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

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

Mechanisms of Ferromagnetism in DMS TM 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

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 82 2302 (2003)

 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?

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!

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.

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

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

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

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

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

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

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

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 -3.3624 4 Co 0.2902 -3.0955 3 Ni 0.3780 -6.0480 2 Cu 0.3961 -12.6752 1 GaAs:Mn 0.3231 -2.0678 5

Magnetic Interaction in Larger Supercell 216-atom supercell

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

Experimental Confirmation Ion Implantation Nanowire Appl. Phys. Lett. 90, 032504 (2007). NanoLett, Accepted 2007

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)

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

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

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

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

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

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

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

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

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

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