1. First Principle Design of Diluted Magnetic Semiconductor: Cu doped GaN
S.-C. Lee*, K.-R. Lee, and K.-H. Lee
Computational Science Center
Korea Institute of Science and Technology, KOREA

2. Diluted Magnetic Semiconductors
Diluted Magnetic Semiconductor (DMS)
A ferromagnetic material that can be made by doping of impurities, especially transition metal elements, into a semiconductor host.
Conducting spin polarized carriers of DMS are used for spin injection.
Compatible with current semiconductor industry.
Spin Field Effect Transistor

3. Mechanisms of Ferromagnetism in DMSTM
Long-ranged interaction of transition metals via delocalized carrier can stabilize ferromagnetic phase.
All valence electrons of the anion atoms between TM should be spin polarized.
The spin polarized carrier can deliver information

4. Success and Failure of Mn doped GaAs
Mn substitutes Ga in zincblende structure
Structure is compatible with GaAs 2DEG
Tc is correlated with carrier density
Ferromagnetic semiconductor with ordering temperature ~ 160 K
Finding a new DMS material having high Tc Mn
Ku et al., APL (2003)

5.  What will happen if other transition elements are used as dopants?
Beyond GaMnAs
언급의 말미에는 GaN을 사용한다는 이야기를 할 것
Among various materials GaN and ZnO have been much attraction. 블라블라
T. Dietl, Semicond. Sci. Technol. 17 (2002) 377
 What will happen if other transition elements are used as dopants?

6. Material specific or chemical effect has not been considered!
Mn doped GaN: Can it be a DMS?
Positive
Theoretically predicted by Dietl
High Tc was observed above room T.
Ferromagnetic behavior by SQUID experiments
Negative
Possibility of precipitates
XMCD or anomalous Hall Effect has not been observed.
Ferromagnetism can be achieved by short ranged double exchange mechanism
여기에서는 우리가 놓친 것이 있다. 아무래도 처음부터 다시 시작해야 할 것 같다는 것으로 넘어가야 함.
Material specific or chemical effect has not been considered!

7. Local Moments and Splitting Valence Bands Simultaneously
Let’s Back to the DMS Basics
Local Moments and Splitting Valence Bands Simultaneously TM
다시 한 번 DMS에 필요한 성질이 무엇인가를 생각해 볼 필요가 있다.

8. Start from Scratch
Design Rule: Finding a TM that induces spin polarization of valence band
Transition Element
(V, Cr, Mn, Fe, Co, Ni and Cu)
5th Nitrogen
1st NN Nitrogen
4th Nitrogen
3rd NN Nitrogen
2nd NN Nitrogen

9. Calculation Methods Planewave Pseudopotential Method: VASP.4.6.21
XC functional: GGA(PW91)
Cutoff energy of Planewave: 800 eV
4X4X4 k point mesh with MP
Electronic Relaxation: Davidson followed by RMM-DIIS
Structure Relaxation: Conjugate Gradient
Force Convergence Criterion: 0.01 eV/A
Gaussian Smearing with 0.1 eV for lm-DOS
Treatment of Ga 3d state
Semicore treatment for GaN
Core treatment for GaAs
TM dopant: V, Cr, Mn, Fe, Co, Ni, and Cu
Ferromagnetism by clustering can be excluded

10. due to p-d Exchange Interaction Localized Moment due to Mn
Electronic Structure of Mn doped GaAs
Delocalized Carrier
due to p-d Exchange Interaction
Localized Moment due to Mn

11. Magnetic Moments of TM in GaN Host
Less-Than Half filled
More-than Half filled

12. Spin Density of TM doped GaN
Less-Than Half filled
GaN:Cr
GaN:Mn
More-than Half filled
GaN:Co
GaN:Cu

13. GaFeN: Magnetic Insulator GaNiN: Magnetic Insulator
Partial DOSs having More-than Half Filled States
GaFeN: Magnetic Insulator
GaCoN: Half Metal
GaCuN: Half Metal
GaNiN: Magnetic Insulator

14. Valence Band Splitting
SCL et al. JMMM (2007)
SCL et al. Solid State Phenomena (2007)

15. Strength of p-d Hybridization
p-d hybridization results in a spin dependent coupling between the holes and the Mn ions.
TM in GaN
ΔEvalence (eV)
Noβ (eV)
Local Moment(μB) Fe
0.4203 4 Co
0.2902 3 Ni
0.3780 2 Cu
0.3961 1
GaAs:Mn
0.3231 5

16. Magnetic Interaction in Larger Supercell
216-atom supercell

17. Cu is the most probable candidate in GaN host
Co, Cu

18. Experimental Confirmation
Ion Implantation
Nanowire
Appl. Phys. Lett. 90, (2007).
NanoLett, Accepted 2007

19. Stability of Ferromagnetic Cu
Non-Magnetic
Magnetic
Number of electrons in frontier level or unfilled states
Para: 0.98 for Cu, 3.2 for Total
Ferro: 0.27 for Cu, 0.82 for Total
Ferromagnetic alignment drastically decrease the number of electrons in frontier level
“Antibonding conjecture” Dronskowski (2006)

20. Electron Configurations of Non-Magnetic Phase
t2g
Mainly M d
M-N Antibonding EF eg
4 antibonding d-character electrons in frontier level
 Energetically unfavored
M 3d
sp3
Mainly p
M-N Bonding

21. Electron Configurations of Magnetic Phase
t2g up
down eg EF
Only 1 electron in frontier level
Energetically favored
Spin polarized configuration can decrease the number of antibonding electrons
M 3d
sp3

22. And More … Non-Magnetic Magnetic Spin-up Spin-down Contracted Expanded
Small Hybridization
Short-ranged
Spin-down
Expanded
Large Hybridization
Long-ranged

23. Magnetic Moments of TM in GaN Host
Less-Than Half filled
More-than Half filled

24. Why Cu is good Mn is bad? Absolute Electronegativity 6.27 7.3 7.54
5.62
6.22
5.3
5.89

25. Why Cu is Good and Mn is Bad in GaN?
Cu doped GaN
Mn doped GaN 2p 3d σg
σu* TM N 3d 2p TM N Cu Mn
Electronegativity difference
Small
Large
d-character in antibonding state
Weaker
Stronger
Carrier in antibonding state
Delocalized
Localized

26. Summary Electronegativity can help to design a novel DMS material
Cu is a probable candidate. Cu
Quantitative analysis is also needed.

27. Formation Energies of Cu in GaN Host
Formation Energy of Cu
CuGa
0.00
CuN
2.56
CuI
5.42
Cu(in fcc metal)+Ga32N32  Ga(in orthorhombic)+ Ga31Cu1N32
Cu(in fcc metal)+Ga32N32  1/2N2(in N2 molecule)+Ga32N31Cu1
Cu(in fcc metal)+Ga32N32  Ga32N32Cu1

28. Local Moments of Cu Total Magnetic Moment: 2.0 μB
Cu Projected Moment: 0.65 μB
Charge State: Cu+2
Possible for Hole Doping: 3d9+h Cu Cu

29. Roles of Transition Metal Impurities
Local Magnetic Moment
Split Valence Band TM TM
Spin Polarized Carrier!!