Gadolinium-doped gallium-oxide-based polymorphs and alloys: Effect of the crystal field on its ultraviolet emission.

• Applicant: Dr. Manuel Alonso-Orts
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 548722780

Gallium oxide (Ga2O3) is a wide band gap material (E_gap = 4.8 eV) that currently experiences very high research interest due to its excellent material properties, such as a high electrical, chemical and optical stability as well as optical transparency in the entire visual part of the spectrum, along with the availability of crystalline substrates in different orientations. By advanced Molecular Beam Epitaxy (MBE) Techniques available in our research group, we can control the crystalline phase (polymorph) of Ga2O3 by selecting the appropriate Al2O3 substrate and growth conditions [1]. Although its wide band gap could make Ga2O3 suitable as an ultraviolet (UV) emitter, the unintentionally doped material exhibits only a broad defect-related emission that extends into the blue-green region, thus highlighting the importance of incorporating appropriate optically active ions into the material in order to pave the way to possible optical applications. This project focuses on the optical doping of Ga2O3 and related ternary oxide thin films by MBE using gadolinium (Gd) as an optically active impurity, when substitutionally incorporated as Gd3+ on Ga3+ sites. This also enables a fundamental study of the host material’s crystal fields using its impact on the intra-ionic transitions of Gd that are used as a probe. Gd has been selected by virtue of its well-defined emission in the UV-B range, which sensitively depends on the atomic environment of the Gd-ion. In particular, the splitting between the involved energy sub-levels is determined by the crystal structure and thus the crystal field strength and symmetry of the respective Ga2O3 polymorph. In addition, the chemical composition in ternary (Al,Ga)2O3:Gd and (In,Ga)2O3:Gd also impacts the Gd transition energies, not only due to changes in the crystal field but also due to strain between the oxide thin film and the substrate or in heterostructures. In addition to a systematic investigation of the crystal field dependence of the Gd emission in the three polymorphs α-, β- and ε-Ga2O3, we will thoroughly investigate the mechanism leading to this emission in order to evaluate Ga2O3:Gd-based materials as a benchmark for future UV-B emitters based on them. Thus, the major goal of the proposed project is to understand and quantify the effect of the crystal field in gallium oxide-based polymorphs and alloys, using the internal optical transitions of the gadolinium impurity as an optical probe for the crystal field effects and as a possible benchmark for future UV light sources.

• DFG Programme: Research Grants
• International Connection: Sweden
• Cooperation Partner: Dr. Jan Eric Stehr