Local Valence Electronic States of SiO2 Ultrathin Films Grown on Si(100) Studied Using Auger Photoelectron Coincidence Spectroscopy: Observation of Upward Shift of Valence-Band Maximum as a Function of SiO2 Thickness
The local valence electronic states of the surface, interface, and substrate for SiO2 ultrathin films thermally grown on a Si(100)-2×1 have been investigated using Si-L23VV Auger-electron Sin+-2p photoelectron coincidence spectroscopy (n represents the number of oxygen atoms bonded to the Si). A series of Si-L23VV Auger electron spectra (AES) measured in coincidence with Sin+-2p photoelectron indicate that the valence electronic states in the vicinity of the Sin+ sites shift to the deeper binding-energy side as n increases. Furthermore, the Si4+-L23VV AES measured as a function of the thickness of the SiO2, show that the valence-band maximum of SiO2 shifts ∼1.6 eV toward the Fermi level when the thickness of the SiO2 film is decreased to 1.7--1.5 Å. This shift is attributed to a decrease in the number of Si4+ and an increase in the number of Si3+, Si2+, Si1+, and Si0 in the vicinity of the topmost SiO2 layer.