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Luminescence from nano - Si

• Group I
• Maria Szlek
• Maksymilian Schmidt
• Michal Jablonski
• Karol Kyziol

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Luminescence (cold light, annealing) its
ability to emit light waves by solid
states. Generated another reason than heating.
There is a few kind of luminescence e.g.
Photoluminescence (PL), electroluminiescence
(EL). PL exited by photons beam. EL - exited
by electric field
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Photoluminescence
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• Porous Silicon

Porous silicon was discovered by accident. It
was produced by non-uniform etching during the
electropolishing of silicon with an electrolyte
containing hydrofluoric acid. The etching
resulted in a system of disordered pores with
nanocrystals remaining in the inter-pore regions.
Porous silicon is still manufactured by
electrochemical etching of silicon in
hydrofluoric acid (HF) solutions. Aqueous HF is
unsuitable for the etching process because the
silicon surface is hydrophobic. The porous
layer can be made more structurally uniform if an
ethanoic solution is used - this increases the
wettability of the silicon and allows better
surface penetration by the acid. Ethanoic etch
solutions also reduce the formation of hydrogen
gas bubbles as ethanol acts as a surfactant and
prevents bubbles sticking to the silicon surface.

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Scheme of produce PS
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Porous Silicon

• What is porous silicon?
• In the most basic sense, porous silicon is a
  network of air holes within an interconnected
  silicon matrix.  The size of these air holes,
  called pores, can vary from a few nanometers to a
  few microns depending on the conditions of
  formation and the characteristics of the silicon.
   
• The SEM image typical porous silicon sample.

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• The silicon nanocrystals in PS that emits
  visible light vary in size from 10-15Å. Raman
  spectroscopy gives indirect information about the
  microstructure of PS and has shown that the
  nanocrystals alter the selection rules relating
  to the interaction of optical phonons with
  incident photons.

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The nanoporous structures have dimensions in the
low nm-range. If the structure size reaches a
value below, say 3 nm, quantum effects can occur
and therefore nanoporous samples can exhibit
strong visible photoluminescence and
electroluminscence, as can be seen in the picture
below.

• Photoluminescence of a nanoporous silicon
  sample

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• SEM images and spectra of porous Si samples.
  The images are examples of a low porosity (left)
  and high porosity (middle). The spectra (right)
  indicate the fluorescence tunability of porous
  Si.

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Field-effect electroluminescence

• In the silicon field-effect LED, a tunneling
  process sequentially charges the nanocrystals
  embedded in the gate oxide with electrons and
  then with holes.
• The electron-hole pairs radiatively recombine to
  yield light at approximately 750 nm.

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Schematic of the field-effect electroluminescence
mechanism in a silicon nanocrystal floating-gate
tranisistor structure.
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PL and EL emission spectra
The emission spectra are inhomogeneously
broadened due to the distribution of luminescent
nanocrystal sizes.
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• The nanocrystal field-effect light-emitting
  device (FELED) could be used to integrate light
  sources on computer chips. This would allow the
  light sources and control circuits of display and
  communications device to be fabricated together,
  making for a faster, cheaper manufacturing
  process.
• The device is energy efficient a prototype that
  generates several microwatts of optical power
  could be built in an area as small as a few
  hundred square microns, according to the
  researchers. The color light the transistor emits
  depends on the size of its nanocrystals.

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Light emission
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Possible mechanisms that can lead to radiative
light emission in Si QDs.
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• A few words about silicon-based lasers
• Generally silicon is not used for light
  sources because of the lack of efficient light
  emitters but there are some an optimistic note on
  silicon lasing.

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Schematic of injection laser based on simple p-n
junction
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Cross-section of the silicon laser
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Conclusion

• The prospects for a Si laser are quite good.
  Besides the approaches other directions of active
  research consider the use of Si-Ge alloys,
  quantum confinement, alloying effects, or
  nanocrystal formation.
• The expectations of realizing a Si-based
  injection laser in the near future are well
  founded. The variety of approaches that are now
  being followed, if successful, will make Si
  generate a rainbow of colors.

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Conclusion

• Silicon is the material of choice for making most
  electronic devices. In its natural crystalline
  form, however, silicon has a very low optical
  radiative efficiency and produces light only
  outside the visible range.
• If the optical property of crystalline silicon
  could be modified to increase the frequency of
  emitted light, silicon would have even more
  device applications, such as use in lasers or
  solar cells.

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References

• Materialstoday January 2005 Lorenzo Pavesi
• Materialstoday January 2005 Philippe M. Fauchet
• Advanced materials 1992 Volker Lehmann, Urlich
  Gösele
• Nature materials February 2005 Robert J.
  Walters, George I.Bourianoff, Harry A.Atwater
• Nature, February 2005 Jerome Faist
• http//www.chem.ucsb.edu/buratto_group/PorousSili
  con.htm
• http//www.ece.rochester.edu/weiss/Porous_silicon
  .html
• http//www.photonics.com
• http//www.trnmag.com/Stories/2005/020905/Silicon_
  nanocrystal_transistor_shines_Brief_020905.html
• http//www.theledlight.com/led-specs.html

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Thank you for attention ?