Identification of P dopants at nonequivalent lattice sites of the Si(111)-(2x1) surface

Substitutional phosphorus atoms at the Si(111)-(2x1) surface have been studied with scanning tunneling microscopy at 8 K. Four different types of the P-induced contrast pattern are distinguished due to their voltage-dependent contrast. Three of them are identified as substitutional P atoms on distinct lattice sites by their spatial symmetry and by comparison with ab initio calculations of the local density of electronic states of substitutional P atoms. The fourth pattern of a P-induced contrast cannot be attributed to the remaining fourth site of the pi-bonded chain. This raises questions not only on the origin of this pattern but also on the absence of substitutional P atoms on one lattice position in this surface.