Electrical, optical and structural characteristics of gallium oxide thin films deposited by RF-sputtering

Highly conductive n-type nanometric films of copper doped amorphous-Ga₂O₃ (amorphous-Ga₂O₃:Cu) or copper doped β-Ga₂O₃ (β-Ga₂O₃:Cu) were produced by forming amorphous-Ga₂O₃/Cu/amorphous-Ga₂O₃ arrays (amorphous-Ga₂O₃:Cu arrays) at room temperature with electrically insulating amorphous-Ga₂O₃ films grown by RF-sputtering and a Cu film grown by DC-sputtering. To study their electrical properties and to control their structural characteristics the amorphous-Ga₂O₃:Cu arrays were annealed in dry N₂ atmosphere at temperatures between 300 and 400 °C by 100 min. When the amorphous-Ga₂O₃:Cu arrays were annealed at temperatures of 400 °C by longer periods the layers acquire electrical insulating characteristic. The lowest resistivity of the n-type β-Ga₂O₃:Cu films were of 1.83 × 10⁻⁵ Ω cm with an electron concentration of 5 × 10²² cm⁻³ and carrier mobility of 9.7 cm²/Vs. The introduction of the Cu layer allows reducing the temperature required to transform the amorphous Ga₂O₃ films to a nanocrystalline phase β-Ga₂O₃ at ∼350 °C according to the X-ray diffraction analysis. Studies by SEM confirm the phase transition by the formation of β-Ga₂O₃ octahedral shape nanocrystallites with sizes from ∼5 to 50 nm. The optical transmittance of the undoped amorphous-Ga₂O₃ in the visible region is ~ 100% with an optical gap of 4.06 eV. The insertion of the Cu layer reduces the transmittance up to 40% but the structural and electrical properties are useful to build electronic devices.