Carbon in Glass - XPS Analysis
Well, XPS analysis should detect atoms as light as Carbon, but not atoms as light as Hydrogen.

Far be it from me to claim any expertise on microscope sliders, but I have used one or two in my career, and I am hazarding a guess. It is likely a pure glass ($\ce{SiO2}$) sheet laminated with some acrylic glass. The hydrogen in the acrylic polymer are too light to appear on the XPS.

There is a slight possibility, although I consider it very unlikely, that it is glass with a substrate of moissanite on top, which would in theory make it scratch proof, due to the hardness of $\ce{SiC}$.

If you would include whether the %'s are weight percent or atomic percent, it would be easier. If it is atomic, then the case for $\ce{SiC}$ is more likely, since the ratio ($\ce{Si:O} &gt; \ce{1:2}$) and the extra Si would be in SiC. If it is weight percent then there is some math to be done. I still think it is just glass and acrylic glass.