EPITAXY, MICROSTRUCTURE, AND PROCESSING-STRUCTURE RELATIONSHIPS OF TIO2 THIN-FILMS GROWN ON SAPPHIRE (0001) BY MOCVD

TiO2 thin films have been deposited on sapphire (0001) substrates unde r various conditions by metal-organic chemical vapor deposition. The s tructural properties of the deposited films were characterized by x-ra y diffraction and transmission electron microscopy. The important grow th parameters were found to be the deposition temperature and the depo sition rate. The ranges studied for the two parameters were 400 to 850 -degrees-C and 10 to 120 angstrom/min, respectively. Depending on the growth conditions, most of the deposited films were either single-phas e anatase or rutile, or a mixture of the two. These films were all epi taxial, but none of them were single-crystal films. Three distinct epi taxial relationships were observed between the films and the substrate s, and, depending on the growth conditions, a deposited film can conta in one, two, or all three of them. The fact that the films we obtained , although epitaxial, were never single crystal is explained based on the consideration of the difference in the rotational symmetries of th e substrate surface and the film growth plane. We believe that it shou ld be generally true that, in heteroepitaxial growth, a true single-cr ystal film can never be obtained as long as the point symmetry group o f the substrate surface is not a subgroup of that of the film growth p lane.