  We show that the HF acid etch commonly used to prepare SrTiO3(001) for
heteroepitaxial growth of complex oxides results in a non-negligible level of F
doping within the terminal surface layer of TiO2. Using a combination of x-ray
photoelectron spectroscopy and scanned angle x-ray photoelectron diffraction,
we determine that on average ~13 % of the O anions in the surface layer are
replaced by F, but that F does not occupy O sites in deeper layers. Despite
this perturbation to the surface, the Fermi level remains unpinned, and the
surface-state density, which determines the amount of band bending, is driven
by factors other than F doping. The presence of F at the STO surface is
expected to result in lower electron mobilities at complex oxide
heterojunctions involving STO substrates because of impurity scattering.
Unintentional F doping can be substantially reduced by replacing the HF-etch
step with a boil in deionized water, which in conjunction with an oxygen tube
furnace anneal, leaves the surface flat and TiO2 terminated.
