Atomic Scale Intermixing during Thin Film Deposition*, , , ,

, * , , ,

2004. 5. 21. ,

Nanoscience and Nanomaterials

Chracteristics of Nano MaterialsContinuum media hypothesis is not allowed.

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

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

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

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

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

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. Performance of spintronics devices are largely depends on the Interface Structures of the Metal/Metal or Metal/Insulator

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

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

Two Different SystemsCo-Al & Co-Cu

* 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-AlCoAl: B2

Deposition Behavior of Al on Co (001)

Deposition Behavior ofCo on Al (001)

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

Co on Al (100)

Structural Analysis CoAl compound layer was formed spontaneously.

Al on CoCo on Al

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

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)

A Suggested Novel Process

Two Different SystemsCo-Al & Co-Cu

EAM Potential for Co-Cu system** X. W. Zhou et al., Acta. Mater., 49, 4005 (2001).

CoCuExpt.Calc.Expt.Calc.a0 ()2.5072.5013.6153.615Ecoh (eV)4.3864.3663.5133.534B (Gpa)180211.7140137100 (J/m2)N/A2.7892.1661.987110N/A3.0512.2372.166111N/A2.5911.9531.90310002.7752.879N/AN/A-10103.0353.042N/AN/A11-203.7913.350N/AN/A

0.1eV Co on Cu (001) Mixing Ratio : 1.56%128 atoms256 atoms384 atoms

0.1eV Cu on Co (001) Mixing Ratio : 0.0 %128 atoms256 atoms384 atoms

5.0eV Co on Cu (001) Mixing Ratio : 21.1%128 atoms256 atoms384 atoms

5.0eV Cu on Co (001) Mixing Ratio : 0.78 %128 atoms256 atoms384 atoms

Intermixing BehaviorCu on Co (100)Co on Cu (100)

Sheet1

Depostion 1MLDeposition 1 Atom

0.1 eV0.00%0.00%

1.0 eV0.00%0.00%

3.0 eV0.00%0.00%

5.0 eV0.78%1.56%

Sheet2

Sheet3

Sheet1

Depostion 1MLDeposition 1 Atom

0.1 eV1.56%8.59%

1.0 eV7.81%14.06%

3.0 eV14.10%48.43%

5.0 eV21.10%71.88%

Sheet2

Sheet3

Kinetic Energy of Co near Cu (100)Hollow siteTop site2.63 eV1.81 eV

Comparison of Interatomic PotentialCo-AlCo-Cu

Energy Barrier for IntermixingSimilar atomic radius induce a simple substitutional exchange.

Energy Barrier for Intermixing0.553 eV1.21 eVCo on Cu(001)Cu on Co(001)

ConclusionsIn 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.

Conclusions

Thank you Prof. song? chairman. I am Kwang-Ryeol Lee of KIST working in the division of future technology research division. First of all, I would like to thank organizing committee for inviting me to this excellent symposium that covers very wide range of science and technology.

Today, I will present our recent molecular dynamics simulation work of thin film deposition with the title, . This is the part of Mr. Sangpils work for his master degree. And another coworker of this result is Dr. Seung-cheol Lee in my group.

This is a beautiful figure showing the schematic structure of human skin in various scales. If we can see the structure in nanometer scale, here, maybe we will see some clusters of molecules or atoms. In order to describe a phenomena this scale using our present knowledge about materials phenomena, we should face two big and serious problems. Second difficulty is that large fraction of the atom lies at the surface or interface. This is evident in this cartoon of nano-scale polycrystalline materials. The fraction of the white atom which placed in the grain boundary region cannot be neglected, here. Many abnormal behaviors in nanoscale materials such as strange wetting behavior, low melting temperature and chemical instability seems to be due to this feature. However, unfortunately, we do not fully understand the surface and interfacial phenomena. In the nanoscience or nano-technology, we needs to understand the structure, kinetics and properties in the atomic scale. However, as we can see in the previous view graphs, our present theoretical method is highly limited for this investigations. O.K. Then, do we have sufficient experimental tools to characterize the phenomena in atomic scale? In the field of nanotechnology, many researchers are considering the atomic or subatomic scale simulation such as first principle calculation to calculate the electronic structure, MD or Monte Carlo simulation as an important tool for their research. Of course, this trend is due to their advantages in understanding the physical phenomena in the nanometer scale. I wanna spend a few minutes of this presentation to review the characteristics of the nanoscale materials and try to identify the reason for this new trend. This would be an extreme case where the interface becomes very significant in the nano-scale devices. This is the schematic of GMR devices, one of typical nanodevice. GMR is composed of multilayers of thickness of few nanometer. Hence, the atomic structure of the interface should be well controlled to obtain high performance devices, and actually one of the major material issue in the development of GMR device is the atomic scale structure of the interface and interdiffusion.

depositon diffusion nucleation and growth . In the present work, we employed the molecular dynamic simulation to understand the atomic scale phenomena during thin film process for spintronic devices. This is the schematic of spin FET devices. The spintronic devices utilize the transfer a spin information from here to there using a nanoscale magnetic elements. Typically, this device is composed of nanoscale multilayer of various materials. Molecular dynamic simulation uses interatomic potential to calculate the interatomic force. Using newtons second law, we can simulate the time evolution of each atoms position and velocity. Hence, the success of the MD simulation is totally dependent on the interatomic potential. In this work, we used embedded atom mothod potential of Co-Co, Al-Al and Co-Al atoms. In order to check the validity of the potential, we calculated the physical properties of Co, Al and CoAl B2 structure. This schematic shows the simulation condition used in the present work. Atdatom is deposited on the substrate with normal incident. The initial kinetic energy of the adatom was 0.1eV which is to simulate the case of evaporation or effusion in the MBE cell. The substrate was 6x6x4 lattice composed of 576 atoms. Periodic boundary condition was applied in x, y direction and the atoms in bottom 1 lattice was fixed to simulate wide and thick substrate. The temperature of bottom 2 lattice was kept at 300K to dissipate the heat generated on the growing surface. However, all other atoms were fully relaxed with initial temperature at 300K. Time step for MD calculation of 0.5 fs. We observed that after arrival of adatom, some agitation occurs on the surface. But most of significant reaction occurs within 1 ps, after that, the system becomes to a steady state. Every 5 ps, atom was added to the system.