ICMCTF2005 Session TS1: The Atomistics of Thin Film Growth: Computational and Experimental Studies

Thursday, May 5, 2005 1:30 PM in Room Sunrise

Thursday Afternoon

Time Period ThA Sessions | Abstract Timeline | Topic TS Sessions | Time Periods | Topics | ICMCTF2005 Schedule

Start Invited? Item
1:30 PM TS1-1 The Atomic-Scale Processes Underlying Nanoscale Pattern Formation on Solid Surfaces
N.C. Bartelt (Sandia National Laboratories)

The deposition of Pb atoms on Cu(111) produces two distinct surface phases: a random Pb-Cu surface-alloy phase and a Pb-overlayer phase. Within a specified range of Pb coverage and temperature, the two phases co-exist and spontaneously order into domain patterns with tens-of-nanometer periodicity [1]. The continuous evolution of the domain structures with increasing Pb coverage agrees well with theoretical descriptions based on competing long- and short-range interactions. The system thus provides an ideal model system for probing the interactions underlying two-dimensional self-assembly. Although a self-consistent picture of the thermodynamic driving forces responsible for pattern formation now exists a comprehensive understanding of the kinetic processes underlying self-assembly is only beginning to emerge. In this talk I will discuss recent low energy electron microscope/scanning tunneling microscope experiments and first-principles calculations that address the key question concerning the kinetics of pattern formation: how do individual domains of each phase, which contain 10s of thousands of atoms, assemble into ordered patterns in matters of seconds to minutes? I will show that for Pb/Cu(111) the self-assembly occurs by the collective motion of entire domains as a unit across the surface. I will present evidence that this motion is facile because (1) Pb atoms move quickly across the surface alloy due to a Pb-Cu exchange mechanism and (2) Cu atoms move quickly through the overlayer phase due to a high concentration of surface vacancies and mobile Cu atoms embedded within the overlayer. The latter finding is not at all intuitive and may explain why the remarkable pattern formation seen for Pb on Cu(111) has not been observed for other metal-metal systems.

*Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin company, for the U. S. Department of Energy under Contract #DE-AC04-94AL85000. [1] R. Plass, J. A. Last, N. C. Bartelt, and G. L. Kellogg, Nature 412, 875 (2001).

2:10 PM TS1-5 Growth of Epitaxial CrN on MgO(001): Role of Deposition Angle on Surface Morphological Evolution
J.R. Lynch, J. D'Arcy-Gall, D. Gall (Rensselaer Polytechnic Institute)
Epitaxial CrN layers, 7 to 300 nm thick, were grown on MgO(001) at 600-700 °C by ultra-high-vacuum magnetron sputter deposition in pure N2 discharges at 20 mTorr. Layers grown at 600 °C nucleate as single crystals with a cube-on-cube epitaxial relationship with the substrate. However, above a critical thickness of ~50 nm, misoriented CrN grains nucleate and develop into cone-shaped grains that protrude out of the epitaxial matrix to form triangular faceted surface mounds, leading to a dramatic increase in the root-mean-square surface roughness, from 3 to 19 nm, for a 300-nm-thick layer. CrN layers grown at Ts = 700 °C are complete single crystals. Their surfaces exhibit dendritic mounds with fingers extending along orthogonal <110> directions. In addition, square shaped "super"-mounds protrude out of the underlying matrix. They are caused by a growth instability related to atomic shadowing effects which were purposely enhanced by non-normal deposition. Cross-sectional transmission electron microscopy reveals that the super-mounds are due to pyramidal epitaxial protrusions which are terminated by a pair of oppositely tilted nanopipes. The nanopipes are 1-nm-wide open structures that form due to anisotropic atomic shadowing under limited adatom mobility conditions. They are terminated on one side by perpendicular {100} facets which form a nanoscale staircase, and by a smooth tilted plane on the opposite side. The dramatic variation in the morphology of nanopipe termination is related to asymmetric shadowing and highly anisotropic surface diffusion, leading to a growth mode where perpendicular {100} planes simultaneously advance causing competing in-plane and out-of-plane directional growth.
2:30 PM TS1-6 Mechanisms of Forming the Microstructure of Niobium Films under Ion-Atomic Deposition
I.G. Marchenko, I.M. Neklyudov (National Scientific Center, Kharkov Physics and Technology Institute, Ukraine)

The work is aimed at the experimental and theoretical study of the radiation effect on the microstructure of niobium films being irradiated with their own ions during deposition.

Films were formed by the method of ion-atomic deposition. The essence of this method consists in film deposition from partially ionized metals vapor onto the substrate. Metal vapors were obtained by electron-beam heating of the material. The ion current density was controlled by the degree of vapor ionization in the low-voltage discharge of direct current. The ion energy was varied by applying the potential onto the substrate. The degree of ionization of the flow deposited was ranged from 0 to 0.45. The ion energy was from 5 to 500 eV.

The structure of niobium films was studied as a function of the degree of ionization of the flow deposited and of the ion energy. The X-ray diffraction methods were used to determine the changes in the mean-size block structure and microstrains of the crystalline lattice depending on the ion flow parameters. The measurements of the film density change with ion energy changing were carried out. The microstructure of films was examinated using the electron microscopy.

The computer simulation of the ion-atomic deposition of niobium films was performed under conditions corresponding to the experimentally observed ones. It is shown that the ion action exerts an influence on the growing film relief. An effect of ion densification is studied. The mechanisms of atomic ordering under ion irradiation are studied. It is shown that the radiation-induced defects have an influence on the process of film growth. The relation between the change in density and the size of the block film structure under ion densification has been revealed.

2:50 PM TS1-7 Two Examples of Synergy between Experiment and Computation in Nano-Science
S.V. Khare (The University of Toledo)

Nano-science and nano-technology have become the locus of attention of materials scientists in recent years. This has led to fruitful synergy between contemporaneous experimental and theoretical work. Two distinct examples from my own research where this has materialized will be shown.

1. Medium range order in amorphous silicon (a-Si): It has not been possible to measure atomic medium range order in disordered materials by conventional diffraction transmission electron microscopy experiments (TEM). A recently developed experimental technique of fluctuation TEM can be related to medium range order. Simulations for a family of atomistic structural models for unhydrogenated amorphous silicon suggest that fluctuation electron microscopy experiments have observed orientational order of para-crystalline grains in amorphous silicon. This order may consist of correlations in the orientation of nearby paracrystalline grains or anisotropy in the grain shape. This observation makes a natural connection to the known growth modes of microcrystalline silicon and may be useful for other materials systems. [Appl. Phys. Lett. 85, 745 (2004)].

2. Structure of Pt and RuPt clusters on graphite: Experimental results from deposition of Pt and Ru-Pt nano-clusters on carbon substrates show that (i) a cluster is formed of layers with fcc(111)-stacked cubo-octahedral geometry; (ii) Pt (in the Ru-Pt clusters) segregates to the top layer; and (iii) very small clusters (37 atoms) surprisingly show bulk-bond distances, despite their average co-ordination number being smaller than that in the bulk. Using density functional theory calculations, we explain these observations and provide a detailed characterization of the structure of the clusters. Surprisingly, we find that the graphitic carbon substrate causes metal-metal bonds in small clusters to expand to their bulk values despite the smaller surface lattice constant of graphite, contradicting a long-held belief that graphite is an inert growth template.

3:30 PM TS1-9 Co/CoAl/Co Trilayer Fabrication using Spontaneous Intermixing of Co and Al: Molecular Dynamics Simulation
S.-P. Kim, Y.-C. Chung (Hanyang University, South Korea); S.-C Lee, K.-R. Lee (Korea Institute of Science and Technology, South Korea); S. Seo, D.-S. Kim (Hanyang University, South Korea)
In spintronic device such as GMR (Giant Magneto-Resistance), its performance is largely dependent on the characteristics of interface region between thin film layers. It is generally composed of two ferromagnetic layers and, in between, nonmagnetic layer. The spin polarization of the junction is critically affected by the crystal structure of the thin films, and slight intermixing at the interfaces is known to pose enormous problems in the magnetic properties of the device. In the present work, we present a novel process for the fabrication of Co/CoAl/Co trilayer which has ordered and sharp interfaces at 300 K using spontaneous intermixing between Co and Al. When Al atom of 0.1 eV was deposited on Co(11-20), Al (001) thin film was grown without any intermixing. Moreover, when Co atom of 0.1 eV was deposited on Al(001) thin film, intermixed layer of ordered B2 structure was spontaneously formed and highly oriented Co(11-20) crystalline phase was subsequently grown on the intermixed region with Co adatom of 3.0 eV. Voronoi diagram was utilized for the quantitative structural analysis of each layer of thin films.
3:50 PM TS1-10 A Comparison of the Bonding in Nanolayered Ternary Carbides and Nitrides
J.M. Schneider, Z. Sun, D. Music (RWTH Aachen University, Germany); R. Ahuja (Uppsala University, Sweden)
We have investigated the elastic properties of nanolayered M2AX with M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and A = Al, Ga, Ge, Sn as well as X = C, N by ab initio calculations. We suggest that M2AC can be classified into two groups: One where the bulk modulus of the binary MC is conserved and another group where the bulk modulus is decreased. This classification can be understood in terms of coupling between MC and A layers which is defined by the valence electron population. The coupling in the here investigated M2AN is in general weaker which can be understood based on the DOS. A comparison of the valence electron concentration effect on the bulk modulus and the shear properties is discussed.
4:10 PM TS1-11 The Kirkendall Effect: Void Formation During Low Temperature SiO2 Formation in the Au/Si(100) System
B.A. Julies, D. Adams (University of the Western Cape, South Africa); J.W. Mayer (Arizona State University)
This study utilizes Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) in the characterization of aging Au/Si(100) samples. It was found that the Au/Si structure is extremely reactive, even at room temperature. Void formation has been encountered in the Si(100) substrate which we attribute to the migration of Si from the substrate, through the Au layer and ultimately to the surface where it oxidizes to form a surface SiO2 layer. We propose that the void formation in the Si substrate just below the Au/Si interface assist in the diffusion of Au into the substrate. This results in a Au front diffusing into the substrate to form a new Au-Si(100) interface. The Surface Potential Model and Kirkendall Effect is used to explain these observations.
Time Period ThA Sessions | Abstract Timeline | Topic TS Sessions | Time Periods | Topics | ICMCTF2005 Schedule