Strain-induced κ-to-β phase transition and intermediate layer formation at the κ-Ga2O3/β-Ga2O3 interface

Polarization engineering can enable high-density two-dimensional electron gas (2DEG) at the interface of ferroelectric/non-polar κ-Ga2O3/β-Ga2O3 hybrid polymorph structures, whereas the formation of a disordered intermediate layer has prevented the experimental observation of 2DEG, with phase transition being the primary obstacle. In this work, we investigate κ-Ga2O3 epilayers of varying thickness grown on β-Ga2O3 substrates to elucidate the strain-driven phase transition within the intermediate layer at the κ-Ga2O3/β-Ga2O3 polymorph interface. Reciprocal space mapping and high-resolution transmission electron microscopy analyses reveal that a pseudomorphic wetting layer forms during the early growth, followed by the nucleation of mixed κ/β-phase islands. As the film thickens, strain drives further κ-to-β transitions and thickens the disordered intermediate β-phase layer. First-principles calculations confirm that the κ-to-β phase transition is triggered when the in-plane strain exceeds the threshold value of 2.7%. These findings provide critical insights into phase transitions in Ga2O3 hybrid polymorph structures and suggest future strategies for enhancing interface quality, which is crucial for enabling a high-density 2DEG channel through polarization engineering toward the next generation of Ga2O3-based electronic devices.