http://www.wiretechworld.com/the-future-of-optical-fibres/

EE 443 Optical Fiber Communications

Dr. Donald EstreichFall Semester

1

Lecture 10

Heterostructures &Semiconductor Lasers

2

Highlights from Lecture 9 (September 19, 2019)

1. Bandgaps play a dominating role in semiconductors2. Intrinsic semiconductors (undoped) have equal numbers of electrons

and holes3. Electrons are mobile in the conduction band and holes in the valence

band4. Concentrations are controlled by introducing impurity atoms (donors

provide electrons and acceptors provide holes)5. Each electron or hole state have both energy level and crystal

momentum6. Formation of pn junction: a depletion layer creates a potential barrier7. In the depletion layer immobile donors are positive and immobile

acceptors are negative8. Donor dominated material is n-type and acceptor dominated material

is p-type

3

Highlights from Lecture 9 (continued)

9. Minority carriers exist along with the majority carriers following the law of mass action: pn = ni

2

10. Spontaneous emission from electron-hole recombination with wavelength =(1.24/EG)

11. Semiconductors can be either direct bandgap and indirect bandgap12. Crystal momentum k is quantized into discrete values as phonons13. In indirect bandgap recombination both a photon (mainly energy) and

a phonon (mainly momentum) must be emitted for the recombination to occur

14. Thus, minority carrier lifetime is generally much longer that for direct bandgap recombination because it is harder to balance both energy and momentum simultaneously

15. Semiconductor lasers predominantly use direct bandgap semiconductors

4

Stimulated Emission From Degenerate pn-Junction

From: Section 6.3.4, Figure 6.16 on page 318, in Senior

Degenerately doped

No applied bias

Strong forward bias

n

n

p

p

Ef

EG

pn-junction narrows withhigh electronconcentrationin conductionband allowingfor stimulatedemission

At equilibrium

5

Basic Construction of a Laser Diode

https://www.researchgate.net/figure/6-Laser-diode-construction-The-Heterojunction-5_fig6_325262396

I

6

https://blog.rpmclasers.com/laser-diodes-and-vcsels-differences

Example: Compound Semiconductor AlGaAs-GaAs Laser Diode

7

Why Compound Semiconductors and Heterojunctions?

Compound semiconductors are well suited for semiconductor lasers. The most basic, necessary condition required of laser materials is that the input energy can be converted into light energy with a reasonably high efficiency. The injected electron and hole concentrations should be higher than approximately 2 · 1018 cm-3 for sufficient optical gain to reach the lasing threshold.

In a semiconductor structure we need

(a) Direct bandgap(b) A bandgap consistent with the desired wavelength (c) Heterojunctions allow for quantum wells to aid in population

requirement and radiation confinement within the device(d) Desire to have high thermal conductivity (for heat removal)

8

Semiconductor Material Wavelength (m) Bandgap Energy EG (eV)

Indium Phosphide InP 0.92 1.35

Indium Arsenide InAs 3.6 0.35

Gallium Phosphide GaP 0.56 2.24

Gallium Arsenide GaAs 0.87 1.42

Aluminum Arsenide AlAs 0.59 2.09

Gallium Indium Phosphide GaInP 0.64 to 0.68 1.82 to 1.94

Aluminum Gallium Arsenide AlGaAs 0.80 to 0.90 1.40 to 1.55

Indium Gallium Arsenide InGaAs 1.0 to 1.3 0.95 to 1.24

Indium Gallium Arsenic Phosphide InGaAsP 0.9 to 1.7 0.73 to 1.35

Some Selected Compound Semiconductors ( and EG)

9

Semiconductor Materials From Periodic Table

B CBoron Carbon

Elemental

https://www.researchgate.net/figure/The-elements-in-the-periodic-table-to-form-possible-semiconductor-solid-solution-in-this_fig1_320116083

Consider: InGaAsP

10

3.0

0.0

1.0

2.0

5.4 5.6 5.8 6.0 6.2 6.4 6.6

AlP

GaP

AlAs

GaAs

Al.3Ga.7As

In.49Ga.51P

Lattice Constant (Å)

Bandgap

Energ

y

(eV

)

Ga.47In.53As

InAs

GaSb

InP

Al.48In.52As

AlSb

InSb

Direct gap

Indirect gap

T = 300 K

Si

Ge

Si1-xGex

Unstrained

Compound Semiconductor Bandgap Engineering

Lattice matching atomic spacings is important for producing defect free crystals.

11https://slideplayer.com/slide/4665641/

12http://antoine.wojdyla.fr/projects/projects.html

Herbert Kroemer’s Nobel Sketch

"Certainly, when I thought of the heterostructure laser, I did not intend to invent compact disc players," he says. "I could not have anticipated the tremendous impact of fiber-optic communications. I really didn't give a damn about what the uses were."

“Not Just Blue Sky” IEEE Interview in 2002 https://spectrum.ieee.org/semiconductors/design/not-just-blue-sky

13

https://slideplayer.com/slide/7245886/

Eg2 = 1.43 eV

Eg1 = 1.83 eV

AlGaAs

GaAs

~

Heterojunction Energy Band Diagram

14

https://www.sciencedirect.com/science/article/pii/B9780857095077500124

Band Diagram of the Double Heterojunction Diode

pn-junction

Doubleheterostructure

Quantum wellheterostructure

15

https://www.semanticscholar.org/paper/Quantum-Well-Infrared-Photodetector-Technology-and-Gunapala-Bandara/7f2e0f59ae3c9d0702be4a372fc2d1fe494e7f43

Semiconductor Quantum Wells (Both for Electrons and Holes)

What can we use quantum wells for?

And how do we fabricate them?

16

Molecular Beam Epitaxy (MBE) Deposition

Effusion

cells

Wafer

MBE is used for the growth of

precisely controlled atomic layers

of various compositions and

thicknesses.

Heated effusion cells produce

molecular beams of atoms or

impinging upon heated substrate

surfaces where these atoms

physically rearrange themselves to

Form atomic layers with a targeted

controlled composition and thickness.

MBE is essentially controlled evap-

oration. Growth rate and deposition

time determine the layer’s thickness.

UHV Chamber

Reference: http://www.riber.com/en/public/mbe_technology_info.htm

1

17

Molecular Beam Epitaxy (MBE) Deposition

https://link.springer.com/chapter/10.1007/978-3-642-32970-8_7

Ga

As

Al

Sb

Reflection high-energy electron diffraction

18

Molecular Beam Epitaxy (MBE) Deposition

http://www.chtm.unm.edu/facilities/mbe.html

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MOCVD allows for the deposition

of thin material layers of controlled

composition and thickness onto

semiconductor surfaces.

The atoms to be deposited are combined

with organic gas molecules which are

passed over a locally heated semi-

conductor surface. The applied heat

breaks up the molecules which end up

on the surface of the wafer surface.

Different layer compositions are controlled

by varying the gas composition.AsH3

Dopant

H2MFC

TEGa

TMAl

TMx Heated

Wafer

Vertical

Chamber

MFC

MFC

MFC

MFCHeteroepitaxy

Ga(CH3)3 + AsH3 = GaAs + 3CH4

Exhaust

Metalorganic Chemical Vapor Deposition (MOCVD)

HigherVolumeOutput

2

20

3 3 3 4AsH + Ga(CH ) GaAs + 3CH→

TMGa = Trimethylgallium (liquid)TBAs = Tertiarybutyl arsine (liquid)AsH3 = Arsine (gas)

Metalorganic Chemical Vapor Deposition (MOCVD)

http://laserboyfriend.blogspot.com/2013/09/mass-flow-controllers-and-finding-ones.html

21

Electron and Photon Rate Equations – I

We desire to have a relationship between optical output power and thediode drive current. To so this we use photon and electron rate equations.Given a carrier confinement region of depth d, then the number of photonsis represented by and its rate equation is

=3 1[m sec ]sp ph

d nCn

dt

−= + −

Time rate ofchange innumber of

photons

= + -

Change innumber of

photonsfrom

stimulatedemission

Change innumber of electrons

fromspontaneous

emission

Loss innumber ofphotons

from cavitylosses

= number of photons; n = number of electronsC = coefficient describing strength of optical absorption & emissionph = photon lifetime in the cavity of the lasersp = spontaneous emission lifetime ( = 21 ) = a small constant of proportionality (generally quite small)

From: Section 6.3.4 pages 317 to 323, in Senior

1

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Electron and Photon Rate Equations – II

Given a carrier confinement region of depth d, then the number of photonsis represented by and its rate equation is

=3 1[m sec ]sp

dn J nCn

dt qd

−= − −

Time rate ofchange innumber ofelectrons

= + -

Change innumber of electrons

from current

injection

Loss innumber of electrons

fromspontaneous

emission

Change innumber ofelectrons

from stimulatedemission

= number of photons; n = number of electronsJ = injected current density (in units of amperes/unit area)sp = spontaneous emission lifetime ( = 21 )q = charge on electrond = depth of electron confinement region

From: Section 6.3.4 pages 317 to 323, in Senior

2

23

Electron and Photon Rate Equations – Combining Equations

We begin by considering the steady-state (non-transient) with the assumptionthat spontaneous emission is negligible (that is a good assumption it turns out),thus, we let = 0. At steady-state both

The time derivative of (d/dt) can only be positive if

Thus, there is a threshold value of the electron concentration, call it nth,such that if n > nth, then can grow, where nth is given by

0dn d

dt dt

= =

10

ph

Cn

−

-3th

1[m ]

ph

nC

=

From: Section 6.3.4 pages 317 to 323, in Senior

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Electron and Photon Rate Equations – Final Result

The threshold current Jth required to maintain nth is given by

And the steady-state photon number s is found from

Substituting for Cnth gives

Note: Optical output power is proportional to s

-3 -1[m sec ]th th

ph

J n

qd =

-3

( )0

1[m ]

thth S

thS

th

J JCn

qd

J J

Cn qd

−= −

− =

-3( ) [m ]ph

S thJ Jqd

= −

From: Section 6.3.4 pages 317 to 323, in Senior

25

Light Output vs. Current Characteristic of a Laser Diode

Equivalent to Figure 6.17 (page 321) in Senior

https://odicforce.com/Background-and-Projects/Laser-Diodes

26

Temperature Behavior of Output Power vs. Current for a Laser Diode

https://odicforce.com/Background-and-Projects/Laser-Diodes

Increasing Temperature

Temperature characteristic of P vs. Iwhich behaves like ( )0 0( ) exp /thI T I T T=

StabilizationNeeded?

27

Laser current (mA)

Light Output vs. Current With Internal heating of Laser

From: Coldren, Corzine and Mašanović, Diode Lasers and Photonic Integrated Circuits, 2nd ed.,J. Wiley & Sons, Inc., New York, 2012; Section 2.8.4, Figure 2.17, page 84. © Wiley

Requirement:Get heat out!

28

The threshold current density Jth for stimulated emission can be related to thethreshold gain gth by the simple relationship,

where gain factor is a constant tailored to a specific device (i.e., devicedependent). Substituting for gth from the equation

gives us an expression for Jth

The mirror reflectivities are r1 and r2. They may be calculated using the Fresnelreflection relationship (eq. 5.1, page 219)

~

ththg J=

~

~

( )Fractional gain exp 2gL= ~

1 2

1 1 1log

2th eJ

L r r

= +

~

Laser Diode Threshold Current Expression

2

1

1

n nr

n n

−=

+

From: Section 6.3.4 pages 317 to 323, in Senior

29

Laser Diode Efficiency – DEQE

The definition of “differential external quantum efficiency” D is the ratio ofthe change in photon output rate to the change in the number of injected

electrons. Let Pe be the optical power output of the device, I the current, q is the magnitude of the charge on an electron (or hole) and hf is the energyof a photon at the laser’s wavelength, then

where EG is the bandgap energy of the material. We can interpret this as theslope of the P-I characteristic shown on the next slide.

Note: There are other definitions of laser efficiency. 1. Total efficiency (external quantum efficiency) – eq. (6.39)2. Internal quantum efficiency – eq. (6.36)3. External power efficiency – eq. (6.41)

( )( )

/ 1

/e e

D

G

d P hf dP

d I q E dI = =

From: Section 6.4.1, pages 328 to 330, in Senior

30

https://slideplayer.com/slide/7919267/

Differential External Quantum Efficiency

31

https://www.researchgate.net/figure/Light-current-characteristic-and-total-efficiency-of-the-laser-diode-at-a-wavelength-of_fig1_276890055

Common Laser Diode Output Power and its Efficiency

Efficiency

Typically40% to 60%

Power

32

Stripe Geometry of AlGaAs DH Injection Laser Diode

From: Section 6.4.2, Figure 6.21, pages 330 to 332, in Senior (3rd ed.)

Typically L < ½ mm

33

Structures for Confining Optical Waves in the Lateral Dimension

From: G. Keiser, 3rd ed., Section 4.3.5, Figure 4-22, Page 172; 2000.

(1) Gain-guided laser;injected carriers changeindex of refraction along

stripe.

(2) Positive-index guided;stripe has high index

of refraction.

(3) Negative-index guided;stripe has lower indexof refraction in active

area.

W < 8 m Single mode

Astigmatic

34http://www.industrial-electronics.com/solid-state-elec-dev_8b.html

AlGaAs-GaAs LASER Diode: Double Heterojunction (Heterostructure)

< 1 m

Protonbombardedregions

Active layerstripe

ActiveRegion

35

GaAlAs LASER Diode: Double Heterojunction (Heterostructure)

➢ AlGaAs has a bandgap energy of about 1.85 eV (depends upon Al to Ga ratio)➢ GaAs has a bandgap energy of 1.43 eV➢ The p- GaAs layer is thin (0.1 to 0.2 m) and is called the “active layer” where

the lasing recombination occurs➢ Both p regions are heavily doped and are degenerate within the valence band➢ Under adequate forward bias, the conduction band edge of the n-AlGaAs

layer moves above the valence band edge of the p-GaAs, developing a large injection of electrons from the conduction band of the n-AlGaAs to the conduction band of the p-GaAs

➢ These electrons are confined to the conduction band of the p-GaAs becauseof the difference in potential barriers of the two compound semiconductors – this allows for laser operation

https://image.slidesharecdn.com/semiconductorlasers-130227095755-phpapp01/95/semiconductor-lasers-24-638.jpg?cb=1361959112

See slide 34 (prior slide) for corresponding diagram.

36From: G. Keiser, 3rd ed., Section 4.3.5, Figure 4-23, Page 173; 2000.

Buried Heterostructure Index-Guiding Lasers (Short & Long Wavelength)

800 to 900 nm 1300 to 1600 nm

Selectively diffused structures

37http://britneyspears.ac/physics/fplasers/images/Image70.jpg

Band Diagram of a n+pp+ AlGaAs/GaAs/AlGaAs Heterostructure(a) Equilibrium band structure

(b) Under high forward bias

38

https://www.ecse.rpi.edu/~schubert/Light-Emitting-Diodes-dot-org/chap07/chap07.htm

Comparison: Homojunction vs. Heterojunction Under Forward Bias

“Double heterojunction”

39

Distributed Feedback Laser Diodes (DFL)

A distributed-feedback laser is a laser where the whole resonator consists of a

periodic structure, which acts as a distributed reflector in the wavelength range of

laser action and contains a gain medium. Typically, the periodic structure is made

with a phase shift in its middle. This structure is essentially the direct concatenation

of two Bragg gratings with optical gain within the gratings.

Semiconductor DFB lasers are available for emission in different spectral regions at

least in the range from 0.8 μm to 2.8 μm. Typical output powers are some tens of

milliwatts.

https://www.photonics.com/Products/DFB_Laser_Diodes/pr62736

Distributed feedback (DFB)

semiconductor lasers emit

light in a single mode which

is essential to providing the

carrier in long haul high bit-

rate optical communication

systems.

40https://perg.phys.ksu.edu/vqm/laserweb/Ch-6/F6s3p21.htm

Optical Cavities For Producing Narrow Emission Linewidths

41

Standard & Quarter-Wave Shifted DBF Laser Configurations

Active region

Active region

From: Coldren, Corzine and Mašanović, Diode Lasers and Photonic Integrated Circuits, 2nd ed.,J. Wiley & Sons, Inc., New York, 2012; Section 3.7.1, Figure 3.25, page 142. © Wiley

42

http://www.invocom.et.put.poznan.pl/~invocom/C/P1-9/swiatlowody_en/p1-1_8_4.htm

Distributed Feedback Heterostructure InP-InGaAsP Diode Laser

43http://eng.thesaurus.rusnano.com/wiki/article23844

High reflection Ratio mirror

AntireflectingCoating

Optical Arrangement of a Distributed Feedback Semiconductor Laser

44

Laser Diode Chip (Top View)

DFB(Distributed-Feedback) LD(Laser Diode)

Access network and Gigabit Ethernet

http://www.sjphotons.com/index.php?m=content&c=index&a=show&catid=100&id=141&lag=en

45

https://www.skipprichard.com/ask-questions-to-improve-your-leadership/