ORDER-DISORDER TRANSITIONS AT THE GE(111) SURFACE

Medium-energy ion scattering is used to study the order-disorder transitions occurring at the Ge(111) surface around 573 K and 1050 K. The measurements show that after the low-temperature transition the adatom concentration has been reduced to about 60% of the concentration at room temperature. In the high-temperature transition, a thin film of positionally disordered atoms is formed on the surface. The number of disordered atoms per unit area is found to be (0.7-1.1) x 10(15) atoms cm-2, corresponding to 1 to 1.5 Ge(111) monolayers. The thickness of the film remains constant up to temperatures to within 25 K of the melting point. Our findings are in good agreement with the predictions of a thermodynamic model that is based on a theory due to Mikheev and Chernov.