E-Photon One Curriculum
2B- Optical Technologies
Coordinator: António Teixeira, Co-Coordinator: K. Heggarty
António Teixeira, Paulo André, Rogério
Nogueira, Tiago Silveira, Ana Ferreira,
Mário Lima, Ferreira da Rocha, João
Andrade
© 2005, it - instituto de telecomunicações. Todos os direitos reservados.
This tutorial is licensed under the Creative Commons
http://creativecommons.org/licenses/by-nc-sa/3.0/
Program
1.
2.
3.
4.
Basic Photonic Measurements
Material growth and processing
Semiconductor materials
Transmission systems
performance assessment tools
5. Optical Amplifiers
a)
b)
c)
d)
Semiconductor Optical Amplifiers
(SOAs)
Erbium Doped Fiber Amplifiers
(EDFAs)
Fiber Amplifiers- Raman
Other Amplifiers
6. Emitters
a)
b)
Semiconductor
Fiber
7. Receivers
a)
b)
PIN
APD
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2 Jan 2006
8. Modulators
a) Mach Zehnder
b) Electro-absorption
c) Acoust-optic
9. Filters
a) Fiber Bragg gratings
b) Fabry Perot
c) Mach-Zehnder
10. Isolators
11. Couplers
12. Switches
a) Mechanical
b) Wavelength converters
c) Multiplexers/ Demultiplexers
Paulo André
Semiconductor Materials
© 2005, it - instituto de telecomunicações. Todos os direitos reservados.
This tutorial is licensed under the Creative Commons
http://creativecommons.org/licenses/by-nc-sa/3.0/
3. Semiconductor Materials
1.1. Crystalline and Semiconductor Materials (3)
1.2. Optoelectronics Devices (4)
1.3. Semiconductors (5)
1.4. Physic's Background Theories (15)
1.5. Metals, Semiconductors and Isolator’s Band Structures (4)
1.6. Lattice Defects (2)
1.7. Alloys (3)
1.8. Confinement (1)
1.9. Temperature Effects (3)
1.10. Carrier Population Distribution (5)
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Crystalline and Semiconductor Materials
 The electrical, optical and mechanical properties of a
material depend on the internal organization of its atoms
and the inter-atomic forces that bind them;
 Most optoelectronic devices are constituted by
crystalline materials where the long range order between
atoms is dominant;
 Optoelectronics it’s mainly (but not exclusively)
supported on Semiconductor materials.
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Semiconductors
The semiconductor conductivity can be changed through :
 Temperature
 Optical Excitation
 Impurity Doping
 Devices based on semiconductors are fast and consume
low energies;
 Semiconductors devices are compact and can be
integrated into IC’s;
 Semiconductors devices are cheap.
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Semiconductors
 They have an electrical conductivity whose value is in
between the metal and the isolators conductivity.
 -1 cm -1
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Cu
Fe
Metais
Ge
Si
Glass
semicondutores
Diamond
Silica
Isoladores
Optoelectronics
Optoelectronic Devices– they involve electronic and optical
(photonic) processes :
Electronic exciting through foton absorption
Foton emission through electron relaxation
Conversion between electrical and optical energy (and vice versa)
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Classification
Light Emitting Devices
- LED’s, laser;
Light Absorbing Devices
- Photodetectors, solar cells
Light Manipulation
- Modulation, switching, waveguides
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Aplications
Fotovoltaic
- convert solar light into electricity
Screens/Pannels
- liquid crystals, plasma, actives and totally colloured
Communications
- optical communications
Sensors e monitors
- CCD, IV
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Photonics
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11 Jan 2006
Semiconductors
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12 Jan 2006
Semiconductors
 Elementary Semiconductors : Si and Ge
 Semicondutores Composites:
Binary: ZnO, GaN, SiC, InP,GaAs
Ternary: AlGaAs, GaAsP, HgCdTe,
Quaternarys: InGaAsP, AlInGaP.
 Transistors, diodes and ICs: Si e Ge
 LEDs: GaAs,GaN, GaP
 Lasers: AlGaInAs, InGaAsP, GaAs, AlGaAs
 Detectors: Si, InGaAsP, CdSe, InSb, HgCdTe
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The hetero-epitaxial growth techniques mentioned early have great impact
on the physics and technology of semiconductors.
Most optoelectronic devices are made of hetero-structures
- the use of different materials allows the electron and hole
localization control in the materials, being crucial to manufacture
efficient lasers;
- different materials have different refraction indexes, broadening the
manufacture span of waveguides and mirror structures.
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Gap Energy (ev)
Semiconductors
Lattice Constant (Å)
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15 Jan 2006
Gap Energy (ev)
Wavelength (m)
Semiconductors
Lattice Constant (Å)
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16 Jan 2006
From atoms to bands
The behavior of solid state devices is intimately related to the concepts
of:
Atomic Theory;
Quantum Mechanics;
Electronic Models
Therefore, comes the necessity to remember some of the important
properties of the electrons:
The atoms electronic structure;
The atom interaction with the radiation fields
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Classic Physics
Hundreds of years ago, nature’s knowledge was based on Classic Physics.
- Newton Laws for the movement, and Maxwell equations
Important Concepts from Classic Physics : Particles and Waves
- Particles: solid objects such as Planets and atoms;
- Mechanical waves : property of propagation through a given media
(Interference and Diffraction)
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Wave-Particle Duality
In the end of the XIX century, Maxwell
formulated the equations that allow us
to describe electromagnetic wave
propagation.

D  

B  0


These equations are a solid proof to the   E   B
t
wave nature of light and to the

ondulatory phenomenon
  D
 H  j 
t
interference and diffraction
They support this ondulatory
nature.
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
 
D  oE  P

 
B  o H  M
Modern Physics’ Experiments
Blackbody radiation, H atom,
photoelectric effect
E  hf 
hc

 
h  6.626110
34
h
J .s;  
2
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Photon Radiation
Quantification
Atomic Spectrum
The emission spectrum results made Niels Bohr build a model for the H
atom, based on the planetary mathematical model.
The electrons exist only at
certain stable orbits around the
nucleus
The electron can make transitions
between orbits by absorption or emission
of an energy photon:
hf  E  E
n
m
The angular momentum of the electron
on an orbit satisfies:
l  n
E  hf 
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hc

Considering an electron on a steady
orbit with radius r around an H atom
proton

4o n 
rn 
2
mq
2
2
mv 2


2
r
4o r
q2
l  mvr  n
2 2

n
2 2
mv  2
rn
1 n 2 2
 2

2
mrn rn
4o r
q2
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22 Jan 2006
The
electron energy at
the n-th orbit is
n
nq 2
q2
v


2 2
4
mrn 4o n 
4o n
1 1
mq
  2 
En2  En1 
2 2  2
1 2
mq 4
24o    n1 n2 
Ec  mv 
2
2
24o  n 2  2
E  hf

 1 1 
mq 4
  2  
f 21  
2 2  2
 24o   h   n1 n2 
 1 1
 Ry  2  2 
 n1 n2 
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q2
mq 4
Ep  

4o r
4o 2 n 2 2
E n  Ec  E p  
mq 4
24o  n 2  2
2

H Atom Bohr Model
r and E quantification
with a0 = 0.0529nm (Bohr radius)
with E0 = 13,6eV (Ionization Energy H)
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The Double-Slit Experiment … a Closer Look

• In reality however when we perform the double-slit experiment with electrons we ob
Light – Wave - Particle Dual
Behavior
Double
Slit experiment
INTERFERENCE FRINGES similar to those found
in the corresponding
performed using light waves

* This experiment therefore demonstrates that the principle of wave-particle duali
holds for ELECTRONS just as we found it to hold for light
Interference
Pattern
Particles

Experiment
 That is under appropriate circumstances electrons may be viewed as WAV
Do they exhibit ondulatory behavior?
 E.g. Electrons
 The Duality principle is valid
 On the appropriate circumstances,
the electrons are treated as waves
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ELECTRON
SOURCE
SLITS
SCREEN
PATTERN

How to describe them? (Quantum Mechanics)
What  ?
The De Broglie relation
(suggested from studies of photon properties)


Photon: relativistic particles with null mass at rest, that move at c
The
relativistic mechanic relates the energy and the momentum
according to
E = mc2= mc c = pc
E = photon energy
p=mc : photon momentum
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A photon’s energy is quantified in accordance to the
Planck relation by

E  hf

Combining the previous equations
E  hf 
De Broglie Relation
 p
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h

hc

 h
c

 pc
Consider the case of a particle in a 1-D potencial well,
with width L e infinite barriers

V(x) = 0 for 0  x  L
 V(x) =  for x<0, x>L

Schrödinger equation
Inside the well (V=0)

d 2 x  2m
 2 E  x   0
2
dx

Schrödinger Equation – free particle
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28 Jan 2006
Potential Well
If we consider that the electrons are confined in a 1D potential
well whose energy levels are quantified by a set of discrete values
in accordance to En.

Notice that the in between spacing of consecutive levels is
bigger the lesser L is.

ENERGY
n 2 2  2
En 
2mL2
(x)
n=4
U=
n=1
U=
n=3
n=3
n=2
n=1
0
L
n=2
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x
Graphical
w.d.f presentation and probability density function
for the three first modes
2
 nπ 
ψ n(x)  in sen
x

Electron
a
Box
L
 L 
 (x )
 (x)
0
n=2
n=2
n=1
n=1
n=0
n=0
L
THE WAVEFUNCTIONS LOOK LIKE THE MODES
OF A VIBRATING STRING
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30 Jan 2006
2
0
L
PROBABILITY DENSITY DISTRIBUTION FOR
THE FIRST THREE QUANTIZED MODES
Energy Bands - Solids
When atoms approach to form molecules, Pauli’s exclusion
principle assumes a fundamental role.

When two atoms are completely isolated from each other, in a
way that there’s no interaction of electrons w.d.f, they can have
identical electronic structures.

As the space between the atoms becomes smaller, electron f.d.o
superposition occurs.

As stated previously, Pauli’s Exclusion Principle says that two
different electrons can not be described by the same quantum
state; so, an unfolding of the isolated atom’s discrete energy levels
into new corresponding levels to the electron pair occurs.

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In order to form a solid, many atoms are brought together.
Consequently, the unfolded energy levels form, essentially,
continuous energy bands.
 As an example, the next
picture shows an imaginary
Si crystal formation from
isolated Si atoms.
 As
the
distance
between atoms approaches
the
equilibrium
interatomic separation of the
Si crystal, this band
unfolds into two bands
separated by an energy
gap, Eg.

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Metals, semiconductors and isolators
In metals, bands overlap or are partially filled and the
electrons easily move under the action of an electric field.

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Real
crystal’s band structures
Shur, Michael, “Physics of Semiconductor Devices”, Prentice Hall,1990
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Two semiconductors types :
direct and indirect gap;

On
direct gap semiconductor,
such as GaAs, an electron at the
minimal CB (Conduction Band)
can unexcite to an unoccupied
state in the VB (Valence Band)
through the emission of a
photon with energy equaling Eg.
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An electron at the indirect minimum of the Si CB doesn’t unexcite
directly to the maximum of the VB e suffers a momentum and
energy change. For example, it can be captured by a gap flaw state.

On an indirect transition that involves a k variation, the energy is
usually freed to the lattice (phonons) instead of emitting photons.

These differences between direct and indirect band structures,
are particularly important on deciding which semiconductors should
be used in light emitting devices;


LED’s and lasers should be based on direct gap semiconductors.
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Lattice Defects
 P acts as a doner
on the Si lattice;
Boron is an
acceptor impurity
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On semiconductor technology, the concept of randomly mixing two
or more semiconductors has two main objectives:

Altering the gap energy to a previously determined value (e.g.
laser/detectors)
 ex. HgCdTe – iv detectors;
 ex. InGaAsP – lasers
 ex. AlGaAs – laser layer confinement;
 ex. InGaN; AlGaN

Creating a material with an adequate lattice constant that
mismatches the available substrates
 e.g. In0.53Ga0.47As – matches InP

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Alloys
When two semiconductors A and B are mixed using a proper
growth technique, the following alloy information should be
obtained:

The lattice crystalline structure: on most semiconductors
the two (or more) alloy components have the same crystalline
structure in a way that the final alloy has the same structure.
For materials having the same structure, the lattice constant
obeys the:

Vergard law
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
E.g. Solid Solution of type AxB1-xC
x atom elements A and (1-x) atoms of element B, randomly
distributed over one of the sublattices
(e.g. In the one of group III);


Element C occupies the other sub lattice (e.g. Group V);

x varies between 0 and 1

E.g.. AlxGa1-xAs, GaAs1-xPx, InxGa1-xN, AlxGa1-xN
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On the case of direct gap
semiconductors, the gap energies are
also linearly weighted in accordance
to:

Bowing (C) pictures
the deviation from
the truly random
behavior
Eg ,liga  xEg , A  (1  x) Eg , B
hc 1239.8
E g ( eV ) 

e  (nm)
Eg (InAs)=0.4 eV
Eg (GaAs)=1.4 eV
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Eg(GaEnergy
As)of Ga11-xInx gap
xInxAs
optical fiber communications–
Excellent to use with 0.8 eV (1.5 m)
lasers.
Gotten for x=0.48
Confinement
Correspond to the material’s
class
that
is
between
monocrystalline and molecular
boundaries

 Average
sizes between 1 and
50 nm
E.g. Drastic changes
optical properties terms

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42 Jan 2006
in
Temperature Effects
As the crystal temperature rises, the crystal expands and
the gap energy gets lower
 By submitting to pressure, the crystal is compressed and
the gap energy rises

Varshni
Equation
T 2
Eg (T )  Eg (0) 
T 
Bandgaps of silicon, germanium and gallium arsenide
S.M.Sze, “Physics of Semiconductor Devices”,
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2nd Edition, John Wiley&Sons, 1981
 For T > 0K, some VB electrons get enough thermical energy to be
excited (through Eg) up to the CB.
 Consequently, the semiconductor material will have some electrons in
the previously empty CB and some unoccupied states in the previously
full VB
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dopant in a rather simple way
* In the case where the dopant is a DONOR the
a core with a net POSITIVE charge
 The donor is thus essentially similar to a
energy of the surplus electron can be co
IONIZATION ENERGY of the hydrogen a
 Intrinsic Semiconductor
n  p  ni
• THE IONIZA
(SUBJECT_1
Doping of Semic
Si:As
8
 Extrinsic Semiconductor • A similar argument to+ that above may also •be
FOR A DON
mad
BY INCLUDI
THE DIELECT
the pseudo hydrogen atom
consists of a POSITIVE
e-
 Donor impurities – provide extra
8
* This givesAaGROUP
SIMILAR
estimate for the binding
electrons to conduction
V DOPANT IN A SILICON LATTICE
CAN BE VIEWED AS A PSEUDO HYDROGEN ATOM!
(type n)
 Acceptor impurities – provide
exceeding holes to conduction
(type p)
• A GROUP I
BE VIEWED
Si:B
• IN THIS CA
NEGATIVEL
B
e+
* The ACTUAL donor and acceptor binding ene
reasonably WELL with these SIMPLE estimat
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BINDING ENERGY FOR DONORS (eV)
Doners and acceptors
 At 0K, the energy level is filled with
electrons and too little thermical
energy is needed in order to excite
these electrons up to the CB. So,
between
50-100K,
electrons
are
virtually “donated” to the CB.
 Likewise, acceptor levels can be
thermically settled with VB electrons,
by there generating holes.
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 On semiconductors there are two charge carriers: electrons and
holes
 Electrons on solids obey the Fermi-Dirac statistics. In equilibrium,
the electron distribution over the allowed energy level interval obeys
f (E) 
1
1  e  E  EF / kT
 where EF is called Fermi level
 For T > 0K the probability to have a state with E=EF occupied, is
f ( EF ) 
1
1 e
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48 Jan 2006
 E F  E F  / kT
1
1


11 2
 A more detailed review of f(E) indicates that at 0K, the
distribution assumes the rectangular form pictured on the next
graphic
 That means that at 0K any
available energy state from up
to EF is filled with electrons
and every states over EF are
unoccupied
f ( E) 

1 se E  EF
0 se E  EF
For T> 0K there’s a finite probability, f(E), that the states over EF
are filled (e.g. T=T1) and a corresponding probability, [1-f(E)], that
the states below EF are unoccupied

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E2
Probability
BANDGAP
Emission
Population
Distribution
E
B
A
hf
C
Lesser energy
Transition (A)
Probability
More Probable
Transition (B)
E1
Population
Distribution
E
Bigger energy,
Less probable (C)
Used by permission from VPIphotonics, a division of VPIsystems,
the Photonics Curriculum version 4.0 (copyright) 2007
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