Crystal Structure and Ferroelectric Properties of ε-Ga₂O₃ Films Grown on (0001)-Sapphire

The crystal structure and ferroelectric properties of ε-Ga₂O₃ deposited by low-temperature MOCVD on (0001)-sapphire were investigated by single-crystal X-ray diffraction and the dynamic hysteresis measurement technique. A thorough investigation of this relatively unknown polymorph of Ga₂O₃ showed that it is composed of layers of both octahedrally and tetrahedrally coordinated Ga³⁺ sites, which appear to be occupied with a 66% probability. The refinement of the crystal structure in the noncentrosymmetric space group P6₃mc pointed out the presence of uncompensated electrical dipoles suggesting ferroelectric properties, which were finally demonstrated by independent measurements of the ferroelectric hysteresis. A clear epitaxial relation is observed with respect to the c-oriented sapphire substrate, with the Ga₂O₃ [10-10] direction being parallel to the Al₂O₃ direction [11-20], yielding a lattice mismatch of about 4.1%.