Low-Energy Electron Microscopy
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followed in real time. The following will give examples obtained with a UHV LEEM designed and built in our laboratory. This machine has been in use for about three years and has been applied to the study of surface morphologies, phase transitions, etching, and growth. This article will concentrate on growth. Experimental A schematic outline of our microscope4 is shown in Figure 1. A 15 keV electron beam emitted from a field emission gun is focused by a condensor lens in the backfocal plane of a magnetic cathode-type objective lens. Between condensor and objective the beam passes through a magnetic sector to spatially separate the condensor and projector optics. The sample, 2 mm above the top of the objective lens, is held at a potential close to that of the field emission tip, so that the well-collimated, coherent electron beam strikes the surface with an energy in the range of 0-100 eV The reflected electrons are accelerated back into the objective lens, where a focused low-energy electron diffraction (LEED) pattern is formed in the backfocal plane. A real space image is formed in the center of the sector field. A transfer lens
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Figure 1. Schematic of a low-energy electron microscope (LEEM).
transfers the LEED pattern and the image to fixed locations in the projector column. An aperture in this conjugate diffraction plane selects the desired diffracted beam for dark field or bright field imaging. An intermediate lens selects either diffraction pattern or image for further magnification by a double-gap projector lens onto a channelplate-intensified phosphor screen. Total image magnification ranges from 400 X to 30,000 X; measured spatial resolution is -80 A. Examples One of the systems we have studied extensively5 is the growth of Ag on Si(lll), a classical example of Stranski-Krastanov growth. Initially, the Ag atoms cover the entire surface, transforming the clean 7X7 surface structure into a (V3 X V3)R30° structure with three Ag atoms per unit cell, at one monolayer (ML) coverage. Depending on the temperature at which the experiment is performed, the V3 structure may be preceded by a low coverage (3 X 1) phase, which strongly modifies the nucleation and growth of the V3 phase at higher coverage. Any Ag adsorbed in excess of one ML agglomerates into three-dimensional islands. With an almost exact 3:4 lattice match between Si and Ag, the Ag islands grow epitaxially, exhibiting the Ag(lll) plane. As these islands grow, a curious phenomenon is observed: The shape oscillates with the addition of each layer to the island thickness. Figure 2a shows a hexagonal island with sharp corners. The surface of the island is free of steps. (The bright/dark line across the center of the island is a step at the Si/Ag interface, as discussed in the next paragraph.) In Figure 2b a new twodimensional (2D) terrace has formed on top of the island and the corners are now rounded. With continued growth this terrace advances by step-flow until it reaches the edges of the island, again leaving the top surface free of steps
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