The gallium arsenide (001) surface has been widely studied because of its importance in quantum dot formation during the deposition of indium arsenide. At conditions most commonly found during molecular-beam homoepitaxy and some heteroepitaxial experiments, the GaAs(001) surface reconstructs into rows of arsenic dimer pairs separated by trenches of As dimers. This is known as the β2(2x4) reconstruction. The atomic positions of the β2(2x4) unit cell have been accurately determined. However, recent studies have shown inconsistencies in experimental RHEED images of the reconstruction due to long range disorder at elevated temperatures relevant for growth¹. It has been suggested that phase shifts of the β2(2x4) unit cell are responsible for this disorder, but this has not yet been confirmed¹. The goal of this research is to use accelerated molecular dynamics (MD) to examine the structure of the β2(2x4) reconstruction as a function of temperature and to identify the underlying processes which lead to the long-range disorder.

The analytical bond-order, Tersoff type potential² used in the simulations was tested by comparing calculated bulk and surface properties to values from experiment and first-principles DFT calculations³. Using NPT MD, the density as a function of temperature was calculated and matches experimental values to within 10%. The potential slightly underestimates the bulk melting point and successfully describes Cv above the melting point. Conjugate gradient minimization was used to calculate binding energies for a Ga atom on the β2(2x4) reconstruction. The locations of the deepest minima match the DFT calculations, while the binding energies are slightly underestimated. The nudged elastic band method⁴ was used to calculate diffusion barriers for a Ga atom on the β2(2x4) reconstruction and these closely match the DFT values.

The bond boost method⁵ was used as the acceleration scheme for the MD simulations. The acceleration was tested by comparing the rates of trench As dimer splitting from accelerated MD simulations to those from NVT MD simulations. The rate of the splitting is preserved by the acceleration scheme.

Many processes, such as the As dimer splitting, come into play on the surface of the β2(2x4) reconstruction. We examined the rates of these processes as a function of temperature and calculated energetic barriers using the nudged elastic band method. The surface comes more disordered as the temperature increases. At lower temperatures, the As dimers in the trench pull apart and form new dimer pairs, and as the temperature increases, the As dimers in the row do the same. We have also probed the structure of the surface as a function of As₂ deposition.

[1] D.W. Pashley, J.H. Neave, B.A. Joyce, Surf. Sci., 582, 189 (2005) [2] K. Albe, K. Nordlund, J. Nord, A. Kuronen, Phys. Rev. B, 66,035205 (2002) [3] M.H. Mignogna, K.A. Fichthorn, in preparation [4] G. Henkelman,H. Jónsson, J. Chem. Phys. 113, 9978 (2000) [5] Miron, R.A., Fichthorn, K.A., J. Chem. Phys., 119, 6210 (2003)