Surface Segregation Study of Transparent ZnGa2O4 Films by XPS

Sol-gel colloidal suspensions were used to elaborate ZnGa2O4 films by spin coating on glass and MgO substrates. Those films were annealed at 600 °C (10°/min) for l h in air and showed good mechanical resistance as well as very good transparency. XPS spectra recorded from 0 to 1200 eV indicated that the films contained zinc, gallium, oxygen, and a significant amount of carbon. In the absence of depth profiling analysis of the film composition, it has been impossible to determine whether this is due to surface contamination or film incorporation. Surprisingly, the data show that the Zn:Ga stochiometry of ZnGa2O4 is disrupted by interaction with the glass substrate with a Ga/Zn ratio of 4. The result is a multiphasic material namely crystallites of ZnGa2O4 (as shown by AFM measurements) in an amorphous matrix of zinc and gallium oxides. This interaction was confirmed by the fact that deposition on MgO substrate and thermal annealing in similar conditions than for the previous deposits gave XPS data confirming the retention of the Zn:Ga 1:2 stoichiometry.