Theoretical investigation of electronic structure and thermoelectric properties of AlGa point defects in two-dimensional β-Ga2O3

As a wide-bandgap semiconductor, β-Ga₂O₃ possesses excellent physical and chemical properties, such as ultra-low lattice thermal conductivity for thermoelectric applications. We investigated the electronic structure and thermoelectric properties of two-dimensional (2D) β-Ga₂O₃ with Al substitutions at four sites (surface/inner tetrahedral Ga(I) and octahedral Ga(II)) using density-functional theory (DFT) and Boltzmann transport theory. It was found that: (1) Compared to intrinsic 2D β-Ga₂O₃, the bandgap of surface tetrahedral Al-doped 2D β-Ga₂O₃ (I-2D) is reduced by 0.035 eV, and the density of states (DOS) for four distinct Al_Ga point defects in 2D β-Ga₂O₃ indicates that Al(p) orbitals contribute mainly to the valence band maximum (VBM). (2) 2D β-Ga₂O₃ with Al_Ga enhances n-type Seebeck coefficients. Especially, I-2D achieves the maximum power factor of 2.70 × 10¹⁰ W/(m·K²·s) of power factor for n-type near 0.8 eV (300 K), which is higher than the intrinsic value of 1.36 × 10¹⁰ W/(m·K²·s). (3) For n-type, Ⅰ-2D β-Ga₂O₃, the electrical conductivity is 4.66 × 10¹⁸/(Ω·m·s), at 1.0 eV and the electronic thermal conductivity (κ_e) is 1.9 × 10¹³ W/(m·K) near 0.8 eV at 300 K. The Lorenz numbers for the five 2D β-Ga₂O₃ structures all conform to the Lorenz distribution, indicating that the calculation results of thermal conductivity and electrical conductivity are accurate and reliable. (4) Type I-2D β-Ga₂O₃ achieves a peak n-type ZT_e of 1.79 at 900 K (vs. 0.96 for intrinsic). The results demonstrate that Al doping in 2D β-Ga₂O₃ optimizes thermoelectric properties for applications.