Observation of mixed-mode behavior of Raman active phonon modes for β-(AlxGa1−x)2O3 alloys

Abstract

Raman spectroscopy has been used to investigate the phonon mode behavior of a β-(AlxGa1−x)2O3 alloy up to Al compositions of x = 0.474. The alloy displays a mixed-mode behavior, where low frequency (100–200 cm−1) modes corresponding to translation and libration of (Ga1O4) tetrahedral and (Ga2O6) octahedral chains and high frequency (500–800 cm−1) modes related to stretching and bending of (Ga1O4) tetrahedral chains show a monotonic behavior throughout the studied composition range, and display one-mode behavior. On the other hand, medium frequency (300–500 cm−1) phonon modes related to the deformation of (Ga2O6) octahedra and (Ga1O4) tetrahedra chains show sudden change such as the appearance (disappearance) of new (existing) phonon modes corresponding to Al–O (Ga–O) sublattices with Al substitution, and they display a two-mode behavior. The existence of two sublattices is further supported by the observation of preferential occupation of the Al atom at the octahedral atomic site in comparison to the tetrahedral atomic site. The observation of phonon modes related to translation and libration of (Ga1O4) tetrahedral and (Ga2O6) octahedral chains up to the Al composition of x = 0.474 clearly indicates that the long-range periodicity of the monoclinic lattice is maintained throughout the investigated Al composition range. Our results provide an insight into the phonon mode behavior of β-(AlxGa1−x)2O3 alloys, which is necessary to have a better understanding of their physical properties.