H.L.M. Chang

Variant structure in metal-organic-chemical-vapor-deposition-derived SnO 2 thin films on sapphire (0001)

Tin oxide (SnO{sub 2}) thin films were deposited on sapphire (0001) substrate by metal-organic chemical vapor deposition (MOCVD) at temperatures of 600 and 700 {degree}C. The microstructure of the deposited films was characterized by x-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). At the growth conditions studied, films were single-phase rutile and epitaxial, but showed variant structures. Three distinct in-plane epitaxial relationships were observed between the films and the substrate. A crystallographic model is proposed to explain the film morphology. This model can successfully predict the ratio of the width to the length of an averaged grain size based upon the lattice mismatch of the film-substrate interface.

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.

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.

PREPARATION OF SINGLE-CRYSTAL Y3AL5O12 THIN FILM BY METALORGANIC CHEMICAL VAPOR DEPOSITION

Single‐crystal yttrium aluminum garnet (Y3Al5O12 or YAG) thin films have been grown on (111) Gd3Ga5O12 substrate by metalorganic chemical vapor deposition. X‐ray diffraction, selective‐area electron diffraction, and transmission electron microscopy were used to confirm the single‐crystal nature of [111]‐oriented single‐crystal Y3Al5O12 thin films. We determined that the epitaxial relationship between the film and the substrate is (111)[110]Y3Al5O12//(111)[110]Gd3Ga5O12.

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.

Study of defects and interfaces on the atomic scale in epitaxial tio2 thin films on sapphire

Abstract TiO2 thin films grown on (11 0) sapphire (α-Al2O3) at 800°C by the metallo-organic chemical vapour deposition technique have been characterized by transmission electron microscopy. The TiO2 thin films are single-crystal rutile. The epitaxial orientation relationship between the rutile thin films (R) and the sapphire substrates (S) is (101)[010]R‖(110)[0001]s. Growth twins are commonly observed in the films with (101) twin planes and twinning directions. The atomic structure of twin boundaries and TiO2-α-Al2O3 interfaces have been investigated by high-resolution electron microscopy. When the interfaces are viewed in the direction of [010]R-[0001]s, the interfaces appear structurally coherent along the direction of [10]R-[100]s. The small misfit (0.5%) is accommodated at interface steps. In contrast, in the direction of [10]R-[100]s, the interfaces are semicoherent. Growth mechanisms are discussed, based on information about the atomic structure of the interfaces.