Atomic Scale Understanding of the Surface Intermixing during Thin Metal Film Growth1,2, 1, 2, 1, 1

1, 2, , 2007. 2. 7.

Nanoscience and Nanomaterials

Devices with Thin MultilayersMajor materials issue is the interfacial structure in atomic scale1~2nmGMR Spin Valve

Nanoscience or NanotechnologyTo 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.

Nanoscience or NanotechnologyTo 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.

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

Molecular Dynamics Simulation

Insufficient Experimental Tools

Methodology of Nanotechnology

Devices with Thin MultilayersMajor materials issue is the interfacial structure in atomic scale1~2nmGMR Spin Valve

Thin Film Growth Model (conventional)

Adatom (normal incident 0.1 eV)Co-Al EAM potential*x,y-axis : Periodic Boundary Conditionz-axis : Open SurfaceDeposition rate: 1.306 10-1 nm/nsecMD calc. step : 0.1fs300K Initial Temperature300K Constant TemperatureFixed Atom PositionCalculation MethodsR. Pasianot et al, Phys. Rev. B45, 12704 (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 SystemCo on Al (001)Al on Co (001)

Asymmetric Interface Formation

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

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

Asymmetry in Interfacial Intermixing

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

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

Asymmetry in Interfacial Intermixing Deposition at 300K Initial kinetic energy 0.1eV

Interface Energy Argument

Snapshots of Co penetration

Activation Barrier for IntermixingReaction Coordinate

Adatom (normal incident 0.1 eV)Co-Al EAM potential*x,y-axis : Periodic Boundary Conditionz-axis : Open SurfaceDeposition rate: 1.306 10-1 nm/nsecMD calc. step : 0.1fs300K Initial Temperature300K Constant TemperatureFixed Atom PositionCalculation MethodsR. Pasianot et al, Phys. Rev. B45, 12704 (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

Local accleration of deposited atomsJ. 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, 0814011 (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

Contour of Acceleration on (001) Surface

Kinetic Criteria for IntermixingLocal Acceleration3.5eV(1)(2)(3)(4)Activation Barrier for MixingReaction Coordinate

Intermixing : Al on Co (001)

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

Asymmetry of Surface ReactionDo you have experimental evidence?

CoAxial Impact Collision Ion Scattering Spectroscopy (CAICISS)Energy range of ~ keV penetration depth : < 10 Ion Scattering Spectroscopy

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

Polar scan of Al added Co (0001)DFT calculation results

Al on Co (0001) Surface

A132 (32.7)A121 (31.7)A122 (26.4)A132 (20.4)A122 (11.52)A11 (12.9)A121 (58.3)A130 (79.1)Polar Scan in [100] on Al (001) Surface

Polar Scan of Co added Al (001)

FCC - AlMagnetic Properties of Co-Al systemSpin resolved DOS

Magnetic properties of Co-Al Thin LayerSi substrateSi substrateSi substrateCu buffer layer (1500)Cu buffer layer (1500)Co (30)Co (30)Co (30)Al (30)Al (840)Cu Capping layer (50)Cu Capping layer (50)Cu Capping layer (50)MOKE(Magneto-Optic Kerr effects)

Effect of Coating SequenceAlSi substrateCu buffer layer (1500)Co (30)Capping layer (50)

Co Thickness Effect

Effect of Coating SequenceAlSi substrateCu buffer layer (1500)Co (30)Capping layer (50)

How thick is the nonmagnetic (B2) interlayer?

Thickness of B2 Layer : 3ML3ML ~ 10

ConclusionAl on CoCo on AlAsymmetry in interfacial intermixing was observed in both MD simulation and experiment, which is governed by a kinetic criteria

AcknowledgementFinancial SupportCore Capability Enhancement Program of KIST (V00910, E19190)

CAICISS and MOKE MeasurementProf. Chungnam Whang Dr. Jae Young ParkMs. Hyunmi Hwang

Thank you Mr. Chairman.

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I would like to thank organizing committee and especially prof sahashi for inviting me to this excellent symposium. This is really interdiciplinary symposium, and I enjoyed the presentations both oral and poster presentations and learned a lot from them.

Title of my talk is .. 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. Now, I would like to talk a real stuff of the simulation work.

Extremely thin mulilayers are quite common in advanced devices. For example, spintronics devices as shown in this figure is essentially a stack of ferromagentic and nonferromagnetic thin films. The thickness of the film is in the order of a few nanometers. This is the TEM microstructure of the stack. It is clear that the interfacial structure is not sharp, but some degree of intermixing occurs in the interface. Because the performance of the device is strongly dependent on the interfacial structure, the major materials issue in this device is to understand and control the interfacial structure in atomic scale. 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 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? This is the simple principle.

We know the interatomic potential between two atoms as a function of distance between them. This is the typical curve of the interatomic potential. This is the equilibrium distance between two atoms. If the atoms are separate larger than this equilibrium distance, it feels attraction. In opposite case, it feels repulsion. The stronger the interatomic bond, the deeper the potential well. We can obtain this curve in real system by various methods, such as empirical fitting of the formalism of this curve with experimental data or we can calculation this curve by first principle calculation, in other word by solving Schrodinger equation.

Once we know this potential curve, we can simulate the dynamics of the atoms by this method. We start with known values of atomic position and its velocity. Then we can calculate the net force imposing on atom i by the surrounding atoms. Using the Newtons second law, we can calculate the new position and velocity after a certain time interval. As a result we can obtain time evolution of R and V, or dynamics of the complex system.

Because these principles (potential curve and Newtons second law) are reasonable, we believe that the calculation results should be right.

However, we dont have sufficient experimental tools in this range.

We only have TEM and STM. Of course many sientists are trying to develop new technology, this is the present situation.

However, the atomic scale simulation can play a significant role in this range as you saw in the beginning of this presentation.However, in nanotechnology, modeling and simulation becomes more significant.

And I hope Korean government spend more money in this area. Now, I would like to talk a real stuff of the simulation work.

Extremely thin mulilayers are quite common in advanced devices. For example, spintronics devices as shown in this figure is essentially a stack of ferromagentic and nonferromagnetic thin films. The thickness of the film is in the order of a few nanometers. This is the TEM microstructure of the stack. It is clear that the interfacial structure is not sharp, but some degree of intermixing occurs in the interface. Because the performance of the device is strongly dependent on the interfacial structure, the major materials issue in this device is to understand and control the interfacial structure in atomic scale. This is the cartoon used in the conventional thin film growth model. In the conventional model, atomic scale intermixing is not a major concern. So the substrate was considered as an inert template. The model just focused on the movement of adatoms on the substrate, such as surface diffusion, island nucleation, attachment at a step. This model was very successful in understanding micrometer scale thin films. However, this is not sufficient in the regime of nanometer scale.

In the present work, we focused on the surface mixing behavior during deposition, and we believe that this is essential to understand the thin film structure evolution in nanometer scale.We choose MD simulation to assess the phenomena. This is the schematics of the simulation. We depostited a metal atom of the kinetic energy 0.1eV at normal incident angle. This kinetic energy is typical of MBE or evaporation. The adatoms were generated at random position on this surface. We used a single crystal as the substrate. The bottom two layer of the substrate was fixed to mimic a thick substrate. Next three layer was relaxed but the temperature was fixed during the deposition. This layer is used for heat sink of upper layers. The other upper layers were fully relaxed at 300K and the structure was evolved as the atom deposited.

Today, I will show you the results of Co-Al system. This system is very important for TMR spin valve where two ferromagnetic thin layers are separated by extremley thin Al oxide layer. EAM potentials developed by these peoples were used for the calculation. We benchmarked the potential using pure Al, Co and AlCo B2 phase. The potentials were very good in predicting the physical properties of each phase and the relative stability between these phases.

We applied periodic boundary condition in xy direction. But Z direction is open to make surface. 1 atom was deposited for 5 psec. This is very high deposition rate, 13cm/sec. Even if the deposition event is very rare in MD time scale, the most significant structure evolution occurs during first few psec after deposition. No further change can be expected at such a low temperature. Hence, in this simulation we rescale the system kinetic energy to 300K after 5 psec. This is a kind of accelerated MD but not very different from the real physics. We used 0.1fs as the time step for accurate calcualtion.

OK. Here is the result movies for one atom deposition. Left is the case of Al deposition on Co (001) surface. The observation is not different from our prediction from conventional thin film model. Atoms sit on the hollow site of the surface. Because of low temperature, surface diffusion is hard to occur. These are nothing new, everything is as predicted.

Right is the opposite case, Co deposition on Al (001) surface. Very unexpected phenomenon occurs, here. Severe intermixing.

RDF analysis of the layer also showed the interface has the B2 structure. This is the RDF spectrum of interface. This is the RDF of ideal defect free B2 structure. Even if the RDF is quite smeared in the MD simulation, the characteristic peaks agree very well with those of B2 structure.

What happens on the close packed surface. These are the results. ..

Same no difference. This is the thermodynamic phase diagram of Co-Al system. CoAl B2 structure is the most stable phase. However, it is quite surprised that the stable phase can be obtained at RT with very small kinetic energy. Furthermore, asymmetry of the intermixing cannot be understood in thermodynamic veiw point. So, this asymmetry is totally governed by kinetics of the deposition.

This is the thermodynamic phase diagram of Co-Al system. CoAl B2 structure is the most stable phase. However, it is quite surprised that the stable phase can be obtained at RT with very small kinetic energy. Furthermore, asymmetry of the intermixing cannot be understood in thermodynamic veiw point. So, this asymmetry is totally governed by kinetics of the deposition.

Now I will show you again the movies that you saw before. I hope you can see now the acceleration near the substrate.

There is a chinese proverb.

This means that Seeing once is much better than hearing hundred times.

Now, I would like to add new proverb like this.

One learning is much better than seeing hundred times, if you trust your teacher. Ok. Anyway, Is this real? Fortunately, we could find an experimental evidence using magnetic properties of this system. This is spin resolved density of state of the phases in Co-Al system. Al is has no spin momentum. Co-Al has a symmetric DOS showing no spin momentum. Co is famous magnetic element. We measure the magentic properties of these three multilayers using MOKE measurement. The left one is the reference. The same thickness of Co but difference sequence. This sample also has 30A thick Co layer and then deposite Al. The final specimen is in opposite sequence. 30A thick Co was deposited on Al substrate. All samples are capped by Cu Here, are the results of multilayers. Red data is for monolayer of Co. Blue triangle is identical to the red, which means that the Al deposition on Co did not affect the magnetic properties, in other word, thickness of Co layer. However, when Co was deposited on Al surface, the MOKE hysteresis curve shows that of thinner Co layer. This results definitely shows that in this case nonmagnetic interfacial layer, here it should be CoAl B2 phase, was formed in this case. On the other hand, this case no intermixing occurs at the interface. This is the result of the MOKE measurement for Co monolayers. This is the typical hysteresis curve of ferromagnetic materials. We tried two different thickness, 30A and 5A. The hysteresis becomes wider as the thickness increased. Here, are the results of multilayers. Red data is for monolayer of Co. Blue triangle is identical to the red, which means that the Al deposition on Co did not affect the magnetic properties, in other word, thickness of Co layer. However, when Co was deposited on Al surface, the MOKE hysteresis curve shows that of thinner Co layer. This results definitely shows that in this case nonmagnetic interfacial layer, here it should be CoAl B2 phase, was formed in this case. On the other hand, this case no intermixing occurs at the interface. How thick is the layer? In order to measure the intermixing layer thckness, we measure the MOKE with various amounts of Co deposition.No magnetic signal for

.