Alternate Assignment for the 250 cm^-1 Raman Mode in β-Ga₂O₃

Monoclinic gallium oxide (β-Ga₂O₃) single crystals have a Raman mode at ~250 cm^-1 (~31 meV) that is strongly correlated with free-electron density and therefore assigned to an electronic excitation of a shallow donor impurity band. The present work, however, demonstrates that heavily n-type epilayers grown by metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) do not contain this peak, contradicting the shallow-donor assignment. An alternate model is proposed: the 250 cm^-1 Raman peak arises from gallium clusters, defined as two or more Ga atoms that form Ga–Ga bonds. Raman mapping reveals variations in the frequency that are consistent with a distribution of cluster sizes. The intensity of the peak decreases as temperature is raised, attributed to melting or dissociation of the Ga clusters. First-principles calculations on clusters at the two extremes, Ga metal and an isolated point defect, indicate that the 250 cm^-1 mode is due to a Ga–Ga bond-stretching vibration. As the Fermi energy is raised, the formation of Ga–Ga dimers becomes energetically favorable, explaining the correlation between n-type conductivity and the Raman peak.