Stabilizing the metastable γ phase in Ga2O3 thin films by Cu doping

Highlights: • The metastable γ-Ga₂O₃ was stabilized by Cu doping through sol-gel method. • A single γ-phase Ga₂O₃ can be obtained with 12.5 at % Cu doping at 700 °C. • The band gap of Ga₂O₃ thin films displays a clear red-shift with Cu doping. • The PL intensity of γ-Ga₂O₃ at UV-blue emission is obviously stronger than β-Ga₂O₃. The metastable cubic γ phase Ga₂O₃ with a defect spinel structure presents several fascinating properties, which may be a more attractive material than the stable β form. Herein, we found that the metastable γ-Ga₂O₃ can be stabilized by copper (Cu) doping through a simple operation and low cost method of sol-gel. The γ-Ga₂O₃ thin films were prepared under the annealing of 700 °C in nitrogen atmosphere for Cu doping. The β phase Ga₂O₃ ones were obtained at the same condition without the Cu dopant. While the Cu doped γ phase Ga₂O₃ will transform to β phase for a high annealing temperature above 800 °C. The grain size of Cu doped γ-Ga₂O₃ and undoped β-Ga₂O₃ thin films increases with the increase of the annealing temperature. The UV–Vis absorption spectrum of the β-Ga₂O₃ thin film exhibits a sharp intrinsic absorption edge at ∼250 nm, whilst Cu doped γ-Ga₂O₃ thin film displays an obvious red-shift. The band gap decreases from 4.90 eV to 4.38 eV for the Cu doping. The photoluminescence intensity at the ultraviolet and blue region of Cu doped γ-Ga₂O₃ thin films are stronger than undoped β-Ga₂O₃, which can be attributed to the introduction of more defects such as oxygen vacancies in γ phase by Cu doping.