III-N Quantum Dots
Nicolas Grandjean
IPEQ-LASPE
Ecole Polytechnique Fédérale de Lausanne, Switzerland
CRHEA-CNRS, Sophia-Antipolis, France
Outline
„ III-V nitrides: properties and applications
„ QDs in nitride-based optoelectronics devices
„ QD Growth
„ Optical properties
Group-III Nitride Semiconductors
„ Direct bandgap from 0.7 eV to 6.2 eV (wurtzite phase)
Emission over a large spectral range
„ Large band-offset: ∆VCB/∆Eg ≈ 0.7
Strong carrier confinement
∆VCB = 2 eV for GaN/AlN Ö 1.3-1.55 µm ISBT
„ Refractive index
Bragg reflectors, waveguides
Lower than GaAs Ö better photon extraction (2.4 vs 3.6)
„ Strong light-matter interaction
Large exciton binding energy (27 meV)
Large oscillator strength (10x GaAs)
Short-wavelength Optoelectronics
Laser diodes
„ Data storage (magneto-optic, CD-ROM, DVD)
„ Printing: high resolution
Light emitting diodes
„ Bio-sensing, chemical detection, air purification
„ Color Displays: brighter and more efficient
„ Lighting (white LEDs): efficient ,compact, long lifetime
UV detectors
„ “Solar blind” 280 nm
„ flam detection
State-of-the-art: summary
„ LEDs
• External Quantum Efficiency >35% (at 400 nm)
• 65 lm/W White LEDs (12 lm/W for bulb lamps)
• mW-class 275-340 nm UV LEDs
„ LDs
• 360 - 480 nm
• 30 mW at 400 - 415 nm (10 000 h lifetime)
„ « Solar blind » UV detectors (280-290 nm)
• High detectivity, low noise, spectral selectivity (>105)
Quantum Dots in III-N devices
QDs have been first invoked in 1997 to explain
the high-performance of nitride-based light
emitters - Narukawa et al. APL (1997)
InGaN/GaN QWs are in
fact InGaN/GaN QDs
resulting from In-rich
clusters induced by
alloy phase segregation
TEM image of InGaN/GaN
QWs grown by MOCVD
But why QDs so
important ?
III-N epilayers = poor material quality
GaN substrates are not available Ö devices are
grown on Sapphire, Silicon Carbide, Silicon, …
Very Large Latticemismatch
Al2O3 (0001): +16 %
SiC (0001): -3,5 %
Si (111): -17 %
Dislocations
GaN
GaAs/Si: +4 %
Al2O3
Ö Dislocations
Plastic deformation
Hetero-Epitaxy of GaN on Sapphire
Dislocation density = 108-1010 cm-2
Surface
AFM
Dislocations
High density of
dislocations in the
active region of
LEDs or LDs
TEM
250 nm
Cross-section
LED Efficiency versus
Dislocation Density
GaN
1E+08
Are dislocations inactive in GaN?
Efficiency of InGaN/GaN based
light emitters
Carriers are strongly localized in QDs and can
therefore no longer diffuse toward non-radiative
recombination centers (dislocations)
Ø
InGaN/GaN QDs - formed by alloy phase
segregation – are thus responsible for the high
efficiency of nitride-based light emitters.
Narukawa et al., APL (1997)
Weisbuch and Gérard – Patent (1990)
Quantum Dot Growth
Strain-induced 2D-3D transition
Stranski-Krastanov growth mode
h < hc
Pseudomorphic
growth
h > hc
Elastic energy
relaxation by 3D
island formation
2D wetting
layer
3D islands
are buried to
form QDs
3D islands = balance between
surface and elastic energies
QDs: III-N versus III-As
Lattice-mismatch
7
Bandgap (eV)
6
AlN
Arsenides:
Nitrides
5
GaAs/AlAs ≈ 0 %
4
3
GaN
2
AlAs
1
0
InAs/GaAs = 7.2 %
GaAs
InN
Arsenides
-6 -4 -2
0
2
InAs
4
6
8 10 12 14
Lattice mismatch (%)
Nitrides:
GaN/AlN = 2.5 %
InN/GaN = 11 %
GaN/AlN Quantum Dots
Surface energy (meV/Ų)
Surface energy of GaN(0001)
GaN surface energy
increases from Ga-rich
to N-rich conditions
185
N-rich
45
Rapcewicz et al. PRB (1997)
µGa- µGa(bulk)
Ga-rich
Elsner et al., SSC (1998)
Lattice-mismatch of 2.5 %
InAs/GaAs: 7.2 % and Ge/Si: 4.2 %
Modelling of the 2D-3D transition
γ0
θ
H
γf
L
Island energy
N-rich
2D-3D transition
0
θ = 30° and H/L = 0.2
Tersoff and LeGoues PRL (1994)
Ga-rich
0
2
4
6
8
GaN thickness (ML)
3D islands are formed under Ga-rich
conditions but not under N-rich conditions
10
GaN on AlN under N-rich conditions
GaN/AlN mismatch (%)
2.5
Plastic relaxation at 14
MLs (3.6 nm) without
any 3D islands
RHEED
2.0
1.5
Plastic relaxation
1.0
Energetics = high
surface energy
0.5
0.0
-0.5
Tg = 800°C
-1.0
1
10
100
GaN thickness (ML)
MBE growth with NH3 as
nitrogen precursor
1000
and/or
Kinetics = small
surface diffusion
length
Ga-rich conditions by N desorption
AlN epilayer
3D islands
Ga-rich
5 MLs of GaN
N-rich
NH3 Off
N (NHx) species
may strongly
increase the
surface free
energy
Growth interruption
with NH3 On
N, NHx
Ga
GaN
Ga
GaN
Reversibility of the 2D-3D
transition
6 MLs of GaN on AlN(0001)
NH3 flux: On (N-rich) Ö Off (Ga-rich) Ö On (N-rich)
2D
Ö
3D
Ö
2D
GaN/AlN QD Characterization
Critical thickness of 2-3 MLs
0.4 x 0.4 µm²
TEM
WL
2 nm
(0001)
(1013)
AFM
30°
High QD density: 3-5x1011 cm-2 (MBE)
Control of the QD nucleation
AFM
Growth of AlN on offaxis Si substrates
3-7 nm
Step-bunching
AlN on Si(111) 2°off
Control of the QD nucleation
AFM
1 x 1 µm²
0.3 x 0.3 µm²
Control of the QD density and size
QD density varies from 2x108 cm-2 to 4x1010 cm-2
with the GaN coverage (MOVPE)
InGaN/GaN Quantum Dots
InGaN/GaN QDs by SK growth mode
11% lattice mismatch between GaN and InN
15
2D-3D transition (ML)
MBE
Tg = 550°C
3D
islands
10
SK growth mode
depends on the strain
and therefore on the In
composition
Ø
5
2D
0
0
10
Critical In composition
for 3D islanding
20
30
40
50
In composition (%)
Rem: coherent InN QDs ?
InGaN growth – Surfactant effect
RHEED intensity (arb. units)
AFM
In 0.13 G a 0.87 N
2D-3D
25 sccm
QDs
2D-3D
100 sccm
QWs
200 sccm
T = 500°C
0
1
TEM
2
3
4
5
InG aN thickness (nm )
Suppression of the 2D-3D transition when increasing the
NH3 flux H and/or NHx may act as surfactant
Optical Properties
GaN/AlN QD Photoluminescence
+
Frequency doubled Ar laser (244 nm/5eV)
T = 10 K
PL intensity (arb. units)
GaN/AlN QDs
Wetting layer
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Photon energy (eV)
4.5
5.0
Emission far below
the bandgap of GaN
Quantum Confined Stark Effect
-σ
+σ
CB
F
Ö Red-shift of the QW
transition energy
Ö Reduction of the
oscillator strength
hνhυ
VB
Pb
Pw
AlGaN
GaN
QCSE
Pb
AlGaN
F = σ/εε0 = (Pb-Pw)/εε0
P: macroscopic polarization
QCSE in AlGaN/GaN QWs
Al0.17Ga0.83N/GaN QWs
Eg(GaN)
1.5 nm
4 nm 2.5 nm
3.8
3.7
QW energy (eV)
Photoluminescence intensity (a.u.)
T = 10 K
1 nm
6 nm
8 nm
80-LO
x 20
60-LO
Barrier
40-LO
3.6
GaN
3.4
3.3
3.1
3.2
3.3
3.4
3.5
3.6
3.7
Photon energy (eV)
3.8
3.9
F = 710 kV/cm
3.2
3.1
3.0
F=0
3.5
0
2
4
6
8
Well thickness (nm)
QW transition energies below the
bandgap of GaN Ö QCSE
10
Giant Polarization Fields
Wurtzite structure
+
Growth along [0001]
+
Large lattice mismatch
Bernardini et al. PRB (1997)
GaN/AlN = 10 MV/cm
PZ = 5 MV/cm
SP = 5 MV/cm
GaN/AlN = 2,5 %, InN/GaN = 11 %
+
huge piezoelectric (PZ) and
spontaneous (SP)
constants
=
Electric field of several
MV/cm
InN/GaN = 16 MV/cm
PZ = 15 MV/cm
SP = 1 MV/cm
GaN/AlN QW: 1 monolayer
fluctuation (2.6 Å) ⇒ 260 meV
Photoluminescence of GaN/AlN QDs
10 mW HeCd laser
300 K
1 mm
50 Å
GaN QD height
25 Å
Stark-shift of 1.5 eV
QD plane
stacking
Electric Field Screening
PL intensity (arb. units)
T = 13 K
I0 = 10 W/cm²
100 %
10 %
Electric field
screening by
photo-generated
carriers
1%
0.1%
0.01%
2.0
2.2
2.4
2.6
Photon energy (eV)
2.8
3.0
Electric field value in GaN/AlN QDs
QDs Iex=Io
QDs Iex=Io/1000
Wetting layer
Energy (eV)
4.0
3.5
3.0
F = 3.9 MV/cm
2.5
1
2
3
4
Thickness (nm)
Thickness measured by TEM
Once corrected from
geometrical effect
F∞ = Lb/(Lb+hQD) x F
F = 4.8 MV/cm
2.0
Electric field of 4.8
MV/cm in x10
GaN/AlN QD planes
5
6
F∞ = 7 MV/cm
Radiative Efficiency of GaN/AlN QDs
Integrated PL intensity (arb. units)
Growth on Si(111)
Exciton localization
I10K/I300K = 4
GaN/AlGaN QWs
GaN/AlN QDs
0.00
0.02
0.04
0.06
-1
1/T (K )
0.08
0.10
High radiative
efficiency despite a
huge dislocation
density (2x1010 cm-2)
and a giant polarization
field (5 MV/cm)
Single GaN QD emission
3.5 K
Single GaN QD emission
The biexciton binding
energy is negative and the
magnitude is about 30 meV
Ø
Effect of the strong built-in
electric field
Kako et al., APL (2004)
PHOTO-INDUCED ABSORPTION (a. u.)
Intersubband absorption in
GaN/AlN QDs
e1 - e2
IR absorption from
1.27 µm to 2.36 µm
T = 300K
λexc = 351 nm
N361
4 nm
N361
N365
N365
4.5 nm
e1 - e3
e3
e2
0.5
0.6
0.7 0.8 0.9
ENERGY (eV)
(coll. F. Julien, IEF-CNRS)
1
1.1
e1
Conclusion
QD Growth
SK growth mode for both GaN/AlN and InGaN/GaN
Critical In composition for 3D islanding
Surface energy plays a key role
High QD density: > 1011 cm-2
Control of the density, the size, and the nucleation site
QD Optical Properties (GaN/AlN)
Internal electric field of 7 MV/cm (piezo+spont.)
Strong exciton localization
Intersubband absorption (1.3-1.5 µm)
SQD: negative biexciton binding energy (30 meV)
The author acknowledges the financial support of
and B. Damilano, A. Dussaigne, J. Massies, and F. Semond from
CRHEA-CNRS