We have investigated the influence of the external strain on the surface structure of Si(001) and on the dynamics of the dissociation reaction of O2 on the Si(001) surface, using low energy electron diffraction and supersonic molecular beam techniques.
On a clean Si(001) surface, the phase transition from c(4×2) to the (2χ1) structure occurs at about 200 K [1, 2]. Below the critical temperature, Tc, the buckled dimer is arranged in an anti-ferromagnetic order, which results in a c(4χ2) structure. Above Tc, however, the buckled dimer appears symmetric due to the time average of the thermal flip-flop motion of the buckled dimer, resulting in a (2χ1) structure [3-6]. Thus this transition is an order-disorder phase transition with respect to the arrangement of the buckled dimer. Externally applied tensile strain on the Si(001)-c(4χ2) surface was found to induce the flip-flop motion of the buckled dimers [3]. Thus the external tensile strain lowers the transition temperature (Tc).
In the dissociation reaction of O2 on Si(001), there are two distinctive channels: a trapping-mediated channel and a direct–activated channel [4, 5]. For a low translational energy (Ei) of the incident O2, with Ei less than 170 meV, a trapping-mediated mechanism is dominant, exhibiting an increase in the initial dissociation probability of O2 (S0) with decreasing Ei. In contrast, S0 increases with increase in Ei above an Ei of 170 meV, where a direct-activated mechanism is dominant. We found that at the surface temperature below Tc, the S0 increases with the external tensile strain in the low Ei regime where the dissociation reaction dominantly proceeds vie a trapping precursor.
Here we report how the external anisotropic strain affects the order-disorder phase transition and how the strain-induced phase transition influences trapping-mediated dissociation of O2 on the Si(001) surface.
[1] J. Ihm et al., Phys. Rev. Lett. 51, 1872(1983).
[2] T. Tabata, T. Aruga, and Y. Murata, Surf. Sci. 179, L63(1987).
[3] M. Yata, Phys. Rev. B 74, 165407(2006).
[4] T. Miyake et al., Phys. Rev. B 42, 11801(1990).
[5] B. A. Ferguson, C. T. Reeves, and C. B. Mullins, J. Chem. Phys. 110, 11574(1999).
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