Transcript ECE 371 – Chapter 1

ECE 371 –
Chapter 1
Crystal Structure of solids
Classifying materials on the basis of
their ability to conduct current.
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ECE 317 Chapter 1
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Conductor – allows for flow of current ex:
copper
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Insulator – prevents flow of current ex:
rubber
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Semiconductor - A semiconductor is a
substance, usually a solid chemical element
or compound, that can conduct electricity
under some conditions but not others,
making it a good medium for the control of
electrical current.
Crystal structure of solids
Classification of
semiconductors
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On the basis of the periodic chart
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Group IV
Elemental
ECE 317 Chapter 1 Crystal structure of solids
III-V
II-VI
Compound
Group IV semiconductors
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Consists of Carbon, Silicon and Germanium.
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Silicon is the dominant semiconductor
material.
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Germanium has certain niche uses in high
speed electronics, optoelectronics and
photovoltaics.
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Carbon semiconductor research is currently
being conducted with very promising results
with carbon nanotube, diamond and
graphene based semiconductors.
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ECE 317 Chapter 1 Crystal structure of solids
III-V compound
semiconductors
Group V
Group III
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ECE 317 Chapter 1 Crystal structure of solids
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Consists of group III and group V elements.
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This class of material is considered as alloys.
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III-N also referred to as nitrides are the basis of most
visible light emitting diodes and lasers in the blue to
green range. Ex: Blue-ray DVD players
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III-P alloys are called phosphides – mainly used for
red lasers and solar cells.
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III-As are referred to as arsenides used for a variety
of near-IR opto-electronic and electronic
technologies.
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III-Sb alloys are called antimonides these are used
for high speed electronics and mid-IR technologies
like countermeasures lasers and thermal cameras.
CD Vs DVD Vs Blue-ray
AlGaAs
laser
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InGaP
laser
InGaN
laser
II-VI semiconductors
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ECE 317 Chapter 1 Crystal structure of solids
Group VI
Group II
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Mainly used in detectors made of HgCdTe.
These detectors are very useful for MWIR
and LWIR applications such as thermal
sensing and night vision.
classification for compound
semiconductors based on number
of constituent elements
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ECE 317 Chapter 1 Crystal structure of solids
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Binary: One group III and one group V. Simplistic
model consists of one layer of group III and one layer
of group V. Group III and V atomic site are mutually
exclusive to their respective elements. Ex: GaAs, InP.
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Ternary: Three elements in all. Could be two group
IIIs and one group V or vice-versa. Again group III
sites and group V sites are exclusive thus in ternary
with two group III species the group III atoms divide
the spots up amongst themselves.
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Ex1: Al0.7Ga0.3As. Here 70% of the group III sites are
occupied by Al and the rest by Ga and 100% of the
group V sites are taken by As.
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Ex2: GaAs0.6P0.4. Here 100% of the group III sites are
occupied by Ga and 60% of the group V sites are
occupied by As and the rest and 40% of the group V
sites are taken by P.
Binaries and ternaries (cont.)
GaAs – Binary alloy
Group III
Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga
Group V
As As As As As As As As As As
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Group V
Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga
As As As As As As As As As As
Al0.7Ga0.3As – Ternary alloy
Group III
Al Ga Al Al Ga Al Al Ga Al Al
Group V
As As As As As As As As As As
GaAs0.6P0.4 – Ternary alloy
Group III
Ga Ga Ga Ga Ga Ga Ga Ga Ga Ga
Group V
As P As As P As As P As P
ECE 317 Chapter 1 Crystal structure of solids
Quaternary alloys
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ECE 317 Chapter 1 Crystal structure of solids
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Three group IIIs one group V Ex: Al0.3Ga0.3In0.4As
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Two group IIIs and two group Vs Ex:
Al0.4Ga0.6As0.2Sb0.8
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One group III and three group Vs. Ex:
GaAs0.8Sb0.1P0.1
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Verify this yourself – in the above examples all the
group III constituents add to give a 100% and all the
group V constituents add to give 100%.
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Can you think of a quintinary (5 element) alloy? Is
Al0.1Ga0.9In0.1As0.7Sb0.2 a valid composition? (hint: its
not ). Feel free to change the compositions of this
alloy to make it correct.
Types of solids
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Amorphous – no order in the atoms.
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Poly-crystalline – short range order.
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Single crystal – Long range order.
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See fig. 1.1 in neamen.
Lattice and basis
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See fig.
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ECE 317 Chapter 1 Crystal structure of solids
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The lattice is a periodic arrangement of
points in space. Each point on the lattice is
called a Lattice point. (duh!)
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The basis consists of the simplest
arrangement of atoms which is repeated at
every point in the lattice to build up the
crystal structure.
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Translation to produce the lattice: Each
lattice point can be translated by a1 in one
direction and b1 in another non-colinear
direction. This results in a 2-D lattice. A third
translation along another non-colinear
direction results in a 3-D lattice.
Unit Cell
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Mathematical Definition (from P.K. Bhattacharya): A unit cell is
the region of a crystal defined by vectors a, b and c and the
angles α, β and γ such which when translated by integral
multiples of those vectors reproduce a similar region of the
crystal.
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OR A unit cell is a small volume of the crystal that can be used
to reproduce the entire crystal.
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See fig. 1.3
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Translation property:
r = ha + kb + lc
a,b,c are basis vectors.
r is the translational vector.
a, b and c could be inter-atomic distances in which case they are
called lattice-constants.
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ECE 317 Chapter 1 Crystal structure of solids
Primitive Cell: A primitive cell is the smallest unit cell in volume
that can be defined for a specific lattice. See fig. 1.4
Bravais Lattices
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The number of ways in which lattice points
can be specified in space while maintaining
translational symmetry, is limited.
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Auguste Bravais demonstrated 14 types of
such point lattices in 1848. Nobody has
come up with new ones since.
Auguste Bravais
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The 14
bravais
lattices
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Cubic lattices
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Simple cubic (SC)
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Body-centered cubic (BCC)
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Face centered cubic (FCC)
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See fig 1.5 in the text.
Class problem #1
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Calculate the packing fraction of a BCC cell
assuming spherical atoms.
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If the interatomic distance is 5 Å what is the
density of atoms in the crystal.
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Do the same for
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ECE 317 Chapter 1 Crystal structure of solids
SC
FCC
Defining planes (hkl)
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ECE 317 Chapter 1 Crystal structure of solids
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See Fig. 1.6 for an example of a plane.
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Miller indices are an effective nomenclature
for naming planes.
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Miller indices refer to the integers (hkl). Ex:
(110), (111), (100) See fig. 1.7
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All parallel planes have the same indices
and are equivalent to each other. So avoid
planes through the origin.
Class problems
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Example 1.3, see fig. 1.8
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Problem #2: TYU E 1.3
Determine the distance between the nearest (110) planes in a SC lattice with
a lattice constant of ao = 4.83 Å.
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Problem #3: TYU E 1.4
The lattice constant of a FCC structure is 4.75 Å. Calculate the surface
density of atoms for (a) a (100) plane and (b) a (110) plane.
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Expressing directions
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Fig. 1.9
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So (hkl) is the plane, [hkl] is the direction.
Diamond structure
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GaAs - ZincBlende
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Atomic bonding
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Ionic bond: Na+Cl-
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Covalent bond – sharing e- to complete an
octet
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ECE 317 Chapter 1 Crystal structure of solids
H need only one atom to complete the
octet and therefore we only have H2.
Silicon needs 4 e- and so can bond to four
other Si atoms, forming a crystal.
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Metallic bond
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Van der Waals
Imperfections in solids
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Lattice vibrations
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Point defect
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Vacancy
Interstitial
Frenkel defect (vacancy-interstitial)
Line dislocation
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Point defect
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Impurities in solids
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Substitution
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Interstitial
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Doping
Semiconductor growth
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From a melt
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Epitaxy - MOCVD
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Epitaxy -MBE
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