1. Atomic Scale Intermixing during Thin Film Deposition
김상필*, 유승석§, 이승철, 이규환, 이광렬
한국과학기술연구원, 미래기술연구본부
* 한양대학교, 세라믹스 공학과, § 서울대학교, 재료공학부
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. Sangpil’s work for his master degree. And another coworker of this result is Dr. Seung-cheol Lee in my group.
표면공학회 춘계학술대회, 서울대학교

2. Nanoscience and Nanomaterials
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.

3. Chracteristics of Nano Materials
Continuum media hypothesis is not allowed.
Large fraction of the atom lies at the surface or interface.
Abnormal Wetting
Abnormal Melting of Nano Particles
Chemical Instabilities
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.

4. Nanoscience or Nanotechnology
To 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
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?

5. Scientific Computation & Simulation in (sub) 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.
First Principle Calculation
Molecular Dynamic Simulation

6. Case III : GMR Spin Valve
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.
Major Materials Issue is the interfacial structure
and chemical diffusion in atomic scale

7. Conventional Thin Film Growth Model
흥미롭게도 기존의 증착메커니즘은 기판 위에서 원자는 depositon diffusion nucleation and growth만이 발생한다고 하였지만 본 실험에서는 기존의 개념과는 다른 새로운 형태의 증착 메커니즘을 발견할 수 있었습니다.
Conventional thin film growth model simply assumes that intermixing between the adatom and the substrate is negligible.

8. The Present Work
Performance of spintronics devices are largely depends on the Interface Structures of the Metal/Metal or Metal/Insulator
We 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.
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.

9. MD Simulation Interatomic Potentials Time Evolution of Ri and vi
Molecular dynamic simulation uses interatomic potential to calculate the interatomic force. Using newton’s 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.
Interatomic Potentials
Lennard-Jones: Inert Gas
Embedded Atom Method: Metals
Many Body Potential: Si, C

10. Substrate Adatom (0.1eV, normal incident) Program : XMD 2.5.30
300K Initial Temperature
Substrate
300K Constant Temperature
Fixed Atom Position
[100]
[001]
[010] z y x
Program : XMD
x,y-axis : Periodic Boundary Condition
z-axis : Open Surface
Atom flux : 5ps/atom
MD calc. step : 0.5fs
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.

11. Two Different Systems Co-Al & Co-Cu

12. EAM Potential for Co and Al
HCP - Co
FCC - Al
Property
Al*
Co**
Expt.
Calc.
A0 (Å)
4.05
4.049
2.507
2.512
Ecoh (eV)
3.36
3.39
4.39
4.29
B (GPa) 79
79.4
180
185
* A. Voter et al. MRS Symp.Proc. , 175 (1987)
** R. Pasianot et al , PRB (1992)

13. EAM Potential for Co–Al
Property
CoAl(B2)
Expt.*
Calc. **
Calc. ***
A0 (Å)
2.86
2.867
2.994
Ecoh (eV)
4.45
4.468
4.083
B (GPa)
162
178
169
CoAl B2
* Intermetallic Compound , Vol 1, 885 (1994)
** C. Vailhe et al. J. Mater. Res., 12 No (1997)
*** R.A. Johnson, PRB (1989)

14. Phase Diagram of Co-Al
CoAl: B2

15. Deposition Behavior of Al on Co (001)

16. Deposition Behavior of Co on Al (001)

17. Deposition Behavior on (111)
Al on Co
Co on Al
TOP VIEW

18. Co on Al (100)

19. CoAl compound layer was formed spontaneously.
Structural Analysis
CoAl
CoAl compound layer was formed spontaneously.

20. CoAl: B2
Al on Co
Co on Al

21. Co on Al (100)
1.4 ML
2.8 ML
4.2 ML
N.R. Shivaparan, et al Surf. Sci. 476, 152 (2001)

22. Energy Barrier for Co Penetration
(1)
(2)
(3)
(1)
(2)
(3)
Reaction Coordinate
Activation 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?

23. Acceleration of Deposited Co
Near Al Substrate 1 2 3 4
3.5eV Co
Hollow site Al
(1)
(2)
(3)
(4)

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

26. Contour of Acceleration
Al on Co (001)
Co on Al (001)

27. Depostion Behavior on (001)
Co on Al (001)
Reaction Coordinate

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

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

30. A Suggested Novel Process
2ML Al on Co(001)
B2-like on Co(001)
Nano-scale Sandwich Structure Co/B2/Co

31. Two Different Systems Co-Al & Co-Cu

32. EAM Potential for Co-Cu system*
Expt.
Calc.
a0 (Å)
2.507
2.501
3.615
Ecoh (eV)
4.386
4.366
3.513
3.534
B (Gpa)
180
211.7
140
137
γ100 (J/m2)
N/A
2.789
2.166
1.987
γ110
3.051
2.237
γ111
2.591
1.953
1.903
γ1000
2.775
2.879
γ-1010
3.035
3.042
γ11-20
3.791
3.350
* X. W. Zhou et al., Acta. Mater., 49, 4005 (2001).

33. 0.1eV Co on Cu (001) 128 atoms 256 atoms 384 atoms
 Mixing Ratio : 1.56%

34. 0.1eV Cu on Co (001) 128 atoms 256 atoms 384 atoms
 Mixing Ratio : 0.0 %

35. 5.0eV Co on Cu (001) 128 atoms 256 atoms 384 atoms
 Mixing Ratio : 21.1%

36. 5.0eV Cu on Co (001) 128 atoms 256 atoms 384 atoms
 Mixing Ratio : 0.78 %

37. Intermixing Behavior
Cu on Co (100)
Co on Cu (100)

38. Kinetic Energy of Co near Cu (100)
Hollow site
Top site
2.63 eV
1.81 eV

39. Comparison of Interatomic Potential
Co-Al
Co-Cu

40. Energy Barrier for Intermixing
Similar atomic radius induce a simple substitutional exchange.

41. Energy Barrier for Intermixing
0.553 eV
1.21 eV
Co on Cu(001)
Cu on Co(001)

42. Conventional Thin Film Growth Model
Conclusions
In nano-scale processes, the model need to be extended to consider the atomic intermixing at the interface.
Conventional Thin Film Growth Model
Calculations of the acceleration of adatom and the activation barrier for the intermixing can provide a criteria for the atomic intermixing.

43. Conclusions