SSL Seminar Series 2005 No.8

Combined talks (two speakers)

Date: 1 September (Thurday), 2005
Time: 4:00pm-5:00pm
Venue: Physics Resource Room (Blk S13 # 02-16)

Speaker I: Mr Abdul Kader Zilani
Title: STM study of the reactive Co/Si(111)-(7x7) interface structure and silicide island evolution

We have studied the interaction of reactive metal Co with Si(111)-(7×7) starting from ultra low coverages ~0.05 ML to higher coverages ~1.5 ML and at different annealing conditions. At lower coverages Co-induced clusters are formed on Si(111)-(7×7) at specific adsorption sites in the faulted half of the unit cell. In situ scanning tunneling microscopy (STM) studies identify the interaction sites of the clusters, distinguishing this system from other group-III metal clusters. Based on first-principles total energy calculations and experimental observations, we propose a model to explain the atomic structure and formation mechanism of these clusters. The electronic structure of these clusters has been unveiled by a combined study of bias dependent STM and scanning tunneling spectroscopy. At higher coverages we explain the terrace width dependence of silicide nucleation on Si(111)-(7×7). The size, shape transition, and positioning of self assembled cobalt silicide islands are discussed. By analyzing height of the atomically flat CoSi 2 (111)-(2×2) islands, we deduce the CoSi 2 (111)/Si(111) interface structure.

Speaker II: Mr. Ong Wei Jie
Title: Co/Si(111) - a STM study

We report the observation of 3D silicide triangular islands and arrays of Co-Si tnano-clusters on Si(111)-(7x7) reconstruction using STM. The formation of these clusters appear to be highly sensitive to the surface coverage of Co, as formation of large 3-D Co silicide islands tend to dominate the surface morphology. It is also interesting to note that while the spatial distribution of clusters appear generally disordered, ordering of the clusters in root 7 x root 7 unit cell can be observed upon annealing at high temperature. STM biasing experiments revealed a ring-like structure in the empty state imaging and a spherical feature in the filled state imaging. STM line profile analysis show that these cluster possess a uniform shape and size (~9.0 ± 0.5Å), which remained unchanged over a wide temperature regime. This suggests an enhanced stability associated with Co-Si atoms when arranged in the cluster geometry observed, akin to other similar magic cluster phenomena observed in other material systems (1 Ga/Si(111), 2 In/Si(111), 3 Pt/Pt(111), 4 Ag/Ag(001) ).

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