Device-grade ultrathin (9-22 Angstrom) films of silicon dioxide, prepared from crystalline silicon by remote-plasma oxidation, are studied by soft x-ray photoelectron spectroscopy (SXPS). The 2p core-level spectra for silicon show evidence of five distinct states of Si, attributable to the five oxidation 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 weaker signals found for the three intermediate states (I-1, I-2, and I-3) are attributed to silicon atoms at the abrupt interface between the thin SiO2 film and substrate. Estimates of the interface state density from these interface signals agree with the values reported earlier of similar to 2 monolayers (ML). The position and intensity of the five peaks are measured as a function of post-growth annealing temperature, crystal orientation, and exposure to He/N-2 plasma. We find that annealing produces more abrupt interfaces (by reducing the suboxide interface state density), but never more abrupt than similar to 1.5 monolayers. We observe a 15%-20% drop in the interface thickness (in particular the "Si2+" peak intensity) with increasing annealing:temperature. Somewhat different behavior is observed with small amounts of nitrogen in the SiO2 film where an apparent increase in interface state density is seen. A quantitative analysis is presented which explores the effects of these sample preparation parameters in terms of relative interface state density and modeling of the SXPS data.