Difficulty for oxygen to incorporate into the silicon network during initial O[sub 2] oxidation of Si(100)-(2×1) (original) (raw)
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Density functional theory simulations are used to investigate the reaction mechanism of oxidation of the bare Si͑100͒-͑2ϫ1͒ surface by molecular oxygen. O 2 adsorbs molecularly on the ''up'' surface Si atom with no activation barrier and an adsorption energy of 35 kcal/mol. Adsorbed O 2 is found to be negatively charged. O 2 (a) then transforms into the peroxide bridge structure with a barrier of 10 kcal/mol and exothermicity of 33 kcal/mol. The bridged peroxide O 2 then dissociates by first inserting one oxygen atom into the Si-Si dimer bond followed by insertion of the remaining oxygen atom into a Si-Si backbond. The activation barriers are 36 kcal/mol and 13 kcal/mol for the first and second oxygen insertions, respectively. We have also calculated the activation barriers for SiO 2 film decomposition, which becomes prevalent at high temperatures, in which SiO(g) desorbs from SiO 2 films. The SiO desorption barriers are found to be in the range of 65-67 kcal/mol.
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.
Surface Science, 2007
The incorporations and migrations of the atomic oxygen in the topmost layer Si(1 0 0)-p(2 · 2) silicon surface, are investigated theoretically using density functional theory. We show that the diffusion is dependent on the starting and the final surrounding environment and does not simply consist in hops from one silicon-silicon bond to another. The activation energies range from 0.11 eV to 2.59 eV.
Physical Review B, 2009
Density functional theory calculations reveal a two-step scenario for silicon oxidation nucleation. We detail a quasibarrierless semihexagonal oxide nucleus, involving an unexpected adjacent dimer oxygen bridging bond. It is formed upon O 2 chemisorption at 0.5 monolayer on Si͑100͒-͑2 ϫ 1͒. This structure arises from the difficulty to systematically insert oxygen atoms into first neighbor Si-Si bonds. While silanone structures, characterized by a Si= O strand, effectively accommodate oxygen at lower coverages, the stabilization of this hexagonal-like pattern on a cubic substrate at low temperatures and at higher coverages demonstrates the ability of oxygen atoms to deeply modify the arrangement of silicon atoms on the surface and to impose a specific structure. It is believed to offer a key natural pathway toward the formation of an abrupt crystalline semiconductor/amorphous oxide transition.
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.
Surface Science, 2011
We have studied the initial oxidation of Hand styrene-terminated Si(100)-2 × 1 films in O 2 atmosphere at 500 K using molecular dynamics (MD) simulations based on a reactive force field. Our simulations show that for both surface terminations the primary reactions observed are the dissociation of the oxygen molecules and the simultaneous insertion of atomic oxygen in the Si\Si back-bonds. On the H:Si(100)-2 × 1 surface, another reaction is the formation of isolated Si\OH bonds via the insertion of an oxygen atom in a Si\H bond. Detailed analysis of MD configurations shows that different vibrational modes of the surface Si\H and the tilting of Si dimers at 500 K facilitate the breaking of the O 2 molecule and the oxygen attack at backbonds. The combination of these reactions leads to increased amorphization of the surface as the oxidation proceeds. In the case of styrene-terminated Si(100)-2 × 1, the rate of O 2 attack was much lower than on H-terminated surface and O-atom insertions were not observed in back-bonds of Si\C bonds. In addition to lesser number of Si\H sites on styrene-Si(100)-2 × 1, another significant reason for the lower rate of O 2 attack was the repulsion of oxygen molecules resulting from the movement of phenyl rings in styrene at 500 K.
Layer-By-Layer Oxidation of Silicon Surfaces
MRS Proceedings, 1999
ABSTRACTLayer-by-layer oxidation of Si(111) and (001) surfaces has been studied by using scanning reflection electron microscopy (SREM). We found that SREM images reveal interfacial structures of the SiO2/Si system. Our results showed that the initial step structure of Si substrates was preserved at SiO2/Si interfaces and that interfacial steps did not move laterally during oxidation. We also observed a periodic reversal of terrace contrast in SREM images during the initial oxidation of Si(001) surfaces. These results indicate layer-by-layer oxidation of Si surfaces, which is promoted by the nucleation of nanometer-scale oxide islands at SiO2/Si interfaces. In addition, we investigated the kinetics of initial layer-by-layer oxidation of Si(001) surfaces. We found that a barrierless oxidation of the first subsurface layer, as well as oxygen chemisorption onto the top layer, occur at room temperature. The energy barrier of the second-layer oxidation was found to be 0.3 eV. The initial...
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 initial stages of the oxidation of Si(100)2 x 1 studied by STM
Ultramicroscopy, 1992
The initial stages of the oxidation of the Si(100)2x 1 surface was studied at room temperature by scanning tunneling microscopy (STM). We found that "type-C defects" which are believed to be two half-dimers have a strong preference for oxidation, compared with areas having no defects. Oxidized type-C defects appear to be depressions in both positive and negative sample bias voltages. We also found two oxidized sites having no defects. Single steps are quite stable against oxidation. The oxidation of the Si(100)2 x 1 surface is discussed in terms of these sites.
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]