Haixia Gao

The structural, electronic, and transport characteristics of Ge-doped ferroelectric-zinc blende phase (FZB)-Ga ₂ O ₃ monolayer are systematically investigated by first-principles calculations, complemented by deformation potential theory and Boltzmann transport theory. Ge dopant preferentially occupies the six-coordinated GaII site with an excellent thermodynamic stability under O-rich condition. The higher electronegativity and the smaller ionic radius of Ge ⁴⁺ relative to Ga ³⁺ promote the stronger Ge–O ionic bonding than that of Ga–O bonds, accompanying with the substantial electron depletion of Ge dopant to the system (losing ∼2.30e quantitatively). A pronounced downward displacement of conduction band minimum (CBM) suggests the n -type conductivity. Applying biaxial strains from –6% to 6%, the bandgap undergoes a steep drop from 3.98 eV to 2.62 eV, which is primarily attributable to the band shifts due to the variations of Ge–O bond lengths. The electron mobility retains strong isotropy under ±6% strain modulations, where the highest μ _ex and μ _ey are of respectively 965.55cm ² V ^–1 s ^–1 and 1093.09 cm ² V ^–1 s ^–1 under +4% strain. Moreover, the mobility anisotropy values range from 1.04 to 1.16, illustrating a robust degree of isotropy. Electrical conductivities show the similar trends with the variations of electron mobility. Our present work demonstrates the pronounced tunability of bandgap and highly isotropic electron mobility in Ge-doped 2D FZB-Ga ₂ O ₃ monolayer, which underscores its strong potential for applications in nanoscale electronic devices.

Doping engineering is crucial for both fundamental science and emerging applications. While transition metal (TM) dopants exhibit considerable advantages in the tuning of magnetism and conductivity in bulk Ga₂O₃, investigations on TM-doped two-dimensional (2D) Ga₂O₃ are scarce, both theoretically and experimentally. In this study, the detailed variations in impurity levels within 3d TM-doped 2D Ga₂O₃ systems have been explored via first-principles calculations using the generalized gradient approximation (GGA) +U method. Our results show that the Co impurity tends to incorporate on the tetrahedral GaII site, while the other dopants favor square pyramidal GaI sites in 2D Ga₂O₃. Moreover, Sc³⁺, Ti⁴⁺, V⁴⁺, Cr³⁺, Mn³⁺, Fe³⁺, Co³⁺, Ni³⁺, Cu²⁺, and Zn²⁺ are the energetically favorable charge states. Importantly, a transition from n-type to p-type conductivity occurs at the threshold Cu element as determined by the defect formation energies and partial density of states (PDOS), which can be ascribed to the shift from electron doping to hole doping with respect to the increase in the atomic number in the 3d TM group. Moreover, the spin configurations in the presence of the square pyramidal and tetrahedral coordinated crystal field effects are investigated in detail, and a transition from high-spin to low-spin arrangement is observed. As the atomic number of the 3d TM dopant increases, the percentage contribution of O ions to the total magnetic moment significantly increases due to the electronegativity effect. Additionally, the formed 3d bands for most TM dopants are located near the Fermi level, which can be of significant benefit to the transformation of the absorbing region from ultraviolet to visible/infrared light. Our results provide theoretical guidance for designing 2D Ga₂O₃ towards optoelectronic and spintronic applications.

Iron impurities are believed to act as deep acceptors that can compensate for the n-type conductivity in as-grown Ga₂O₃, but several scientific issues, such as the site occupation of the Fe heteroatom and the complexes of Fe-doped β-Ga₂O₃ with native defects, are still lacking. In this paper, based on first-principle density functional theory calculations with the generalized gradient approximation approach, the controversy regarding the preferential Fe incorporation on the Ga site in the β-Ga₂O₃ crystal has been addressed, and our result demonstrates that Fe dopant is energetically favored on the octahedrally coordinated Ga site. The structural stabilities are confirmed by the formation energy calculations, the phonon dispersion relationships, and the strain-dependent analyses. The thermodynamic transition level Fe³⁺/Fe²⁺ is located at 0.52 eV below the conduction band minimum, which is consistent with Ingebrigtsen’s theoretical conclusion, but slightly smaller than some experimental values between 0.78 eV and 1.2 eV. In order to provide direct guidance for material synthesis and property design in Fe-doped β-Ga₂O₃, the defect formation energies, charge transitional levels, and optical properties of the defective complexes with different kinds of native defects are investigated. Our results show that V_Ga and O_i can be easily formed for the Fe-doped β-Ga₂O₃ crystals under O-rich conditions, where the +3 charge state Fe_GaGa_i and −2 charge state Fe_GaO_i are energetically favorable when the Fermi level approaches the valence and conduction band edges, respectively. Optical absorption shows that the complexes of Fe_GaGa_i and Fe_GaV_Ga can significantly enhance the optical absorption in the visible-infrared region, while the energy-loss function in the β-Ga₂O₃ material is almost negligible after the extra introduction of various intrinsic defects.