Effect of Oxide Charge Trapping on X-ray Photoelectron Spectroscopy of HfO2/SiO2/Si Structures

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We examined the effects of interfacial SiO2 layers and a surface metal layer on the photoelectron spectra of HfO2/SiO2/Si structures by hard X-ray photoemission spectroscopy with synchrotron radiation as well as conventional X-ray photoelectron spectroscopy (XPS). The Hf 4f and Hf 3d photoelectron peaks broadened and shifted toward a higher binding energy with increasing thickness of the interfacial SiO2 layer, even though photoelectrons may have been emitted from the HfO2 layer with the same chemical composition. Thinning the interfacial Si oxide layer to approximately one monolayer and depositing a metal layer on the HfO2 surface suppressed these phenomena. The O 1s photoelectron spectra revealed marked differences between the metal- and nonmetal-deposited HfO2/SiO2/Si structures; HfO2 and SiO2 components in the O 1s photoelectron spectra for the metal-deposited structures were observed at reasonably separated binding energies, but those for the nonmetal-deposited structures were not separated clearly. From this behavior concerning the effects of interfacial SiO2 and surface metal layers, we concluded that the Hf 4f, Hf 3d, and O 1s spectra measured from the HfO2/SiO2/Si structures did not reflect actual chemical bonding states. We consider that potential variations in the HfO2 film owing to charge trapping strongly affect the measured photoelectron spectra. On the basis of angle-resolved XPS measurements, we propose that positive charges are trapped at the HfO2 surface and negative charges are trapped inside the HfO2 layer.