Internal strain mechanism of the incomplete phase separation of nonstoichiometric silicon oxide films

•Extension of the Gibbs free energy to describe additional mechanism is required.•Mathematical structure of extending term to describe this mechanism is derived.•Compliance of strain Gibbs free energy with found mathematical structure is shown.•Strain mechanism allows explaining experimental equilibrium stoichiometry behavior.•Temperature dependence of strain Gibbs free energy and its parameters are found.

The role of internal strain on the formation of equilibrium states during phase separation of nonstoichiometric silicon oxide films is demonstrated. The strain contribution to the Gibbs free energy of Si/Si oxide systems has suitable mathematical structure to provide an explanation of the experimentally observed dependence of the equilibrium stoichiometry index of Si oxide phase on its initial value and annealing temperature. The strain free energy is shown to exponentially decrease with the increase in annealing temperature, which is consistent with strain relaxation. Obtained results make a step forward toward a development of a comprehensive thermodynamic theory of phase separation in nonstoichiometric silicon oxide films.