Strain-Induced Metastable Phase Stabilization in Ga2O3 Thin Films

It is well known that metastable and transient structures in bulk can be stabilized in thin films via epitaxial strain (heteroepitaxy) and appropriate growth conditions that are often far from equilibrium. However, the mechanism of heteroepitaxy, particularly how the nominally unstable or metastable phase gets stabilized, remains largely unclear. This is especially intriguing for thin-film Ga2O3, where multiple crystal phases may exist under varied growth conditions with spatial and dimensional constraints. Herein, the development and distribution of epitaxial strain at the Ga2O3/Al2O3 film-substrate interfaces is revealed down to the atomic resolution along different orientations, with an aberration-corrected scanning transmission electron microscope. Just a few layers of metastable alpha-Ga2O3 structure were found to accommodate the misfit strain in direct contact with the substrate. Following an epitaxial alpha-Ga2O3 structure of about couple unit cells, several layers (4-5) of transient phase appear as the intermediate structure to release the misfit strain. Subsequent to this transient crystal phase, the nominally unstable kappa-Ga2O3 phase is stabilized as the major thin-film phase form. We show that the epitaxial strain is gracefully accommodated by rearrangement of the oxygen polyhedra. When the structure is under large compressive strain, Ga3+ ions occupy only the oxygen octahedral sites to form a dense structure. With gradual release of the compressive strain, more and more Ga3+ ions occupy the oxygen tetrahedral sites, leading to volumetric expansion and the phase transformation. The structure of the transition phase is identified by high-resolution electron microscopy observation, complemented by the density functional theory calculations. This study provides insights from the atomic scale and their implications for the design of functional thin-film materials using epitaxial engineering.