Study on topographic images of Sn/Si(1 1 1)-(√3 × √3)R30° surface by non-contact AFM

Various contrast of topographic images depending on a state of a tip apex on Sn/Si(1 1 1)-(√3 × √3)R30° surface was investigated using a low temperature non-contact AFM. With the type A tip, the image of the ring-type Sn, composed of six Sn atoms surrounding substitutional Si defect, was observed when the frequency shift (∣Δf∣) was small (the tip-sample distance, Ztip-sample, was long), while the ring-type Sn was not observed and all the Sn atoms have the same contrast when ∣Δf∣ was large (Ztip-sample was short). On the other hand, with the type B tip, modified from the type A tip by the tip-sample contact, the image of the ring-type Sn atoms was not observed regardless of variation of Δf. It is the first experimental result on the low temperature NC-AFM observation in the Sn/Si(1 1 1) system, which depends on short-range chemical bonding force or electrostatic force acting between the tip and the sample surface. In addition, the substitutional Si defects on the surface were seen as a dim spot or were not seen, also depending on the tip state.