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J. Faist Quantum Electronics

Semiconductor lasers

J. Faist

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Bibliography

Optoelectronics:

E. Rosencher, B. Vinter Optolectronique, Masson

Jasprit Singh Semiconductor Optoelectronics Mc Graw Hill

A. Yariv Quantum electronics Wiley

Semiconductor lasers (multi-authors):

P. Zory Quantum Well lasers Academic Press

E. Kapon Semiconductor lasers (I, II), Academic Press

Semiconductor lasers (monographs, out of print):

G. P. Agrawal, N. K. Dutta Semiconductor lasers, Kluwer

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Outline

Semiconductor materials: Bands and Gaps

direct/indirect, gap size

Waveguides

Heterojunctions


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Materials

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Essential feature: bands and gaps

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Energy (meV)

transmission

Electron

The modulus of thewavefunction is proportionalto (1-R)n

Why gaps? Bragg reflection

A periodic modulation of the potential opens gaps in the energy spectrum

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Why bands?: coupling isolated states

A periodic array of coupled isolated states forms bands

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Direct semiconductors

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IV: indirect semiconductors

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Semiconductor elements

Characteristics:

-Crystalline structure and lattice

-Nature and size of the gap

-Doping

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Absorption edge semiconductors

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III-V family

- Nitrideslacking

- Availability of

substrates

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Blue lasers: III-V vs II-VI

-very hard material

-GaN substrates rares and expensive

-Can be grown on GaAs

-Defects not solved

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Injection of minority carriers

Key number:

Diffusion length of electron

and holes

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A key element: the heterojunction

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Heterojunctions

The junction between two semiconductors with different

bandgaps may align differently:

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Even stranger heterojunctions

In this case, the energy

of the GaSb hole islarger than the one of

the InAs electron!

One may use this for

tunneling from

valence to conduction

band

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Waveguiding

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Waveguiding

Simplest, total internal reflection

Helmholz equation

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Two set of modes: TE and TM

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Solution of the transandental equation

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TE vs TM modes

m

!= 1.55m

The TE mode has

a better confinement,

so will lase before

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Gain in bulk semiconductor

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Fermi-Dirac distribution

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Adding doping

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Quasi-Fermi levels

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Gain condition

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Gain computation

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Gain versus injection

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Density of states

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Quantum well

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Threshold condition

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Threshold condition

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1984 InGaAs / AlGaAs strained QW laser

1988 AlGaAs / GaAs VCSEL

(CW, 300K)(Tokyo Institute of Technology)

1994 InGaAs / AlInAs / InP Quantum Cascade Laser

(pulsed operation, cryogenic temperatures)

(Bell Labs)

1995 InGaN/AlGaN/GaN blue laser diode

(pulsed operation, cryogenic temperatures)

(Nichia Chemicals)

1996 InGaN/AlGaN/GaN blue laser diode

(CW, 300K)

(Nichia Chemicals)

1998 AlGaAs / GaAs Quantum Cascade Laser

(pulsed operation, cryogenic temperatures)(Thomson-CSF)

2002 InGaAs / AlInAs / InP Quantum Cascade Laser

(CW, 300K)(University of Neuchtel)

1962 First GaAs laser diode

(pulsed operation, cryogenic temperature)

(General Electric Research Labs)

1970 AlGaAs / GaAs DH laser diode

(CW, 300K)

(Ioffe Institute, Bell Labs)

1974 AlGaAs / GaAs DFB laser diode

1976 GaInAsP / InP DH laser diode at 1.2!m

(CW, 300K)

(Lincoln Labs)

1977 InGaAsP / InP QW laser

(Urbana University)

1978 AlGaAs / GaAs QW laser

(Urbana University)

1979 InGaAsP / InP VCSEL

(pulsed operation, 77K)

(Tokyo Institute of Technology)

Laser diodes:milestones

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1962: GaAs homojunction laser

GaAs p-n-junctiondoped with Te (n-type)

and Zn (p-type)

Polished facets

Pulsed operation @ 77

K

Jth= 20000 A/cm2

Quist, APL, 1, 91, 1962

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1970: single-/double-heterostructure laser

SH-structure provides e-confinement only

Holes are less mobile

Reduce Ith

to 11000 A/cm2

DH-structure provides e-and hole confinement

Ithgets reduced to 2300 A/cm2

Smaller active region area

Panish, APL, 16, 326, 1970

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1979: SQW laser

QW structure thanks toMOCVD growth

20 nm thickness Ith= 2600 A/cm

2(today10 x better !)

No waveguide layers

(QW guides optically)

Quantization in QW

Dupuis, APL, 34, 265, 1979

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1986: Strained QWs

Strained InGaAs/GaAsQW

Optical waveguide(GRINSCH-structure)

Ithnow at 200 A/cm2

>30 mW output power

Low loss optical WG

Fekete, APL, 49, 1659, 1986

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Development of semiconductor lasers

Development triggeredby growth techniques

1962: Bulk crystals

1970: LPE thin layers

1979: MOCVD/MBEQWs

1985: strained layers byMOCVD

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Separate confinement

"= 0.2m

- More or less

Gaussian mode

- Strongly confined

#= 1.7%

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Materials and processing

Material choice: wavelength

Growth technology

Processing steps Cavity geometries: DFB, DBR, VCELS,

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Molecular Beam Epitaxy

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MBE growth reactor

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FABRICATION STEPS FOR A SEMICONDUCTOR LASER

1- SUBSTRATE 2- EPITAXIE 3- LASER PROCESSING

4- FACETS CLEAVING 5- SINGLE CHIPPREPARATION 6- MOUNTING, BONDING

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Semiconductor ridge laser

n-dopedSubstrat

eActive region

Polymer BCB

or different typeof insulator

Ridge laser

AuGeNiAu

CrAu

AuGeNiAu

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RIDGE LASER WITH POLYMER

10 m

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Laser characteristics

Power

Threshold current

Slope efficiency

Beam properties

Far-field/near field

Linewidth (Henry)

Modulation, noise

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Applications

Telecommunications

modulation, tunability

Sensing

wavelength, tunability

Pumping of Solid-state lasers

Power, efficiency

Processing

Power, wavelength, efficiency

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Semiconductor lasers features

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Optical Power

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Light vs current

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Beam Profile


have wide divergence

because of narrow emitter

120 deg for the example!

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Quantum Cascade Lasers

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Material coverage

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Interband vs intersubband

intersubband:

E

k||

Interband:

k||

E

flexibility in tailoring wavefunctionsand energiesatomic-like joint density of stateshort lifetime (~1ps)

Photon energy limited by gap2D joint density of statelong lifetime (~1ns)

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Are these semiconductors transparent?

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Intersubband absorption

0

0.5

1.0

1.5

120 130 140 150 160

Photon energy (meV)

Absorbance

10K

!"= 5meV

Atomic - like transition!= 3-300m

Mid-Infrared Terahertz

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Needed for a laser:

An optical transition

Population inversion:

need to engineer lifetimes

$up > $dn

Low loss optical resonator

long lifetime

short lifetime

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Milestones: proposals

1986-93: Proposals for QCs usingresonant tunneling in superlattices:

F. Capasso et al, JQE (1986)

H. C. Liu et al, JAP (1988)

1971: R. Kazarinov and R.

Suris propose usingintersubband transitions in a

biased superlattice for lightamplification

R. F. Kazarinov, R.A. Suris, Sov. Phys. Semicond. 5, 707 (1971)

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Quantum cascade laser

1994: First intersubband laser (quantum cascade laser) isdemonstrated in Bell Labs

Tmax = 125K (pulsed), Pmax = 10mW, != 4.26m

J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A.L. Hutchinson, A. Y. Cho, Science 264, 553 (1994)

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Cascade

Cascade:-1 electron may generate many photons

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High temperature DFB-CW QC source

Narrow gain 2Ph design

Low active region doping

Standard mounting

Uncoated device

Jth=1.29 kA/cm2

Tmaxcw of 400 K !!!

A. Wittmann et al., unpublished

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QCL Performances

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