Cross-Sectional Scanning Tunneling Microscopy of Silicon-Based Heterostructures Andp-N Junctions
Tao, Meng
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https://hdl.handle.net/2142/82914
Description
Title
Cross-Sectional Scanning Tunneling Microscopy of Silicon-Based Heterostructures Andp-N Junctions
Author(s)
Tao, Meng
Issue Date
1998
Doctoral Committee Chair(s)
Lyding, Joseph W.
Department of Study
Materials Science and Engineering
Discipline
Materials Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
Abstract
This thesis describes the application of cross-sectional STM to the characterization of silicon-based device structures. Several difficulties have been overcome in the process of developing this new structural characterization method. A V-groove cleaving technique has been developed to help produce atomically-flat cleaved Si(111) surfaces, and cross-sectional STM has been performed on various samples prepared with this V-groove technique, including Si1-xGe x/Si heterostructures, ZnS/Si heterostructures, and Si p-n junctions. It is found on Si(111)2x1 that there is no preferential orientation for atomic steps, and the step structures are determined by the hexagonal symmetry of the atomic arrangement and the pi-bonded chains on Si(111)2x1. Two types of Si adatoms on Si(111)2x1 are identified. One sits on top of a single pi-bonded chain, and the other sits between two pi-bonded chains. Atomic-resolution images of cross sections of Si1-xGex/Si heterostructures reveal two strain relaxation mechanisms on the cleaved (111) surface. One is through stressing the substrate. The strained region in the substrate can extend several hundred Angstroms from the interface. The other is through defect formation, i.e. atomic steps, on the cleaved surface. This is equivalent to misfit dislocations in bulk strain relaxation. Cross-sectional STM of cleaved ZnS/Si heterostructures reveals that the ZnS layer sandwiched between Si substrate and Si cap layer has a high density of gap states. Possible reasons for the gap states include surface states on the cleaved ZnS(111) surface, and bulk defect states in the ZnS layer due to antiphase domains and intermixing of Si and ZnS during the high-temperature Si cap growth. The intermixing also makes the ZnS layer thicker than its targeted thickness. Atomic-resolution images of cross sections of p-n junctions and As dopant atoms indicate that As atoms on Si(111)2x1 are localized features about 7 A in diameter. Both As atoms in the top and second surface layers are observed. They actively participate in conduction in filled-state imaging, but are less active in empty-state imaging. Band structure models are proposed to explain these observations.
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