InAs/GaAs quantum dots foroptoelectronics devices

Vittorianna Tasco

National Nanotechnology Laboratory, Istituto di National Nanotechnology Laboratory, Istituto di

Nanoscienze-CNR, Lecce, Italy

XCVI Congresso Nazionale

Bologna, 20 - 24 Settembre, 2010 1

OutlineOutline

• Physical properties of semiconductor QDs

• Device applications of QD systems

• InAs/GaAs QDs

• Synthesis of InAs/GaAs QDs by epitaxial growth

• Structural properties of InAs/GaAs QDs• Structural properties of InAs/GaAs QDs

• Carrier dynamics in InAs/GaAs QDs

• Performances of QD laser

• Conclusions

SIF-XCVI Congresso Nazionale

Bologna, 20 - 24 Settembre, 2010

10 nm

2

Display and Lightning technologylooking for bright, high-resolution,

multi-color or white emitting light sources

Development of:

- cheap

- spectrally tunable LIGHT LIGHT

Semiconductor Quantum Dot Based Semiconductor Quantum Dot Based Photonic DevicesPhotonic Devices

Telecommunications and information technology demand

fast and reliable data exchange on wide-world area networks

Development of optical data transmission systems requiring:

- low-cost

- spectrally pureLASER LASER

SOURCES & SOURCES &

Pioneering studies on spintronics, quantum computing

and non-classical photon sources

Development of:

- reliable

- spectrally ultra-pureSINGLE SINGLE PHOTON PHOTON

- spectrally tunable

- long-lasting

- bright

- controlled at micron-scale

LIGHT LIGHT SOURCESSOURCES

- spectrally pure

- low-power operating

- temperature insensitive

- fast modulating

-Wide operation band

SOURCES & SOURCES & AMPLIFIERSAMPLIFIERS

- spectrally ultra-pure

- long-lasting

- RT working

PHOTON PHOTON SOURCES & SOURCES & OPTICAL OPTICAL MEMORIESMEMORIES

3

Quantum Quantum dotsdots asas a a breakthroughbreakthrough in in

optoelectronicsoptoelectronics

Appl. Phys. Lett. 40, 939 (1982)4

QD Electronic structureQD Electronic structure

2 22 2 21 2 32

9( ) 1,2,3

2 2n gr

E E n n n nm L

π= + + + =h

E

Quantized Energy levels

Quasi zero-dimensional systems

Fewer discrete states depending on QD size, shape and material

Electron Quantization Energies ≥ kBT

Limited number of trapped carriers

rL ~λDB

5

PropertiesProperties ofof quantum dot quantum dot laserslasers

)(1

)(0

0 i

n

izyxD EE

LLLE −∑=

=δρ

Advantages:

• Low threshold current=>low consumption

• High gain and high-frequency operation

• Temperature insensitive

• Reduced CHIRP

Asada et al., J QE 22, 1915(1986)

6

QD QD SemiconductorSemiconductor Optical Optical

AmplifiersAmplifiers (SOA)(SOA)

� Inhomogeneous broadening leading

to broad gain spectrum and wide band

amplification ( > 100nm)

� High saturation power

� High speed response (>10Gbit/s)

� Compact devices� Compact devices

� Low cost

� Low power consumption

Gain properties must show polarization independence

7M. Sugawara et al., Meas. Sci. Technol. 13 (2002) 1683–1691

M. Sugawara et al., PHYSICAL REVIEW B 69, 235332 (2004)

QDs for PhotovoltaicsQDs for Photovoltaics

o Intermediate Band (IB) solar cells

• IB arising from QD confined states

• Photocurrent increases while photovoltage is the

difference between CB and VB quasi Fermi levels

• QDs ideally isolate the IB from the CB through a

true zero density of states

• QDs exhibit longer carrier lifetime than QWs

A. Luque, A. Martì, Phys. Rev. Lett. , 1997, 78, 5014

A. Luque, A. Martì, Adv. Mater., 2010, 22, 160-174

o Hot Carrier Solar Cells

o Quantum confinement provided by QD strongly modifies carrier

relaxation dynamics, via an important reduction of hot carrier cooling

rates

o Enhanced photocurrent, with hot carriers generating a second e-/h+

pair through impact ionization (multiple exciton generation or inverse

Auger effect)

o Enhanced photovoltage, collecting hot carriers before they cool

Nozik A.J., Physica E 14 (2002) 115-1208

Self assembled Quantum DotsSelf assembled Quantum DotsEpitaxial growth ((Bottom-up approach)

Growth mode

substrate

epilayer

nk-van der Merwe

(layer by layer)

Key parameters:�Strain energy�Surface energy

substrate

islands

substrate

Wetting layer

Volmer-Weber (no wetting)

Stranski-Krastanov

(strain-driven) Different material combinations:InAs/GaAs, InAs/InP, GaSb/GaAs, Si/Ge…

Lattice parameter (Å)

9

Optical Telecommunication SystemsOptical Telecommunication Systems

In(Ga)As/GaAs QDs as active medium for high

Fibre to the Home networks

Gigabit local area networks

In(Ga)As/GaAs QDs as active medium for high

performance devices at telecom wavelengths (1.3 µm)

10

Material composition + growth conditions determines:

• Size

• Shape

Material issues for QD devicesMaterial issues for QD devices

MOCVDMBE

• Shape

• Orientation

• Strain

� Controlled density between 1x109 cm-2 and 1x1011 cm-2

�� Size uniformity: lateral < 10% and vertical < 5%Size uniformity: lateral < 10% and vertical < 5%

�� Control of wavelength emission (composition, shape, size)Control of wavelength emission (composition, shape, size)11

Material gain gm ∝∝∝∝ DOSQD(E) ≈ ρρρρQD/∆∆∆∆Einh

1 QD layer

3x1010 ÷ 1x1011 cm-2

1) Increase of single layer density

2) Stacking of multiple QD layers

QD densityQD densityWith: ρρρρQD⇒⇒⇒⇒ area dot density

∆∆∆∆Einh ⇒⇒⇒⇒ Inhomogeneous

broadening (∼20 meV @RT)

Decreasing density

with stacking

⇓⇓⇓⇓

• Sublinear increase of

optical efficiency

• Gain Saturation

Size inhomogenity

⇓⇓⇓⇓

Increased FWHM and lower efficiency

12

2 2

2

9

2 2 r

E E n n n nm L

π= + + + =hEn=Eg+

QD emission wavelength tuning QD emission wavelength tuning

Growth and Overgrowth Dynamics

Different shape due to growth technique:

�Truncated pyramid by MOCVD

�Lens shape by MBE

A.Passaseo et al., Appl. Phys. Lett. 82, 3632 (2003)

Low RV/III

Delayed GaAs growth on of islands top

High RV/III

GaAs growth directly on top of islands

1000 1100 1200 1300 1400

T=300 K

Wavelength (nm)

PL I

nten

sity

(a.

u.)

RV/III

A. Passaseo et al., Appl. Phys. Lett. 84, 1868 (2004)13

GaAs

Optical properties of Optical properties of InAsInAs//GaAsGaAs QDsQDs

GaAs

GaAs InAs QDs with strain reducing layer

InGaAs QW for emission wavelength

QW N1 N2

14

Carrier relaxation

GaAs

GaAs

QW

N=2

N=1

PL

Inte

nsity

0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5Energy (eV) IPL(t)=A·[1-exp(-t/tR)] · exp(-t/tD)

300K Rise Time (ps) Decay Time (ps)

QD StateQD State--Filling Filling Mechanism Mechanism

Time resolved PL with

subpicosecond resolution

-5 0 5 10 15 20 25 30 35 40 45

Time (ps)

N=2

N=1

QW

Nor

mal

ized

Inte

nsity GaAs 1.4 ± 0.6 4

QW 6.1 ± 0.3 24

N=2 6.3 ± 0.2 80

N=1 5.9 ± 0.2 227

�No sequential PL rise starting from the top QD energy state occurs

�Fast relaxation occurring through a finite continuum density of states

� Crossed transitions 0D-2DG. Rainò et al.., Appl. Phys. Lett. 90, 111907 (2007)

15

RT

Waveguide Planar absorption measurements

20

25

30

N=2

300 K

abs-H abs-V

Inte

nsity

WLQD

G. Visimberga et al., APPLI. PHYS. LETT. 93, 151112 2008

QD StateQD State--filling filling Mechanism Mechanism

0.88 0.96 1.04 1.12 1.20 1.280

5

10

15N=2

N=1

Inte

nsity

Energy (eV)

V

H

bound-to-

bound

WL-to-WL

bound-to-WL

WL-to-bound

� In-plane polarized light much more absorbed by QDs as an effect of flat lens shape

and quasi-biaxial compressive strain (heavy-hole like QD transition)

� Broad smooth absorption background attributed to crossed 2D-0D transitions 16

Vertically Vertically stacked stacked QDs QDs

Material gain gm ∝∝∝∝ DOSQD((((E) ≈ ρρρρQD/∆∆∆∆Einh

Proper spacer thickness to balance for

modal gain(confinement factor) and

electronic coupling

g002

40nm-thick barrier

20 nm5nm-thick barrier

20 nm

Lens-shaped QDs with flat bottom, base length of ~15 nm, height of ~5 nm, randomly distributed within each layer

Perfect vertical alignment triggered by the strain field of the buried dots of the underlying layer.

17

29

0.9 1.0 1.1 1.2 1.3

0.01

0.1

1

15 meV

15 nm 5 nm

75K

30 meV

PL

Inte

nsity

(a.

u)

Energy (eV)

Vertically Vertically stacked stacked QDs QDs cw Photoluminescence

0 10 20 30 40 50 60 70400

450

500

550

600

650

700

750

Dec

ay T

ime

(ps)

Barrier thickness (nm)0 100 200 300 400 500 600 700 800 900

1

5 nm 15 nm 25 nm 65 nm

75 K

PL

Inte

nsity

(a.

u)

Time (ps)

TRPL measurements @ 75K

0 10 20 30 40 50 60 70

23

24

25

26

27

28

29

FW

HM

(m

eV)

Barrier thickness (nm)

1.030

1.035

1.040

1.045

1.050

Energy (eV

)

• FWHM almost constant for samples with thick barrier with an increase for 5nm spacer layer due to QD molecule antibonding state

• GS Blueshift due to lower electron-hole wavefunction overlap , strain, intermixing…

Time (ps)

radiative decay time ~ 430 ps for thick barrierlonger exciton radiative lifetime ( 750 ps) for the thinnest spacer => electron-heavy hole wavefunctionoverlap reduction.

G. Rainò et al., J. APPL. PHYS., 103, 9, 096107 (2008)

18

Linearly polarized PL spectra at RT taken from cleaved sides of the three-fold stacked QDs (65nm and 5 nm barrier thickness)

Vertically Vertically coupled QDs coupled QDs

0.6

0.8

1.0 60

90

120 TM/TEuncoupled

= 0.4TM/TE

coupled= 0.69

Nor

mal

ized

PL

Inte

nsity

(a.

u)

65nm-thick barrier (ρ=43%) 5nm-thick barrier (ρ=18%)

Doubling of the Degree of linear polarization

L. Fortunato et al., SUPERLATT. AND MICROSTRUCT., 47,

1, pp. 72-77 (2010)

0.0

0.2

0.4

0.6

0

30150

180

210

240

270

300

330

0.0

0.2

0.4

0.6

0.8

1.0

N

orm

aliz

ed P

L In

tens

ity (

a.u)

QD multistacking with thin spacer

modifies strain symmetry

20 nm

19

0.0

5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

3.0x104

3.5x104

300K

3 layers 5 layers 7 layers

LAP

L in

tens

ity (

a.u)

QD multistacking with thick spacer

1.2

70K

PL

Inte

nsity

(a.

u)

3 layers 5 layers 7 layers

Vertically Vertically stacked stacked QDs QDs

0.9 1.0 1.1 1.2 1.30.0

Energy (eV)

2 3 4 5 6 70.0

5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

3.0x104

3.5x104

4.0x104

PL

Inte

nsity

(a.

u)

Number of QD layers

0 100 200 300 400 500 600 700 800 9000.0

0.6

PL

Inte

nsity

(a.

u)

Time (ps)

Radiative decay time of about 450 ps for all the

samples

Linear increase in PL intensity :

No saturation 20

Vertically Vertically stacked QDsstacked QDs

TEM analysis by LADIATEM analysis by LADIA

• QW with sharp interfaces• Strain field: confined to well width

Bright-field TEM image: strain + material contrast

[001] 50 nm

First QD

layer

5.5 nm

10.9 nm6.7 nm

10.4 nm

5.4 nm

7.5 nm

7th QD

layer21

Broad area QD Broad area QD laserslasersStructure and device fabrication

p+ -GaAs

p –AlGaAs

cladding

n –AlGaAs

cladding

n+ -GaAs

� 1-7 InAs/In0.18Ga0.82As QD layers

n+ -GaAs

� Stripe geometry devices

� Cavity length (100 µm – 4 mm)

� Tested in pulsed regime

1000 1100 1200 1300 14000,0

5,0x102

1,0x103

1,5x103

2,0x103

2,5x103

3,0x103

3,5x103

PL

inte

nsity

(a.

u)

Wavelength (nm)

29 meV

Room temperature

Photoluminescence and lasing from a structure containing 7 stacked QDs layers

[A.Salhi et al., Journal of Applied Physics, 100, 123111 (2006)]22

Broad area QD Broad area QD laserslasers

5

10

15

20

25

30

35

40

45

7 layers

5 layers

3 layers

M

odal

gai

n at

thre

shol

d (c

m-1)

RT

2 3 4 5 6 7 80

5

10

15

20

25

30

35

40

45

50

Sat

urat

ion

Mod

al G

ain

(cm

-1)

Number of QD layers

RT1.3 µm

Variation of gain with the number of QD layers

0 200 400 600 800 10000

5

Mod

al g

ain

at th

resh

old

Threshold current density (A/cm2)

0 20 40 60 80 100 1200

1

2

3

4

5

6

7

8

9

Nor

mal

ized

mod

al g

ain

(cm

-1)

Threshold current density (Acm-2)

gsat

=6 cm-1

Jtr= 10 Acm-2

Jtr = e Ns/ττττr in which τr is the radiative lifetime

∼∼∼∼ 0.5nsNs the QDs surface density ∼3.2×1010 cm -2

Modal Gain = gsat[1-exp(-γ (Jth-Jtr)/Jtr)]

A. Salhi et al., IEEE Photonics Technol. Lett. 18, 1735 (2006)

� Lasing from the ground state for 360 µm-long cavity

� 1/ηd vs L: ηi=67% and αi=8 cm-1

� Saturation modal gain ~ 42 cm-1 (~ 6 cm-1 per QD layer)

23

Single mode Single mode QD QD laserslasersStructure and device fabrication

p+ -GaAs

p –AlGaAs

cladding

n –AlGaAs

cladding

n+ -GaAs

� InAs/In0.18Ga0,82As QD layers

� 6 stacked layers

� Al0.7Ga0.3As cladding layers

� 40 nm-thick GaAs barriers

� 55 nm-thick GaAs spacer

� 500 nm-3.5 µm wide narrow stripe reverse mesas n+ -GaAsstripe reverse mesas

� Reverse mesas:selective wet etching of GaAs and AlGaAs layers

� Au 2 µm-thick on the top contact by electrodeposition

� 600 µm-long cavities

� As cleaved and HR (50%)/HR (80%) coated devices

24

Single mode QD lasers Single mode QD lasers Static characteristics

� internal losses of 4.8 cm-1

� internal quantum efficiency of 23%

� GS saturation modal gain of 36.3 cm-1 � 6 cm-1 per

QD layer

� P vs I in 15°C-85°C range

� Threshold current of 7.52 mA @ 15°C

� Maximum power around 15 mW@ 80mA

� T0= 109.4 K in the whole T range

25

QD Laser QD Laser Performances Performances

High Frequency modulation

40 GHzModulation

Γ = 20 - 30 meV

Γ< 10 meV

10 GHzModulation

Per

form

ance

15°C 85°C

Sugawara M., Semiconductors and Semimetals, vol 60 (1999)

M. T. Todaro et al., IEEE Phot. Technol. Lett. 19, 191 (2007)

0.1 1 10 100 1000

Γ = 20 - 30 meVHigh Power

High efficiency

Per

form

ance

Relaxation Lifetime (ps)

@ 5 Gb/s Extinction ratio of 4.8 dB @ 15°C and 6 dB @ 85°C

15°C 50°C

@10 Gb/s Extinction ratio of 4 dB @ 15°C and 4.5 dB @ 85°C

26

Conclusions

• Growth dynamics to achieve high density and narrow size

dispersion

• Carrier dynamics investigation showing fast capture and

relaxation processes

High, controllable gain with linear dependence on QD stacked• High, controllable gain with linear dependence on QD stacked

number

• Frequency modulation up to 10 Gbit/s

• Temperature insensitive lasing (T0=110K)

27