EE530 Advanced
Semiconductor Device
Physics
Lecture – 02
Crystal Structure
1
Materials

Solid state materials are classified as:
1) Conductors, 2) Insulators, & 3) Semiconductors
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2
Structure
“Spatial arrangements of atoms within a material”
Solid State materials exist in three forms
 Amorphous

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Crystalline
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No recognizable long-range order in positioning of atoms
Atoms are arranged perfectly in a orderly three
dimensional arrangement
Polycrystalline
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Multiple crystalline sections, placed along with each
other, either disjoined or misaligned
Intermediate case between Amorphous and Crystalline
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Structure
Amorphous
Polycrystalline
Crystalline
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Crystal Structure

Crystal Lattice
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Periodic and systematic arrangement of atoms in the
crystal
Unit Cell

A small portion of any given crystal that could be used to
reproduce the crystal
a
Unit Cell
a
Crystal Lattice
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Crystal and unit
cells
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Crystal Structure

Unit Cell



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We need unit cell only to reproduce the crystal structure
Both the configurations given in figure are acceptable
a
a
Unit Cell
Unit Cell
A unit cell need not to be primitive cell, i.e., the smallest
possible unit cell
Idea is to employ unit cells with orthogonal sides even if it
is bigger than primitive cell
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Unit Cell vs. Primitive Cell


A red colored primitive cell within a unit cell
The length of an edge of unit cell is called lattice
constant, unit is Å (angstrom). 1 Å = 0.1 nm
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1.1 Simple Cube (SC)
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6 neighboring atoms
Total 8 atoms contributes
Example: Polonium
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1.2 Body Centered Cube (BCC)
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8 neighboring atoms
Total 9 atoms contributes
Example: Sodium and Tungsten
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1.3 Face Centered Cube (FCC)
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12 neighboring atoms
Total 14 atoms contributes
Example: Aluminum, Copper, Gold and
Platinum
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1.4 Diamond Lattice
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Unit Cell of Silicon
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Si has the diamond lattice unit cell
Unit Cell
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Primitive Cell
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Unit Cell of Gallium Arsenide


GaAs has the zincblende lattice unit cell, similar to that of Si
However, the lattice constant is larger than that of Si
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1.5 Zincblende Lattice
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Example: GaAs
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1.5 Zincblende Lattice 3D Animation
http://www.youtube.com/watch?feature=player_embedded&v=MD79L2
W9sp4#!
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1.5 Zincblende Lattice 3D Animation
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Summary
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1.6 Example

At 300K the lattice constant for Silicon is
5.43A. Calculate the number of Silicon
atoms per cubic centimeter and the density
of Silicon at room temperature.
8/a3 = 8/(5.43x10-8)3 = 5x1022 atoms/cm3
Density = no. of atoms/cm3 x atomic weight
(g/mol)/Avogadro Constant (atoms/mol) = 2.33g/cm3
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