AVS2001 Session SS2-ThA: Nucleation & Growth

Thursday, November 1, 2001 2:00 PM in Room 122

Thursday Afternoon

Start	Invited?	Item
2:00 PM	Invited	SS2-ThA-1 The Use of Instabilities in Epitaxial Growth for Lateral Patterning of Surfaces H.-J. Ernst (CEA Saclay, France) The use of intrinsic instabilities in growth processes is currently actively explored as a promising pathway to reach a lateral patterning of surfaces at the nanometerscale. The origin of these instabilities is traced back to the presence of an excess energy barrier for adatom diffusion over descending steps, the Ehrlich-Schwoebel barrier. Structural patterns can be created either by spontaneous self-organization of the material deposited, or by guided growth on prestructured substrates. The deposition of Cu on singular and vicinal Cu surfaces illustrates this approach, using Helium Atom Scattering (HAS), Scanning Tunneling Microscopy (STM) and Low Energy Electron Microscopy (LEEM) as structural probes. Surprisingly, the interaction of green laser light with these surfaces leads as well to atomic scale restructuring. I like to acknowledge the contributions of F. Charra, L. Douillard, R. Gerlach, and T. Maroutian at various stages of this project.
2:40 PM		SS2-ThA-3 Evolution of Mounds during Ag/Ag(100) Homoepitaxy: Temperature Dependence of Pre-asymptotic Behavior K.J. Caspersen, A.R. Layson (Iowa State University); C.R. Stoldt (University of California at Berkley); V. Fournee, P.A. Thiel, J.W. Evans (Iowa State University) Step edge barriers are known to induce unstable epitaxial growth characterized by "mound" formation, but a detailed understanding of the roughening and coarsening dynamics is lacking. Most theoretical studies aim to elucidate asymptotic behavior for simple models in the regime of (mound) slope selection. We instead perform realistic atomistic modeling of Ag/Ag(100) growth where slope selection is often slow, and the experimentally relevant "pre-asymptotic" behavior is then characterized by slow coarsening and rapid roughening. To describe observed 25ML morphologies from 150-300K ,¹ our model includes irreversible island formation (with a 0.40eV barrier for terrace diffusion), distinct step edge barriers for straight and kinked step edges (0.07eV and ~0eV), and a realistic description of periphery diffusion which controls island shapes and coalescence (rapid diffusion along straight steps; 0.41eV barrier for kink rounding). This model then reproduces the key features of mound evolution observed at various temperatures for growth up to 60-100ML, it allows a precise characterization of evolution of the mound distribution (as quantified by a suitable tessellation), and also reveals the crossover to slope selection for thicker films. ¹K.J. Caspersen et al., Phys. Rev. B 63 (2001) 085401.
3:00 PM		SS2-ThA-4 Mechanisms for Hole Formation in Surface Alloy Systems: Rh/Ag(001)¹ L.D. Roelofs (Haverford College); R.J. Behm (University of Ulm, Germany); D.A. Chipkin (Haverford College) We present a study of mechanisms for surface hole formation during heterogeneous, metal-on-metal epitaxial growth of surface alloy systems. We consider specifically the system Rh/Ag(001) for which a detailed STM study of the structures formed during epitaxy at room temperature is available.² Three mechanisms are proposed and investigated via a kinetic Monte Carlo simulation based on a simple model for the atomic-level energetics. It is found that the dominant mechanism of hole formation in this system involves the growth of vacancy islands via an upward exchange diffusion move. First principles total energy computations using the VASP suite of programs confirm the plausibility of the explanation. Our simulation also accounts, via the same mechanism, for the observation of the growth of Ag islands on top of deposited material. An alternative mechanism for hole formation, coalescence of point vacancies, is found not to contribute appreciably to formation of vacancy clusters, but point vacancies do catalyze structure formation by other means. The mechanism identified in the present study should be applicable to other soft substrate surface alloy systems. L.D.R. thanks the DOE for supporting a visit to Sandia Livermore in order to carry out the VASP calculations.. ¹Funded by the NSF via grant DMR - 9974545 ²S.-L. Chang, et al., Phys. Rev. B53, 13747 (1996).
3:40 PM		SS2-ThA-6 Molecular Dynamics Simulations of Thin Film Nucleation through Molecular Cluster Beam Deposition: Effect of Incident Angle Y. Hu (The University of Kentucky); S.B. Sinnott (The University of Florida) Deposition of organic cluster beams on surfaces leads to the creation of polymeric thin films through rapid chemical reactions. These reactions occur over timescales on the order of a few picoseconds and therefore are suitable for study by molecular dynamics. Besides such factors as incident energy and cluster size, the deposition angle is believed to have important effects on the nucleation of thin films grown through molecular, ionic and cluster beam deposition. In this work, angular effects on molecular organic beam deposition are studied extensively through classical molecular dynamics simulations. The reactive empirical bond potential developed by Brenner et al.¹ is used in this simulation. Various organic cluster beams are deposited on the hydrogen terminated diamond (111) surface at room temperature. The beam impacts the surface along different crystallographic orientations at incident angles of 0Â°, 15Â°, 45Â° and 60Â° with respect to the normal to the surface. Two beam energies are considered: one corresponding to 25eV/cluster molecule and one corresponding to 50eV/cluster molecule. As the angle increases from the normal, the amount of energy deposited along the surface normal decreases. Therefore we have also considered cases where the energy normal to the surface is constant while the total energy varies. The results show the dependence of the angle effects on the crystallographic orientations, the incident energies and the reactivity of the impacting species. This work is supported by the National Science Foundation (CHE-9708047). ¹S.B.Sinnott, L.Qi, O.A.Shenderova, D.W.Brenner, in Chaper 1 Volume 4 of Advances in Classical Trajectory Methods, Molecular Dynamics of Clusters, Surfaces, Liquids, and Interfaces, Ed. W. Hase(JAI Press, Inc. Stamford, CT, 1999), p. 1-26.
4:00 PM		SS2-ThA-7 Scanning-tunneling/Atomic-force Microscopy Study of the Growth of KBr Films on InSb(001) J.J. Kolodziej, B. Such, P. Czuba, P. Piatkowski, F. Krok, M. Szymonski (Jagiellonian University, Poland) Thin epitaxial KBr films have been grown on InSb (001) surface. Scanning tunneling and non-contact atomic-force microscopy in ultra-high vacuum has been used to study surface structures generated during growth, for coverages ranging from 0.3 to 120 ML. It is found that in submonolayer coverage regime oval-shaped islands of monatomic thickness are formed. These islands are often cut along <1 1 0> crystallographic direction and the distribution of these islands on the substrate surface is anisotropic reflecting anisotropic diffusion of KBr molecules during the growth. The KBr/InSb interface is likely to be stabilized by a bond between the halide ion and AIII atoms arranged in chains on InSb. At 1 - 1.5 ML coverage continuous KBr film is formed and the material in excess of 1 ML forms rectangular islands with edges oriented along <100> and <010> directions on the surface. For multilayer coverages pyramidal structures of rectangular bases are formed indicative of slow diffusion of KBr molecules down across steps. These rough KBr films can be, as a result of thermal annealing, converted to flat films exposing large (> 0.1 micrometer), atomically flat (100) terraces.
4:20 PM		SS2-ThA-8 In situ Variable Temperature-pressure STM on Selected Nanoparticles: From Nucleation and Growth to High Pressure Stability A.A. Kolmakov, D.W. Goodman (Texas A&M University) The nucleation and stability of metal nanoparticles on oxides are of great importance in catalysis, gas sensors and microelectronics. Using variable temperature and pressure scanning tunneling microscopy (STM) in conjunction with in-situ deposition techniques, a versatile imaging approach has been developed that allows the nucleation, growth and alloying of individual nanoparticles to be followed in-situ. Selected nanoparticles can be exposed to reactive gas mixtures and the evolution of their morphology followed while spanning a pressure range of over eleven orders of magnitude. Since the size and composition of the particles can be controlled individually, direct comparison of various particles with similar treatments can be carried out in a single experiment. In particular, the stabilities of Au and Ag nanoparticles supported on TiO₂ (110) have been investigated while carrying out a catalytic reaction (CO oxidization) and while exposing the sample to an aqueous environment.
4:40 PM		SS2-ThA-9 Characterization by XPS, LEED and STM of Silicon Deposited onto HfB₂ (0001) R. Singh (University of Illinois at Chicago); W. Hayami, T. Tanaka (National Institute for Materials Science, Japan); M.W, Trenary (University of Illinois at Chicago) In the microelectronics industry, transition metal diborides like TiB₂ and HfB₂ have received a great deal of attention as possible diffusion barriers because both diborides are refractory materials that have high melting points, high degrees of hardness, and are chemically very stable. While studies have been done to assess the ability of these materials to prevent the diffusion of copper into silicon, there have not been any studies of the actual bonding, or interface, between silicon and a diboride. Since the structures and composition of the interfacial region necessarily dictate the properties and morphology of the subsequent film, this region is of great importance. Also, while HfB₂ on Si more closely resembles the actual application of a diffusion barrier, many experimental advantages are gained from studying Si on HfB₂, while yielding the same results as HfB₂ on Si. Silicon was deposited onto a clean and well ordered single crystal by the reaction of silane, SiH₄(g), at 800C. Two phases of hafnium and silicon, were identified on the surface. The Hf₅Si₃ phase has a hexagonal unit cell and was found to form hexagonal islands that were more than 100Å in width and scattered over the surface. This surface exhibited a complex (√7x√7)R19.1 LEED pattern and two distinct XPS peaks in the Si2p region at 99.8 and 99.4 eV assigned to silicon and the silicide respectively. The HfSi₂ phase has an orthorhombic unit cell and formed nanometer-wide lines which, at higher coverages, form a "wagon wheel" structure. STM also shows rows of silicon dimers both before and after the 900C anneal, growing side by side the silicide features. These dimers are "bean-like" protrusions that have a (√7x√3) silicon structure. After annealing the dimer covered surface to 1300C the Si-Si bond was cleaved and the individual silicon atoms relaxed to form a honeycomb-type structure, occupying the three fold hollow sites.
5:00 PM		SS2-ThA-10 Heteroepitaxy of a Manganese Carbonate on Calcite in Aqueous Solutions A.S. Lea, A. El-Azab, D.R. Baer, J.E. Amonette (Pacific Northwest National Laboratory) Heteroepitaxy of a manganese carbonate phase on the (1014) surface of calcite using an AFM has been observed in solution when the ion activity product of Mn²⁺ and CO₃^2- exceeds the solubility limit of MnCO₃. Thermodynamic data indicates that the resulting phase is a Mn_0.5Ca_0.5CO₃ phase and is consistent with our XPS and EPR measurements. These islands, while growing many microns in length along the [221] direction, have a uniform width of 150-220 nm and a uniform height of only 2.5 nm, corresponding to eight atomic layers. The islands cease growing when they encounter a step edge and have been observed to dissolve when undercut by a growing etch pit. Comparison of the crystal lattices of calcite and the Mn_0.5Ca_0.5CO₃ phase, indicate the direction of preferred growth is along the direction of greatest lattice mismatch, 3.3% as opposed to a mismatch of 2.2% along the direction of island width, [010]. A 25% decrease in stiffness along the [221] direction compared to the stiffness along the [010] direction is sufficient to account for this discrepancy. We have used a glued wetting layer model with conditions of constant surface chemical potential to model the observed morphology of the heteroepitaxial layer. Although not all the required parameters are accurately known, the model accurately depicts the measured cross-sectional profiles of the islands. This result implies that the models and considerations associated with nano-phase formation on surfaces in vacuum apply to a significant degree to growth in solution. Pacific Northwest National Laboratory is a multiprogram national laboratory operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract DE-AC06-76RL0 1830.