Magneto-Optical Spectroscopy of Cr³⁺ and Red Emission in β-Ga₂O₃

β-Ga₂O₃ is emerging as a promising wide bandgap semiconductor for numerous applications ranging from power electronics to solar-blind UV photodetectors and gas sensors [1]. The successful development of Ga₂O₃-based devices requires understanding of dopants and residual contaminants. Previous electron paramagnetic resonance (EPR) studies have identified one of such common contaminants as Cr that is usually present in the 3+ charge state (Cr³⁺). It was also reasoned that internal transitions within the Cr³⁺ center give rise to the so-called R-lines at around 1.78 eV and 1.80 eV at room temperature, though the same emission was also argued to stem from Fe³⁺.<br/><br/>In this work we determine the exact electronic states and thereby unambiguously identify the chemical origin of the transition-metal impurity responsible for the R-line emissions in β-Ga₂O₃. This is done by combining the EPR spectroscopy, which can determine the spin states of the ground state involved in the optical transitions, with magneto-photoluminescence (PL) that is sensitive to both excited and ground state. Commercially available undoped b-Ga₂O₃ bulk crystals from Tamura and undoped b-Ga₂O₃ bulk crystals grown by the Czochralski method were studied. Unambiguously evidence is provided that the R₁ and R₂ PL lines that can be detected in all investigated materials are due to the intracenter transitions between the ²E excited state and the ⁴A₂ ground state of Cr³⁺. This conclusion is based on the identical spin-Hamiltonian parameters of the ground state involved in the R1 emission measured by both EPR and magneto-PL techniques. The Cr³⁺ center is concluded to have monoclinic local symmetry and exhibits a large zero-field splitting of ∼ 147 meV in the ground state, which can be directly measured from the splitting of the R1 transition at low temperatures. Furthermore, the spin-Hamiltonian parameters of the ground state and the lowest-lying excited state are accurately determined. Our results [2] have, therefore, contribute to a better understanding of the electronic structure of Cr in b-Ga₂O₃.<br/>[1] S.J. Pearton et al, Appl. Phys. Rev. <b>5</b>, 011301 (2018).<br/>[2] J. E. Stehr et al, Appl. Phys. Lett. <b>119</b>, 052101 (2021)