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Dr. Khalil-Ur-Rehman

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Quantum Dot Applications Different Generations of Solar Cells First generation: Single crystal silicon wafer. Advantages: high carrier mobility. – PowerPoint PPT presentation

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Title: Dr. Khalil-Ur-Rehman


1
Class-14 Quantum Dots Quantum Wires
  • By
  • Dr. Khalil-Ur-Rehman

2
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.

3
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.

4
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.

5
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.

6
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.

6
7
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)

8
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9
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.

10
Outline
  1. Quantum Confinement and Quantum Dots
  2. Fabrication of Quantum Dots
  3. Quantum Dot Applications

11
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.

12
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.

13
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.

14
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.

15
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.
16
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

17
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.

18
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).

19
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.

20
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.

21
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

21
22
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.

23
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.

24
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

25
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.

25
26
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.

26
27
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.

27
28
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.

28
29
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.

30
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.

31
Applications
  • The fluorescence intensity can be enhanced by a
    factor of up to 108 compared with quantum dots on
    an unpatterned surface.

32
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.
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