Growth mechanism and ferroelectric properties of ε-Ga₂O₃ heteroepitaxially grown on sapphire substrates

This study systematically investigates the influence of growth temperature on the structural, morphological, optical, electrical, and ferroelectric-like properties of ε-Ga₂O₃ thin films heteroepitaxially grown on c-plane sapphire substrates via metalorganic chemical vapor deposition. Films were deposited within a temperature range of 330 °C–380 °C, revealing a clear temperature-dependent phase evolution: amorphous at 330–340 °C, high-quality ε-phase at 350–360 °C, and thermodynamically stable β-phase at 380 °C. The films grown at 360 °C exhibit optimal properties including sharp XRD peaks, minimum FWHM, pronounced six-fold symmetry, vertically aligned columnar grains, smooth surface morphology, and the largest thickness. Optical assessments indicate a direct bandgap of approximately 4.81 eV and transmittance exceeding 80 % in the visible spectrum. XPS analysis shows complete oxidation of gallium and low oxygen vacancies. Electrical characterization reveals the lower resistivity at 360 °C as compared with those of grown at higher temperature, indicative of enhanced charge transport. Additionally, a nonlinear polarization-voltage hysteresis with stable thermal behavior is observed, attributed to the non-centrosymmetric hexagonal P63mc structure of ε-Ga₂O₃. These findings identify 360 °C as the optimal growth temperature for balancing structural integrity and functional performance, emphasizing the potential of ε-Ga₂O₃ as a wide-bandgap polar semiconductor for applications in ferroelectric memory, deep-ultraviolet optoelectronics, and high-power electronics.