ORDER-DISORDER C(4X2)-(2X1) TRANSITION ON GE(001) - AN INSITU X-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(4X2) 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 (2X1) surface. Due to pinning of the c(4X2) 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 (4X2) reconstruction is conserved through the transition. This implies that above room temperature the (2X1) surface consists of a random array of buckled dimers.