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