First-principles thermodynamic theory of epitaxial alloys: Prediction of spontaneous rotation of epitaxial habits in InGaN and GaAsSb alloys

A general-purpose method for calculating the stablest ground state structures and finite-temperature thermodynamics of AC-BC alloys grown \emph{epitaxially} on a coherent substrate is presented. In addition to the fact that such coherent epitaxy stabilizes certain ordered phases, we discovered that depending on the substrate and the film concentration there is a spontaneous rotation of the stablest film microstructure. This general behavior is revealed for both mixed-cation (In, Ga)N and mix-anion Ga(As, Sb) alloys on a variety of substrates. Such spontaneous rotation of the epitaxial habits can be understood by the tetragonal ratio $\eta\neq1$ of the corresponding bulk structure.