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Laser III
Device Design &
Materials Selection
EBB424E
Dr Zainovia Lockman
Laser 3- Lecture Layout
By the end of the course you would be able to answer
the following questions:
1.
2.
3.
4.
5.
What is homojunction laser?
What is heterojunction laser?
Explain the principles of heterojunction laser.
Sketch a typical stripe geometry laser diodes.
What is quantum well laser?
Introduction

In the pervious lectures you have been explain about two
important conditions for designing laser:
Optical Gain
 Medium which possess the desired energy level structure to
support laser action  in the case of diode laser this will be
the active region of the p-n junction
 To establish a population inversion in a laser system  the
forward bias current supplied to the diode laser.
2. Optical Feedback
 Homojunction laser with one end cleaved and the other
roughned. This is to achieve the optical feedback (optical gain)
of the laser system. Such system is often termed Fabry-Perot
Cavity.
1.
Threshold Current Density

Consider a diagram showing the active region and mode volume of a semiconducting laser:
p
Mode volume, thickness, d
Active region, thickness, t
n
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Recall that when forward biased, with eV > Eg of the material, electrons (from degenerately
doped n) and holes (from degenerately doped p) will be injected across the junction to
create population inversion.
The population inversion is created in a region called active region. Radiative transition
may occur resulted in stimulated emission when the photon is absorbed by the electrons in
the conduction band.
The radiation generated will be spread out in the vicinity of the active region and is almost
confined in the thin layer shown above (mode volume).
Schematic construction of a homojunction GaAs diode laser.
Metal contact (+)
Cleaved end (110)
Natural crystal planes of the junction
so that the end faces are parallel
p+ GaAs
The laser beam output
n+ GaAs
Roughened
end
Junction (active region and mode volume)
Metal contact (-)
•The carriers in the active region increases refractive index of GaAs
•The refractive index increment is only ~0.02, hence is not a good dielectric waveguide
•The beam therefore can be spread out to the surrounding region – mode volume
•Vigorous pumping is therefore needed to enhance lasing
•The threshold current for the pumping action exceeds 400Amm-2
Threshold Current Density
Definition
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If the injected carrier concentration become large enough, the
stimulated emission can exceed absorption so optical gain can
be achieved in the active region. With appropriate configuration
to achieve optical feedback, laser oscillation occurs when gain
exceeds losses.
For significant gain, a high current density is necessary.
The onset of lasing is characterised by the a specific injection
current known as the Threshold Current
Since the simple homojunction laser has high threshold current,
it is considered not efficient.
The onset of laser action at the threshold current density is
indicated by an abrupt increase in radiance of the emitting
region, leading to marked decrease in spectral width.
Threshold Current Density
The typical output spectrum
Optical power
Optical
Power
Stimulated
Emission
Optical power
Spontaneous
Emission
LED
JTH

I
laser

In conclusion about the
homojunction laser….


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The main problem with the homojunction laser diode is that the threshold
current density, Jth is far too high for practical applications.
JTH increases with temperature, too high at room temperature, not
continuous but pulsed laser output.
Homojunction laser has:

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If Jth is low: improve rate of stimulated emission & improve efficiency of
optical cavity
To get low Jth:

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Poor optical
Less carrier confinement
Confined carriers in a narrow region  carrier confinement
Build dielectric waveguide around the optical gain region (increase photon
concentration hence stimulated emission)  photon confinement
How do we achieve that?

heterostructured laser diodes
The Heterojunction Laser
Single & Double
Metal
contact (+)
GaAs sandwiched
between the higher band
gap AlGaAs
n GaAlAs
1m
p GaAs
p GaAlAs
N GaAs
n GaAlAs
p GaAs
P GaAlAs
Metal
contact (-)
N-p-P
GaAs sandwiched between
the higher band gap AlGaAs.
GaAs is the active region
where lasing takes place
N-n-p-P
Homojunction laser
(a)
n
p
p
AlGaAs
GaAs
AlGaAs
(a) A double
heterostructure diode has
two junctions which are
between two different
bandgap semiconductors
(GaAs and AlGaAs).
(~0.1 m)
Electrons in CB
Ec
Ec
Ec
1.4 eV
2 eV
2 eV
Ev
(b)
Ev
Holes in VB
Refractive
index
(c)
Photon
density
Active
region
n ~ 5%
(d)
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
(b) Simplified energy
band diagram under a
large forward bias.
Lasing recombination
takes place in the pGaAs layer, the
active layer
(c) Higher bandgap
materials have a
lower refractive
index
(d) AlGaAs layers
provide lateral optical
confinement.
Carriers & Photons
Confinement
N-Ga1-xAlxAs|p-GaAs|P-Ga1-xAlxAs
N |ACTIVE LAYER|P
GaAs and GaAlAs:
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1.
Have different refractive index

2.
Have different Eg

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nGaAlAs < nGaAs
Eg (GaAlAs) > Eg(GaAs)
Band gap difference  forms barriers for e and h to diffuse from GaAs to the
sandwich layers of GaAlAs  CARRIER CONFINEMENT
Step difference in the refractive index  waveguide (Optical/Photons
Confienment)
Eg (GaAlAs) > Eg(GaAs)  Photons produced in GaAs will not be absorbed
by GaAlAs.
Stripe Geometry DHJ Laser

Features:

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Oxide layer or high resistive layer (produced by proton
bombardment) between metal contact and the
semiconductor.
Restrict current along the junction into narrow stripe (few
microns)
Small JTH with high Power  continuous operation
Used largely in Optical Fibre Communication
The configuration is shown to you in Wilson page 217
for DHJ with oxide that isolate the metal contact to the
GaInAsP (figure 2.17)
Double Heterojunction StrpeLaser
Diode
Cleaved reflecting surface
W
L
Stripe electrode
Oxide insulator
p-GaAs (Contacting layer)
p-AlxGa 1-xAs (Confining layer)
p-GaAs (Active layer)
n-AlxGa 1-xAs (Confining layer)
n-GaAs (Substrate)
Elliptical
laser
beam
2
1
Current
paths
Substrate
3
Substrate
Electrode
Cleaved reflecting surface
Active region where J > Jth.
(Emission region)
Schematic illustration of the the structure of a double heterojunction stripe
contact laser diode
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Materials Criteria & Selection
To date GaAs and GaAlAs are largely used.
Advantages of AlGaAs/GaAs system is that:


1.
2.
3.
4.
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GaAs is direct band gap material
Ga1-xAlxAs is direct when x < 0.45
Lattice match between Ga1-xAlxAs & GaAs is very small (0.1%)
therefore epi growth can be achieved
The band gaps of both materials can be manipulated to produce SH or
DH junctions lasers for high optical and carrier confinemnts
For optical fibre communication, wavelength of 1.1-1.6m is
preferred.
Refer to Wilson page 216 (figure 5.33) or see the next slide
Typical Exam Question  on
GaAs/GaAlAs
Eg(x) = 1.424 + 1.247x (eV)  Empirical relationship
Calculate the band gap if GaAlAs is to be used as emitter
for fibre optics communication at wavelength 1.4m.
Calculate compositions of the GaAlAs ternary alloy for
peak emission at wavelength 1.4m.
Band Gap Engineering
To answer: What other system can be used?
Quantum Well Lasers
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Structure similar to the DH laser except thickness of active layer is very small (1020nm)
E.g. narrow Eg GaAs sandwich between larger band gap GaAlAs
With this configuration, density of states near the bottom of the conduction band and
the top of the valance band increased significantly the hence enhance the
population inversion
Better population inversion, smaller active layer hence JTh is smaller.
BUT, in single quantum well (SQW) extreme narrowness of the active region created
poor optical confinement.
So… Solve by Multiple Quantum Well Structure (MQW)
SQW can be coupled to produce the MQW
Overall active region is now thicker
Carriers which are not captured in one well can be captured by the second well etc.
MQW has JTH higher than SQW (~ 1mA) but the more optical power due to better
optical confinement
Cladding Layer and Separate
Confinement Heterostructure
Preparation for Next
week (Monday)
Test on Laser and LED