Red photoluminescence from Mn-doped <b> <i>β</i> </b> -Ga2O3: Optical signature of an Mn3 <b>+</b> ion?

A sharp photoluminescence (PL) line at ∼710 nm, commonly present in Mn-doped β-Ga2O3, has previously been attributed to an intra-center transition of an Mn4+ ion with the d3 electronic configuration, based on similarities of its properties with the well-known 2E → 4A2 transitions of Cr3+. In this study, we present spectroscopic data that challenge this interpretation and suggest that the 710-nm emission likely originates from Mn3+ in a high-spin d4 configuration. Specifically, temperature-dependent magneto-PL data indicate that the emission originates from internal transitions between a non-degenerate S = 0 excited state and the S = 2 ground state of the involved center, incompatible with the S = 3/2 spin of Mn4+. Time-resolved PL measurements reveal a long emission lifetime (∼800 μs), characteristic of spin-forbidden internal d–d transitions. These observations, along with the determined spin-Hamiltonian parameters of the center, are consistent with a transition from a low-lying spin-singlet excited state to a high-spin 5E ground state of Mn3+, orbitally split by a local crystal field. A second emission line at ∼694 nm, which is much weaker and spatially inhomogeneous, exhibits nearly identical magnetic and thermal behavior, suggesting that it arises from the Mn3+ ion perturbed by a nearby defect. Our findings, therefore, provide a spectroscopic signature of the previously undocumented Mn3+ ion in β-Ga2O3.