D.J. Lam

Microstructure of PbTiO[sub 3] thin films deposited on (001) MgO by MOCVD

PbTiO[sub 3] thin films deposited on (001)MgO by the MOCVD technique have been characterized by x-ray diffraction and transmission electron microscopy (TEM). The PbTiO[sub 3] films grown at temperatures below [ital T][sub [ital C]] ([similar to]500 [degree]C) are c-axis oriented polycrystals, while the PbTiO[sub 3] films grown at temperatures above [ital T][sub [ital C]] are single crystals with a bi-layer structure at room temperature. The top layers of the films near the free surface are [ital c]-axis oriented with the orientation relationship of (001)[100]PbTiO[sub 3][parallel](001)[100]MgO. The bottom layers of the films near the substrate are [ital a]-axis oriented with the orientation relationship of (100)[00[bar 1]]PbTiO[sub 3][parallel](001)[100]MgO. The formation of the bi-layer microstructure of the PbTiO[sub 3] films will be discussed in terms of the effect of the cooling rate and the substrate on the phase transition of PbTiO[sub 3] from the cubic to the tetragonal phase. When the PbTiO[sub 3] films were grown with large excess of the Pb precursor vapor in the vapor mixture, PbO and PbTiO[sub 3] coexisted in the films. Both PbO and PbTiO[sub 3] were grown epitaxially on (001)MgO. The epitaxial orientation relationships were found to be (100)[001]PbO[parallel](001)[100]MgO and (001)[100]PbTiO[sub 3][parallel](001)[100]MgO. The 90[degree] domains were observedmorexa0» in the PbTiO[sub 3] films deposited at temperatures above [ital T][sub [ital C]]. The domain walls are the [l brace]101[r brace] and [l brace]011[r brace] twin boundaries, which were formed during the phase transition from the cubic to the tetragonal phase.«xa0less

Structure and composition of grain boundary dislocation cores and stacking faults in MOCVD-grown YBa2Cu3O7-x thin films

Abstract The dislocation cores of a low-angle grain boundary in MOCVD-grown YBa2Cu3O7−x have been studied by high-resolution electron microscopy and image simulation. It was found that the low-angle boundary consists of a wall of discrete edge dislocations separated by relatively perfect lattice matching regions. Lattice reconstruction has been observed at the dislocation cores. The dislocation cores appear to be Cu-rich, with a core radius of about 1 nm. These observations are used to discuss the transition from strong to weak coupling behavior across grain boundaries. Stacking faults in the a-b and b-c planes have been observed for the first time in the MOCVD-grown YBa2Cu3O7−x thin films. HREM image analysis indicates that the stacking faults contain an extra Cu-O layer. The thickness of the stacking faults is about 1.6 (100) interplanar spacing, or 0.6 nm, which is smaller than the coherence length in a-b plane. Thus, the stacking faults are not expected to strongly affect superconducting properties.

Epitaxial TiO2 and VO2 films prepared by MOCVD

Abstract Titanium and vanadium oxide systems were selected to study the growth of thin film in a metal-organic chemical vapor deposition (MOCVD) process. Epitaxial TiO 2 and VO 2 films were obtained on sapphire (112¯0), (0001), and (11¯02) but not on Si(111). Eight distinct substrate-film epitaxial relationships have been determined by X-ray diffraction studies using a four-circle diffractometer. It was found that none of the eight epitaxial systems had a good lattice match between substrate and film. But further investigation revealed that substantial similarity existed in the local atomic patterns of the substrate and the film for all these systems. Nevertheless, it should be emphasized that mismatches of the local atomic patterns for these systems are, in general, substantially larger than those observed in the epitaxial systems containing semiconductor materials such as silicon and GaAs.

The relationship between the MOCVD parameters and the crystallinity, epitaxy, and domain structure of PbTiO 3 films

Lead- and titanium-based oxide thin films were prepared by the metal-organic chemical vapor deposition technique (MOCVD) and the relationship between the film structures and the processing parameters, such as the ratio of Pb/Ti precursors in the gas phase, substrate materials, substrate surface orientation, and growth temperature, was systematically studied. It was found that whether a single-phase stoichiometric PbTiO[sub 3] film could be obtained depended on both the Pb/Ti precursor ratio in the gas phase and the deposition temperature. Under appropriate conditions, stoichiometric PbTiO[sub 3] films could be obtained on all the substrates including silicon, MgO, [alpha]--Al[sub 2]O[sub 3], SrTiO[sub 3], and LaAlO[sub 3]. The PbTiO[sub 3] films grown on silicon substrates were always polycrystalline, whereas epitaxial PbTiO[sub 3] films were obtainable on all the other substrates. For epitaxial PbTiO[sub 3] films, the epitaxial relationship, crystallinity, and domain structures were found to be a function of both the substrate materials and surface orientation as well as the deposition temperature. X-ray rocking curves ([omega] scan) of the (100) and (001) planes of PbTiO[sub 3] epitaxial films and PbTiO[sub 3] single crystal revealed the inherent nature of the domain structures in PbTiO[sub 3].

Preparation and structure of PbZrO3 epitaxial films by metalorganic chemical vapor deposition

Single‐crystal PbZrO3 thin films have been grown on (001) SrTiO3 substrates by metalorganic chemical vapor deposition. The single‐crystal nature of c‐axis oriented PbZrO3 film is confirmed by x‐ray diffraction, selective area electron diffraction, and transmission electron microscopy. The epitaxial relationship between the PbZrO3 film and the SrTiO3 substrate was found to be (001)[100]PbZrO3∥(001)[110]SrTiO3.

Microstructure and defects in a-axis oriented YBa2Cu3O7−x thin films

Abstract MOCVD-grown, a -axis oriented YBa 2 Cu 3 O 7− x thin films have been studied by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). The majority of the thin film material is oriented with the a -axis perpendicular to the substrate surface, while about 20% of the material has the c -axis perpendicular to the substrate. The c -axis oriented grains are disconnected from each other, and are separated by a -axis oriented grains. There are two types of a -axis oriented grains, differing by a 90° rotation of the c -axis. HREM images in conjunction with image simulation analysis are used to derive the atomic structures of the 90° rotation grain boundaries and of defect structures in the film. The majority of the grain boundaries is asymmetrical, bounded by (010)(001) lattice planes. However extended steps, whose segments are mostly parallel to the (013)(013) planes, are also present, thus forming short facets of symmetric grain boundaries. The latter are believed to be important in forming weak-link-free connections between the grains. The misfit at (010)(001) grain boundaries is accomodated by the formation of stacking faults, which were identified to be 124-type defects. The grain boundary structures are discussed in terms of correlations with the electric transport properties across the boundaries.

Epitaxy‐induced phase of near‐stoichiometry PbTiO3 films prepared by metalorganic chemical vapor deposition

Ferroelectric PbTiO3 thin films have been prepared by a low‐pressure MOCVD process. Epitaxial films with a crystal structure of a bulk equilibrium tetragonal phase were readily obtained within a range of growth conditions. However, by providing excess Pb in the growth process, we have observed an epitaxy‐induced phase for near‐stoichiometry PbTiO3 which possesses a perovskite‐like, but possible, orthorhombic structure.Ferroelectric PbTiO3 thin films have been prepared by a low‐pressure MOCVD process. Epitaxial films with a crystal structure of a bulk equilibrium tetragonal phase were readily obtained within a range of growth conditions. However, by providing excess Pb in the growth process, we have observed an epitaxy‐induced phase for near‐stoichiometry PbTiO3 which possesses a perovskite‐like, but possible, orthorhombic structure.

Substrate surface step effects on microstructure of epitaxial films

Sapphire (0001) basal plane is a commonly used substrate for film depositions. Due to the threefold rotational symmetry of the (0001) substrate surface, epitaxial films deposited are expected to form at most three variants with relative orientations of 120° and 240°. However, epitaxial TiO2 (tetragonal) and VO2 (monoclinic) films deposited on sapphire (0001) substrates by the metalorganic chemical vapor deposition technique were found to have six variants of relative orientations of 60°, 120°, 180°, 240°, and 300° based on the x‐ray diffraction studies. Furthermore, epitaxial MgO (cubic) films deposited on sapphire (0001) substrates by the molecular beam epitaxy technique were found to have two variants which are mirror images about the (1210) plane from the high resolution electron microscopy. We show that these unconventional film microstructures found experimentally are caused by the substrate surface steps.

A neutron diffraction study of a single crystal of YBa2Cu3O7-δ

Abstract A single crystal neutron diffraction study of a superconducting sample (Tc=∼90K) of YBa2Cu3O7-δ was performed at 295K. Despite complications due to the small sample size (∼0.3×0.2×0.1mm3, about 10-2 of optimal) and twinning, a satisfactory refinement of the structure has been carried out based on an orthorhombic unit cell (a=3.810, b=3.883, c =11.674 A ). The structure parameters are similar to those found for ceramic powders, indicating that well-annealed, oxygen-site ordered single crystals can be prepared. Anisotropic Debye-Waller factors (U11 =U22 ≠U33) for all atoms are derived for the first time and compared with isotropic values found in previous studies. The results are not consistent with an alternative structure recently proposed by Reller, Bednorz, and Muller.

Electromagnetic modes and prism-film coupling in anisotropic planar waveguides of epitaxial (101) rutile thin films

We report optical waveguiding in single‐crystal, epitaxial (101) oriented rutile (TiO2) thin films grown on (1120) sapphire (α‐Al2O3) substrates using the metal‐organic chemical vapor deposition technique. The electromagnetic field distributions and propagation constants for asymmetric planar waveguides composed of an anisotropic dielectric media applicable to these films are derived. Modifications to the prism‐film coupling theory for this anisotropic case are also discussed. By application of this model to (101) oriented rutile thin films, we directly obtain values of the ordinary and extraordinary refractive indexes, n0 and ne, of the rutile thin films as well as film thicknesses. We obtain typical values of the refractive indexes (n0=2.5701±0.0005; ne=2.934±0.001) near to those for bulk rutile single crystals indicating the exceptional quality of these films.