The reaction of ammonia with GaAs(110) studied by scanning tunneling microscopy /
We have used STM to obtain a detailed understanding of the chemisorption of NH3 on GaAs(110). By developing a new understanding of the bare surface relaxation and analyzing semi-empirical calculations on ammonia-indane, we conclude that, consistent with acid-base theory predictions, the reaction li...
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| Format: | Thesis Book |
| Language: | English |
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[Place of publication not identified] :
[publisher not identified] ;
1997.
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| Subjects: | |
| Online Access: | http://proxy.library.tamu.edu/login?url=http://proquest.umi.com/pqdweb?did=739887651&sid=1&Fmt=2&clientId=2945&RQT=309&VName=PQD |
| Summary: | We have used STM to obtain a detailed understanding of the chemisorption of NH3 on GaAs(110). By developing a new understanding of the bare surface relaxation and analyzing semi-empirical calculations on ammonia-indane, we conclude that, consistent with acid-base theory predictions, the reaction likely occurs between the dangling p-like cation orbital and the NH3 lone-pair. In the molecule, this involves a significant charge transfer that induces a large dipole moment. Our understanding of the surface subsequently suggests that the dipole is tilted with respect to the surface normal and points along the [001] direction. There is complete agreement between our STM results and these predictions. Chemisorption on both p- and n-type material results in a set of filled- and empty-state features whose intact migration indicates that they reflect undissociated molecules. The symmetry of both filled- and empty-state features about a common [110] mirror plane reveals the ammonia is adsorbing to the surface gallium sites. The long range screening signatures associated with the features demonstrate that the adsorption complex is uncharged but has an associated dipole moment. In addition, the details of the dipole signature confirm that the molecule is oriented along the [001] direction and tilted with respect to the surface. Analysis of the distribution of ammonia pairs on the surface yields an anisotropic, repulsive adsorbate-adsorbate interaction. The repulsion at [110] nearest neighbor (nn) sites is the strongest as no pairs are observed at this separation. Weaker repulsion occurs at [110] second nn and [001] nn sites, while the diagonal nn site shows no interaction at all. This lack of diagonal interaction and the anisotropy that it introduces are inconsistent with dipole-dipole repulsion, suggesting that the dipole moment is much less than that calculated for ammonia-indane and that substrate-mediated effects prevail over electrostatic phenomena. This conclusion resonates with the observation that there is a one-to-one correspondence between sites of strongest lateral interaction and sites of strongest substrate perturbation in the empty-state topography. A broader view of the effect that adsorption has on the surface is obtained from STS results where it is obvious that the electronic energy bands have been perturbed by the adsorption. |
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| Item Description: | Vita. "Major Subject: Physics". In title, numerals are used. |
| Physical Description: | xiii, 145 leaves : illustrations ; 28 cm. Issued also on microfiche from University Microfilms Inc. |
| Bibliography: | Includes bibliographical references: pages 140-144. |