DEGENERATE EPITAXY, COINCIDENCE EPITAXY AND ORIGIN OF SPECIAL BOUNDARIES IN THIN-FILMS

When the substrate crystal is of a higher symmetry than the crystal of the overgrown film, a finite number of differently oriented island-nuclei (n) develop, which ensues a finite number of misorientations (nl) across boundaries in the film where m = C(n, 2). This phenomenon of degenerate epitaxy is observed in the double positioning of the [111] f.c.c. metal films on MoS2 with (112) twin boundaries and the triple positioning of the tetragonal YBa2Cu3 O-7-x (YBCO) on lattice matched cubic SrTiO3 with 90 degrees-boundaries of Sigma 3. The in situ growth of YBCO films on (001) MgO is a good example of coincidence epitaxy, where the lattice misfit is large. Differently oriented island-nuclei form to minimize interfacial energy by maximizing the lattice coincidence according to the Near Coincidence Site Lattice (NCSL) model and minimizing the misfit and strain energy across the film/substrate interface. Thus the number of possible misorientations (m) From the number of differently oriented island-nuclei (n) is given as m = n(2)/2 when n is even and m = (n - 1)(n + 1)/4 when n is odd. Only nine misorientations have been observed compared to the calculated value of 64; extensive grain growth and grain annihilation are believed to be responsible for the disappearance of many misorientations. Those [001] tilt boundaries corresponding to Sigma 5, Sigma 17, Sigma 29, off-Sigma 13 and 45 degrees have all been observed.