Scanning tunnelling microscopy was used to identify P that was present at the clean Si (100)-(2x1) surface as a result of the thermal cycling necessary for preparation of samples cut from heavily doped wafers. Substitutional P was observed in top layer sites as buckled Si-P heterodimers. A second type of feature was also observed that appeared as a single depressed dimer site. Within this site, the atoms appear as a pair of protrusions in the empty states and a single protrusion in the filled states. These properties were not consistent with known adsorbate signatures or previously reported observations of P-P dimers on the (100)-(2x1) surface. The lack of other impurity sources suggested that they were due to either P or silicon. The symmetry of the features and their magnitude were consistent with one of those elements residing in an interstitial site just below the top layer of atoms. In order to identify the type of interstitial, density functional theory calculations were performed for both P and Si located below a surface dimer. The resulting charge density plots and simulated scanning tunnelling microscopy images were consistent with interstitial P and not interstitial silicon.Observation of Substitutional and Interstitial Phosphorus on Clean Si(100)-(2x1) with Scanning Tunneling Microscopy. G.W.Brown, B.P.Uberuaga, H.Grube, M.E.Hawley, S.R.Schofield, N.J.Curson, M.Y.Simmons, R.G.Clark: Physical Review B, 2005, 72[19], 195323 (5pp)