Phase stability of (AlxGa1-x)2O3 polymorphs: A first-principles study

Monoclinic Ga2O3 and (AlxGa1-x)(2)O-3 alloys are wide-band-gap semiconductors with promising applications in power electronics. Although the physical properties of monoclinic Ga2O3 (beta phase) have been extensively explored, information is lacking about other phases (alpha, gamma, kappa) of Ga2O3 and related alloys. Here we use density functional theory to assess the phase stability of different polymorphs of Ga2O3 and (AlxGa1-x)(2)O-3 alloys at both zero and finite temperatures. Due to the preference of Al for the octahedral site, the gamma and kappa phases of (AlxGa1-x)(2)O-3 exhibit a minimum enthalpy of formation at 62.5% and 50% Al concentrations, respectively. Relative to the other phases, the enthalpy of formation of the gamma phase is the highest over the entire range of alloy compositions. We examined the effect of strain arising from pseudomorphic growth of (010)-oriented (AlxGa1-x)(2)O-3 films on beta-Ga2O3 substrates and found that the alloys become less stable. At finite temperature we found that the lattice vibrations tend to stabilize the kappa phase and destabilize the alpha and gamma phases, referenced to the monoclinic phase. This can be attributed to the greater phonon density of states of the kappa phase at low frequencies. A unique configurational entropy that is present in the gamma phase due to the cation vacancy disorder plays a substantial contribution in stabilizing the gamma phase at finite temperature, particularly at 50% Al concentration. Our study provides a comprehensive overview of stability of different phases of (AlxGa1-x)(2)O-3, offering insights into the driving forces for polymorph formation that should prove useful for improved control over phase-pure growth of these important alloys.