Transcript 10 20 [cm

13aXD-14
希薄磁性半導体CdMnTeにおける強励起効果
High excitation effects in dilute magnetic semiconductor CdMnTe
橋本佑介A,B
三野弘文A、山室智文A、蒲原俊樹A、神原大蔵A、松末俊夫B
Jigang WangC、Chanjuan SunC、河野淳一郎C、嶽山正二郎D
千葉大院自然A、千葉大工B、ライス大ECEC、東大物研D
Y. HashimotoA,B
H. MinoA, T. YamamuroA, T. KamoharaA, D. KanbaraA, T. MatsusueB, J.
WangC, C. SunC, J. KonoC, S. TakeyamaD
Graduate School of Science and Technology, Chiba Univ.A 、
Department of Engineering, Chiba Univ.B, ECE Dept., Rice Univ.C 、
ISSP, Univ. of TokyoD
Magnetic Polarons
Free Exciton Magnetic Polaron (FEMP)
h
e
Mn spin
Exciton spin
A Golnic, et. al. J. Phys. C16, 6073 (1983)
M. Umehara, Phys. Rev. B 68, 193202 (2003)
Localization only by sp-d exchange interaction
Photo-induced ferromagnetism
via the FEMP
Localization energy
Free exciton magnetic polaron (FEMP) in
CdMnTe
Localization energy of
Magnetic Polaron
Alloy Potential
fluctuation
Current work :
High quality CdMnTe sample
with low Mn concentration
5 10
Mn Concentration [%]
S. Takeyama, J. of Crys. Growth,
184-185 (1998) 917-920
x = 5 ~ 10% → FEMP energy : Large
Alloy potential fluctuation : Small
He-Ne laser
76 MHz Ti:Sapphire laser
250 kHz OPA laser
1 kHz OPA laser
Exciton density 1012 – 1020 [cm-3]
CW and Time-resolved
Photoluminescence
Free Exciton Magnetic Polarons
FEMP
Bipolaron
Ferromagnetic Phase Transition via
Free Exciton Magnetic Polarons ?
Experimental Setup for PL measurements
Bulk Cd1-xMnxTe
x = 5%
Laser
Cd0.95Mn0.05Te
Cd1-yMgyTe
CCD
or
Streak camera
GaAs
Spectrometer
Sample 1.4 K
Lasers
Excitation intensity: 1mW, Focus size: 200mm, O.D. 1
Laser
Exciton density [/cm3]
rs
Wavelength
He-Ne
2.2 x 1013
33
634 nm
Ti: Sapphire
2.8 x 1015
6.6
400 nm
250KHz OPA
8.6 x 1017
1
700 nm
1KHz OPA
2.2 x 1020
0.15
634 nm
He-Ne
n
rs
1012
100
Ti: Sapphire 250 kHz OPA 1 kHz OPA
1015 1016 1017 1018 1019 1020
0.1
1
2
10
aB = 6.7 nm  nMott = 7.9 x 1017 [cm-3]
1013
1014
Low Excitation Limit
Exciton Density 1012 - 1014 [cm-3]
1017
1016
Ti: S
1015
1014
1012
PL
Absorption: 4.2 K, PL: 1.4K
PL Light source：He-Ne 633nm
FEMP
BMP
FX
BX
BMP'
He-Ne
1013
Absorption
1018
Absorption
Photoluminescence
1019
1 kHz OPA 250 kHz OPA
1020
1.650
1.660
1.670
1.680
Photon energy [eV]
Distinct PL line of the
FEMP appear !!
FEMP binding
energy  1.8 meV
Photoluminescence
Exciton Density 1015 – 1016 [cm-3]
1018
1017
1016
1014
1012
BX
FX
10
16
10
FEMP
1.668
He-Ne
1013
15
Ti:S
1015
Exciton density 1015, 1016[cm-3]
PL intensity
1019
Excitation intensity normalized PL
1 kHz OPA 250 kHz OPA
1020
1.676
1.672
Photon energy [eV]
FEMP PL intensity: Saturate
FX PL intensity:
Increase
Time Resolved Photoluminescence
Exciton Density 8.6 x 1017 cm-3
1018
1017
1016
Ti: S
1015
He-Ne
1012
EHP
B
BX
A
0
100
200
300
1014
1013
1.674eV
1.667eV
Time [ps]
1019
1 kHz OPA 250 kHz OPA
1020
400
1.66
1.67
1.68
Photon energy [eV]
Time Resolved Photoluminescence
Exciton Density 8.6 x 1017 cm-3
1017
1016
He-Ne
1012
A
2
10
6
4
2
1
0
200
400
600
800
Time Delay [ps]
1000
1.64 1.66 1.68 1.70
Photon energy [eV]
Inverse Boltzman
1014
1013
t = 40 - 140 ps
6
4
Ti: S
1015
6
4
PL intensity
1018
A (1.6768 ~ 1.6723 eV)
B (1.6611 ~ 1.6656 eV)
100
PL intensity
1019
1 kHz OPA 250 kHz OPA
1020
A: 1.674 eV t ~ 150 ps  Biexciton
B: 1.667 eV t < 30 ps 
?
Many Body Effect of FEMPs
Bi-polaron
Coupled two FEMPs has been expected
to be more stable than single FEMP
Bi-exciton
Photoluminescence
Exciton Density > Mott Density
1018
1017
1016
1014
1012
3.3I
1.62
2I
1.64
I
1.66
1.68
Photon energy [eV]
He-Ne
1013
I = 5.6 × 1018 [cm-3]
EHP
Biexciton
Ti: S
1015
12 K
Photoluminescence
1019
1 kHz OPA 250 kHz OPA
1020
Electron hole plasma  I4.2
Biexciton  I1.6
Exciton Density Dependence
of Origin of Photoluminescence
FEMP
1019
1018
1017
1016
Ti: S
1015
1 kHz OPA 250 kHz OPA
1020
1014
1012
He-Ne
1013
Biexciton
Electron hole
Plasma
Summary
PL measurements
Exciton density: 1012 – 1020 [cm-3]
FEMP  Biexciton
Electron hole plasma
Future work
Spin Dynamics Under Strong Excitation
Free Exciton Magnetic Polaron
Mn spin
Electron
Hole
14.4Å
Hole mass:
Electron mass:
mh
64Å

 0 . 81
me
me

 0 . 096
me
N 0  220 meV
N 0   880 meV
The number of Mn ion
electron:
481
hole:
~5.5
Exciton Density Dependence
1017
1016
Ti: S
1015
1014
1012
He-Ne
1013
0
0.1
1
10
100
Exciton density (× 10 ) [cm-3]
16
FEMP binding energy [meV]
1018
Excitation intensity
normalized FEMP PL int.
Normarized FEMP PL Int.
1019
1 kHz OPA 250 kHz OPA
1020
FEMP binding energy
1.5
1.0
0.5
0.0
0.1
1
10
100 1000
16
Exciton density (× 10 ) [cm-3]
When the exciton density is above 1018 cm-3
FEMP may disappear
Spin Relaxation Dynamics
n ~ 4  10 [ cm
16
1020
1018
T/T
1019
10x10
1017
3
]
-3
5K
8
6
4
2
0
0
10
20
Time delay [ps]
1016
n ~ 4  10 [ cm
17
3
n ~ 1  10 [ cm
18
]
3
]
1015
1013
-4x10
1012
T/T
1014
T/T
4
0
-3
0
-4
-8x10
0
10
20
Time delay [ps]
-3
0
10
20
Time delay [ps]
Time Resolved Photoluminescence
1018
1017
1016
Ti: S
1015
1014
1012
He-Ne
1013
1.4K
250 kHz OPA laser
76 MHz OPA laser
0
0
Time [ps]
1019
1 kHz OPA 250 kHz OPA
1020
100
100
200
200
300
300
1.4K
400
1.66
1.67
1.68
Photon energy [eV]
1.665
1.670
1.675
1.680
1.685
1.690
Experimental Setup for PL measurements
Sample 13 K
1kHz
OPA&CPA
chopper
He-Ne
Movable
mirror
Lock-in
Amplifier
Photodiode
Spectrometer
PL peak position [eV]
Discussions
1.665
1.660
1.655
1.650
1.645
0
5
10
15
18
-3
Exciton density ( x 10 ) [cm ]
-15
Mott transition
80
EHP
Exciton
PL intensity
100x10
60
40
20
00
0
5
10
15 -3
18
Exciton density ( x 10 ) [cm ]
Normarized PL intensity
Excitation Dependence of the PL Intensity
Excited with Ti:Sapphire Laser
2
10
1.0
I
4
2
1
I
1.28
I
FEMP
FX
4
2
6
2
0.1
1.04
3 4 56
16 1
Exciton density (× 10 ) [cm-3]
2X
Peak position
[eV]
BX
X
FEMP
Binding Energy
[meV]
Absorption
1.6748
Biexciton
1.6741
0.7
FEMP
1.6722
2.6
E FEMP  2 . 8 meV
G
E BX  E FEMP  1meV
E BX  3 . 8 meV  3 . 3 meV
Estimate by the EBX (4.1 meV) on CdSe
Purpose
BX
2X
t BX  1 / n
X
2
t FEMP ~ 10 ps
FEMP
G