Lisa A. Tietz

Crystallography of YBa 2 Cu 3 O 6+ x thin film-substrate interfaces

The epitactic nature of the growth of YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}} (YBCO) superconducting thin films on ceramic substrates has been studied using high-resolution electron microscopy (HREM) and selected-area diffraction (SAD) of cross-sectional specimens. The films were grown {ital in} {ital situ} on (001)-oriented MgO and (001)-oriented Y{sub 2}O{sub 3}-stabilized cubic ZrO{sub 2} (YSZ) single-crystal substrates by electron beam evaporation. Both of these materials have large lattice misfits with respect to YBCO. Different orientation relationships were observed for films grown on the two types of substrates. These orientation relationships are shown to provide the best matching of the oxygen sublattices across the substrate-film interfaces. A crystalline intermediate layer, 6 nm thick, between the YBCO film and YSZ substrate was observed by HREM and shown by EDS to be a Ba-enriched phase, possibly barium zirconate formed by a reaction. In contrast, the YBCO--MgO interface was found to be sharp and free of any intermediate layers.

Observation of the early stages of growth of superconducting thin films by transmission electron microscopy

A method for the direct observation of the early stages of growth of superconducting films by transmission electron microscopy (TEM) is reported. The technique uses well‐characterized, single‐crystal TEM foils as substrates for the deposition process. Ultrathin films of YBa2Cu3O6+x (YBCO) were prepared by pulsed laser deposition from stoichiometric bulk samples directly onto (001) oriented MgO thin‐foil substrates. Observation of the film by TEM is possible without any post‐deposition specimen preparation. The epitactic nature of the film growth is shown by analysis of the moire fringe pattern and by selected area diffraction. In addition to an interconnected film, copper oxide particles and stoichiometric ablated clusters were observed.

Special grain boundaries in YBa2Cu3O7−x thin films

Abstract Grain boundaries described by rotations about the c [001] axis are discussed in terms of a near-CSL model for orthorhombic materials. Grain boundaries in an oriented YBa 2 Cu 3 O 7- x (YBCO) thin film grown on an (001) ZrO 2 substrate by electron-beam coevaporation of the metals have characterized by transmission electron microscopy. Bright-field and dark-field images and selected-area diffraction patterns recorded from near-Σ=1,5,13, 17, and 29 [001] tilt boundaries and near-Σ=3 and 13 [100] tilt boundaries are presented. It is also shown that low-angle grain boundaries with misoreintations near 7° [001] can be introduced at triple junctions containing two high-angle near-CSL boundaries. A table including coincidence, near-coincidence, and “low-index’ facet planes expected for near-CSL misorientations is given and some preliminary observations of the faceting behaviour of such grain boundaries are made.

Early stages of the heteroepitactic growth of hematite on (0001) Al2O3 by transmission electron microscopy

A new method for studying the early stages of the growth of oxide epilayers by transmission electron microscopy (TEM) is described. The technique uses well‐characterized, single‐crystal TEM foils as substrates for the deposition process. In the present study, chemical vapor deposition was used to form small islands (50–300 nm) of α‐Fe2O3 on (0001) oriented Al2O3 thin‐foil substrates. The preferential nucleation of islands at surface steps on the alumina is clearly demonstrated. Selected area diffraction and moire fringe pattern analysis are used to show the epitactic nature of the growth and to study the island morphology.

The effect of substrate orientation on the chemical vapour deposition growth of αfe2o3 on α-al2o3

Abstract The early stages of growth of haematite (α-Fe2O3) on four orientations of sapphire (α-Al2O3), namely (0001), {102}, {110} and {100}, has been studied by transmission electron microscopy. Specimens were made by a new technique in which the haematite is deposited directly onto specially prepared electron-transparent sapphire substrates using low-pressure chemical vapour deposition. Growth was epitaxial and occurred by the formation of islands on all four substrate orientations. On the (0001) surface, nucleation and growth of the islands occurred preferentially at surface steps. The islands grew parallel to the substrate surface and most rapidly along the step directions. On the {102} surface, the nucleation rate was higher and extensive coalescence of small islands was observed. The islands also exhibited small tilts away from exact epitaxy. On the {110} and {100} substrates, the islands tended to grow normal to the substrate surfaces. The growth behaviour of the haematite on the four substrate ori...

Pulsed Laser Deposition of High TC Superconducting Thin Films

In this paper we present results of studies of the effect of process variations and substrate surface conditions on the microstructure and electrical properties of laser deposited YBa 2 Cu 3 O 7‐δ thin films. The principle conclusions are as follows. The film properties of YBa 2 Cu 3 O 7‐δ are relatively constant over a large range of substrate temperatures (620C‐720C) and oxygen pressures (150‐800 mT). There are small compositional variations with substrate temperature similar to, but much smaller than, those seen in reactively sputtered films. The best films were grown at 670C in 400mT of oxygen and have a zero resistance T c of 89K and J c > 1x10 6 A/cm 2 at 77K. There are, however, variations in the T c , J c , and orientation of films grown on MgO under similar deposition conditions. These variations reflect differences in the nucleation and growth process of YBa 2 Cu 3 O 7‐δ on MgO which can arise from subtle changes in substrate surface properties. In particular, while the films are predominantly oriented with c axis perpendicular and a,b axes in the plane of the substrate along the MgO[100] direction, there are variable amounts of grains whose in‐plane orientations are at large angles to the MgO [100] direction, leading to large angle grain boundaries in some films. These large angle grain boundaries are correlated with the presence of Josephson weak links in some small constrictions for films on MgO.

Structure of the Fe2O3–Al2O3 (1102) interface

Abstract The interface structure of α-Fe2O3 islands grown directly on an electron transparent Al2O3 (1102) substrate by low-pressure chemical vapour deposition has been characterized by weak-beam imaging in the transmission electron microscope. The islands exhibited irregular moire patterns, suggesting that tilting of the Fe2O3 occurs as a means of partially accommodating the 5·5% lattice misfit at the interface. Consistent with this observation, numerous types of misfit dislocation network were observed. One type of network was identified as an othogonal array of b=⅓ 〈1120〉 and b=⅓〈1101〉 perfect edge dislocations. A second type of network is most probably an array of perfect dislocations with Burgers vectors inclined to the interface which has been directly associated with tilting of the Fe2O3 island. The observation suggests that the introduction of misfit dislocations at the (1102) interface is more difficult than at the (0001) interface.

TOP-BOTTOM EFFECTS IN DOUBLE DIFFRACTION

Abstract A top-bottom effect in double diffraction has been observed in selected-area diffraction (SAD) patterns recorded from epitactic overgrowths. In SAD patterns recorded from epitactic α-Fe2O3 islands grown on an (0001) Al2O3 substrate, the double-diffraction reflections were found always to surround the primary reflections associated with the bottom crystal. This observation is explained using simple geometric constructions and the concepts of kinematical diffraction in a deformed crystal. The effect may be useful as a simple method for determining on which side of a TEM specimen a particular epitactic feature lies.

Grain Boundaries in Yba 2 Cu 3 O 7-δ Thin Films

Grain boundaries in YBa 2 Cu 3 O 7-δ thin films deposited on (001)-MgO by pulsed laser ablation have been characterized using selected area diffraction. In addition to twin boundaries, the presence of both low-angle and high-angle grain boundaries were characteristic of the films microstructure. The presence of certain grain boundaries may be responsible for the weak-link Josephson behavior observed in small constrictions formed in superconducting films on MgO.