Upload
others
View
0
Download
0
Embed Size (px)
Citation preview
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