The electrical and physical properties of atomic-layer-deposited EryHf1-yOx thin films have been investigated with different stoichiometries of erbium oxide (Er2O3) and hafnium oxide (HfO2). The as-deposited and annealed EryHf1-yOx films exhibit much higher dielectric constants than the reported k-values of the corresponding binary oxides. The highest k-value of 37.6 ± 1 is achieved with 13 at.% of erbium in the film. The enhancement in dielectric constant is due to the formation of the cubic HfO2 phase stabilized by erbium, as revealed by x-ray diffraction experiments. The annealed mixed oxide films exhibit remarkably low oxide charges, low interface states, low leakage, and good breakdown electric fields.