Thermodynamics and Phase Stability in the Ga–N System

Abstract

The decomposition of GaN powder was studied experimentally using two different customized thermogravimetric methods, dynamic oscillation TGA, and isothermal stepping TGA for a higher resolution of the decomposition start. A reproducible mass gain at slightly lower temperature suggests the equilibrium temperature to be at 1110±10 K under 1 bar of nitrogen. A CALPHAD type thermodynamic analysis of all available phase equilibrium and thermodynamic data is performed. This includes the determination of the absolute entropy of GaN, 30±4 J/mol K, based on a Debye- and Einstein-analysis of the experimental data on the heat capacity. An explicit equation for the fugacity–pressure relation of nitrogen, f( P), is developed, which is useful for the conversion of complex phase diagram calculation output. This is crucial because f can be several orders of magnitude higher than P for the high pressures encountered during GaN decomposition. Based on the consistent thermodynamic description, developed for the Ga–N system, all thermodynamic data and various phase diagrams are calculated. They indicate a good overall agreement between the different types of experimental data (calorimetric, vapor pressure, phase equilibrium). The high pressure part of the decomposition pressure of GaN is actually predicted from the thermodynamic model in good agreement with the experimental data.