The formation of disordered regions is observed on the Ge(lll)c2x8 surface, at temperatures in the range 150-350 °C. The disorder occurs by the diffusion of surface adatoms in the (01T) directions. The disordered regions form at domain boundaries, and grow continuously with temperature until the entire surface becomes disordered at 300 °C. We argue that this phase transition is an example of premelting in two dimensions, i.e., "edge melting." PACS numbers: 61.16.Di, 68.35.Rh At room temperature, the lowest-energy structure of the (111) surface of germanium is a centered 2x8 reconstruction, consisting of Ge adatoms bonded on top of a bulk-terminated (111) bilayer. ! " 3 At a temperature near 300°C, the surface is known to undergo a reversible phase transition in which the c2x8 structure disorders, forming a structure characterized by an apparent 1 x 1 diffraction pattern with weak half-order spots. This transition has been studied by a number of techniques, 3 " 7 with the definitive work arguably being the low-energy electron diffraction (LEED) study of Phaneuf and Webb. 3 Above the transition temperature, they find that the weak half-order spots split and broaden, indicating that the disordered phase is actually incommensurate with the underlying lattice. They also argue that the transition is first order, based on an observed hysteresis in the LEED spot intensity between warming and cooling scans. These conclusions were supported by subsequent Monte Carlo simulations. 8 In this work, we use the scanning tunneling microscope (STM) to directly observe the 300 °C phase transition of the Ge(l 11) surface. Since few high-temperature STM measurements have previously been reported, 9 " 11 it is necessary for us to establish the conditions under which atomic motion can be identified from STM images. To this end, we first study the surface at temperatures of 150-220°C, where the motion of individual surface adatoms and rows of adatoms can be clearly seen. As the temperature is increased, this activity accelerates, and the rapidly moving atoms form disordered regions on the surface. These disordered regions are found to form at surface domain boundaries or steps. The regions grow continuously in size as the temperature is increased, and above 300 °C the entire surface is disordered. As discussed below, this type of behavior is precisely what is expected for a two-dimensional phase transition which is first order for an infinite-size domain, but is continuous near domain boundaries due to the occurrence of premelting at the boundaries. 12,13 This work was performed on a newly built STM, incorporating a tube scanner and inch worm 14 approach, with a symmetrical design to minimize thermal drifts.Germanium samples, p-typc with resistivity of 0.1 n cm, were prepared by cleaving and then annealing at a temperature of about 400 °C. Cooling at a rate of 2°C/min following the anneal resulted in
