Wide bandgap tunability of N-alloyed ZnGa₂O₄ thin films

Abstract

ZnGa2O4 films with different nitrogen impurity concentrations were deposited using radio frequency magnetron sputtering in an ammonia (NH3)/Ar gas mixture and were post-annealed at different temperatures, ranging from 600 °C to 900 °C, in NH3 atmosphere. The influence of ammonia partial pressure ratio and nitridation temperature on the microstructure, surface morphology, nitrogen doping profile, and optical properties has been investigated. The optical band gap of ZnGa2O4, under in-situ nitrogen-doping, decreases from 4.6 to 4.1 eV but the primitive ZnGa2O4 crystalline structure does not change. On nitridation, the optical band gap gradually decreases from 4.6 to 2.1 eV and the ZnGa2O4 changes into a zinc gallium oxynitride quaternary alloy structure. Secondary ion mass spectrometry analysis revealed that nitrogen atoms were uniformly distributed in the film. The narrowing, by more than 40%, of the optical band gap is attributed to the hybridization of Zn3d and N2p orbits promote p-d repulsion in the top of the valence band, and the formation of hexagonal wurtzite phase.