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

      Researchers from the Shihezi University have published a dissertation titled "Theoretical investigation of electronic structure and thermoelectric properties of Al_Ga point defects in two-dimensional β-Ga₂O₃" in Results in Physics.

Project Support

      This work was financially supported by National Natural Science Foundation of China (Grant nos. 62104162, 12264041, 12164038).

Background

      Thermoelectric materials, which enable direct interconversion between thermal and electrical energy, hold unique value in waste heat recovery and solid-state refrigeration technologies. Among emerging wide-bandgap semiconductors, two-dimensional (2D) monoclinic gallium oxide (β-Ga₂O₃) has attracted significant attention due to its ultra-low lattice thermal conductivity (0.71 W/(m·K) at 300 K, nearly 20 times lower than bulk β-Ga₂O₃). H. J. Wu et al. reported a sucrose-derived β-Ga₂O₃ with intentionally engineered porosity through low-temperature sintering (600 °C), achieving low thermal conductivity of 0.45 W/(m·K) (room temperature), which further decreases to 0.29 W/(m·K) at 600 °C. In addition, the intrinsic wide bandgap of β-Ga₂O₃ in bulk crystals and thin films, which is typically 4.70 eV ∼ 4.85 eV at room temperature, fundamentally limits carrier concentration and electrical conductivity. Studies show 2D β-Ga₂O₃ is an indirect-bandgap semiconductor like bulk β-Ga₂O₃. The bandgaps of 2D β-Ga₂O₃ calculated by GGA, HSE and GGA + U methods are 2.30 eV,4.69 eV and 4.66 eV respectively. Nan Chen et al. calculated the band gap of 2D β-Ga₂O₃ to be 1.89 eV using the PBE method, and 4.91 eV using the GGA + U method. The researchers found that defects can regulate the bandgap of β-Ga₂O₃. 

Abstract

      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.

Highlights

      ● Electronic structure regulation: In Al-doped 2D β-Ga₂O₃, surface tetrahedral doping (Ⅰ-2D) reduces the bandgap by 0.035 eV. Al(p) orbitals contribute significantly to the valence band maximum (VBM). All Al-doped structures exhibit negative formation energies, indicating the thermodynamic spontaneity of the doping process.

      ● Optimization of thermoelectric properties: Al doping significantly enhances the n-type Seebeck coefficient. Specifically, Ⅰ-2D achieves a power factor of 2.70 × 10¹⁰ W/(mK²s) near 0.8 eV at 300 K, which is twice the intrinsic value. Its n-type electrical conductivity reaches 4.66 × 10¹⁸/(Ωms) at 1.0 eV, and the electronic thermal conductivity (κ_e) is 1.9 × 10¹³ W/(mK) near 0.8 eV.

      ● Significant improvement in ZT_e: Ⅰ-2D attains a peak n-type ZT_e of 1.79 at 900 K, much higher than the intrinsic structure’s value of 0.96. The Lorenz numbers conform to the theoretical distribution, verifying the reliability of the calculated electrical and thermal conductivities, suggesting that Al-doped 2D β-Ga₂O₃ is a promising material for high-efficiency thermoelectric applications.

Conclusion

      This study systematically investigates the electronic structure and thermoelectric properties of intrinsic and 2D β-Ga₂O₃ with four distinct Al_Ga point defects through first-principles calculations and Boltzmann transport theory. The key findings are summarized as follows:

      (1) Under the GGA-PBE framework, the β-Ga₂O₃ bandgap calculated by us is lower than the experimental value, but it can still correctly reflect the trend of bandgap changes before and after Al doping and does not affect subsequent experimental studies related to thermoelectric data. The band structure calculation results show that when aluminum is doped into the surface tetrahedral Ga(II) sites, the calculated bandgap decreases from the intrinsic value of 1.595 eV to 1.560 eV. (2) For n-type, the power factor of 2D β-Ga₂O₃ with Al_Ga point defects is significantly enhanced at 300 K. Especially, the power factor of Ⅰ-2D β-Ga₂O₃ is enhanced from an intrinsic value of 1.36 × 10¹⁰ W/(m·K²·s) to 2.70 × 10¹⁰ W/(m·K²·s) near 0.8 eV at 300 K. This PF enhancement is attributed to the combined effect of the Seebeck coefficient (−2570 μV/K) and electrical conductivity (4.66 × 10⁸/(Ω·m·s)). Meanwhile, the Lorentz number analysis shows that the calculation results of thermal conductivity and electrical conductivity are accurate and reliable. (3) For n-type, ZT_e of surface tetrahedral Al-doped 2D β-Ga₂O₃ (Ⅰ-2D) increases from 0.96 to 1.79 at 900 K. Therefore, the results indicate that Al doping engineering in 2D β-Ga₂O₃ can provide a theoretical reference for thermoelectric applications.