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OPTI510R: Photonics
Khanh Kieu
College of Optical Sciences,
University of Arizona
kkieu@optics.arizona.edu
Meinel building R.626
Announcements
HW #6 is assigned, due April 23rd
Final exam May 2
Semiconductor Lasers
Introduction
p-n junction
Lasers based on p-n junction
Lasers based on heterostructure
Semiconductor materials
Fabrication
Examples of semiconductor lasers and performance
Semiconductor Lasers
Metal
Semiconductor
Insulator
Valence band is completely full
Periodic Table
Group IV
semiconductor
Group III-V
semiconductor
Group II-VI
semiconductor
Metal
Devised 1869 by Dmitri Mendeleev
118 confirmed elements as of 2011
# of proton
Direct and indirect bandgaps
• Energy and momentum must be conserved
• Transition appear as vertical lines in dispersion diagram
• In indirect gap materials, phonon is needed for momentum conservation
• Low probability for light emission, 3 body process
Semiconductor doping
• Intrinsic (pure) semiconductor, n(Si) ~ 1010
cm-3
• Extrinsic (doped) semiconductor
• Control of resistivity of silicon by over 9 orders of magnitude by adding small amount of impurites or dopants
• p-type, majority carrier is hole
• n-type, majority carrier is electron
This is what makes everything possible!
Transistors, semiconductor lasers, diode detectors, CCDs …
which lead to computer, internet, mp3 players, digital camera …
Density of Si 5x1022cm-3
The Nobel Prize in Physics 1956
"for their researches on semiconductors
and their discovery of the transistor effect"
William Bradford
Shockley
1910–1989
John
Bardeen
1908–1991
Walter Houser
Brattain
1902–1987
The Nobel Prize in Physics 1956
p-n junction
p-n junction
p+ n+
EF n
(a)
Eg
Ev
Ec
Ev
Ho les in V B
Electro ns in C B
Junction
Electro nsE
c
p+
Eg
V
n+
(b)
EF n
eV
EF p
The energy band diagram of a degenerately doped p-n with no bias. (b) Banddiagram with a sufficiently large forward bias to cause population inversion andhence stimulated emission.
In vers ionreg io n
EF p
Ec
Ec
eVo
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Lasers based on p-n junction
Lasers based on p-n junction
• January 1962: observations of super-lumenscences in GaAs p-n junctions
(Ioffe Institute)
• Sept.-Dec. 1962: laser action in GaAs and GaAsP p-n junctions
(General Electric , IBM, Lebedev Institute)
LElectrode
Current
GaAs
GaAsn+
p+
Cleaved surface mirror
Electrode
Active region(stimulated emission region)
A schematic illustration of a GaAs homojunction laserdiode. The cleaved surfaces act as reflecting mirrors.
L
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)Wavelength
Lig
ht
inte
nsity
Lasers based on heterostructure
• p-n junction design requires cryogenic temperature to lase
• Large current density needed to create population inversion
Solution: Double Heterostructure! (DHS)
Refractiveindex
Photondensity
Active
region
n ~ 5%
2 eV
Holes in VB
Electrons in CB
AlGaAsAlGaAs
1.4 eV
Ec
Ev
Ec
Ev
(a)
(b)
pn p
Ec
(a) A doubleheterostructure diode hastwo junctions which arebetween two differentbandgap semiconductors(GaAs and AlGaAs).
2 eV
(b) Simplified energyband diagram under alarge forward bias.Lasing recombinationtakes place in the p-GaAs layer, theactive layer
(~0.1 m)
(c) Higher bandgapmaterials have alower refractiveindex
(d) AlGaAs layersprovide lateral opticalconfinement.
(c)
(d)
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
GaAs
Lasers based on heterostructure
• AlGaAs has Eg of 2 eV
• GaAs has Eg of 1.4 eV
• p-GaAs is a thin layer (0.1 – 0.2 um) and is the Active Layer
where lasing recombination occurs.
• Both p regions are heavily doped and are degenerate within the
VB.
• With an adequate forward bias, Ec of n-AlGaAs moves above
Ec of p-GaAs which develops a large injection of electrons from
the CB of n-AlGaAs to the CB of p-GaAs.
• These electrons are confined to the CB of the p-GaAs due to the
difference in barrier potential of the two materials.
Lasers based on heterostructure
1. Due to the thin p-GaAs layer a minimal amount of current is
required to increase the concentration of injected carriers at a
fast rate. This is how threshold current is reduced for the
purpose of population inversion and optical gain.
2. A semiconductor with a wider bandgap (AlGaAs) will also have
a lower refractive index than GaAs. This difference in refractive
index is what establishes an optical dielectric waveguide that
ultimately confines photons to the active region.
Two important advantages:
Room temperature operation possible!
Lasers based on heterostructure
250 µm
120
µm
200 mA
Copper
Metal
Metal
SiO2
p Al Ga As 3 µm0.25 0.75
p Al Ga As 3 µm0.25 0.75
p GaAs 0.5 µm
p GaAs 3 µm+
n GaAs
Schematic representation of the DHS injection laser in the first CW-
operation at room temperature
Credit: Zhores I. Alferov
The Nobel Prize in Physics 2000
"for basic work on information and communication technology"
Zhores I. Alferov b. 1930
Herbert Kroemer b. 1928
Jack S. Kilby 1923–2005
“for his part in the
invention of the
integrated circuit”
“for developing semiconductor
heterostructures used in high-speed- and
opto-electronics”
(by I. Hayashi, 1985)
Heterostructure Tree
HighPower
Electronics
LD
LED
APD
PIN
DetectorArray
FET
HEMTHBT
GaAsIC
HSSolarCell's
PhasedArrayLD
Multi-Wavelength
LDPIN-FETLD-Driver
One ChipRepeater
MonolithicOEICSwitch Optical
ConnectionBetween
LSIsOpticalWiringInside
LSI
SSI
MSI
LSIIntegrationof Optical
and ElectronicDevices
Integrationof OpticalDevices
Integration ofBifunctional
Devices
Wide BandOptical Transition
Wavelength DivisionMultiplexity
All Optical Link
Laser DiskLaser Printer
Optical Sensor
Advanced LAN
BidirectionalVideo Network
Super High SpeedComputer
One ChipComputer
IntegrationTechnology
Device Technology
ProcessTechnology
SubstrateCrystal
EpitaxiThin Film
MaterialCharacterization
Semiconductor materials
Semiconductors allow fabrication of
electrically active devices
Semiconductors belonging to III-V group
often used
Two semiconductors with different
refractive indices needed
They must have different bandgaps but
same lattice constant
Nature does not provide such
semiconductors.
Useful for semiconductor lasers, modulators, and photodetectors
Ternary and quaternary compounds
A fraction of the lattice sites in a binary semiconductor (GaAs, InP,
etc.) is replaced by other elements
Ternary compound AlxGa1-xAs is made by replacing a fraction x of Ga
atoms by Al atoms
Band-gap varies with x as: Eg(x) = 1.424+1.247x (0 < x < 0.45)
Quaternary compound In1-xGaxAsyP1-y useful in the wavelength range
1.1 to 1.6 µm
For matching lattice constant to InP substrate, x/y = 0.45
Band-gap varies with y as: Eg(y) = 1.35-0.72y+0.12y2
Wavelength coverage
Blue light emitting diodes
Blue light emitting diodes
http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/advanced.html
Impact of dimensionality on
density of states
Lz
Lx
Lz
3D
0D
1D
2D
Ly
Lz
Lx
Egap
E00 E01
E0 E1
E000 E001
De
nsity o
f sta
tesP
N
P
N
P
N
P
N
Energy
http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/advanced.html
Quantum dot: artificial atom
Atom Semiconductor Quantum dot
photon
kT
photon
valenceband
conductionband
phonon
forbidden gaps
electronlevels
holelevels
http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/advanced.html
Impact of dimensionality on
density of states
Quantum well laser Quantum dot laser
Homework: Quantum cascade lasers
LD, SLD, LED
Superluminescent diodes
(SLDs) are semiconductor
laser diodes with strong
current injection so that
stimulated emission outweighs
spontaneous emission.
Output of SLD is generally
greater than LED and lower
than LD. Spectrum is narrower
than LED and broader than
LD.
Application in sources with low
coherent time, such as optical
coherence tomography, fiber
optic gyroscopes and fiber
optic sensors
Liquid Phase Epitaxy of
III–V compounds
5 nm
InAsGaP thin layer in
InGaP/InGaAsP/InGaP/GaAs
(111 A) structure with
quantum well grown by LPE.
TEM image of the structure.
H2Heater coils
Pull rod
Solution
GaAssource
GaAssubstrate
Quartz reactor
Credit: G. Khitrova’s group
Molecular Beam Epitaxy (MBE)
of III–V compounds
e-gun
ion gauge
ion pump
RHEEDscreen
shutters
effusioncells
substrateunit
residual gasanalyzer
Schematic view of MBE machine
Riber 32P
MESFET, HEMT
QCL
PD, LED, LD
....
MBE — high purity of materials, in
situ control, precision of structure
growth in layer thickness and
composition
Credit: G. Khitrova’s group
Progress
4.3 kA/cm(1968)
2
900 A/cm(1970)
2
160 A/cm(1981)
2
40 A/cm(1988)
2
6 A/cm(2002)
2
19 A/cm(2000)
2
Impact of SPSL QW
105
104
103
102
10
01960 00 200565 70 75 80 85 90 95
Years
Jth
2 (
A/c
m)
Impact of DoubleHeterostructures
Impact of Quantum Wells
Impact of Quantum
Dots
SPSL: short period superlatticeCredit: Zhores I. Alferov
Progress
high power diode array
stacks
Main problem: heat management
Credit: Zhores I. Alferov
Distributed feed-back laser
• Single frequency operation
• Low noise performance
• Suitable for WDM networks
DFB laser
Vertical cavity surface emitting laser
Optical cavity
mirrors
• Low threshold currents (<1mA)
• Narrow emission lines (often single frequency operation). This is caused by
the very short cavity length, which results in large longitudinal mode spacing
• Circular beam, efficient coupling into single mode optical fiber
• The possibility of fabricating 2 dimensional arrays of lasers (eg. 103 x103
diodes) on the same chip, with each laser individually addressable
Vertical external cavity surface
emitting laser
VECSELs
http://www.rp-photonics.com/vertical_external_cavity_surface_emitting_lasers.html
DFB laser with integrated modulator
• 10Gb/s module, Ith = 20mA, Pmax = 4mW @80mA , extinction ratio
= 15dB for -2.5V.
Pump laser diodes
980nm single mode pump laser (14-pin)
Up to 750mW power range
Air-cooled
-20 to 75C operating temperature
Remaining challenges
• High power singlemode pump lasers
• UV semiconductor lasers
• Wavelength gaps coverage
• Ultrafast semiconductor lasers
• …
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