ICMCTF2007 Session H4-2: The Atomistics of Thin Film Growth: Computational and Experimental Studies

Monday, April 23, 2007 1:30 PM in Room Tiki Pavilion

Monday Afternoon

Start	Invited?	Item
1:30 PM	Invited	H4-2-1 Emergence of Macroscopic Structure from Atomistic Processes in Thin Film Growth K. Robbie, C. Elliott, T. Brown, C. Buzea (Queen's University, Canada) It is well known that variations in thin film growth conditions can create films with different physical properties such as hardness, crystallinity, etc. Recent advances in the understanding of atomic processes have clearly shown how the dynamics of atomic condensation determine atomic scale structure, and therefore thin film properties. Coupling atomic scale modeling and simulation with experimental analysis, we are now approaching the point where film properties can be accurately predicted for many film growth systems. There are, however, examples of unique phenomena that occur in film growth, due to condensation effects, that are beyond existing models. The emergence of crystalline whiskers in some growth systems is one example, and our recent discovery of so-called 'pyramids' in copper and silver films is another. We will present an experimental study of emergent phenomena in thin films deposited with glancing vapour incidence, showing how macroscopic geometry and atomic-scale dynamics join forces to create complicated macroscopic structures under conditions where existing models predict nothing out of the ordinary. We will present current results from this ongoing study, and new proposals for an automata-like model of growth that has the potential to capture these complicated phenomena.
2:10 PM		H4-2-3 Ta Nanopillar Arrays Grown by Glancing Angle Deposition C.M. Zhou, D. Gall (Rensselaer Polytechnic Institute) Periodic Ta nanopillar arrays were grown onto patterned substrates by Glancing Angle sputter Deposition (GLAD). Both the effects of surface patterning and surface diffusion on morphological evolution were studied by varying the pattern length-scale created by Nanosphere lithography (NSL) and by growing at temperatures T_s ranging form 200 to 900 °C, respectively. Statistical size analyses from nanopillars grown on NSL patterns show that the distribution in width (w) broadens with decreasing interspacing (D) and increasing height (h), but remains constant with a fixed h/D-ratio. This is attributed to an intercolumnar growth competition that exacerbates nanopillar size fluctuations but scales with pillar height, indicating that the overall pillar morphologies are determined by geometric shadowing and are independent of material parameters such as the characteristic length-scale for surface diffusion at room temperature. The pillar tops branch into subcolumns that form on surface mounds caused by kinetic roughening under limited adatom mobility conditions. The fraction of branched pillars decreases with increasing T_s, as described with a simple nucleation model that provides an effective activation energy for Ta surface diffusion of 2.0 eV. At high T_s ≥ 500 °C, sub-pillars nucleate at the pillar bottom. This is attributed to a competitive growth mode that is caused by an increased adatom diffusion length and results in an increase in the average pillar width, a decrease in the pillar separation and pillar number density, the accelerated growth of some pillars at the cost of others which die out, and an increased probability for the merging of neighboring pillars.
2:30 PM		H4-2-4 Kinetics of Compound Layer Formation During Microwave Post-Discharge Nitriding J. Oseguera (ITESM-CEM, Mexico); F. Castillo (ITESM, Mexico); A. Fraguela (BUAP, Mexico); J.A. Gomez (UFRO, Chile) Different approaches have been developed concerning growth description of the compact nitride layers, especially those produced by ammonia. The atomic nitrogen flux from the surface to the solid is produced by ammonia dissociation. Microwave post-discharge nitriding is carried out in iron and steel samples. Nitriding by this process involves molecular nitrogen dissociation in the post-discharge and on the sample surface. The thermo-chemical simulation of the concomitant evolution nitride layers relates the representation of reference states of neutral and excited nitrogen in a gas mixture, surface nitrogen concentration produced by molecular dissociation on the surface and nitrogen flux from the surface into the solid. This work presents a mathematical model of the kinetics of compound layer formation during a post-discharge nitriding process. The model is related to a moving boundary value problem which takes into account the observed qualitative behavior in a laboratory. The model assumes several steps: diffusion process, formation of the layers, layer growth and quasi-stabilization of the layer growth. An analytical approximate solution of the Goodman's type is proposed, which allows us to obtain a representation of the motion of the interfaces and the nitrogen concentration profiles. Through this approach the thermodynamics of the irreversible diffusion process is applied in order to describe the evolution towards equilibrium between the growing layers.
2:50 PM		H4-2-9 Kinetic Pathways Leading to Layer-by-Layer Growth from Hyperthermal Atoms: a Multibillion Time Step Molecular Dynamics Study D. Adamovic, V. Chirita, E.P. Münger, L. Hultman (Linköping University, Sweden); J.E. Greene (Frederick Seitz Materials Research Laboratory) We employ multibillion timestep embedded-atom molecular dynamics simulations to study the homoepitaxial growth of Pt(111) from hyperthermal Pt atoms (E_Pt = 0.2 - 50 eV) using deposition fluxes approaching experimental conditions. Calculated antiphase diffraction intensity oscillations, based upon adatom coverages as a function of time, reveal a transition from a three-dimensional (3D) multilayer growth mode with E_Pt < 20 eV to layer-by-layer growth with E_Pt ≥ 20 eV. We isolate the effects of irradiation-induced processes and thermally activated mass transport during deposition in order to identify the mechanisms responsible for promoting layer-by-layer growth. Direct evidence is provided to show that the observed transition in growth modes is primarily due to irradiation-induced processes which occur during the 10 ps following the arrival of each hyperthermal atom. The kinetic pathways leading to the transition involve both enhanced intralayer and interlayer adatom transport, direct incorporation of energetic atoms into clusters, and cluster disruption, leading to increased terrace supersaturation.
3:10 PM	Invited	H4-2-7 First Principles Predictions on Structures, Properties, and Dynamics of Coatings and Thin Films W.A. Goddard (California Institute of Technology) Thin films and coatings often have amorphous, irregular, or disordered structures making it difficult to extract detailed atomistic structural information from experiment. Moreover the lack of long-range order makes it difficult to use traditional first principles methods of quantum mechanics (QM) to determine the structure. To circumvent this problem we have been developing the ReaxFF reactive force field that describes chemical reaction pathways (including barriers) nearly as accurately as QM but with computational costs nearly as fast as traditional nonreactive force fields. This allows us to simulate the reactive processes involved in building films and coatings from chemical vapor deposition, oxidation, atomic layer epitaxy, and molecular beam epitaxy. The structures of the films from these structures can then be used to predict properties of the films such as surface tension, reactivity, hardness, and wear. We will illustrate the use of ReaxFF for a variety of films and coatings and for a variety of such properties.
3:50 PM		H4-2-10 Mathematical Simulation of Atomic Nitrogen Transport in a Microwave Post-Discharge J.A. Sanchez, L. Mora (Huracan CFD); J. Oseguera (ITESM-CEM, Mexico) Mathematical simulation of the growth of compact and concomitant nitride layers in microwave post discharges has been performed considering a mass balance in each interface and nitrogen concentration on surface. The reactivity of the postdischarge strongly depends on the atomic nitrogen concentration in the postdischarge. Transport of this specie through the postdischarge is affected by gas and surface reactions. CFD was used as a basic virtual tool to understand the behavior of gas inside of vessel and accomplish the desired target. In this study, 3D mathematical simulation of atomic nitrogen transport in a microwave postdischarge is presented, N₂, N₂-H₂, and N₂-H₂-Ar gas mixtures were considered. The convergence of the coupled mass, energy and momentum equations were obtained using appropriate mesh and models in CFD FLUENT code. Several configurations are described considering the residence period of atomic nitrogen in the post-discharge. Mathematical simulation is contrasted with experimental results.