This study presents comprehensive results on the structural modifications of Er2O3/n-Si hetero-structures under post-deposition annealing (PDA) and the effects of these changes on the electrical characteristics of the Al/Er2O3/n-Si/Al MOS capacitors. The Er2O3 films were grown on n-Si substrate with RF magnetron sputtering and annealed under nitrogen at RT, 300 degrees C, 500 degrees C, 600 degrees C, 700 degrees C. The increasing grain size of the films up to 300 degrees C did not show a significant change in other annealing temperatures. The erbium silicate content in the RT and 600 degrees C-Er2O3/n-Si interface is quite higher than those of 500 degrees C and 700 degrees C, while a silicate-like layer was not found at 300 degrees C. The highest dielectric constant (epsilon) value was obtained from the 500 degrees C-Er2O3 MOS capacitor due to the lowest oxygen deficient bond content. It was found that Q(eff) values tend to increase as the oxygen concentration decreases in the film. It was determined that Er-Er oxygen deficient bonds may have acted as negative charge trap centers. Although the Er-M content in the 700 degrees C-Er2O3/n-Si is higher than that of 500 degrees C, lower Q(eff) values were obtained from the 700 degrees C-Er2O3 MOS capacitor due to the higher Si-Si oxygen deficient bond content, which is most likely act as a positive charge trap center. It was concluded that the contributions of oxide trap and interface trap charges should be evaluated together in establishing a link between electrical characteristics and structural analyses.