Phases that are in equilibrium with BaR2CuO6+x (R=lanthanides and Y), such as the “green-phase” and “brown-phase” structural variants of BaR2CuO5 in bulk samples, are attractive choices for flux-pinning for coated conductor applications because of the guaranteed chemical stability. In films, high-temperature x-ray diffraction studies of Ba2RCu3O6+x superconductor deposited on SrTiO3 substrate using the trifluoroacetate solution method demonstrate that while BaNd2CuO5 (“brown-phase” structure) develops at 735 °C and 100 Pa pO2, neither BaGd2CuO5 nor Ba(Nd1/3Eu1/3Gd1/3)2CuO5 (both green-phase structure) form at these conditions. As a result, Ba2(Nd1/3Eu1/3Gd1/3)Cu3O6+x in thin films is in equilibrium with the brown-phase, and Ba2GdCu3O6+x is in equilibrium with Gd2O3 in the Ba–Gd–Cu–O system, in contrast to the bulk systems. Different phase relationships in the vicinity of the Ba2RCu3O6+x phase imply different phases are available for flux-pinning applications. These differences will need to be considered carefully in designing optimized superconducting coated conductors.

Commensurate BaTiO3/SrTiO3 superlattices were grown by reactive molecular-beam epitaxy on four different substrates: TiO2-terminated (001) SrTiO3, (101) DyScO3, (101) GdScO3, and (101) SmScO3. With the aid of reflection high-energy electron diffraction (RHEED), precise single-monolayer doses of BaO, SrO, and TiO2 were deposited sequentially to create commensurate BaTiO3/SrTiO3 superlattices with a variety of periodicities. X-ray diffraction (XRD) measurements exhibit clear superlattice peaks at the expected positions. The rocking curve full width half-maximum of the superlattices was as narrow as 7 arc s (0.002°). High-resolution transmission electron microscopy reveals nearly atomically abrupt interfaces. Temperature-dependent ultraviolet Raman and XRD were used to reveal the paraelectric-to-ferroelectric transition temperature (TC). Our results demonstrate the importance of finite size and strain effects on the TC of BaTiO3/SrTiO3 superlattices. In addition to probing finite size and strain effects, these heterostructures may be relevant for novel phonon devices, including mirrors, filters, and cavities for coherent phonon generation and control.

We report a detailed study of the grain orientations and grain boundary (GB) networks in YBa2Cu3O7-δ (YBCO) films ∼0.8 μm thick grown by both the in situ pulsed laser deposition (PLD) process and the ex situ metalorganic deposition (MOD) process on rolling-assisted biaxially textured substrates (RABiTS). The PLD and MOD growth processes result in columnar and laminar YBCO grain structures, respectively. In the MOD-processed sample [full-width critical current density Jc(0 T, 77 K) = 3.4 MA/cm2], electron back-scatter diffraction (EBSD) revealed an improvement in both the in-plane and out-of-plane alignment of the YBCO relative to the template that resulted in a significant reduction of the total grain boundary misorientation angles. A YBCO grain structure observed above individual template grains was strongly correlated to larger out-of-plane tilts of the template grains. YBCO GBs meandered extensively about their corresponding template GBs and through the thickness of the film. In contrast, the PLD-processed film [full width Jc(0 T, 77 K) = 0.9 MA/cm2] exhibited nearly perfect epitaxy, replicating the template grain orientations. No GB meandering was observed in the PLD-processed film with EBSD. Direct transport measurement of the intra-grain Jc(0 T, 77 K) values of PLD and MOD-processed films on RABiTS revealed values up to 4.5 and 5.1 MA/cm2, respectively. As the intra-grain Jc values were similar, the significantly higher full-width Jc for the MOD-processed sample is believed to be due to the improved grain alignment and extensive GB meandering.

It has been generally accepted that YBa2Cu3O7−x (YBCO) films deposited on deformation textured polycrystalline metal tapes result in YBCO grain boundary (GB) networks that essentially replicate the GBs of the underlying substrate. Here we report that for thicker YBCO films produced by a BaF2 ex situ process, this is not true. Using electron backscatter diffraction combined with ion milling, we have been able to map the evolution of the YBCO grain structure and compare it to the underlying template in several coated conductors. For thin (≤0.5 μm) YBCO films deposited on rolling-assisted biaxially textured substrates (RABiTS), the YBCO GBs nearly directly overlap the substrate GBs. For 0.7–1.4 μm YBCO films, the GBs were found to meander along the substrate GBs and along the sample normal, with displacements several times the film thickness. In very thick films (2.5–2.9 μm), the YBCO grains can completely overgrow substrate grains and GBs, resulting in a substantial disconnection of the YBCO and substrate GB networks. Similar behavior is found for BaF2 ex situ YBCO films on ion-beam-assisted deposition-type templates. The ability of the YBCO to overgrow substrate grains and GBs is believed to be due to liquid-phase mediated laminar grain growth. Although the behavior of the YBCO GB networks changes with YBCO film thickness, the samples maintained high critical current density (Jc) values of >2 MA/cm2 for films up to 1.4 μm thick, and up to0.9 MA/cm2 for 2.5–2.9-μm-thick films.

YBa2Cu3Oy (YBCO) films produced by the ex situ conversion of BaF2-based precursors deposited by physical vapor deposition on ion-beam assisted deposited (IBAD) yttrium-stabilized zirconia (YSZ) and rolling-assisted biaxially textured substrates (RABiTS) templates are characterized by a bi-axially aligned, laminar grain structure that results from the anisotropic growth characteristics of the YBCO phase and its precipitation from a transient liquid phase during the conversion process. A bimodal microstructure characterizes these films and is defined by large, well-formed YBCO grains with Y2O3 precipitates in the bottom region of the film and small YBCO grains with a high density of stacking faults in the upper half. Ba2Cu3Oy or Ba–O–F/CuO second phase layers were often found between large YBCO grains in the bottom half of the films. YBCO grain sizes exceeded 50 μm within the plane of the film in some cases. Conversely, discrete secondary phases of Y2Cu2O5, Y2O3, and Ba2Cu3Oy/Ba–O–F could be found among the much smaller YBCO grains in the top portion of the bimodal structure. The dividing line of the bimodal structure was generally at one half of the film thickness, although exceptions to this trend were found. The highest critical current densities (Jc) and best film alignments for a given film thickness were found in samples where the layers of Ba2Cu3Oy or Ba–O–F were minimized or eliminated from the films. Samples quenched after partial conversion show the segregation of CuO to the top region of the film and the lateral growth of large YBCO grains from a precursor mix of Y2Cu2O5 and Ba–O–F. The data demonstrate that transient liquid phases are part of the conversion process of BaF2-based YBCO films. The control of both CuO segregation and the amount of liquid phases generated during the initial stages of phase formation is needed for optimizing the ex situ conversion process for high-Jc coated conductors.

The growth and surface reconstructions of AlN(0001) films were studied. For moderately Al-rich surfaces, the 2×6 structure is commonly observed in reflection high energy electron diffraction. It is found that this pattern consists of 2√3×2√3-R30° and 5√3×5√3-R30° reconstructions according to scanning tunneling microscopy. Similar to the Ga-rich GaN(0001) surface, these structures are determined to contain 2–3 monolayers of excess Al terminating the surface. Based on first-principles theory the structures are believed to contain a laterally contracted Al layer. At higher Al coverage a thick, flat Al film is found to form on the surface. A high density of growth spirals (associated with threading dislocations having full or partial screw character) is found to be present on the surface, although this density decreases with increasing growth temperature.

GaN films have been grown homoepitaxially by MOCVD on MBE-grown GaN template layers, using both porous and nonporous SiC substrates. The effect of the porous SiC substrates on dislocations in the MBE and MOCVD GaN layers has been studied using TEM and x-ray characterization. A reduction in dislocation density from ≥1×1010 cm-2 in the MBE template to 2.5×109 cm-2 at the top of the MOCVD film is found, with similar final values in the MOCVD films for both porous and nonporous substrates. We discuss various mechanisms by which dislocation density is reduced in the MOCVD layers.

M-plane GaN(1100) is grown by plasma assisted molecular beam epitaxy on ZnO(1100) substrates. A low-temperature GaN buffer layer is found to be necessary to obtain good structural quality of the films. Well oriented (1100) GaN films are obtained, with a slate like surface morphology. On the GaN(1100) surfaces, reconstructions with symmetry of c(2×2) and approximate “4×5” are found under N- and Ga-rich conditions, respectively. We propose a model for Ga-rich conditions with the “4×5” structure consisting of ≥ 2 monolayers of Ga terminating the GaN surface.

Gallium nitride films are grown by plasma-assisted molecular beam epitaxy (MBE) on 6H-SiC(0001) substrates with no miscut and with 3.5° miscuts in both the [1 0 0] and [1 1 0] directions. The hydrogen-etched substrates display straight or chevron shaped steps, respectively, and the same morphology is observed on the GaN films. X-ray rocking curves display substantially reduced width for films on the vicinal substrates compared to singular substrates, for the same Ga/N flux ratio used during growth.

We have explored the growth of GaN on porous SiC substrates by plasma-assisted molecular beam epitaxy. The porous 4H- and 6H-SiC(0001) substrates used in this study contain 10 to 100-nm sized pores and a thin skin layer at the surface. This skin layer was partially removed prior to the growth by H-etching. Transmission electron microscopy (TEM) observations indicate that the epitaxial GaN growth initiates from the surface areas between pores, and the exposed surface pores tend to extend into GaN as open tubes and trap Ga droplets. Plan-view TEM observations indicate that the GaN layers grown on porous substrates contain fewer dislocations than layers grown on non-porous substrates by roughly a factor of two. The GaN layers grown on a porous SiC substrate were also found to be mechanically more relaxed than those grown on non-porous substrates; electron diffraction patterns indicate that the former are free of misfit strain or are even in tension after cooling to room temperature.

A review of surface structures of bare and adsorbate-covered GaN (0001) and (000) surfaces is presented, including results for In, Mg, Si, and H adsorbates. Emphasis is given to direct determination of surface structure employing experimental techniques such as scanning tunneling microscopy, electron diffraction, and Auger electron spectroscopy, and utilizing first principles computations of the total energy of various structural models. Different surface stoichiometries are studied experimentally by varying the surface preparation conditions (e.g. Ga-rich compared to N-rich), and the stoichiometry is included in the theory by performing calculations for various chemical potentials of the constituent atoms. Based on the work reviewed here, surface reconstructions for plasma-assisted molecular beam epitaxy growth of GaN (0001) and (000) surfaces are fairly well understood, but reconstructions for reactive molecular beam epitaxy or for metal-organic vapor phase epitaxy (both involving H, at moderate and high temperatures, respectively) are less well understood at present.

InGaN alloys with (0001) or (000) polarities are grown by plasma-assisted molecular beam epitaxy. Scanning tunneling microscopy images, interpreted using first-principles theoretical calculations, show that there is strong indium surface segregation on InGaN for both (0001) and (000) polarities. Evidence for the existence and stability of a structure containing two adlayers of indium on the In-rich InGaN(0001) surface is presented. The dependence on growth temperature and group III/V ratio of indium incorporation in InGaN is reported, and a model based on indium surface segregation is proposed to explain the observations.

Gallium nitride films are grown by plasma-assisted molecular beam epitaxy (MBE) on vicinal 6H-SiC(0001) substrates with [1 1 00] and [11 2 0] miscut directions. The hydrogen-etched substrates display straight, or chevron shaped steps respectively, and the same morphology is observed on the GaN films. X-ray rocking curves display substantially reduced width for films on the vicinal substrates compared to singular substrates, for the same Ga/N flux ratio used during growth.

The interplay of melting equilibria and reaction kinetics is important during formation of the Ba2YCu3O6+x (Y-213) phase from starting materials in the quaternary reciprocal system Ba,Y,Cu//O,F. For experimental investigation of the process we used a combination of differential thermal analysis (DTA) for study of melting equilibria, and in-situ high-temperature x-ray diffraction (HTXRD) for study of the phase formation and reaction kinetics. DTA investigation of compositions spaced along compositional vectors extending from the oxide end to the fluoride end of the reciprocal system have given evidence of low melting liquids (∼600 °C) near the fluorine-rich end. Work is continuing to determine whether similar thermal events observed in the interior of the system also indicate low temperature liquids, and on the extent to which low-melting liquids could be involved in Y-213 phase formation. HTXRD investigations have been initiated on the conversion of 0.3 μm and 1.0 μm thick BaF2-Y-Cu precursor films to Y-213 in the presence of water vapor. Preliminary results indicated that the thickness of film has a strong influence on the texture of the Y-213 film: a 0.3 μm film showed mainly (001) texture, whereas a 1.0 μm film showed a greater volume fraction of (h00) texture. While the HTXRD method cannot directly reveal the presence of liquid, we are working to combine DTA and HTXRD data for a unified picture of Y-213 phase formation during the “BaF2 ex-situ” process for coated-conductor fabrication.

A low-cost, non-vacuum reel-to reel dip-coating system has been used to continuously fabricate epitaxial Gd2O3 buffer layers on mechanically strengthened, biaxially textured Ni- (3at.%W-1.7at%Fe), defined as Ni-alloy, metal tapes. X-ray diffraction analysis of the seed Gd2O3 layers indicated that well textured films can be obtained at processing temperatures (Tp) between 1100 and 1175°C. Processing speed did not significantly affect the crystalline quality of the Gd2O3. Scanning electron microscopy revealed a continuous, dense and crack-free surface morphology for these dip-coated buffers. The Gd2O3 layer thickness led to remarkable differences in the growth characteristics of the subsequent YSZ and CeO2 layers deposited by rfmagnetron sputtering. Epitaxial YBCO films grown by pulsed laser deposition on the short prototype CeO2/YSZ/Gd2O3/Ni-(3at%W-1.7at%Fe) conductors yielded self-field critical current densities (Jc) as high as 1.2×106 A/cm2 at 77 K. Pure Ni tapes were used to asses the viability of dip-coated buffers for long length coated conductor fabrication. The YBCO films, grown on 80 cm long and 1 cm wide CeO2/YSZ/Gd2O3 buffered Ni tapes by the industrially scalable ex-situ BaF2 precursor process, exhibited end-to-end self-field Jc of 6.25×105 A/cm2 at 77 K.

Chemical solution epitaxy was used to deposit an epitaxial film of Gd2O3 on roll-textured nickel. A 2-methoxyethanol solution of gadolinium methoxyethoxide was used for spin-coating and dip-coating. Films were crystallized using a heat treatment at 1160 °C for 1 h in 4% H2/96% Ar. Single-layer films were approximately 600 Å in thickness, and thicker films could be produced using multiple coatings. θ/2θ x-ray diffractograms revealed only (0041) reflections, indicating a high degree of out-of-plane texture. A pole-figure about the Gd2O3 (222) reflection indicated a single in-plane epitaxy. Scanning electron microscopy showed that the films were smooth, continuous, and free of pin holes. Atomic force microscopy revealed an average surface roughness of 53 Å. Electron diffraction indicated that the misalignment of the majority of the grains in the plane was less than 10°. High-current (0.4 MA/cm2) Yba2Cu3O7–δ films were grown on roll-textured nickel substrates using Gd2O3 as the base layer in a three-layer buffer structure.

InGaN alloys with (0001) or (000) polarities are grown by plasma-assisted molecular beam epitaxy. Scanning tunneling microscopy images, interpreted using first-principles theoretical cal- culations, show that there is strong indium surface segregation on InGaN for both (0001) and (000) polarities. The dependence on growth temperature and group III/V ratio of indium incorporation in InGaN is reported, and a model based on indium surface segregation is proposed to ex- plain the observations.