1. Study on the Diluted Magnetic Semiconductors QSRC, Dongguk University
Nammee Kim
QSRC, Dongguk University

2. Current Research Topics
Magnetic Quantum Structures (Dot, Ring)
Diluted Magnetic Semiconductors (DMS)
Ferro-Electric Semiconductors (FES)

3. Contents 1. Motivation 2. Review on DMS 3. My Research on DMS
4. Future Research Plan
5. Conclusion

4. 1. Motivation 1947-point contact transistor 1956-Nobel Prize
(Brattain, Bardeen, Shockley)
Central Processing Unit (CPU)
size: the wedge is 1.25 inches to a side.

5. 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

6. What device structures?
Semiconductor Device
Limitation of size reduction
( energy quantization, quantum interference etc.)
Limitation of Conventional Semiconductor Device
What physics?
What materials?
What device structures?

7. 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

8. Ferromagnetic materials Conventional semiconductors
This technology exists between the magnetism and electronics of semiconductors.
Ferromagnetic materials
Conventional semiconductors
Hybrid e
charge
spin e
Spin-Electronics
Capable of much higher speed at very low
power, higher density, and nonvolatile
Spin FET, spin LED, Spin RTD, etc.

9. 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…)

10. Main Issues in DMS Enhance Tc (Curie Temp.) above Room temperature
Structures and Materials
Control of ferromagnetism

11. 1. Optimal Doping Rate in As grown sample
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

12. 3. Effect of selective doping and annealing
M. Tanaka et al . APL 80, 3120 (2002) Tc=170 K
Cond-matt: – 192 K (I-HEMT), 250 K (N-MEMP)

13. 4. Structural Method (Digital alloy)
Result of TEM GaSb (12 ML)/Mn (0.5ML)
5 K
100K
285 K
layer
containing Mn
H. Luo et al., Appl. Phys. Lett. 81, 511 (2002)

14. T. Dietl, SCIENCE 287, 1019 (2000)

15. Electric-field Control of Ferromagnetism
H. Ohno, Nature 408, 944 (2000)

16. 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, (2005))
3. Magnetic properties of p-doped GaMnN diluted magnetic semiconductor containing clusters
(Solid State Commun. 133, (2005))
4. Numerical study of ferromagnetism of a GaMnN quantum well
(J. Korean Phys. Soc. 45, (2004))
5. Curie Temperatures of Magnetically Heavily Doped III-V/Mn Alloys
(J. Korean Phys. Soc. 45, (2004))
6. Effect of cluster-type on the Ferromagnetism of a GaMnN quantum well
(Phys. Lett. A , 329, (2004))

17. 7. Curie temperature modulation by electric fields in Mn delta-doped asymmetric double quantum well
(Phys. Rev. B 69, (2004))
8. Model study on the magnetization of digital alloys
(Phys. Rev. B 68, (2003))
9. Growth of ferromagnetic semiconducting Si:Mn film by Vacuum  Evaporation Method
(Chem. Mater.15, (2003))
10. Study on phase transitions of III-Mn-V diluted magnetic semiconductor quantum wires
(Phys. Lett. A 302, (2002))
11. Finite-Temperature Study of a Modulation-Doped DMS Quantum Well  with Broken Spin Symmetry
(Physica E 12, (2002))
12. Magnetization of a diluted magnetic semiconductor quantum well in a  parallel magnetic field
(J. Korean Phys. Soc. 39 , (2001)

18. 1. Ferromagnetic properties of Mn-doped III-V semiconductor quantum wells
(J. Superconductivity/Novel Magnetism, 18, (2005))
Previous theoretical studies on III-V DMS quantum wells have predicted ….
B. Lee, T.Jungwirth, A.H.MacDonald
PRB 61, (2000)
L.Bery and F. Guinea PRL 85 ,2384 (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.

19. Hamiltonian

20. * Spin- polarization:
* Hole-density:

21. Self-Consistent Calculation

22. 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.

23. 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.

24. Purpose of this work: to suggest a quantum structure to enhance Tc and
2. Curie temperature modulation by electric fields in Mn delta-doped asymmetric double quantum well (Phys. Rev. B 69, (2004))
Purpose of this work: to suggest a quantum structure to enhance Tc and
to control ferromagnetism by the external electric field. V B 1 2 3 4 5
Kim-fig1
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)

25. 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.

26. 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.

27. 3. Model study on the magnetization of digital alloys
(Phys. Rev. B 68, (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
containing Mn
Isolated Mn ions
Quasi-2D Islands
H. Luo et al., Appl. Phys. Lett. 81, 511 (2002)

28. Hamiltonian

29. Total magnetization

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

31. 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

32. 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

33. 1. Spin polarized field effect transistor
Rashba Hamiltonian
(LS coupling)
Suggested by S. Datta and B. Das,
Appl. Phys. Lett. 56, 665(1990)

34. 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

35. 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 > micro m
4. control of spin precession
coherent propagation of spin

36. Metal GaAs (channel) AlGaAs S.I. GaAs(100) InMnAs Q.D. G DMS DMS
InAs wetting layer
GaAs (channel)
AlGaAs
DMS
DMS

37. 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

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

39. FES FM Multilayer Structures ZnCrTe ZnLiMnO ZnCdTe ZnLiO CMS:Au CMS
Diluted Magnetic Semiconductors
(DMS)
Ferromagnetic
Ferro-Electric Semiconductors
(FES)
Ferroelectric
ZnCrTe ZnLiMnO
ZnCdTe ZnLiO FM
CMS:Au
CMS
FES
ZnCrTe
CdZnS

40. FES DMS Quaternary Dipole Valve FES의 dipole Quaternary DMS의 spin
Gate(Au)
FES
FES
FES
Insulator
DMS Si ID
VD-S
Parallel polarization
Anti-parallel polarization
(VG = constant)
Quaternary
FES의 dipole
DMS의 spin

41. Thank you for your attention!!!!!
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!!!!!