crystal properties and growth of semiconductors-lecture-1
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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 betweenmetals and insulators
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What distinguishes semiconductors?
- resistance/resistivity
- crystalline structure
The resistance of a bar of material with dimensionsL, W,t:
Resistivity is:metals: < 10-3 -cminsulators: >102 -cmsemiconductors: 10-3 -cm < < 102 -cm
RL
Wt
W
tL
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Figure 1: Electrical conductivities of some commonmaterials.
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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-VCompound semiconductors example:GaAs
II-VICompound semiconductors example:CdTe
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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
AmorphousPolycrystalline
(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
Diamond lattice is an FCC plus placing atoms a, a, a(in x, y and z direction) from each atom in the FCC.
Construction of a diamond lattice from two interpenetrating
FCC sublattices.
Top view
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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.
*Characteristic 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 chemicalproperties of the material.
Crystals cleaved along certain atomic planes,resulting in exeptionally planar surfaces
Chemical reactions-etching of a crytal oftentake place preferentially along certaindirections