1' Crystal Properties and Growth of Semiconductors

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1. Crystal Properties and Growth of Semiconductors

• Electrical behavior of solids
• Arrangement of atoms

1.1. Semiconductor Materials

• Electrical conductivity intermediate between
• metals and insulators

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What distinguishes semiconductors?  - resistance/r
esistivity - crystalline structure
The resistance of a bar of material with
dimensions L, W, t Resistivity ?
is metals ?
lt 10-3?-cm insulators
? gt102?-cm semiconductors 10-3?-cm lt ? lt
102?-cm
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Figure 1 Electrical conductivities of some
common materials.
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Conductivity of semiconductor material can be
varied over orders of magnitude by changes
in -temperature -optical excitation -impurity
content
Periodic table Column IV Si, Ge Elemental
semiconductors III-V Compound semiconductors
exampleGaAs II-VI Compound semiconductors
exampleCdTe
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The Portion of Periodic Table Where
Semiconductors Occur
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Binary compounds GaAs, GaP, GaN, CdTe, InP
etc. Ternary compounds AlGaAs, GaAsP, HgCdTe,
etc. Quaternary compounds InGaAsP, AlGaInAs,etc.
Applications transistors, integrated circuits
(Si) light emitting diodes (LEDs) (GaAs,GaN,
GaP) lasers (AlGaInAs, InGaAsP, GaAs,
AlGaAs) light detectors(Si, InGaAsP, CdSe, InSb,
HgCdTe)
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1.2. Crystal Lattices

• Solids classified according to atomic
  arrangement
• Crystalline
• Amorphous
• Polycrystalline

 (a) Crystalline (b) Amorphous (c)
Polycrystalline
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The atoms making up the CRYSTAL are arranged in
a periodic fashion called LATTICE.
UNIT CELL is a representative of the entire
lattice and is generally repeated throughout the
structure.
PRIMITIVE CELL is the smallest unit cell that can
be repeated to form the lattice.

• Crystal lattice determines
• Density of solid (mechanical property)
• Allowed energy bands of electrons (electrical
  property).

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Cubic Lattices
Simple (SC) Body-centered (BCC)
Face-centered (FCC)
a lattice constant
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Diamond Lattice
Top view
Diamond lattice is an FCC plus placing atoms ¼ a,
¼ a, ¼ a (in x, y and z direction) from each atom
in the FCC. Construction of a diamond lattice
from two interpenetrating FCC sublattices.
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Diamond Lattice of Si from Different Angles of
View
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Diamond (zincblende) lattice - typical of most
of the commonly used semiconductors. Characterist
ic of Si, Ge, III-V compounds. Possible to vary
the composition of alloy by choosing the fraction
of numbers of the atoms, par example AlxGa1-xAs
(x of Al, (1-x) of Ga) InxGa1-xAsyP1-y
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Conclusion

• Atomic arrangement in certain planes in crystal
  is important to many of the mechanical,
  metallurgical, and chemical properties of the
  material.
• Crystals cleaved along certain atomic planes,
  resulting in exeptionally planar surfaces
• Chemical reactions-etching of a crytal often take
  place preferentially along certain directions