title: Quasi-Direct Optical Transitions in Silicon Nanocrystals with Intensity Exceeding the Bulk.
abstract: Comparison of the measured absolute absorption cross section on a per Si atom basis of plasma-synthesized Si nanocrystals (NCs) with the absorption of bulk crystalline Si shows that while near the band edge the NC absorption is weaker than the bulk, yet above ∼ 2.2 eV the NC absorbs up to 5 times more than the bulk. Using atomistic screened pseudopotential calculations we show that this enhancement arises from interface-induced scattering that enhances the quasi-direct, zero-phonon transitions by mixing direct Γ-like wave function character into the indirect X-like conduction band states, as well as from space confinement that broadens the distribution of wave functions in k-space. The absorption enhancement factor increases exponentially with decreasing NC size and is correlated with the exponentially increasing direct Γ-like wave function character mixed into the NC conduction states. This observation and its theoretical understanding could lead to engineering of Si and other indirect band gap NC materials for optical and optoelectronic applications.
authors.affiliation: National Renewable Energy Laboratory , Golden, Colorado 80401, United States.
authors.forename: Benjamin G
authors.initials: BG
authors.lastname: Lee
authors.affiliation: State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences , Beijing 100083, China.|Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China , Hefei, Anhui 230026, China.
authors.forename: Jun-Wei
authors.initials: JW
authors.lastname: Luo
authors.affiliation: National Renewable Energy Laboratory , Golden, Colorado 80401, United States.
authors.forename: Nathan R
authors.initials: NR
authors.lastname: Neale
authors.affiliation: National Renewable Energy Laboratory , Golden, Colorado 80401, United States.
authors.forename: Matthew C
authors.initials: MC
authors.lastname: Beard
authors.affiliation: Laboratory of Nanotechnology, IMTEK, Albert Ludwigs University , Freiburg 79110, Germany.
authors.forename: Daniel
authors.initials: D
authors.lastname: Hiller
authors.affiliation: Laboratory of Nanotechnology, IMTEK, Albert Ludwigs University , Freiburg 79110, Germany.
authors.forename: Margit
authors.initials: M
authors.lastname: Zacharias
authors.affiliation: National Renewable Energy Laboratory , Golden, Colorado 80401, United States.
authors.forename: Paul
authors.initials: P
authors.lastname: Stradins
authors.affiliation: Renewable and Sustainable Energy Institute, University of Colorado , Boulder, Colorado 80309, United States.
authors.forename: Alex
authors.initials: A
authors.lastname: Zunger
keywords: Silicon nanocrystals; absorption cross section; atomistic screened pseudopotential; optical absorption; quantum dots