Accelerated molecular dynamics study of theGaAs(001)β2(2×4)/c(2×8)surface

Abstract

We develop an accelerated molecular dynamics method that has several advantages over conventional rare-event methods. We apply our method using a semiempirical Tersoff-Abell potential to understand the disordered structure of the $\mathrm{GaAs}(001)\ensuremath{\beta}2(2\ifmmode\times\else\texttimes\fi{}4)$ surface at various temperatures. Our simulations indicate that the self-reconstruction of this surface is mediated by As-row-dimer and As-trench-dimer shifting events that typically occur over microsecond-millisecond times at temperatures of interest and that trench-dimer shifting occurs more rapidly than row-dimer shifting. Our calculations indicate that the equilibrium surface consists of an approximately equal mixture of $\ensuremath{\beta}2(2\ifmmode\times\else\texttimes\fi{}4)$ and $c(2\ifmmode\times\else\texttimes\fi{}8)$ domains, in agreement with an experimental scanning-tunneling microscopy study. Due to strain along the $[1\overline{1}0]$ direction, there is a tendency for the surface to form long runs of in-phase As-row- and As-trench dimers. There are weak correlations between As-row-dimer pairs along the $[110]$ direction such that the free energy of the $c(2\ifmmode\times\else\texttimes\fi{}8)$ surface is slightly lower than that of the $\ensuremath{\beta}2(2\ifmmode\times\else\texttimes\fi{}4)$ surface. This finding is also in agreement with the experiment and indicates the dynamical nature of this important substrate for thin-film growth.