1998 MRS Spring Meeting & Exhibit

Chairs

George Comsa
Inst fur Physik & Theoretische Chem
Univ Bonn
Bonn, D-53115 GERMANY
49-228-733297

Kiyoyuki Terakura 
National Inst for Adv Interdisc Res 
Ibaraki, 305 JAPAN 
81-298-542704

Luc Wille
Dept of Physics
Florida Atlantic Univ
Boca Raton, FL 33431
561-297-3379

* Invited paper

SESSION AA1: ALLOYING AND EFFECTS OF IMPURITIES 
Chairs: Fereydoon Family and Jerry Tersoff 
Monday Morning, April 13, 1998 
Pacific I

8:30 AM *AA1.1 
SELF-ORGANIZED COMPOSITION MODULATION IN ALLOY GROWTH. J. Tersoff, IBM T. J. Watson Center, Yorktown Heights, NY. 

In epitaxial growth of unstable systems (such as strained layers or immiscible alloys), the competition between thermodynamics and kinetics can lead to pattern formation and self-organization. The specific case of step-flow growth will be discussed from a theoretical perspective. When significant diffusion occurs only at the surface, either stress or spinodal decomposition is predicted to cause a self-organized surface morphology. This is accompanied by a composition modulation, which reflects the surface pattern. For a strained alloy film, computer simulations show the formation of a periodic modulation of the alloy composition. Experimental evidence for this effect will be briefly discussed. 

9:00 AM AA1.2 
COSEGREGATION-INDUCED EPITAXIAL GROWTH OF TWO- AND THREE-DIMENSIONAL COMPOUNDS ON MULTICOMPONENT ALLOY SURFACES. E. Clauberg, A. Dziakova, B. Eltester, B. Huning, C. Uebing, Max-Planck-Institut fur Eisenforschung, Department of Physical Chemistry, Dusseldorf, GERMANY. 

The chemical reactivity of solids depends to a large extent on structure and composition of their surfaces. Therefore, segregation and adsorption effects decisively influences many surface processes and material properties, such as heterogeneous catalysis, high-temperature oxidation, adhesion of coatings, etc.. In recent years it has been realized that the segregation of dissolved elements from the bulk of the crystal may change the surface composition and in many cases also the surface structure. Thus, it is of great fundamental and technological interest to investigate such processes in detail. Cosegregation denotes the joint enrichment of different solute atoms from the bulk of a condensed phase at interfaces. This process may cause the formation of two-dimensional chemical compounds ( surface compounds). The cosegregation-induced formation of two-dimensional chemical compounds must be distinguished from the precipitation of three-dimensional compounds at surfaces. According to the Gibbs phase rule, three-dimensional precipitates can be formed upon oversaturation in coexistence with the saturated solid solution. Precipitation can take place at free surfaces and grain boundaries where the nucleation is favored. In this contribution the cosegregation-induced epitaxial growth of two- and three-dimensional chromium nitrides on ferritic Fe-15%Cr-N(100) ( wt-ppm) single crystal surfaces will be discussed. The two-dimensional CrN surface compound is stable between 600 and C. From the  LEED pattern it is inferred that the surface compound is epitaxed to the bcc(100) alloy surface. X-ray photoelectron diffraction (XPD) and low energy electron diffraction with intensity analysis (LEED-IV) have revealed that this surface compound consists of a single CrN compound layer plus an additional subsurface chromium layer with a huge interlayer expansion between both layers. The CrN surface precipitate formed at temperatures C is also epitaxed to the bcc(100) substrate surface. Its structure corresponds to the rocksalt structure, i.e. the structure of the well-known bulk CrN. Starting from a sputter cleaned alloy surface the growth of the epitaxial CrN surface precipitate proceeds via the two-dimensional CrN surface nitride. After completion of the first CrN layer the nucleation and growth of the three-dimensional CrN surface precipitate takes place. 

9:15 AM AA1.3 
PARAMETERS THAT DRIVE CHEMICAL ORDERING ALONG THE GROWTH DIRECTION IN EPITAXIAL (0001) Co- Ru AND (0001) OR (111) Co-Pt ALLOY THIN FILMS. Laziz Bouzidi, Mireille Maret, Véronique Pierron-Bohnes and Marie-Claire Cadeville, IPCMS-GEMM, Strasbourg, FRANCE. 

The occurrence of long range order (LRO) or anisotropic short range order (ASRO) along the growth direction in epitaxial hcp (0001) Co-Ru and hcp or fcc (111) Co-Pt alloy thin films is now a well established result. The great interest in preparing such new structures, that are metastable, is their ability to develop large perpendicular magnetic anisotropy. In the present paper we first discuss the optimal preparation conditions that determine the structures and microstructures. In addition to the role played by the nature, symmetry and crystalline quality of the substrate and of the buffer layer, the growth temperature (T_G) is a determinant parameter. Secondly, the concentration dependencies of the chemical ordering are presented. In Co_1-XRu_X LRO chemical ordering is present over the whole explored concentration range (), whereas in Co_1-XPt_X LRO is observed for  and anisotropic ASRO is observed around x = 0.75. In both systems, the optimal concentration for the occurrence of LRO is around x = 0.25. We present a thermally activated model that takes into account the competition between the surface and the bulk diffusion during the growth time. It fits well the dependencies of structural and magnetic properties with T_G. The competition between surface and bulk interactions that originates the concentration dependencies of the chemical ordering is qualitatively discussed.The systems that could develop equilibrium LRO along a growth direction, as the L1₀ CoPt structure along a (001) axis are not concerned by the present study. 

9:30 AM AA1.4 
COMPARATIVE STUDIES OF Pb/Cu(001) BY TEM, AFM AND STM. Franck Bocquet, Camille Cohen, Didier Schmaus, GPS, Universites Paris VI et VII, FRANCE; Andre Rocher, Jacques Crestou, Sebastien Gauthier, CEMES-CNRS, Toulouse, FRANCE. 

The Pb/Cu system is interesting for studying FCC heteroepitaxy because of the immiscibility of the two elements. Above room temperature, Pb (a=4.951) grows on the (001) surface of Cu (a= 3.615) according to the Stranski-Krastanov mode : individualised islands on a wetting monolayer. We have studied by TEM, STM and AFM, the same specimens prepared under UHV condition with deposition temperature between 300 to 450K. STM was performed in situ and AFM in dry nitrogen atmosphere. Specimens for TEM study, protected by an amorphous carbon film, have been prepared for plan view observations. We show that the information obtained by these techniques are consistant when comparable. They are also complementary, in particular: 
- AFM reveals the external three dimensional shape of the islands: tabular bordered by well defined facets. This information coupled with STM atomic resolution images permits the precise determination of their atomic structure. 
- The moirÈ patterns reveal that the Pb islands are well relaxed. We observed three island morphologies, which correspond to different specific orientations: 
i) rectangular shape with Pb(111),<110>//Cu(001),<100>; these islands are in fact bicrystals; 
ii) square shape with Pb(001),<110>// Cu(001),<100>; 
iii) elongated island along the <110> Cu directions, constituted by three different grains. 
The relative population of these three types of islands depends markedly on growth temperature. At 300K the majority of islands are rectangular and square shaped. At 450K, the third type of islands is mainly observed. 

9:45 AM AA1.5 
ON THE ATOMISTIC MECHANISMS OF SURFACTANT ACTION DURING EPITAXIAL GROWTH OF METALS. V. Cros, J. Camarero, A.L. Vázquez de Parga, J.E. Prieto, J.J. de Miguel, R. Miranda, Dpto. Física de la Materia Condensada, C-3, Univ. Autónoma, Madrid, SPAIN; J.M. Gallego, ICMM-CSIC, Madrid, SPAIN; L.Gómez, IFIR, Rosario, ARGENTINA; J. Ferrón, INTEC-UNL, Santa Fe, ARGENTINA. 

Epitaxial growth of metals, even in its simplest case (homoepitaxy) has to deal with several difficulties, specially on fcc-(111) faces. Firstly, due to the existence of two three-fold sites with very similar adsorption energies, a certain amount of stacking faults is formed during growth. Also, high Ehrlich-Schwoebel barriers at atomic steps nearly suppress interlayer diffusion, producing rough, 3D films. These undesirable effects can be prevented by using a suitable surfactant [1]. Our systematic study of the effect of Pb during the growth of Co and Cu on Cu(111) sheds some light onto the mechanisms of surfactant action. Our data demonstrate that this is a purely local effect, and show that the optimum surfactant coverage is one full monolayer. These results contradict existing theoretical models and have profound implications on the atomistic mechanisms of the surfactant effect. 

10:30 AM *AA1.6 
DYNAMICS OF SUBMONOLAYER EPITAXIAL GROWTH: INFLUENCE OF A SECOND SPECIES. Pushkar S. Ranade, University of California, Dept. of Materials Science and Mineral Engineering, Berkeley, CA; and D.C. Chrzan University of California, Department of Materials Science and Mineral Engineering, Berkeley, CA and Lawrence Berkeley National Laboratories, Materials Sciences Division, Berkeley, CA. 

It is now established that the evolution of island sizes during submonolayer epitaxial growth is consistent with a dynamic scaling hypothesis. However, most models of the phenomenon neglect the presence of impurities which may influence the subsequent nucleation and growth process. Our work is motivated specifically by experiments performed by R. Q. Hwang et al. in which Au islands are nucleated and grown on a Ru(0001) substrate exposed to O. The atom interactions are modeled using a simple lattice gas Hamiltonian. The Hamiltonian reproduces essential features of the O on Ru(0001) phase diagram. In addition, the Hamiltonian allows for the mutual attraction of Au atoms on the surface, and an attractive or repulsive Au-O interaction. Kinetic Monte Carlo simulations are performed. The dynamic evolution of island size distributions is explored as a function of the parameters governing the interactions, including the temperature and the O concentration. 

11:00 AM AA1.7 
ATOMIC INTERACTIONS IN THE STRUCTURE AND STABILITY OF CLUSTERS: Pd ON W(110). Seong Jin Koh, Gert Ehrlich, Univ of Illinois at Urbana-Champaign, Dept of Materials Science and Engineering, Urbana, IL. 

Atomic interactions responsible for the stability and shape of surface clusters have been studied quantitatively using a low temperature field ion microscope. The probability distribution of interatomic separations between two Pd atoms on the (110) plane of tungsten has been measured. This distribution in turn yields the free energy of pair interactions. Interactions are found to be highly anisotropic, varying strongly with the orientation of the pair on the surface, and to be of quite long range, extending over many atomic spacings. Along with pair interactions, it has been shown that many-atom interactions decisively influence the shape and cohesion of surface clusters in the early stages of growth. The pronounced anisotropy of interactions is also found to significantly affect the kinetics of incorporation. (Supported by the UIUC Campus Research Board and by the U.S. Department of Energy under Grant No. DEFG02-96ER-45439) 

11:15 AM AA1.8 
EPITAXIAL STABILIZATION OF PSEUDOMORPHIC PHASES IN METALLIC MULTILAYERS. Rajarshi Banerjee, Xiao-Dong Zhang, Suliman A. Dregia and Hamish L. Fraser, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH. 

Sputter deposited Ti/Al multilayers with varying layer thicknesses of Ti and Al have been characterized by high resolution transmission electron microscopy (HRTEM) and low angle x-ray diffraction (LAXRD). Structural features in these multilayers, such as stacking of close-packed planes, interfacial misfit dislocations and other defects, have been probed at the atomic resolution in order to develop a better understanding of the structural transitions occurring in these multilayers. Expanding on classical thermodynamics, a new model for structural stability in thin film multilayers based on the competition between bulk and interfacial contributions to the free energy will be discussed with special emphasis on experimental results from the Ti/Al system. The thermodynamics results in a biphase stability diagram for multilayers in which concurrent phase stabilities are mapped as a function of two degrees of freedom, volume fraction and a size scale, associated with thin film multilayers. A biphase diagram for Ti/Al will be presented. The applicability of this thermodynamic model to other binary systems is currently being explored, and it has been successfully applied to the Co/Cr system. A biphase diagram for Co/Cr multilayers, which has been constructed based on experimental results available in literature, will be presented. In addition, applicability to other candidate systems will be discussed. 

11:30 AM *AA1.9 
EPITAXIAL GROWTH AND MAGNETISM. H. Dreyssé, M. Freyss and D. Stoeffler, IPCMS-GEMME, Université Louis Pasteur, Strasbourg, FRANCE. 

Growth phenomena and magnetism are not easy to describe at the same level of precision. A small variation of distances can modify drastically the magnetic properties of thin metallic films. A good example is given by the different magnetic arrangements of small Cr clusters grown on Fe versus the cluster size and shape. In this contribution we review different aspects encountered in growth of magnetic metallic films. Interdiffusion during the growth is very common in metallic systems. A microscopic mechanism is presented where the Point and Pair Interactions are computed from first-principles band structures calculations. For instance the Cr interdiffusion into Fe(001) can be well understood within this framework. We show also how the magnetic properties of thin films can be strongly affected during the growth and we emphasize the role of the magnetic frustrations. For the Fe/Cr/Fe system, taken here as a generic example, due to the large number of available experimental data, the growth of Fe overlayer can change totally the magnetic order of the Cr film, as observed experimentally. Using simple model to simulate the growth, we point out the influence of the growth mode on the total magnetization of the film. Finally we discuss the possibility of non-collinear magnetic order and his evolution during the growth process. 

SESSION AA2: ISLAND DISTRIBUTION 
Chairs: Gert Ehrlich and Thomas Michely 
Monday Afternoon, April 13, 1998 
Pacific I

1:30 PM *AA2.1 
SCALING AND COARSENING IN SUBMONOLAYER AND MULTILAYER EPITAXIAL GROWTH. Fereydoon Family, Emory University, Department of Physics, Atlanta, GA. 

The results of recent theoretical and simulational studies of submonolayer and multilayer homoepitaxial growth will be discussed. In the submonolayer regime, the results of kinetic Monte Carlo simulations and a self-consistent rate equation approach will be presented. Our results provide a quantitative explanation for the variation of the submonolayer island density, critical island size, island-size distribution and morphology as a function of temperature and deposition rate found in recent experiments. In multilayer growth, a realistic model for homoepitaxial growth on fcc and bcc lattices which takes into account the correct crystal structure will be introduced. The effects of instabilities which lead to mound formation and coarsening will be discussed and a unified picture of the effects of attractive and repulsive interactions at ascending and descending steps on surface morphology and island nucleation will be presented. An accurate prediction of the observed mound angle for Fe/Fe(100) deposition is obtained analytically and by kinetic Monte Carlo simulations. The general dependence of the mound angle, and mound coarsening behavior on temperature, deposition rate, and strength of the step barrier in bcc(100) and fcc(100) growth will also be presented and compared with recent experiments. 

2:00 PM *AA2.2 
RIPENING MECHANISMS IN ULTRATHIN METAL FILMS . Georg Rosenfeld, Faculty of Applied Physics, University of Twente, AE Enschede, The NETHERLANDS. 

The talk will discuss new experimental results on diffusion and decay of two-dimensional clusters. Both processes play a key role in ripening and annealing of thin metal films. An important example is Ostwald ripening where an island ensemble coarsens as small islands decay in favour of larger ones. Also island motion leads to coarsening as islands meet and coalesce. Island motion and decay have been studied in model experiments on fcc(111) metal surfaces in UHV using temperature-variable Scanning Tunneling Microscopy. The results are suited to develop a detailed understanding of the microscopic diffusion processes involved, and quantitative information on activation energies for relevant atomic processes has been derived from the data. 

2:30 PM AA2.3 
SELF-CONSISTENT RATE EQUATION APPROACH TO TRANSITIONS IN CRITICAL ISLAND SIZE IN METAL (100) AND METAL (111) HOMOEPITAXY. Mihail N. Popescu, Fereydoon Family, Emory Univ., Dept. of Physics, Atlanta, GA; Jacques G. Amar, Univ. of Toledo, Dept. of Physics and Astronomy, Toledo, OH. 

The development of new experimental techniques has made possible the real-time probing of microscopic details of the surface during the early stages of thin-film growth. These developments have renewed both experimental and theoretical interest in the scaling properties of the island density and island-size distribution in submonolayer epitaxial growth. One concept which has been extensively used is that of a critical island size i corresponding to one less than the number of atoms in the smallest stable island. With increasing temperature, a transition from a critical island size i=1 at low temperature to a higher critical island size at high temperature is expected to occur. Such transitions have been observed in metal (111) and metal (100) homoepitaxy for a variety of systems. In the present work we consider a self-consistent rate equation approach to submonolayer growth for a restricted pair-bond model that is relevant to low and intermediate temperature metal (100) and (111) homoepitaxy. In contrast to previous standard rate equation results, the average island density and monomer density are well predicted along with the transition temperature from i=1 to a higher critical island size. It is shown that the method's implicit short-range correlations between attachment/detachment rates, together with a careful estimate of the escape rates for small clusters, are important factors for a good agreement with the kinetic Monte Carlo simulation results. 

2:45 PM AA2.4 
ELASTIC INTERACTIONS IN AN ORDERED ARRAY OF VACANCY ISLANDS IN A STRAINED METAL FILM. Karsten Pohl, Juan de la Figuera, Norman C. Bartelt, Maria C. Bartelt, and Robert Q. Hwang, Sandia National Labs, Livermore, CA; Jan Hrbek, Brookhaven National Lab, Upton, NY. 

Ordered nanoscale arrays in thin films have attracted increasing research interest due to their potential technological applications. Elastic interactions are expected to be important, both in defining their properties and as a fundamental ordering mechanism. However, there have been no direct measurements of these interactions. In this work, we have probed the ordering forces by measuring thermal fluctuations in an array of vacancy islands in a strained metal film using time-resolved scanning tunneling microscopy. The vacancy island arrays are formed by reacting sulfur with sub-monolayer films of Ag on Ru(0001) at room temperature. We find that sulfur preferentially etches the highly ordered dislocation pattern in the Ag adlayer. This results in an ordered triangular lattice of 20‰ wide vacancy islands with a superlattice constant of about 50‰. At room temperature, the vacancy islands are mobile, leading to vibrations of the vacancy island solid. By performing a normal mode decomposition of these vibrations ñ i.e. measuring the thermal phonons ñ we are able to determine the elastic constants of the island lattice, thereby providing insight into the cohesive interactions of the nanoscale array. The observed order of magnitude for the elastic forces are consistent with strain induced interactions between vacancy islands. These results will be interpreted in a general framework of step-step elastic interactions in a heteroepitaxial system. This work is supported by the Office of Basic Energy Sciences Division of Material Science of the U.S. Department of Energy under Contract No. DE-AC-04-94AL85000. 

3:00 PM AA2.5 
STEP EDGE VELOCITIES FOR MBE GROWTH. R.E. Caflisch, Department of Mathematics, UCLA; M.F. Gyure and C. Ratsch, Hughes Research Laboratories, Malibu, CA. 

Determination of the velocity of a step edge is essential for macroscopic modeling of epitaxial growth for a non-rough surface. The velocity of any step edge is determined by the density of kinks on it, i.e. a rough step edge captures more adatoms and thus grows faster than a smooth step edge. In thermodynamic equilibrium, it is appropriate to use the velocity formula of Burton-Cabrera-Frank that relies on a Gibbs distribution for the kinks in the step edge. During MBE growth, however, the equilibrium assumption needs to be relaxed since many slow processes are kinetically frozen out. We derive a kinetic rate theory that predicts the nonequilibrium kink density and resulting velocity of a step edge under MBE conditions. This theory is compared to results from kinetic Monte Carlo simulations of a solid-on-solid model, and excellent agreement is obtained. We will also show how the velocity can be used in an island dynamics model for step flow and layer-by-layer growth. 

3:15 PM AA2.6 
MECHANISMS AND KINETICS OF THE INITIAL STAGES OF ISLAND FILM GROWS BY CVD. D.A.Grigoriev, S.A.Kukushkin, Institute of Mechanical Engineering Problems, Russian Academy of Sciences, Sankt-Petersburg, RUSSIA. 

Kinetics of initial stages of film and coatings growth by CVD is investigated. Complete closed equation system bounding the characteristics of growing island film with process parameters. As a result of this system solution all basic relations of growing film such as island size distribution function, mean radius of island and ration of substrate occupation on the dependence of time and spatial coordinate are obtained. The causes of phenomenon that films produced by CVD show an ununiformity of characteristics in terms of substrate surface are revealed and mathematical description is provided. Some recommendations on the development of films with prearranged properties are provided. 

4:00 PM *AA2.7 
COARSENING SIMULATIONS OF TWO-DIMENSIONAL ISLANDS ON SOLID SURFACES. Horia Metiu, Thomas Mattsson, Gregory Mills, Dept of Chemistry, University of California, Santa Barbara, CA. 

Using simulations, we have studied the coarsening of an ensemble of atom-high two-dimensional Ag islands on the Ag(100) surface. We consider first the elementary processes controlling coarsening: the evaporation of atoms from islands of various sizes and the migration of these islands due to material transport along their edges. The information obtained from these preliminary studies is used to produce a very efficient simulation of coarsening. This allows us to study how coarsening modifies the density of the islands, the distribution of their sizes, and the positions of the islands on the surface. 

4:30 PM AA2.8 
FORMATION AND MIGRATION ENERGIES OF A VACANCY APPROACHING A METAL SURFACE. F. Willaime, Y. Piquet, M. Nastar, B. Legrand, Section de Recherches de Metallurgie Physique, CEA Saclay, FRANCE. 

The energy variations associated to a vacancy approaching a metal surface have been studied by atomistic calculations. Vacancy formation-energies at the surface and migration energies for a surface atom jumping towards the bulk into a nearest-neighbour vacancy have been investigated for various surfaces of FCC (Cu, Ag, Au) and BCC (Fe) metals modelled by semi-empirical potentials (Second Moment Approximation and EAM). The formation energies are discussed as a function of crystaline sructure and surface density. A relaxation procedure has been developped to find the exact saddle point configurations. A giant softening of the migration energies with respect to the bulk is observed. 

4:45 PM AA2.9 
OUT-OF-PLANE TEXTURE CONTROL IN PVD. Liang Dong, David. J. Srolovitz, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI. 

Two and three dimensional molecular dynamics (MD) simulations were performed to determine the effect of ion beam conditions on the evolution of the out-of-plane film texture during the ion beam assisted deposition (IBAD) of metallic films. The ion beam is capable of modifying out-of-plane film texture through the dependence of the sputter yield on the ion beam orientation with respect to the crystal lattice. Ion beam orientations corresponding to ion channeling directions result in less sputtering from the film surface and, therefore, higher growth rates during IBAD. This sputtering effect can change the out-of-plane texture from one corresponding to a low surface energy to one corresponding to a high surface energy. For fcc crystals, simulations showed <110> oriented films can be grown by directing the ion beam normal to the substrate in order to take advantage of the fact that the <110> direction is the strongest channeling direction in most fcc crystals. We present IBAD growth rate and sputtering rate results for a range of ion beam orientation, ion beam energy, and ion-to-atom arrival rates. The simulation results demonstrate that higher ion beam energy results in larger sputter yield difference between films oriented in channeling and non-channeling orientations. 

SESSION AA3: STRAIN AND DISLOCATION GROWTH 
Chair: Dimitri D. Vvedensky 
Tuesday Morning, April 14, 1998 
Pacific I

8:30 AM *AA3.1 
EFFECTS OF MISFIT AND MISFIT RELAXATION ON YOUNG'S ANGLE AND ISLAND MORPHOLOGY. David J. Srolovitz, Jurgen Schnitker, Liang Dong, Univ of Michigan, Ann Arbor, MI. 

The equilibrium shape of an island on a substrate is determined by several factors, including surface and interfacial energies and misfit strain. The surface and interfacial energies can lead to faceting and determine the equilibrium wetting or Young's angle. Elastic energy associated with misfit can modify the island shape but is generally assumed not to affect Young's angle. In this presentation, we examine the effect of misfit strain and misfit strain relaxation on the equilibrium Young's angle using both analytical, continuum concepts and atomistic simulations. The continuum analysis, based on the order of the divergence of the elastic fields and the interfacial energy, suggests that Young's angle is independent of misfit. On the other hand, the atomistic simulations show some effects, not included in the continuum analysis, indicating that misfit may indeed change the equilibrium Young's angle. 

9:00 AM AA3.2 
THE GROWTH MODE AND ELECTRONIC STRUCTURE OF STRAINED THIN FILMS OF GADOLINIUM. C. Waldfried, P. A. Dowben, Department of Physics & Astronomy and Center for Materials Research and Development, University of Nebraska-Lincoln, Lincoln, NE. 

Strained thin films of gadolinium have been obtained by growing Gd on the corrugated surface of Mo(112).