In the last week I did some surface measurements with Zygos NewView8000 microscope on samples of Sapphire, SiO₂-glass, and a GaAs-wafer from LMA.

Attached to this file are figures of the surface profile of each sample taken from the center and an area close to the edge of each sample.
The calculated surface roughnesses (rms) are as follows:

Sapphire-center: 1 nm
Sapphire-edge: 2nm
SiO₂-center: 1nm
SiO₂-edge: 1nm
GaAs-center: 2nm
GaAs-edge: 3nm