Preferred orientation in carbon and boron nitride: Does a thermodynamic theory of elastic strain energy get it right?

We address whether the elastic strain-energy theory (minimizing the Gibbs e nergy of a stressed crystal) of McKenzie and co-workers [D. R. McKenzie and M. M. M. Bilek, J. Vac. Sci. Technol. A 16, 2733 (1998)] adequately explai ns the preferred orientation observed in carbon and BN films. In the formal ism, the Gibbs energy of the cubic materials diamond and cubic boron includ es the strain that occurs when the phases form, through specific structural transformations, from graphitic precursors. This treatment violates the re quirement of thermodynamics that the Gibbs energy be a path-independent, st ate function. If the cubic phases are treated using the same (path-independ ent) formalism applied to the graphitic materials, the crystallographic ori entation of lowest Gibbs energy is not that observed experimentally. For gr aphitic (hexagonal) carbon and BN, an elastic strain approach seems inappro priate because the compressive stresses in energetically deposited films ar e orders of magnitude higher than the elastic limit of the materials. Furth ermore, using the known elastic constants of either ordered or disordered g raphitic materials, the theory does not predict the orientation observed by experiment. (C) 1999 American Vacuum Society. [S0734-2101(99)00105-0].