source of smart solutions ©2007 3s photonics s.a. all rights reserved. confidentiality notice: the...

Source of Smart Solutions

©2007 3S PHOTONICS S.A. All rights reserved. CONFIDENTIALITY NOTICE: The information contained in this presentation is 3S PHOTONICS confidential information. Any dissemination, distribution or copying of this presentation or disclosure of the information contained within by any unauthorized person is strictly prohibited.

Epitaxy : an enabling technology for confined optical

structures

Sept. 22nd, 2007

Maratea, Italy

F. Laruelle



2

OutlineOutline

IntroductionFrom liquid phase to vapor phase epitaxy :

• thermodynamic background

Atomistic view of interfaces• Focus on element III and element V

Low dimensional structures• Quantum well, wires and dots

Growth on non planar surface• Grating and butt-joint overgrowth

Conclusion



3

IntroductionIntroduction

“Materials are those solid state system who have plaid an essential role in the human civilization”

• Wood, Bronze, Iron, Concrete, Silver Emulsion for photography,

• Semiconductors leading to the Information Age

Recognized by the year 2000 Nobel Prize in Physics– « for basic work on information and communication technology »

– Z. Alferov & H. Kroemer : « for developing semiconductor heterostructures used in high-speed- and opto-electronics »

– J. Kilby : « for his part in the invention of the integrated circuit »



4

IntroductionIntroduction

“Semiconductors are materials whose properties are determined by their impurities”



5

DefinitionDefinition

From the greek : = on = orderThe science to preserve crystal order from the

substrate to the grown material



6

Band Gap EngineeringBand Gap Engineering

Ga-V/Al-V : very similar lattice parameter wrt to In-VBandgap Gap energies follow Al-Ga-In atomic radiusEpitaxy of III-V lattice mismatch and strain effectsOptical index to follow similar trends

0,0

1,0

2,0

3,0

0,53 0,55 0,57 0,59 0,61 0,63 0,65 0,67

Lattice Parameter (nm)

Ban

d g

ap E

ner

gy

(eV

)

GaAs

AlAs

InAsInSb

AlSb

GaSb

AlP

GaP

InP



7

Band Offset RulesBand Offset Rules

Defined for bands at identical point in the Brillouion zone (, X, L) Experimentally documented for simple materials pairs : GaAs/AlAs,

GaAs/GaInAs, ... Ga – Al - In / As - P : group III : CB offset, group V : VB offset

Growth axisB

an

d e

ne

rgy

Type I Type II

Valence band

Conduction band



8

Lattice Mismatch, Strain and StressLattice Mismatch, Strain and Stress

Lattice mismatch defined wrt to substrate :

al as ; a/a = (a l –a s) / a s

Pseudomorphic growth implies :

axy = as & az determined by elasticity theory

Strain defined wrt to bulk material & used to calculate stress:

xy = (a xy-al) / al a/a



9

Strain and Band StructureStrain and Band Structure

Strain decomposed in hydrostatic & shear componentsConduction Band : E = |S> , isotropicValence band : strongly directionalHH = 1/2 |X+iY> : in plane LH = 1/6 |X+iY>+ 2/ 3 |Z> : along growth axis

E

Compressive : al > as

HH

LH

axy < al , az > al

TETE

E

Tensile : al < as

HHLH

axy > al , az < al

TMTM



10

Crystal StructureCrystal Structure

Blende Zinc structure : 2 fcc sublattices : III and V[110] and [-110] axes : cleave directions

• Equivalent in bulk, not at the surface or in 2D

• [-110] refers to As surface dangling bond

• Rotate by 90º between top and bottom surfaces !

As

Ga

[-110]

[-110]



11

Wafer OrientationWafer Orientation

One single convention• Two flats : major and minor

• Major flat is used to align optical lithography wrt crystal plane typ. : [110] ± 0,05°

• Two possibilities :

CW

EU JPN

[110]

[-110]

CCW

US

[110]

[-110]



12

Thermodynamic BackgroundThermodynamic Background

Liquid-solid equilibrium for GaAs

Random alloy solution frame with xs = 1/2

Given T : low pressure solution x << 1/2

• Liquid rich in Ga, poor in As



13

III-V Thermodynamic AsymmetryIII-V Thermodynamic Asymmetry

High Temperature :

- Low pressure solution more favourable

- PAs<PGa excess Ga will not evaporate

- Excess As with very high pressure :

will evaporate in low pressure reactor

Low Temperature :

- Below Tcs, PAs<PGa

- Sublimation is congruent

PGa/GaAs

PAs/GaAs

Tcs

PAs/GaAs

PGa/Ga

PAs/As



14

III Alloys : Miscibility StatusIII Alloys : Miscibility Status

Follow mostly Vegard’s law with small but noticeable deviations

• Si/Ge, GaAs/AlAs, ...

In general for III-V : Free energy against miscibility, reinforced by lattice parameter difference

GaAs/AlAs : no miscibility gap

GaAs/InAs : weak alloy ordering

GaP/InP : strong alloy ordering



15

III Alloys : Miscibility StatusIII Alloys : Miscibility Status

Follow mostly Vegard’s law with small but noticeable deviations

• Si/Ge, GaAs/AlAs, ...

GaAs/AlAs = no alloy ordering

GaAs/InAs : weak ordering

GaP/InP : strong alloy ordering

GaInAsP/InP “forbidden zone”

• Depends on growth temp

• Metastable growth at the edge



16

V Alloys : Temperature SensitivityV Alloys : Temperature Sensitivity

High T : As replaced by P Low T : P replaced by As Due to element V volatility : xsolid/xgaz = f(T)

PAs/GaAs

PP/GaP

850ºC

600ºC



17

Doping : BackgroundDoping : Background

Unintentional or background doping determined by growth conditions, reactor design & source materials purity (8N Ga & As)

• |Na-Nd| ~ 1013 cm-3 in world record MBE GaAs

• |Na-Nd| ~ 1014 cm-3 in world record MOVPE GaAs

Acceptable background doping level determined by device performance

• Na-Nd ~ 1015 - 1016 cm-3 generally suitable for optoelectronic applications in undoped regions

• Na-Nd ~ 1/10 intentional doping level for doped layers



18

Doping : IntentionalDoping : Intentional

p-type and n-type shallow impurities in :

AlGaInAs & GaInAsP

All impurities exhibit a solubility limit :

n free_carrier as n impurities

Zn:InP 2 1018 cm-3, Zn:GaAs : 5 1019 cm-3 C : GaAs : >1 1020 cm-3 Si : GaAs : 4 1018 cm-3



19

Technologies : OverviewTechnologies : Overview

Toxic compounds : As and Be, AsH3, PH3, ...Inflammable or explosive substances : P, H2 , ...Cryogenic liquid : Liquid N2

SAFETY IS MANDATORY AND EXPENSIVE

Typical budget :• Equipement : 0.5 - 2.0 M€

• Infrastructure & safety : 0.5 - 1.0 M€

• Process development : 0.5 - 1.0 M€

• Total : 2.0 - 4.0 M€

• Yearly budget 0.25 - 1.0 M€



20

Technologies : Technologies : Molecular beam Epitaxy (MBE)Molecular beam Epitaxy (MBE)

Ballistic vapour phase transport for Knudsen cell to substrate

Ultra high vacuum : 10-10 Torr Evaporation of metals :

• Liquids : Ga, Al, In

• Solids : As, P, Be, Si



21

Technologies : MBETechnologies : MBE

Industrially mature :

• 9x4” or 4x6”

Advantage :

• Safety : closed system

• Floor space

Drawback :

• Downtimes due to high temperature bake-out



22

Technologies : Technologies : Metal Organic Vapour Phase Epitaxy (MOVPE)Metal Organic Vapour Phase Epitaxy (MOVPE)

Vapour phase transport and thermal decomposition in H2 flow

Primary vacuum : • 100 mbar

H2 charged with OM precursors or hydride gases

• Al-Ga-In-(CH3)3 , CBr4 ,...

• AsH3, PH3, SiH4 , H2S... Advantage :

• Primary vacuum

• Surface selectivity Drawback :

• Toxic gases

• Floor Space Industrially mature :

• 12x4” or 7x6”



23

Technologies : Hybrid TechniquesTechnologies : Hybrid Techniques

GSMBE Gas Source MBE:• III : MBE

• V : AsH3, PH3& thermal cracking

• Operating pressure : 10-5 Torr

• Outstanding composition uniformity of GaInAsP compounds wrt MOVPE

• Combined drawbacks of MOVPE & MBE

CBE : Chemical Beam Epitaxy• UHV analog of MOVPE

• III : Direct sublimation of precursors Al-Ga-In-(CH3)3 , CBr4 ,...

• V : AsH3, PH3 & thermal cracking

• Operating pressure : 10-5 Torr

• Combined drawbacks of MOVPE & MBE



24

Process ControlProcess Control

In situ• MBE : UHV allows Reflection High Energy Electron Diffraction

• Atomic scale growth process

• MOVPE : polarized reflectance spectroscopy

• Optical walength scale (100 nm) and interference pattern of vertical stack (10 nm)

• Mostly used for growth studies or calibration purposes Reactor Calibration

• In situ

• With calibration samples measured outside the growth chamber

• Reactor stability mandatory Wafer characterization

• Wafer parameters determined either by calibration or by characterization : PASS / FAIL basis.



25

Wafer CharacterizationWafer Characterization

Surface defects density : for further lithography process

Doping : to ensure E/O operation (mostly by calibration)

Photoluminescence : to ensure wavelength is OKX-Ray diffraction : to ensure strain is OK

X-Ray & PL Quaternary composition



26

Surface Control Surface Control & Particule Contamination& Particule Contamination

Optical microscope• Qualitative

• Defect attribution

– Growth conditions

– Reactor

– Substrate quality

– Chemical cleaning

– …

Optical diffusion• (SurfscanTM)

• Quantitative in arbitrary

• (equipment related) units

Haze (Roughness)Defects



27

Doping : Electrochemical Doping : Electrochemical CV Profiler (PolaronCV Profiler (PolaronTMTM))

Semiconductor-Electrolyte diode Biased to flat band :

• CV measure

• Doping determined

Biased to accumulate holes :

• At liquid / SC interface

• Etching p-type

• Etching with illumination n-type

Etch / CV sequences to establish

the doping profile



28

X-ray DiffractionX-ray Diffraction

Strained compensated Quaternary AlGaInAs MQW example : Fringe : MQW period & average strain (blue) Growth time : Qw & Barr thcikness Calibrated composition & Wavelength : QW & barr strains Checked by diffraction simulation software (red)

Tensile

Barrier

Compressive

QW



29

Photoluminescence : 300 KPhotoluminescence : 300 K

AlGaInAs 1,55µm MQW MOVPE

I pl

GaInAsP 1,55 µm MQW GSMBE

300 K PL mapping

300 K PL spectrum

WL, Intensity, FWHM



30

Others Others

Hall effect : fully destructive for the wafer• 300 K & 77 K

• Simple materials (InGaAs, GaAs, InP, AlGaAs) purity through carrier density and mobility

• New dopant source qualification

• …

SIMS : diagnostic & external tool• To get atomic profile (impurity diffusion, …)

• To track unintentional impurities

• To get a qualitative idea of contaminant at regrowth interfaces

• …



31

Low-Dimensional StructuresLow-Dimensional Structures

3D : bulk material : > 30- 10 nm2D : interface1D : quantum wires0D : quantum dots



32

III Interfaces : Al-Ga-In segregationIII Interfaces : Al-Ga-In segregation

Atomic scale process

Broken bond energy driven

Biggest atoms stay at the surface In - Ga – Al

Atomic abruptness cannot be reached except with clever tricks

GaAs

GaAs

AlAs

GaAs

AlAs

GaAs

AlGaAs

AlGaAs

GaAs

GaAs

AlAs



33

2D QWs and Interfaces2D QWs and Interfaces

Square well model : popular and simple mathsNor quite exact though…Element III segregation evidence

• TEM

• Xray

• Raman

• PL

• In situ RHEED

• …

TEM Image &

comp. reconstruction



34

V interfaces : As-P VolatilityV interfaces : As-P Volatility

InGaAs+P- / InGaAs-P+ or InP/InGaAs interface

Determined by As / P switching conditions & growth temperature

Interfacial layer with intermediate composition might be present seen in Xray diffraction span

As/P

InP

InAsP

InPInP

As/P

InAsP

InP

As/P



35

1D Quantum Wires : T approach1D Quantum Wires : T approach

Cleaved edge overgrowth (L. Pfeiffer AT&T Bell labs, WSI Garching, …)

MBE approach

Weak 1D confinement

No lithography!

For the fun of physics only



36

1D Quantum Wires : V grooves1D Quantum Wires : V grooves

Growth on different plane : growth rate & composition changes (E. Kapon, EPFL)

V shape wires by etching & MOVPE growth

No nanolithography



37

1D Quantum Wires : Organized 1D Quantum Wires : Organized Growth on Vicinal SurfacesGrowth on Vicinal Surfaces

Smooth atomic surface Crystal miscut induce step arrays P=a/ Nanoscale template without lithography

0,5° step period 32 nm1°step period 16 nm



38

1D Quantum Wires : Organized 1D Quantum Wires : Organized Growth on Vicinal SurfacesGrowth on Vicinal Surfaces

Fractional layer growth & segragation : step induced alloy ordering dense array of coupled quantum wires

Demonstrated by MOVPE & MBE



39

0D : Quantum Dots 0D : Quantum Dots

P. Petroff, A. Lorke, and A. Imamoglu. Epitaxially self-assembled quantum dots. Physics Today, May 2001.

Relief of strain energy by creating more surface Growth mode transition

• From layer by layer mode

• To three dimensional

Capping process critical

• Cristalline quality preserved



40

OvergrowthOvergrowth

Aim : to add functionality after 1st epi and wafer processing :• Stripe overgrowth : to confine current injection

• Grating overgrowth : to select wavelength by DFB action in the FP cavity

• Butt-joint overgrowth : to integrate to different active or guiding layer ( bulk quaternary or MQW sections laser & modulator)



41

Stripe overgrowthStripe overgrowth

P/n BH : thyristor effect to force current flow through MQW mesa

AuSn

p contact

Zn doped InP MOVPE cladding 3rd epi step

S doped InP selective MOVPE 2nd epi step blocking layer

Zn doped InP selective MOVPE 2nd epi step

n (Si) InGaAsP (GSMBE) 1st epi step

n (Sn or S) InP substrate

RIE etched mesa

InGaAsP MQWs



42

Grating OvergrowthGrating Overgrowth

Mixed III-V compound InP and InGaAsPExposed to a mixed As/P gas phase during annealing

EPI AsH3 flow Grating overgrowth

1

2

3

4

InP

InGaAsP

InP

As InGaAsP



43

Butt-Joint OvergrowthButt-Joint Overgrowth

SEM & selective etching cross section

MODULATOR

MQW

LASER

MQW

GRATING



44

Butt-Joint OvergrowthButt-Joint Overgrowth

2 MQW section offset by 57 nm Section length typ. 300µm 1 MQW section : strained QW laser 1 MQW section : electro absorption modulateur (QCSE)

Modulator WL

x4

Laser WL

x4



45

Conclusion Conclusion

This technology is

• Expensive but Enabling new applications :

– High speed, High power Micro- and Opto- Electronics

• Involved in large volume consumer electronic markets :

– CD players

– DVD players

– Mobile phones

– Low consumption lightning

• A good example : from the LAB to the FAB

Key issue :

• Reliability at affordable price : a long way to go

• No failure in 3-4 years : MTTF > 50 000 hrs !



46

Conclusion Conclusion

More to come from the LAB to the FAB

Based on new physical concepts

On new organisational schemes aimed at reducing innovation costs :• Academic lab : Universities, CNR, CNRS, ...

• Industrial R&D centers : Fraunhofer Inst., III-V lab, ...

• Industrial and commercial : OSRAM, BOOKHAM, 3S PHOTONICS, ...



47

ConclusionConclusion

Only part of the story to get a real optoelectronic device such as this one :



48

Credits & ThanksCredits & Thanks

B. Etienne, R.Planel, F.Mollot, L. Goldstein, F. Alexandre, J. Decobert, H. Sik, G. Beuchet, V. Cargemel, I. Sagnes, J.-P. Hirtz, J.-M. Gérard, J. Massies, M. Quillec, J.-C. Harmand.

GB Stringfellow Academic Press Hermann & Sitter Springer

A Ourmazd AT&T Bell labs A. Ponchet J.-M. Moison F. Lelarge P.M. Petroff E. Kapon T. Fukui Riber, Aixtron.

Special tahnks to the organizers of this school

source of smart solutions ©2007 3s photonics s.a. all rights reserved. confidentiality notice: the...

Documents

information age

source of smart solutions

confidentiality notice

unauthorized person

laruelle slide

grown material slide

integrated circuit slide

introduction semiconductors