50 Years of Semiconductor Lasers - BostonPhotonics.org€¦ · • Zinc (Zn) diffused pn - junction...

IPS Laser Workshop - 1 PWJ121024

Paul W. Juodawlkis Electro-Optical Materials and Devices Group

24 October 2012

50 Years of Semiconductor Lasers

This work was sponsored by the Assistant Secretary of Defense for Research and Engineering under Air Force Contract No. FA8721-05-C-0002. The opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Government.


2012 CLEO Symposium on the “50th Anniversary of the Semiconductor Laser”

• Organized by Seth Bank (UT Austin), Dan Wasserman (Illinois), and Tom Koch (U. Arizona)

• Speakers: – Marshall Nathan, IBM, “The Invention of the Semiconductor Laser” – Herbert Kroemer, UCSB, “The Double Heterostructure” – Russel Dupuis, Georgia Tech, “Materials Development for

Semiconductor Lasers” – Charles Henry, Bell Labs, “Quantum Well Lasers” – Amnon Yariv, CalTech, “Semiconductor Lasers & OEIC's” – Don Scifres, SDL Ventures, “High Power Semiconductor Lasers, – Thomas Koch, U. Arizona, “Telecom & DFB Semiconductor Lasers” – Jack Jewell, Green VCSEL, “VCSELS” – David Welch, Infinera, “Semiconductor Photonic Integrated Circuits” – Jerome Faist, ETH Zurich, “Quantum Cascade Lasers (QCLs)” – Yasuhiko Arakawa, The University of Tokyo, “Quantum Dot Lasers” – Ming Wu, UC Berkeley, “Antenna-Coupled Nanolasers and Nano-LEDs”

• 3 sessions of standing-room-only attendance (> 200 people)


• The First Semiconductor Laser(s) - 1962 • Semiconductor Laser Advances • Other Semiconductor Laser Structures

– Vertical-cavity surface-emitting lasers (VCSELs) – High-power lasers (Tapered lasers, SCOWLs, coherent combining) – Quantum cascade lasers (QCLs)

• Recent Results – InGaN lasers emitting at green wavelengths – “Silicon” based lasers (hybrid, strained germanium) – Large-scale integration of semiconductor lasers – Thresholdless nano-scale lasers

• Summary

Outline


R. J. Keyes and T. M. Quist, “Recombination radiation emitted by gallium arsenide diodes,” presented at the Solid-State Device Research Conf., Durham, NC, July 1962.

R. J. Keyes and T. M. Quist, “Recombination radiation emitted by gallium arsenide,” Proc. IRE, vol. 50, pp. 1822-1823, Aug.1962.

Before the Semiconductor Laser: Light Emitting Diodes (LEDs) at Lincoln Laboratory

LED Emission Spectra • “When appropriately diffused

GaAs diodes are biased in the forward direction at 77 K, nearly all of the injected carriers, upon recombination, emit a photon whose energy is slightly but significantly smaller than the optically measured band-gap of GaAs.”

• “Our efficiency lies somewhere between 0.48 and 1 and perhaps closer to the latter.”


R. H. Rediker, R. J. Keyes, T. M. Quist, M. J. Hudson, C. R. Grant, and R. G. Burgess, “Gallium arsenide diode sends television by infrared beam,” Electronics, vol. 35, pp. 44–45, Oct. 5, 1962.

R. J. Keyes, T. M. Quist, R. H. Rediker, M. J. Hudson, C. R. Grant, and J. W. Meyer, “Modulated infrared diode spans 30 miles,” Electronics, vol. 36, pp. 38–39, Apr. 5, 1963.

Free-Space Optical Communications with Light-Emitting Diodes, 1962-1963

Map showing 30-nmi path from Mt. Wachusett, Princeton, Mass. to Lincoln Laboratory, Lexington, Mass.

Reflecting Telescope LED Transmitter

5-Foot Searchlight + PMT Receiver Keyes Quist Rediker


Published: November 1, 1962 Copyright The New York Times

• “In a striking coincidence, two companies announced independently yesterday the same scientific achievement, which, they said, promises to open the door to communication by light waves”

• “Compounding the coincidence was the fact that a third group of researchers, at a university laboratory, had reached similar success a week or less after one company”

• General Electric Research Laboratories – Robert N. Hall and associates “used an electric

current to ‘pump’ a semiconductor, a transistor-like device, to get it to emit coherent or ‘in-step’ light waves”

• International Business Machines Corp. – Marshall I. Nathan and associates “succeeded in

operating a new laser, using a semiconductor diode, that is powered directly by an electrical current rather than an external light source”

• MIT Lincoln Laboratory – Robert J. Keyes and Theodore M. Quist – “…the Lincoln team was not claiming priority. G.E.

achieved success ‘a couple of days or a week’ before the MIT group…” said a MIT spokesman

First Semiconductor Lasers Reported in the Popular Press


First Semiconductor Lasers Reported in the Technical Literature

Organization Reference Material System

Date Received

Date Published

General Electric

(Schenectady)

R. N. Hall, G. E. Fenner, J. D. Kingsley, T. J. Soltys, and R. O. Carlson, “Coherent light emission from GaAs junctions,” Phys. Rev. Lett., v. 9, p. 366, 1962.

GaAs 24 Sept. 1962

1 Nov. 1962

IBM

M. I. Nathan, W. P. Dumke, G. Burns, F. H. Dill, Jr., and G. Lasher, “Stimulated emission of radiation from GaAs p-n junctions,” Appl. Phys. Lett., v. 1, p. 62, 1962.

GaAs 4 Oct. 1962

1 Nov. 1962

General Electric

(Syracuse)

N. Holonyak, Jr., and S. F. Bevacqua, “Coherent (visible) light emission from Ga(As1-xPx) junctions,” Appl. Phys. Lett., v. 1, p. 82, 1962.

GaAsP 17 Oct. 1962

1 Dec. 1962

MIT Lincoln Laboratory

T. M. Quist, R. H. Rediker, R. J. Keyes, W. E. Krag, B. Lax, A. L. McWhorter, H. J. Zeigler, “Semiconductor maser of GaAs,“ Appl. Phys. Lett, v. 1, p. 91, 1962

GaAs 5 Nov. 1962

1 Dec. 1962


Semiconductor Laser - Version 1.0

Gallium Arsenide (GaAs) Homojunction Diode Laser

• Material grown via vapor transport

• Zinc (Zn) diffused p-n junction

• Cleaved or polished facets

• Operated at cryogenic temperatures (77 K)

• Operated under pulsed conditions (I > 10 A, τ ~ few µs, Jth ~ 10 kA/cm2)

• High optical losses (αi ~ 104 cm-1)

• Electrical-to-optical conversion efficiency ~ 0.01%

• Problems: (i) Large active volume, (ii) Large losses outside gain region

MIT-LL

GaAs





• Summary

Outline


Semiconductor Laser Functional Overview

Optical Amplifier

What are the properties of the optical gain medium? • Spectral distribution (center wavelength, spectral width) • Relationship between gain and injected carrier density



Waveguide Optical Amplifier

Heterostructure provides both optical and electronic confinement


Fabry-Perot Laser Longitudinal

Modes


• Laser = Gain + Feedback • Multiple longitudinal modes limit applications


Distributed Bragg Reflector (DBR) Laser


Use intracavity filter to obtain single-longitudinal-mode operation


Distributed Bragg Reflector (DBR) Laser


Electrically adjust grating index via current injection to tune center wavelength


Direct Modulation


• Modulate injected current to generate time-varying optical signal • Bandwidth limited by photon-carrier dynamics


External Modulation


• Monolithically integrate modulator external to cavity to obtain higher bandwidth & lower chirp

• Laser operates continuous-wave (CW)


Integrated Mode-Locked Laser


• Monolithically integrate modulator internal to cavity to enable active mode-locking operation

• Generate periodic train of short optical pulses


Semiconductor Energy-Band Diagrams

Direct Bandgap Indirect Bandgap

Examples: GaAs, InP, GaN, InGaAsP Examples: Si, Ge


Semiconductor Energy-Band Diagrams

Direct Bandgap Indirect Bandgap

Electrons

Holes

Direct Transition => Photon Emission

Photon

Indirect Transition Requires Phonon => Low Probability of Photon Emission

Photon Phonon


Bandgap Energy vs. Lattice Constant for Common III-V Semiconductors

Lattice-Matched GaxIn1-xAsyP1-y

Ga0.47In0.53As

InP

Strained GaxIn1-xAs

Advances in Semiconductor Lasers Dependent on Improved Material Growth

• Liquid-Phase Epitaxy (LPE) • Vapor- Phase Epitaxy (VPE) • Molecular Beam Epitaxy (MBE) • Organometallic VPE (OMVPE, aka MOCVD)

Lattice-Matched AlxGa1-xAs


Evolution of Carrier and Optical Confinement

Courtesy of Jim Coleman, University of Illinois

GaAs/AlGaAs Double Heterostructure

Double-Heterostructure Advantages: • Reduce the thickness of the active

region in the growth direction • Increase the spatial overlap between

electrons and holes • Provide index-based waveguide • Reduce the optical absorption due to

transparency of wide-gap material

Double Heterostructure

Separate-Confinement Heterostructure (SCH)

Graded-Index Separate-Confinement

Heterostructure (GRIN-SCH)

Conduction-Band Energy Profiles

Single Quantum-Well Heterostructure (QW)

Multiple Quantum-Well Heterostructure

(MQW) Ref. Herbert Kroemer, “A proposed class of hetero-junction injection lasers,” Proc. IEEE, 1963

2000 Nobel Prize


Why Heterostructure Semiconductor Lasers Work

Holes

Electrons Intrinsic QW Region

p-type n-type

MQW Energy-Band Profile

Semiconductor Laser Cross-Section

Electrons

Conduction-Band Energy Profile

Refractive-Index Profile Transverse

Optical Mode

Material refractive index is inversely proportional to bandgap energy

=> Both carriers and optical field confined in the same region


Impact of Spatial Carrier Confinement

Joint Density of States

3 Dimensions = Bulk

2 Dimensions = Quantum Well

1 Dimension = Quantum Wire

0 Dimension = Quantum Dot

Reduced Dimension => Reduced Density of States => Reduced Threshold Current


First CW GaInAsP/InP Laser

Jim Hsieh

Development of near-IR (1-2 µm) lasers driven by low fiber & atmospheric losses + eye safety


First Quantum-Well (QW) Laser

Theory (1974) Demonstration (1978)

Quantum-Well Heterostructure


First Quantum-Dot (QD) Laser

Theory (1982) Demonstration (1994)

• Predicted negligible threshold current vs. temperature dependence for QDs

• InGaAs QDs grown via MBE using Stranski-Kranstanov method


Basic Semiconductor Laser Fabrication: Strip-Loaded Rib Waveguide

Ridge Mask Deposition &

Patterning

Ridge Etching

Ridge Mask Removal

Insulation Deposition

Initial Epitaxial Material

Via Mask Deposition

Via Mask Patterning

Via Etching

Via Mask Removal

Top-Metal Deposition &

Patterning

Backside Wafer

Thinning

Bottom- Metal

Deposition


Semiconductor Laser Threshold

1 2

1 1ln2

th i m

i

g

L R R

Γ α α

α

= +

= +

Threshold Modal Gain Γ = Optical Confinement Factor

gth = Threshold Material Gain Coefficient

αi = Internal Loss Coefficient

L = Cavity Length

R1,R2 = Mirror Power Reflection Coefficients

Laser Threshold Condition Round-Trip Gain = Round-Trip Loss

0 ln thth

tr

ng gn

i thth

InqV

η τ

=

=> G2A2R1R2 = 1


Power vs. Current Characteristics

( )

( )

mO i th

i m

d th

hP I Iq

h I Iq

α νηα ανη

= − +

= −

Laser Output Power

I = Injection Current Ith = Threshold Current ηi = Internal Quantum Efficiency ηd = Differential Quantum Efficiency αi = Internal Loss Coefficient

αm = Mirror Loss Coefficient hν = Photon Energy q = Electronic Charge

O md i

i m

dPqh dI

αη ην α α

= = +

Spontaneous Emission

Stimulated Emission

Differential Quantum Efficiency


Power vs. Current: Evidence of Lasing

T. M. Quist et al., Appl. Phys. Lett, 1962

Power-Current (L-I) Characteristic for GaAs Homojunction Laser

Amplified Spontaneous

Emission (ASE)

Spontaneous Emission

Lasing Operation

• Analysis of slope changes (“kinks”) in L-I curves are one metric for identifying lasing operation


High-Speed Modulation

1 mW 2 2.7

5

L = 125 µm

Relaxation Oscillation Frequency Short-Cavity Laser Frequency Response

A = Differential Optical Gain Constant p0 = Steady-State Photon Density τp = Photon Lifetime

Methods to Increase Modulation Bandwidth • Increase differential gain (cooling, QWs, strain) • Increase photon density => Higher optical power • Reduce photon lifetime => Shorter cavity

Note: Also need to minimize parasitics

Bandwidth ~ Relaxation Oscillation Frequency


Longitudinal-Mode Control

Fabry-Perot Laser Distributed Feedback

(DFB) Laser Distributed Bragg

Reflector (DBR) Laser

• Cavity formed by high- and low-reflectivity-coated facets

• Multiple longitudinal modes lasing simultaneously

• Grating fabricated along entire length of device

• Single longitudinal mode operation

• Tunable grating outside of active region

• Single longitudinal mode operation





• Summary

Outline


Vertical-Cavity Surface-Emitting Lasers (VCSELs)

• Vertical confinement = High-reflectivity mirrors (> 99%) • Lateral confinement = Mesa etch + oxide layers • Cavity length ~ laser wavelength => Single longitudinal mode • Surface-emitting structure enables on-wafer testing, eliminates

the need for facet cleaving, and supports two-dimensional arrays

First Demonstration by Ken Iga and Co-Workers:

H. Soda, K. Iga, C. Kitahara, and Y. Suematsu, “GaInAsP/InP Surface Emitting Injection Lasers, Jpn. J. Appl. Phys., 1979


High-Power Semiconductor Laser Structures

Standard Rib

Waveguide

Output Facet Cross-Section Top View Attributes

Tapered Laser

Slab-Coupled Optical

Waveguide Laser

(SCOWL)

• Low power (< 100 mW) • High gain (30 dB) • Mode size: 1 x 3 µm • High loss: 5-10 cm-1

• Stable mode profile • Simple lens-coupling

• High power (> 1 W) • High gain (30 dB) • Mode size: 1 x 200 µm • High loss: 5-10 cm-1

• Unstable mode profile • Complex lens-coupling

• High power (> 1 W) • Moderate gain (15 dB) • Mode size: > 5 x 5 µm • Low loss: ~ 0.5 cm-1

• Stable mode profile • Simple lens-coupling

Slab


Tapered Lasers/Amplifiers

• InGaAs/AlGaAs quantum-well graded-index separate-confinement heterostructure (GRINSCH)

• Wavelength ~ 970 nm


Slab-Coupled Optical Waveguide Concept

β = (2π/λ) x Modal Index

β z-guide

z-slab β

Continuum

Continuum Discrete Modes

E. A. J. Marcatili, Bell Syst. Tech. J., 53, 645 (1974)

x n1

n 1

n2

y

z

T

D

W

n3

• Large-diameter fiber by itself supports multiple transverse modes

• Slab guide effectively filters

out higher-order transverse modes in composite structure

• Composite structure is single-

moded (only one bound transverse mode)

Fiber Waveguide

Slab Waveguide


Slab-Coupled Optical Waveguide Laser*

Slab-Coupled Optical Waveguide Laser (SCOWL) Cross-Section

• Small index-contrast (∆n/n) + Coupled-mode filtering => Large optical mode • Small mode-overlap with QWs => High saturation power • Small mode-overlap with p-InP => Low optical loss

Key Characteristics:

Demonstrated SCOWL wavelengths: • 9xx, 1060, 1300, 1550, and 2100 nm

Jim Walpole

Joe Donnelly


Slab-Coupled Optical Waveguide Technology

Slab-Coupled Optical Waveguide Amplifier

(SCOWA)

Demonstrated SCOW Applications Watt-Class

Semiconductor Optical

Amplifiers (SOAs)

High-Power, Low-Noise

Mode-Locked Lasers

Single-Frequency, Narrow-Linewidth

Lasers

• Small index-contrast (∆n/n) + Coupled-mode filtering => Large optical mode (5x7 µm) • Small mode-overlap with QWs => High output power (~1 W) • Small mode-overlap with p-InP => Low optical loss (~0.5 cm-1)

High-Current Waveguide

Photodiodes P. W. Juodawlkis et al., IEEE JSTQE, 2011


Coherent Beam Combining (CBC) of High-Power Semiconductor Optical Emitters

Seed

47 SCOWA Array 12.5 mm x 5 mm

Lincoln CBC Architecture 47-Element SCOWA Array

G. M. Smith et al., IEEE Summer Topical on High-Power Semiconductor Lasers, 2012

Upcoming Boston IPC Laser Workshop Talks: • 7 November: T. Y. Fan, MIT-LL • 17 November: Gary Smith, MIT-LL

• Coherently combined 47 SCOWA elements to produce 50-W total power with > 90% combining efficiency

• Diffractive optical element (DOE) used to achieve single beam with M2 ~ 1.2 x 1.3 λ = 1060 nm


Interband vs. Intersubband Transitions

• Absorption above E=hν

• Broad absorption features • Long lifetime (>1 ns) • Very high radiative efficency • Transition energy <->gap

• Absorption at E = hν

• Narrow absorption features • Short lifetime (<1 ps) • Very low radiative efficiency • Transition energy <->QW thickness

Courtesy of Jérôme Faist, ETH Zurich

Interband Intersubband


Quantum Cascade Laser (QCL) Operation


• Unipolar operation (electrons only) • Each injected electron produces

multiple photons • Requires precise control of many

(~500-1000) material layers

Conduction Band Energy Profile


Quantum-Cascade Laser Milestones


1971 1994 1997 2002 2012

R. F. Kazarinov, R.A. Suris, Sov. Phys. Semicond., 1971

First Theoretical Proposal: Use intersubband transitions

in semiconductor QWs

First QCL Demonstration: Tmax = 125K (pulsed)

Pmax = 10 mW λ = 4.26 µm

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

Single-Mode CW Operation at Room Temperature

J. Faist et al., Appl. Phys. Lett.,1997

Improved Injector DFB Structure

Terahertz QCL Demo

R. Köhler et al., Nature,2002.

• Improvements in designs, growth and technology

• High wall-plug efficiency (27%) at 300K

• High output power • Low power (< 1 W) dissipation • QCL arrays





• Summary

Outline


Bandgap Energy vs. Lattice Constant for III-N Wurtzite Materials

• The most stable form of III-N materials is the wurtzite structure – Energy gap vs. “a-plane” lattice constant shows bowing effects


Gallium Nitride Lasers: From Ultra-Violet to Green Wavelengths

Semi-Polar 2021 Plane

λ = 530 nm, P > 100 mW

Sumitomo & Sony (2012)

Shuji Nakamura

(Nichia/UCSB)

S. Nakamura et al., Jpn. J. Appl. Phys., 1996

GaN Blue Diode Lasers and LEDs

• Quantum-well materials grown on c-plane (0001) GaN exhibit large built-in electric fields due to polarization discontinuity

• Use of nonpolar or semi-polar orientations provide improved electron-hole wavefunction overlap

Crystal Structure of GaN


Hybrid Silicon-InP Laser

p-InP n-InP

Passive Si-Waveguide

SiO2

Si Substrate

AlGaInAs MQW Active Region

“A Chip That Can Transfer Data Using Laser Light”

Published in the New York Times September 18, 2006

• Silicon waveguide + III-V active region

• Bonding between InP and Si samples performed using low-temperature oxygen plasma assisted wafer bonding

• Bonding performed prior to III-V material patterning

John Bowers UCSB

A. W. Fang et al., Opt. Exp., 2006

Power-vs-Current Characteristic

Potential approach for realizing lasers on silicon photonic platforms for optical interconnects


Obtaining Direct-Gap Optical Transition in Germanium (Ge)

<111>k

E

ΓL

0.80

0 eV

0.66

4 eV

(a)

<111>k

E

L

(b)

<111>k

E

L

(c)

electrons

Γ Γ

bulk Ge tensile strained i-Ge tensile strained n+ Ge

<111>k

E

ΓL

0.80

0 eV

0.66

4 eV

(a)

<111>k

E

L

(b)

<111>k

E

L

(c)

electrons

Γ Γ

bulk Ge tensile strained i-Ge tensile strained n+ Ge

Courtesy of Jurgen Michel, MIT

• Germanium (Ge) is pseudo-direct gap => ∆E between L and Γ valley is only 136 meV • Steps to achieve direct-gap optical transition in Ge:

– Add tensile strain: Ge fully relaxed at 650C growth temp; 0.2-0.3% tensile strained at room temp – Add heavy n-type doping (~1019 cm-3): Fill L-valley, scatter electrons to Γ-vally thermally

• Use novel device design to efficiently inject holes into n+ Ge

injected holes


Electrically Pumped Germanium (Ge) Laser

Courtesy of Jurgen Michel, MIT

0 10 20 30 40 50 60 70 80

1500 1550 1600

90 kA/cm2

Inte

nsity

(a.u

.)

0 10 20 30 40 50 60 70 80

1500 1550 1600 Wavelength (nm)

511 kA/cm2

Wavelength (nm)

Inte

nsity

(a.u

.)

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400

Current Density (kA/cm2)

Out

put P

ower

(mW

)

Ge Laser Cross-Section Emission Spectra

L-I Characteristic

• Use delta-doping + indiffusion of phosphorous to achieve high n-doping concentration

• Use heavily doped n+ and p+ silicon contacts => results in high optical losses (100-1000 cm-1)

• Device length ~ 300 µm • Lasing achieved at room temperature under pulsed

conditions (~50 µs @ 1000 Hz)

< 1.2 nm

Jth ~ 270 kA/cm2

POUT ~ 1 mW T = 300K


Large-Scale Integration of Semiconductor Lasers for Optical Communications

Monolithic InP Photonic Integrated Circuit (PIC) Transmitters and Receivers

100-Gb/s Transmitter Components • 10 xTunable DFB Lasers • 10 x 10-Gb/s EA Modulators • 10 x Variable Optical Attenuators • 10 x Optical Power Monitors • 1 x Wavelength Division Mux

D. F. Welch et al., IEEE JSTQE, 2007

Terabit Transmitter PIC

• 1.12 Tb/s Coherent PM-QPSK • 10 Wavelengths • > 450 Photonic Components

P. Evans et al., Opt. Exp, 2011


Thresholdless Nano-Laser

Ag/Al Alloy

M. Khajavikhan et al., Nature, 2012

InGaAsP Gain

Region

• Nano-scale, metal-clad coaxial structure supports only 1 mode (TEM-like) within the bandwidth of the optically pumped InGaAsP gain medium

• Evidence of “thresholdless” lasing: (i) no kink in L-I curve, (ii) Lorentzian lineshape over 105 pump-current increase, (iii) behavior or linewidth vs. pump power (i.e., no subthreshold narrowing, line narrowing only due to carrier pinning => no 1/P)


Spectral Coverage of Semiconductor Lasers

λ (µm)

f (THz)

QCLs

InGaN/GaN GaN/AlGaN

THz QCLs

InGaAsP

InGaAs/AlGaAs AlGaInP

InGaAsSb

PbS/PbSnSe/PbSnTe HgCdTe

GaAs

= Lasers and Laser Materials Developed at Lincoln Laboratory


Courtesy of Jim Coleman, University of Illinois

Applications of Semiconductor Lasers and LEDs

more than $7B in sales!


• NASA’s first space lasercom

• Space terminal to fly on Lunar Atmosphere and Dust Environment Explorer (LADEE) – 2013 launch

• Key demonstration objectives - 622 Mbps optical downlink - 20 Mbps optical uplink - 200–ps time-of-flight measurement

Lunar Laser Communication Demonstration (LLCD) Program

Contains 8 Semiconductor Lasers: • 1 Master Laser for Down-Link • 4 Pump Lasers for Down-Link EDFA • 2 Pump Lasers for Up-Link EDFA • 1 Laser for Uplink Self-Test

Courtesy of Bryan Robinson, MIT-LL


• Semiconductor lasers have gone through dramatic improvements and changes over the past 50 years through the contributions of many groups around the world

• Semiconductor lasers are having an increasing impact on many areas of technology – Ultra-high capacity optical networks – Optical interconnects for data centers, supercomputers, and

processors – Medical diagnostics and treatments – Industrial cutting and welding

• MIT Lincoln Laboratory has been actively engaged in the development and application of semiconductor lasers for 50 years

Summary


Back-Up Charts


• What are the properties of the optical gain? – How are the carriers injected and confined? – What is the relationship between carrier density and optical gain? – What is the spectral distribution of the gain?

• How is the optical field confined within the laser? – Spatial and spectral confinement – Overlap between optical field and the gain region (Γ)

• What are the loss mechanisms?

• How is the heat removed from the laser structure?

• What performance parameters are most critical for application? – Output power – Noise – Modulation bandwidth

Key Considerations for Semiconductor Laser Operation


A Few Words About Modes…

Transverse Optical Modes of a Cylindrical Waveguide

Longitudinal Optical Modes of a Fabry-Perot Cavity

M = 1

M = 2

M = 23

L

=2MMcnL

ν• Frequency of Mth mode:

• For λ0 = 1.55 µm, L = 10 mm, n = 3.5: => νM = 193.4 THz and M = 45161

• Solutions to 2D Maxwell’s equations w/ appropriate boundary conditions

Spatial Domain: Intensity Profile Frequency Domain: Single-Frequency Mode-Locking


Radiative and Non-Radiative Transitions


Interband Optical Gain

Energy Band Diagram Carrier Distributions

EFc = Quasi-Fermi energy for electrons EFv = Quasi-Fermi energy for holes

Population Inversion Condition (Gain > 0): EG < EPH < EFc - EFv

Bulk Material


Strained Semiconductor Materials Compressive Strain Tensile Strain

Unstrained Band Diagram

Strained Band Diagram • Material strain modifies the

band-structure of semiconductor materials

• Desirable impact on gain medium: –Strain reduces hole effective mass –Reduces density of states –Reduces carrier density required to

achieve population inversion

• Thickness of strained material limited by relaxation


Compound Semiconductor Material Growth

• Liquid-Phase Epitaxy (LPE)—Developed 1960-62 • Vapor-Phase Epitaxy (VPE)—Developed 1960-62

– Developed for Si, Ge, and III-V’s at about the same time – Halide VPE process (mostly at atmospheric pressure) – Hydride VPE process (mostly at atmospheric pressure) – Metalorganic chemical vapor deposition (MOCVD)—1968

• MOCVD is the name first used for this VPE process in the January 1968 APL paper by Manasevit in an original series of work demonstrating the growth III-V’s on sapphire—A.K.A.: o Metalorganic vapor-phase epitaxy (MOVPE) o Metal-alkyl vapor-phase epitaxy (MAVPE) o Organometallic vapor-phase epitaxy (OMVPE) o Organometallic chemical vapor deposition (OMCVD)

• Molecular-beam epitaxy (MBE)—Developed 1968

Courtesy of Russ Dupuis, Georgia Tech


Molecular-Beam Epitaxy (MBE)



Organometallic Vapor Phase Epitaxy (OMVPE)

Ref. C. Wang, Lincoln Laboratory Journal, 1990


OMVPE Reactors: Then and Now

Russ Dupuis

VEECO TurboDisc MOCVD System for Gallium-Nitride (GaN)

• 45 x 2”-diameter wafers • 12 x 4”-diameter wafers • 5 x 6”-diameter wafers

MOCVD at Rockwell International for GaAs/AlGaAs

1975

• Single 2”-diameter wafer Courtesy of Russ Dupuis, Georgia Tech

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