Matthew T. Johnson

THIN-FILM REACTIONS

Thin-film geometries, where at least one of the reactants is in the form of a thin film, are particularly useful for studying the earliest stages of solid-state reactions in oxides. In the present work, a thin-film approach has been developed to study the earliest stages of reactions in two model oxide systems. The first being NiO/Al203, where the NiO is an epitactic film on various orientations of single-crystal A1203. In this case, both interfacialreaction and diffusion control of the kinetics have been observed and correlated with the interfacial structure. In the second, a thin-film approach has also allowed reaction kinetics in the system Fe20,/MgO to be measured. In this case, the Fe203 is an epitactic film on

Iron oxide on (001) MgO

Abstract Transmission and scanning electron microscopy have been used to characterize thin films of Fe oxide grown on (001)-oriented MgO using pulsed-laser deposition. Several films, each with a longer deposition time, and hence increasing thickness, were grown under similar conditions of oxygen pressure, deposition rate and substrate temperature. For shorter deposition times, i.e. thinner films, a spinel-structured Fe oxide film occurred, while for longer deposition times a corundum-structured Fe oxide film formed. In some instances it was found that the film had a mixture of the two structures of Fe oxide. The analysis of these films determined that the corundum-structured Fe oxide was being formed via a transformation from the spinel structure. This transformation process led to interesting thin-film morphologies and epitactic alignments.

Thin-Film Reaction between [alpha]-Fe2O3 and (001) MgO.

The kinetics of a thin-film, solid-state reaction were investigated in the spinel-forming oxide system Fe2O3/MgO. In this study, epitactic thin films of Fe2O3 (α, or corundum, structure) were deposited on (001)-oriented MgO using pulsed-laser deposition (PLD). The resulting diffusion couples were then reacted at elevated temperatures in air to induce the reaction between the thin-film and bulk substrate to form the spinel, MgFe2O4. Both the as-deposited and reacted diffusion couples were characterized using low-voltage scanning and transmission electron microscopy. These techniques allow the kinetics of the reaction and the structural properties of the spinel to be investigated.

AlN films grown by electric field induced flux of Al cations

Abstract Thin films of aluminum nitride (AlN) have been grown on (0001) oriented sapphire ( α -Al 2 O 3 ) substrates through the use of an externally applied electric field. By applying an electric field with an appropriate electrode material, at elevated temperatures, across Al 2 O 3 , a flux of Al 3+ cations toward the cathode was induced. The cations arriving at the surface of the Al 2 O 3 then reacted with a nitrogen gas atmosphere to form a thin epitactic film of AlN on the Al 2 O 3 .

Movement of Pt markers in MgO during a solid-state reaction

The effectiveness of Pt in marking the initial location of heterophase boundaries between two reactants in a thin-film solid-state reaction is discussed. The Pt markers were produced by dewetting a continuous Pt film at elevated temperatures, thus forming an array of small isolated particles. These particles can then serve as fine-scale markers for tracking the motion of interfaces. The thin-film diffusion couples with Pt markers were used, in this study, to investigate the effect of an applied electric field on a spinel-forming reaction. The reacted diffusion couples with Pt markers were analysed using both transmission and scanning electron microscopy. Analysis of the diffusion couples showed that the Pt markers could be affected by, or themselves affect, the reaction process. The interface between the Pt marker and surrounding oxide matrix plays an important role in the reliability of Pt as an effective marker.

Crystal Defects In Gan On (0001) Sapphire

Defect structures in GaN thin films grown on (0001) sapphire have been studied using a combination of different transmission electron microscopy (TEM) techniques. Two fundamentally different types of defects are found in these films. Planar defects which lie on planes perpendicular to the growth surface are common. In some regions of the films, other planar defects are present which run parallel to the surface of the substrate. The terminology used to describe these different defects varies quite widely in the literature and includes combinations of antiphase (inversion) domain boundaries and stacking faults. The second type of defect is generally referred to as a threading dislocation since many thread through the whole thickness of the film. Dislocations with different Burgers vectors have been identified in this work and in previous studies; these dislocations usually have a component of their Burgers vector lying normal to the (0001) plane. The overall defect structures in these films have been characterized using conventional bright-field and dark-field imaging. The detailed structure of the individual defects have been examined using weak-beam microscopy both in plan view and in cross section. This paper illustrates the different types of defects, both planar and linear, compares them to defects which have been characterized more thoroughly in related materials, and discuss the nomenclature of the different defect configurations.