The Structure of Interfaces in Oxide Heterojunctions Formed by CVD
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THE STRUCTURE OF INTERFACES IN OXIDE HETEROJUNCTIONS FORMED BY CVD LISA A. TIETZ, SCOTT. R. SUMMERFELT, AND C. BARRY CARTER
Cornell University, Department of Materials Science and Engineering, Ithaca, NY 14853
ABSTRACT Weak-beam imaging is used to characterize the interface structure of hematite (a-Fe203) / sapphire (a-A1203) heterojunctions parallel to (0001). The heterojunctions were prepared by lowpressure chemical vapor deposition of hematite directly onto electron-transparent sapphire substrates. Bright-field imaging and selected-area diffraction show that the growth is epitactic. The 5.5% lattice misfit at the interface is found to be accommodated by one of two different hexagonal dislocation networks: (1) b = 1/3 or (2) b = 1/3. The latter are associated with a "basal twin"-type orientation relationship of the hematite and the alumina. INTRODUCTION Interface structures at heterojunctions in metals and semiconductors have been extensively studied. Networks of lattice misfit-accommodating dislocations are commonly observed at such interfaces. In the case of interfaces parallel to two close-packed planes, hexagonal arrays of edge dislocations are typically observed. In addition to dislocation networks, planar defects such as stacking faults and twins are also common at such interfaces [1]. Other materials with one or more close-packed planes in their structures are expected to exhibit similar behavior. Hematite (ac-Fe20 3 ) and sapphire (a-A1203) were chosen as a model system for the study of the formation and structure of oxide heterojunctions. Both materials have the corundum-type crystal structure (Space Group = R3c); hematite having a 5.5% larger lattice spacing. The (0001) plane is a close-packed plane in the oxygen sublattice. Defects in the individual materials, particularly in ot-A120 3 , have been the subject of many investigations [e.g., 2-5]. In addition, these materials exhibit little interdiffusion and no reaction at the temperatures used in this study. The present work forms part of a larger investigation into the relationship between substrate surface structure and the formation of defects in thin films [6,7]. Here, the structure of the (0001) hematite / sapphire interface is characterized using transmission electron microscopy (TEM).
HETEROJUNCTION PREPARATION Hematite / alumina heterojunctions were prepared using low-pressure chemical vapor deposition (LPCVD) to deposit a thin film of hematite directly onto electron-transparent, (0001)-oriented, sapphire substrates. The method has been described in detail elsewhere and is
summarized here [6,7]. The thin-foil alumina substrates were prepared by conventional dimpling and ion-thinning to perforation of (0001)-oriented, 3-mm. sapphire discs. The samples were then acid-cleaned and annealed at 1400TC according to the method described by Susnitzky, et.al., to produce a stepped surface structure that could be imaged in the transmission electron microscope [8]. Hematite was produced by reacting FeC13 vapor, from a solid FeC13 source, with water vapo
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