— —   Atomic Motion at Germanium Surfaces: Scanning Tunneling Microscopy and Monte Carlo Simulations JEAN-POL VIGNERON, ABBOUD BENAISSA,* ISABELLE DERYCKE, ALAIN WIAME, R. SPORKEN Institute for Studies in Interface Sciences, Facultes Universitaires Notre-Dame de la Paix, rue de ´ Bruxelles 61, 5000 Namur, Belgium Received 21 March 1998; accepted 2 April 1998 ABSTRACT: The observation of the motion of an adatom on the reconstructed Ge Ž . 111 is a rare event, which will be examined by way of simple adatom surface interaction models. Estimations of the residence time of adatoms on energetically favorable sites indicate that a thermal excitation can account for casual adatom motion and that a strong tip surface interaction is not obviously needed to explain the changes Ž . found in sequences of scanning tunneling microscopy STM images of the same surface areas.  1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 10931097, 1998 Key words: surface; STM; germanium Introduction Ž . he 111 surface of elemental semiconductors T like Ge usually assumes a rather complex structure, and several types of reconstructions Ž .  1  2,2  8 ... can be identified 1 . The long- range regularity of the reconstructions encoun- tered on this surface is easily disrupted by the presence of defects, and this fragility of the surface order is expected to have some impact on the *Facultes des Sciences et Techniques de Settat, Universite ´ ´ Hassanier, Settat, Marokko. Correspondence to: J.-P. Vigneron. Contract grant sponsor: Belgian Programs on Interuniversity Attraction, Belgian National Fund for Scientific Research.  thermally activated self-diffusion 2 . The direct Ž . observation of the Ge 111 surface by scanning Ž . tunneling microscopy STM , and the new com- puter tools that can be used to achieve a better interpretation of the images provide a new direct way of studying the diffusion processes that take place on these systems. Crystalline Structure of the ( ) Germanium 111 Surface The crystalline structure of bulk Ge can be viewed as a succession of parallel, equidistant,   rumpled planes lying normal to the 111 direction. The thickness of these rumpled planes is about ( ) International Journal of Quantum Chemistry, Vol. 70, 1093 1097 1998  1998 John Wiley & Sons, Inc. CCC 0020-7608 / 98 / 041093-05