The Si 100 -2 x 1 surface

The Si(100) surface is used in a large majority of semiconductor devices, and has been extensively studied. The 2x1 periodicity of the surface shown in 

---

The pseudoexcitation is induced by the delocalization from alkenes to the Si(100)-2 X 1 surface [133]. Electron-accepting alkenes undergo different reactions. For acrylonitirile, a [4+2] cycloaddition reaction was found to be kinetically most favorable [135]. 

Boland, J. J. (1991c). Erratum to "Evidence of pairing and its role in the recombinative desorption of hydrogen from the Si(100)-2 X 1 surface." Phys. Rev. Lett. 67, 2201. 

Figure 1.16. Plan view of the Si(100)(2 x 1) surface (with symmetric dimers) showing four possible sites for the adsorption of C2H2 and C2H4. Only the C atoms of the adsorbate are shown as small dark spheres. The Si atoms are shown with successively darker shading for deeper layers.

Due to the historical importance of the initial stages of silicon oxidation to microelectronics fabrication, there has been a great deal of interest in the reaction of the water oxidant on the Si(100)-2 x 1 surface. A number of studies have shown that water adsorbs in a dissociated state consisting of OH(a) and H(a) species adsorbed on the Si surface dimer at room temperature [60-69]. More recent studies have closely investigated the mechanism of water oxidation. A series of density functional theory calculations (DFT) calculations by Konecny and Doren indicated that water first molecularly adsorbs through one of its lone pairs in a weakly bound precursor state, then transfers a proton to form OH(a) and H(a) species on the surface dimer [43]. The pathway to proton transfer is found to be unactivated with respect to the entrance channel, which suggests that OH(a) and H(a) are the dominant surface species at room temperature, in agreement with the previous experimental work [60-69]. 

Due to the relative ease of carrying out the reaction and the versatility of the process, the hydrosilylation reaction has been used in a number of interesting extensions and applications. Here several of them are highlighted. In one report, Lop-inski and coworkers used the same concept of the radical-initiated hydrosilylation reaction on the Si(100)-2 x 1 surface to induce self-directed growth of molecular wires on the surface [141]. On the Si(100)-2 x 1 surface, the radical chain reaction propagates primarily along the direction of the dimer row, leading to lines of... 

If the analogy that is drawn between the Si=Si dimer on the Si(100)-2 x 1 surface and an alkene group is reasonable, then certain parallels might be expected to exist between cycloaddition reactions in organic chemistry and reactions that occur between alkenes or dienes and the silicon surface. In other words, cycloaddition products should be observed on the Si(100)-2 x 1 surface. Indeed, this prediction has been borne out in a number of studies of cycloaddition reactions on Si(100)-2x1 [14], as well as on the related surfaces of Ge(100)-2 x 1 (see Section 6.2.1) and C(100)-2 x 1 [192-195]. On the other hand, because the double-bonded description is only an approximation, deviations from the simple picture are expected. A number of studies have shown that the behavior differs from that of a double bond, and the asymmetric character of the dimer will be seen to play an important role. For example, departures from the symmetry selection rules developed for organic reactions are observed at the surface. Several review articles address cycloaddition and related chemistry at the Si(100)-2 x 1 surface the reader is referred to Refs. [10-18] for additional detail. 

Figure 5.13. Cycloaddition products at the silicon dimer of the Si(100)-2 x 1 surface, (a) shows the [2 + 2] cycloaddition product formed in the reaction with ethylene, and (b) shows the [4 + 2], or Diels-Alder, cycloaddition product formed in the reaction with 1,3-butadiene.

One example is the work by Hamers and coworkers. Hamers et al. showed with IR spectroscopy that cyclic alkenes, such as cyclopentene [216] and 1,5-cyclooctadiene [217], also bond to the Si(100)-2 x 1 surface to give a [2 + 2] —C cycloaddition product. Furthermore, they demonstrated with STM that cyclopentene and 1,5-cyclooctadiene form well-ordered monolayers, as shown in Figure 5.14 [218]. The cyclopentene experiments by Hamers and workers were performed on a vicinal Si surface that was intentionally cut slightly (4°) off-axis... 

Figure 5.15. Density functional theory study of the reaction of 1,3-butadiene with the Si(100)—2 x 1 surface, modeled using a nine silicon atom cluster and examining two possible products the product of the Diels-Alder cycloaddition, and the product of the [2 + 2] cycloaddition [237,238].

Studies by Teplyakov et al. provided the experimental evidence for the formation of the Diels-Alder reaction product at the Si(100)-2 x 1 surface [239,240]. A combination of surface-sensitive techniques was applied to make the assignment, including surface infrared (vibrational) spectroscopy, thermal desorption studies, and synchrotron-based X-ray absorption spectroscopy. Vibrational spectroscopy in particular provides a molecular fingerprint and is useful in identifying bonding and structure in the adsorbed molecules. An analysis of the vibrational spectra of adsorbed butadiene on Si(100)-2 x 1 in which several isotopic forms of butadiene (i.e., some of the H atoms were substituted with D atoms) were compared showed that the majority of butadiene molecules formed the Diels-Alder reaction product at the surface. Very good agreement was also found between the experimental vibrational spectra obtained by Teplyakov et al. [239,240] and frequencies calculated for the Diels-Alder surface adduct by Konecny and Doren [237,238]. 

Hetero-Diels-Alder products (i.e., Diels-Alder products involving an atom other than carbon) have also been observed for a number of systems at the Si(100)-2 x 1 surface. Some examples include the reaction of unsaturated ketones RC=C—CR=0 (e.g., ethylvinylketone) [255], 2-propenenitrile [256-260], and dicarbonyls 0=C—R—C=0 [261]. For a review specifically of heteroatom chemistry at the silicon surface, the reader is referred to Ref. [262]. Here we use the dicarbonyl example to provide an illustration of this class of surface chemistry. Hamers and coworkers have used the [4 + 2] heteroatom cycloaddition reaction of... 

Interestingly, Cao and Hamers found in the same set of studies that trimethylamine also forms a dative-bonded adduct on the Si(l 11)—7 x 7 surface, reacting similarly to the Si(100)-2 x 1 surface [47]. They note that the positively charged adatoms on Si(l 11)—7 x 7 act as Lewis acids, and are the most likely site for the nucleophilic amine to bond. The main difference observed between the two surfaces is that the coverage of trimethylamine molecules on Si(lll)-7 x 7 is only about half the coverage on Si(100)-2 x 1. 

The participation of the germanium dimers in nucleophilic/electrophilic or Lewis acid/base reactions has been the subject of several investigations on the Ge(100)-2x1 surface [16,49,255,288,294,313-318]. As for the case of silicon, adsorption of amines has provided an excellent system for probing such reactions. Amines contain nitrogen lone pair electrons that can interact with the electrophilic down atom of a tilted Ge dimer to form a dative bond via a Lewis acid/base interaction (illustrated for trimethylamine at the Si(100)-2 x 1 surface in Ligure 5.17). In the dative bond, the lone pair electrons on nitrogen donate charge to the Ge down atom [49]. 

Konecny, R. and Doren, D. J. Theoretical prediction of a facile Diels-Alder reaction on the Si(100)—2 x 1 surface. Journal of the American Chemical Society 119, 11098-11099 (1997). 

Teplyakov, A. V., Kong, M. J. and Bent, S. F. Vibrational spectroscopic studies of Diels-Alder reactions with the Si(100)-2 x 1 surface as a dienophile. Journal of the American Chemical Society 119, 11100-11101 (1997). 

Fig. 9. Filled state topographic STM images showing the two types of step on the Si(100)-2 X 1 surface (a) type Sa steps and (b) type Sg steps. Schematics showing the step types are shown below. Images courtesy of S. R. Schofield.

Using high-resolution STM Hamers et al. [46] first characterized the three most common defects of the Si(100)-2 x 1 surface. The type A defect is more commonly known as the single-dimer vacancy (DV) defect. In STM images (see Fig. 10) it appears as a dark region in both polarities and is due to the absence of a dimer [47]. The type B defect is a combination of two dimer vacancies (double dimer vacancy) and also appears as a larger... 

---