Passivation hydrogen termination

The only cathodic process in HF solutions is the hydrogen evolution reaction (HER), which is important in that it is involved in almost all reactions at both anodic and cathodic potentials. The silicon electrode can be passivated by hydrogen termination... 

Here we describe some of the results. In each of these studies, the compound semiconductor was first etched in either acid or base to remove the oxide. The specific surface groups following the etch are not well understood. However, Pluchery et al. have followed the acid etching of InP by in situ infrared spectroscopy [175] and observed the removal of the oxide. Unlike Si, for which an acid (HF) etch leaves the surface hydrogen-terminated and temporarily passivated, acid etching of InP does not produce a chemically passivated surface. Presumably, the surface is left unprotected, and quickly oxidizes if not passivated by another process. Similar results showing reduction or removal of the oxide are seen for GaAs [174,176,177]. 

The procedure for preparing atomically smooth, hydrogen-terminated silicon surfaces involves a number of steps removal of hydrocarbon contamination, formation of a uniform oxide, oxide removal, etching of the silicon surface, and the formation of the passivation layer. The uniformity of the oxide is important in developing a smooth surface at the Si/Si02 interface. 

Scanning probe lithography on metal or silicon substrates is a well known technique and can be supported by a self-assembled monolayer (SAM) [1,2], Such monolayers are of great interest e.g. for passivation of silicon surfaces [3]. Covalently bound monolayers by Si-C bonds that are formed by the reaction of 1-alkenes and a hydrogen terminated silicon surface [4,5], are known to show high thermal [6] as well as chemical stability [3,7]. 

HCl to NH40H). Polyhydrides are found to be more stable than monohydride. Hydrogen termination also serves to passivate grain boundaries. Terrace monohydride has different stability from step monohydride. As a result of hydrogen passivation, HF-treated silicon surface exhibits a very low density of surface states in various acids over a wide concentration range. ... 

Boukherroub R, Morin S, Wayner DDM, Bensebaa F, Sproule Gl, Baribeau JM, Lockwood DJ (2001) Ideal passivation of luminescent porous silicon by thermal, noncatalytic reaction with alkenes and aldehydes. Chem Mater 13 2002-2011 Boukherroub R, Wojtyk JTC, Wayner DDM, Lockwood DJ (2002) Thermal hydrosilylation of undecylenic acid with porous silicon. J Electrochem Soc 149 59-63 Boukherroub R, Petit A, Loupy A, Chazalviel JN, Ozanam F (2003) Microwave-assisted chemical functionalization of hydrogen-terminated porous silicon surfaces. J Phys Chem B 107 13459-13462... 

In contrast to the (100) surface, hydrogen termination of the surface danghng bonds stabihzes the diamond (111) surface in its bulk-terminated form as shown in Figtu-e 10.12a without any notable deviation of the carbon atoms from their respective bulk positions. Instead of carrying a danghng bond, each surface atom is passivated by a hydrogen atom. The formation of the covalent carbon-hydrogen... 

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. 

The surface condition of a silicon crystal depends on the way the surface was prepared. Only a silicon crystal that is cleaved in ultra high vacuum (UHV) exhibits a surface free of other elements. However, on an atomistic scale this surface does not look like the surface of a diamond lattice as we might expect from macroscopic models. If such simple surfaces existed, each surface silicon atom would carry one or two free bonds. This high density of free bonds corresponds to a high surface energy and the surface relaxes to a thermodynamically more favorable state. Therefore, the surface of a real silicon crystal is either free of other elements but reconstructed, or a perfect crystal plane but passivated with other elements. The first case can be studied for silicon crystals cleaved in UHV [Sc4], while unreconstructed silicon (100) [Pi2, Ar5, Th9] or (111) [Hi9, Ha2, Bi5] surfaces have so far only been reported for a termination of surface bonds by hydrogen. 

Although Si(100) and Ge(100) undergo similar dimer reconstructions, the Ge(l 11) surface reconstructions differ from those of Si(lll). As described above, Si(lll) reconstructs into a (7 x 7) structure that contains 49 surface atoms in the new unit cell. Ge(lll) is found in various reconstructed forms depending on surface preparation, but the most common reconstruction under vacuum is Ge(lll)-c(2 x 8) [51-53]. This structure involves charge transfer from adatoms to restatoms [5]. On the other hand, most of the passivation and functionalization studies reviewed here lead to the Ge(lll)-1 x 1 surface structure. This structure, in which the surface Ge atoms retain their bulk positions, can be achieved by hydrogen, chlorine, or alkyl termination of the surface (discussed below). The structure is analogous to that for H-terminated Si(lll). 

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