Influence of lattice constraint from InN and GaN substrate on relationship between solid composition of In_xGa_1-xN film and input mole ratio during molecular beam epitaxy

Thermodynamic analyses were carried out to understand the influence of lattice constraint from InN and GaN substrates on the relationship between solid composition x of In_xGa_1-xN films and input mole ratio R_In (= P_In⁰/(P_In⁰ + P_Ga⁰), where P_i⁰ is the input partial pressure of element i) during molecular beam epitaxy. The calculation results suggest that a compositionally unstable region is found at the GaN-rich region for InGaN on InN at higher temperatures while that for InGaN on GaN can be seen at the InN-rich region. This is because the maximum enthalpy of mixing shifts toward x ∼ 0.10 for InGaN on InN and toward x ∼ 0.80 for InGaN on GaN compared with x ∼ 0.50 for stress-free InGaN.