flux motion, we consider a Josephson-coupling model which is consistent with the broadened resistive transitions and the lack of Lorentz-force dependence. We found that the Josephson-coupling model agrees with the temperature dependences of the activation energy and J(c) and is better matched to the weak field dependence of J(c) than the flux creep model. Possible origins of Josephson junctions in high-quality films and single crystals are discussed.

Magnetoconductance (MC) has been measured above the mean-field transition temperature T(c) in the highly anisotropic high-temperature superconductor Tl2Ba2CaCu2O(x) for fields parallel and perpendicular to the CuO plane. Recent theoretical expressions for the magnetoconductance in the layered superconductors provide a good fit for the temperature range T(c) + 10 K < T < 160 K. Using the additional constraint of the zero-field fluctuation conductivity, we obtain the in-plane coherence length xi-ab(0) almost-equal-to 11.8 +/- 0.4 angstrom and a linear temperature dependence for the phase relaxation rate, with 1/tau-phi-almost-equal-to (3.5 +/- 0.5) X 10(13) sec-1 at T(c). For T(c) < T < T(c) + 10 K, we found a significant enhancement of the negative MC compared to the prediction based on the Gaussian approximation. A possible origin of such deviation is discussed.

NbN films have been exposed to reactor neutron irradiation at 4.6 K to a fluence of 5.3×1022 m-2 (E.>0.1. MeV). The critical current densities were measured at 4.2 K in magnetic fields up to 23 T prior to irradiation, after 5 K irradiation following a transfer at 77 K and again after an annealing cycle to room temperature. The results show an identical field dependence of JC up to 15 T in all three cases and radiation-induced enhancements at higher fields. The latter is explained quantitatively by the observed (small) increase of the upper critical fields HC2.