Cleaning hydrogen termination

This chapter will focus on organic/silicon interfaces formed via solution phase reactions using hydrogen-terminated crystalline silicon surfaces as a starting point. While some of the surface chemistry issues have been reviewed previously [7,8], more recent developments will be emphasized here. We will not discuss the considerable literature of reactions with porous silicon [8], or studies of molecules reacting with clean silicon surfaces under ultrahigh vacuum (UHV) conditions [9-11] which have been reviewed elsewhere. 

To examine the adsorption properties, we have first optimized the geometric structures of all the clean tubes under consideration. Secondly, for each tube we have found the optimal adsorption distance d(J between the H atom and the tube. To do this, we have calculated the total energy I(1 [BNNT + 2 H] of the hydrogen-terminated BNNTs with the adsorbed H atom on the top of the B or N atom as a function of the distance d between the adatom and the tube. As an example, in... 

Thus, unless in a vacuum, the surface of silicon is never clean because of the adsorption by foreign species. As will be seen in the following sections, the type of termination, in terms of chemical nature, thickness, and composition, is a function of how the surface is prepared and cleaned. In particular, the surface cleaned with HF solutions, which are widely used for preparation of silicon surfaces, is known to be hydrogen terminated. On the other hand, in water and non-HF solutions, silicon surfaces tend to be covered with an oxide film. Employing certain treatment processes, the surface of siheon can be terminated by different species—and/or F and/or OH—using different cleaning solutions as shown in Table 2.7. ... 

However, even the ideal buLk-terminated SDB-type surface (Figure 9.21b) is not a stable surface configuration. It has never been observed as a (1 x 1) reconstruction of the clean surface. It could be stabihzed by hydrogen termination, which saturates aU the danghng bonds and results in a quasi ideal bulk-terminated Si(lll)-(1 x 1) structure, which does not even possess surface states [43]. The ground state of the bare (111) surfaces is the famous (7x7) reconstruction for Si and a c(2x8) reconstruction for Ge. 

Figure 10.8 Angle-resolved photoelectron domains rotated by 90°. The resonances B spectra of hydrogen-terminated and clean di- and C are associated with surface states. The amend (100) [57]. Progression of the polar dispersion relationships evaiuated from the angle 9 indicated at the spectra corresponds spectra of the clean surface have been added to a variation of ky along X in one set of do- to the surface band structure of Figure 10.7. mains and simultaneously to Y in equivalent...

Figure 10.9 Near-edge X-ray absorption fine structure (NEXAFS) for clean (a) and hydrogen-terminated (b) diamond (100). The resonances Spi, Srj,4, and Sh imply unoccupied surface states in the band gap for the respective surfaces. For the clean surface, the symmetry of the surface core excitons could even be determined from...

Figure 10.14 Angle-resolved photoelectron spectra of the clean C(lll)2xl surface scanning the surface Brillouin zone along a line through K. The resonance S corresponds to the surface state, K is reached for a polar angle of ft 45. The zero of the energy axis is the Fermi level. In the upper panel one spectrum for the hydrogen-terminated surface is shown to demonstrate the absence of S for that surface. (Data from Ref [57].)...

Figure 10.17 Sketch of the bulk-terminated diamond (110) surface. Both the clean and the hydrogen-terminated surfaces are found in this 1x1 geometry. Note the similarity to C(lll)2xl (Figure 10.12), which is also reflected in the...

Hydrogen termination of the diamond (110) surface maintains the 1x1 geometry [63] but reduces the relaxation of the clean surface considerably. The distance between surface atoms is now smaller by only 1.7% compared to the bulk-terminated structure. All other atomic distances that deviate by less than 0.6% form the corresponding bulk values [60]. No occupied (donorlike) surface states are found in the gap (compare Table 10.2), neither by band structure calculations [60] nor by photoemission [64]. Unoccupied (acceptorlike) surface states are predicted by theory, ranging from 2.0 eV above the VBM to the CBM and extending as pronounced surface resonances up to 2.8 eV above the CBM [60]. As for the other diamond surfaces, hydrogen can thus provide a successful passivation of the (110) surface for p-type bulk material, but leaves electronically active surface states on -type diamond. 

Summarizing the three most important diamond surfaces, the (100) and the (111) surfaces undergo characteristic 2x1 reconstructions in their clean, adsorbate-free form, whereas the (110) surface remains unreconstructed albeitwith a considerable relaxation. The (111)2x1 and the (110)1x1 surfaces form characteristic jr-bonded chains, whereas the (100)2x1 surface atoms arrange in rows of double-bonded dimers. Hydrogen termination leads to unreconstructed and almost perfectly... 

For example, a mild HF etch (1 100) is used in cleaning glass for flat panel display production. Hydrofluoric acid solution is a common etchant for sihcon and can leave a sihcon surface either hydrogen- or hydroxyl-terminated. The silicon surface is hydrophobic ( wata--hating ) if hydrogen-terminated, and hydrophilic ( water-loving ) if hydroxyl-terminated. 

For a substrate surface heated at -2°C per second, most of the dihydride surface will desorb H2 and convert to a monohydride surface at -400°C, while the monohydride will desorb H2 and yield a clean surface at -520°C. Slower heating or a static anneal will allow to desorption at lower temperatures if sufficient time is available. Hydrogen-terminated Si (100) surfaces can also be created Ifom normal air-exposed oxidized surfaces by dipping the wafer in HF. The strength of the Si-H bond is illustrated by the observation that this surface is stable in laboratory air under standard conditions for several minutes to several hours. 

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