Nammee Kim
QSRC, Dongguk University
Quantum-functional Semiconductor Research Center, Dongguk University
Current Research Topics
• Magnetic Quantum Structures (Dot, Ring)
• Diluted Magnetic Semiconductors (DMS)
• Ferro-Electric Semiconductors (FES)
Quantum-functional Semiconductor Research Center, Dongguk University
Contents
1.
Motivation
2.
Review on DMS
3.
My Research on DMS
4.
Future Research Plan
5.
Conclusion
Quantum-functional Semiconductor Research Center, Dongguk University
1. Motivation
1947-point contact transistor
Central Processing Unit (CPU)
1956-Nobel Prize
(Brattain, Bardeen, Shockley)
size: the wedge is 1.25
inches to a side.
Quantum-functional Semiconductor Research Center, Dongguk University
Moore’s law: With price kept constant, the processing power
of microchips doubles every 18 months.(1965)
Year of introduction
Transistors
4004
1971
2,250
8008
1972
2,500
8080
1974
5,000
8086
1978
29,000
286
1982
120,000
386™ processor
1985
275,000
486™ DX processor
1989
1,180,000
Pentium® processor
1993
3,100,000
Pentium II processor
1997
7,500,000
Pentium III processor
1999
24,000,000
Pentium 4 processor
2000
42,000,000
Quantum-functional Semiconductor Research Center, Dongguk University
Semiconductor Device
Limitation of size reduction
( energy quantization, quantum interference etc.)
Limitation of Conventional Semiconductor Device
What physics?
What materials?
What device structures?
Quantum-functional Semiconductor Research Center, Dongguk University
Spintronics? Spintronics involves the study of active control and
manipulation of spin degree of freedom in solid-state system.
• Electronics – charge
metal, doped semiconductors
• Spintronics – charge+ spin
metal, doped semiconductors, magnetic materials
Quantum-functional Semiconductor Research Center, Dongguk University
This technology exists between the magnetism and electronics of semiconductors.
Ferromagnetic
materials
Hybrid
Conventional
semiconductors
charge
spin
e
e
Spin-Electronics
• Capable of much higher speed at very low
power, higher density, and nonvolatile
• Spin FET, spin LED, Spin RTD, etc.
Quantum-functional Semiconductor Research Center, Dongguk University
2. Diluted Magnetic Semiconductors (DMS)
History
• II-VI DMS
CdMnSe, ZnMnTe, HgMnTe...
J. K. Furdyna, J. Appl. Phys. 64, R29 (1988)
• III-V DMS
InMnAs, GaMnAs, GaMnN, ZnMnO…
H. Munekata et al., PRL 63, 1849 (1989)
H. Ohno et al., J. Magn. Magn. Mater. 200, 110 (1999).
Conventional non-magnetic semiconductors (II-VI, III-V..)
PLUS Magnetic Elements (Mn, Co, Ni, Fe…)
Quantum-functional Semiconductor Research Center, Dongguk University
Main Issues in DMS

Enhance Tc (Curie Temp.) above Room temperature
Structures and Materials
 Control of ferromagnetism
Quantum-functional Semiconductor Research Center, Dongguk University
Research progresses
 Enhance Tc of GaMnAs
2. Effect of annealing
1. Optimal Doping Rate
in As grown sample
H. Ohno et al., J. Magn. Magn. Mater. 200, 110(1999) Ku et al., APL 82, 2302 (2003)
Tc = 110 K with x=0.05
Tc = 160 K with x=0.085
Quantum-functional Semiconductor Research Center, Dongguk University
3. Effect of selective doping and annealing
M. Tanaka et al . APL 80, 3120 (2002) Tc=170 K
Cond-matt:0503444 – 192 K (I-HEMT), 250 K (N-MEMP)
Quantum-functional Semiconductor Research Center, Dongguk University
4. Structural Method (Digital alloy)
Result of TEM GaSb (12 ML)/Mn (0.5ML)
6
5K
4
100K
2
-5
M (10 emu)
layer
containing
Mn
285 K
0
-2
-4
-1500 -1000
-500
0
500
1000
1500
M agnetic Field (Gauss)
H. Luo et al., Appl. Phys. Lett. 81, 511 (2002)
Quantum-functional Semiconductor Research Center, Dongguk University
T. Dietl, SCIENCE 287, 1019 (2000)
Quantum-functional Semiconductor Research Center, Dongguk University
 Electric-field Control of Ferromagnetism
H. Ohno, Nature 408, 944 (2000)
Quantum-functional Semiconductor Research Center, Dongguk University
3. My Research on DMS
1. Controllable spin polarization of carriers in a DMS quantum dot
(ssc submitted)
2. Ferromagnetic properties of Mn-doped III-V semiconductor quantum wells
(Superconductivity/Novel Magnetism, 18, 189-193 (2005))
3. Magnetic properties of p-doped GaMnN diluted magnetic semiconductor containing clusters
(Solid State Commun. 133, 629-633 (2005))
4. Numerical study of ferromagnetism of a GaMnN quantum well
(J. Korean Phys. Soc. 45, 568-571 (2004))
5. Curie Temperatures of Magnetically Heavily Doped III-V/Mn Alloys
(J. Korean Phys. Soc. 45, 647-649 (2004))
6. Effect of cluster-type on the Ferromagnetism of a GaMnN quantum well
(Phys. Lett. A , 329, 226-230 (2004))
Quantum-functional Semiconductor Research Center, Dongguk University
7. Curie temperature modulation by electric fields in Mn delta-doped asymmetric double quantum well
(Phys. Rev. B 69, 115308.1-115308.4 (2004))
8. Model study on the magnetization of digital alloys
(Phys. Rev. B 68, 172406.1-172406.4 (2003))
9. Growth of ferromagnetic semiconducting Si:Mn film by Vacuum Evaporation Method
(Chem. Mater.15, 3964 (2003))
10. Study on phase transitions of III-Mn-V diluted magnetic semiconductor quantum wires
(Phys. Lett. A 302, 341-344 (2002))
11. Finite-Temperature Study of a Modulation-Doped DMS Quantum Well with Broken Spin Symmetry
(Physica E 12, 383-387(2002))
12. Magnetization of a diluted magnetic semiconductor quantum well in a parallel magnetic field
(J. Korean Phys. Soc. 39 , 1050-1054 (2001)
Quantum-functional Semiconductor Research Center, Dongguk University
1. Ferromagnetic properties of Mn-doped III-V semiconductor quantum wells
(J. Superconductivity/Novel Magnetism, 18, 189-193 (2005))
Previous theoretical studies on III-V DMS quantum wells have predicted ….
xN0 S S  1 2
1
Tc 
12k B  2 1 e 2  3 2 1/ 3  2 / 3

 n2 d

mt* 3  0  w 
L.Bery and F. Guinea PRL 85 ,2384 (2000)
xN S S  1 2 mt*
Tc  0
dz  n( 0) ( z )
2 
12k B
 0
w
4
B. Lee, T.Jungwirth, A.H.MacDonald
~ 1/ d
PRB 61, 15606 (2000)
Purpose of this work:
To know the dependence of Tc on free carrier density, magnetic impurity
density and spin-exchange interaction energy!!!
To compare the magnetic properties of In1-xMnxP and Ga1-xMnxN.
Quantum-functional Semiconductor Research Center, Dongguk University
Hamiltonian
H  H K .E.  Vconf  Vpd  VH  Vxc
Quantum-functional Semiconductor Research Center, Dongguk University
* Spin- polarization:
 z    p z   p z  / p2 D z 
* Hole-density:
p2 D z   p z   p z 
Quantum-functional Semiconductor Research Center, Dongguk University
Self-Consistent Calculation
 n , n ( z )
p z , p z 
 vs. T at P2D
Tc vs. P2D
Quantum-functional Semiconductor Research Center, Dongguk University
Case of In1-xMnxP quantum well
The dependence of the Tc on the carrier density of In1-xMnxP exhibits step-like behavior due to
the discrete energy subbands by confinement effects.
The Tc of the p-type In1-xMnxP quantum wells increases as the magnetic impurity density and
the spin-exchange interaction energy increase.
Quantum-functional Semiconductor Research Center, Dongguk University
Case of Ga1-xMnxN quantum well
Ga1-xMnxN shows weak step-like behavior compared to other III-Mn-V DMS quantum wells
because the hole effective mass of Ga1-xMnxN is very large and the large hole effective mass
reduces the energy splitting due to the confinement effects.
Contributions: Verify the relation between Tc and the carrier density quantitatively.
Surely Ga1-xMnxN has Tc above room temperature as predicted by Dietl.
Quantum-functional Semiconductor Research Center, Dongguk University
2. Curie temperature modulation by electric fields in Mn delta-doped asymmetric double
quantum well (Phys. Rev. B 69, 115308.1-115308.4 (2004))
Purpose of this work: to suggest a quantum structure to enhance Tc and
to control ferromagnetism by the external electric field.
V
Fg
Vh
zh
D1
W1
D2
B
T. Dietl et al. PRB 55, R3347(1997)
A.H.MacDonald et al. PRB 61,15606(2000)
M. Tanaka et al . APL 80, 3120 (2002)
W2
1

2
3 4
5

Kim-fig1
Quantum-functional Semiconductor Research Center, Dongguk University
The change of the Tc as a function of the applied electric fields
The change of the fourth power of the growth direction envelope
function of carriers at the lowest subband.
2.0
10
w1=10nm, w2=10nm, B=5nm
W1=10nm, W2=0nm
center-doped
edge-doped
center-doped
edge-doped
9
8
Tc/Tc02
Tc/Tc01
1.5
1.0
0.020
Fg=0.0 meV/nm
7
6
5
0.020
4
0.016
Fg = 0.1 meV/nm
Fg=5.1 meV/nm
Fg=7.0 meV/nm

 
0.016
Fg = 3.0 meV/nm
0.012
0.012

3
0.008
 
0.5
Fg = 0.5 meV/nm
0.008
2
0.004
0.004
0.000
-15.0 -12.5 -10.0 -7.5 -5.0 -2.5
Zh(nm)
0.0
0.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
1
0.000
-20 -15 -10 -5
Fg(meV/nm)
Kim-fig2
0
5
10 15 20
zh(nm)
0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Fg(meV/nm)
Kim-fig3
The Curie temperature is enhanced up to eight times higher than
the case of no external electric fields for both of the Mn edge-doped
and Mn center-doped samples.
Quantum-functional Semiconductor Research Center, Dongguk University
Effect of the well width
10
9
8
Tc/ Tc02
7
6
5
4
3
W1=10nm, B=5nm, Fg=0.5meV/nm
2
center-doped
edge-doped
1
0
Kim-fig4
5
6
7
8
9
10
11
12
13
14
15
W2(nm)
The Curie temperature is controlled not only by applied electric fields
but also by asymmetry (or amount of p-dopants) of wells.
Contributions: Propose a quantum structure to enhance Tc of
DMS by applying
an electric field to a Mn-delta-doped asymmetric double quantum well structure.
Quantum-functional Semiconductor Research Center, Dongguk University
3. Model study on the magnetization of digital alloys
(Phys. Rev. B 68, 172406.1-172406.4 (2003))
Purpose of this work: To propose a new model of 2D system applied to the individual Mn
layer in digital alloys to explain ferromagnetism of digital alloys.
Model
layer
containin
g
Mn
Isolated Mn ions
Quasi-2D Islands
H. Luo et al., Appl. Phys. Lett. 81, 511 (2002)
Quantum-functional Semiconductor Research Center, Dongguk University
Hamiltonian
Quantum-functional Semiconductor Research Center, Dongguk University
Total magnetization
Quantum-functional Semiconductor Research Center, Dongguk University
6
-7
M (10 em u)
5
4
3
2
1
0
50 100 150 200 250 300 350 400
T(K )
The magnetization of digital alloys also strongly depends on the carrier and Mn ion
concentrations and distribution of Mn ions in the system.
Quantum-functional Semiconductor Research Center, Dongguk University
5K
4
100K
2
-5
M (10 em u)
6
285 K
0
-2
-4
-1500 -1000
-500
0
500
1000
1500
Magnetic Field (Gauss)
Appl. Phys. Lett. 81, 511 (2002)
This model produces temperature dependent magnetization as a function of
external magnetic field qualitatively.
Contributions: Propose a new model for the digital alloys to explain the ferromagnetic
mechanism and magnetic properties of the digital alloys successfully
Quantum-functional Semiconductor Research Center, Dongguk University
4. Future Research Plan
Purpose: to achieve new concept quantum structures and Devices.
1. SPFET (Spin Polarized Field Effect Transistor)spin polarization, spin injection, spin transport
2. Multi-ferroic material and quantum structurescombine DMS and FES
Quantum-functional Semiconductor Research Center, Dongguk University
1. Spin polarized field effect transistor
•
Rashba Hamiltonian
(LS coupling)



H R     k  zˆ

E ( )   2 k x1 / 2m  k x1
2
E (  )   2 k x 2 / 2 m   k x2
2
Suggested by S. Datta and B. Das,
k x1  k x2  2m /  2
Appl. Phys. Lett. 56, 665(1990)
  (k x1  k x2 ) L  2mL /  2
Quantum-functional Semiconductor Research Center, Dongguk University
Schematic idea of the spin transistor
With a gate voltage V1, spin
of electrons precess with π
between two ferromagnets.
Expect high resistance
With a gate voltage V2, spin
of electrons precess with 2π
between two ferromagnets.
Expect low resistance
Quantum-functional Semiconductor Research Center, Dongguk University
Requirements for a spin transistor
1. spin polarizer & spin detector (collector)
cf> Ferromagnetic material such as permalloy (Ni80Fe20) or iron
polarize about 45% of electron spins
2. High spin injection rate - low resistivity mismatch
3. 2 dimensional electron gas(2DEG) channel- 1dimensional channel
high mobility
high carrier concentration
large spin-orbit interaction parameter
cf>Surface states of semiconductor, 2DES----InAs, GaAs……
spin life time > 100 ns, coherent travel distance > 100 micro m

4. control of spin precession
coherent propagation of spin
Quantum-functional Semiconductor Research Center, Dongguk University
InMnAs Q.D.
Metal
G
Metal
InAs wetting layer
GaAs (channel)
AlGaAs
S.I. GaAs(100)
DMS
DMS
Quantum-functional Semiconductor Research Center, Dongguk University
2. Multi-ferroic materials
Example 1: Mutiferroic BaTiO3-CoFe2O4 nanostructures
H. Zheng et al., Science 303,661 (2004).
CoFe2O4-spinel
BaTiO3-perovskite
SrTiO3 (001) Substrate
By Pulsed laser deposition
Quantum-functional Semiconductor Research Center, Dongguk University
Example 2: Epitaxial BiFeO3 multiferroic thin film heterostructures,
J. Wang et al.,Science 299, 1719 (2003).
Quantum-functional Semiconductor Research Center, Dongguk University
Multilayer Structures
Diluted Magnetic Semiconductors
(DMS)
Ferromagnetic
Ferro-Electric Semiconductors
(FES)
Ferroelectric
FM
ZnCrTe
ZnLiMnO CMS:Au
FES
ZnCdTe
ZnLiO
FM
ZnCrTe
ZnLiMnO CMS:Au
CMS
ZnCrTe
CdZnS
ZnCrTe
Quantum-functional Semiconductor Research Center, Dongguk University
Quaternary
Dipole Valve
Gate(Au)
FES
FES
FES
Insulator
DMS
Si
ID
Parallel polarization
Anti-parallel polarization
(VG = constant)
FES의 dipole
DMS의 spin
Quaternary
VD-S
Quantum-functional Semiconductor Research Center, Dongguk University
5. Conclusion
 Spintronics will find a breakthrough to overcome the limitation of
semiconductor devices.
 DMS is a good candidate of spintronics materials.
 We have accomplished good contributions to the
developments of DMS materials and structures experimentally
as well as theoretically.

Future plans developing spintronics devices based on
these study will open the new concept quantum computers
and artificial intelligence, which are expected to change
the paradigm of the future information society.
Thank you for your attention!!!!!
Quantum-functional Semiconductor Research Center, Dongguk University