GdxGa0.4-xO0.6/Ga2O3 dielectric stacks have been grown on (001)GaAs to form a III-V/oxide with a low interface state density and a high conduction band offset. Photoluminescence is used to compare the stacks with low interface state density Ga2O3-GaAs layers. Rutherford backscattering and electron energy loss spectroscopy are used to investigate the Gd compositional variation with depth and this is related to the interface state density. The effect of Gd flux and atomic oxygen on the growth rate is reported. The leakage current through GdxGa0.4-xO0.6/Ga2O3 stacks is compared with ones using only Ga2O3 as the oxide. (c) 2007 American Vacuum Society.