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FM. FM. ?. Interface Atomic & Electronic Structure Uniform Layer Growth Optimization of Operating Conditions Novel Materials & Novel Properties.
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FM FM ? Interface Atomic & Electronic Structure Uniform Layer Growth Optimization of Operating Conditions Novel Materials & Novel Properties Differences in Thin Film Growth Morphologies of Co-Al Binary Systems using Molecular Dynamics Simulation: In cases of Co on Co(001), Al on Al(001), Co on Al(001), and Al on Co(001) Sang-Pil Kim1,2, Seung-Cheol Lee1, Kwang-Ryeol Lee1 Kyu-Hwan Lee1 and Yong-Chae Chung2 1 Future Technology Research Division, KIST, Seoul, Korea, 2 CPRC, Department of Ceramic Engineering, Hanyang University, Seoul, Korea IV. EAM potentials for Co, Al, and CoAl Binary System I. Abstract - Understanding and control of thin films interfaces are important issues for application to the magnetoelectric devices. Morphology evolutions of thin film of Co-Al binary system were studied by molecular dynamics simulation. In this simulation four combinations of deposition and substrate elements were used: Al on Al(001), Al on Co(001), Co on Co (001), Co on Al(001). Morphologies of thin films is quite different each other. Al thin film was grown by layer-by-layer on Al or Co substrate. While Co thin film of island type was grown on Co surface, 3 ML thickness of CoAl surface compound was initially formed on Al substrate before pure Co thin film growth. In addition to the study on morphologies, the degrees of mixing of interface were studied quantitatively. No surface mixing and a sharp interface were observed when Co was used as a substrate regardless of deposited atoms. On the contrary, large amount of surface mixing or compound formation were observed when Al was used as a substrate. HCP - Co B2 - CoAl FCC - Al 1) R. Pasianot et al. , PRB 45 12704 (1992) 2) C. Vailhé et al. J. Mater. Res., 12 No. 10 2559 (1997) 3) A. Voter et al. MRS Symp.Proc. , 175 (1987) V. Thin Film Morphologies Al films on Al(001) Al films on Co(001) II.Introduction Al films Basic Structures of Magnetic Multilayers Conductor: GMR, Spin Valve Insulator: TMR, MRAM(Co or CoFe/Al2O3/Co ) Semiconductor: Spin Transistor, Spin FET Superconductor: … Spacer - When Al atoms are deposited on Al or Co substrates, Al thin films are grown by layer-by-layer growth mode regardless of the substrate used. Co films on Co(001) Co films on Al(001) } Co films Order of 1 nm • In homogeneous Co deposition, deposited Co atoms grew as an island shape, which is predicted to grow as a layer-by-layer mode by conventional crystal growth theory. This growth morphology is due to the ‘steering effect’. • In Co on Al (001), most of the deposited Co atom formed a CoAl B2 structured surface compound. Considering the low incident energy (0.1 eV) and the low operating temperature (300 K), the formation of the B2 structure is quite surprising. Considering the thickness of thin film used for MRAM device approaches to a nanometer which corresponds to 4 ~ 5 monolayer, 40 ~ 50 % of the interface atoms is subject to the surface alloying. This study was focused on the structural evolution of Co-Al binary system using molecular dynamics simulation. Cobalt is used for a ferromagnet of TMR junction and aluminum is used for making an insulating barrier after oxidation. To understand the effects of a substrate and an adatom interaction, various combinations of the Co and Al substrate was used. VI. Mixing Behavior Al films on Co (001) Co films on Co (001) III. Computational Methods Co or Al - random position to the x,y axis Deposition energy : 0.1eV Normal Incident Program : XMD 2.5.30* x,y-axis : Periodic Boundary Condition z-axis : Open Surface dt : 0.2fs , calculation time : 5ps/atom Number of substrate atom : 576 (Al or Co) 300K Constant Temp. Al or Co(001) Substrate Fix Position • Though thermodynamics stable phase of Co-Al binary system is CoAl B2 structure, mixing behaviors of Co on Al and Al on Al is largely different. When Al is deposited, no mixing is observed. When Co atoms are deposited on Al substrate, on the contrary, the modulation of layer densities of Co and Al were observed, which indicates the formation of well-defined surface compound. *XMD : Molecular Dynamics Program (http://www.ims.uconn.edu/centers/simul) VIII. Conclusions • When Co atoms were deposited, an island growth on Co substrate and the formation of the surface compound on Al surface were observed. • When atoms on Co substrate, no mixing was observed. When Co atoms deposited on Al surface, modulation of layer density was observed, which indicates the formation of well-defined surface compound. • Evolutions of thin film morphologies were studied by molecular dynamics simulation: Al/Al(001), Al/Co(001), Co/Co(001), and Co/Al(001) • Al thin film was grown by layer-by-layer growth mode regardless of substrates used.
