Study on the Diluted Magnetic Semiconductors QSRC, 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 1947-point contact transistor 1956-Nobel Prize (Brattain, Bardeen, Shockley) Central Processing Unit (CPU) size: the wedge is 1.25 inches to a side.

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

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?

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

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.

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…)

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

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

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)

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)

T. Dietl, SCIENCE 287, 1019 (2000)

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

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))

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)

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

Hamiltonian

* Spin- polarization: * Hole-density:

Self-Consistent Calculation

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 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.

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, 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 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)

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.

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.

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 containing Mn Isolated Mn ions Quasi-2D Islands H. Luo et al., Appl. Phys. Lett. 81, 511 (2002)

Hamiltonian

Total magnetization

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

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

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 Rashba Hamiltonian (LS coupling) Suggested by S. Datta and B. Das, Appl. Phys. Lett. 56, 665(1990)

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

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

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

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

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

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

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!!!!!