semiconductor optical communication components and devices
TRANSCRIPT
Professor, Department of Electrical Engineering,
Laser Technology Program,
Indian Institute of Technology, Kanpur
Prof. Utpal Das
http://www.iitk.ac.in/ee/faculty/det_resume/utpal.html
Lecture 19: Introduction to Diode Lasers - II
Semiconductor Optical Communication
Components and Devices
-2
-1
0
1
2
3
4
5
-40 -20 0 20 40 60 80 100
Dis
tanc
e Y
(x 1
0-6 m
)
Electric Field E
TE
TM
nc=1
n=3.65
n=3.56
n=3.51
n=3.5
0 order
1st order2nd order
4
5
6
7
8
9
2 2.5 3 3.5 4 4.5 5 5.5 6
Mo
de W
idth
(
m)
Mask Width (m)
Measurement
Wh
Wv
Index Guided Semiconductor
lasers
Wh
Wv
Guided Field
Electrical field profile for a
6-layer slab waveguide
Waveguides are employed to confine the
emitted light for better interaction with
the injected carriers. These layers have
to be epitaxially grown first.
Multilayer Slab WaveguideLayer 1 (n1)
Layer p-2
Layer p-1
Layer p (np)
t1
tp-2
tp-1
htp
X
Y
Waveguide
Front Mirror
Back Mirror
For lasing action to occur there
also must be a resonant cavity.
For edge emitting lasers this
cavity is formed at the junction
between air and the active
material in the longitudinal
direction; to achieve a sharp
reflective boundary, the crystal
is then cleaved along crystal
planes.
When the substrate cleavage planes
do not match the stripe geometry of
the laser, dry etching with smooth
surfaces may be used to form the
reflective facet.
Semiconductor laser structure: introduction
Nickel mask
Etched Rib
waveguide
facet5m
Cleaved
Facet Mirror
Etched Facet Mirror
Emitting Region Bottom contact
n GaAs substrate
n AlGaAs
Active region
p AlGaAs
p+ GaAs
Proton bombarded
Semi-insulating
barrier
Top metallic
contact
Electric Field Profiles
for a rib w/g structure
A 2-D Rib waveguide may
not be rectangular. Specially
when wet chemical etching
is used to cut out the rib
(i.e., etch a depth of ‘h’)
ncore
ncladdin
g
θinc θincθc
θtran θtran
-2
-1
0
1
2
3
0 0.5 1 1.5 2
Lateral SliceCentral Slice
Dis
tanc
e Y
(x10
-6)
Electric Field E
-2
-1
0
1
2
3
4
0 0.5 1 1.5 2
Dis
tan
ce Y
( x
10-6
)
Electric Field E
neff2=3.449 neff2=3.449neff1=3.465
2m nc=1
nf=3.5
ns=3.4
1m
t=0.1m
2m
neff2=3.449
neff1=3.465
neff2=3.449
h
Typical index guided Semiconductor Laser Structures
Positive contact
p InP
n+ InP
p+ InGaAsP
InGaAsP
InGaAsP
n InP
p+ InP
n+ InP
(1.3-μm, 0.85 eV)
(1.3-μm, 0.85 eV)
Contact layer
Blocking and
Capping layer
Guide layer
Active layer
Buffer layer
Negative contact
Ammelback layer
p - GaAlAs
Confining layerp+- GaAlAs
Contact layerp- GaAs
Active layer
p-n
junction
n - GaAlAs
Confining layer
Positive contact
And heat sink
SiO3
Regrown
n - GaAlAs
Negative
contact
p- GaAs
substrate
Substrate
Zn contact diffusion
Positive contact
Oxide (SiO2)
n-InP
confining layer
Negative
contact
Confining layer (p-InP)
Active region
(n-InGaAsP)
All these
structures
require one
or more
steps of
REGROWTH
It is a
process
where the
first
epitaxial
multilayer
is
processed
and then
few more
layers are
epitaxially
grown in
the growth
chamber
Channeled substrate planar LaserOxide insulating layer
Laser Stripe
(Waveguide
Active layer
Low Index
CladdingHigh Index
Substrate
Laser Stripe
Buried Crescent Laser
p-type material
n-type material
p-type material
Blocks current flow
Ridge Waveguide Laser
Oxide layer
p-type cladding
Active layer
n-type cladding
n-type substrate
Active stripe
Dual-Channel planar
buried-heterostructure
laser
p-type cap
p-type embedding
Active layer
n-type confining layer
p-type material
p-type blocking layer
n-type buffer
Substrate
Laser Stripe
Current flow blocked by
reverse junction except for
laser stripe
Ef
Ec
Ev
p-(AlGa)Asn-(GaAl)As
∆Ec
p-GaAs
p-GaAs
∆Ec
p-(AlGa)Asn-(GaAl)As
Ec
Ev
Efc
Efv
(b) Forward Biased
(a) Equilibrium Band structure
Semiconductor LaserThe structure of the edge-
emitting heterostructure laser,
consists of a thin region of
semiconductor with a small
energy bandgap, called the
active layer, sandwiched in
between two oppositely doped
semiconductor with wide band
gap energy. When the laser is
forward biased, carriers flow
into the active region and
recombine creating light that
gets emitted out of the side of
the structure. A thin metal stripe
is often used for the contact to
confine the current to a small
region of the device, which will
be the laser output
Heterostructure Bands
Note the collection of Electrons and Holes in the active
region which increases the probability of recombination
w/2-w/2
x
Vo
mW mbmb
p+ GaInAsP
n+ InP
p InP
n InP
Active Region
GaInAsP (d=20Å)
InP (d=20Å)
Multi-Quantum-Well Laser
Semiconductor
Quantum Well LaserOften the step-like
density of states in
a quantum well is used in
the lower band-gap layer
sandwiched between higher
to increase the photon gain
in the active region of the
laser. This gives to separate
confinement (SC) of carriers
in the quantum well
whereas the photons are
confined in the double
heterostructure (DH).0
2
4
6
8
10
12
0 20 40 60 80 100 120
Bulk (3D)
Quantum Well (2D)
Quantum Wire (1D)
Quantum Dot (0D)
Den
sity
of
Sta
tes
(x1
020 )
Energy (meV)
Strained Quantum
Well Lasers - I
Strained Layer Epitaxy for
Lattice Mismatched
Materials, such as
InGaAs/GaAs for Optical
Fiber Amplifier Pumping
used earlier.
Thin Epitaxial Layer
Defects in the Epitaxial Layer Biaxially Coherent Compressively
Strained Epitaxial Layer
Thick Substrate
Through the Poission’s
ratio it expands
uniaxially due to
biaxial compressive
strain as the density
remains constant
Critical thickness of an InxGa1-xAs/GaAs
single quantum well on GaAs according
to the theory of Matthews and Blakeslee.
The strain can be coherently contained
such that generation of defects due to
mismatch is minimum.
Strained Quantum
Well Lasers - II
50
Indium Content (%)
InxG
a1-x
As c
riti
ca
l
thic
kn
es
s o
n G
aA
s(n
m)
010
20
30
40
40200 60 40 100
CB
VB
DH-
Structure
QW-Gain
Region
Modal
Profile
Modal
Overlap
Integral (G)
Single Quantum Well Laser (QW could be strained)
Bi-axial strain can be resolved to a
Hydrostatic Strain + Shear Strain. Shear Strain
splits the HH-LH degeneracy (mJ= 3/2, 1/2) levels.
Moreover the dispersion relation of the valence band is
modified such that the density of states of the VB comes
closer to that of the CB. This helps the electron in the CB
to easily find a hole in the VB with Dk=0. Results in higher
efficiency of recombination.
Strained Quantum Well Lasers - III
VB
mJ= 3/2, 1/2
CB
CompressiveTensile
VB
mJ= 3/2
mJ= 1/2
mJ= 1/2
VB
mJ= 3/2
CB
VB
E
k
1. How should the direction of the optical waveguide oriented with
respect to the crystal axes for the formation of cleaved cavity
mirrors for the diode lasers?
2. What is the effect of the introduction of Dopuble Heterostructure
in the active region on the performance of the Laser diode?
Calculate the optimum thickness of a GaAs/Al0.3Ga0.7As DH
structure from a consideration of the optical mode – carrier
overlap.
3. What is the advantage of introducing QWs in the DH active
region? How does the number of QWs determine the speed of
operation of the device?
4. How does compressive strain in the active region improve the
efficiency of a diode laser?
Review Questions