LEEM investigations of surfaces using a beam of energetic self-ions

This article reviews recent research using a low-energy electron microscope, built by Tromp at IBM, and equipped with an accelerator that permits in situ irradiation with a beam of self-ions. The available ion energies of 20 eV to 5 keV span the range from epitaxial growth by a hyperthermal beam to sputtering at the level of ∼10 atoms per incident ion. The design criteria and instrument calibration are described. The research described is surface science that requires a vacuum maintained below 10 ^-10 Torr, with all components contained in the same vacuum. Two general categories of applications are sketched. Experiments that accurately measure important physical quantities include surface mass diffusion over an extended temperature range; determining the critical chemical potential at which island nucleation occurs; observation and explanation of the universal evolution by which adatom and advacancy islands both grow and shrink by beam-driven processes; and the study of sublimation (regarded as negative ion beam intensity). Experiments described here with other goals include beam-assisted synthesis first of large pans and mesas for isolating surface experiments (e.g., nucleation) from the surrounding crystal, and second of Fourier waves on steps, for studies of diffusive relaxation. Operation of exotic structures including Bardeen-Herring sources and Frank growth spirals deformed by crystal anisotropy are also described.