1. Phonon-Activated Electron-Stimulated Desorption of Halogens from Si(100)-(2x1)John H. Weaver, University of Illinois at Urbana-Champaign,DMR-0301821 This research led to the discovery of a novel mechanism for the desorption of adatoms from surfaces. A scanning tunneling microscope (STM) was used to show that Br desorbs from the Si(100) surface upon heating in the temperature range 600 - 800 K, far too low for a temperature for conventional bond breaking. Moreover, the desorption rate was found to be sensitive to the doping of the silicon sample, implicating electronic processes in the desorption channel. The results can be explained by a new mechanism that involves the thermal excitation of an electron into a long-lived antibonding state, followed by ejection of the adatom. Analysis of the data indicated that 8-20 phonons combine to provide the energy for the promotion of an electron to the excited state. The broader implication is that the commonly held assumption of the separability of electronic and thermal processes is not valid. Instead, a convolution of lattice vibrations and electronic transitions is required to remove the Br from the Si surface. Electronic excitations should then play an important role in other surface reactions, such as adsorption and catalysis, where they have previously been overlooked. The study also raises the possibility of using the doping of the sample to selectively control chemistry at the surface during semiconductor processing. Atomic-resolution STM images show Br-covered Si(100) before and after heating the sample to 725 K for 20 min. The dimer rows of Si run diagonally across the image. They are bright and are separated by dark lines. A dimer vacancy, DV, appears as a dark feature that spans the dimer row. Single Br vacancies, BrVs, appear as dark spots on one side of the dimer. Pairs of Br vacancies or bare dimers, BDs, appear as bright features in the right image. The schematic shows multiple phonons exciting an electron into the antibonding state of the Si-Br molecule. The capture of an electron repels the Br away from the surface leading to desorption.