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.

No abstract

A bandgap bowing parameter of 0.4 ± 0.2 eV for β-(AlxGa1−x)2O3 alloys, with Al compositions (x) up to 0.35, has been determined from the bandgap obtained from low temperature optical reflectivity, which suppresses the effect of electron–phonon interaction on the bandgap. A length scale of inhomogeneity of 0.21 ± 0.03 times of the electron–hole mean free path length has been estimated for β-(AlxGa1−x)2O3 alloys. The unit cell of β-(AlxGa1−x)2O3 alloys compresses, and the lattice parameters vary linearly with Al substitution. Our results provide insight into bandgap engineering and alloy disorder for β-(AlxGa1−x)2O3 alloys, which are an important material system for applications in deep ultraviolet opto-electronic devices.

Al occupancy and local structure (bond lengths and bond angles) for monoclinic β-(AlxGa1−x)2O3 alloys, with Al compositions (x) up to 90%, have been determined from Rietveld refinement of x-ray diffraction data. An Al atom preferentially occupies an octahedron (Oh) atomic site in comparison with a tetrahedron (Td) atomic site. However, a sizable number of Td atomic sites, i.e., 20% for Al composition of 5%, remain occupied by Al atoms, which is found to increase sharply with Al composition. The Oh atomic sites are not fully occupied by Al atoms even for the Al composition of 90%. The lattice parameters (bandgap) of the β-(AlxGa1−x)2O3 alloy decrease (increase) linearly with Al composition, but a change in slope of the variation of both lattice parameters and bandgap is observed at around Al composition of 50%. The lattice is found to be distorted for Al compositions more than 50% as indicated by a large change in the bond angles. The lattice distortion is determined to be the origin for the observed change in slope of the variation of both lattice parameters and bandgap for a monoclinic β-(AlxGa1−x)2O3 alloy system. Our results provide an insight into the local structure of β-(AlxGa1−x)2O3 alloys, which are required to have better understanding of their physical properties.