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UBC Theses and Dissertations

Characterization of epitaxial semiconductor films by scanning tunneling microscopy at ambient pressure Pinnington, Thomas H.


Epitaxial layers grown by molecular beam epitaxy on both silicon and gallium arsenide substrates are exposed by cleaving and studied by scanning tunneling microscopy (STM) at ambient pressure. The cleaved surfaces are prepared for imaging by wet chemical treatments. GaAs/AlGaAs and Si/SiGe multilayer structures are imaged with —1 nm resolution. The contrast in the STM images is believed to be topographic in origin, resulting either from selective etching during surface preparation or strain-relaxation at the cleaved surface. Layers of alternating dopant-type are also resolved. In particular, alternately doped layers on GaAs are imaged by STM for the first time in air, and the pn junctions located to within 20 nm. The apparent topographic contrast in these images is explained in terms of an electronic contrast mechanism associated with the carrier type. Current-voltage (IV) characteristics of the STM tunnel junction are obtained at selected locations in the epitaxial layers. IV curves acquired over n- and p-type regions resemble the IV characteristics of Schottky barrier diodes. A new imaging method is proposed which exploits the contrast between the n- and p-type IV curves. Imaging in this mode is achieved by repetitively interrupting the constant-current feedback loop during the scan and recording the tunnel current at a new tip-sample voltage setting, preselected to yield high conductivity-type contrast. This method, which decouples the electronic contrast associated with the carrier type from the topography, is demonstrated for an npn structure on GaAs. Modification of the treated GaAs and Si surfaces is observed during imaging, and is attributed to a chemical change in the surface enhanced by the tunneling process. Photoemission spectroscopy of the treated GaAs surface indicates that the treatment inhibits oxidation, possibly by formation of a thin sulfur passivation layer. The photoemission results also suggest that selective removal of the arsenic atoms occurs during the treatment, which may also help to produce a more stable surface.

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