Si surface

Atom abstraction occurs when a dissociation reaction occurs on a surface in which one of the dissociation products sticks to the surface, while another is emitted. If the chemisorption reaction is particularly exothennic, the excess energy generated by chemical bond fomiation can be chaimelled into the kinetic energy of the desorbed dissociation fragment. An example of atom abstraction involves the reaction of molecular halogens with Si surfaces [27, 28]. In this case, one halogen atom chemisorbs while the other atom is ejected from the surface. 

In corrosion, adsorbates react directly with the substrate atoms to fomi new chemical species. The products may desorb from the surface (volatilization reaction) or may remain adsorbed in fonning a corrosion layer. Corrosion reactions have many industrial applications, such as dry etching of semiconductor surfaces. An example of a volatilization reaction is the etching of Si by fluorine [43]. In this case, fluorine reacts with the Si surface to fonn SiF gas. Note that the crystallinity of the remaining surface is also severely disrupted by this reaction. An example of corrosion layer fonnation is the oxidation of Fe metal to fonn mst. In this case, none of the products are volatile, but the crystallinity of the surface is dismpted as the bulk oxide fonns. Corrosion and etching reactions are discussed in more detail in section A3.10 and section C2.9. 

For some materials, the most notable being silicon, heating alone sufiBces to clean the surface. Commercial Si wafers are produced with a thin layer of silicon dioxide covering the surface. This native oxide is inert to reaction with the atmosphere, and therefore keeps the underlying Si material clean. The native oxide layer is desorbed, i.e. removed into the gas phase, by heating the wafer in UHV to a temperature above approximately 1100 °C. This procedure directly fonus a clean, well ordered Si surface. 

Chemical reactions of aimnonia with the silicon surface have also been clearly observed using STS [21], where the disappearance of the it and it states characteristic of the clean surface coincides with the fomiation of Si-H antibonding states corresponding to the dissociation of the ammonia on the Si surface. 

The dangling bonds of a Si surface abstract one F atom from an incident F2 molecule while the complementary F atom is scattered back into the gas phase [20]. This abstractive mechanism leads to F adsorjDtion at single sites rather than at adjacent pairs of sites, as observed directly by scanning tunnelling microscopy [21]. Br atoms adsorb only to Ga atoms in the second layer of GaAs(001)-(2 x 4) where empty dangling bonds on the Ga atoms can be filled by electrons from the Br atoms [22]. 

Engel T 1996 Fundamental aspects of the reaction of thermal and hyperthermal F, F2, Cl, and CI2 with Si surfaces Japan. J. Appl. Phys. 35 2403-9... 

McFeely F R, Morar J F, Shinn N D, Landgren G and Himpsel F J 1984 Synchrotron photoemission investigation of the initial stages of fluorine attack on Si surfaces relative abundance of fluorosilyl species Phys. Rev. B 30 764-70... 

Feil H, Dieleman J and Garrison B 1993 Chemical sputtering of Si related to roughness formation of a Cl-passivated Si surface J. Appi. Phys. 74 1303-9... 

InP, in the range 10 —10 cm . Boron, phosphorus, and other shallow impurities can be detected in silicon in concentrations approaching 10 cm . Copper contamination at Si surfaces has been detected down to 10 cm levels. ... 

An example of interaction stiffness and force curves for a Si surface with a native oxide at 60% relative humidity (RH) is shown in Fig. 12 [104]. The stiffness and force data show an adhesive interaction between the tip and substrate. The hysteresis on retraction is due to a real change in contact area from surface oxide deformation and is not an experimental artifact. The adhesive force observed during retraction was consistent with capillary condensation and the surface energy measured from the adhesive force was close to that of water. 

Follow the above procedures, we obtained the friction coefficient of the unclean single crystal Si surface is about 0.06, which agrees very well with the result obtained under the same experimental condition by Bhushan et al. It is validated with the feasibility of the calibration method. Based on the friction coefficient, we can easily obtain the friction forces under the loads. 

The cross section of the collision region that the particle impacts with the Si surface with an incident angle of 45° at a speed of 2,100 m/s is shown in Fig. 16 [28]. As the particle impacts into the Si surface layer, the contact region of the Si surface layer transforms from crystal into amorphous phase immediately. The area of the depressed region and the thickness of the amorphous layer increase with the penetration depth of the particle (Figs. 16(a)-16(c)). After it reaches the deepest position, the particle then moves both upwards and rightwards, and some silicon atoms ahead of the particle are extruded out and result in a pileup of atoms. Then the released elastic deformation energy of the Si surface pushes... 

Cu and Ag on Si(lll) surfaces. In the last example, we come back to surfaces. It is well known (44-46) that Cu catalyzes the formation of dimethyl-dichlorosilane from methylchloride and solid silicon, which is a crucial technological step in the synthesis of silicone polymers. Even today, the details of the catalytic mechanism are unclear. Cu appears to have unique properties for example, the congener Ag shows no catalytic activity. Thus, the investigation of the differences between Cu and Ag on Si surfaces can help in understanding the catalytic process. Furthermore, the bonding of noble metal atoms to Si surfaces is of great importance in the physics and chemistry of electronic devices. 

In addition, the calculations also provide evidence for differences in the electronic structures Cu weakens the Si-Si bond between adjacent surface and sub-surface atoms to a larger extent than does Ag. Thus, Cu promotes the Si-Si bond-breaking without blocking access to the surface whereas Ag has a smaller electronic effect and blocks the Si surface from a direct interaction with methylchloride molecules. 

Finally, the investigation of noble metal bonding on semiconductor surfaces provides evidence that at moderate temperatures Cu diffuses easily into the Si surface whereas the penetration barrier for Ag is almost as large as its binding energy. The theoretical results help in the understanding of an important catalytic process in the synthesis of silicone polymers and shed light on the Cu/Si and Ag/Si interface formation. 

Scheme 3. Schematic illustration of SCVP of an AB inimer from a functionalized Si surface...

In 1993, Linford firstly reported a quite useful method to prepare monolayers of alkyl chains by thermal hydros-ilylation of hydrogen-terminated silicon surfaces [25]. Alkyl chains are covalently bound to Si surface by Si-C bonds. This thermal hydrosilylation could be attributed to a free-radical process with 1-alkene. First, a diacyl peroxide initiator was used to produce free radicals. However, at higher temperature, only hydrogen-terminated silicon and a neat solution of 1-alkene or 1-alkyne can form Si-C linkages [26]. Furthermore, lately it is found that such Si-C covalent links can be observed even in dilute solutions of 1-alkenes [27]. In that case, the density of monolayer packing strongly depends on the reaction temperature. 

Figure 5. High-resolution SEM images of the Si surface covalently linked to the nanoparticles stabilized by (A) C3, (B) C6, and (C) Cll. (Reprinted with permission from Ref [11a], 2004, American Chemical Society.)...

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