THE EFFECT OF SUBSTRATE ORIENTATION ON THE CHEMICAL VAPOR-DEPOSITION GROWTH OF ALPHA-FE2O3 ON ALPHA-AL2O3

The early stages of growth of haematite (alpha-Fe2O3) on four orientations of sapphire (alpha-Al2O3), namely (0001), {1102BAR}, {1120BAR} and {1010BAR}, 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 {1102BAR} 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 {1120BAR} and {1010BAR} substrates, the islands tended to grow normal to the substrate surfaces. The growth behaviour of the haematite on the four substrate orientations is discussed in terms of the known crystal growth habits of haematite, the interface energies and the influence of the substrate surface structure.