Study on the Diluted Magnetic Semiconductors

1. Study on the Diluted Magnetic Semiconductors 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. 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.

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

6. 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. 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.

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. 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:0503444 – 192 K (I-HEMT), 250 K (N-MEMP)

13. 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)

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, 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))

17. 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)

18. 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.

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. 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)

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. 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)

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. Rashba Hamiltonian • (LS coupling) 1. Spin polarized field effect transistor 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 > 100 micro m • 4. control of spin precession • coherent propagation of spin

36. InMnAs Q.D. Metal Metal G InAs wetting layer GaAs (channel) AlGaAs S.I. GaAs(100) 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 BiFeO3multiferroic thin film heterostructures, J. Wang et al.,Science 299, 1719 (2003).

39. 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

40. 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

41. 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!!!!!