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The anisotropic dielectric functions (DF) of corundum structuredm-planeα-(Al_xGa_1−x)₂O₃thin films (up tox= 0.76) grown onm-plane sapphire substrate by metalorganic CVD have been investigated. IR and visible–UV spectroscopic ellipsometry yields the DFs, while X-ray diffraction revealed the lattice parameters (a,m,c), showing the samples are almost fully relaxed. Analysis of the IR DFs from 250 to 6000 cm⁻¹by a complex Lorentz oscillator model yields the anisotropic IR active phononsE_uandA_2uand the shift towards higher wavenumbers with increasing Al content. Analyzing the UV DFs from 0.5 to 6.6 eV we find the change in the dielectric limitsε_∞and the shift of the Γ-point transition energies with increasing Al content. This results in anisotropic bowing parameters forα-(Al_xGa_1−x)₂O₃ofb_⊥= 2.1 eV andb_∣∣= 1.7 eV.

The simplest picture of excitons in materials with atomic-like localization of electrons is that of Frenkel excitons, where electrons and holes stay close together, which is associated with a large binding energy. Here, using the example of the layered oxide V₂O₅, we show how localized charge-transfer excitations combine to form excitons that also have a huge binding energy but, at the same time, a large electron-hole distance, and we explain this seemingly contradictory finding. The anisotropy of the exciton delocalization is determined by the local anisotropy of the structure, whereas the exciton extends orthogonally to the chains formed by the crystal structure. Moreover, we show that the bright exciton goes together with a dark exciton of even larger binding energy and more pronounced anisotropy. These findings are obtained by combining first principles many-body perturbation theory calculations, ellipsometry experiments, and tight binding modelling, leading to very good agreement and a consistent picture. Our explanation is general and can be extended to other materials.