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Study on the Diluted Magnetic Semiconductors. Nammee Kim QSRC, Dongguk University. Current Research Topics Magnetic Quantum Structures (Dot, Ring) D iluted M agnetic S emiconductors ( DMS ) F erro- E lectric S emiconductors ( FES ). Contents 1. Motivation 2. Review on DMS

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slide1

Study on the Diluted Magnetic Semiconductors

Nammee Kim

QSRC, Dongguk University

slide2

Current Research Topics

    • Magnetic Quantum Structures (Dot, Ring)
    • Diluted Magnetic Semiconductors (DMS)
    • Ferro-Electric Semiconductors (FES)
slide3

Contents

1. Motivation

2. Review on DMS

3. My Research on DMS

4. Future Research Plan

5. Conclusion

slide4

Central Processing Unit (CPU)

1. Motivation

1947-point contact transistor

1956-Nobel Prize

(Brattain, Bardeen, Shockley)

size: the wedge is 1.25 inches to a side.

slide5

Moore’s law: With price kept constant, the processing power

of microchips doubles every 18 months.(1965)

slide6

Semiconductor Device

Limitation of size reduction

( energy quantization, quantum interference etc.)

Limitation of Conventional Semiconductor Device

What physics?

What materials?

What device structures?

slide7

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
slide8

e

e

This technology exists between the magnetism and electronics of semiconductors.

Ferromagnetic materials

Conventional semiconductors

Hybrid

charge

spin

Spin-Electronics

  • Capable of much higher speed at very low
  • power, higher density, and nonvolatile
  • Spin FET, spin LED, Spin RTD, etc.
slide9

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…)

slide10

Main Issues in DMS

  • Enhance Tc (Curie Temp.) above Room temperature
  • Structures and Materials
  • Control of ferromagnetism
slide11

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)

Tc = 110 K with x=0.05

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

Tc = 160 K with x=0.085

slide12

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)

slide13

5 K

100K

285 K

4. Structural Method(Digital alloy)

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

layer

containing

Mn

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

slide15

Electric-field Control of Ferromagnetism

H. Ohno, Nature 408, 944 (2000)

slide16

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))

slide17

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 semiconductorquantum wires

(Phys. Lett. A 302, 341-344 (2002))

11. Finite-Temperature Study of a Modulation-Doped DMS Quantum Wellwith 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)

slide18

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

B. Lee, T.Jungwirth, A.H.MacDonald

PRB 61, 15606 (2000)

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 ….

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.
slide20

*Spin- polarization:

*Hole-density:

slide22

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.
slide23

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.

slide24

V

B

1

2

3

4

5

Kim-fig1

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.

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)

slide25

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.

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.

slide26

Effect of the well width

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.

slide27

layer

containing

Mn

Isolated Mn ions

Quasi-2D Islands

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

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

slide30

The magnetization of digital alloys also strongly depends on the carrier and Mn ion concentrations and distribution of Mn ions in the system.

slide31

5 K

100K

285 K

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

slide32

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

slide33

Rashba Hamiltonian

  • (LS coupling)

1. Spin polarized field effect transistor

Suggested by S. Datta and B. Das,

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

slide34

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

slide35

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
slide36

InMnAs Q.D.

Metal

Metal

G

InAs wetting layer

GaAs (channel)

AlGaAs

S.I. GaAs(100)

DMS

DMS

slide37

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

slide38

Example 2: Epitaxial BiFeO3multiferroic thin film heterostructures,

J. Wang et al.,Science 299, 1719 (2003).

slide39

CMS:Au

ZnCrTe ZnLiMnO

ZnCrTe

FM

CMS

CdZnS

ZnCdTe ZnLiO

FES

FM

CMS:Au

ZnCrTe

ZnCrTe ZnLiMnO

Multilayer Structures

Diluted Magnetic Semiconductors

(DMS)

Ferromagnetic

Ferro-Electric Semiconductors

(FES)

Ferroelectric

slide40

FES의 dipole

DMS의 spin

Parallel polarization

ID

Quaternary

Anti-parallel polarization

VD-S

(VG = constant)

Quaternary

Dipole Valve

Gate(Au)

FES

FES

FES

DMS

Insulator

Si

slide41

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!!!!!