QWIUnivLinz.pdf

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Quantum Well Intermixing and Its Applications

Shu YuanSchool of Materials Engineering

Nanyang Technological University

Singapore

Thanks to the following friends for providing some slides:

Dr. B. S. Ooi, Phosistors Inc., CA, USA

Dr. C. Jagadish & Dr. L. Fu, Australian Natl Univ.

Dr. S. F. Yu, NTU & late Dr. E. H. Li (Hong Kong Univ.)


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Contents

Introduction

Techniques for QW intermixing Device applications

Conclusions


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IntroductionIntroductionIntroduction

What is QW intermixing?

A non-square QW produced by

thermal induced interdiffusion ofconstituent atoms through the QWheterointerface


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Quantum Well Intermixing

before

1

2

after

1

2


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Column III self diffusion through a Column III

interstitial mechanism.Ga

As Ga

As

Ga

As Ga

As

Ga

As Ga

As

Ga

As Ga

As

Ga

As Ga

As

Ga

As

As

Ga

As Ga

As

Ga

As Ga

As

Al

As Al

As

Al

As VAl

As

Al

As Al

As

Al

As Al

As

Al

As Al

As

Al

As Al

As

Al

As Al

As

Al

As Al

As

GaAs AlAs

IGa

IAlVGa


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As atom

Ga atom

Al atom

interface

GaAs AlAsBefore QWI


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AlxGa1-xAs AlyGa1-yAsAfter QWI

As atom

Ga atom

Al atom


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Quantum Well Intermixing Techniques

Impurity Induced Intermixing e.g., Zn diffusion induced intermixing

Impurity Free Intermixing

e.g., intermixing induced by SiO2 cap

Laser Induced Intermixing

Especially for InGaAsP/InP QW structures


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QWI techniquesHow to produce QWI?

By slow (hrs) / rapid (sec-min) thermal

annealing (Group III atoms self-diffusion

with the help of Group III interstitial andvacancy diffusion)

By controlled interdiffusion using impurity or

impurity-free / vacancy disordering

Laser-assisted disordering


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Thermal Annealing

(n-Sample, p-Sample, SI sample PL results)Transfer energy to crystal latticeTransfer energy to crystal lattice increase defect migration and enhanced defect formationincrease defect migration and enhanced defect formation

700 720 740 780

0

3

6

9

12

As-grown

N-sample (on Si-doped GaAs)

P-Sample(on Zn-doped GaAs)

SI-Sample (on semi-insulating GaAs)

RelativePLIntensity(a.u.)

Wavelength (nm)

RTA@880C

760 800

50nm Al0.24Ga0.76As (undoped)

100nm GaAs (undoped)

4nm GaAs (undoped)

50nm Al0.24Ga0.76As (undoped)

200nm GaAs buffer (undo ed

(001) GaAs substrate


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Rapid thermal Annealing of GaAs/AlGaAsquantum wells

0 30 60 90 120 150

0

20

40

60

80

100

120

N-Sample

P-Sample

SI-Sample

PLEnergyShift(meV)

RTA Time (s)

RTA @ 876 C

180


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Diffusion equations in quantum well intermixing

The group III atom profile W across the quantum well can

be described as:

where z is the displacement in the growthdirection and is centered at the quantum wellcenter, Lz is the well width, Ld = sqrt (Dt) is thediffusion length, D is the diffusioncoefficient, and t is the diffusion time.

Al profile of a GaAs/AlGaAsquantum well before and afterintermixing.

S. Yuan et al, J. Appl. Phys., Vol. 83, No. 3, 1 February 1998


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Impurity Induced Intermixing

Zn or Si diffusion enhanced QWI


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Impurity Induced IntermixingMechanism

Zn induced intermixing:

Column III interstitial is generated directly by an Interstitial Zn moving into a Column IIIlattice site through a kick-out mechanism:

++++ ++ hIZnZn IIIIIII 2

Si induced intermixing:

The donor binds with the Group III vacancy to form a

complex and they move together.

+GaSi

IIIV

+GaSi( )

IIIV

The group III vacancy and interstitial diffusion causes Column III atoms (like Ga, Al, In)to diffuse through the sample.


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Ion implantation Induced Intermixing

Vacancy

Interstitial

Ion implantation

Point defects:

Intermixing

QW

(1) Ion implantation(2) Rapid thermal annealing


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Impurity induced intermixing: ion implantation

Ion energies from a few keV to several MeV

Implant doses from 1010 to more than 1016

ions/cm2

Great combinations of ions and substratespossible, e.g.

p-type ion (Zn, Be) n-type ion (Si)

neutral-type ion (O)

constituent ion (Al, Ga, As) on AlGaAs/GaAs


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Ion implantation Induced Intermixing

40-keV proton implantation profile in a 4-well

multiple quantum well structure.

Photoluminescence photon energy

shift as a function of irradiation dose.RTA: 900C for 30s.

H. H. Tan, C. Jagadish et alAppl. Phys. Lett. 68, 2401(1995)


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Lateral Selectivity of QW Intermixing (Ion

implantation induced intermixing)

MOCVD grown

InGaAs/InGaAsP unstrained laserstructure

P+ at 1 MeV with 1014 cm-2 dose

at 200 C prior to implantation, a 2.0m

SiO2 layer as a selective area mask

5m mask stripe width giving a

lateral selectivity of 2.5 mJ.Vac. Sci. Tech. A, vol.16, no.2 (1998)


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Impurity free induced intermixing

(a) As-grown GaAs/AlAs QW

(b) A SiO2 cap layer is depositedon top of the sample. The sampleis then annealed. Ga atoms aresocked into the SiO2 cap, leavingbehind some Ga vacancies, thuspromoting interdiffusionbetween Al and Ga atoms.


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Selective area QWI

Degree of intermixing is in some way proportional to the area ofcontact between the sample and the SiO2 cap.

One-step lithography (e-beam), lift-off, and one-step RTP.

B.S. Ooi, et.al IEEE. J. Quantum Electron. , 33 (10), pp1784-1793, (1997)

L ower cladding AlGaAs

1m SrF2 mask

SiO2 capp ing layer

Up per cladd ingAl GaAs

GaAs D QW

Intensity(arb.

units)

830 840 850 860 870

Wavelength (nm)

0% 15% 25% 50% As-grown

SiO2: promotes intermixingSrF2: surpresses intermixing

SrF2 area percentage

Vacancy diffusion front


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Impurity free intermixing(Pulsed anodic oxide induced intermixing)

Pulsed anodic oxidation set-up


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Pulsed anodic oxide induced intermixing

Effects of the distance between the oxide andthe quantum well


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Impurity free intermixing application toquantum wire photoluminescence(Pulsed anodic oxide induced intermixing)

Quantum well intermixing in the side walls enhances the lateral confinementof electrons in the quantum wire, resulting in the observation of PL signal from thequantum wire (QWR).


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Laser induced intermixing

Quantum Well Intermixing

Bandgap increases as intermixing proceeds

h

Eg disordered

hhh h

Eg starting

h


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Laser induced intermixing methods:

Direct laser radiation: thermal intermixing (poorspatial resolution ~ 100 micron )

Two step Photo-absorption induced intermixing:

(1) Laser radiation at low power and low temperature togenerate defects

(2) High temperature annealing.

(better spatial resolution ~20 micron)


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Laser induced intermixing: experimental set-up

Operation: AlGaAS-GaAs as example

Encapsulation layer Si3N4 (90nm) Light source Ar+ laser beam,

= 488nm

scan speed 85 m / s

Setup consists of computer-controlled x-y table and a laser

Appl. Phys. Lett. 52, 1371 (1988)


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Laser induced intermixing (pulsed photon-absorptionassisted interdiffusion, PPAID)

Power-current characteristics of three lasersAs-grown, control (annealed, without pulsedlaser exposure), and PPAID lasers.

Emission spectrum of these threelasers.

B. S. Ooi et al, IEEE Photonics Technology Letters, 9, 587(1997)


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Quantum Well Intermixing Applications

Laser diodes: non-absorbing mirrors (windows)

Laterial confinement lasers

Photonic Integration


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Nonabsorbing mirror laser diodes

Ti/Aumetalcontact

SiO 2insulationlayer

MQWInGaAs-InGaAsP

Au/Ge/Au/Ni/Au

backcontact

QWI in a region near the laser facet (window/mirror)

to increase the band gap of that region, making thatregion transparent to the laser beam, reducing the light absorptionand reducing facet temperature==> higher light output and better reliability

Selected area QWI


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Lateral confinement in laser diodes

Lateral electrical and optical confinements can both be enhanced by QWI in the regionsoutside of the ridge wave guide.

When this region is intermixing, the effective band gap is increased, and effective

refractive index is reduced.


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Lateral confinement in laser diodes

Lateral leakage current is reduced by impurity induced intermixing (IID).S. Y. Hu et al, PTL 7, 712(1995)


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Photonic Integration Laser action occurs just above the band-edge

i.e. passive section is highly absorbing at lasing wavelength

Solution: modify the bandgap energy at the passive region

Laser Waveguide couplerModulator

Absorpt

ion

QWs

Laser

Energy

modulatorwaveguide


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Photonic integration by using QW Intermixing

Excellent alignment of active and passive waveguides

Reflection at the joint can be negligible (~10-6) Mode matching is intrinsic to the process

Low-loss waveguideQuantum well amplifier


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Extended Cavity Lasers Two-section Integration

Active cavity = 500 m

Passive cavity lengths = 0 600 m

Activ

elaser

sectio

n

Passiv

e

waveg

uides

ection


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Multiple wavelength laser array

p-type contact

Oxide isolation

QW

n-type contact

Substrate

un-implanted dose A dose B

1

1

> 2

> 3

dose A < dose B

2 3

Ion implantation


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Multiple wavelength laser array (results)

< 20% increase in Jth, and


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Multiple Wavelength Integration Capability

Multiple bandgaps monolithically integrated on a single chip

Wavelength spans over entire C-band

1250 1300 1350 1400 1450 15000.000

0.001

0.002

0.003

0.004

0.005

FWHM=113 nm

Enveloped curve

PL

intensi

ty

(a.u.)

Wavelength (nm)

Confidential


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Multiple-Bandgap Integration

Ion-Implantation Induced Disordering

One-step implantation, one-step RTP, multiple-step oflithography and dry etching.

E.S. Koteles, et. al, IEEE Selected Topics in Quantum Electron, 1998

Semiconductor wafer

SiO2


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One-Step Multiple-Bandgap Integration

One photolithography step to achieve different thicknessof resist.

One dry etching step to create different thickness of SiO2

implant mask. One implantation and RTP step to create multiple bandgap

across a laser chip.

B.S. Ooi, et. al, Multiple Bandgap Photonic Integration, PTC Patent pending, 1999

Semiconductor wafer Semiconductor wafer

SiO2SiO2

Photoresist


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Monolithic Multiple Wavelength Lasers

.

Isolation

Lasersection

Multiple wavelength lasers (10-channel) fabricated using theone-step postgrowth bandgap engineering technique.

B.S. Ooi, et. al, A technique for fabricating WDM laser sources, PTC Patent pending, 2000


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Monolithic Multiple Wavelength Lasers

1470 1480 1490 1500 1510 1520 1530 1540 1550 1560

Intensity

(a.u.)

Wavelength (nm)

1 2 3 4 5 6 7 8 9 101460

1480

1500

1520

1540

1560

Lasingwavelength(nm)

Channel number

Lasing wavelength

0.0

0.2

0.4

0.6

0.8SiO2thickness(nm)

SiO2 thickness

A correlation between SiO2 thickness and the emission has been observed.

17% increase in Jth, i.e. from 1.2 kA/cm2 (channel 1) to 1.4

kA/cm2 (channel 10).

Only small change in slope efficiency has been observed.


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Tuning the wavelength of QWIP

Metal contact

Multi-QWsMetal contact

Bottom contact

Substrate

un-implanted dose A < dose B

1

23

1 < 2 < 3

Top contact


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Conclusions

Impurity induced intermixing

Impurity free vacancy induced intermixing

Laser induced intermixing

Improving device performance

Photonic integration

Quantum well intermixing: post-growth modification of band gap

==> usually larger band gap (0-200meV) andlower refractive index (0-5% change)

Its applications

Vielendank !

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QWIUnivLinz.pdf