quantum-functional semiconductor research center, dongguk university nammee kim qsrc, dongguk...

Quantum-functional Semiconductor Research Center, Dongguk University

Nammee Kim

QSRC, Dongguk University


Current Research Topics

• Magnetic Quantum Structures (Dot, Ring)

• Diluted Magnetic Semiconductors (DMS)

• Ferro-Electric Semiconductors (FES)


Contents

1. Motivation 2. Review on DMS 3. My Research on DMS

4. Future Research Plan

5. Conclusion


1. Motivation

size: the wedge is 1.25 inches to a side.

1947-point contact transistor

1956-Nobel Prize

(Brattain, Bardeen, Shockley)

Central Processing Unit (CPU)


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

Moore’s law: With price kept constant, the processing power of microchips doubles every 18 months.(1965)


Limitation of Conventional Semiconductor Device

Semiconductor Device

Limitation of size reduction( energy quantization, quantum interference etc.)

What physics?

What materials?

What device structures?


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


• Capable of much higher speed at very low

power, higher density, and nonvolatile

• Spin FET, spin LED, Spin RTD, etc.

Conventional semiconductor

s

Spin-Electronics

Ferromagnetic materials

spin echarge

e

Hybrid

This technology exists between the magnetism and electronics of semiconductors.


Conventional non-magnetic semiconductors (II-VI, III-V..)

PLUS Magnetic Elements (Mn, Co, Ni, Fe…)

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).

2. Diluted Magnetic Semiconductors (DMS)


Main Issues in DMS

Enhance Tc (Curie Temp.) above Room temperature

Structures and Materials

Control of ferromagnetism


Research progresses

Enhance Tc of GaMnAs

H. Ohno et al., J. Magn. Magn. Mater. 200, 110(1999)

Tc = 110 K with x=0.05

1. Optimal Doping Rate in As grown sample

2. Effect of annealing

Ku et al., APL 82, 2302 (2003)

Tc = 160 K with x=0.085


3. Effect of selective doping and annealing

M. Tanaka et al . APL 80, 3120 (2002) Tc=170 KCond-matt:0503444 – 192 K (I-HEMT), 250 K (N-MEMP)


4. Structural Method (Digital alloy)

Result of TEM GaSb (12 ML)/Mn (0.5ML)

-1500 -1000 -500 0 500 1000 1500

-4

-2

0

2

4

6

Magnetic Field (Gauss)

M (

10-5

em

u)

5 K100K

285 K

layer containing Mn

H. Luo et al., Appl. Phys. Lett. 81, 511 (2002)


T. Dietl, SCIENCE 287, 1019 (2000)


Electric-field Control of Ferromagnetism

H. Ohno, Nature 408, 944 (2000)


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))

3. My Research on DMS


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)


Previous theoretical studies on III-V DMS quantum wells have predicted ….

B. Lee, T.Jungwirth, A.H.MacDonald PRB 61, 15606 (2000)

4

0

)0(2

*20 )(

12

1

w

nt

Bc zdz

m

k

SSxNT

d/1~

L.Bery and F. Guinea PRL 85 ,2384 (2000)

3/2

2

3/12

0

2

*

2

20

331

1

12

1

dt

Bc

nw

em

SSxN

kT

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.

1. Ferromagnetic properties of Mn-doped III-V semiconductor quantum wells (J. Superconductivity/Novel Magnetism, 18, 189-193 (2005))


xcHpdconfEK VVVVHH ..

Hamiltonian


zpzpzpz D2/ * Spin- polarization:

zpzpzp D 2

* Hole-density:


)(, znn

zpzp ,

Pat T vs. 2D P vs. 2DcT

Self-Consistent CalculationSelf-Consistent Calculation


Case of In1-xMnxP quantum well 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.


Case of Ga1-xMnxN quantum well 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.


V

B

hz

1 2 3 4 5

1D 1W 2W 2D

hV

Kim-fig1

gF

T. Dietl et al. PRB 55, R3347(1997)

A.H.MacDonald et al. PRB 61,15606(2000)

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.

M. Tanaka et al . APL 80, 3120 (2002)


0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.00.0

0.5

1.0

1.5

2.0

-15.0 -12.5 -10.0 -7.5 -5.0 -2.5 0.00.000

0.004

0.008

0.012

0.016

0.020

Fg(meV/nm)

W1=10nm, W

2=0nm

center-doped edge-doped

Tc/T

c01

Zh(nm)

Fg=0.0 meV/nm

Fg=5.1 meV/nm

Fg=7.0 meV/nm

Kim-fig2

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.00

1

2

3

4

5

6

7

8

9

10

-20 -15 -10 -5 0 5 10 15 200.000

0.004

0.008

0.012

0.016

0.020

Fg(meV/nm)

w1=10nm, w

2=10nm, B=5nm


Tc/T

c02

zh(nm)

Fg = 0.1 meV/nm

Fg = 0.5 meV/nm

Fg = 3.0 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.

The change of the Tc as a function of the applied electric fieldsThe change of the fourth power of the growth direction envelope function of carriers at the lowest subband.


Effect of the well width

5 6 7 8 9 10 11 12 13 14 150

1

2

3

4

5

6

7

8

9

10

Tc/ T

c02

W2(nm)

W1=10nm, B=5nm, F

g=0.5meV/nm


Kim-fig4

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.


Isolated Mn ions

Quasi-2D Islands

Model

3. Model study on the magnetization of digital alloys (Phys. Rev. B 68, 172406.1-172406.4 (2003))

layer containing Mn

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.

H. Luo et al., Appl. Phys. Lett. 81, 511 (2002)


Hamiltonian


Total magnetization


0 50 100 150 200 250 300 350 400

1

2

3

4

5

6

M (

10

-7 e

mu

)

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.


-1500 -1000 -500 0 500 1000 1500

-4

-2

0

2

4

6

Magnetic Field (Gauss)

M (

10

-5 e

mu

)

5 K100K

285 K

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

Appl. Phys. Lett. 81, 511 (2002)


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 structures- combine DMS and FES


1. Spin polarized field effect transistor

Suggested by S. Datta and B. Das,

Appl. Phys. Lett. 56, 665(1990)

2

2

22

2

22

/2)(

/2

2/)(

2/)(

ˆ

21

21

2

11

LmLkk

mkk

kmkE

kmkE

zkH

xx

xx

xx

xx

R

• Rashba Hamiltonian(LS coupling)


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


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


G

S.I. GaAs(100)

Metal MetalInMnAs Q.D.

InAs wetting layer

GaAs (channel)AlGaAs

DMS DMS


Example 1: Mutiferroic BaTiO3-CoFe2O4 nanostructures H. Zheng et al., Science 303,661 (2004).

CoFe2O4-spinel BaTiO3-perovskiteSrTiO3 (001) SubstrateBy Pulsed laser deposition

2. Multi-ferroic materials


Example 2: Epitaxial BiFeO3 multiferroic thin film heterostructures, J. Wang et al.,Science 299, 1719 (2003).


Diluted Magnetic Semiconductors (DMS)Ferromagnetic

Multilayer Structures

Ferro-Electric Semiconductors(FES)Ferroelectric

ZnCrTe ZnLiMnO

ZnCdTe ZnLiO

ZnCrTe ZnLiMnOFM

FM CMS:Au

CMS:Au

CMSFES

ZnCrTe

ZnCrTe

CdZnS


FES FES

Gate(Au)

ID

VD-S

Parallel polarization

Anti-parallel polarization

(VG = constant)

Si

Dipole Valve

DMS

FES

Insulator

QuaternaryQuaternaryFESFES 의 의 dipole dipole DMSDMS 의 의 spinspin

Quaternary


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.

5. Conclusion

Thank you for your attention!!!!!

quantum-functional semiconductor research center, dongguk university nammee kim qsrc, dongguk...

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