Constant temperature molecular dynamics simulations of Si(100) and Ge(100): Equilibrium structure and short-time behavior

The structures of the (100) surfaces of silicon and germanium generally have been interpreted in a static manner in the past. We present molecular dynamics (MD) simulations that show these surfaces to consist of a mixture of rapidly interconverting buckled and unbuckled dimers. Over a time average, the surface is found to have long p(2X1) rows of symmetric, unbuckled dimers, as seen in recent scanning tunneling microscopy images of silicon. However, higher order unit cells are observed in He scattering and low energy electron diffraction experiments at low temperatures. We present a dynamical interpretation of the structure to explain both sets of observations. The simulations have been performed on different size slabs at both constant energy and constant temperature utilizing a new method for effective removal of heat from an exothermic system while retaining the correct dynamics. Several different interaction potentials were analyzed in an attempt to find the most realistic one for simulations of these surfaces. The effect of surface defects and annealing were also investigated. The surface phonon densities of states were calculated and for Si (100) are in good agreement with experiments and other theoretical treatments. Such simulations and structural analyses are reported for the first time for Ge(100).