Thermodynamic and Kinetic Effects on the Nucleation and Growth of epsilon/kappa- or beta-Ga2O3 by Metal-Organic Vapor Phase Epitaxy

In this paper, we focus on the growth of ?- and ?/?-Ga2O3 thin films via metal-organic vapor phase epitaxy on c-plane sapphire using water and trimethyl-gallium at temperatures between 610 and 650 °C. Using these precursors, the monoclinic ?-phase is usually obtained only at temperatures higher than 700 °C. We show here, for the first time, that both ?- and ?-Ga2O3 can also be obtained by tuning the growth rate of the film, that is, by controlling the supersaturation of the vapor phase. The experimental findings are discussed in the framework of classical nucleation theory and Ostwald's step rule, showing the interplay of thermodynamic (related to different chemical potentials for the metastable ?/? phase and the stable ? phase) and kinetic effects (mainly related to different surface energy barriers for nuclei of different crystallographic phases/ planes). The experimental conditions that permit the nucleation and growth of the desired Ga2O3 polymorph are identified and thoroughly explained, giving to this work a fundamental as well as a technological relevance.