Atomic hydrogen easily adsorbs on semiconductor surfaces saturating surface dangling bonds and removing gap surface states. Molecular hydrogen, on the contrary, is generally considered
to hardly react with semiconductor surfaces due to the substantial energy barrier involved in breaking
molecular H-H bonds. Against this background, the reaction of H and H2 with SiC(001) surfaces is
very unique and surprising. Recent experiments [1] astonishingly find that the SiC(001)-(3×2)
surface becomes metallic upon H adsorption rather than semiconducting. Concomitantly, the amazing observation
has been made [2] that H2 readily adsorbs dissociatively on the c(4×2) surface at room temperature
while it scarcely reacts with the more Si-rich 3×2 surface.
To elucidate these unexpected phenomena, we have studied a series of hydrogen reaction scenarios within
density functional theory.
As to H-induced metallization of the 3×2 surface, we find that the
metallicity may result from adsorption of H atoms in bridge positions above Si dimers in either the second
or third surface layer, forming angular Si-H-Si bonds [3]. In particular, we have studied H reaction pathways
from vacuum to these bridge positions to assess whether they can actually be reached.
Our results reveal a direct pathway to the bridge positions on the second but not on the third
layer because the H atoms tend to become captured by the second layer before they can reach the third.
Concerning H2 adsorption, we find that intradimer adsorption is unlikely at both the c(4×2) and 3×2 surfaces while interdimer adsorption depends crucially on the distinct spatial arrangement and dangling-bond topology of the Si dimers at the surfaces [4]. The results clearly reveal barrierless reaction pathways for dissociative H2 adsorption on the c(4×2) as opposed to pathways with significant energy barriers on the 3×2 surface.
[1] V. Derycke, et al., Nat. Mater. 2, 253 (2003)
[2] V. Derycke, et al., Phys. Rev. B 63, 201305(R) (2001)
[3] X. Peng, P. Krüger and J. Pollmann, Phys. Rev. B 72, 245320 (2005)
[4] X. Peng, P. Krüger and J. Pollmann, Phys. Rev. B 75, 073409 (2007)
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