The role of surface defects in multi-exciton generation of lead selenide and silicon semiconductor quantum dots

Jaeger, Heather M.; Fischer, Sean; Prezhdo, Oleg V.

doi:doi:10.1063/1.3682559

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Multi-exciton generation (MEG), the creation of more than one electron-hole pair per photon absorbed, occurs for excitation energies greater than twice the bandgap (E_g). Imperfections on the surface of quantum dots, in the form of atomic vacancies or incomplete surface passivation, lead to less than ideal efficiencies for MEG in semiconductor quantum dots. The energetic onset for MEG is computed with and without surface defects for nanocrystals, Pb₄Se₄, Si₇, and Si₇H₂. Modeling the correlated motion of two electrons across the bandgap requires a theoretical approach that incorporates many-body effects, such as post-Hartree-Fock quantum chemical methods. We use symmetry-adapted cluster with configuration interaction to study the excited states of nanocrystals and to determine the energetic threshold of MEG. Under laboratory conditions, lead selenide nanocrystals produce multi-excitons at excitation energies of 3 E_g, which is attributed to the large dielectric constant, small Coulomb interaction, and surface defects. In the absence of surface defects the MEG threshold is computed to be 2.6 E_g. For lead selenide nanocrystals with non-bonding selenium valence electrons, Pb₃Se₄, the MEG threshold increases to 2.9 E_g. Experimental evidence of MEG in passivated silicon quantum dots places the onset of MEG at 2.4 E_g. Our calculations show that the lowest multi-exciton state has an excitation energy of 2.5 E_g, and surface passivation enhances the optical activity of MEG. However, incomplete surface passivation resulting in a neutral radical on the surface drives the MEG threshold to 4.4 E_g. Investigating the mechanism of MEG at the atomistic level provides explanations for experimental discrepancies and suggests ideal materials for photovoltaic conversion.

Article Outline

INTRODUCTION
THEORETICAL METHODS
RESULTS AND DISCUSSION
1. Atomic vacancies in lead selenide quantum dots
2. Surface-passivated silicon quantum dots
CONCLUSIONS

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KEYWORDS, PACS, and IPC

Keywords

configuration interactions, electron-hole recombination, elemental semiconductors, energy gap, excited states, excitons, impurity states, IV-VI semiconductors, lead compounds, nanostructured materials, passivation, semiconductor quantum dots, silicon, surface states

PACS

71.55.Cn

Elemental semiconductors
71.55.Ht

Other nonmetals
73.20.Mf

Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
73.21.La

Quantum dots
71.35.Gg

Exciton-mediated interactions
71.35.Lk

Collective effects (Bose effects, phase space filling, and excitonic phase transitions)

International Patent Classification (IPC)

B82B1/00
Nano-structures
B82B3/00
Manufacture or treatment of nano-structures
H01L21/02
Manufacture or treatment of semiconductor devices or of parts thereof
H01L21/70
Manufacture or treatment of devices consisting of a plurality of solid state components or integrated circuits formed in or on a common substrate or of specific parts thereof; Manufacture of integrated circuit devices or of specific parts thereof

ARTICLE DATA

Digital Object Identifier

http://dx.doi.org/10.1063/1.3682559

PUBLICATION DATA

ISSN

0021-9606 (print)

1089-7690 (online)

Publisher

$abstract.society.value is a Member of CrossRef

American Institute of Physics

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