Thin Er-oxide films were prepared by oxidation of pure Er films grown on glass and Si (p) substrates. The oxide films were characterised by X-ray fluorescence (XRF), X-ray diffraction (XRD), and optical absorption spectroscopy. The XRD analysis of the as-prepared oxide film and the vacuum annealed film demonstrates the formation of Er2O3 phase with about 4.6% of ErO phase, which totally transforms into Er2O3 phase under annealing at 600 degrees C in dry oxygen. Therefore, the phase-structural changes in the prepared Er-oxide films because of the annealing and the long time Storage in vacuum were studied. The constructed Al/Er-oxide/Si MOS devices were characterised by measuring gate-voltage dependence of their capacitance and ac conductance, from which the surface states density (D-it) of insulator/semiconductor interfacial charges and the density of fixed charges in the oxide, were determined, which were within the device-grade range. The ac-electrical conduction and dielectric properties of the of the Er oxide-silicon structure were studied at room temperature. The data of ac conductivity measurements were found to follow the correlated barrier-hopping (CBH) model and the model's parameters were calculated, while the Kramers-Kronig (KK) relations explain the high-frequency dependence of the capacitance. (c) 2006 Elsevier B.V. All rights reserved.