Molecular Dynamic Simulation of Atomic Scale Intermixing in Co-Al Thin MultilayerSang-Pil Kim*, Seung-Cheol Lee and Kwang-Ryeol Lee

Future Technology Research Division Korea Institute of Science and Technology, Seoul, Koreakrlee@kist.re.kr http://diamond.kist.re.kr/DLC

* also at Ceramics Engineering Division, Hanyang University

2004. 12. 5. CISAS 2003, Changwon National University

Scientific Computation & Simulationin (sub) Atomic ScaleFirst Principle CalculationMolecular Dynamic Simulation

Nanoscience and Nanomaterials

Characteristics of NanomaterialsContinuum media hypothesis is not allowed.Diffusion & MechanicsBand Theory

Case I : Size Dependent PropertiesHan et al, Nature Biotech., 19, 631 (2001).

Chracteristics of NanotechnologyContinuum media hypothesis is not allowed.

Large fraction of the atom lies at the surface or interface.Abnormal WettingAbnormal Melting of Nano ParticlesChemical Instabilities

Case III : GMR Spin ValveMajor Materials Issue is the interfacial structure and chemical diffusion in atomic scale

Nanoscience or NanotechnologyTo develop new materials of devices of novel properties by understanding a phenomenon in the scale of atoms or molecules and manipulating them in an appropriate manner.Needs Atomic Scale Understandings on the Structure, the Kinetics and the Properties

Insufficient Experimental Tools

Scientific Computation & Simulationin (sub) Atomic ScaleFirst Principle CalculationMolecular Dynamic Simulation

KIST 1024 CPU Cluster System Top 22nd supercomputer in the world

The Present WorkWe employed the molecular dynamic simulation to understand the atomic scale phenomena during thin film process in spintronic devices. .We focused on the interfacial intermixing behavior in atomic scale. New device utilize the electron spin to differentiate electrical carriers into two different types according to their spin projection onto a given quantization axis, .By transferring a magnetic information from one part of the device to another by using nanoscale magnetic elements.

J.F.Gregg et al., J. Phys. D: Appl. Phys. 35(2002) R121

Performance of spintronics devices are largely depends on the Interface Structures of the Metal/Metal or Metal/Insulator Controlling & UnderstandingThe atomic behavior at the interface are fundamental to improve the performance of the nano-devices!

Adatom (0.1eV, normal incident)SubstrateProgram : XMD 2.5.30x,y-axis : Periodic Boundary Conditionz-axis : Open SurfaceAtom flux : 5ps/atomMD calc. step : 0.5fs300K Initial Temperature300K Constant TemperatureFixed Atom Position

MD Simulation Lennard-Jones: Inert GasEmbedded Atom Method: MetalsMany Body Potential: Si, CInteratomic Potentials

* A. Voter et al. MRS Symp.Proc. , 175 (1987)** R. Pasianot et al , PRB 45 12704 (1992)EAM Potential for Co and Al

PropertyAl*Co**Expt.Calc.Expt.Calc.A0 ()4.054.0492.5072.512Ecoh (eV)3.363.394.394.29B (GPa)7979.4180185

EAM Potential for CoAl* Intermetallic Compound , Vol 1, 885 (1994) ** C. Vailhe et al. J. Mater. Res., 12 No. 10 2559 (1997)*** R.A. Johnson, PRB 39 12554 (1989)CoAl B2

PropertyCoAl(B2)Expt.*Calc. **Calc. ***A0 ()2.862.8672.994Ecoh (eV)4.454.4684.083B (GPa)162178169

Phase Diagram of Co-Al

Deposition Behavior of Al on Co (001)

Deposition Behavior ofCo on Al (001)

Al on CoCo on Al

Deposition Behavior on (111)Al on CoTOP VIEWCo on Al

Deposition Behavior on (001)Al on AlCo on Co

Al on Al (100)

Co on Al (100)6144 substrate atoms (16x16x6)a0(512 atoms/ML)Deposition Energy : 0.1eV

Co on Al (100)

1.4 ML2.8 ML4.2 MLN.R. Shivaparan, et al Surf. Sci. 476, 152 (2001)Co on Al (100)

Structural Analysis CoAl compound layer was formed spontaneously.

Structural Analysis

Energy Barrier for Co Penetration(1)(2)(3)(1)(2)(3)Reaction CoordinateActivation barrier is larger than the incident kinetic energy (0.1eV) of Co.

How can the deposited Co atom get that sufficient energy to overcome the activation barrier?

Acceleration of Deposited Co Near Al SubstrateHollow siteCoAl(2)(3)(4)(1)

Deposition Behavior on (001)Co on AlAl on Co

Contour of AccelerationCo on Al (001)

Depostion Behavior on (001)Co on Al (001)

Deposition Behavior on (001)Al on Co (001)

Deposition Behavior on (001)Al on Al (100)Al on Al (001)

ConclusionsConventional thin film growth model assumes negligible intermixing between the adatom and the substrate atom. In nano-scale processes, the model need to be extended to consider the atomic intermixing at the interface. Conventional Thin Film Growth ModelCalculations of the acceleration of adatom and the activation barrier for the intermixing can provide a criteria for the atomic intermixing.

Energy Barrier for Intermixing

Contour of AccelerationCo on Co (001)

Co Deposition on Al (001)

Deposition Behavior on (001)Co on AlAl on Al

Co on Co (0001)1344 substrate atoms(168atoms/ML)After 10ML (1680atoms) in 5 eVCo-substrate

a misfit!cbcbca misfit!cbcbabababaRemove misfits

Spintronic NanodevicesFigures from Scientific American, June 2002New device utilize the electron spin to differentiate electrical carriers into two different types according to their spin projection onto a given quantization axis, .By transferring a magnetic information from one part of the device to another by using nanoscale magnetic elements. J.F.Gregg et al., J. Phys. D: Appl. Phys. 35(2002) R121

Case II : Scale Down IssuesKinetics based on continuum media hypothesis is not sufficient.

Methodology of Nano-R&D

Conventional Thin Film Growth Model Conventional thin film growth model simply assumes that intermixing between the adatom and the substrate is negligible.

depositon diffusion nucleation and growth .