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