Transcript Optical Properties of Ga1

Optical Properties of Ga1-xMnxAs
C. C. Chang, T. S. Lee, and Y. H. Chang
Department of Physics, National Taiwan University
Y. T. Liu and Y. S. Huang
Department of Electronics, National Taiwan University of Science and Technology
J. Furdyna
Department of Physics, University of Notre Dame
Outlines:
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Review on semiconductor spintroincs
Basic properties of III-Mn-V
Optical properties of GaMnAs
Experimental results and discussions
Summary
Review on semiconductor spintroincs
• Classical device: use the electrical and
particle properties of the electron.
• Quantum device: use the wave properties
of electron.
• Spin-properties- non volatile memory,
integration of memory and logic devices,
spin-FET, quantum computing, etc.
Requirement for spintronic devices
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1. spin-injection
2. spin-manipulation
3.spin-detection
An example: spinFET
Problem with the spin-injection
• Conductance
mismatch
Basic knowledge about magnetism
• Paramagnetism:
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Atoms have magnetic moments
but the coupling between the
magnetic moments is small and
the magnetic moment of the
atoms are randomly oriented.
Origin of ferromagnetism
• Direct exchange
interaction
• Super-exchange
interaction
• Indirect exchange
Nature Choice for magnetic semiconductor
II1-x-Mnx-VI (Diluted magnetic semiconductor)
• 1. Anti-ferromagnetic at high x,
paramagnetic at low x.
• 2. large spin g-factor
• 3. Spin polarized LED, Spin
superlattice, etc.
MBE phase diagram of Ga1-xMnxAs
Some basic knowledge about the material
properties of Ga1-xMnxAs
• The samples were grown at low temperature with MBE, the
quality of the material is usually very poor
• Mn is an acceptor and in principle could donate a hole for
electrical conduction.
• Mn in GaMnAs is a substitutional acceptor? (yes, Soo et al.
APL 80, 2654 (2002)
• Metal-insulator transition and Anderson localization are
essential ingredient of the problem
Basic Properties of Ferromagnetic
Semiconductors
• Magnetic property
• Magneto-transport
property (Anomalous Hall
Effect)
• RH=(R0/d) B+(RM/d) M
Carrier induced ferromagnetism?
• Dependence of TC on x
Metal-insulator transitions
Summary of optical studies
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InMnAs (Hirakawa et al.,, Physica E10, 215
(2001))
The conductivity could be well fitted with Drude
model, indicating the holes are delocalizd.
Localization length of hole estimated to be 3-4
nm, close to the average inter-Mn distance.
Add figure from their paper
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GaMnAs:(Hirakawa et al. PRB 65, 193312,
(2002))
Non-Drude-like FIR response observed.
Broad conductivity peak near 200meV observed.
Estimated mean free path of 0.5nm implies that
even for the metallic sample the hole
wavefunction is localized. RKKY?
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GaMnAs (Singley et al. PRL 89, 097203 (2002))
A broad band centered at 200 meV is observed.
From the sum rule analysis it was found that the
charge carrier has a very heavy effective mass
0.7me< m*< 15me. for the x=.052 sample. It is
suggested that the holes reside in the impurity
band
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GAMnAs (Yang et al., PRB 67, 04505 (2003))
A non-perturbative self-consistent study which
treat both disorder and interaction on equal
footing.
The broad peak centered at 220 meV is present
even in a one band approximation.
Non-Drude behavior could be accounted for if
multiple scattering is taken in to consideration
A new feature at around 7000 cm-1, originated
from the transition from heavy hole to split off
band is predicted.
Meatal-insulator transitions in
doped semiconductor
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Impurity level broaden into an impurity
band.
Impurity band merge with the valence
band.
Where are position of the Fermi level and
the position of the mobility edge?
Samples:
Sample
Mn concentration
Structure
(Bottom>Top)
Growth time
Thickness
10529A
X=1.4%
GaAs
LT-GaAs
LT-GaMnAs
8 min
8 min
24 min
100 nm
100 nm
300 nm
01016A
X=2.4%
GaAs
LT-GaAs
LT-GaMnAs
8 min
8 min
24 min
100 nm
100 nm
300 nm
30422A
X=3.3%
GaAs
LT-GaAs
LT-GaMnAs
30min
11sec
800 sec
400 nm
2nm
200 nm
11127A
X=4.8%
GaAs
LT-GaAs
LT-GaMnAs
30min
11sec
900 sec
400 nm
2 nm
210 nm
21028G
X=6.2%
GaAs
LT-GaAs
LT-GaMnAs
20min
12sec
500 sec
300 nm
2.6 nm
120nm
T-dependent magnetization
T-dependent RXX
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Source
Beam splitter
NIR (13000~4000 cm-1) Tungsten
Si/Ca
MIR(4000~400 cm-1)
Globar
KBr
FIR(400~10 cm-1)
Hg Lamp
Mylar 6μm
Detector
InSb (LN2)
MCT (LN2)
Bolometer (LHe)
FIR transmission data
0.8
Ga1-xMnxAs
Transmittance (a.u.)
T=300 K
0.6
x=1.4%
x=2.4%
x=3.3%
x=4.8%
0.4
0.2
0.0
0
50
100
150
200
250
300
-1
Frequency (cm )
• Flat response in the low energy region
• Zero transmission for x=4.8% sample
350
400
Transmission data –IR and near IR
1.2
Ga1-xMnxAs
T=300 K
Transmittance (a.u.)
1.0
x=1.4%
x=2.4%
x=3.3%
x=4.8%
0.8
0.6
0.4
0.2
0.0
0
2000
4000
6000
8000
10000
12000
-1
Frequency (cm )
Absorption dips for low x samp[learound 2000 cm -1.
Peculiar behavior of x=4.8% sample: below opaque below about 1500cm-1 but become
transparent above 1500 cm-1
Plasma frquency
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ωp=(4πn e2/ m*) ½
n=m* ωp2 / 4πn e2
Take ћ ωp= 2000cm-1, m*= 0.5 me, we get
n=5* 10 19 cm-3
AB =- log TR
3.0
x=1.4%
Ga1-XMnxAs
Absorption (a.u.)
2.5
x=2.4%
T=300 K
x=3.3%
x=4.8%
2.0
1.5
1.0
0.5
0.0
0
2000
4000
6000
8000
10000
12000
-1
Frequency (cm )
• Three peaks could be identified: 1648 cm-1 for X=.14% sample,
1712 cm-1 for 2.4% sample and 1872 cm-1 for x=3.35% sample.
Absorption spectra in mid and near
IR
2.0
x=1.4%
Ga1-XMnxAs
x=2.4%
T=300 K
x=3.3%
Absorption (a.u.)
1.5
x=4.8%
1.0
0.5
0.0
4000
6000
8000
-1
Frequency (cm )
10000
Refletance spectra in FIR
3.0
Reflectance (a.u.)
2.5
GaxMn1-xAs
x=1.4%
T=300 K
x=2.4%
x=3.3%
x=4.8%
2.0
1.5
1.0
0.5
0.0
0
50
100
150
200
250
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Frequency (cm )
300
350
400
Reflectance spectra in mid-IR and near IR
2.0
Ga1-xMnxAs
x=1.4%
T=300 K
x=2.4%
x=3.3%
x=4.8%
Reflectance (a.u.)
1.5
1.0
0.5
0.0
0
2000
4000
6000
8000
-1
Frequency (cm )
10000
12000
Real part of conductivity in the mid and near IR
8.0x10
2
x=1.4%
x=2.4%
x=3.3%
GaxMn1-xAs
6.0x10
4.0x10
2
2.0x10
2
-1
-1
σ 1(ω )(Ω m )
T=300K
2
0.0
0
2000
4000
6000
8000
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Wave Number (cm )
10000
12000
4.0x10
2
x=1.4%
x=2.4%
x=3.3%
GaxMn1-xAs
-1
-1
(ω )(Ω m )
T=300K
2
σ
1
2.0x10
0.0
4000
6000
8000
10000
-1
Wave Number (cm )
Black Box
Focus Lens
Sample
VNDF
Light Source
Power
Supply
Long Pass
Filter
Monochromator
Focus Lens
Detector
Motor
Driver
High Voltage
Amplifier
High Voltage output
Voltage Monitor
Ref.out
Oscillo
Scope
Signals in
Input
GPIB
Lock-in
Amplifier
PC
CER Measurement System
圖四
Input
Band filling effect for InP heavily doped with Se
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P.M. Raccah et al., APL 39, 496 (1981)
High doping concentration > 10 20 cm-1
Optical gap increases from 1.34 to 1.9 eV
Contactless electro-reflectance (CER) results
01016A CER Mn=2.4%
R/R
15K
30K
50K
77K X 1.5
100K X 2
125K X 2
150K X 3
175K X 4
200K X 5
225K X 5.8
250K X 7
275K X 18
300K X 24
1.35
1.40
1.45
1.50
1.55
Photon Energy (eV)
1.60
1.65
11127A CER Mn=4.8%
R/R
15K
30K
50K
77K
100K
125K
150K
175K
200K
225K
250K
275K
300K
1.30
1.35
1.40
1.45
1.50
Photon Energy (eV)
1.55
1.60
1.65
X
X
X
X
X
X
X
X
1.5
2
3
4
8
15
25
13
Band gap VS Temperature
1.52
X=1.4%
X=2.4%
X=3.3%
X=4.8%
X=6.2%
Energy (eV)
1.50
1.48
1.46
1.44
1.42
1.40
0
50
100
150
200
Temperature (K)
250
300
Above bandgap feature in the CER spectra: band filling
effect?
30422A CER 30K Mn=3.3%
10529A CER 30K Mn=1.4%
0.0008
0.004
0.0006
0.002
0.0004
0.000
X 20
R/R
R/R
0.0002
0.0000
1.546 eV
-0.002
X3
-0.0002
-0.004
-0.0004
-0.0006
1.3
1.4
1.5
1.6
1.7
-0.006
1.40
1.8
1.45
Photon Energy (eV)
1.50
1.55
1.60
1.65
Photon Energy (eV)
11127A CER 30K Mn=4.8%
0.0020
0.0004
0.0015
0.0002
0.0010
21028G CER 30K Mn=6.2%
0.0005
R/R
R/R
0.0000
-0.0002
1.5394 eV
0.0000
-0.0005
-0.0004
-0.0010
-0.0006
-0.0015
X6
1.532 eV
-0.0020
-0.0008
1.40
1.45
1.50
1.55
Photon Energy (eV)
1.60
1.65
1.40
1.45
1.50
1.55
1.60
Photon Energy (eV)
1.65
1.70
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EF= ћ 2kF2/2m*
kF=(3π2n)1/3
n= (2m* EF/ ћ 2)3/2/3π2
Take m*=0.5me , EF=30 meV,
We find n=0.3×10 20 cm-3
Summary and Conclusions:
• The FIR response of the sample appears to be flat
• Non-Drude-like optical conductivity behavior is observed in the midIR spectra
• Clear Absorption peaks observed for sample with x<=3.3%
• Metallic behavior obseved for samples with x>=4.8%
• From the the transmission data, plasma frequency and carrier
concentration could be obtained.
• Indication of band filling effect observed for some samples with EF
about 30 meV high than the valence band edge.
• The carrier concentration obtained from plasma frequency are
consistent with the carrier concentration obtained from the band
filling effect.