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

The crystal structure and ferroelectric properties of ε-GaO 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 GaO showed that it is composed of layers of both octahedrally and tetrahedrally coordinated Ga sites, which appear to be occupied with a 66% probability. The refinement of the crystal structure in the noncentrosymmetric space group P6mc 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 GaO [10-10] direction being parallel to the AlO direction [11-20], yielding a lattice mismatch of about 4.1%.