SSL Seminar Series 2003 No.3

Combined talks (Two speakers)

Date: November 13 (Thursday), 2003
Time: 4:00-5:00pm
Venue: Physics Resource Room (Blk S13 # 02-16)

Speaker I: Mr. Chen Wei (PhD Candidate)
Co nanocluster deposition on 6H-SiC(0001)- templates
We demonstrate that the carbon-rich 6H-SiC(0001)- honeycomb-like reconstructed surface can be used as an effective template for the growth of high density, monodispersed and uniform-sized nanoclusters. In-situ UHV (ultrahigh vacumm) STM (scanning tunneling microscopy) is used to systematically study the nucleation of cobalt (Co) nanoclusters on this SiC honeycomb template. During the initial stages of Co cluster growth, the boundaries between neighboring honeycomb structures serve as active sites for Co cluster adsorption. After 2? thick Co deposition at room temperature, high density, monodispersed and regular-sized (3-4 nm) Co nanoclusters are formed on this reconstructed 6H-SiC(0001) surface without the nucleation of Co single crystal 3D islands due to restricted adatom surface diffusion on the underlying honeycomb template. The regeneration of the honeycomb template is achieved by annealing the as-deposited 2?Co/SiC sample to higher than 800K. It is found that this carbon-rich SiC honeycomb template is chemically inert to Co and thermally stable, and acts as a diffusion barrier preventing Co from reacting with underlying Si atoms to form cobalt silicide during annealing. A narrow size distribution of Co nanoclusters (~6.8nm in diameter) is achieved by annealing the as-deposited 2?Co/SiC sample at 800K for 20 minutes. Deposition of Co on the SiC honeycomb template at different temperatures is also presented and discussed.

Speaker II: Dr. Xu Hai (Research Fellow)
6H-SiC(0001) Surface Reconstruction and Oxidation
Silicon carbide (SiC) has many potential applications in high temperature, high power and high frequency devices compared to traditional silicon (Si) materials because of its large breakdown field and high electron mobility. For not only the heteroepitaxial growth but also the nanostructure formation at compound semiconductor surface it is essential to control the surface chemical composition and the surface atomic structure. In this present study using scanning tunneling microscopy(STM) and low -energy electron diffraction (LEED), we investigated Si terminated 6H-SiC(0001) surface reconstruction and transition after heat treatment at increasing temperature. Especially, we reported on our STM and LEED study of the 6 ¡Á 6 superstructure of 6H-SiC(0001) surface and propose a evolution model which explained very well the formation of the 6 ¡Á 6 superstructure and previous STM results .
Secondly, obtaining high quality oxides on SiC is an important step in the fabrication of metal-oxide-semiconductor devices. For this reason, studies of the oxidation mechanism and chemical composition of thermally grown oxides on SiC have attracted a lot of attention recently. In this work, we also systematically study the energetic, equilibrium geometry and possible reaction pathways of O2 chemisorption using STM and XPS. In order to study the stability of oxide products grown by different oxidation paths on SiC, a comparative experiment on the interaction of molecular and atomic oxygen with Si-terminated 6H-SiC(0001)3x3 surface was carried out. It was found that different non-stoichiometric silicon oxycarbide species were formed via different oxidation paths. In molecular thermal oxidation, in addition to SiO2, the oxidation product is mainly SiOC3. However, silicon oxycarbides are presented as SiO3C when the sample was prepared by atomic oxygen oxidation. Furthermore, this SiO3C phase shows a good thermal stability under 1000 C annealing.