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Investigation of Single Boron Acceptors at the Cleaved Si:B (111) Surface

The cleaved and (2 x 1) reconstructed (111) surface of p-type Si is investigated by scanning tunneling microscopy (STM). Single B acceptors are identified due to their characteristic voltage-dependent contrast which is explained by a local energetic shift of the electronic density of states caused by the Coulomb potential of the negatively charged acceptor. In addition, detailed analysis of the STM images shows that apparently one orbital is missing at the B site at sample voltages of 0.4 - 0.6 V, corresponding to the absence of a localized dangling-bond state. Scanning tunneling spectroscopy confirms a strongly altered density of states at the B atom due to the different electronic structure of B compared to Si.