Strain-Induced Metastable Phase Stabilization in Ga₂O₃ Thin Films.

Area of Science:

• Materials Science
• Solid State Physics
• Crystallography

Background:

• Metastable and transient structures can be stabilized in thin films using epitaxial strain.
• The stabilization mechanisms of these phases, especially in thin-film gallium oxide (Ga₂O₃), are not well understood.
• Gallium oxide thin films exhibit multiple crystal phases influenced by growth conditions and constraints.

Purpose of the Study:

• To investigate the atomic-scale development and distribution of epitaxial strain at Ga₂O₃/Al₂O₃ interfaces.
• To elucidate the mechanism by which unstable or metastable phases are stabilized in thin-film gallium oxide.
• To understand the role of strain accommodation in phase transformation for designing functional thin-film materials.

Main Methods:

• Aberration-corrected scanning transmission electron microscopy (STEM) for atomic-resolution imaging of Ga₂O₃/Al₂O₃ interfaces.
• High-resolution electron microscopy (HREM) to identify transition phase structures.
• Density functional theory (DFT) calculations to complement experimental observations.

Main Results:

• Metastable α-Ga₂O₃ accommodates misfit strain directly at the substrate interface.
• A transient crystalline phase (4-5 layers) acts as an intermediate to release strain.
• The unstable κ-Ga₂O₃ phase is stabilized as the predominant thin-film phase.
• Epitaxial strain is accommodated by oxygen polyhedra rearrangement, influencing Ga³⁺ ion site occupancy (octahedral vs. tetrahedral) and driving phase transformation.

Conclusions:

• Epitaxial strain engineering is crucial for stabilizing desired phases in thin-film materials.
• The observed phase transformation in Ga₂O₃ thin films is driven by strain-induced changes in atomic structure and Ga³⁺ ion distribution.
• Understanding these atomic-scale mechanisms provides insights for designing advanced functional thin films.