Single Si dopants in GaAs studied by scanning tunneling microscopy and spectroscopy

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We present a comprehensive scanning tunneling microscopy and spectroscopy study of individual Si dopants in GaAs. We explain all the spectroscopic peaks and their voltage dependence in the band gap and in the conduction band. We observe both the filled and empty donor state. Donors close to the surface, which have an enhanced binding energy, show a second ionization ring, corresponding to the negatively charged donor D^-. The observation of all predicted features at the expected spectral position and with the expected voltage-distance dependence confirms their correct identification and the semiquantitative analyses of their energetic positions.