STRUCTURAL PHASE-TRANSITION ON SI(001) AND GE(001) SURFACES

Among a variety of solid surfaces, Si(001) and Ge(001) have been most extensively studied. Although they seem to be rather simple systems, t here have been many conflicting arguments about the atomic structure o n these surfaces. We first present experimental evidence indicating th at the buckled dimer is the basic building block and that the structur al phase transition between the low-temperature c(4x2) structure and t he high-temperature (2x1) structure is of the order-disorder type. We then review recent theoretical work on this phase transition. The real system is mapped onto a model Ising-spin system and the interaction p arameters are derived from total-energy calculations for different arr angements of buckled dimers. The calculated critical temperature agree s reasonably well with the experimental one. It is pointed out that th e nature of the phase transition is crucially affected by a small amou nt of defects on the real surfaces.