The strain in Si, on which a single-crystalline CeO 2 gate oxide was epitaxially grown, was investigated by evaluating lattice spacings in CeO 2 and Si. In-plane X-ray diffraction measurements and observations of electron diffraction patterns by a transmission electron microscope were performed to examine the lattice spacings precisely. It was found that the lattice spacings in epitaxial CeO 2 isotropically expanded by ϳ1%, compared with those in bulk polycrystalline CeO 2 . The oxygen-defect-induced state was observed in the CeO 2 valence bandedge by X-ray photoelectron spectroscopy measurements. The decrease of the coulombic interaction in ionic oxide due to the oxygen defects may induce the expansion of the lattice spacings in CeO 2 . It was clarified that Si at 50 nm depth from the CeO 2 /Si interface was tensile strained owing to the expansion of the lattice spacings in CeO 2 . Oxygen defects in epitaxial crystalline gate dielectrics must be controlled by taking account of Si channel properties.
We have investigated the reason behind the enhancement of dielectric constant (ε), which occurred in CeO2 directly grown on Si(111). ε of directly grown CeO2 is enhanced to 52, which is twice as large as the reported value. From in-plane X-ray diffraction measurements and electron diffraction pattern observations using a transmission electron microscope, it has been found that the lattice spacings in CeO2 were isotopically expanded by 0.6%, as compared with the reported values in bulk CeO2. In addition, from X-ray photoelectron spectroscopy measurements, the existence of oxygen defects in CeO2 was confirmed. The oxygen defects in CeO2 may cause the decrease in coulomb interaction in the ionic crystal, resulting in the expansion of lattice spacings. The enhancement of ion movability, due to the expansion of lattice spacings is considered as the reason behind the enhancement of ε.
The structure of ultrathin epitaxial CeO 2 films grown on Si(111) by molecular beam epitaxy was investigated using highresolution Rutherford backscattering spectroscopy. The observed elemental depth profiles showed that there were oxygen defects in the CeO 2 films and the amount of oxygen defects was about 10%. The strain distribution in the CeO 2 film was measured using a channeling technique. It was found that the [100] axis of CeO 2 was tilted toward the ½2 1 1 1 1 direction by $0:7 . The observed tilt angle was more than twice larger than the expected tilt angle calculated from the lattice mismatch by assuming a pseudomorphic growth of CeO 2 . The present results suggest that the crystal structure of the CeO 2 film was changed from a cubic structure due to the oxygen defects.
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