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.

• Received 3 December 1992

DOI:https://doi.org/10.1103/PhysRevB.47.10375