First principles analysis of the initial oxidation of Si(001) and Si(111) surfaces terminated with H and CH3

Abstract

Transition state analyses have been carried out within a density functional theory setting to explain and quantify the distinctly different ways in which hydrogen and methyl terminations serve to protect silicon surfaces from the earliest onset of oxidation. We find that oxidation occurs via direct dissociative adsorption, without any energy barrier, on Si(111) and reconstructed Si(001) that have been hydrogen terminated; oxidation initiates with a barrier of only 0.05 eV on unreconstructed Si(001). The commonly measured protection afforded by hydrogen is shown to derive from a coverage-dependent dissociation rate combined with barriers to the hopping of adsorbed oxygen atoms. Methyl termination, in contrast, offers an additional level of protection because oxygen must first undergo interactions with these ligands in a three-step process with significant energy barriers: adsorption of O(2) into a C-H bond to form a C-O-O-H intermediate; decomposition of C-O-O-H into C-O-H and C=O intermediates; and, finally, hopping of oxygen atoms from ligands to the substrate.

Optimal Size Regime for Oxidation-Resistant Silicon Quantum Dots

“…This requires that a specific chain of adsorption and diffusion steps be followed. Since E a inw = 2.8 eV for HÀSi(111), 14 we expect that the inward barrier for any Si QDs with d > 1.0 nm is in the range of 2.8À3.7 eV. This analysis of lateral and inward hopping indicates that O atoms will be frozen on the surface of otherwise defect-free passivated Si QDs, regardless of their size.…”

“…Aside from the geometry-induced apex site vulnerability, methyl passivation therefore provides much better protection against oxidation than does H, in agreement with our previous analysis of bulk Si surfaces. 14 The coverage dependence of apex oxidation was investigated by allowing oxygen to occupy back bond sites on five of the six apexes, as illustrated in Figure 5. Oxidation attack on the remaining apex site was found to have a barrierless second step, but the activation energies for the first and third steps rise to 1.3 and 0.7 eV, respectively.…”

“…For instance, Si 66 (CH 3 ) 40 H 24 dots offer an apex site barrier of 2.3 eV, close to the bulk value of 3.0 eV for methylterminated flat Si(111) surfaces. 14 This has remarkable implications for the protection of QDs from oxidation. Just as for bulk surfaces, hydrogen passivation does not offer any barrier to initial oxidation, 14 although the barrier increases rapidly with the surface coverage of oxygen in both settings.…”

“…14 This has remarkable implications for the protection of QDs from oxidation. Just as for bulk surfaces, hydrogen passivation does not offer any barrier to initial oxidation, 14 although the barrier increases rapidly with the surface coverage of oxygen in both settings. For methyl and siloxane passivation, only Si QDs with a diameter less than 1.2 nm will be oxidized significantly faster than bulk Si surfaces, while QDs with d > 1.2 nm provide strong oxidation resistance nearly identical to that of (defect-free) bulk surfaces.…”

“…14 This barrier increases rapidly with surface coverage, though, and because the mobility of O is low, it is possible to achieve stable surfaces that exhibit submonolayer O coverage. 14 Significantly, this level of protection is only achieved in the absence of surface defects. 15 Surface dangling bonds, in particular, can greatly reduce the dissociative barrier (E a dis ) for O adsorption.…”