2005 - 11 - 11
1
Properties Related to Band Theory
History of Semiconductors
Color
Conductivity
Photoluminescence
P-n Junction
Photoelectricity
Semiconductors
Historical Timelines
Semiconductor Products are Proliferating
LANs
WANs
Routers
Hubs
Switches
Workstations
Internet Servers Video
Games
Voice Over IP
Digital
Cameras
Wireless
Handsets PDAs
PCsStorageSystems
Set-Top Boxes
Internet
Browsers
Scanners
Digital Copiers
Internet
The Next Big Thing… A Lot of Little Things
$27.0
$12.8
0
5
10
15
20
25
30
2000 2005
Billions US$
China Semiconductor Consumption
China to grow faster than the world at CAGR
(Compound Annual Growth Rate) of 17% from
2001?2005, reaching $27B in 2005
China currently
produces only 1 of
every 4 chips it
consumes
The Electronics Ecosystem
Materials
Semiconductor
Equipment
Semiconductors
Electronic End Equipment
SEMI
MEMBERSHIP
$990 B
2001 Estimate 2004
$21B
$28B
$139 B
$879 B
$28B
$46B
$218 B
2005 - 11 - 11
2
Band Gap Energy and Color
.
Bandgap energy (eV)
Color that
corresponds to
band gap energy
Apparent color
of material
(unabsorbed light)
4
3
2
1
red
yellow
green
blue
violet
colorless
black
yellow
orange
ultraviolet
infrared
red
Bandgap and Conductivity in Group 4A
as Unit Cell increases, band gap energy (Eg) decreases
major factor is orbital overlap; as it increases in tighter
lattices, the band splitting also increases
Element Unit Cell(?) Eg (eV) λ (nm)
C 3.57 5.5 230 insulator
Si 5.43 1.1 1100 semiconductor
Ge 5.66 0.66 1900 semiconductor
α-Sn 6.49 <0.1 12000 conductor
Band Gap and Periodic Properties
Note that Eg increases as the Pauling electronegativity
difference, ?χ, increases (the compound gets more polar).
Also, Eg increases as the unit cell size decreases.
Material Unit Cell (?) ? χ Eg (eV) λ (nm) Color
Ge 5.66 0 0.66 1900 black
GaAs 5.65 0.6 1.42 890 black
ZnSe 5.67 0.8 2.70 460 yellow
CuBr 5.69 0.9 2.91 430 white
Delocalized Bonding Model
energy
Conduction band
Valence band
electrons
holes
Bonding Picture of Silicon
Delocalized bonding picture
Electrical Conductivity
Conductivity of metals decreases with temperature
as atomic vibrations scatter free electrons.
Conductivity of semiconductors increases with
temperature as the number of carriers increases.
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Three Types of Solid Materials
?? Based on Electrical Conductivity
10 84-4-8-12-16-20 101010 10 10 10 10
glass
diamond
fused
silica
silicon
germanium
iron
copper
insulators semiconductors metals
0-2410
Conductivity (? -1cm-1 )
= isolator
= alloy → increasing resistivity
→ resistivity below Tc = 0 !!
→ decreasing resistivity
Temperature Dependence of the Electrical
Conductivity of Metals and Semiconductors (Isolators)
Creation of Carriers in Intrinsic
Semiconductors by Thermal Excitation
Thermally induced electrical conductivity
T=0 K
Conduction band empty
Valence band completely filled
No electrical conductivity
T>>0 K
The thermal energy is responsible for the
promotion of electrons to the conduction
band.
Creation of electron?hole pairs:
carriers
Electrical conductivity
Experimental Observation:
Conductivity of Semiconductors
Semiconductor block connected
to the terminals of a battery
No conductivity observed at low
or room temperature or in the
dark.
When we increase the
temperature or expose the
semiconductor to light, we
observe that it starts conduction.
Semiconductors
Intrinsic Semiconductors: If a semiconductor
crystal contains no impurities, the only charge
carriers present are thus produced by thermal
breakdown of the covalent bonds. The conducting
properties are thus characteristic of the pure
semiconductor. Such a crystal is termed an
intrinsic semiconductor.
Extrinsic Semiconductors: If a semiconductor
crystal contains n-type or p-type impurities, the
conducting properties are chiefly due to the
impurities. Such a crystal is termed an extrinsic
semiconductor.
Conductivity of Intrinsic Semiconductors
§The valence band of semiconductors is
completely filled. However, the band gap
between the valence and conduction bands
is small, and electrons can be promoted to
the conduction band.
§In semiconductors, only the electrons
promoted to the conduction band and the
holes created in the valence band will be
carriers.
§The smaller the gap, the easier to promote
electrons to the conduction band.
§At the same temperature, smaller gap
semiconductors will show a larger
conductivity.
§The higher the temperature, the larger the
number of carriers.
§Conductivity increases with temperature
TK2
E
B
g
Ce
?
=σ
2005 - 11 - 11
4
the Response of Equilibrium to
Temperature
The van’t Hoff equation
12
0
12
2
0
11
1
T/T/
R/HKlnKln
RT
H
dT
Klnd
R
H
)T/(d
)K(lnd
o
P
?
??=?
?=
??=
Temperature Effects
Intrinsic semiconductors
Concentration of holes and
free electrons increase with
temperature. Because
increasing thermal energy will
excite more e- across the band
gap.
Ge has a greater charge
concentration than Si. Because
Ge has a smaller band gap than
Si (0.67 vs 1.11)
Carrier Mobility
Similar to metals, charge carriers in semiconductors
lose mobility with increasing dopant concentration.
The intrinsic carrier
mobility is defined as
the drift velocity per
unit electric field.
Carrier Mobility
Temperature also affects carrier mobility. Note regardless
of dopant concentration, high temperatures reduce mobility.
mobile holes:acid species
electrons:basic species
Semiconductors and Acid-Base Analogy
Chemical Equilibrium in Solution
H2O → H++OH-
Kw=[H+][OH-]
[H+]≈1014 ions/cm3
Chemical Equilibrium in Solid
Si(crystal)→h++e-
K=[h+][e-]=p?n
[h+]≈1.5x1010cm-3
Donor States:n?type Semiconductors
If an atom in the lattice is
substituted by an atom of a
different element with more
valence electrons, once the
impurity is accommodated to
the lattice and the new bonds
are formed, there will be a
remaining negative charge.
Example: Pentavalent Sb
impurity in a silicon crystal
(tetrahedrally coordinated)
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Effect of Doping on Conductivity
Valence Band
Acceptor States: p?type Semiconductors
If an atom in the lattice is
substituted by an atom of a
different element with less
valence electrons, once the
impurity is accommodated to
the lattice and the new bonds
are formed, there will be a
remaining positive charge.
Example: Trivalent boron (B)
impurity in a silicon crystal
(tetrahedrally coordinated)
Effect of Doping on Conductivity
Valence Band
Intrinsic vs. Extrinsic
Valence
Band
Conduction
Band
Intrinsic Extrinsic (doped)
Conduction
Band
Valence
Band
h+
e-
Conduction
Band
Valence
Band
n?type p?type
Silicon Si:P Si:Al
Extrinsic (Doped) Semiconductors
We can enhance the electrical
properties of a semiconductor by
adding impurities to it. The
addition of impurities is called
doping and the doped
semiconductor is called extrinsic.Example: the addition of 1
boron atom every 105 silicon
atoms enhance the conductivity
of silicon by a factor of 103 at
room temperature.
Extrinsic semiconductors are
the basic materials in the
electronics technology. Great
importance in current technology: lasers, solar cells, rectifiers,
transistors, ...
p?type
semiconductors
Addition of
acceptor states
n?type
semiconductors
Addition of donor
states
Examples:
P, As, or Sb
impurities
in Si or Ge.
Examples: B,
Al, Ga, or In
impurities in
Si or Ge.
Band Diagram (n and p type)
Electrons can jump
to Al atom
Electrons can jump from P atom to Conduction Band
n type
p type
Acceptor level in Band Gap
Ea
Donor Level in Band Gap
Ed
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Temperature Effects
undoped
Extrinsic n-type
Semiconductors
Low Temperatures
Thermal energy is
insufficient to excite electrons
from the donor state
Intermediate Temperatures
e?’s from donor state are
excited into the conduction
band. e? concentration equal
to dopant concentration.
High Temperatures
Enough thermal energy to
excite an effective amount of
valence e?’s into the
conduction bandp?types behave similarly
with temperature
Fermi Level
Ef
metal
Ef
undoped semiconductor
Ef
Ef
p?type semiconductor n?type semiconductor
EdE
a
The pH of aqueous
solutions and the
Fermi level in
semiconductors
play analogous
roles in
determining the
extent of ionization
in the two media.
Analogy Between pH and Fermi Level (Ef) Extent of Ionization:
Weak Acid ? Acceptor Analogy
When pH = pKa
[HA] = [A?]
Acid?base system
Semiconductor
When Ef ~ Ea
[A] ~ [A?]
Ea = acceptor energy level
aa
a
KlogpK
]HA[
]A][H[K
AHHA
?=
=
+?
?+
?+
?+ +? AhA
Energy Levels for Impurities in Silicon
Donors
Acceptors
e-
h+
shallow
shallow
deep
Interaction of Light and Electrons
absorption
Spontaneous
emission
Stimulated
emission
2005 - 11 - 11
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Optical Properties of Semiconductors
longest wavelength absorption to promote e?
corresponds to Eg
Eg is energy between “HOMO ”of valence band and
“LUMO”of conduction band
Eg
Absorption Emission
Eg
Semiconductor Glossary
Direct Bandgap Semiconductor: semiconductor in which the
bottom of the conduction band and the top of the valence
band occur at the momentum k=0; in this case, energy
released during band?to?band electron recombination with
a hole is converted primarily into radiation (radiant
recombination); wavelength of emitted radiation is
determined by the energy gap of semiconductor.
e.g. GaAs , InP, etc.
Indirect Bandgap Semiconductor: semiconductor in which
bottom of the conduction band does not occur at effective
momentum k=0, i.e. is shifted with respect to the top of the
valence band which occurs at k=0; energy released during
electron recombination with a hole is converted primarily
into phonon; e.g. Si, Ge, GaP.
An important property of direct semiconductors is that electrons
may easily drop from the conduction band to the valence band by
emitting a photon
This process is known as electron-hole recombination since the
electron drops to occupy a hole state in the valence band
? the energy of the photon emitted by the semiconductor is
determined by the size of its energy gap
? recombination is therefore analogous to the level transitions that
occur in atomic systems
Bandstructure in Three Dimensions
E
k
PHOTON
Electron-hole Recombination in a direct
semiconductor such as GaAs
An electron drops from the conduction band
to the valence band and its excess energy is
emitted in the form of a photon
Note that in the figure shown here the initial
and final wavevector states are the same …
this is an important property of direct
semiconductors
In indirect semiconductors, we have seen that the bottom of the
conduction band and the top of the valence band occur at different points in
k?space
An electron cannot therefore drop from the conduction band to the valence
band just by emitting a photon since this would violate momentum
conservation
? instead the electron must simultaneously emit a photon and exchange
momentum with the crystal lattice
? the probability of this double process occurring is very small,so indirect
semiconductors turn out to be much poorer emitters of light than direct ones
Bandstructure in Three Dimensions
E
k
PHOTON
Electron-hole recombination in an indirect
Semiconductor
In order to conserve energy and momentum, an
electron must drop to the valence band by emitting a
photon and exchanging momentum with the crystal
Because this process has a low probability, indirect
semiconductors such as Si or Ge cannot be used in
optoelectronic applications as light emitters
The opposite process to recombination is electron?hole generation
in which an electron is excited from the valence band into the
conduction band by absorbing a photon
Since this process also must conserve momentum the electron is
excited into a state with the same k?value as the initial
valence?band state
? Both direct and indirect semiconductors may therefore be used as
photodetectors to detect electromagnetic radiation
? The absorption of these materials strongly increases once the
photon energy exceeds the direct band gap
Bandstructure in Three Dimensions
E
k
PHOTON
absorptionof light by direct (left) and indirect (right) semiconductors
E
k
PHOTON
Luminescence
2005 - 11 - 11
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Solid line:
direct bandgap
materials
Dotted line:
indirect
bandgap
materials
Matched system to
reduce the strain
effect and epitaxial
growth defects!
What's Luminescence?
The spontaneous emission of light upon electronic excitation is
called luminescence.
Absorption and Luminance p?n Junction
What happens if we bring a
p?type semiconductor in
contact with a n?type
semiconductor?
Electrons close to the junction
diffuse across the junction into
the p?type region. Holes are
filled by recombination.
Equilibrium is established resulting in a potential
difference.
If the two regions are connected in a circuit a variety of
applications are possible.
pn
-
-
-
-
e-
+
+
+
+
h+
Energy? diagram of p?n Junction
When p?type and n?type semiconductors touch, the
Fermi levels do not align until equilibrium is reached.
Biasing the p?n Junction
Biasing ?? introduction of
a voltage into the circuit
containing the p?n junction.
Forward bias ?? negative
voltage is applied to n?type
side. Decreases energy barrier
for electrons and holes to flow
through the junction.
Reverse bias ?? positive
voltage applied to n?type side.
Raises energy barrier for
current flow.
pn
V +—
e-
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“Majority Carrier”and Current
Flow in p?type Silicon
p Type Silicon+ -
Hole Flow
Current Flow
“Majority Carrier”and Current
Flow in n?type Silicon
Electron Flow
n Type Silicon+ -
Current Flow
The p?n Junction
p n 0 Volts
Hole Diffusion
Electron Diffusion
Holes and Electrons
“Recombine”at the Junction
A Depletion Zone (D) and a Barrier
Field Forms at the p?n Junction
The Barrier Field Opposes
Further Diffusion
(Equilibrium Condition)
p -- ++ n
0 Volts
Hole (+) Diffusion
Electron (-) Diffusion
D
Barrier
Field
Donor IonsAcceptor Ions
“Forward Bias”of a p?n Junction
?Applied voltage reduces the barrier field
?Holes and electrons are “pushed”toward the junction
and the depletion zone shrinks in size
?Carriers are swept across the junction and the
depletion zone
?There is a net carrier flow in both the P and N sides =
current flow!
p ? + n
+ Volts - Volts
Current
“Reverse Bias”of a p?n Junction
p ??? +++ n
- Volts
D
+ Volts
Current
?Applied voltage adds to the barrier field
?Holes and electrons are “pulled”toward the terminals,
increasing the size of the depletion zone.
?The depletion zone becomes, in effect, an insulator for
majority carriers.
?Only a very small current can flow, due to a small
number of minority carriers randomly crossing D (=
reverse saturation current)
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Applications of p?n Junction Diode
Rectifier
Photodetectors
solar cells
LEDs
diode lasers
Optoelectronics
Why call it p?n Junction as a Diode?
Simple Application: Rectifier
One of the most important uses of a diode is rectification.
The normal PN junction diode is well?suited for this
purpose as it conducts very heavily when forward biased
(low?resistance direction) and only slightly when reverse
biased (high? resistance direction). If we place this diode in
series with a source of ac power, the diode will be forward
and reverse biased every cycle. Since in this situation
current flows more easily in one direction than the other,
rectification is accomplished.
PN junction
p?n Rectifying Junction
Forward Bias
Holes and free electrons
flow together and
recombine at the junction.
Current flows.
Reverse Bias
Holes and free electrons
flow away from each
other. The center of the
diode quickly becomes a
dead zone with no charge
carriers. Current is
reduced.
p?n Rectifying Junction
A diode’s properties can be seen when the voltage is examined
Optoelectronics
In optoelectronic applications of semiconductor devices,
the basic idea is that the device is used to either detect or to
emit electromagnetic radiation
In detection the incident light is converted into a
measurable electrical signal by exploiting internal carrier
processes within the device. ? Examples of such devices
include photodetectors and solar cells
In emission on the other hand the internal processes
allow the conversion of an electrical signal into detectable
light and examples of such devices include LEDs and lasers
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Photodetector
Photodetector converts optical energy into electrical
energy, thus making possible data reading in the optical
storage systems, such as CD or DVD drives. Modern
photodetectors are typically semiconductor photodiodes.
so?called "reverse ?bias p?n photodiode" with the carriers
flowing away from the p?n junction thus creating a
depletion region. There is very little current flowing
through this junction until the light illuminates the surface
of the photodiode. Then, the absorbed photons create pairs
of electrons and holes ? mostly in the depletion
area. Those new carriers move quickly in opposite
directions, and moving electrons create current in the
external circuit.
LED (Light?Emitting Diodes)
LEDs are p?n junction devices constructed of gallium
arsenide (GaAs), gallium arsenide phosphide (GaAsP), or
gallium phosphide (GaP). Silicon and germanium are not
suitable because those junctions produce heat and no
appreciable IR or visible light.
The junction in an LED is forward biased and when
electrons cross the junction from the n- to the p-type material,
the electron?hole recombination process produces some
photons in the IR or visible in a process called
electroluminescence. An exposed semiconductor surface can
then emit light.
Light?emitting diodes (LEDs) operate in the opposite
manner to photodetectors by exploiting the enhanced
diffusion of carriers that occurs across the depletion region
in forward?biased junctions
? in direct semiconductors, the additional carriers
recombine through band?to?band processes giving rise to
the emission of light from the junction
PHOTON
PHOTON
PHOTON
when the p-n junction is
forward biased large numbers of
electrons and holes diffuse
across the depletion region
in a direct semiconductor these
carriers may recombine by
emitting photons
the photon flux increases as
the forward?bias voltage ? and
so the corresponding diffusion
current ? is increased
LEDWith forward bias electrons
reaching the p?type side can
recombine with the abundant holes
and emit light according to the
energy difference. Holes reaching
the n?type side can recombine with
the abundant electrons emitting
light.
Color of luminescence is
controlled by the composition of the
solid solutions in the
semiconductors.
A great advantage of
semiconductor optoelectronic
devices is that they can be
fabricated in a highly compact
manner and can even be
incorporated into integrated circuits
semiconductor
lens
In contrast to photodetectors, an important
requirement for light emitting diodes is that
they be fabricated from a direct semiconductor
While gallium arsenide is a direct
semiconductor, its energy gap (1.42 eV)
corresponds to a photon wavelength (870 nm)
that lies outside of the visible spectrum
For display applications it is therefore
necessary to use alloys of GaAs which allow
access to photon frequencies in the visible range
of the spectrum.
ALLOY COLOR
GaAs0.6P0.4Ga RED
As0.35P0.65:N ORANGE-RED
GaAs0.14P0.86:N YELLOW
GaP:N GREEN
GaP:ZnO RED
AlGaAs RED
AlInGaP ORANGE
AlInGaP YELLOW
AlInGaP GREEN
SiC BLUE
GaN BLUE
color characteristics of
commercial LEDs
changes in alloy
composition are exploited
to modify the energy band
gap or to introduce
impurity levels that
mediate photon emission
in indirect
semiconductors
the development of the
blue GaNLED (from
1994) now allows the
possibility of
manufacturing full color
LED-based displays
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Band Gap Engineering in Semiconductors:
Solid Solutions
Trends in cubic unit cell
lattice parameter and Eg as a
function of the composition x for
the solid solution ternary
semiconductor AlxGa1-xAs:
Band gap engineering enables a range of optical and
electronic devices to be fabricated.
The roughly linear dependence of the physical properties on
composition is known as Vegard’s law and proves that the
distribution of the Al and Ga is random :
P(AlxGa1-xAs) = xP(AlAs ) + (1?x)P(GaAs )
P = physical property
Any physical property is the atomic fraction weighted
average of the two end members.
Semiconductor Heterostructures
In addition to alloying, fabricating artificial structures with
tailored optical and electronic properties has been possible using
crystal growth techniques, such as molecular beam epitaxy (MBE)
and metal?organic chemical vapor deposition (MOCVD). These
techniques allow monolayer control in the chemical composition of
the growing crystal. When two different semiconductors are grown
into a single structure, the structure is called heterostructure. One
such structure is superlattice in which two (or more) semiconductors
A and B are grown alternately with thickness dA and dB,respectively.
GaAs?AlGaAs superlattice. On the left is a sequence of nearly thirty
different layers, while on the right the individual atomic resolution is
indicated.
What is a solar cell?
A solar cell is a kind of semiconductor device that takes
advantage of the photo?voltaic effect, in which electricity is
produced when the semiconductor's PN junction is
irradiated.
Solar Cell (Photovoltaic)
If light of sufficient energy
strikes the semiconductor,
electrons are promoted into
the high energy state and
move toward the n?type
semiconductor. Holes are also
generated and move toward
the p?type semiconductor.
This creates an electric
current.
Magnitude of current
depends on intensity of light.
pn
e-
h+
hg
V
Electron Flow in a Solar Cell
n?typep?type
hg
e-
h+
How A Solar Cell Works
When sunlight strikes a solar cell, only certain bands (or
wavelengths) of light will cause electrons to move within the
semiconductor, thereby producing electric current. The energy
"band gap" of the semiconductor determines the ideal portion
of the light spectrum that will create this effect. To allow it all
to happen, the semiconductor layers must be constructed so as
to produce an electric field (shown as the layer above).
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Amorphous Silicon Solar Cells
Amorphous silicon solar cells are
cells containing noncrystalline silicon,
which are produced using
semiconductor techniques.
Amorphous silicon solar cells are
mostly used as power sources for
devices requiring little electricity or
as modulated light sensors. They are
common in pocket calculators,
watches, light detectors for cameras,
and television and car navigator
screens.
Solar Vehicle Project
“Spirit of Canberra”II solar
vehicle (Australian)
Amorphous Silicon
and Solar Cell House
Self-supplying Solar Cell House in Germany
Water + primary energy sources Hydrogen + oxygen → water
Clean Energy by Means of Advanced Materials
Transistor
Collector (n)Emitter (n)
Base (p)
Both p?n junctions are
reversed biased
First transistor ? 1947
John Bardeen, Walter
Brattain, William Shockley.
MOSFET
(Metal-Oxide-Semiconductor Field Effect Transistor)
MOSFET transistor: consists of two small islands of p-type
semiconductor created within n-type silicon substrate. Islands
are connected by narrow p-type channel.
Metal contacts are made to islands (source and drain), one
more contact (gate) is separated from channel by a thin (< 10
nm) insulating oxide layer.
Gate serves the function of the base in a junction transistor
(the electric field induced by the gate controls the current
through the transistor)
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MOSFET
(Metal-Oxide-Semiconductor Field Effect Transistor)
Voltage applied from source encourages carriers (holes in the
case shown below) to flow from the source to the drain through
the narrow channel.
Width (and hence resistance) of channel is controlled by
intermediate gate voltage. For example, if positive voltage is
applied to the gate, most of the holes are repelled from the
channel and conductivity is decreasing.
Current flowing from the source to the drain is therefore
modulated by the gate voltage (amplification and switching)
The MOSFET dominates the microelectronic industry
(memories, microcomputers, amplifiers, etc.)
Large Si single crystals are grown and purified. Thin
circular wafers (“chips”) are then cut from the crystals
Circuit elements are then constructed by selective
introduction of specific impurities (diffusion or ion
implantation)
A single 8”diameter wafer of silicon can contain as
many as 1010 ? 1011 transistors in total
Cost to consumer ~ 0.00001cent each.
Transistors and Microelectronic Devices
Sand à Silicon à Computer Chips Silicon Wafer Preparation
Evaluation
Melting
Preparation
Body growth
Cool?down
Ingot removal
Silicon Wafer Preparation
Slicing
Lapping
Etching
Heat
Treatment
Polishing
Silicon Wafer Preparation
Epitaxial
Processing
Cleaning
Inspection
Packing
2005 - 11 - 11
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Four Stages of Semiconductor Manufacturing
raw semiconductor material mined and purified
crystal growth and wafer preparation
the devices or integrated circuits are actually
formed in and on the wafer surface. Chip
fabrication!
packaging
Why wide band?gap semiconductors?
Needs: Solid?state amplifiers for:
Broad band wireless comunications
Sophisticated controllers for electric grids,
Radars
Base stations of future wireless network
Multifunction RF Systems
Military Applications
Why Wide Band-gap Semiconductors?
Requirements:
Ultra high power
High efficiency
High Frequency
Linearity
Manufacturability
Low Cost
High Temperature (300°C?400°C )
& Hostile Surrondings Endurance
Silicon Devices can
not substain these
requirements
Gallium Arsenide GaAs
Indium phosphide InP
Silicon Carbide SiC
Gallium Nitride GaN
GaN will replace GaAs devices and all of its properties comes
from electrical and physical characteristics
Important Compound
Semiconductors Materials
Good1030GaN
(like Si)0.610SiC
Very Good0.5100?200InP
Good110?50GaAs
NOISE
FIGUREPOWER W/mm
SPEED
GHz
Commercial devices available Not commercial devices available
Front-end Devices Semiconductor characteristics
Semiconductor
characteristics
Silicon
Galliun
Arsenide
Indium
Phosphide
Silicon
Carbide
Gallium
Nitride
Bandgap(eV) 1.1 1.42 1.35 3.26 3.49
Electron mobility
(cm2/Vs)
1500 8500 5400 700 1000-2000
Saturated (peak)
electron velocity
(x107 cm/s)
1.0(1.0) 1.3 (2.1) 1.0 (2.3) 2.0 (2.0) 1.3 (2.1)
Critical breakdown
field MV/cm
0.3 0.4 0.5 3.0 3.0
Thermal
conductivity
1.5 0.5 0.7 4.5 >1.5
Relative dielectric
constant (er)
11.8 12.8 12.5 10 9
2005 - 11 - 11
16
Technology development costs can be amortized over
several large electronic and opto-electronic applications,
like BLUE & WHITE LED and BLUE LASER.
Cost Advantages
Nanocrystal ? 3 D
Quantum layer ? 2 D
Quantum wire ? 1 D
Quantum dot ? 0 D
Nanoscaled Semiconductor
Quantum Confinement
Trap particles and restrict their motion
Quantum confinement produces new material
behavior/phenomena
“Engineer confinement”? control for specific
applications
Structures
Quantum dots (0?D) only
confined states, and no freely
moving ones
Nanowires (1?D) particles
travel only along the wire
Quantum wells (2?D)
confines particles within a
thin layer
Variation of the Optical Properties
with the Crystal Size
1.0 1.5 2.0 2.5
1600 1200 800
Wavelength (nm)
Absorbance (a.u.)
Energy (eV)
Photoluminescence
5.8 nm
5.0 nm
4.6 nm
4.0 nm
3.3 nm
2.9 nm
2.4 nm
6.4 nm
600
Quantum
Confinement in
InAs Nanocrystals
Particle in a box model
E n e r g y
r
1Sh
1Ph
1Se
1Pe
1Sh
1Ph
1Se
1Pe
Luminescence from Indirect
Gap Semiconductors
It is possible to observe
luminescence from indirect
gap semiconductors when
their crystal size is very
small. The origin of this
emission is the
modification of the
electronic structure due to
the size, although some
theories support some
other possible radiative
paths in nanocrystals
(defects, surface effects,...)
2005 - 11 - 11
17
Quantum Wells
The optical properties of a semiconductor are
altered by quantum size effects; at least one of
the dimensions of material is on the order of De
Broglie’s wavelength of an electron: λ = h/mν;
if mν ~ eV ? λ = ~ a few nm;
lSuperlattices based on
Bi2Te3,Si/Ge, GaAs/AlAs
Ec
Ev
x
E
Quantum well (QW)Barrier
Top View
Nanowire
Al2O3 template
lNanowires based on
Bi, BiSb ,Bi2Te3,SiGe
Nature, June 10, 2004
Outlook for Nanocrystal LEDs Brightens
Victor Klimov and colleagues at Los
Alamos National Laboratory assembled
their cadmium selenide dots on top of a so-
called quantum well, a thin sheet of
semiconductor sandwiched between two
barrier layers. A quick flash of laser light
aimed at the well generates pairs of
electrons and positively charged "holes" in
the middle layer. Normally the pairs would
recombine and emit a photon, but by
making the top layer of the well thinner
than 30 Angstroms, the researchers forced
the recombined pairs to release their
energy as a wiggling electric field. This
field generated electron-hole pairs in the
adjacent dots; these pairs recombine,
producing photons.
Important Semiconductor Materials for Optoelectronics
Materials Type Substrate Devices Wavelength range(mm)
Si
SiC
Ge
GaAs
AlGaAs
GaInP
GaAlInP
GaP
GaAsP
InP
InGaAs
InGaAsP
InAlAs
InAlGaAs
GaSb / GaAlSb
CdHgTe
ZnSe
ZnS
IV
IV
IV
III- V
III- V
III- V
III- V
III- V
III- V
III- V
III- V
III- V
III- V
III- V
II- VI
II- VI]
II- VI
II- VI
Si
SiC
Ge
GaAS
GaAS
GaAs
GaAS
GaP
GaP
InP
InP
InP
InP
InP
GaSb
CdTe
ZnSe
ZnS
Detectors, Solar cells
Blue LEDs
Detectors
LEDs, Lasers, Detectors, Solar
Cells, Imagers, Intensifiers
LEDs, Lasers,
Solar Cells, Imagers
Visible Lasers, LEDs
Visible Lasers, LEDs
Visible LEDs
Visible LEDs
Solar Cells
Detectors
Lasers, LEDS
Lasers, Detectors
Lasers, Detectors
Lasers, Detectors
Long wavelength Detectors
Short wavelength LEDs
Short wavelength LEDs
0.5- 1
0.4
1-1.8
0.85
0.67 -0.98
0.5- 0.7
0.5- 0.7
0.5- 0.7
0.5- 0.7
0.9
1-1.67
1-1.6
1-2.5
1-2.5
2-3.5
3-5 and 8 -12
0.4- 0.6
0.4- 0.6
Commercial Applications of Optoelectronic Devices
Materials Devices Applications
Remote control TV, etc., video disk
players, range- finding, solar energy
conversion, optical fiber communication
systems (local networks), image
intensifiers
Space solar cell
Optical fiber communications
(long - haul and local loop)
Optical fiber communications,
Military applications, medicine, sensor
Displays, control, compact disk players,
laser printers/scanners, optical disk
memories, laser medicine equipment
Solar energy conversions, e.g. watches,
calculators, cooling, heating, detectors
Detectors
Displays, optical disk memories, etc.
Infrared imaging, night vision sights,
missileseekers, other military applications
Commercial applications (R&D stages
only)
Detectors, Infrared
LEDs and Lasers
Solar cell
Infrared LEDs,
Lasers (1-1.6mm)
1-1.67mm Detectors
1.67 -2.4mm Detectors
0.5- 0.7mm LEDs
and Lasers
Detectors and
Solar Cells
Detectors
Blue LEDs
Long wavelength
detectors/smitters
Visible LEDs
GaAs/ AlGaAs
InP /InP
InP /InGaP
InP /InGaAs
InGaAlAs/InGaAs
GaAs/ GaInP /
GaInAlP
Si
Ge
SiC
GaSb / GaAlSb /InSb
ZnSe / ZnS
