【International Papers】Self-Trapped Hole Migration and Defect-Mediated Thermal Quenching of Luminescence in α- and β-Ga₂O₃

      Researchers from the Leibniz-Institut im Forschungsverbund Berlin e.V. and University of Oslo have published a dissertation titled "Self-Trapped Hole Migration and Defect-Mediated Thermal Quenching of Luminescence in α- and β-Ga₂O₃" in Advanced Functional Materials.

Background

      Ultrawide bandgap (UWBG) semiconductors offer compelling opportunities for the development of high-power and high-frequency solid-state electronic devices. Among these materials, the gallium oxide (Ga₂O₃) system, which comprises several polymorphs, stands out as one of the most promising UWBG candidates. Ga₂O₃ exhibits unique electronic and optical properties, including high breakdown electric fields (8 MV cm⁻¹) and bandgaps in the deep UV region (exceeding 4.7 eV). These characteristics make Ga₂O₃ a strong competitor to established wide bandgap semiconductors such as silicon carbide (SiC) and gallium nitride (GaN). While its monoclinic β-phase is the only thermodynamically stable polymorph, increasing attention is being directed toward the metastable corundum α-phase. In particular, α-Ga₂O₃ offers an even larger bandgap (>5.3 eV) and higher crystal symmetry which results in a reduced anisotropy of physical properties. Such attributes could facilitate device fabrication and improve performance in practical applications. Moreover, α-Ga₂O₃ can be grown epitaxially on widely available substrates like sapphire (Al₂O₃), offering significant advantages in terms of cost and scalability for device manufacturing.

Abstract

      Gallium oxide (Ga₂O₃) is a promising ultrawide bandgap semiconductor for next-generation power electronics and optoelectronic devices. Here, temperature-dependent and polarization-resolved photoluminescence excitation spectroscopy data, complemented by hybrid-functional first-principles calculations, are presented, and a microscopic model is derived that explains the interplay of hole migration, defect trapping, and carrier recombination at defects underlying thermal quenching phenomena in α- and β-Ga₂O₃. In α-Ga₂O₃, the UV emission is attributed to self-trapped holes, while the blue luminescence arises from defect-related processes, including gallium split vacancies and their defect complexes. Calculations reveal an energy barrier of 88 meV for self-trapped hole migration in α-Ga₂O₃, consistent with activation energies from temperature-dependent photoluminescence. This enables efficient trapping by defects, enhancing blue luminescence and quenching UV emission. In β-Ga₂O₃, a higher migration barrier of 0.36 eV reduces the defect trapping, allowing the UV self-trapped hole emission to remain intense, with blue luminescence emerging only at elevated temperatures. These results establish a direct link between self-trapped hole migration, defect trapping, and thermal quenching of emission in both phases. The insights advance the understanding of carrier dynamics in ultrawide bandgap oxides and may guide defect engineering for high-performance functional devices.

Conclusion

      In this work, we have investigated the temperature-dependent luminescence mechanisms in high-quality α-Ga₂O₃ thin films and monoclinic β-Ga₂O₃ single crystals using polarization-dependent photoluminescence excitation spectroscopy, complemented by hybrid-functional first-principles calculations. Our results demonstrate that UV emission in both materials originates from self-trapped hole recombination. Blue emission is linked to defect-related processes, which include deep-level defects such as gallium split vacancies and their complexes. We also find that the excitation energy has a decisive influence on the emission behavior. When the excitation energy is above the bandgap, the UV emission is selectively enhanced. In contrast, sub-bandgap excitation preferentially activates blue luminescence through defect-assisted channels. This observation highlights how the optical response in Ga₂O₃ depends on both intrinsic and extrinsic states and demonstrates the potential for tuning emission properties by varying the excitation energy.

      A key advance of this study is the identification of self-trapped hole migration as the microscopic mechanism responsible for the temperature-dependent luminescence behavior. Our combined experimental and theoretical approach shows that, in α-Ga₂O₃ thin films, the low migration barrier for self-trapped holes, calculated at 88 meV, enables efficient hopping and trapping at defects even at low temperatures. This process leads to rapid quenching of UV emission and enhances blue luminescence, thereby explaining the experimentally observed negative thermal quenching. In β-Ga₂O₃ single crystals, the migration barrier is higher, with a value of 0.36 eV. This results in greater thermal stability of self-trapped hole related emission and a delayed onset of defect-assisted recombination. These findings clarify how carrier localization, migration, and defect trapping interact and establish a direct link between microscopic polaron dynamics and the macroscopic optical properties of Ga₂O₃.

      This improved understanding of excitation-dependent self-trapped hole dynamics and their impact on luminescence quenching advances the fundamental knowledge of recombination processes in ultrawide bandgap oxides. The insights gained from this work provide valuable guidance for defect engineering and the design of Ga₂O₃-based optoelectronic devices. Furthermore, these concepts are broadly relevant to other wide-bandgap oxide semiconductors.