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Si(100) is one of the most thoroughly studied surfaces because of the ease it provides to investigate fundamental surface processes. In my graduate research, I have studied atomic level processes involved during interaction of halogens with Si(100) and Si(114). I have extensively used room-temperature scanning tunneling microscopy to gain insights into various surface reactions and processes. I have also used low-temperature scanning tunneling microscopy, as well as the variable-temperature instrument available in the Center of Microanalysis of Materials at the University of Illinois at Urbana-Champaign.
In the work presented here, it is shown that, contrary to conventional wisdom, Cl2 dosing of Cl-saturated Si(100)-(2x1) surfaces at elevated temperature can lead to uptake beyond "saturation". The surface then evolves along a new etching pathway. This process involves Cl insertion in Si-Si dimer bonds or back-bonds, diffusion of the inserted Cl, and ultimately desorption of SiCl2. Using STM, the etch kinetics were investigated and the results showed that insertion occurs via a novel form of Cl2 dissociative chemisorption that is mediated by dangling bond sites. Upon dissociation, one Cl atom adsorbs at the dangling bond while the other can insert.
In collaboration with senior lab-mate G.J. Xu, the role of steric interactions during thermally-activated roughening reaction for I-Si(100)-(2x1) and thermally-activated diffusion of I atoms on Si(100) were studied. Iodine atoms, being larger than Cl and Br, experience stronger adsorbate-adsorbate repulsive interactions. This manifests itself in distinctly different surface patterns observed during I-induced roughening of Si(100)-(2x1) compared to Cl and Br. Vacancy lines running perpendicular to the dimer row direction were favored for I, as opposed to vacancy lines running parallel to the dimer row direction for Cl and Br. In the dilute limit of surface coverage, iodine adatoms on Si(100) dimers were observed to hop pairwise during scanning at room temperature. Detailed studies revealed that diffusion is thermally activated and the diffusion parameters were deduced.
In collaboration with senior lab-mate B.R. Trenhaile, spontaneous halogen desorption for Cl and I on Si(100), and initial stage of oxidation for H 2O exposed surface were studied. Studying the desorption rates as a function of temperature revealed that Cl behaved in a similar manner as Br and followed the phonon-activated electron-stimulated desorption pathway. However, for I this was not the case. During the H2O-induced oxidation studies, Cl was used to image the resulting site of O atoms on the surface. It was demonstrated that Cl allowed the bridge-bonded oxygen atoms to be imaged.
Finally, along with R.E. Butera, dissociative chemisorption of Cl 2 on Si(114) was studied at room temperature. Si(114) is much more complex than the (100) surface. (114) has three distinct building block units as opposed to just one, a dimer, for (100). During Cl adsorption, the sequence in which these units get occupied and the reasoning behind this trend was studied.