X-ray photoelectron spectroscopy analysis and band offset determination of CeO₂ deposited on epitaxial (100), (110), and (111)Ge

The oxidation states, interface, and band alignment properties of physical vapor deposited CeO₂ films on epitaxial (100), (110), and (111)Ge were investigated by x-ray photoelectron spectroscopy (XPS). The cross-sectional transmission electron microscopy demonstrated the polycrystalline nature of the CeO₂ film. XPS analysis showed multiple Ce3d and Ce4d oxidation states with a mixture of Ce³⁺ and Ce⁴⁺ components existing in CeO₂. Angular resolved XPS investigations indicate that the CeO₂ films mostly consist of Ce⁴⁺ oxidation states while the Ce³⁺ oxidation states are preferentially present near the surface. The CeO₂/(100)Ge, CeO₂/(110)Ge, and CeO ₂/(111)Ge structures showed almost identical valence band offset (VBO) values of 1.6, 1.5, and 1.6 eV, respectively, using XPS measurements from Ce3d core level (CL) peaks. These (VBO) values were also supported by XPS measurements from shallow Ce4d CL binding energy peaks. The conduction band offset values between CeO₂/Ge were ∼1.3 eV using the measured optical bandgap of CeO₂. The XPS spectral analysis of cerium oxidation states and the measured band offset parameters for carrier confinement would offer an important path for the future design of Ge-based metal-oxide semiconductor devices.