K. L. Merkle

Substrate effects on the structure of epitaxial PbTiO3 thin films prepared on MgO, LaAlO3, and SrTiO3 by metalorganic chemical‐vapor deposition

Epitaxial PbTiO3 films were prepared by metalorganic chemical‐vapor deposition on MgO(001)‐, SrTiO3(001)‐, and LaAlO3(001)‐oriented substrates. Four‐circle x‐ray diffraction, transmission electron microscopy, Rutherford backscattering (RBS) channeling, and optical waveguiding were performed to characterize the deposited films. Epitaxial, single‐crystal films were obtained on all three substrate materials under the same growth conditions. However, the defect structure of the films, including grain tilting, threading dislocation density, and 90° domain formation, was strongly dependent on the choice of substrate material. Films grown on MgO(001) and LaAlO3(001) (pseudocubic indices) substrates are nominally c‐axis oriented; however, the PbTiO3 grains in the film form a fourfold domain structure, with the grains tilted ∼0.6° and ∼0.7°, respectively, toward the [100] directions (cubic or pseudo‐cubic) of the substrates. In addition, these films contain a significant volume fraction of 90°‐domain (a‐axis) stru...

High-resolution electron microscopy of interfaces in fcc materials

Abstract Modern high-resolution electron microscopy (HREM) instruments, which are capable of a point-to-point resolution of better than 0.2 nm, have allowed atomic-scale observations of a variety of internal interfaces. The application of the HREM technique to fcc model systems for the purpose of addressing a number of interface issues will be examined in this paper. Atomic structure observations for heterophase interfaces of metal/metal and metal/metal-oxide systems as well as HREM studies of grain boundaries in NiO and Au will be discussed with emphasis on generic structural features and the role of the interface plane. Comparisons between observed interface structures and atomistic computer modeling results have shown agreements for some interfaces, and certain differences in others. A number of structural features are common to both metal and oxide grain boundaries, as well as certain heterophase boundaries. Of particular importance in close-packed solids appears to be the tendency to preserve, as much as possible, local atomic coordination, giving rise to atomically well-matched regions that alternate along the interface with regions of misfit. It is commonly observed that heterophase interfaces are preferentially formed on dense-packed planes. Low-index planes are also frequently observed in asymmetric grain boundaries. In addition to the observation of misfit dislocations in heterophase boundaries, misfit-dislocation-like defects have also been found in asymmetric, incommensurate grain boundaries. The tendency for maintaining coherence between dense-packed planes across the interface has resulted in the formation of novel three-dimensional grain boundary structures. HREM observations have brought new insights into the correlations between macroscopic geometry, interfacial energy, and microscopic atomic relaxations.

Atomic resolution electron microscopy of NiO grain boudaries

Atomic-scale detail in 〈100〉 tilt grain boundaries (GB) of high-purity bicrystals of NiO has been studied with a 400 kV high-resolution electron microscope. Crystallinity is always maintained right up to the grain boundary and there is a strong tendency for coherent matching of atomic planes across the GB in symmetric and asymmetric GBs, and for all misorientations. Low-angle boundaries are characterized by their primary dislocation structure which is, depending on the GB plane, composed of a〈100〉 and 12a〈110〉 type dislocations. High-angle boundaries, even those close to σ = 5, often take on asymmetric configurations with the boundary close to a (100) plane in one of the crystals. The observation of symmetric as well as highly asymmetric facets at high angles suggests that both configurations correspond to local minima in the free energy. These results are discussed in terms of current models of tilt GBs of NiO.

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.

Atomic-scale observation of grain boundary motion.

Atomic-scale grain boundary (GB) migration mechanisms are investigated by high-resolution transmission electron microscopy (HREM). For the first time, atomic-scale motion of high-angle twist as well as general GBs is directly observed by HREM. GB motion was found to proceed by the propagation of atomic-scale steps for two quite different GB geometries. Time-resolved HREM indicates that migration mechanisms of high-angle GBs also include cooperative effects, whereby groups of atoms spontaneously rearrange their lattice positions to be incorporated into the growing grain.

Correlations between ion and neutron irradiations: Defect production and stage I recovery

Abstract A comparison is made between results of recent neutron- and ion-irradiation experiments on metals. Specifically, lowtemperature defect production and stage-I annealing are discussed. Defect production efficiencies observed for neutron irradiations are shown to be consistent with results for ion irradiations. A phenomenological model describing both defect production and stage-I annealing in terms of thermal-spike induced recombinations is introduced. The stage-I recovery fraction for ion and neutron irradiations is found to correlate with cascade-energy density as represented by an appropriate nuclear stopping power.

The Chemical Composition of a Metal/Ceramic Interface on an Atomic Scale: The Cu/MgO {111} Interface

The chemical composition profile across a Cu/MgO {111}-type heterophase interface, produced by the internal oxidation of a Cu(Mg) single-phase alloy at 1173 K, is measured via atom-probe field-ion microscopy with a spatial resolution of 0.121 nm; this resolution is equal to the interplanar spacing of the {222} MgO planes. In particular, we demonstrate directly that the bonding across a Cu/MgO {111}-type heterophase interface, along a <111> direction common to both the Cu matrix and an MgO precipitate, has the sequence Cu|O|Mg... and not Cu|Mg|O...; this result is achieved without any deconvolution of the experimental data. Before determining this chemical sequence, it was established, via high-resolution electron microscopy, that the morphology of an MgO precipitate in a Cu matrix is an octahedron faceted on {111} planes with a cube-on-cube relationship between a precipitate and the matrix; that is, {111}Cu//{222}MgO and <110>Cu // <110>MgO.

Efficiency of defect production in cascades

In copper and silver, the defect production by energetic ions has been investigated for projectile masses between 1 and 209 in the energy range between 10/sup 4/ and 10/sup 6/ eV. Thin-film residual-resistivity measurements were used to determine the defect production below 10K. Calculations of the number of Frenkel pairs produced by various projectiles as a function of energy were performed, based on Lindhards cross sections and a modified Kinchin-Pease expression. A comparison of measured and calculated damage rates shows that the defect-production efficiency decreases with ion mass for light projectiles, whereas at masses >20 the cascade efficiency remains rather constant and assumes a value of xi approximately 0.3 for both Ag and Cu. The decrease in defect efficiency indicates a transition from isolated Frenkel-pair formation to the production of energetic displacement cascades.

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.

Atomic structure of grain boundaries

Abstract Atomic-scale structures of fcc oxide and metal grain boundaries are investigated by highresolution electron microscopy. This paper focuses on the examination of structures that evolve when the grain boundary deviates from the ideal planar and periodic geometry. Examples are given for atomic-scale grain boundary steps with and without dislocation character. The misorientation dependence of atomic-scale grain boundary structures is investigated in 〈110〉 tilt grain boundaries of gold near the (113)(113) low-energy cusp. Common grain boundary relaxation modes include those of stacking disorder, low-index asymmetric facets and microfacets. All of the observed structures and atomic relaxation modes in grain boundaries are consistent with the short-range nature of the interatomic interactions which lead to misfit localization and a tendency to preserve as much as possible the atomic coordination of the defect-free solid.