The initial stages of the oxidation of Si(100)2 x 1 studied by STM (original) (raw)
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STM studies of Si(100)-2×1 oxidation: defect chemistry and Si ejection
Ultramicroscopy, 1992
The initial stages of oxidation of the Si(100)-2× 1 surface have been studied using STM and STS. In contrast to the Si(111)-7 x 7 surface, which has a metallic density of states (DOS), the dangling bonds of the Si dimers on the 2 × 1 surface are paired, leading to the formation of a surface gap and a vanishing DOS near E F. We observe a reduced reactivity of Si dimers towards 0 2 compared to that of Si adatoms on Si(111)-7 × 7, consistent with the reduced DOS near E F. Defects on the Si(100)-2× 1 surface which have a metallic DOS dominate the reactivity towards 0 2 in the early stages of the reaction. Among the new sites generated by the exposure to 0 2 are 1.4 A high bumps on top of the surface. Upon annealing of the O2-exposed surface or upon 0 2 exposure at an elevated temperature these bumps form elongated islands. Evidence is presented suggesting that the bumps and islands likely are due to silicon ejected to the surface by the oxidation reaction. Possible mechanisms and implications are discussed.
The Journal of Physical Chemistry, 1990
We use scanning tunneling microscopy, atom-resolved tunneling spectroscopy, and electronic structure calculations to determine the nature of the adsorption state of oxygen in the initial stages of the oxidation of Si(11 1). We are able to directly image two states of adsorbed oxygen. One of them is identified as a Si adatom site with one oxygen atom inserted in one of the back bonds, while the other involves an oxygen atom tying up the adatom dangling bond with, most likely, another oxygen inserted in one of the back bonds. As the coverage is increased toward the monolayer, the latter site becomes the dominant one.
Transformations of C-type defects on Si(100)-2 × 1 surface at room temperature – STM/STS study
Surface Science, 2008
We present a scanning tunneling microscopy study of the C-type defects on the Si(100)-2 Â 1 surface and their transformations into other defect forms at room temperature. A model of the C defect as a dissociated water molecule was adopted for interpretation of the observed transformations. We explained the transformations by hopping the H or OH between bonding sites on Si dimers. Newly, the most stable defect form, corresponding to the H and hydroxyl group adsorbed on the same dimer, is reported. Real time observations provided an explanation for the defect C2-C2 described earlier. A reversible transition of this defect into another form, not revealed yet, is presented. Electronic structure of the observed defects is studied by means of scanning tunneling spectroscopy. Measured spectra show semiconducting character of the C defect. Spectra of the other defect forms are discussed.
Microscopic mechanisms of initial oxidation of Si(100): Reaction pathways and free-energy barriers
Physical Review B, 2012
Various reaction pathways and corresponding activation barriers in the initial oxidation of Si(100) surfaces are clarified by free-energy sampling techniques combined with the Car-Parrinello molecular dynamics. We find a crucial stable geometry which is ubiquitous during the oxidation and links the dissociation of O 2 molecules and the oxidation of subsurfaces. The calculated free-energy landscape provides a comprehensive picture of the various competing reaction pathways.
The Journal of Chemical Physics, 2007
First principles calculations and scanning tunneling microscopy studies of the oxidation of Si͑100͒-͑2 ϫ 1͒ surfaces by molecular oxygen reveal that the surface silanone ͑O͒͑Siv O͒ species is remarkably stable, constituting the key intermediate for initial oxidation. The propensity for oxygen to remain within the top surface layer as opposed to incorporating within Si-Si backbonds is surprisingly high. This resistance to incorporation into a cubic lattice even at higher coverages could be a factor to facilitate surface amorphization in subsequent steps.
Physical Review B, 2004
The Si͑100͒ surface structures on n-type degenerately doped samples (ϳ0.005 ⍀ cm) have been investigated with a scanning tunneling microscope ͑STM͒ at very low temperature ͑ϳ5 K͒. We have developed a method to monitor quantitatively the proportion of the various observed surface structures ͓ p(2ϫ2), c(4 ϫ2) and flickering͔. This study has been performed as a function of the tunnel current and the presence ͑or not͒ of surface defects in the observed areas. The normal surface areas having a low density of defects ͑ϳ1%͒ have been observed to vary from the p(2ϫ2) to the c(4ϫ2) structures when the tunnel current increases. This indicates that the STM tip-surface interaction strongly influences the observed structures. Furthermore, surface areas completely free of any defects are dominated by flickering structures.
Electronic characterization of defect sites on Si(001)-(2 × 1) by STM
Surface Science, 1997
A study of the defect sites on Si(001 )-(2 x 1 ) was undertaken by employing comparative scanning tunneling spectroscopy (CSTS). Ni induced defects, the A, B, and C defects, and the A and B steps were studied. The STM tip induced band bending significantly affects tunneling spectra. This is evident from a comparison of LDOS and local surface potentials obtained for high (10 l~ cm) and low (100 f~ cm) doped Si(001) crystals. Decay in the local surface potential was found around the Ni induced "split off dimer" defect site and in the "vacancy channel" defect. In agreement with previous studies, a reduction in the surface states energy gap was observed for the C defect. The B step and the C terrace defect reveal similar LDOS. This is interpreted as indicating that the C defect on the terraces migrates preferentially to the B step edge, and that it is due to a surface impurity such as CI or a product of H20 decomposition. This may be responsible for preferential etching phenomena or evaporation at step edges. © 1997 Elsevier Science B.V.
Oxidation of Si(100)2 × 1: thermodynamics of oxygen insertion and migration
Surface Science, 1997
Accurate quantum chemical calculations on model cluster molecules are used to identify and compare the most probable surface structures involved in the oxidation of the Si(100)2 x 1 surface. The energetics of various reaction channels for insertion of oxygen into Si-Si bonds are evaluated. The dimer bond is clearly shown to be the initial target of O entry and an oxygen-inserted dimer is proposed as the most likely structure at lower temperatures. At higher temperatures an asymmetrically oxidized dimer unit with three oxygen atoms inserted into Si-Si bonds at the same silicon is the dominant feature.
Kinetics of Initial Layer-by-Layer Oxidation of Si(001) Surfaces
Physical Review Letters, 1998
Layer-by-layer oxidation of Si(001) surfaces has been studied by scanning reflection electron microscopy (SREM). The oxidation kinetics of the top and second layers were independently investigated from the change in oxygen Auger peak intensity calibrated from the SREM observation. A barrierless oxidation of the first subsurface layer, as well as oxygen chemisorption onto the top layer, occurs at room temperature. The energy barrier of the second-layer oxidation was found to be 0.3 eV. The initial oxidation kinetics are discussed based on first-principles calculations. [S0031-9007(97)04959-4]