Dr. Khalil-Ur-Rehman

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Class-14 Quantum Dots Quantum Wires

• By
• Dr. Khalil-Ur-Rehman

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Quantum Dots

• A quantum dot is a very small structure.
• A semiconductor nanocrystal embedded in another
  semiconductor material.
• Quantum dots are confine electrons or other
  carriers in all three dimensions.
• Quantum dots can be fabricated from
  semiconductors.

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Quantum dots

• Self-assembled quantum dots are typically between
  5 and 50 nm in size. Quantum dots defined by
  lithographically patterned gate electrodes
• The energy spectrum of a quantum dot can be
  engineered by controlling the geometrical size,
  shape, and the strength of the confinement
  potential.

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Quantum dots

• Also, in contrast to atoms, it is relatively easy
  to connect quantum dots by tunnel barriers to
  conducting leads, which allows the application of
  the techniques of tunneling spectroscopy for
  their investigation.

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Quantum dots

• Conventional, small-scale quantum dot
  manufacturing relies on a process called "high
  temperature dual injection" which is impractical
  for most commercial applications that require
  large quantities of quantum dots.

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How Can Quantum Dots Improve the Efficiency?
3. Quantum Dot Applications

• Quantum dots can generate multiple exciton
  (electron-hole pairs) after collision with one
  photon.

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Exciton(the electron-hole pair)

• In a simplified model of the excitation, the
  energy of the emitted photon can be seen as a sum
  of the band gap energy between occupied level and
  unoccupied energy level
• the confinement energies of the hole and the
  excited electron, and the bound energy of the
  exciton(the electron-hole pair)

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Introduction

• Quantum dots are semiconductors whose excitons
  are confined in all three dimensions of space.
• Quantum dots have properties combined between
• Those of bulk semiconductors
• Those of atoms
• Different methods to create quantum dots.
• Multiple applications.

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Outline

1. Quantum Confinement and Quantum Dots
2. Fabrication of Quantum Dots
3. Quantum Dot Applications

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Thin Film Semiconductors

• Electrons in conduction band (and holes in the
  valence band) are free to move in two dimensions.
• Confined in one dimension by a potential well.
• Potential well created due to a larger bandgap of
  the semiconductors on either side of the thin
  film.
• Thinner films lead to higher energy levels.

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Quantum Wire

• Thin semiconductor wire surrounded by a material
  with a larger bandgap.
• Surrounding material confines electrons and holes
  in two dimensions (carriers can only move in one
  dimension) due to its larger bandgap.
• Quantum wire acts as a potential well.

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Quantum Dot

• Electrons and holes are confined in all three
  dimensions of space by a surrounding material
  with a larger bandgap.
• Discrete energy levels (artificial atom).
• A quantum dot has a larger bandgap.
• Like bulk semiconductor, electrons tend to make
  transitions near the edges of the bandgap in
  quantum dots.

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Discrete Energy Levels

• The energy levels depend on the size, and also
  the shape, of the quantum dot.
• Smaller quantum dot
• Higher energy required to confine excitons to a
  smaller volume.
• Energy levels increase in energy and spread out
  more.
• Higher band gap energy.

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Quantum Dot
1. Quantum Confinement and Quantum Dots

• 5 nm dots red
• 1.5 nm dots violet

B.E.A. Saleh, M.C. Teich. Fundamentals of
Photonics. fig. 13.1-12.
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How to Make Quantum Dots
2. Fabrication of Quantum Dots

• There are three main ways to confine excitons in
  semiconductors
• Lithography
• Colloidal synthesis
• Epitaxy
• Patterned Growth
• Self-Organized Growth

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Lithography
2. Fabrication of Quantum Dots

• Quantum wells are covered with a polymer mask and
  exposed to an electron or ion beam.
• The surface is covered with a thin layer of
  metal, then cleaned and only the exposed areas
  keep the metal layer.
• Pillars are etched into the entire surface.
• Multiple layers are applied this way to build up
  the properties and size wanted.
• Disadvantages slow, contamination, low density,
  defect formation.

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Colloidal Synthesis
2. Fabrication of Quantum Dots

• Immersion of semiconductor microcrystals in glass
  dielectric matrices.
• Taking a silicate glass with 1 semiconducting
  phase (CdS, CuCl, CdSe, ).
• Heating for several hours at high temperature.
• Formation of microcrystals of nearly equal size.
• Typically group II-VI materials (e.g. CdS, CdSe)
• Size variations (size dispersion).

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Epitaxy Patterned Growth
2. Fabrication of Quantum Dots

• Semiconducting compounds with a smaller bandgap
  (GaAs) are grown on the surface of a compoundwith
  a larger bandgap (AlGaAs).
• Growth is restricted by coating it with a masking
  compound (SiO2) and etching that mask with the
  shape of the required crystal cell wall shape.
• Disadvantage density of quantum dots limited by
  mask pattern.

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Epitaxy Self-Organized Growth
2. Fabrication of Quantum Dots

• Uses a large difference in the lattice constants
  of the substrate and the crystallizing material.
• When the crystallized layer is thicker than the
  critical thickness, there is a strong strain on
  the layers.
• The breakdown results in randomly distributed
  islets of regular shape and size.
• Disadvantages size and shape fluctuations,
  ordering.

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Applications
3. Quantum Dot Applications

• Photovoltaic devices solar cells
• Biology biosensors, imaging
• Light emitting diodes LEDs
• Quantum computation
• Flat-panel displays
• Memory elements
• Photodetectors
• Lasers

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Applications (cont)

• Quantum dots make possible the fabrication of
  laser diodes with very low threshold pump power
  and/or low temperature sensitivity.
• Quantum dots can be used in white light-emitting
  diodes (LEDs) they are excited with a blue or
  near-ultraviolet LED and emit e.g. red and green
  light (acting as a kind of phosphor), so that
  overall a white color tone is achieved.

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Applications (cont)

• In semiconductor saturable absorber mirrors,
  quantum dots can serve as absorbers with a very
  low saturation fluence.
• Such quantum dot absorbers can also be contained
  in a glass matrix.
• Quantum dots can be parts of very sensitive
  photodetectors, and in the future they may
  function in efficient photovoltaic cells.

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Applications (cont)

• In the context of quantum cryptography, quantum
  dots can serve as single-photon emitters.
• Quantum dots might also be suitable for
  performing quantum computations.
• The mentioned functions can also be realized in
  the context of quantum nanophotonics

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Solar Cells
3. Quantum Dot Applications

• Photovoltaic effect
• p-n junction.
• Sunlight excites electrons and creates
  electron-hole pairs.
• Electrons concentrate on one side of the cell and
  holes on the other side.
• Connecting the 2 sides creates electricity.

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Different Generations of Solar Cells
3. Quantum Dot Applications

• First generation
• Single crystal silicon wafer.
• Advantages high carrier mobility.
• Disadvantages most of photon energy is wasted as
  heat, expensive.
• Second generation
• Thin-film technology.
• Advantages less expensive.
• Disadvantages efficiency lower compared with
  silicon solar cells.
• Third generation
• Nanocrystal solar cells.
• Enhance electrical performances of the second
  generation while maintaining low production costs.

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Solar Cells Efficiency
3. Quantum Dot Applications

• What limits the efficiency
• Photons with lower energy than the band gap are
  not absorbed.
• Photons with greater energy than the band gap are
  absorbed but the excess energy is lost as heat.

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How Can Quantum Dots Improve the Efficiency?
3. Quantum Dot Applications

• The quantum dot band gap is tunable and can be
  used to create intermediate bandgaps. The maximum
  theoretical efficiency of the solar cell is as
  high as 63.2 with this method.

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Applications

• Colloidal quantum dots display a wide range of
  novel optical properties that could prove useful
  for many applications in photonics.
• The enhancement of fluorescence emission from
  quantum dots on the surface of two-dimensional
  photonic crystal slabs.

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Applications

• The enhancement is due to a combination of
  high-intensity near fields and strong coherent
  scattering effects.
• By fabricating two-dimensional photonic crystal
  slabs that operate at visible wavelengths and
  engineering their leaky modes so that they
  overlap with the absorption and emission
  wavelengths of the quantum dots.

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Applications

• The fluorescence intensity can be enhanced by a
  factor of up to 108 compared with quantum dots on
  an unpatterned surface.

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Conclusion

• Quantum dot
• Semiconductor particle with a size in the order
  of the Bohr radius of the excitons.
• Energy levels depend on the size of the dot.
• Different methods for fabricating quantum dots.
• Lithography
• Colloidal synthesis
• Epitaxy
• Multiple applications.