Thermodynamics of Al_1-xGa_xN solid solution: Inclination for phase separation and ordering

Although Al_1-xGa_xN semiconductors are used in lighting, displays and high-power amplifiers, there is no experimental thermodynamic information on nitride solid solutions. Thermodynamic data are useful for assessing the intrinsic stability of the solid solution with respect to phase separation and extrinsic stability in relation to other phases such as metallic contacts. The activity of GaN in Al_1-xGa _xN solid solution is determined at 1100 K using a solid-state electrochemical cell: Ga + Al_1-xGa_xN/Fe, Ca₃N₂//CaF₂//Ca₃N₂, N₂ (0.1 MPa), Fe. The solid-state cell is based on single crystal CaF₂ as the electrolyte and Ca₃N₂ as the auxiliary electrode to convert the nitrogen chemical potential established by the equilibrium between Ga and Al_1-xGa_xN solid solution into an equivalent fluorine potential. Excess Gibbs free energy of mixing of the solid solution is computed from the results. Results suggest an unusual mixing behavior: a mild tendency for ordering at three discrete compositions (x = 0.25, 0.5 and 0.75) superimposed on predominantly positive deviation from ideality. The lattice parameters exhibit slight deviation from Vegard's law, with the a-parameter showing positive and the c-parameter negative deviation. Although the solid solution is stable in the full range of compositions at growth temperatures, thermodynamic instability is indicated at temperatures below 410 K in the composition range 0.26 ≤ x ≤ 0.5. At 355 K, two biphasic regions appear, with terminal solid solutions stable only for 0 ≤ x ≤ 0.26 and 0.66 ≤ x ≤ 1. The range of terminal solid solubility reduces with decreasing temperature.