전이금속의 도핑에 따른 질화물계의 전자구조 및 자기적 특성 변화
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전이금속의 도핑에 따른 질화물계의 전자구조 및 자기적 특성 변화
S.C. Lee1, K.R. Lee1, K.H. Lee1, W. Temmerman2
1 Future Technology Research Division, KIST, Korea
2 Band Theory Group, Daresbury Laboratory, UK
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Spintronics DevicesSpintronics Devices
D. Awschalom et al, Sci.Am.(2002)
Magnetic Tunneling Junction Spin Field Effect Transistor
• Magnetic RAM• GMR: HDD Read Head
• Semiconductor based device• Next Generation of Spintronics
Control of Spin and Charge of Electrons Simultaneously
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2
0
02
)12(
2
fm
scmffm
scmf
x
x
G. Schmidt et al., Phys. Rev. B 62, 4790 (2000)
2 0 : Injection probability at : Spin polarization of FM metal
: Spin coherence length : Carrier conductivity in materials
fm
i
x
i
abls
Spin Injection from FM Metal to SCSpin Injection from FM Metal to SC
FMMetal
PMSC
Spin Injection
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Possible SolutionsPossible Solutions
• Diluted magnetic semiconductors (DMS): σsc/ σfm ~ 1 Fielderling et al., Nature 402 787 (1999) Ohno et al. Nature 402 790 (1999)
• Half metallic ferromagnets: β ~ 1
• Tunneling barrier at the FM/SC interaction Rashiba, Phys. Rev. B 62, 16267 (2000)
• Intrinsic Schottky barrier Zhu et al., Phys. Rev. Lett. 87, 016601 (2001) Hanbicki et al. Appl. Phys. Lett. 80, 1240 (2002)
• Spin-dependent interface resistance Fert and Jaffres, Phys. Rev. B 64, 184420 (2001) Zwierzycki et al. arXiv:cond-mat/0204422 (2002)
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Conditions for Successful DMSConditions for Successful DMS
• DMS should show ferromagnetism.– Origin of FM should be the diluted transition metal.– Clustering or third phase formation should be avoided.
• Curie temperature should be higher than room temperature.– Ferromagnetic behavior should operate at room temperature.
• Carrier of semiconductor should be spin polarized.– Spin polarized carrier is essential for application.
• Selected material should be compatible to the semiconductor process.
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Magnetic Properties of Ga1-xMnxAsMagnetic Properties of Ga1-xMnxAs
• Mn can substitute Ga in GaAs of zinc blende structure.
• Tc is correlated with carrier density.
• Ferromagnetic semiconductor with ordering temperature ~ 160K Max.
Matsukura et. al. PRB (1998)
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Ku et al., Appl. Phys. Lett. 82, 2302 (2003).
Magnetic Properties of Ga1-xMnxAsMagnetic Properties of Ga1-xMnxAs
Impurity induced polarization in the host (RKKY type)
TM
Induced hole
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DMSs of High TcDMSs of High Tc
T. Dietl, Semicond. Sci. Technol. 17 (2002) 377
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General Overview of GaNGeneral Overview of GaN
• Wide band gap semiconductor: – Direct band gap with Eg=3.5 eV (W), 3.29~3.35 (ZB)
• Generally wurtzite, but zinc blend structure is also possible.
• Intrinsic n-type semiconductor– Mg has known to be the only one element for p-type do
ping. • Applications
– Short-wavelength LED– High power/high temperature electronics
Possibility of High Tc DMS when doped with Mn.Possibility of High Tc DMS when doped with Mn.
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Calculation MethodCalculation Method
• 64 atoms (2x2x2 supercell)• VASP (Vienna Ab-initio Simulation Package)
– Planewave pseudopotential– GGA(PW91) exchange-correlation potential– Ecut: 400 eV
• Fully relaxed atomic structure• 4x4x4 Monkhorst-Pack k-point mesh• Wurzite and zinc blende GaN structures
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Wurtzite
ΔCR
ΔEX
Ga0.97Mn0.03NGa0.97Mn0.03N
• Fermi level locates at the unpaired and localized Mn t2g orbital with large ΔCR (1.5 eV)
• Possibly high magnetic moment (4B)– Large Exchange Splitting: ΔCR (1.5 eV) < ΔEX (2.1 eV)
Up Spin Down Spin
t2g
eg
GaN:Mn(7-3)
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2zd 2 2x y
d
xyd xzd yzd
2gt
ge
3d Orbital Configuration3d Orbital Configuration
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Orbital Degeneracy wrt Crystal StructureOrbital Degeneracy wrt Crystal Structure
Tetrahedral Configuration Octahedral Configuration
Crystal Field Splitting
t2g
eg
Crystal Field Splitting,ΔCR
t2g
eg
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Wurtzite
ΔCR
ΔEX
Ga0.97Mn0.03NGa0.97Mn0.03N
• Fermi level locates at the unpaired and localized Mn t2g orbital with large ΔCR (1.5 eV)
• Possibly high magnetic moment (4B)– Large Exchange Splitting: ΔCR (1.5 eV) < ΔEX (2.1 eV)
• No valence band splitting : no carrier polarization : Major problem
Up Spin Down Spin
t2g
eg
GaN:Mn(7-3)
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Zinc Blende Wurtzite
ΔCR
ΔEX
Ga0.97Mn0.03NGa0.97Mn0.03N
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Comparison of GaMnAs and GaMnNComparison of GaMnAs and GaMnN
GaMnAs GaMnN
• Mn in GaAs polarize the host valence band edge, which results in the formation of spin polarized carrier (hole) in host GaAs.
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Ga0.97Ni0.03NGa0.97Ni0.03NWurtzite
• Ni doped GaN exhibits an insulating behavior.• Spin down eg state is fully occupied by electrons.• Relatively larger exchange splitting behavior.
GaN:Ni(10-3)
Up Spin Down Spin
t2g
eg
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Wurtzite
• Fermi level locates at the unpaired spin down Cu t2g orbital.• Degree of localization is much smaller than that of GaN:Mn.• Exchange splitting is smaller.• Stronger hybridization between Cu 3d – N 2p state
Ga0.97Cu0.03NGa0.97Cu0.03N
GaN:Cu(11-3)
Up Spin Down Spin
t2g
eg
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SummarySummary
GaMnN GaCuN