Sol-gel synthesis and optical band structure modulation of wafer-scale Cr-doped β-Ga₂O₃ thin films

Abstract The structural, optical, and electronic properties of wafer-scale pure and chromium (Cr)-doped β-Ga₂O₃ thin films are comprehensively investigated via sol-gel spincoating method combined with first-principles calculations. High-quality, uniform 2inch films were successfully deposited on sapphire substrates. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses confirmed the formation of the monoclinic β-phase at annealing temperatures of 600 °C and 700 °C, with Cr ions effectively incorporated into the Ga₂O₃ lattice at Ga substitutional sites. Optical characterization revealed that all films exhibit high transmittance (>90% in 300-1100 nm). Increasing annealing temperature sharpened the absorption edge and induced a red shift, reducing the optical band gap from 5.00 eV (400 °C) to 4.93 eV (700 °C). Cr doping further decreased the band gap to 4.85 eV and enhanced absorption in the visible region. Theoretical calculations elucidated that Cr³⁺ doping introduces localized midgap states from Cr-3d orbitals, while oxygen vacancies create deep donor levels. The dielectric function and absorption spectra from DFT align with experimental trends. This work establishes the sol-gel technique as a viable route for large-area β-Ga₂O₃ film fabrication and provides a validated theoretical framework for understanding how Cr doping and point defects tailor the band structure and optoelectronic properties of Ga₂O₃ for advanced device applications.