Effect of Thickness on Morphology, Structural, and Optical Properties of Nb-Doped β-Ga₂O₃ Films Prepared by RF Magnetron Sputtering

Here, niobium-doped monoclinic gallium oxide thin films of different thicknesses were deposited on p-Si (100) and quartz substrates by radio-frequency magnetron sputtering. All films were annealed in argon ambient. The crystal structure and surface morphology of the films were researched using x-ray diffraction and scanning electron microscopy. Then, their crystallite size was evaluated via the Debye–Scherrer formula. The results demonstrated that the films had a good crystal structure and a flat surface when the thickness was around 300 nm. The films’ optical properties were also investigated, and the results showed that all of the films’ transmittance is above 80% to ultraviolet–visible light whose wavelength is above 350 nm. Meanwhile, the films’ optical band gap decreased as their thickness increased. The Urbach energy of all films was calculated by the Urbach rule, and the results indicated that the best crystal quality occurred when the thickness was around 300 nm. The films’ electrical characteristics showed that the current was larger when the thickness was around 300 nm and that the contact between the Au electrode and films was Ohmic contact, independent of the film thickness and test conditions. These findings will provide useful information for the practical application of Nb-doped β-Ga₂O₃ thin films.