전이금속의 도핑에 따른 질화물계의 전자구조 및 자기적 특성 변화
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Transcript of 전이금속의 도핑에 따른 질화물계의 전자구조 및 자기적 특성 변화
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
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
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
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)
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.
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)
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
DMSs of High TcDMSs of High Tc
T. Dietl, Semicond. Sci. Technol. 17 (2002) 377
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.
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
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)
2zd 2 2x y
d
xyd xzd yzd
2gt
ge
3d Orbital Configuration3d Orbital Configuration
Orbital Degeneracy wrt Crystal StructureOrbital Degeneracy wrt Crystal Structure
Tetrahedral Configuration Octahedral Configuration
Crystal Field Splitting
t2g
eg
Crystal Field Splitting,ΔCR
t2g
eg
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)
Zinc Blende Wurtzite
ΔCR
ΔEX
Ga0.97Mn0.03NGa0.97Mn0.03N
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.
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
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
SummarySummary
GaMnN GaCuN