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Nanoscale Dopant Pile-up near Semiconducting Oxide Surfaces Explained

Edmund G. Seebauer, University of Illinois at Urbana-Champaign, DMR 1005720 Goal: to control concentration and

movement of defects in semiconducting ceramics using surfaces

Outcome: explanation developed for dopant pile-up near the surfaces of several semiconductors including oxides of titanium and zinc

Impact: improve the performance of devices for producing hydrogen fuel from sunlight, photo-catalysts for wastewater cleanup, gas sensors

Summary: electric charge that normally resides on a ceramic surface can interact directly with extra atoms (interstitial atoms) or voids (atomic vacancies) in the underlying bulk that are charged. Several years ago, Prof. Edmund G. Seebauer and his graduate students showed that this interaction can cause dopant atoms to pile up within the first nanometer or two of a silicon surface in micro-electronics applications. They have now shown experimentally that analogous effects occur in the ceramics zinc oxide and titanium dioxide. Mathematical modeling has indicated what conditions of charge build-up lead to this effect, as well as the way it evolves over time.

oxygen / titanium dioxide

oxygen / zinc oxide

boron in silicon

Pile-up

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Modeling Near-Surface Dopant Pile-up in SemiconductorsEdmund G. Seebauer, University of Illinois at Urbana-Champaign, DMR 1005720

Background: Point defects such as interstitials and vacancies control migration of dopants in semi-conductors, and those defects can change their charge state depending upon local electron richness.

Experimental observations: Dopant pile-up is confined to within a few

nanometers of the surface. The amount of pile-up builds up quickly (a

few minutes) during annealing, and then remains roughly constant over hours.

Approach: Software simulator tracks the charge states of defects that mediate diffusion of boron in silicon, as well as their temporal behavior

Results: Pile-up occurs only in a narrow surface region where the electric potential (quantified by the Fermi energy EF) changes the defect charge state. This implies that the amount of excess charge at the surface must exceed a critical material-dependence value. Temporal behavior depends upon the evolution of background defect concentration.

Implications: The basic principles should generalize across many semiconductors, with the details depending upon host material and dopant.

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Promoting “Virtue Ethics” in STEM DisciplinesEdmund G. Seebauer, Universotu of Illinois at Urbana-Champaign, DMR1005720

Background: Imparting a sense of research ethics represents an important and sometimes difficult task for a graduate advisor.

Recent activities: Maintenance of ethics Web

site www.scs.uiuc.edu/ethics/

Seminars to national/inter-national audiences

Written articles (e.g., forth-coming chapter on virtue ethics in Practical Ethics for Food Professionals, ed. Clark & Ritson, Wiley-Blackwell publ.)