Calculated strain energy of hexagonal epitaxial thin films

Generalized formulae for the strain energy of hexagonal thin films on both hexagonal and rhombohedral substrates have been developed. These formulae require knowledge of the elastic stiffness coefficients and the lattice parameters of the film and only the lattice parameters of the substrate. Example calculations of the strain energy present in the strained film-substrate material combinations GaN/Al₂O₃ and GaN/LiGaO₂ are presented for different film crystallographic directions and rotation with respect to the substrate. Finally, phase equilibrium calculations are performed for the Ga-N binary system which show the substantial influence of strain energy on equilibrium in the system.