L. Nazar

High‐temperature superconductivity in ultrathin films of Y1Ba2Cu3O7−x

We have grown ultrathin films of Y1Ba2Cu3O7−x in situ on (001) SrTiO3 by pulsed laser deposition. The zero resistance transition temperature (Tc0) is >90 K for films >300 A thick. The critical current density (Jc at 77 K) is 0.8×106 A/cm2 for a 300 A film and 4–5×106 A/cm2 for a 1000 A film. The Tc0 and Jc deteriorate rapidly below 300 A, reaching values of 82 K and 300 A/cm2 at 77 K, respectively, for a 100 A film. Films only 50 A thick exhibit metallic behavior and possible evidence of superconductivity without showing zero resistance to 10 K. These results are understood on the basis of the defects formed at the film‐substrate interface, the density of which rapidly decreases over a thickness of 100 A. We have studied these defects by ion channeling measurements and cross‐section transmission electron microscopy. Our results suggest that the superconducting transport in these films is likely to be two dimensional in nature, consistent with the short coherence length along the c axis of the crystals.

Microwave properties of highly oriented YBa2Cu3O7−x thin films

We have performed intra‐ and extra‐cavity microwave frequency (1–100 GHz) measurements on high quality Y1Ba2Cu3O7−x superconducting thin films on (100) LaAlO3 substrates. The ∼0.3 μm thin films fabricated by the pulsed laser deposition technique exhibit superconducting transition temperatures >90 K, as determined by resistivity and ac susceptibility measurements, and critical current densities of 5×106 A/cm2 at 77 K. Moreover, ion beam channeling minimum yields of ∼3% were measured, indicating the extremely high crystalline quality of films grown on the LaAlO3 substrate. Microwave surface resistance values at 77 K for these films are found to be more than one to two orders of magnitude lower than for copper at 77 K for almost the entire frequency range explored. We postulate that the reason we observe such low surface resistances in these films is the virtual absence of grain and phase boundaries coupled with the high degree of crystallinity. Furthermore, we believe that the residual resistance measured b...

Epitaxy of Y‐Ba‐Cu‐O thin films grown on single‐crystal MgO

The epitaxy of a thin‐film Y‐Ba‐Cu‐O (YBCO) superconductor deposited on a single‐crystal [001] MgO substrate was examined by transmission electron microscopy. The large lattice mismatch (8–10%) in the basal plane of YBCO and MgO is accommodated mainly by the formation of a polycrystalline, mosaic structure. The grain boundaries correspond to unique crystallographic interfaces, determined by the crystal symmetry of the substrate and the thin film.

High critical currents in epitaxial YBa2Cu3O7−x thin films on silicon with buffer layers

As‐deposited superconducting thin films (∼0.1 μm) of YBa2Cu3O7−x have been prepared by pulsed laser deposition on (100) Si with buffer layers of BaTiO3/MgAl2O4. X‐ray diffraction studies reveal that the films grow epitaxially with the c axis preferentially oriented normal to the substrate surface. This is confirmed by ion channeling measurements along the (100) (normal to the surface) and (110) directions of the Si substrate showing a minimum yield of 54% along the (100), and 78% along the (110) axes using 2.8 MeV He++. Preliminary transmission electron microscopy study also supports these results. The as‐deposited films have zero resistance temperatures of 86–87 K, and critical current densities of 6×104 A/cm2 at 77 K and 1.2×105 A/cm2 at 73 K. Our results indicate that the superconducting properties of the films are limited primarily by the quality and degree of epitaxal growth of the buffer layers on the silicon substrate.

Epitaxial Y1Ba2Cu3O7−y/Y1−xPrxBa2Cu3O7−y heterostructures

For a variety of device applications, junction devices in particular, we have demonstrated a heterostructure system of Y{sub 1}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{ital y}}/Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{ital y}} which maintains epitaxy over the entire Pr composition range {ital x}=0--1. We have grown both trilayer and multiperiod superlattices which show nearly single crystalline helium ion backscattering minimum yields of {lt}6% in the topmost layer. X-ray diffraction measurements indicate {ital c}-axis orientation by a transverse scan across (005) line with a full width at half maximum of 0.6{degree} and 0.4{degree} on MgO and SrTiO{sub 3} substrates, respectively. Scanning Auger electron depth profiles and cross-sectional transmission electron micrographs indicate abrupt Pr/Y interfaces within one unit cell and virtually no disruption of the layered structure at the interface. These results indicate the potential for the growth of excellent heterostructures and superlattices of the high-temperature superconductors.For a variety of device applications, junction devices in particular, we have demonstrated a heterostructure system of Y1Ba2Cu3O7−y/Y1−xPrxBa2Cu3O7−y which maintains epitaxy over the entire Pr composition range x=0–1. We have grown both trilayer and multiperiod superlattices which show nearly single crystalline helium ion backscattering minimum yields of <6% in the topmost layer. X‐ray diffraction measurements indicate c‐axis orientation by a transverse scan across (005) line with a full width at half maximum of 0.6° and 0.4° on MgO and SrTiO3 substrates, respectively. Scanning Auger electron depth profiles and cross‐sectional transmission electron micrographs indicate abrupt Pr/Y interfaces within one unit cell and virtually no disruption of the layered structure at the interface. These results indicate the potential for the growth of excellent heterostructures and superlattices of the high‐temperature superconductors.

Microstructure of in situ epitaxially grown superconducting Y‐Ba‐Cu‐O thin films

The microstructure of in situ epitaxially grown Y‐Ba‐Cu‐O thin films on (001) SrTiO3 substrates was studied using cross‐sectional transmission electron microscopy. The films, prepared by pulsed laser deposition at substrate holder temperature of 650 °C without post‐annealing, exhibit zero resistivity above 90 K and critical currents exceeding 106 A/cm2 at 77 K. The films are of heavily faulted single crystalline structure with the c axis approximately perpendicular to the substrate (001) surface. We suggest that, due to the fast quenching and low substrate temperature, crystalline defects and chemical fluctuations are locked into a faulted structure after each laser pulse. Despite their rather imperfect microstructure, the films are free from macroscopic grain boundaries and secondary phases and possess superb superconducting properties.

InGaAs/InP superlattice mixing induced by Zn or Si diffusion

Recent studies have shown that Zn diffusion preferentially induces mixing (interdiffusion) of In and Ga in unstrained InGaAs/InP superlattices, with little diffusion of the anions. In the present study, a 3.1% lattice mismatch is accommodated in the mixed superlattice with no observable defects in layers on the order of the predicted critical layer thickness. At high concentrations, Zn resides preferentially in the InP layers in the form of Zn3P2. In marked contrast to this behavior of Zn, Si diffusion is observed to cause comparable interdiffusion on the cation and anion sublattices within a narrow range of dopant concentration. This result is at odds with some recent mixing models and is consistent with a divacancy mixing mechanism.

Wet chemical etching of high-temperature superconducting Y-Ba-Cu-O films in ethylenediaminetetraacetic acid

A new wet chemical etchant for high‐temperature superconducting films is reported, which leaves transition temperature unaffected within experimental accuracy (1 K) and does not require reoxygenation. The solution consists of ethylenediaminetetraacetic acid (EDTA) in water, and is suitable for micropatterning using standard photolithography. We have fabricated 3–50 μm patterns on laser‐deposited Y‐Ba‐Cu‐O films. The bulk of the films etches at 0.14 μm/min in a saturated solution at room temperature. Porous surface layers are removed three times faster than the dense portions of the films. Etch rate depends linearly on the solution concentration and exponentially on the solution temperature. These rates are reduced if etching is interrupted and the samples are exposed to atmosphere.

Conversion of InP/In0.53Ga0.47As superlattices to Zn3P2/In1−xGaxAs and Zn3P2/Zn3As2 superlattices by Zn diffusion

A standard 600 °C closed‐tube Zn diffusion into an unstrained InP/In0.53Ga0.47As superlattice was found to produce new superlattices containing Zn3P2 layers, and in some cases Zn3As2 layers. Crystalline properties and diffusion profiles were examined by transmission electron microscopy and secondary‐ion mass spectrometry. Initial doping of Zn enhances the diffusion of In and Ga and results in a superlattice of uniform In and Ga distribution. Upon further infusion of Zn, Zn3P2 forms selectively in the phosphorus layers and propagates from the surface while maintaining an atomically abrupt Zn3P2/In1−xGaxP interface. Zn3As2 conversion is also observed to occur under sufficiently stringent conditions. Diffusion of P and As was not observed.

Transmission electron microscopy studies of superconducting Y‐Ba‐Cu‐O films prepared by laser deposition

The polycrystalline structure of pulsed‐laser deposited Y‐Ba‐Cu‐O thin films was studied using transmission electron microscopy. Many grains of the superconducting films on (001) SrTiO3 substrates had the c axis normal to the surface, while grains on (110) facets had their c axis oriented preferentially along the interface. Observation of an amorphous layer of thickness ∼6 nm at some oriented grain‐substrate interfaces suggests that the amorphous layer is formed subsequent to the formation of the crystalline structures. Surface ridges were found to facilitate the formation of triple points where the grain boundaries between nonepitaxial grains were pinned.