Tuning of crystal structure and electronic band gap of the monoclinic Ga2O3 by simultaneous alloying with Al2O3 and In2O3

The paper presents the study of phase and structural behavior of pseudoternary compound (Ga_1-x-yIn_xAl_y)₂O₃ with focus on the compositional cross-section where the x/y ratio is a fixed at 0.31/0.69. This specific ratio ensures that the average cation radius, equal to that of Ga³⁺ ions, remains unchanged. Through the combination of experimental XRD studies and DFT calculations, the stability region of the monoclinic phase within the Ga₂O₃-Al₂O₃-In₂O₃ ternary system was established. Detailed analysis of the crystal lattice parameters and unit cell volume of the monoclinic structure was carried out across a wide range of compositions. An empirical relationship was derived linking the monoclinic lattice parameters to the average ionic radius of the cations (Ga³⁺, Al³⁺, In³⁺) enabling prediction of lattice parameters in monoclinic (Ga_1-x-yIn_xAl_y)₂O₃ solely from chemical composition. The experimental crystal structure studies and the electronic structure calculations suggest that in the monoclinic (Ga_1-x-yIn_xAl_y)₂O₃ structure the tetrahedral positions of Ga1 atoms are preferentially occupied by Ga³⁺ and Al³⁺ cations, while the octahedral Ga2 sites accommodate a mixture of Ga³⁺, Al³⁺ and In³⁺ cations. Additionally, the presence of unidentified phase(s) was confirmed in the central region of the Ga₂O₃-Al₂O₃-In₂O₃ triangle. Comparison of the calculated optical absorption spectra and the Tauc-plots derived from the diffuse reflectance spectra indicate that the monoclinic (Ga_1-x-yIn_xAl_y)₂O₃ compounds have a direct band gap.