Scott R. Summerfelt

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.

Surface preparation for the heteroepitactic growth of ceramic thin films

The morphology, composition, and crystallographic orientation of the substrate influence the nucleation and growth of deposited thin films. A method for the preparation of controlled, characteristic surfaces is reported. The surfaces are suitable for the heteroepitactic growth of thin films. When used in the formation of electron‐transparent thin foils, the substrates can be used to investigate the very early stages of film growth using transmission electron microscopy. The substrate preparation involves the cleaning and subsequent annealing to generate a surface consisting of a series of steps. The step terraces are formed on the energetically stable surface, and controlled nucleation and growth of films at step edges is found. The substrate materials prepared using this technique include (001) MgO, (001) SrTiO3, and (001) LaAlO3.

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...

Thin-film NiOAl2O3 reaction couples prepared by CVD

Summary A CVD technique for preparing thin-film reaction couples for examination in the TEM has been described. Epitactic relations between NiO, NiAl 2 O 4 and Al 2 O 3 have been studied with this technique. Preferred growth directions in the NiAl 2 O 4 reaction layer have been determined and defects associated with the phase transformation have also been monitored.

The movement of the spinel-NiO interface in thin films

Abstract The effect of heat-treatment on precipitate morphology has been studied by transmission electron microscopy. In this study, a thin-film sample was prepared from bulk material in which spinel (NiFe2O4) particles had been precipitated within a NiO matrix. Individual precipitates were then characterized by dark-field imaging in the electron microscope between a series of annealing treatments. These coherent precipitates transform from dendrites, which have “arms” in the 〈001〉 directions bounded by {111} and {011} planes, to octahedra bounded by {111} planes. In addition, smaller precipitates have also been observed to shrink and even disappear between a series of anneals.

Effect of a Ge Barrier on the Microstructure of BaTiO3 Deposited on Silicon by Pulsed Laser Ablation

BaTiO 3 (BT) thin films deposited using pulsed laser ablation on substrates of (100) Si and (100) Si with 0.3 μm Ge were examined. For one set of samples, approximately 2000 A of BT was deposited at 70°C at O 2 pressures of 0, 1.0 and 10.0 mT. In a second set, O 2 pressures of 0 and 1.0 mT were used during deposition of 200 A of BT at 450°C followed by a 5 minute anneal and deposition of an additional 2000 A at 750°C. This gave a total sample matrix of 10 samples. Cross-sectional TEM revealed that an interfacial layer formed in the BT on Si samples but not in the BT on Ge samples. HREM analysis of the interfaces showed that the interfacial layer was amorphous. On Ge, the BT films were found to have large areas of epitactic growth along the interface. This was confirmed by diffraction tilt angle experiments which showed a strong preferred orientation of BT on Ge. No preferred orientation was found for BT on Si. Statistical grain size analysis of the films using multiple regression showed that the film microstructures were affected most strongly by the substrate type (Ge or Si) followed by the deposition temperature of the substrate. Only a weak effect of O 2 pressure was observed.

Reaction couples of ceramic thin films prepared by CVD

Abstract A thin-film reaction-couple experiment has been designed for monitoring solid-state reactions in the electron microscope. NiO particles were deposited by chemical-vapor deposition onto bulk α-Al 2 O 3 substrates and electron-transparent α-Al 2 O 3 thin films. The NiO-α-Al 2 O 3 reaction couples were then annealed and the formation and growth of NiAl 2 O 4 were analyzed. Special crystallographic relations have been determined, and effects associated with phase transformation were examined. The experiment allows both NiO/NiAl 2 O 4 and NiAl 2 O 4 /α-Al 2 O 3 interfaces to be analyzed from the same reaction couple and provides the opportunity to characterize phase transformation kinetics for a particular interface structure.

Metal-Ceramic Interphase Interfaces: Preparation and Structural Characterization

The earliest stages of epitaxy between metals and ceramics have been characterized using a novel transmission electron microscope experiment. Metal particles were deposited onto ceramic substrates which were previously thinned to electron transparency. Illustrations of epitactic relations, the influence of surface defects on particle nucleation and the growth of metal particles on single-crystal ceramic substrates are provided for the Cu / A1 2 O 3 , W / β-SiC and Cu / MgO systems.

Imaging of III-V Compound Superlattices by Hrem and Rem

Two techniques for the analysis of III-V compound superlattices are examined. It has been proposed that high-resolution TEM of [100]-oriented thin foils would give an improvement in layer contrast compared with [110]-oriented thin foils; it is shown here that the contrast of [100]-oriented superlattices is not necessarily better. Moreover, both high resolution and conventional dark-field imaging may be subject to significant diffraction contrast effects resulting from the bending of the reflecting planes near the surface of the sample. Reflection electron microscopy (REM) of cross-sectional (110) cleavage planes can also yield dark-field superlattice images and selected area RHEED patterns can in principle be used to determine reliably the superlattice strain as surface effects are minimized.