Tailoring the structural and functional properties of spray-pyrolyzed Ga₂O₃ thin films via Al doping

Undoped and Al-doped α-Ga₂O₃ thin films (1-3 vol % of Al in precursor solution) were successfully deposited on Si substrates by spray pyrolysis at 500 °C and systematically investigated in terms of morphological, structural, compositional, optical, and electrical properties. Atomic force microscopy analysis revealed that moderate Al incorporation significantly improves surface morphology, reducing the root mean square roughness from 2.0 nm (undoped) to a minimum of 1.25 nm at 2 vol % Al, followed by a slight increase at 3 vol % Al due to possible lattice strain and compositional inhomogeneity. Cross-sectional scanning electron microscopy images confirmed the formation of dense, crack-free, and well-adhered films with thicknesses ranging from 445 to 540 nm, indicating that Al incorporation affects growth kinetics. Energy-dispersive X-ray analysis verified successful Al incorporation and slight compositional variations, suggesting modifications in defect chemistry. X-ray diffraction results demonstrated that all films crystallize in the α-Ga₂O₃ phase with a corundum-type structure, while Al doping significantly influences crystal orientation, crystallite size, and lattice strain without altering the phase. Optical measurements revealed high transparency (≈90%) in the visible region for all samples. The optical bandgap increased slightly from 4.82 eV (undoped) to 4.87 eV (3 vol % Al), indicating successful substitutional incorporation of Al³⁺ and tunable optical properties. Electrical measurements showed linear current-voltage characteristics, confirming ohmic conduction. The resistance decreased from 17.32 MΩ (undoped) to 0.47 MΩ (3 vol % Al), demonstrating enhanced conductivity while preserving the semiconducting nature of the material. Overall, controlled Al doping effectively tailors the microstructural, optical, and electrical properties of α-Ga₂O₃ thin films, with 2 vol % Al identified as the optimal concentration for achieving improved surface morphology and balanced functional performance.