Correlation between small polaron tunneling relaxation and donor ionization in Ga₂O₃

Pulsed laser deposition is employed to fabricate as-grown amorphous and post-growth annealed crystalline β-Ga₂O₃ films. The films annealed at temperatures above 600 °C are found to exhibit a pure monolithic phase with a bandgap of 4.7 eV. The thermally activated donor ionization and dielectric relaxation of these films are systematically investigated by temperature-dependent DC and AC conductivity measurements, and complex electric modulus analysis. A donor level at ∼180 meV below the conduction band edge and a small polaron tunneling (SPT) relaxation with an activation energy of ∼180 meV are observed in the as-grown amorphous Ga₂O₃ film but not in the monolithic β-Ga₂O₃ film. The SPT occurs between donor sites with its thermal relaxation of polarization being associated with the thermal ionization of the donor state. Thermal annealing of the amorphous films removes the 180 meV donors as well the corresponding SPT relaxation.