materials - concordia universityusers.encs.concordia.ca/~mojtaba/elec...

Materials

• Almost all optoelectronic light source depend upon epitaxial crystal growth

techniques where a thin film (a few microns) of semiconductor alloys are

grown on single-crystal substrate; the film should have roughly the same

crystalline quality. It is necessary to make strain-free heterojunction with

good-quality substrate. The requirement of minimizing strain effects arises

from a desire to avoid interface states and to encourage long-term device

reliability, and this imposes a lattice-matching condition on the materials

used.

Schematic illustration of the the structure of a double heterojunction stripecontact laser diode

Oxide insulator

Stripe electrode

SubstrateElectrode

Active region where J > Jth.

(Emission region)

p-GaAs (Contacting layer)

n-GaAs (Substrate)

p-GaAs (Active layer)

Current

paths

L

W

Cleaved reflecting surfaceElliptical

laser

beam

p-AlxGa

1-xAs (Confining layer)

n-AlxGa


12 3

Cleaved reflecting surface

Substrate

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Solid State Optoelectronic Devices

Optical Sources; Laser,

LED

Switches

Photodiodes

Photodetectors

Solar Cells

Materials

• The constraints of bandgap and lattice match force that more complex compound must be chosen.

These compounds include ternary (compounds that containing three elements) and quaternary

(consisting of four elements) semiconductors of the form AxB1-xCyD1-y; variation of x and y are

required by the need to adjust the band-gap energy (or desired wavelength) and for better lattice

matching. Quaternary crystals have more flexibility in that the band gap can be widely varied while

simultaneously keeping the lattice completely matched to a binary crystal substrate. The important

substrates that are available for the laser diode technology are GaAs, InP and GaP. A few

semiconductors and their alloys can match with these substrates. GaAs was the first material to

emit laser radiation, and its related to III-V compound alloys, are the most extensively studied

developed.

Materials

III-V semiconductors•Ternary Semiconductors; Mixture of binary-binary semiconductors; AxB1-xC; mole fraction, x, changes from 0 to 1(x will be adjusted for specific required wavelength). GaxAl1-xAs ; In0.53Ga0.47As; In0.52Al0.48As

-Vegard’s Law: The lattice constant of AxB1-xC varies linearly from the lattice constant of the semiconductor AC to that of the semiconductor BC.

-The bandgap energy changes as a quadratic function of x.

-The index of refraction changes as x changes.

•The above parameters cannot vary independently

•Quaternary Semiconductors; AxB1-xCyD1-y (x and y will be adjusted for specific wavelength and matching lattices).GaxIn1-xPyAs1-y ; (AlxGa1-x)yIn1-yP; AlxGa1-xAsySb1-y

2cxbxaEg

Materials

• II-VI Semiconductors

CdZnSe/ZnSe; visible blue lasers.Hard to dope p-type impurities at

concentration larger than 21018cm-3 (due to

self-compensation effect). Densities on this

order are required for laser operation.

Materials

• IV-VI semiconductors

PbSe; PbS; PbTe

• By changing the proportion of Pb atoms in these materials semiconductor changes from n- to p-type.

• Operate around 50 Ko

• PbTe/Pb1-xEuxSeyTe1-y operates at 174 Ko

Materials

Materials

• In the near infrared region, the most important and certainly the most

extensively characterized semiconductors are GaAs, AlAs and their

ternary derivatives AlxGa1-xAs.

• At longer wavelengths, the materials of importance are InP and ternary

and quaternary semiconductors lattice matched to InP. The smaller

band-gap materials are useful for application in the long wavelength

range.

Energy Band Structure of Semiconductors

Materials

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

0.54 0.55 0.56 0.57 0.58 0.59 0.6 0.61 0.62

Lattice constant, a (nm)

GaP

GaAs

InAs

InP

Direct bandgap

Indirect bandgap

In0.535Ga0.465AsX

Quaternary alloys

with direct bandgap

In1-xGaxAs

Quaternary alloys

with indirect bandgap

Eg (eV)

Bandgap energy Eg and lattice constant a for various III-V alloys ofGaP, GaAs, InP and InAs. A line represents a ternary alloy formed withcompounds from the end points of the line. Solid lines are for directbandgap alloys whereas dashed lines for indirect bandgap alloys.Regions between lines represent quaternary alloys. The line from X toInP represents quaternary alloys In1-xGaxAs1-yPy made fromIn0.535Ga0.465As and InP which are lattice matched to InP.


Example:

III-V compound semiconductors in optoelectronics Figure in the

previous page represents the bandgap Eg and the lattice parameter a

in the quarternary III-V alloy system. A line joining two points

represents the changes in Eg and a with composition in a ternary alloy

composed of the compounds at the ends of that line. For example,

starting at GaAs point, Eg = 1.42 eV and a = 0.565 nm, Eg decreases

and a increases as GaAs is alloyed with InAs and we move along the

line joining GaAs to InAs. Eventually at InAs, Eg = 0.35 eV and a =

0.606 nm. Point X in Figure 3Q6 is composed of InAs and GaAs and

it is the ternary alloy InxGa1-xAs. It has Eg = 0.7 eV and a = 0.587

nm which is the same a as that for InP. InxGa1-xAs at X is therefore

lattice matched to InP and can hence be grown on an InP substrate

without creating defects at the interface.

Further, InxGa1-xAs at X can be alloyed with InP to obtain a quarternary alloy

InxGa1-xAsyP1-y whose properties lie on the line joining X and InP and therefore

all have the same lattice parameter as InP but different bandgap. Layers of

InxGa1-xAsyP1-y with composition between X and InP can be grown epitaxially

on an InP substrate by various techniques such as liquid phase epitaxy (LPE) or

molecular beam expitaxy (MBE) .

The shaded area between the solid lines represents the possible values

of Eg and a for the quarternary III-V alloy system in which the bandgap is direct

and hence suitable for direct recombination.

The compositions of the quarternary alloy lattice matched to InP follow

the line from X to InP.

a Given that the InxGa1-xAs at X is In0.535Ga0.465As show that quarternary

alloys In1-xGaxAsyP1-y are lattice matched to InP when y = 2.15x.

b The bandgap energy Eg, in eV for InxGa1-xAsyP1-y lattice matched to

InP is given by the empirical relation,

Eg (eV) = 1.35 - 0.72y + 0.12 y2

Find the composition of the quarternary alloy suitable for an emitter

operating at 1.55 mm.

Materials

y = 2.2 x

Basic Semiconductor Luminescent Diode Structures

LEDs (Light Emitting Diode)

• Under forward biased when excess minority carriers diffuse into the neutral semiconductor regions where they recombine with majority carriers. If this recombination process is direct band-to-band process, photons are emitted. The output photon intensity will be proportional to the ideal diode diffusion current.

• In GaAs, electroluminescence originated primarily on the p-side of the junctionbecause the efficiency for electron injection is higher than that for hole injection.

• The recombination is spontaneous and the spectral outputs have a relatively wide wavelength bandwidth of between 30 – 40 nm.

• = hc/Eg = 1.24/ Eg

Photon Emission in Semiconductors

Light output

Insulator (oxide)p

n+ Epit axial layer

A schematic illustration of typical planar surface emitting LED devices . (a) p-layergrown epitaxially on an n+ substrate. (b) Firs t n+ is epitaxially grown and then p regionis formed by dopant diffusion into the epitaxial layer.

Light output

pEpit axial layers

(a) (b)

n+

Substrate Substrate

n+

n+

Met al electrode


Basic Semiconductor Luminescent Diode Structures

2 eV

2 eVeVo

Holes in VB

Electrons in CB

1.4 eVNo bias

With

forward

bias

Ec

EvEc

Ev

EFEF

(a)

(b)

(c)

(d)

pn+ p

Ec

GaAs AlGaAsAlGaAs

ppn+

~ 0.2 m

AlGaAsAlGaAs

(a) A doubleheterostructure diode hastwo junctions which arebetween two differentbandgap semiconductors(GaAs and AlGaAs)

(b) A simplified energyband diagram withexaggerated features. EF

must be uniform.

(c) Forward biasedsimplified energy banddiagram.

(d) Forward biased LED.Schematic illustration ofphotons escapingreabsorption in theAlGaAs layer and beingemitted from the device.


GaAs

Ec

Ev

E1

E1

h = E1 – E

1

E

In single quantum well (SQW) lasers electrons areinjected by the forward current into the thin GaAslayer which serves as the active layer. Populationinversion between E1 and E1 is reached even with a

small forward current which results in stimulatedemissions.


0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

Blu

e

Gre

en

Ora

nge

Yel

low

Red

1.7

Infrared

Vio

let

GaA

s

GaA

s 0.5

5P0.4

5

GaAs1-yPy

InP

In0.1

4Ga 0

.86A

s

In1-xGaxAs1-yPy

AlxGa1-xAs

x = 0.43G

aP(N

)

GaS

b

Indirect

bandgap

InG

aNS

iC(A

l)

In0.7G

a 0.3A

s 0.6

6P0.3

4

In0.5

7Ga 0

.43A

s 0.9

5P0.0

5

Free space wavelength coverage by different LED materials from the visible spectrum to theinfrared including wavelengths used in optical communications. Hatched region and dashedlines are indirect Eg materials.

In0.49AlxGa0.51-xP


E

Ec

Ev

Carrier concentration

per unit energy

Electrons in CB

Holes in VB

h

1

0

Eg

h

h

h

CB

VB

Relative intensity

1

0

h

Relative intensity

(a) (b) (c) (d)

Eg + kBT

(2.5-3)kBT

1/2kBT

Eg

1 2 3

2kBT

(a) Energy band diagram with possible recombination paths. (b) Energy distribution ofelectrons in the CB and holes in the VB. The highest electron concentration is (1/2)kBT above

Ec . (c) The relative light intensity as a function of photon energy based on (b). (d) Relativeintensity as a function of wavelength in the output spectrum based on (b) and (c).


LED Characteristics

V

2

1

(c)

0 20 40

I (mA)0

(a)

600 650 700

0

0.5

1.0

Relative

intensity

24 nm

655nm

(b)

0 20 40I (mA)0

Relative light intensity

(a) A typical output spectrum (relative intensity vs wavelength) from a red GaAsP LED.(b) Typical output light power vs. forward current. (c) Typical I-V characteristics of ared LED. The turn-on voltage is around 1.5V.


800 900

–40°C

25°C

85°C

0

1

740

Relative spectral output power

840 880

Wavelength (nm)

The output spectrum from AlGaAs LED. Valuesnormalized to peak emission at 25°C.


Light output

p

Electrodes

Light

Plastic dome

Electrodes

Domed

semiconductor

pn Junction

(a) (b) (c)

n+

n+

(a) Some light suffers total internal reflection and cannot escape. (b) Internal reflectionscan be reduced and hence more light can be collected by shaping the semiconductor into adome so that the angles of incidence at the semiconductor-air surface are smaller than thecritical angle. (b) An economic method of allowing more light to escape from the LED isto encapsulate it in a transparent plastic dome.

Substrate


Electrode

SiO2 (insulator)

Electrode

Fiber (multimode)

Epoxy resin

Etched well

Double heterostructure

Light is coupled from a surface emitting LEDinto a multimode fiber using an index matchingepoxy. The fiber is bonded to the LEDstructure.

(a)

Fiber

A microlens focuses diverging light from a surfaceemitting LED into a multimode optical fiber.

Microlens (Ti2O3:SiO2 glass)

(b)


Schematic illustration of the the structure of a double heterojunction stripe

contact edge emitting LED

InsulationStripe electrode

SubstrateElectrode

Active region (emission region)

p+-InP (Eg = 1.35 eV, Cladding layer)

n+-InP (Eg = 1.35 eV, Cladding/Substrate)

n-InGaAs (Eg = 0.83 eV, Active layer)

Current

paths

L

60-70 m

Light beam

p+-InGaAsP (Eg 1 eV, Confining layer)

n+-InGaAsP (Eg 1 eV, Confining layer) 12 3

200-300 m


qI

hP

a

aext

/

/

IV

PPCE

0

nrr

r

11

1

int

Internal Quantum Efficiency

External Quantum Efficiency

Power Conversion Efficiency

Active layer Barrier layer

Ec

Ev

E

A multiple quantum well (MQW) structure.Electrons are injected by the forward currentinto active layers which are quantum wells.


Multimode fiberLens

(a)

ELED

Active layer

Light from an edge emitting LED is coupled into a fiber typically by using a lens or aGRIN rod lens.

GRIN-rod lens

(b)

Single mode fiberELED


LASERS

• The sensitivity of most photosensitive material is greatly increased at wave-length < 0.7 m; thus, a laser with a short wave-length is desired for such applications as printers and image processing.

• The sensitivity of the human eye range between the wavelengths of 0.4 and 0.8m and the highest sensitivity occur at 0.555m or green so it is important to develop laser in this spectral regime for visual applications.

• Lasers with wavelength between 0.8 – 1.6 m are used in optical communication systems.

LASERS

• The semiconductor laser diode is a forward bias p-n junction. The structure appears to be similar to the LED as far as the electron and holes are concerned, but it is quite different from the point of view of the photons. Electrons and holes are injected into an active region by forward biasing the laser diode. At low injection,these electrons and holes recombine (radiative) via the spontaneous process to emit photons. However, the laser structure is so designed that at higher injections the emission process occurs by stimulated emission. As we will discuss, the stimulated emission process provides spectral purity to the photon output, provides coherent photons, and offers high-speed performance.

• The exact output spectrum from the laser diode depends both on the nature of the optical cavity and the optical gain versus wavelength characteristics.

• Lasing radiation is only obtained when optical gain in the medium can overcome the photon loss from the cavity, which requires the diode current I to exceed a threshold value Ith and gop>gth

• Laser-quality crystals are obtained only with lattice mismatches <0.01% relative to the substrate.

A

B

L

M1 M2 m = 1

m = 2

m = 8

Relative intensity

m

m m + 1m - 1

(a) (b) (c)

R ~ 0.4

R ~ 0.81 f

Schematic illus tration of the Fabry-Perot optical cavity and its properties. (a) Reflectedwaves interfere. (b) Only standing EM waves, modes, of certain wavelengths are allowed

in the cavity. (c) Intens ity vs. frequency for various modes. R is mirror reflectance and

lower R means higher loss from the cavity.


Lm

2

m=1,2,3….

kLRR

IIcavity 22

0

sin41

fm mL

cm )

2(

L

cf

2 Lowest frequency; m=1

Fabry-Perot Optical Resonator

L

m

m - 1

Fabry-Perot etalon

Partially reflecting plates

Output lightInput light

Transmitted light

Transmitted light through a Fabry-Perot optical cavity.


2

0

max1 R

II

kLRR

RII incidentdtransmitte 22

2

sin41

1

R

RF

1

2/1Finesse measures the loss in

the cavity, F increases as loss

decreases

It is maximum when kL=mπ

m

fF

=ratio of the mode separation to

spectral width

LASER Diode Modes of Threshold Conditions

Lasing Conditions:

Population Inversion

Fabry-Perot cavity

gain (of one or several modes) > optical loss

zhvhvg

eIzI

)(

)0()(

I – optical field intensity

g – gain coefficient in F.P. cavity

- effective absorption coefficient

Γ – optical field confinement factor. (the fraction of

optical power in the active layer)

In one round trip i.e. z = 2L gain

should be > loss for lasing;

During this round trip only R1 &

R2 fractions of optical radiation

are reflected from the two laser

ends 1 & 2.

2

21

21

nn

nnR

LASER Diode Modes of Threshold Conditions

From the laser conditions: ,)()2( oILI

)0()0()2(

_

2

21 IeRRILIgL

21

2 1_

RRe

gL

21

_ 1ln2

RRgL

endthRRL

g _

21

_ 1ln

2

1

Lasing threshold is the point at

which the optical gain is equal to the

total loss αt

endtthg _

thth Jgg β– is constant and depends on the

specific device construction.

Thus the gain

Laser Diode Rate Equations

Rate Equations govern the interaction of photons and electrons in

the active region.

ph

spRBndt

d

= stimulated emission + spontaneous emission - photon loss

Bnn

qd

J

dt

dn

sp

= injection - spontaneous recombination - stimulated emission

(shows variation of electron concentration n).

Variation of photon concentration:

The relationship between optical output and the diode drive current:

d - is the depth of carrier-confinement region

B - is a coefficient (Einstein’s) describing the strength of

the optical absorption and emission interactions,;

Rsp - is rate of spontaneous emission into the lasing mode;

τph

– is the photon lifetime;

τsp

– is the spontaneous recombination lifetime;

J – is the injection current density;


Solving the above Equations for a steady-state condition yields an

expression for the output power.

0dt

d0

dt

dnSteady-state => and

n must exceed a threshold value nth in order for Φ to increase.

In other words J needs to exceed Jth in steady-state condition, when the number

of photons Φ=0.

sp

thth n

qd

J

No stimulating emission

This expression defines the current required to sustain an excess

electron density in the laser when spontaneous emission is the only

decay mechanism.


0

0

qd

JnBn

RBn

sp

thsth

ph

sspsth

Now, under steady-state condition at the lasing threshold:

Фs is the steady-state photon density.

0qd

JnR

sp

th

ph

ssp

phph

sp

thphsps

qd

JnR

qd

Jn th

sp

th

phspth

ph

s RJJqd

# of photon resulting from stimulated emission

Adding these two equations: but

The power from the first term is generally concentrated in one or few modes;

The second term generates many modes, in order of 108 modes.


To find the optical power P0:

c

nLt time for photons to cross cavity length L.

s2

1- is the part travels to right or left (toward output

face)

R is part of the photons reflected and 1-R

part will escape the facet

t

Rhvvolume

Ps

12

1

0

th

phJJ

qn

RWhcP

2

)1(2

0

W is the width of active layer

Laser Characteristics

P0

J

Jth

nth

P0 lasing output power ≡ Фs

Threshold

population

inversion

n

Typical output optical power vs. diode current (I) characteristics and the correspondingoutput spectrum of a laser diode.

Laser

LaserOptical Power

Optical Power

I0

LEDOptical Power

Ith

Spontaneous

emission

Stimulated

emission

Optical Power


Laser CharacteristicsResonant Frequency

mL 22 /2 n

mLn

2

22

c

nLv2

nLm

2

So:

n

Lm

2

This states that the cavity resonates (i.e. a standing wave pattern exists

within it) when an integer number m of λ/2 spans the region between the

mirrors. Depending on the laser structures, any number of freq. can satisfy

1&)0()2( 2 LjeILI

Thus some lasers are single - & some are multi-modes.

The relationship between gain & freq. can be assumed

to have Gaussian form:

2

20

2)0()(

egg

where λ0 is the wavelength at the center of spectrum;

σ is the spectrum width of gain & maximum g(0) is

proportional to the population inversion.

Remember that inside the

optical cavity for

zero phase difference :

12 Lje ; ;

Laser CharacteristicsSpacing between the modes:

mvc

Lnm

2 L

n

m

2

1

21 mv

c

Lnm

Lnm

Ln

m

Ln

m

Lnm

2

2

1

22 2

2

122

1 vc

Lnvv

c

Lnmm

Ln

cv

2

vc 2

cv

cv

LnLn

cc

22

2

2

m

Ln2or

Laser CharacteristicsInternal & External Quantum Efficiency

Quantum Efficiency (QE) = # of photons generated for each EHP

injected into the semiconductor junction a measure of the efficiency

of the electron-to-photon conversion process.

If photons are counted at the junction region, QE is called internal

QE (int

), which depends on the materials of the active junction and

the neighboring regions. For GaAs int

= 65% to 100%.

If photons are counted outside the semiconductor diode QE is

external QE(ext

).

Consider an optical cavity of length L , thickness W and width S. Defining a

threshold gain gth as the optical gain needed to balance the total power loss,

due to various losses in the cavity, and the power transmission through the

mirrors .


The optical intensity due to the gain is equal to:

I = Io exp(2Lgth),

There will be loss due to the absorption and the reflections on both

ends by

R1R2 exp(-2Lα)

So: I = Io exp(2Lgth){ R1R2 exp(-2Lα)} = Io (at threshold). Therefore:

1)(2

21 thgL

eRR

where R1 and R2 are power reflection coefficients of the mirrors, is

attenuation constant.


21

1ln

2

1

RRLg th

g is gain constant of the active

region and is roughly

proportional to current density (g

= J). is a constant.

21

1ln

2

1)/1(

RRLJ th

By measuring Jth, , L, R1 and R2 one

can calculate (dependent upon the

materials and the junction structure).

The ratio of the power radiated

through mirrors to the total power

generated by the semiconductor

junction istotal

ra

P

P

RRL

RRL

21

21

1ln

2

1

1ln

2

1

nrr

r

RR

R

int

q

IRR nrr

q

IR

q

I

Rr

rintint

h

PRr

int

hq

IP intint

th

thext

g

g

)(int

Internal efficiencyI is the current in diode

Pint is internal optical power

External effeciency


Therefore ext= int(Pra/Ptotal) which can be determined

experimentally from PI characteristic.

For a given Ia (in PI curve current at point a) the number of

electrons injected into the active area/sec = Ia /q and the number

of photons emitted /second = Pa /h

qI

hP

a

aext

/

/

qI

hP

b

bext

/

/

I

P

h

q

hII

PPq

ba

baext

)(

)(

i.e. ext is proportional to slope of PI curve in the region of I >

Ith.

th

extII

P

h

q

If we choose Ib = Ith , Pb0 and

h Eg and h/q = Eg /q gives voltage

across the junction in volts.

Laser CharacteristicsPower Efficiency

At dc or low frequency the equivalent circuit to a LASER diode may be

viewed as an ideal diode in series with rs.

Therefore the power efficiency =

p =(optical power output)/ (dc electrical power input)

sg

prIqEI

P2)/(

extthIIq

hP

)(

sg

gthext

prIqEI

qEII2)/(

/)(

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


Typical output optical power vs. diode current (I) characteristics and the correspondingoutput spectrum of a laser diode.

Laser

LaserOptical Power

Optical Power

I0

LEDOptical Power

Ith

Spontaneous

emission

Stimulated

emission

Optical Power


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)


GaAs

Schematic illustration of the the structure of a double heterojunction stripecontact laser diode

Oxide insulator

Stripe electrode

SubstrateElectrode

Active region where J > Jth.

(Emission region)

p-GaAs (Contacting layer)

n-GaAs (Substrate)


Current

paths

L

W

Cleaved reflecting surfaceElliptical

laser

beam

p-AlxGa


n-AlxGa


12 3

Cleaved reflecting surface

Substrate


Oxide insulation

n-AlGaAs

p+-AlGaAs (Contacting layer)

n-GaAs (Substrate)


n-AlGaAs (Confining layer)

p-AlGaAs (Confining layer)

Schematic illustration of the cross sectional structure of a buriedheterostructure laser diode.

Electrode


Height, HWidth W

Length, L

The laser cavity definitions and the output laser beamcharacteristics.

Fabry-Perot cavity

Dielectric mirror

Diffraction

limited laser

beam


778 780 782

Po = 1 mW

Po = 5 mW

Relative optical power

(nm)

Po = 3 mW

Output spectra of lasing emission from an index guided LD.At sufficiently high diode currents corresponding to highoptical power, the operation becomes single mode. (Note:Relative power scale applies to each spectrum individually andnot between spectra)


Typical optical power output vs. forward currentfor a LED and a laser diode.

Current0

Light powerLaser diode

LED

100 mA50 mA

5 mW

10 mW


Corrugated

dielectric structure

Distributed Bragg

reflector

(a) (b)

A

B

q(B/2n) =

Active layer

(a) Distributed Bragg reflection (DBR) laser principle. (b) Partially reflected wavesat the corrugations can only constitute a reflected wave when the wavelengthsatisfies the Bragg condition. Reflected waves A and B interfere constructive when

q(B/2n) = .


Active layer

Corrugated grating

Guiding layer

(a)

(a) Distributed feedback (DFB) laser structure. (b) Ideal lasing emission output. (c)Typical output spectrum from a DFB laser.

Optical power

(nm)

0.1 nm

Ideal lasing emission

B(b) (c)


Fiber Bragg grating

Bowei Zhang, Presentation for the

Degree of Master of Applied Science

Department of Electrical

and Computer Engineering

Fiber Bragg grating fabricationPhase Mask: Direct Imprinting

Bowei Zhang, Presentation for the

Degree of Master of Applied Science

Department of Electrical

and Computer Engineering

0th order

(Suppressed)

Diffraction

m = -1

Diffraction

m = +1

Phase MaskΛPM

Ge doped

Fiber

248 nm Laser

Active

layer

L D

(a)

Cleaved-coupled-cavity (C3) laser

Cavity Modes

In L

In D

In both

L and D

(b)


A quantum well (QW) device. (a) Schematic illustrat ion of a quantum well (QW) structure in which athin layer of GaAs is sandwiched between two wider bandgap semiconductors (AlGaAs). (b) Theconduction electrons in the GaAs layer are confined (by ² Ec) in the x-direction to a small length d so

that their energy is quantized. (c) The density of stat es of a two-dimensional QW. The density of statesis constant at each quantized energy level.

AlGaAs AlGaAs

GaAs

y

z

x

d

Ec

Ev

d

E1

E2

E3

g(E)Density of states

E

BulkQW

n = 1

Eg2Eg1

E n = 2² Ec

BulkQW

² Ev

(a) (b) (c)

Dy

Dz


Ec

Ev

E1

E1

h = E1 – E

1

E

In single quantum well (SQW) lasers electrons areinjected by the forward current into the thin GaAslayer which serves as the active layer. Populationinversion between E1 and E1 is reached even with a

small forward current which results in stimulatedemissions.


Active layer Barrier layer

Ec

Ev

E

A multiple quantum well (MQW) structure.Electrons are injected by the forward currentinto active layers which are quantum wells.


materials - concordia universityusers.encs.concordia.ca/~mojtaba/elec...

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