2002 11 13...大阪大学大学院基礎工学研究科 山中一克 宮島顕祐 伊藤正...
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レーザーを用いた赤外時間分解分光レーザーを用いた赤外時間分解分光
半導体量子ドットに閉じ込められた励起子の半導体量子ドットに閉じ込められた励起子の励起状態の研究励起状態の研究
枝松 圭一
大阪大学大学院基礎工学研究科
分子研研究会
赤外放射光の現状と将来計画
2002年11月13日
1. Introduction• Excited states of excitons confined in semiconductor
quantum dots• CuCl quantum dots
2.Experimental procedure• Infrared transient absorption spectroscopy • ns and ps systems
3.Results• Excited-state absorption of confined excitons in CuCl
quantum dots• Excited-state absorption of confined biexcitons
4.Summary
OutlineOutline
3D: Bulk crystal
2D: Quantum well
1D: Quantum wire
0D: Quantum dot (QD)(or nanocrystal)
E
DO
S
Semiconductor quantum structureSemiconductor quantum structure
Photon Energy
Lum
ines
cenc
e In
tens
ity Excitation Laser
~2Γhomo
(b)
Photon Energy
Lum
ines
cenc
e In
tens
ity
Γinhomo
(a)
Photon Energy
Lum
ines
cenc
e In
tens
ity
Γhomo
(b)
Photon Energy
Lum
ines
cenc
e In
tens
ity
Γinhomo
(a)
1. Resonant size-selective excitation
Laser
2. micro-photoluminescence
SizeSize--selective observation of selective observation of QDsQDs
•Electron-hole individual confinement.(aB≫a)
•Exciton confinement.(aB≪a)
M
0 a r
∆E
∆EM a
M m mM
e h
=
= +
h2 2
2π
: translational mass
me
mh
2a
∆EC
∆EV
∆ ∆ ∆E E E
a
m m
C V
e h
= +
=
= +− − −
h2 2
1 1 1
2µπ
µµ:reduced mass
Quantum size effect in spherical Quantum size effect in spherical QDsQDs
Excitons in CuCl quantum dotsaB=0.7 nm (1S); Ry*=197 meVConfinement energy~ 10meV (a* = 4 nm)
⇒ Exciton confinement regime
Two quantum numbers of the confined exciton:
1s1p1d2s2p
1S
2S ,P3S ,P ,D
∞
2a B2a
1) Confinement oftranslational motion (nl)
2) Coulombic states (n’L)
Confined excitons inConfined excitons in CuCl QDsCuCl QDs
T. Uozumi, et al., Phys. Rev. B 59, 9826 (1999)
0 2 4 6 8 10 12 14 16 18
-1.0-0.8-0.6-0.4-0.20.00.20.40.60.81.01.21.4
n=2 (2S,np), (2P,ns)n=3 (3S,np), (3P,ns)
Ene
rgy
[E* R]
Radius [a *B ]
0
5
10
15
20
Ene
rgy
n=1 (1S,1s)
Energy levels of confined excitonsEnergy levels of confined excitons
(Infrared transient absorption)• Mimura et al., J. Lumin. 66&67, 401 (1996)• Yamanaka et al., J. Lumin. 76&77, 256 (1998)• Yamanaka et al., J. Lumin. 87&89, 312 (2000)• Itoh et al., Int. J. Mod. Phys. 15, 3569 (2001) • Miyajima et al., Phys. Stat. Sol. (b), in press.
(Two-photon excitation)• Edamatsu et al., Phys. Rev. B 59, 15868 (1999)
(Theoretical works)• Kayanuma, Phys.Rev. B 38, 9797 (1988)• Uozumi et al., Phys. Rev. B 59, 9826 (1999)• Uozumi et al., Phys. Rev. B 65, 165318 (2002)
ExcitedExcited--state absorption of excitons state absorption of excitons confined inconfined in CuClCuCl quantum dotsquantum dots
CuCl QDs (or nanocrystals) embedded in NaCl matrices
Dried NaClingot
CuCl flakes (~1mol%)
Quartz tube NaCl:Cu+ single
crystal
H2 gas (~0.3atm)
2cm/day
NaClpowder
Cl2gas
Melting (~820°C)
Annealing
500~600°C several hours
Small CuCl QDs(a few nm)
200~300°C
Large QDs(~10nm)
500~600°C
Sample preparationSample preparation
3.2 3.3PHOTON ENERGY(eV)
370380390WAVELENGTH(nm)
0
1
2
3
O.
D.
LUM
INES
CEN
CE
IN
TEN
SIT
Y (
a.u.
)
absorptionluminescence
77Ka*=3.7nm
Absorption and luminescence spectraAbsorption and luminescence spectraofof CuCl QDsCuCl QDs inin NaClNaCl matricesmatrices
Inhomogeneous broadening due to size distribution.}
Probe light
Pump light
g
1S
nP
3.2 3.3PHOTON ENERGY(eV)
370380390WAVELENGTH(nm)
0
1
2
3
O.
D.
pump
Transient absorption spectroscopy Transient absorption spectroscopy under sizeunder size--selective excitationselective excitation
1. Introduction• Excited states in semiconductor quantum dots• CuCl quantum dots
2.Experimental procedure• Infrared transient absorption spectroscopy • ns and ps systems
3.Results• Excited-state absorption of confined excitons in
CuCl quantum dots• Excited-state absorption of confined biexcitons
4.Summary
OutlineOutline
power unit
Xe lamp
monochro-mator(NaCl prism)
photo detectorCdHgTe
amp.
storageoscillo-scope
computer
pulse motor
PD
timingcircuit
YAG laser
Ti-sapphire
trigger
trigger
triggercryostat(77K)sample
SH
SH
•Pump light sourceSecond harmonic of Ti:Sa laser
•Tunability (SH)350∼ 470nm(2.64∼ 3.54eV)
•Pulse energy : ~1mJ•Pulse duration: ~15ns•Excitation power : ~70MW/cm2
•Line width <0.1nm(1meV)
•Probe light sourceXe flash lamp
•Pulse duration: ~8µs•1~8µm (160~1200meV)
Experimental setup (ns system)Experimental setup (ns system)
Probe Light
Pump Light
Mono-chromator
Cryostat
Sample
Detector(cooled MCT)
0 0.2 0.4TIME (µs)
22
23IN
TEN
SITY
(a.u
.) I0
I
pump
Observation of the infrared transient Observation of the infrared transient absorptionabsorption
-0.1 0 0.1 0.2 0.3TIME (µs)
0
0.05
0.1
∆O
.D.
τ f (≤ 1ns)
τ s (>> 100µs)
Fast decay component (τf): originates from the confined exciton (1S → 2P)
Excitation: 3.224 eV(a*~6 nm)
Probe: 246 meV(λ=5.03 µm)
Infrared transient absorption:Infrared transient absorption:temporal profiletemporal profile
R=15.0[a*B]
8.0[a*B]
Inte
nsity
[arb
. uni
ts]
3.0[a*B]
0 200 400 600
2.5[a*B]
Photon Energy [meV]200 400 600PHOTON ENERGY (meV)
345710WAVELENGTH (µm)
∆O
.D.
(A)
(B)
(C)
(D)
77K
a*≥10nm
a*=5.4nm
a*=1.9nm
a*=1.4nm
Experiment Theory (Uozumi and Kayanuma)
Transient absorption spectra Transient absorption spectra
T. Uozumi, et al., Phys. Rev. B 59, 9826 (1999)
0 2 4 6 8 10 12 14 16 18
-1.0-0.8-0.6-0.4-0.20.00.20.40.60.81.01.21.4
n=2 (2S,np), (2P,ns)n=3 (3S,np), (3P,ns)
Ene
rgy
[E* R]
Radius [a *B ]
0
5
10
15
20
Ene
rgy
n=1 (1S,1s)
Energy levels of confined excitonsEnergy levels of confined excitons
0 5 10EFFECTIVE RADIUS a* (nm)
200
400
TRA
NSI
TIO
N E
NER
GY
∆E
(meV
) (1pe, 1sh)(1se, 1ph)
theory
experiment
(2P,1s)
1S 1S →→ PP--like state transition energy:like state transition energy:comparison with theorycomparison with theory
1. Introduction• Excited states in semiconductor quantum dots• CuCl quantum dots
2.Experimental procedure• Infrared transient absorption spectroscopy • ns and ps systems
3.Results• Excited-state absorption of confined excitons in
CuCl quantum dots• Excited-state absorption of confined biexcitons
4.Summary
OutlineOutline
Motivation of theMotivation of the psps experimentexperiment
• Insufficient resolution (in energy and time) of the ns experiment ⇒ ambiguous attribution of the observed transient
absorption
• Excited-state absorption of confined biexcitons?
• Modification of the confined exciton states induced by strong IR pulses?
Ti:sapphire laser
regenerative amplifier
OPAOPA
monochro-
matorMCT
lock-in
amplifier
800 nm, ~2 ps, 1 kHz
Probe pulseIdler (2.1~2.9 µm) or DFM (3~10 µm)
~0.1µJ/pulse
Pump pulse4ωs(~384 nm)
8 µJ/pulse
chopper
CuCl QDs in NaCl
T = 70Ksample
cryostat
ND filter
Experimental setup (Experimental setup (psps system)system)
pump
IRTA
excited exciton
2 excitonbiexcitonIRTA
pump
excited biexciton
exciton
ground state
ExcitedExcited--state absorption of confined state absorption of confined biexcitonsbiexcitons ??
3.10 3.15 3.20 3.25 3.30 3.35
70K laser light (3.227eV)
M 3.18eV
PL
Inte
nsity
(a.u
.)
Photon Energy (eV)
PL spectrum: exhibiting biexciton PL spectrum: exhibiting biexciton luminescenceluminescence
• Excitation : Resonant to the confined excitons with the effective radius ~4.2 nm (3.227eV).
• PL from the biexcitons (M band) at ~3.18eV.
(Ex)
0 200 400 600 8000.00
0.02
0.04
0.06probe 4.89µm (0.25eV)
τ2~490ps
τ1~56ps
experiment calculation
∆O.D
.
Time (ps)
Two decay components
• Fast decayτ1 ~56±15ps⇒ Biexciton
• Slow decayτ2 ~490±290ps⇒ Exciton
Infrared transient absorption:Infrared transient absorption:psps decay profiledecay profile
fit
0.1 1 10
0.01
0.1
∝IEXC0.8
∝IEXC1.3
fast decay slow decay
∆O.D
.
Excitation Intensity IEXC (mJ/cm2)
• Fast decaysuperlinear⇒ Biexciton
• Slow decaylinear~sublinear⇒ Exciton
Infrared transient absorption:Infrared transient absorption:excitation power dependenceexcitation power dependence
100 200 300 400 500 6000.00
0.05
0.10
0.15
biexciton exciton
∆O.D
.
Photon Energy (meV)
100 200 300 400 500 600
a*=3.9 nm
Inte
nsity
(a.u
.)
Photon Energy (meV)
Experiment Theory (exciton)
Infrared transient absorption spectra:Infrared transient absorption spectra:exciton and biexciton componentsexciton and biexciton components
0 5 10EFFECTIVE RADIUS a* (nm)
200
400
TRA
NSI
TIO
N E
NER
GY
∆E
(meV
) (1pe, 1sh)(1se, 1ph)
theory (exciton)
experiment (exciton)
(2P,1s)
experiment (biexciton)
Transition energy:Transition energy:comparison with theorycomparison with theory
ground state
exciton
2 exciton
biexcitonexcited exciton
excited biexciton
pump
IRTA
IRTA
ExcitedExcited--state absorption of the state absorption of the confined biexcitonconfined biexciton
Excited biexcitons:one lowest exciton and one excited exciton.
In QDs, excited biexcitonsare stable because the two excitons are confined together in a QD.
We have investigated the infrared transient absorption spectra of CuCl QDs under size-selective excitation.
• Direct observation of the Rydberg 1S-2P transition of the confined exciton.
• The transition energy depends on the dot size, in agreement with the theoretical calculation.⇒ Deviation from the “exciton confinement” regime
• Finding of the excited-state absorption of the confined biexciton.
SummarySummary
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