【Member Papers】Feasibility of p-type conduction in beta rhodium alloyed gallium oxide

      Researchers from the Shenzhen Pinghu Laboratory have published a dissertation titled "Feasibility of p-type conduction in beta rhodium alloyed gallium oxide" in Journal of Applied Physics.

Project Support

      The authors acknowledge financial support from the Shenzhen Pinghu Laboratory Project (Grant No. 224110) and the computing resources from the National Supercomputing Center in Shenzhen.

Background

      Metal oxides have extensive applications in various fields, including dielectrics, supercapacitor electrodes, photocatalysts, thin-film transistors, and others. However, most oxides' valence band maxima (VBMs) mainly consist of deep and localized oxygen 2p orbitals, leading to low hole mobility and the presence of deep acceptors. In 1997, Kawazoe et al. proposed a chemical design concept for p-type metal oxides. According to this concept, to mitigate the localization of the valence band edge, cationic species should possess orbitals, such as nd¹⁰, whose energy is almost comparable to that of oxygen 2p orbitals. This allows the formation of covalent bonds with oxide ions. Additionally, crystal lattice fields, such as tetrahedral or octahedral coordination of oxide ions, can strengthen these covalent bonds and further reduce the localization of the oxygen 2p orbital. Based on this concept, several p-type metal oxides have been developed, such as Cu₂O, SnO, and spinel ZnRh₂O₄. Despite these advancements, employing these p-type materials in power electronics remains a formidable challenge. Cu₂O often co-mixes with Cu and CuO. SnO easily converts into SnO₂ via oxidation and thus exhibits n-type behavior. ZnRh₂O₄ has a relatively small bandgap, approximately 2.1 eV. NiO, a commonly used p-type oxide, still suffers from insufficient hole mobility. In essence, oxides with both ultrawide bandgaps and favorable p-type conduction characteristics are yet to be discovered and explored.

Abstract

      Beta-gallium oxide (β-Ga₂O₃) has attracted extensive attention in the field of power devices owing to its ultrawide bandgap and well-established synthesis method. However, the lack of p-type conductivity in β-Ga₂O₃ limits the device performance. Recently, rhodium-alloyed β-Ga₂O₃, namely, β-(Rh_xGa_1−x)₂O₃, has been predicted to possess elevated valence band maxima (VBMs) and reduced hole masses. This study explores the feasibility of realizing p-type doping in β-(Rh_xGa_1−x)₂O₃. The acceptor levels for Li, Na, and Cu dopants in β-(Rh_0.25Ga_0.75)₂O₃ are determined to be lower than 0.4 eV from the VBMs. The elevation of the VBM, along with a weak interaction between the dopants' orbitals and the orbitals at the VBMs, plays a crucial role in facilitating the attainment of a shallow acceptor level. At a hole density of 10¹⁷ cm⁻³ and room temperature, the hole mobility of β-(Rh_0.5Ga_0.5)₂O₃ is predicted to reach 10.7 cm² V⁻¹ s⁻¹, which is higher than the values of most p-type metal oxides. Moreover, the hole mobility is determined to be anisotropic, and its magnitude is mainly limited by polar optical phonon scattering. Our work shows that achieving p-type doping in β-(Rh_xGa_1 − x)₂O₃ alloys is indeed feasible, and the formation of p–n homojunctions based on β-(Rh_xGa_1 − x)₂O₃ could significantly extend the application scope of alloy oxides.

Conclusion

      In summary, our research has delved into the feasibility of p-type conduction within the β-(Rh_xGa_1 −x)₂O₃ alloys. We investigated possible p-type doping strategies for β-(Rh_0.25Ga_0.75)₂O₃ and β-(Rh_0.5Ga_0.5)₂O₃, along with the hole mobilities of β-(Rh_xGa_1 −x)₂O₃ over the range of 0 ≤ x ≤ 0.5. Regarding the p-type doping schemes, we considered dopants such as Li, Na, K, Be, Mg, Ca, Cu, Ag, Au, Zn, Cd, Hg, and Ga vacancies. The valence band energies and the bonding behaviors of these dopants play crucial roles in determining the magnitude of acceptor levels. For a specific dopant in a similar location, the acceptor level decreases as the valence energy increases. In β-(Rh_0.5Ga_0.5)₂O₃, the acceptor level of the Mg dopant was determined to be 0.543 eV. The acceptor levels of Li and Na dopants at the octahedral Ga site are significantly lower than those at the tetrahedral Ga site, with values of 0.388 and 0.381 eV, respectively. These lower acceptor levels can be ascribed to the weaker interactions between the dopants' orbitals and the orbitals at the valence band maxima. For the hole mobilities, the β-(Rh_xGa_1 −x)₂O₃ alloys typically show higher values compared to β-Ga₂O₃. Notably, this is particularly true for β-(Rh_0.25Ga_0.75)₂O₃ and β-(Rh_0.5Ga_0.5)₂O₃. At room temperature, the hole mobility of β-(Rh_0.5Ga_0.5)₂O₃ reaches 10.7 cm² V⁻¹ s⁻¹ at a hole concentration of 10¹⁷ cm⁻³, which is higher than the values of most p-type metal oxides. The hole mobility displays strong anisotropy, and its magnitude is limited by polar optical scattering. Overall, the β-(Rh_xGa_1 −x)₂O₃ alloys generally possess much shallower acceptor levels and higher hole mobilities compared to β-Ga₂O₃. Our predicted properties suggest that it is indeed feasible to achieve p-type conduction in β-(Rh_xGa_1 −x)₂O₃ alloys. Given their ultrawide bandgaps, these alloys hold potential applications in high-power devices, deep ultraviolet optoelectronics, CMOS technology, and other areas.