OBSERVATION OF THE AG/SI(111) SYSTEM USING A HIGH-RESOLUTION ULTRA-HIGH-VACUUM SCANNING ELECTRON-MICROSCOPE

Secondary electron (SE) imaging with a new ultra-high vacuum scanning electron microscope (UHV-SEM) has been employed to study the Ag/Si(111) system. Domains of the square-root 3 x square-root 3-Ag or 6 x 1(3 x 1)-Ag structure partially covering a Si(111) surface are clearly contrasted against the 7 x 7 region. No sample bias is applied. Two-dimensional (2D) domain morphology differs completely between square-root 3 x square-root 3-Ag and 6 x 1(3 x 1)-Ag. The square-root 3 x square-root 3-Ag nucleates at any of (1) the steps, (2) the steps plus phase boundaries, and (3) the steps plus phase boundaries plus mid-domains, of the Si(111)-7 x 7 substrate, depending on both the step spacing and the substrate temperature. An SE image for Ag deposition at approximately room-temperature shows regular arrangements having the same periodicity as the 7 x 7 unit meshes. This implies that 2D Ag clusters, approximately 2.3 nm in size and composed probably of 36 Ag atoms, resulting from the preferential Ag deposition onto faulted halves of the 7 x 7 unit meshes are observed. In our previous papers, the presence of areas with three sorts of brightness for samples with the 7 x 7 and the square-root 3 x square-root 3-Ag structure were reported. This is shown to be caused by the remaining 1 x 1 domains among the 7 x 7 domains, resulting from quenching during the substrate cleaning. Difference between 1 x 1 and 7 x 7 domains and contrast due to steps are observed even without Ag deposition.