Hydrogen atom adsorption on Si

It has been postulated by Appelbaum et al. [217] that, while the clean Si 100 2x1 surface is almost certainly row dimerized, the monohydride phase is not (although the equivalent monohydride on Ge is). This conclusion is based on the lack of agreement between UPS data for Si 100 2x 1 H and a self-consistent calculation assuming the pairing model of surface geometry, while there is complete agreement between theory and experiment for the clean Si 100 2 x 1 surface and the Si lll H monohydride phase. No alternative to row dimerization has been proposed for Si 100 2 x 1 H, however. 

Further hydrogen adsorption causes rupture of the Si—Si bond between surface atoms with a hydrogen atom bonded to each additional dangling orbital so produced to give the dihydride phase. This is the sequence for room temperature adsorption. Thermally stimulated desorption from this dihydride phase occurs by the association of adjacent H atoms to give H2 as the desorption product, the surface phase remaining being the monohydride. Adsorption at a substrate temperature of 500 K produces only the monohydride phase, but if the substrate is cooled to room temperature in the presence of H, it is converted completely to the dihydride. 

These results make the idea that the 100 2x1 reconstruction results from a high concentration of surface vacancies [78, 202] or a canted ridged structure [79] rather improbable, since in both models significant atom migration would be required for the change to 100 1 x 1 to occur. 

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Despite the many differences in HF and KOH solutions as shown in Table 5.8, the overall reactions are similar in two important aspects the silicon surface is dynamically terminated by hydrogen and breaking of the silicon-silicon back bond is facilitated by the adsorption of electronegative ligands such as F" or OH". More specifically this means (1) the initial surface is hydrogen terminated (2) the Si-Si back bond requires that the hydrogen termination is first replaced by F" or OH" and (3) the silicon atoms on the newly exposed layer are terminated by hydrogen so that the surface after the dissolution of one silicon layer is identical to that before the dissolution. 

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