ICMCTF2008 Session TS1-2: The Atomistics of Thin Film Growth: Computational and Experimental Studies

Thursday, May 1, 2008 1:30 PM in Room Royal Palm 4-6

Thursday Afternoon

Start	Invited?	Item
1:30 PM		TS1-2-1 Structural Evolution and Electrochemical Performance of LiFePO₄/C Thin Films Deposited by Ionized Magnetron Sputtering K.-F. Chiu, P.Y. Chen (Feng Chia University, Taiwan) Thin films of carbon mixed LiFePO₄ were prepared by ionized magnetron sputter deposition and post anneal. The technique uses a built-in radio frequency coil to generate an inductively coupled plasma (ICP) confined close to the substrate. Therefore, the films were deposited under concurrent ion bombardment, which resulted in an atomic-peening effect during film growth. The evolution of film structures and surface morphologies under different ICP plasma conditions was investigated. The deposited LiFePO₄/C thin films were tested as cathodes for lithium ion batteries. The performances of these thin film cathodes can be related to the modified film structures and surface morphologies. The thin film cathodes deposited under suitable ICP plasma powers exhibited higher discharge voltage and specific capacity.
1:50 PM	Invited	TS1-2-2 Theoretical and Experimental Studies Related to the Compositional and Microstructural Evolution of Oxide Thin Films J. Rosen (Linköping University, Sweden) The correlation between plasma characteristics and the film composition and microstructure of oxides, primarily Al₂O₃, has been investigated by integration of experimental and theoretical methods. Thin film depositions based on cathodic arc as well as magnetron sputtering show a resulting film which is strongly dependent on plasma properties, especially ion energy. Using Density Functional Theory (DFT), the sequence of an ion approaching and being incorporated in the top surface layers during film growth has been studied through ion-surface interaction prior to adsorption, adsorption, surface migration and structural as well as compositional changes induced by ion bombardment. The results may explain experimentally observed formation of amorphous structures during kinetically limited alumina growth. Furthermore, unintentional hydrogen incorporation due to presence of residual gas during material synthesis motivated studies on compositional evolution. In simulated ion-surface collisions, increased ion energy resulted in hydrogen molecule formation and removal, hence suggesting pathways to reduce impurity incorporation during film growth in a high vacuum ambient.
2:30 PM		TS1-2-4 Structure, Elastic Properties, and Phase Stability of Cr_1-xAl_xN P.H. Mayrhofer (Montanuniversität Leoben, Austria); D. Music, J.M. Schneider (RWTH Aachen University, Germany); F.D. Fischer (Montanuniversität Leoben, Austria); H.J. Böhm (Vienna University of Technology, Austria) Cr_1-xAl_xN is established for many applications like in automotive, machining, and semiconductor industries, and hence its structure, elastic properties, and phase stability as a function of the chemical composition are of vital importance. Here we present ab initio calculations of cubic (c), hexagonal (h), and orthorhombic Cr_1-xAl_xN in combination with continuum mechanical investigations of the decomposition process into the stable phases c-CrN and h-AlN with the intermediate step of metastable c-AlN. We show that calculations of Cr_1-xAl_xN in their ferromagnetic (FM) and antiferromagnetic (AFM) state have an excellent correlation to experiments. For AFM c-Cr_1-xAl_xN the lattice parameter decreases from 4.139 to 4.069 Å with increasing x from 0 to 1 in agreement with experiments (4.139 to 4.060 Å). The energy of formation suggests that the cubic phase can be stabilized for x in the range of 0.48-0.75, depending on the metal-sublattice population. This can be understood by considering the Al distribution induced changes in the electronic structure and configurational entropy. Synthesized by plasma-assisted vapor deposition, Cr_1-xAl_xN is reported to crystallize in the cubic modification for x of 0.6-0.71. With increasing Al content from x = 0 to 1 the bulk modulus of c-Cr_1-xAl_xN increases from 245 to 252 GPa, respectively. The calculated enthalpy change for the separation of supersaturated c-Cr_1-xAl_xN into c-CrN and c-AlN has a maximum of ~0.038 eV/at (7.27 kJ/mol) at x ~0.5. Using continuum mechanical investigations the contribution of strain energy for a fully constrained condition of this decomposition process is ~55 J/cm³ (0.58 kJ/mol) at x ~0.5. Estimating the entropy term by an ideal binary mixture the chemical driving force for decomposition of c-Cr_1-xAl_xN into c-CrN and c-AlN is already at temperatures between 800 and 900°C smaller than the strain energy. This can explain the experimental observations that c-Cr_1-xAl_xN decomposes by forming stable h-AlN without the intermediate step of metastable c-AlN.
2:50 PM		TS1-2-5 Mathematical Simulation of the Layer Growth Kinetics During Post-Discharge Nitriding: From Early Stage to Quasi-Steady Stage J. Oseguera, F. Castillo (ITESM-CEM, Mexico); A. Gomez (UFRO, Chile); A. Fraguela (BUAP, Mexico) Nitriding by microwave post-discharge process involves molecular nitrogen dissociation. It has been observed that nitrogen flux from surface to solid during the early stage does not follow a parabolic regime and that the growth rate of concomitant nitride layers is sensitive to atomic nitrogen concentration on the surface. In this work a mathematical model has been developed in order to describe the kinetics of the compound layer formation during a post-discharge nitriding process. The model is related to a moving boundary value problem and considers different stages: diffusion process, formation of the layers, layer growth and quasi-stabilization of the layer growth. An analytical approximate solution of Goodman’s type is sought and representations of the motion of the interfaces and the nitrogen concentration profiles are analyzed.
3:10 PM		TS1-2-7 Atomistic Origin of the Isostructural Decomposition in Multinary Nitride Materials, a First-Principles Study B. Alling, A. Karimi (Swiss Federal Institute of Technology Lausanne (EPFL), Switzerland); I.A. Abrikosov (Linköping University, Sweden) We have used first-principles calculations to study the mixing and decomposition thermodynamics of the two widely used coating materials Ti_1-xAl_xN and Cr_1-xAl_xN. We have calculated the mixing enthalpies of the two systems on a fine concentration mesh and derived their second concentration derivatives. Although the mixing enthalpy is found to be positive for both systems, the Ti_1-xAl_xN material has a much stronger driving force for iso-structural spinodal decomposition, especially when the Al content is high, as compared to Cr_1-xAl_xN. The reason for this as well as the asymmetric shape of the enthalpy curve of Ti_1-xAl_xN is found to have an electronic structure explanation. With increasing Al content, an energetically unfavourable development of the Ti 3d non-bonding states into an atomic like state right at the Fermi level takes place. In the case of CrAlN this effect is absent since the Cr 3d non-bonding states are magnetically split, and the Fermi level falls in the valley between minority and majority spin states regardless of Al content. The results are compared with experimental studies and discussed in the context of age-hardening and thermal stability.
3:30 PM		TS1-2-8 Thickness Uniformity and Morphology of Films Deposited Using a Filtered Pulse Cathodic Arc Source L.H. Li, R.K.Y. Fu, P.K. Chu (City University of Hong Kong) Filtered pulse cathodic arc deposition is a new film fabrication method. One of the merits is that the plasma transport efficacy can be controlled to produce a variety of thin films with different thicknesses. The thickness of the films prepared by this method can be varied from tens of nanometers to micrometers. However, because of plasma confinement in the curved duct that is frequently used to eliminate deleterious macro-particles and divergent diffusion of the plasma from the exit, the characteristics of the films are affected. In this work, parameters such as the magnetic field flux density, curved duct bias, and deposition temperature are investigated from the prospective of the thickness uniformity across the film. Under different conditions, the resulting films have different morphology and microstructures and the optimal parameters for proper film fabrication are discussed.
3:50 PM	Invited	TS1-2-11 A Direct View of Quantum-Stabilized Atomically Flat Films: from Magic Heights to Quantum Mirrors R.S. Miranda (Universidad Autónoma de Madrid, Spain) In metallic ultrathin films, these states influence the Density of States at the Fermi level and, thus, many properties (superconducting transition temperature, resistivity) of thin fims depend on them. We have shown recently that for Pb/Cu(111) the contribution of these states to the total energy of the system determines its structural stability, which oscillates with the thickness, giving rise to islands with preferred or "magic" height. Here, variable temperature STM is used to visualize the thermal evolution of Pb films deposited on Cu(111) at 60 K, a temperature low enough to allow the kinetically hindered initial growth of flat films of even forbidden thicknesses. Tunnelling Spectroscopy is used to determine the local thickness of Pb. This allows the quantitative determination of the temperature dependence of the population of the different atomic layer and the roughening temperature of each atomic height, which oscillates with bilayer periodicity and a longer beating period, as dictated by the Fermi surface of Pb. Conditions are found to stabilize, by this quantum effect, particular thicknesses up to room temperature, resulting in Pb films which are atomically flat over lateral distances of microns, limited only by the perfection of the substrate. The magic height effect is employed to grow an atomically flat Pb film on a 50 micron-thick, ultra-perfect Si (111) wafer, which acts as an ideal mirror for He atoms. This "quantum mirror" will be used to focus a beam of He atom down to 100 nm and it is an essential part of the atom microscope with sub-micron spatial resolution currently under construction.