Nitride-based Semiconductors and their Applications

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Outline of lectures Day 1-2 Research on the
physics of nitride semiconductors Fundamentals
of semiconductor physics Research on
nitrides Day 3-4 Research on the teaching and
learning of physics Research in cognitive
science Research in physics education
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Nitride semiconductors and their
applicationsPart I Basic Semiconductor Physics

• One should not work on semiconductors, that is a
  filthy mess who knows whether they really
  exist.
• Attributed to Wolfgang Pauli (1931)

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What are semiconductors?

• Metals, semimetals, semiconductors, insulators
• Characteristics
• Conductivity increases dramatically with
  temperature (conductivity at T 0 K is zero)
• Conductivity changes dramatically with addition
  of small amounts of impurities
• Applications
• Anything in which you want to control the flow of
  current (transistors, amplifiers,
  microprocessors, etc.)
• Devices for producing light
• Radiation detectors

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History of semiconductors

• 1833 Michael Faraday discovers temperature-depende
  nt conductivity of silver sulfide
• 1873 Willoughby Smith discovers photoconductivity
  of selenium
• 1874 Ferdinand Braun discovers that point
  contacts on some metal sulfides are rectifying
• 1947 John Bardeen, Walter Brattain, and William
  Shockley invent the transistor

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Semiconductor materials
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Semiconductor materials

• Examples
• IV C, Si, Ge
• III-V GaAs, GaN, InP, AlSb, GaAlAs, GaInN
• II-VI ZnSe, CdTe

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Physical Structure
Basic lattice Face-centered cubic (fcc)
Diamond structure Si, Ge
Zincblende GaAs, InP, ZnS,...
Zincblende ABCABC Wurtzite ABABAB

• About 1022 atoms in each cm3.

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Electronic Structure

• Bands analogous to electronic energy levels of
  single atoms
• Band gap between 0 and 5 eV (1 eV 3.83 x 10-23
  Cal)
• Electrons in valence band are involved in atomic
  bonding
• Electrons in conduction band are free to wander
  the crystal
• Temperature dependence of resistance is due to
  thermal excitation of electrons across bandgap

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Band structure of Si
Chelikowski and Cohen, Phys. Rev. B 14, 556 (1976)
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Growth (bulk)

• Czochralski growth (1918)
• Crystals grown near melting point of material (gt
  1410 C for silicon)
• Boules up to 12 diameter and 6 feet long
• Growth rate few mm/min
• Used for Si, Ge, GaAs, InP

From http//kottan-labs.bgsu.edu/teaching/workshop
2001/chapter5.htm
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Growth (layers)

• MOCVD (Metal-Organic Chemical Vapor Deposition)
• Also known as MOVPE, etc.
• Growth temperatures near melting point
• Growth rate 1 mm/min.

From http//kottan-labs.bgsu.edu/teaching/workshop
2001/chapter5.htm
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Fun facts about AsH3

• OSHA Permissible Exposure Limit 0.05 ppm
  (averaged over 8 hour work shift)
• Detection Garlic-like or fishy odor at 0.5 ppm
• IDLH (Immediately Dangerous to Life or Health) at
  6 ppm. (IDLH for other toxic gases such as
  Chlorine or Phosphine are gt1000 ppm.)

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Growth (layers)

• MBE (Molecular-Beam Epitaxy)
• Low growth temperature
• Growth rate few mm/hr.
• Can grow atomically flat surfaces and monolayers

From http//kottan-labs.bgsu.edu/teaching/workshop
2001/chapter5.htm
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Doping

• Adding impurities to alter the electrical
  properties
• n-type (donors) or p-type (acceptors)
• Deep or shallow
• Single/double/triple

n-type
p-type
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Doping

• Shallow donors can be modeled as hydrogen atoms
  in a dielectric medium.
• The donor electron level is only a few (6-50) meV
  below conduction band.
• Hydrogen-like and helium-like levels are observed.

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Doping

• Grown in
• Diffusion
• Neutron transmutation(30Si(n,g)31Si --gt 31P
  b-, T1/22.6 hr.)
• Ion implantation

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Characterization (electrical)

• Hall effect enables determination of
• charge of carriers
• density of carriers
• binding energy of carriers (temperature dependent)

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Characterization (optical)

• Infrared (IR) spectroscopy allows determination
  of
• impurity species
• electronic and vibrational energies of impurities

Agarwal et al., Phys. Rev. 138, A882 (1965).
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Applications

• The pn-junction is the basis of many
  semiconductor devices.
• Three semiconductor devices
• Field effect transistor
• Light-emitting diode
• Laser diode

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pn-junction

• Consists of p-type material next to n-type
  material.
• Electrons from the n-type material fill in the
  acceptors on the p-type side near the junction
  and vice versa.
• Process stops when the layer of negatively
  charged acceptors becomes too think for the
  remaining electrons to get through.

Negatively charged acceptors
Positively charged donors
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pn-junction

• Current will flow if a battery is hooked up as
  shown. The positive terminal of the battery
  attracts electrons, pulling them through the
  depletion region.
• A certain minimum voltage is required to overcome
  the repulsion of the depletion region.

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pn-junction

• If the battery is hooked up in the opposite
  direction, then no current flows. (The depletion
  region actually gets bigger.)
• If too much voltage is applied in this direction,
  current flows, but your junction is unhappy.

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Another view of the pn-junction

• No bias
• Reverse bias
• (no current)
• Forward bias
• (current)

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Field Effect Transistor (FET)
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Light Emitting Diode (LED)

• Is basically a pn-junction
• When an electron and a hole collide, a photon
  (light) is emitted. The energy of the light is
  equal to the bandgap energy.Si bandgap 1.2
  eV (infrared)GaAs bandgap 1.5 eV (red)
• Defects in crystal can cause electron-hole
  collisions to occur without emission of light
  (non-radiative recombination).

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Laser Diode (LD)

• Is basically a pn-junction
• Same principle as LEDs, however, waveguides are
  added to the structure to enable the light to
  reach lasing intensities. Some surfaces are
  polished mirror-flat to allow light to reflect
  back and forth inside the active region.
• Much better material quality (smaller density of
  defects) is required for LDs than LEDs.

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Other applications

• Radiation detectorsRadiation hitting the
  material knocks an electron from the valence to
  the conduction band, creating a free carrier. An
  applied voltage sweeps the carrier out of the
  material where it is detected as current.
• Solar cellsAgain, a pn-junction. Light creates
  an electron-hole pair which is forced out of the
  material as electric current by the electric
  field in the depletion region.