msepostdoc-list Seminar Reminder - N. Anderson, Tue. 7/24 9:00 AM, ARMS 1028

[Stacey, Lisa A]

MATERIALS SCIENCE AND ENGINEERING
SEMINAR


Structural, Thermodynamic, Electronic, and Magnetic Characterization of
Point Defects in Amorphous Silica
by

Nathan L. Anderson
Ph.D. Final Exam

Advisor
Professor A. Strachan

ABSTRACT

A completely first-principles procedure for the creation of experimentally validated amorphous silicon dioxide structures via a combination of molecular dynamics and density functional theory is presented.  Point defects are analyzed within a statistical ensemble of these structures and overcoordinated silicon and oxygen defects are found to have similar formation energies to undercoordinated silicon atoms and oxygen vacancies.  The formation of E' centers is found to occur in the absence of oxygen vacancies, and a single oxygen vacancy is found to lead to two isolated E' center precursors.  Density functional techniques that properly account for the electrostatics in the presence of periodic boundary conditions are then used to add and remove electrons from each defect and the trapping level distributions are identified.  These distributions are the result of the inherent local atomic variability in the amorphous network.  The distribution energies are in good agreement with trap spectroscopy experiments where defect contributions are experimentally indistinguishable.  This ability to distinguish defect contributions is used to provide a physical explanation of the atomic relaxations which occur upon electron or hole capture.  The paramagnetic [cid:image003.png at 01CD669A.0D975270]  and [cid:image005.png at 01CD669A.0D975270]  defects are shown to exist in the neutral charge state and are capable of trapping both electrons and holes.  Statistical support for the oxygen vacancy originated dimerized model of the positively charged [cid:image007.png at 01CD669A.0D975270]  defect is demonstrated.  An overlap of distributions for different defects is also found suggesting the existence of less known trapping mechanisms involving positively charged overcoordinated oxygen defects and overcoordinated silicon floating bond defects.  Further, the uncertainty from the model form that results from exchange-correlation functional choice in density functional theory is quantified and found to be much less than the inherent atomic variability in the amorphous network.  Extending these amorphous structure prediction and defect analysis methods to silicon/silica interfaces and silicon/germanium transistors is also discussed.

Date:   Tuesday, July 24, 2012
Time:   9:00 AM
Place:   ARMS 1028


Lisa Stacey
Secretary/Development Assistant
Purdue University
School of Materials Engineering
765/494-4100

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