American Physical Society, Physical Review Letters, 7(108), 2012
DOI: 10.1103/physrevlett.108.076801
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Donors near the polar (0001) surface of nominally undoped ZnO were investigated with scanning tunneling microscopy at 5 K. Spatially resolved spectroscopy reveals single and double charging. Equidistant peaks in spectra of ionized donors are attributed to polaron excitation. The data are consistent with doping due to Zn interstitials or complexes.