Structure of ultrathin SiO2/Si(111) interfaces studied by photoelectron spectroscopy

Device-grade ultrathin (9-22 Angstrom) films of silicon dioxide, prepared f rom crystalline silicon by remote-plasma oxidation, are studied by soft x-r ay photoelectron spectroscopy (SXPS). The 2p core-level spectra for silicon show evidence of five distinct states of Si, attributable to the five oxid ation states of silicon between Si-0 (the Si substrate) and Si4+ (the thin SiO2 film). The relative binding energy shifts for peaks Si1+ through Si4(with respect to Si-0) are in agreement with earlier work. The relatively w eaker signals found for the three intermediate states (I-1, I-2, and I-3) a re attributed to silicon atoms at the abrupt interface between the thin SiO 2 film and substrate. Estimates of the interface state density from these i nterface signals agree with the values reported earlier of similar to 2 mon olayers (ML). The position and intensity of the five peaks are measured as a function of post-growth annealing temperature, crystal orientation, and e xposure to He/N-2 plasma. We find that annealing produces more abrupt inter faces (by reducing the suboxide interface state density), but never more ab rupt than similar to 1.5 monolayers. We observe a 15%-20% drop in the inter face thickness (in particular the "Si2+" peak intensity) with increasing an nealing:temperature. Somewhat different behavior is observed with small amo unts of nitrogen in the SiO2 film where an apparent increase in interface s tate density is seen. A quantitative analysis is presented which explores t he effects of these sample preparation parameters in terms of relative inte rface state density and modeling of the SXPS data. (C) 1999 American Vacuum Society. [S0734-2101(99)22704-2].