Observation of substitutional and interstitial phosphorus on clean Si(100)-(2x1) with scanning tunneling microscopy

We have used scanning tunneling microscopy to identify phosphorus that is present at the clean silicon (100)-(2x1) surface as a result of the thermal cycling necessary for preparation of samples cut from heavily doped wafers. Substitutional phosphorus is observed in top layer sites as buckled Si-P heterodimers. We also observe a second type of feature that appears 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 are 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 suggests that they are due to either phosphorus or silicon. The symmetry of the features and their magnitude are consistent with one of those elements residing in an interstitial site just below the top layer of atoms. To identify the type of interstitial, we performed density functional theory calculations for both phosphorus and silicon located below a surface dimer. The resulting charge density plots and simulated STM images are consistent with interstitial phosphorus and not interstitial silicon.