In this work, we have demonstrated wide-composition-range \b{eta}-(AlxGa1-x)2O3 thin films with record-high Al compositions up to 77% for \b{eta}-(AlxGa1-x)2O3 covering bandgaps from 4.9 to 6.4 eV. With optimized thermal annealing conditions, the \b{eta}-Ga2O3 binary thin films on sapphire substrates transformed to the \b{eta}-(AlGa)2O3 ternary thin films with different compositions. The binary to ternary transformation resulted from the Al atom diffusion from sapphire into the oxide layers; meanwhile, the Ga atoms diffused into sapphire leading to thicker thin films than the original thicknesses. The interdiffusion processes were confirmed by transmission electron microscopy, which enhanced in proportion to the annealing temperature. The strain states of the \b{eta}-(AlGa)2O3 films have been analyzed showing reduced in-plane compressive strain with higher annealing temperature; and the film eventually became strain-free when the temperature was 1400 oC corresponding to the Al composition of 77%. The proposed method is promising for the preparation of the \b{eta}-(AlGa)2O3 thin films without employing sophisticated direct-growth techniques for alloys.
