We studied the cleaning of the native oxide surfaces of silicon wafers using variable-angle spectroscopic (multiwavelength) ellipsometry (SE) and x-ray photoelectron spectroscopy (XPS). We focused on removing surface contamination, while preserving the oxide layer and minimizing surface roughness. Five minute under a xenon excimer lamp in air was adequate to render "carbon free" (<0.05nm overlayer) oxide surfaces previously cleaned with detergents and/or solvents. We further investigated different ways of storing samples and how quickly carbon-containing contamination returns. With nonvacuum storage conditions, two hours after being cleaned an overlayer of 0.1 to 0.2nm reappeared on the surface as measured by SE. After a week in closed storage conditions an additional 0.2 to 0.4nm was deposited. Between the time span of a week and 90 days, we saw no further significant adventitious overlayer deposition. XPS indicates that the overcoat is most probably hydrocarbon/organic with significant oxygen content. We observed that (impure) vacuum storage can contaminate samples more than air. We traced instrumental hydrocarbon contamination to our XPS antechamber and show how attaching a commercial low-pressure oxygen radical source removes the bulk of the contamination.