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We have used the tunneling microscope to excite electron standing waves over clean and adsorbatecovered surfaces. For smooth, continuous overlayers we observe a resonance in the conductivity spectra that is absent on the clean surface, over film defects, and over film areas that are rough on a length scale of the electron's de Broglie wavelength. By making reasonable assumptions of the adlayer thickness and interface scattering strength, the resonance can be modeled by a quantum-size effect in electron transmission through the adlayer.PACS numbers: 61.16.Di, 68.35.Bs, 73.20.At The tunneling microscope has proven to be a powerful technique for locally probing the atomic structure of clean semiconducting surfaces on a length scale of a few angstroms. ! As interest shifts from the study of clean to adsorbate-covered surfaces, it would be useful to extend the capabilities of the microscope to include localized studies of buried heterointerfaces. 2 We have utilized electron standing waves excited by the tunneling microscope 3 " 5 for the first atomic resolution structural study of a buried heterointerface. We believe that these data provide the first causal connection between geometric structure, the Sn/Si (lll) interface, and how it manifests itself in the measured density of states. The detection of standing-wave formation between a surface and buried interface has been previously reported for thin-film tunnel diodes, 6 and in retarding-field measurements. 7,8 In the following we use the tunneling microscope to investigate these quantum-size effects 6 " 8 (QSE) in tunneling through thin-film overlayers. This technique has the advantage of being localized on an atomic length scale, as well as rendering a real space image of the area that is being probed.The samples used in this study were cut from an arsenic-doped, 0.005-n cm Si(l 11) wafer. Approximately y ML (monolayer) of tin was deposited onto the room-temperature substrate from a tungsten-filament source at a deposition rate of 1 ML/min. After deposition, the sample was annealed at 550°C for 2 min to generate the V3 and 2VJ reconstructions, and at 800 °C for 2 min to generate the VJ mosaic surface. The operation of the microscope to obtain conductivity maps has been described previously. 5 Figure 1 shows topographic images of the VJ, 2VJ, and VJ mosaic regions we have observed on this surface. The 2A/J and VJ reconstructions shown in Fig. 1 (a) have been observed previously by LEED, 9 reflection highenergy electron diffraction, 10 and scanning tunneling microscopy (STM). 11 The STM results 11 indicate that the V3 adatoms lie in threefold-coordinated sites above a second-layer Si atom, in the T4 adsorption site. The different areas shown in this figure were characterized by exciting electron standing waves over the respective areas, with the resulting conductivity spectra shown in Fig. 2.In Fig. 2(a), the standing-wave spectrum was collected over the 2VJ area to the left in Fig. 1 (a). The standingwave spectrum shown in Fig. 2(b) was collected over the \/3 M...

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