Doping and Crystal Growth Techniques

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Doping and Crystal Growth Techniques
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Types of Impurities

• Substitutional Impurities
• Donors and acceptors
• Isoelectronic Defects
• Vacancies
• Charged Vacancies
• Color centers in solids (alkali halides)
• Interstitial Atoms
• Mid Gap Trap
• Antisite Defects

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Back to the Periodic Table
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Column V Atoms

• Have 5 outer shell electrons

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The extra electron on the phosphorous atom is
easily removed and becomes a free electron
without generating a hole. The phosphorous atom
becomes positively charged (ionized).
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Back to the Periodic Table (again)
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Column III Atoms

• Have 3 outer shell electrons

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The boron atom steals an electron from a
neighboring Si atom to complete the four bonds
with the surrounding Si atoms, generating a hole
at the neighboring Si atom. The boron atom
becomes negatively charged (ionized).
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n-type Semiconductors

• Are doped with donor atoms, which have an extra
  electron that they donate to the crystal
• When the concentration of donor atoms is much
  greater than the intrinsic carrier concentration,
  the electron concentration is composed of these
  donated electrons.

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p-type Semiconductors

• Are doped with acceptor atoms, which generate
  holes in the crystal
• When the concentration of acceptor atoms is much
  greater than the intrinsic carrier concentration,
  the hole concentration is composed of the holes
  generated by the acceptors.

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Carrier Concentrations

• n-type semiconductor

• p-type semiconductor

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Bohr model for Hydrogen atom
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Translation to Donor Atom

• Include relative dielectric constant
• Extra electron has a effective mass equal to the
  conduction band electrons

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Translation to Acceptor Atom

• Include relative dielectric constant
• Missing electron has a effective mass equal to
  the valence band electrons

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Heisenbergs Uncertainty Principle

• In quantum mechanics, we talk about the
  probability of finding a particle in a certain
  place.
• DxDp h/2
• DtDn 1/4p
• DtDE h/2

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Impurity Level
DeBroglies relation The deeper the impurity
level from either Ec or Ev, the smaller rn is
i.e, the electron or hole is more tightly bound
to the impurity.
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http//kottan-labs.bgsu.edu/teaching/workshop2001/
chapter6.htm
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GaP LEDs have a low concentration of N impurities
in them. The impurity energy level has a large k
that extends from the X minima to the G minima,
allowing the trapped electrons to radiative
recombine with holes.
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Types of Impurities

• Substitutional Impurities
• Donors and acceptors
• Isoelectronic Defects
• Vacancies
• Charged Vacancies
• Color centers in solids (alkali halides)
• Interstitial Atoms
• Mid Gap Trap
• Antisite Defects

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Types of Crystal Growth

• Product is a boule from which wafers are then cut
• Czochralski (CZ)
• Float Zone (FZ)
• Bridgeman

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Czochralski
www.qahill.com/tz/silicon/silicon.html
http//www.tf.uni-kiel.de/matwis/amat/elmat_en/kap
_6/illustr/i6_1_1.html
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http//www.tf.uni-kiel.de/matwis/amat/elmat_en/kap
_6/backbone/r6_1_2.html_dum_1
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Impurity Segregation
Where Co is the initial concentration of th
impurity in the melt
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Impurity Segregation
Atom Cu Ag Au C Ge Sn As
ko 4 104 106 2.5 105 7 102 3.3 102 1.6 102 0.3
Atom O B Ga Fe Co Ni Sb
ko 0.5 0.8 8 103 8 106 8 106 4 104 2.3 102
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Float Zone
www.mrsemicon.com/crystalgrowth.htm
www.tms.org/pubs/journals/JOM/9802/Li/
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Impurity Segregation
Where Co is the initial concentration of the
impurity in the solid and L is the width of the
melted region within RF coil
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Bridgeman

• Used for some compound semiconductors
• Particularly those that have a high vapor
  pressure
• Produced D shaped boules

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Crystalline Defects

• Point Defects
• Vacancies
• Impurities
• Antisite Defects
• Line Defects
• Dislocations
• Edge
• Loop
• Volume Defects
• Voids
• Screw Dislocations

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Edge Dislocation
http//courses.eas.ualberta.ca/eas421/lecturepages
/mylonite.html
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Screw Dislocation
http//focus.aps.org/story/v20/st3
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Strain induced Dislocations

• The temperature profile across the diameter of a
  boule is not constant as the boule cools
• the outer surface of the boule contracts at a
  different rate than the internal region
• Thermal expansion differences produces edge
  dislocations within the boule
• Typical pattern is a W

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Strain due to Impurities

• An impurity induces strain in the crystal because
  of differences in
• ionic radius as compared to the atom it replaced
• Compressive strain if the ionic radius is larger
• Tensile strain if the ionic radius is smaller
• local distortions because of Coulombic
  interactions
• Both cause local modifications to Eg

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Dislocation Count

• When you purchase a wafer, one of the
  specifications is the EPD, Etch Pit Density
• Dislocations etch more rapidly in acid than
  crystalline material
• Values for EPD can run from essentially zero (FZ
  grown under microgravity conditions) to 106 cm-2
  for some materials that are extremely difficult
  to grow.
• Note that EPD of 106 cm-2 means that there is a
  dislocation approximately every 10mms.

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Wafer Manufacturing

• Boules are polished into cylinders
• Aligned using an x-ray diffraction system
• Cut into slices using a diamond edged saw
• Slices are then polished smooth using a colloidal
  grit
• Mechanical damage from sawing causes point
  defects that can coalesce into edge dislocations
  if not removed

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http//www.tf.uni-kiel.de/matwis/amat/elmat_en/kap
_6/backbone/r6_1_2.html_dum_1
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SCS Manufacturing
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Carrier Mobility and Velocity

• Mobility - the ease at which a carrier (electron
  or hole) moves in a semiconductor
• Symbol mn for electrons and mp for holes
• Drift velocity the speed at which a carrier
  moves in a crystal when an electric field is
  present
• For electrons vd mn E
• For holes vd mp E

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L
H
W
Va
Va
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Resistance
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Resistivity and Conductivity

• Fundamental material properties

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Current Flow
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Resistivity
n-type semiconductor
p-type semiconductor
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Diffusion

• When there are changes in the concentration of
  electrons and/or holes along a piece of
  semiconductor
• the Coulombic repulsion of the carriers force the
  carriers to flow towards the region with a lower
  concentration.

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Diffusion Currents
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Relationship between Diffusivity and Mobility
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Wafer Characterization

• X-ray Diffraction
• Crystal Orientation
• Van der Pauw or Hall Measurements
• Resistivity
• Mobility
• Four Point Probe
• Resisitivity
• Hot Point Probe
• n or p-type material