Direct Bandgap Silicon: Tensile-Strained Silicon Nanocrystals

  • Kůsová K
  • Hapala P
  • Valenta J
 et al. 
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Silicon, a semiconductor underpinning the vast majority of
microelectronics, is an indirect-gap material and consequently is an
inefficient light emitter. This hampers the ongoing worldwide effort
towards the integration of optoelectronics on silicon wafers. Even
though silicon nanocrystals are much better light emitters, they retain
the indirect-gap nature. Here, we propose a solution to this
long-standing problem: silicon nanocrystals can be transformed into a
material with fundamental direct bandgap via a concerted action of
quantum confinement and tensile strain. We document this transformation
by DFT calculations mapping the E(k) band-structure of Si nanocrystals.
The experimental proofs are then given firstly by a 10 000x increase in
the photon emission rate of strained silicon nanocrystals together with
their altered absorbance spectra, both of which point to direct
dipole-allowed transitions, secondly by single nanocrystal spectroscopy,
confirming reduced phonon energies and thus the presence of tensile
strain, and lastly by photoluminescence studies under external
hydrostatic pressure

Author-supplied keywords

  • direct bandgap
  • photoluminescence
  • silicon nanocrystals
  • strain engineering
  • surface termination

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