QD
Nanoscale single quantum dot
devices at 1300 nm
Andrea Fiore
Ecole Polytechnique Fédérale de Lausanne
Quantum Devices group at EPFL:
Postdocs: L.H. Li, C. Monat, V. Zwiller (now at ETHZ)
PhD students: B. Alloing, C. Zinoni
Undergrads: G. Buchs, M. Gobet (now at Texas Univ.)
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
QD
MBE growth of InAs on GaAs:
15 nm
TEM
200 nm
AFM
GaAs InAs GaAs
|1>
|1
>
|0>
|0
>
|0>
|0
|1>
>
Single-photon
emission
(Kiraz et al., PRB 2002)
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
QD
Single QDs in 2 µmdiameter mesa:
QDs
14x
DBR
2
1.8
1.6
1.4
1.2
1
0.8
1260
1280
1300
1320
W avelength (nm)
Low signal-to-noise due to
noisy InGaAs detector
Intensity (a.u.)
Intensity (arb. un.)
2.2
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
5000
Increase light extraction
with a microcavity
0
1150
1200
1250
1300
1350
W avelength (nm )
1400
Andrea Fiore
X-XX spectroscopy at
1300 nm
QD
Wavelength (nm)
1306 1304 1302 1300 1298 1296 1294
QD1
QD1
XXX
-
+
X
50µW
10
XX
XX X
21µW
X
8.3µW
x2
5.3µW
x2
x3
2.5µW
XX X
XX
X
x10
760nW
300nW
952
956
Energy (meV)
960
PL Intensity (cps)
PL Intensity (cps)
X
1
0.1
X
XX
10K
1000
10000
Laser Power (nW)
Alloing et al., APL, to be published
Andrea Fiore
Temperature dependence
QD
Single QD emission above 77 K:
x40
Counta/sec (arb)
xx x
94K
78K
• Population of charged exciton
states at high T
• Homogeneous broadening of
lines due to phonon scattering
• Single lines can be isolated up
to T>77K
xx x
x20
xx x
70K
x15
62K
x10
x
52K
xx
x4
xx
x
42K
xx
Towards LN2-cooled singlephoton sources at 1300 nm
x
31K
936
940
944
Energy (m eV)
948
Andrea Fiore
Fabrication of nano-LEDs
< 1 µm
Al2O3
Au
GaAs
AlGaAs
QDs
QD
Etching:
Defects ⇒ NR recombination
Needs high-res. lithography
QD
300 nm current aperture
oxid. edge
oxid. edge
Oxidation:
☺ Does not create defects
☺ Smaller dimensions (<100 nm
with optical lithography)
Fiore et al., APL 2002
Andrea Fiore
Confining current injection
QD
Current spreading: Lspread = f ( R // ,R ⊥ )
Ldiff = f ( material )
Carrier diffusion:
Ldiff
To suppress current spreading: Bandgap engineering of
hole injector
4
10
2
10
0
2
10
undoped, graded p-injector:
Current density (A/cm )
2
Current density (A/cm )
doped p-injector:
10
-2
10
-4
10
-6
400 µm
200 µm
100 µm
50 µm
0
0.2
0.4
0.6
0.8
Voltage (V)
1
1.2
10
4
10
2
10
0
10
-2
10
-4
10
-6
400 µm
200 µm
100 µm
50 µm
0
0.2
0.4
0.6
0.8
1
1.2
Voltage (V)
Andrea Fiore
Ultrasmall LEDs: Scaling
10
2
10
0
10
-2
10
-4
10
-6
10
-8
Area = aperture area
2
2
Current density (A/cm )
4
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
Current density (A/cm )
InGaAs quantum well:
Quantum dots:
10
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
2
Ld
Ld
Ld
Ld
7
6
5
(2.0 µm )
(940 n m )
(720 n m )
(530 n m )
4
3
2
1
0
0.5
Current density (A/cm )
8
Diffusion length (µ m)
QWs: ≈ 2.7 µm
QDs: < 100 nm
1
1.2 1.4
Voltage (V)
Voltage (V )
Estimating diff.
length from IVs:
QD
QWs
0.6
0.7
0.8
0.9
1
Voltage (V)
1.1
1.2
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:
-3
-4
10
1
10
293 K
9.1 um
1.8 um
1.0 um
830 nm
600 nm
10
QD
10
100
-2
293 K
Efficiency
10
InGaAs quantum well:
-2
10
1000
-3
9.0 µ m
2.0 µ m
940 nm
720 nm
530 nm
-4
10
2
Carrier diffusion +
extraction efficiency:
Fiore et al., PRB 2004
100
QW
1000
10
4
2
C urrent density (A/cm )
Current density (A /cm )
Radiative efficiency
Efficiency
10
QD
10
10
10
0
293 K
-1
9.0 µ m
2.0 µ m
940 nm
720 nm
530 nm
-2
1
10
QW
100
1000
2
C urrent density (A/cmAndrea
)
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 µ-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)
Zinoni et al., APL 2004
Effective
index model:
Loss increases with
increasing confinement
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
QD
Andrea Fiore