In the previous study 23 we showed that doping of ZnO single crystals by Sb from the vapor in the absence of an external oxidant results in the ZnO structure containing exclusively Sb +5 when only surface layers are studied. One would assume that a different situation might hold for the bulk of doped ZnO. XPS spectra recorded with Ar + etching support this idea revealing the presence of Sb +3 and Sb 0 in deeper layers of Sb 2 O 3 -doped samples and no Sb +5 component (Fig. ).