QD
Quantum dot photonic
nanodevices
Andrea Fiore
Ecole Polytechnique Fédérale de Lausanne
Quantum Devices group at EPFL:
B. Alloing, G. Buchs, M. Gobet, L. Li, C. Monat,
C. Paranthoen, C. Zinoni, V. Zwiller
Funding: Swiss National Science Foundation
Outline
QD
electron
photon
Q
Q
Q
Single-photon emitters
Practical issues for single-photon emitters:
X Wavelength & density: Growth of QDs at 1300 nm
X Electrical injection: Nanosized QD LEDs
X Controlling the optical density of states
Conclusions
Andrea Fiore
N. photons
Single photon emitters
Single-φ
emitter
QD
2
1
time
Application: Quantum cryptography
Bob
Alice
01011...
Single-φ
emitter
01011...
BB84
quantum key
distribution
Eve
Andrea Fiore
A close look at single
photons
QD
0,4
0,35
0,3
0,25
0,2
0,15
0,1
0,05
0
<n> =10
1
<n> =1
0,8
P(n)
σ n2 = n
P(n)
"Classical" light sources (e.g. a laser) are Poissonian:
0,6
0,4
0,2
0
0 2 4 6 8 10 12 14 16 18
n
0 1 2 3 4 5 6 7 8 9 10
n
"Nonclassical" light source:
1
<n> =1
Single quantum system:
P(n)
0,8
0,6
0,4
0,2
0
0 1 2 3 4 5 6 7 8 9 10
n Andrea Fiore
The quest for single-ϕ
sources
QD
The simplest single-ϕ sources:
• Single atoms
• Single ions
• Single molecules
• ...
Wish list for single-ϕ sources:
• Compact, electrically pumped
• Efficient
• Emitting at 1300-1550 nm
⇒ A semiconductor LED!
(Diedrich and Walther, PRL 1987)
electron
photon
Andrea Fiore
Semiconductor
Quantum Dots
E
QD
InAs QD in a GaAs matrix:
∆E>>kT
Single-QD LED:
Transmission Electron Microscopy
QDs
Andrea Fiore
Self-assembled QDs
Self-assembly in nature…
QD
...and in semiconductors:
As
In
InAs
GaAs
☺ Simple growth technique
☺ High radiative efficiency at RT
15 nm
☺ 1.3 µm operation possible
Random nucleation sites
Size dispersion
200 nm
Andrea Fiore
Inhomogeneous broadening
QD
1 µm diameter:
11000
40K , 4 µ W
PL counts
8800
300 nm
6600
4400
2200
0
1170
25
20
15
10
E
FW H M =
31 m eV
50K , 4 µ W
600
400
0
1170
1
300 n m
800
200
5
0
0.95
1250
1000
P L counts
PL intensity (arb. un.)
ideal
meas.
E
30
1190
1210
1230
Wavelength (nm )
300 nm diameter:
MacroPL at 5K:
35
1 µm
1.05
E nergy (eV )
1.1
1190
1210
1230
Wavelength (nm )
1250
2
Andrea Fiore
Dot density: 300 dots/µm
Single QDs emit
single photons
GaAs InAs GaAs
|1>
|0>
Total energy
2E0-∆Ecorr
E0
|0>
|1>
QD
XX
X
0
(Kiraz et al., PRB 2002)
Spectral selection of the X
emission ⇒ Photon antibunching:
(Michler et al., Nature 2000)
Andrea Fiore
Isolating single QDs
QD
High areal density
15 nm
200 nm
Need very high spatial
resolution!
Nanomesas:
Shadow-mask apertures:
≈ 100 nm
≈ 1 QD
CNR-IFN Roma
Andrea Fiore
Towards practical
single-φ emitters
Single-QD LED ?
• Electrical pumping ?
QD
≈ 100 nm
QD
• Efficiency ?
• Emission at 1300 nm ?
Our approach:
• Sparse InAs/GaAs QDs at 1300 nm
• Nanostructured LEDs
• Nanocavities for efficient LEDs
Andrea Fiore
Controlling the density by
the coverage
In
As
GaAs
In
As
GaAs
In
QD
As
GaAs
530°C, 0.163 µm/hour:
3*3 µm2
1*1 µm2
1*1 µm2
600nm
200nm
200nm
1.7 MLs< critical thickness
No QDs, streaky RHEED
1.8 MLs ≈ critical thickness
15 µm-2, slightly spotty RHEED
3 MLs>> critical thickness
340 µm-2, spotty RHEED
• Low density at low coverage (Leonard et al., PRB 1994)
• Wavelength?
Andrea Fiore
Low-density QDs
QD
Coverage affects emission wavelength:
In
-2
GaAs
Blue-shift at low coverage
(smaller QD size, lower In
content)
85
80
75
70
65
60
55
50
45
1.8
520C, 0.015 ML/s
1300
1250
1200
1150
293 K
2 2.2 2.4 2.6 2.8
InAs thickness (ML)
3
PL peak (nm)
Density (µ m )
As
1100
Difficult to grow large and In-rich QDs at small In coverage
Andrea Fiore
Growing slowly
QD
Low growth rate ⇒ Increased diffusion length ⇒ Low density
Wavelength vs InAs growth rate
Dots density vs InAs growth rate
1350
PL peak (nm)
100
-2
Density (µm )
293K
10
505C, 2.1 ML
1
1E-3
0.01
InAs growth rate
In atoms
1300
1250
1200
1150
0.1
(µm/h)
1E-3
0.01
0.1
InAs growth rate (µm/h)
0.002 µ/h
InAs islands
0.015 µ/h
2 µm
Joyce et al. PRB 2000, Nakata et al. JCG 2000
Andrea Fiore
The role of capping
InGaAs capping:
InGaAs
In-rich
QD
InGaAs capping:
• Lower strain
• Reduced In segregation
1300 nm at 5K:
0.002
15%
µm/h,
InAs grow
th rate:cap
0.002InGaAs
µ m /h
W avelength (nm )
1400
1350
1300
1250
0.1 5 M L/s
0.009 M L/s
1200
1150
0
0.0 5 0.1 0.15 0.2 0.25 0 .3 0.3 5
In com position in cap
PL intensity (a.u.)
3
1450
25 m eV
2
1
RT
10 K
0
1.0
1.1
1.2
1.3
1.4
1.5
PL w avelength ( µ m )
Andrea Fiore
Single QDs at 1300 nm
QD
2
Exciton-biexciton behaviour:
1.8
1.6
1.4
1.2
18µW
1
0.8
1260
1280
1300
1320
W avelength (nm )
Fit with rate equation model:
Intensity (counts/s)
1000
100
10
X
0.18µW
18nW
9nW
X
3
X2
4.5nW
1.8nW
0.9nW
X2
1
0.1
6
Normalized Intensity
Intensity (arb. un.)
2.2
1
10
Laser Power (nW)
0.18nW
0
1260
1290
1320
Wavelength(nm) Andrea Fiore
QDs in planar microcavities
to increase extraction
QD
⇒ Single QD emission up to 130 K:
QDs
3.5 10
4
3 10
4
2.5 10
4
2 10
4
1.5 10
4
1 10
4
5K
cavity
no cavity
x10
200
12K
35K
50K
70K
90K (x2)
110K (x3)
127K (x4)
155K (x10)
0
0.94
5000
0
1150
Pexc=200nW
Pl intensity (a.u.)
Intensity (a.u.)
14x
DBR
0.95
0.96
0.97
0.98
0.99
Energy(eV)
1200
1250
1300
1350
W avelength (nm )
1400
Andrea Fiore
Time-resolved dynamics of
single QDs at 1300 nm
delay
generator
gate trig.
APD
start
Correlation
card
X
5K
XX
1000
500
1280
1290
1300
W avelength (nm )
stop
(partly resolution-limited by
the APD jitter)
1500
0
1270
1500
1000
Counts
50 ps diode
laser
trigger
2000
Intensity (a.u.)
Time-correlated
fluorescence spectroscopy:
QD
0.8 ns decay time
500
0
Anticorrelation measurements
under way….
-500
20
25
30
35
Time (ns) Andrea Fiore
Fabrication of nano-LEDs
< 1 µm
Al2O3
QD
Etching:
Defects ⇒ NR recombination
Needs high-res. lithography
QD
Au
GaAs
AlGaAs
QDs
300 nm current aperture
oxid. edge
oxid. edge
Oxidation:
☺ Does not create defects
☺ Smaller dimensions (<100 nm
with optical lithography)
Andrea Fiore
Fiore et al., Appl. Phys. Lett. 81, 1756 (2002)
Ultrasmall LEDs: Scaling
10
0
10
-2
10
-4
10
-6
10
-8
2
2
Q Ds
293 K
30 µm
9.1 µm
1.8 µm
1.0 µm
830 nm
600 nm
0
0.2
0.4
0.6
0.8
1
Ldiff
1.2
10
4
10
2
10
0
10
-2
10
-4
10
-6
10
-8
Area = aperture area
QWs
293 K
9.0 µm
2.0 µm
940 nm
720 nm
530 nm
0
0.2 0.4 0.6 0.8
Diffusion length:
≈ 2.7 µm in QWs
< 100 nm in QDs !
1
1.2 1.4
Voltage (V)
Voltage (V )
2
10
Area = aperture area
Current density (A/cm )
2
Current density (A/cm )
4
Current density (A/cm )
InGaAs quantum well:
Quantum dots:
10
QD
Area = aperture area + diffusion
10
2
10
0
10
-2
10
-4
10
-6
10
-8
Q W s, L =2.7 µ m
diff
293 K
9.0 µm
2.0 µm
940 n m
720 n m
530 n m
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Voltage Andrea
(V)
Fiore
Ultrasmall LEDs: Efficiency
Quantum dots:
10
10
InGaAs quantum well:
293 K
10
-3
9.1 u m
1.8 u m
1.0 u m
830 n m
600 n m
-4
50
150
250
350
-2
293 K
QW
Efficiency
Efficiency
QD
QD
450
2
Current density (A/cm )
10
9 um
2 um
940 n m
720 n m
530 n m
-3
50
150
250
2
Current density (A/cm )
QDs: Efficiency ≈ independent of device size
QWs: Efficiency strongly reduced for smallest devices
Efficient QD nanoscale LEDs
Andrea Fiore
Extracting light from
semiconductors
QD
The problem of light extraction:
Outside
medium
(noutside)
Total internal reflection
n(GaAs)≈3.5
θc
Active region
(ninside)
η
extr
=
Ω
c ≈ 2%
4π
Planar m-cavity: no control
in lateral directions
⇒ ηmax ≈ 20%
Need to change carrier-photon interaction
so that light is generated only in useful directions Andrea Fiore
Microcavities & QDs
Spontaneous
emission rate:
free space:
cavity:
Wif ∝ r12
2
g ( E 2 − E1 )
V
8π E 2
gFS ( E ) = 3 3 V
hc
1
max
gcav =
∆Ecav
QD
g(E): Opt. density of
states per unit energy
V: Mode volume
Energy
free space
E 2- E 1
∆Ecav
0
g
Sp. em. rate enhancement: (Purcell, 1946)
Wcav
1
1
λ3
FP =
∝
∝Q
2
WFS ∆EcavV E
V
FP>>1
η ≈ 100%
(all photons emitted
in cavity mode)
Andrea Fiore
Electrical injection?
Micropillars (Gérard et al., PRL 1998) :
Strong optical
confinement,
optical excitation
QD
VCSELs:
Al2O3
Low optical
confinement,
electrical inj.
Serkland et al., APL 2000:
Strong optical
confinement,
and
electrical
injection?
Andrea Fiore
3D microcavity LED
360 nm aperture
QD
Spectroscopy of cavity modes
under electrical pumping:
diameter
a=0.7µm
top DBR
oxid. aperture
oxid. DBR
Design quality factor: Q=1000
Calculated Purcell effect for Φ=300 nm:
FP =
(λ / n)
3
4π
2
Vcav
3
Q ≈ 40 (Ideally!)
Log (Intensity) (arb. un.)
a=1.0µm
a=1.3µm
a=1.6µm
a=2.0µm
a=2.4µm
1100 1120 1140 1160 1180
Wavelength (nm)
Andrea Fiore
3D microcavity LED:
Spectra
800
Experimental data
Model
40
Quality factor
Energy Shift (meV)
50
QD
30
20
600
400
200
10
0.5
1.0
1.5
2.0
2.5
Diameter (µm)
0
5
10 15 20 25 30 35 40 45 50
Energy shift (meV)
Loss increases with
increasing confinement
Effective
index model:
nclad
ncore
nclad
For Fp>>1: Need
Q≈1000 for Φ<1 µm
Andrea Fiore
Summary
QD
• Low growth rate ⇒ sparse, long-wavelength QDs
• Single QD spectroscopy at 1300 nm
• Carrier injection in <300 nm with oxidized apertures
• Strong optical confinement in ≈λ3 volumes
Towards efficient single-QD LEDs
7000
PL Intensity (arb. units)
1*1
µm2
6000
T=10 K
5000
4000
3000
2000
1000
0
-1000
200nm
1270 1280 1290 1300 1310 1320 1330
Wavelength (nm)
Andrea Fiore