OPTI510R: Photonics

Khanh Kieu

College of Optical Sciences,

University of Arizona

kkieu@optics.arizona.edu

Meinel building R.626

Announcements

HW #6 is assigned, due April 23rd

Final exam May 2

Semiconductor Lasers

Introduction

p-n junction

Lasers based on p-n junction

Lasers based on heterostructure

Semiconductor materials

Fabrication

Examples of semiconductor lasers and performance

Semiconductor Lasers

Metal

Semiconductor

Insulator

Valence band is completely full

Periodic Table

Group IV

semiconductor

Group III-V

semiconductor

Group II-VI

semiconductor

Metal

Devised 1869 by Dmitri Mendeleev

118 confirmed elements as of 2011

# of proton

Direct and indirect bandgaps

• Energy and momentum must be conserved

• Transition appear as vertical lines in dispersion diagram

• In indirect gap materials, phonon is needed for momentum conservation

• Low probability for light emission, 3 body process

Semiconductor doping

• Intrinsic (pure) semiconductor, n(Si) ~ 1010

cm-3

• Extrinsic (doped) semiconductor

• Control of resistivity of silicon by over 9 orders of magnitude by adding small amount of impurites or dopants

• p-type, majority carrier is hole

• n-type, majority carrier is electron

This is what makes everything possible!

Transistors, semiconductor lasers, diode detectors, CCDs …

which lead to computer, internet, mp3 players, digital camera …

Density of Si 5x1022cm-3

The Nobel Prize in Physics 1956

"for their researches on semiconductors

and their discovery of the transistor effect"

William Bradford

Shockley

1910–1989

John

Bardeen

1908–1991

Walter Houser

Brattain

1902–1987

The Nobel Prize in Physics 1956

p-n junction

p-n junction

p+ n+

EF n

(a)

Eg

Ev

Ec

Ev

Ho les in V B

Electro ns in C B

Junction

Electro nsE

c

p+

Eg

V

n+

(b)

EF n

eV

EF p

The energy band diagram of a degenerately doped p-n with no bias. (b) Banddiagram with a sufficiently large forward bias to cause population inversion andhence stimulated emission.

In vers ionreg io n

EF p

Ec

Ec

eVo

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

Lasers based on p-n junction

Lasers based on p-n junction

• January 1962: observations of super-lumenscences in GaAs p-n junctions

(Ioffe Institute)

• Sept.-Dec. 1962: laser action in GaAs and GaAsP p-n junctions

(General Electric , IBM, Lebedev Institute)

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

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

Lig

ht

inte

nsity

Lasers based on heterostructure

• p-n junction design requires cryogenic temperature to lase

• Large current density needed to create population inversion

Solution: Double Heterostructure! (DHS)

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)

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

GaAs

Lasers based on heterostructure

• AlGaAs has Eg of 2 eV

• GaAs has Eg of 1.4 eV

• p-GaAs is a thin layer (0.1 – 0.2 um) and is the Active Layer

where lasing recombination occurs.

• Both p regions are heavily doped and are degenerate within the

VB.

• With an adequate forward bias, Ec of n-AlGaAs moves above

Ec of p-GaAs which develops a large injection of electrons from

the CB of n-AlGaAs to the CB of p-GaAs.

• These electrons are confined to the CB of the p-GaAs due to the

difference in barrier potential of the two materials.

Lasers based on heterostructure

1. Due to the thin p-GaAs layer a minimal amount of current is

required to increase the concentration of injected carriers at a

fast rate. This is how threshold current is reduced for the

purpose of population inversion and optical gain.

2. A semiconductor with a wider bandgap (AlGaAs) will also have

a lower refractive index than GaAs. This difference in refractive

index is what establishes an optical dielectric waveguide that

ultimately confines photons to the active region.

Two important advantages:

Room temperature operation possible!

Lasers based on heterostructure

250 µm

120

µm

200 mA

Copper

Metal

Metal

SiO2

p Al Ga As 3 µm0.25 0.75

p Al Ga As 3 µm0.25 0.75

p GaAs 0.5 µm

p GaAs 3 µm+

n GaAs

Schematic representation of the DHS injection laser in the first CW-

operation at room temperature

Credit: Zhores I. Alferov

The Nobel Prize in Physics 2000

"for basic work on information and communication technology"

Zhores I. Alferov b. 1930

Herbert Kroemer b. 1928

Jack S. Kilby 1923–2005

“for his part in the

invention of the

integrated circuit”

“for developing semiconductor

heterostructures used in high-speed- and

opto-electronics”

(by I. Hayashi, 1985)

Heterostructure Tree

HighPower

Electronics

LD

LED

APD

PIN

DetectorArray

FET

HEMTHBT

GaAsIC

HSSolarCell's

PhasedArrayLD

Multi-Wavelength

LDPIN-FETLD-Driver

One ChipRepeater

MonolithicOEICSwitch Optical

ConnectionBetween

LSIsOpticalWiringInside

LSI

SSI

MSI

LSIIntegrationof Optical

and ElectronicDevices

Integrationof OpticalDevices

Integration ofBifunctional

Devices

Wide BandOptical Transition

Wavelength DivisionMultiplexity

All Optical Link

Laser DiskLaser Printer

Optical Sensor

Advanced LAN

BidirectionalVideo Network

Super High SpeedComputer

One ChipComputer

IntegrationTechnology

Device Technology

ProcessTechnology

SubstrateCrystal

EpitaxiThin Film

MaterialCharacterization

Semiconductor materials

Semiconductors allow fabrication of

electrically active devices

Semiconductors belonging to III-V group

often used

Two semiconductors with different

refractive indices needed

They must have different bandgaps but

same lattice constant

Nature does not provide such

semiconductors.

Useful for semiconductor lasers, modulators, and photodetectors

Ternary and quaternary compounds

A fraction of the lattice sites in a binary semiconductor (GaAs, InP,

etc.) is replaced by other elements

Ternary compound AlxGa1-xAs is made by replacing a fraction x of Ga

atoms by Al atoms

Band-gap varies with x as: Eg(x) = 1.424+1.247x (0 < x < 0.45)

Quaternary compound In1-xGaxAsyP1-y useful in the wavelength range

1.1 to 1.6 µm

For matching lattice constant to InP substrate, x/y = 0.45

Band-gap varies with y as: Eg(y) = 1.35-0.72y+0.12y2

Wavelength coverage

Blue light emitting diodes

Blue light emitting diodes

http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/advanced.html

Impact of dimensionality on

density of states

Lz

Lx

Lz

3D

0D

1D

2D

Ly

Lz

Lx

Egap

E00 E01

E0 E1

E000 E001

De

nsity o

f sta

tesP

N

P

N

P

N

P

N

Energy

http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/advanced.html

Quantum dot: artificial atom

Atom Semiconductor Quantum dot

photon

kT

photon

valenceband

conductionband

phonon

forbidden gaps

electronlevels

holelevels

http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/advanced.html

Impact of dimensionality on

density of states

Quantum well laser Quantum dot laser

Homework: Quantum cascade lasers

LD, SLD, LED

Superluminescent diodes

(SLDs) are semiconductor

laser diodes with strong

current injection so that

stimulated emission outweighs

spontaneous emission.

Output of SLD is generally

greater than LED and lower

than LD. Spectrum is narrower

than LED and broader than

LD.

Application in sources with low

coherent time, such as optical

coherence tomography, fiber

optic gyroscopes and fiber

optic sensors

Liquid Phase Epitaxy of

III–V compounds

5 nm

InAsGaP thin layer in

InGaP/InGaAsP/InGaP/GaAs

(111 A) structure with

quantum well grown by LPE.

TEM image of the structure.

H2Heater coils

Pull rod

Solution

GaAssource

GaAssubstrate

Quartz reactor

Credit: G. Khitrova’s group

Molecular Beam Epitaxy (MBE)

of III–V compounds

e-gun

ion gauge

ion pump

RHEEDscreen

shutters

effusioncells

substrateunit

residual gasanalyzer

Schematic view of MBE machine

Riber 32P

MESFET, HEMT

QCL

PD, LED, LD

....

MBE — high purity of materials, in

situ control, precision of structure

growth in layer thickness and

composition

Credit: G. Khitrova’s group

Progress

4.3 kA/cm(1968)

2

900 A/cm(1970)

2

160 A/cm(1981)

2

40 A/cm(1988)

2

6 A/cm(2002)

2

19 A/cm(2000)

2

Impact of SPSL QW

105

104

103

102

10

01960 00 200565 70 75 80 85 90 95

Years

Jth

2 (

A/c

m)

Impact of DoubleHeterostructures

Impact of Quantum Wells

Impact of Quantum

Dots

SPSL: short period superlatticeCredit: Zhores I. Alferov

Progress

high power diode array

stacks

Main problem: heat management

Credit: Zhores I. Alferov

Distributed feed-back laser

• Single frequency operation

• Low noise performance

• Suitable for WDM networks

DFB laser

Vertical cavity surface emitting laser

Optical cavity

mirrors

• Low threshold currents (<1mA)

• Narrow emission lines (often single frequency operation). This is caused by

the very short cavity length, which results in large longitudinal mode spacing

• Circular beam, efficient coupling into single mode optical fiber

• The possibility of fabricating 2 dimensional arrays of lasers (eg. 103 x103

diodes) on the same chip, with each laser individually addressable

Vertical external cavity surface

emitting laser

VECSELs

http://www.rp-photonics.com/vertical_external_cavity_surface_emitting_lasers.html

DFB laser with integrated modulator

• 10Gb/s module, Ith = 20mA, Pmax = 4mW @80mA , extinction ratio

= 15dB for -2.5V.

Pump laser diodes

980nm single mode pump laser (14-pin)

Up to 750mW power range

Air-cooled

-20 to 75C operating temperature

Remaining challenges

• High power singlemode pump lasers

• UV semiconductor lasers

• Wavelength gaps coverage

• Ultrafast semiconductor lasers

• …