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

Abstract

The local valence electronic states of SiO 2 ultrathin films grown on a Si(100)-2×1 surface (SiO 2 /Si(100)) have been studied extensively because in-depth understanding of the electronic properties of surfaces and interfaces from an atomic point of view is of fundamental importance in science. Therefore we investigate the local valence electronic states of the surface of the SiO 2 /Si(100) ultra-thin films by using Si-L 23 VV-Si 4+ -2p Auger electron photoelectron coincidence spectroscopy (Si-L 23 VV-Si 4+ -2p APECS) [1]. Figure 1 shows the Si-2p photoelectron spectrum of a SiO 2 /Si(100) with a thickness of 2.8 Å (≈2 ML, 2.8-Å SiO 2 /Si (100)). The Si-2p peaks are decomposed into the Si n+ -2p photoelectron components (n = 0, 1, 2, 3, 4). The straight dashed line at +4.1 eV represents the Si 4+ -2p photoelectron kinetic energy (PeKE) position taken as the trigger signals for the Si-L 23 VV-Si 4+ -2p APECS measurements. Figure 2 shows a series of Si-L 23 VV-Si 4+ -2p APECS spectra for 13-, 2.8-, 1.7-, and 1.5-Å SiO 2 /Si(100). The Si 4+ -2p PeKE positions taken as trigger signals of these APECS were set to the same value. Every wide-scan Si-L 23 VV-Si 4+ -2p APECS spectrum in Fig. 2(a) shows clear five peaks (P 1 -P 5 ). In Fig. 2(b), we show the enlarged S-L 23 VV-Si 4+ -2p APECS spectra. The intense peaks shift by ≈1 eV to the higher-AeKE side, while the cut-offs shift by ≈4 eV to the higher-AeKE side as the SiO 2 thickness decreases. These results indicate that the binding energies of valence band maximum of 1.5-and 1.7-Å SiO 2 /Si(100) are shifted upwards by ~1.6 eV (toward the Fermi level) in comparison with that of 13-Å SiO 2 /Si(100) [1].