Atomic Scale Understanding of the Surface Intermixing during Thin Metal Film Growth 김상필 1,2, 이승철 1, 정용재 2, 이규환 1, 이광렬 1 1 한국과학기술연구원, 계산과학센터 2 한양대학교, 재료공학부 진공학회, 표면 및 계면과학 심포지움

Nanoscience and Nanomaterials

Major materials issue is the interfacial structure in atomic scale Major materials issue is the interfacial structure in atomic scale 1~2nm Devices with Thin Multilayers GMR Spin Valve

Nanoscience or Nanotechnology To develop new materials or devices of novel properties by understanding a phenomenon in the scale of atoms or molecules and manipulating them in an appropriate manner. CdSe Nanoparticles Smaller Size

Nanoscience or Nanotechnology To develop new materials or devices of novel properties by understanding a phenomenon in the scale of atoms or molecules and manipulating them in an appropriate manner. CdSe Nanoparticles Smaller Size

Nanoscience or Nanotechnology needs atomic scale understandings of structure, kinetics and properties. needs atomic scale understandings of structure, kinetics and properties.

Empirical Approach First Principle Approach Interatomic Potentials Molecular Dynamics Simulation Time evolution of R i and v i i

Insufficient Experimental Tools

Synthesis & Manipulation Analysis & Characterization Analysis & Characterization Modeling & Simulation Modeling & Simulation Methodology of Nanotechnology

Major materials issue is the interfacial structure in atomic scale Major materials issue is the interfacial structure in atomic scale 1~2nm Devices with Thin Multilayers GMR Spin Valve

Thin Film Growth Model (conventional)

Adatom (normal incident  0.1 eV) Co-Al EAM potential* x,y-axis : Periodic Boundary Condition z-axis : Open Surface Deposition rate: × nm/nsec MD calc. step : 0.1fs Co-Al EAM potential* x,y-axis : Periodic Boundary Condition z-axis : Open Surface Deposition rate: × nm/nsec MD calc. step : 0.1fs 300K Initial Temperature 300K Constant Temperature Fixed Atom Position Calculation Methods R. Pasianot et al, Phys. Rev. B45, (1992). A. F. Voter et al, MRS Proc. 82, 175 (1987). C. Vailhe et al, J. Mater. Res. 12, 2559 (1997).

Deposition in Co-Al System Co on Al (001) Al on Co (001)

Asymmetric Interface Formation

 CoAl compound layer of B2 structure was formed spontaneously. Radial Distribution Function of Interface

Al on Co(111)/(0001) Co on Al(111) Atomic deposition behavior

3ML Al on Co(001) 3ML Co on Al(001) Asymmetry in Interfacial Intermixing

Au on Pt (001) Pt on Au (001) Asymmetry in Au-Pt

Deposited Atom of 5.0 eV Co on Cu (100) Cu on Co (100)

3ML Al on Co(001) 3ML Co on Al(001) Asymmetry in Interfacial Intermixing Deposition at 300K Initial kinetic energy 0.1eV

Interface Energy Argument

Snapshots of Co penetration

Activation Barrier for Intermixing (2) (3) (4) (1) Reaction Coordinate

Adatom (normal incident  0.1 eV) Co-Al EAM potential* x,y-axis : Periodic Boundary Condition z-axis : Open Surface Deposition rate: × nm/nsec MD calc. step : 0.1fs Co-Al EAM potential* x,y-axis : Periodic Boundary Condition z-axis : Open Surface Deposition rate: × nm/nsec MD calc. step : 0.1fs 300K Initial Temperature 300K Constant Temperature Fixed Atom Position Calculation Methods R. Pasianot et al, Phys. Rev. B45, (1992). A. F. Voter et al, MRS Proc. 82, 175 (1987). C. Vailhe et al, J. Mater. Res. 12, 2559 (1997).

Acceleration of Adatoms near Surface Co Al 3.5eV (1) (2) (3)(4) (2) (3) (4) (1)

Local accleration of deposited atoms J. R. Manson et al, Phys. Rev. B29, 1084 (1984). R. Wang et al., Phys. Rev. B51, 1957 (1995). F. Montalenti et al., Phys. Rev. B64, (2001). As the atom approaches the surface, its kinetic energy increases significantly due to its attraction to the surface. The initial kinetic energy is negligible compared to the gain due to the acceleration.

Acceleration of Adatoms near Surface Co Al 3.5eV (1) (2) (3)(4) (2) (3) (4) (1)

Contour of Acceleration on (001) Surface Co on Al (001) Al on Co (001)

Kinetic Criteria for Intermixing Local Acceleration 3.5eV Co Al (2) (3) (4) (1) (2) (3) (4) Activation Barrier for Mixing Reaction Coordinate

Intermixing : Al on Co (001)

Deposition in Co-Al System Co on Al (001) Al on Co (001)

百聞不如一見 百見不如一知

Asymmetry of Surface Reaction Al on Co Co on Al Do you have experimental evidence?

CoAxial Impact Collision Ion Scattering Spectroscopy (CAICISS) –Energy range of ~ keV → penetration depth : < 10 Å Ion Scattering Spectroscopy (bond direction) intensity (angle)  o  c1  c2  o

Polar Scan in [1100] on Co (0001) Surface ¯

Polar scan of Al added Co (0001) In [1100] direction DFT calculation results Al atom(s) 1 st Co layer 2 nd Co layer A h2 fcc hcp On Top site(s) Bridge A h2’, A f2’ A h1’, A f1’ A h1,,A f1 1.8 ± 0.05 Å

Al on Co (0001) Surface

Al(001) [100] 1st 2nd 3rd 4th 4.05 Å A 130 A 132 A 121 A 11 A 122 A 123 A 132 (32.7°) A 121 (31.7°) A 122 (26.4°) A 132 (20.4°) A 122 (11.52°) A 11 (12.9°) A 121 (58.3°) A 130 (79.1°) Polar Scan in [100] on Al (001) Surface

Polar Scan of Co added Al (001) 1st 2nd 3rd 4th 4.05 Å Å [100] A 11 C 230 C 231 A 130 A 131 Al Co C 232

Spin-Up Spin-Down FCC - Al HCP - Co B2 - CoAl Magnetic Properties of Co-Al system Spin resolved DOS

Magnetic properties of Co-Al Thin Layer Si substrate Cu buffer layer (1500Å) Co (30Å) Al (30Å) Al (840Å) Cu Capping layer (50Å) MOKE (Magneto-Optic Kerr effects)

Al Si substrate Cu buffer layer (1500Å) Co (30Å) Capping layer (50Å) Al Co Si substrate Cu buffer layer (1500Å) Co (30Å) Al (30Å) Capping layer (50Å) Si substrate Co (30Å) Al (840Å) Capping layer (50Å) Effect of Coating Sequence

Co Thickness Effect Si substrate Cu buffer layer (1500Å) Co (30Å, 5Å) Capping layer (50Å)

Al Si substrate Cu buffer layer (1500Å) Co (30Å) Capping layer (50Å) Al Co Si substrate Cu buffer layer (1500Å) Co (30Å) Al (30Å) Capping layer (50Å) Si substrate Co (30Å) Al (840Å) Capping layer (50Å) Effect of Coating Sequence

How thick is the nonmagnetic (B2) interlayer? 5Å5Å 7Å7Å 10Å 30Å Si substrate Co (variable thickness) Al (840Å) Capping layer (50Å)

Thickness of B2 Layer : 3ML 3ML ~ 10Å

Conclusion Al on Co Co on Al Asymmetry in interfacial intermixing was observed in both MD simulation and experiment, which is governed by a kinetic criteria

Acknowledgement Financial Support –Core Capability Enhancement Program of KIST (V00910, E19190) CAICISS and MOKE Measurement –Prof. Chungnam Whang –Dr. Jae Young Park –Ms. Hyunmi Hwang