Order-disorderc(4×2)-(2×1) transition on Ge(001): Anin situx-ray scattering study

Using in situ x-ray diffraction, we have studied the order-disorder phase transition on the Ge(001) surface by measuring the temperature dependence of a superlattice reflection specific to the c(4×{}2) low-temperature phase. The results indicate that the transition corresponds to a two-dimensional phase transition with anisotropic interaction energies along and perpendicular to the dimer rows that form the (2×{}1) surface. Due to pinning of the c(4×{}2) domains by defects, we are unable to observe any universal critical behavior. The results indicate that the number of buckled dimers involved in the c(4×{}2) reconstruction is conserved through the transition. This implies that above room temperature the (2×{}1) surface consists of a random array of buckled dimers.