Atomic-level revelation of spontaneous polarization orientation and piezoelectricity in <b> <i>ε</i> </b> -Ga2O3

Advancing polarization control in semiconductors is pivotal for next-generation electronics, enabling revolutionary advances in energy and industrial systems. The metastable ε-Ga2O3 holds promise for polarization-engineered devices but remains hindered by unresolved polarization orientation and ambiguous piezoelectric responses. Here, using an integrated experimental and theoretical approach, we demonstrate that ε-Ga2O3 exhibits a spontaneous polarization (Psp) of −24.8 μC/cm2 oriented antiparallel to the crystal growth direction. The piezoelectric coefficient d33 was experimentally measured as 4.125 pm/V, in strong agreement with the theoretical value of 4.93 pm/V. The phase-pure ε-Ga2O3 films were grown via low-pressure mist-CVD with exceptional crystallinity, as evidenced by an x-ray diffractometer rocking curve [full-width-at-half-maximum (FWHM) = 0.08°]. Optimized piezoelectric force microscopy protocols were employed to determine d33, while the orientation of Psp was resolved using pulsed DC bias-dependent amplitude/phase-voltage measurements combined with aberration-corrected scanning transmission electron microscopy. This multimodal methodology enabled direct mapping of bound charge distributions at the film surface and provided atomic-scale visualization of crystal orientation. These findings clarify ε-Ga2O3 polarization ambiguities, establish structure–property relationships, and unlock transformative potential for advancing power electronics, high-frequency communication systems, and energy-efficient memory technologies.