Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N40: Focus Session: Morphology and Evolution at Surfaces: Ion Beams and Instabilities |
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Sponsoring Units: DMP DCMP Chair: Lars Samuelson, Lund University, Sweden Room: LACC 408A |
Wednesday, March 23, 2005 8:00AM - 8:12AM |
N40.00001: Smoothening Mechanism for GaAs(100) Surfaces during Plasma Etching Sang Lee, Harry Gillis, Christian Ratsch Scanning probe microscopy and modeling have advanced the understanding of surface morphology evolution during thermal processing (deposition) and ion bombardment (sputtering). Ion-enhanced plasma etching, where the morphology is determined by the interplay between chemical and ion effects, is less advanced. We have demonstrated the transition from crystallographic to smooth morphology as ion energy increases during etching of GaAs(100) with BCl$_{3}$-Cl$_{2}$ gases. With negligible ion energy, the surface develops $<$110$>$ ridges and {\{}111{\}} facets, as expected from chemistry. With ions at \textit{ca.} 27 eV, ridges and facets are reduced, and the surfaces become smooth (RMS roughness $<$0.5 nm) at ion energy above 100 eV. This transition was simulated using Kinetic Monte Carlo methods, in which morphology is correlated with the relative etch rates at specific types of lattice sites. The simulation results suggest that ion bombardment increases the etch rate at ``ridge'' sites relative to other sites, and enables smooth surface etching, primarily by step flow. [Preview Abstract] |
Wednesday, March 23, 2005 8:12AM - 8:24AM |
N40.00002: Sputter Erosion of Ni(111) studied using Low Energy Electron Microscopy Mahesh Rajappan, Michal Ondrejcek, Wacek Swiech, C. Peter Flynn Sputter erosion of Ni (111) by 1 keV Ar$^{+ }$ions in the temperature range between 400 and 750K has been investigated using low energy electron microscopy (LEEM). Characteristic step profiles are found as a function of sputtering temperatures and durations. Below 500K sputtering leads to ragged steps and small islands. As the sputtering temperature is increased a regular sequence of the surface morphology changes with the sputtering temperatures. Sine-like waves occur at intermediate temperatures , which evolve into sharp peaked ripples resembling interfacial structures reported for the Mullins-Sekerka instability of driven solidification interfaces. This behavior is very similar to our earlier observations on Pd (111) and Pt (111). At high sputtering temperatures, the steps become smoother with only weak periodic structure. Distinctly, erosion of Ni (111) gives rise to spiral structures at screw dislocations exhibiting well-defined crystallographic features. At longer sputtering times with $\sim $ 120 ML removed, the star-like structures form. This research is supported by DOE grants DEFG02-02ER46011 and DEFG02-91-ER45439 through the Center for Microanalysis of Materials, University of Illinois. [Preview Abstract] |
Wednesday, March 23, 2005 8:24AM - 8:36AM |
N40.00003: Optical Studies of the Morphologies of Metal Surfaces during Ion-erosion and Thermal Annealing Petros Thomas, Xiangdong Zhu Ar- and Ne-ion sputtering and thermal annealing of Nb(110) and Cu(111) are studied at different temperatures using the oblique-incidence reflectivity technique (OI-RD). It is found that a step-flow or 2-D removal sets in at about 1073 K for Nb and at 700 K for Cu. In the case of the Cu(111) surface, the ratio of the 3-D/2-D-removal transition temperature to the melting temperature agrees with previously predicted values for fcc(111) metal surfaces. Moreover, using a moderately stepped Ni(111), it is demonstrated for the first time that the OI-RD technique can be used to monitor directly the average slope of a surface morphology during ion erosion. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 8:48AM |
N40.00004: Real-Time X-ray Studies of Si Surface Morphology Evolution during Ar+ Ion Bombardment Gozde Ozaydin, Ahmet Ozcan, Yiyi Wang, Karl Ludwig, Randall Headrick, Hua Zhou A systematic study of Si surface evolution during normal- incidence Ar+ ion bombardment is reported. Real-time grazing incidence small- angle x-ray scattering (GISAXS) measurements were performed at the National Synchrotron Light Source of Brookhaven National Laboratory. \textit{Ex- situ} atomic force microscopy was also used to provide real-space information. Si (100) samples were bombarded at ion energies ranging from 300 to 1000 eV. For normal-incidence sputtering at room temperature, the development of correlated structures with two different characteristic length scales was observed. The shorter length scale features (``dot-like structures'') coarsened with time but approached a limiting value of 25-40 nm at all energies examined. These correlations eventually saturate. The surface roughness development then becomes dominated by the growth of the larger length-scale corrugations, causing kinetic roughening. To study the temperature dependence of the surface evolution, Si (100) samples were bombarded with 500 eV ions at temperatures ranging from 25 -- 700 C. There is a transition with increasing temperature from an amorphized surface to a crystalline surface. At high temperatures, the nanoscale correlations coarsen rapidly and are significantly longer in wavelength than the ``dot'' correlations observed at lower sputter erosion temperatures. No saturation is observed during the time of observation. This work is supported by NSF-DMR and DOE-BES. . [Preview Abstract] |
Wednesday, March 23, 2005 8:48AM - 9:00AM |
N40.00005: Nanopore Sculpting with Low Energy Ion Beam of Noble Gases Qun Cai, Brad Ledden, Eric Krueger, Jene Golovchenko, Jiali Li Experiments show that 3keV Helium, Neon, Argon, Krypton, and Xenon ion beams can be used to controllably ``sculpt'' nanoscale features in silicon nitride films using a feedback controlled ion beam sculpting apparatus. Here we report nanopore ion beam sculpting effects that depend on the inert gas ion species. We demonstrate that: (1) all the noble gas ion beams enable single nanometer control of structural dimensions in nanopores; (2) every ion species above shows similar ion beam flux dependence of nanopore formation, (3) the thickness of nanopores sculpted with different inert gas ion beam is deferent. Computer simulations (with SRIM and TRIM) and an ``adatom'' surface diffusion model are employed to explain the dynamics of nanoscale dimension change by competing sputtering and surface mass transport processes induced by different ion beam irradiation. These experiments and theoretical work reveal the surface atomic transport phenomena in a quantitative way that allows the extraction of parameters such as the adatom surface diffusion coefficients and average travel distances. [Preview Abstract] |
Wednesday, March 23, 2005 9:00AM - 9:12AM |
N40.00006: In situ studies of capillary filling in nano-porous alumina Kyle Alvine, Oleg Shpyrko, Peter Pershan, Kyusoon Shin, Thomas Russell Manipulation of matter on nano-scale requires thorough understanding of fundamental properties of materials defined by surface and interfacial phenomena, rather than bulk structure. We report studies of capillary filling in nano-porous alumina with a liquid solvent (perfluoromethyl-cyclohexane) as a function of the offset in chemical potential from liquid-vapor coexistence. Nano-porous alumina samples contains nearly hexagonally ordered, geometrically aligned cylindrical pores of $\sim $15nm diameter and aspect ratio of 1:4,000. Small-angle x-ray scattering measurements make it possible to observe formation of nano-scale wetting film at pore walls and to characterize the film thickness as the chemical potential is varied. Our data indicates a gradual thickening of a liquid film thickness on the capillary walls followed by a nearly discontinuous jump as pores get entirely filled. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N40.00007: Step bunching kinetics of vicinal silicon (111) surface : A method for nanopatterning Do Young Noh, Myung-Hun Cho, Tae Chul Kim, Jinwook Chung A vicinal Si(111) surface shows a uniform atomic step array above the 7x7 reconstruction temperature. Below the reconstruction temperature, the surface phase separate into step-bunched regions and large (111) terraces. In this experiment, the evolution of the surface morphology during the step bunching is studied using \textit{in-situ} real time x-ray scattering measurement. The period of the nano-scale `step-bunched and terrace' structure at various quench temperatures below the 7x7 transition is obtained as a function of time by grazing incident small angle scattering measurement. The evolution of the surface roughness, measured by x-ray reflectivity, shows that the bunching occurs in two distinct steps. The nano-scale patterns are also examined by \textit{ex-situ} atomic force microscopy. We shall also discuss how to control the size of the step-bunched regions and the separation between them by adjusting kinetic parameters such as the step-bunching temperature and the bunching time. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N40.00008: Self-Assembly, Dynamics, and Stability of Oxide Nano-Rods on the NiAl(110) Surface John Pierce, Kevin McCarty We report the formation of parallel oxide rods upon exposing a clean NiAl(110) surface to oxygen at high temperatures (850- 1350 K). The rods are several microns long, several nanometers wide, and composed vertically of 2-{\AA}-thick atomic layers. We investigate how they grow and measure their thermodynamic stability by following their assembly and decomposition in real- time with low-energy electron microscopy (LEEM). At a fixed temperature and O2 pressure, the rods elongate along their axes at a constant rate. The temperature dependence of this rate yields an activation energy for growth of 1.2 $\pm$ 0.1 eV. The layered nature of the rods leads to sharp changes in their rates of elongation due to their tendency to gain (lose) atomic layers as they descend (climb) atomic steps on the surface. Thermodynamic measurements indicate that alumina in the form of nano-rods on NiAl(110) is far less stable than alumina that forms upon oxidation of aluminum surfaces. This work was supported by the Office of Basic Energy Sciences, Division of Materials Sciences of the U.S. DOE under Contract No. DE-AC04- 94AL85000. [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N40.00009: Intrinsic vacancy induced nanoscale wire structure in heteroepitaxial Ga$_2$Se$_3$/Si(001) T. Ohta, D. S. Schmidt, A. Klust, S. Meng, Q. Yu, M. A. Olmstead, F. S. Ohuchi A highly anisotropic growth morphology is found for heteroepitaxial gallium sesquiselenide (Ga$_2$Se$_3$) on the lattice matched substrate, arsenic-terminated Si(001). This anisotropic, nanowire structure is attributed to surface coalescence of the intrinsic vacancies in $\beta$-Ga$_2$Se$_3$, a defected zinc-blende semiconductor with every third Ga site vacant. Scanning tunneling microscopy of Ga$_2$Se$_3$ films reveals nanoscale, wire-like structures covering the surface in parallel lines, less than 1 nm wide and up to 30 nm long. Core-level photoemission spectroscopy and diffraction reveals the local structure of buried Ga and Se atoms to reflect bulk $\beta$-Ga$_2$Se$_3$, which contains ordered $\langle 110\rangle $ arrays of Ga vacancies. The semiconducting, passivated wires form perpendicular to the underlying As-dimer rows of the Si(001):As substrate, and continue to lie in the same direction, with bilayer height differences, as the film grows thicker. [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N40.00010: Fractal-Mound Growth of Pentacene Thin Films: A Novel Morphology Serkan Zorba, Yonathan Shapir, Yongli Gao We investigated the growth mechanism of pentacene film formation on SiO$_{2}$ substrate with a combination of atomic force microscopy measurements and numerical modeling. It is found that the submonolayer islands are diffusion-limited aggregates (DLA). With increased coverage, the Schwoebel barrier effect disrupts the desired epitaxial growth, leading to mound growth. The terraces of the growing mounds have a fractal dimension of 1.6, indicating a lateral DLA shape. This novel growth morphology thus combines horizontal DLA-like growth with vertical mound growth. [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N40.00011: A Quantitative Understanding of Ge Hut Formation on Si(001) Surface from First-principles Guang-Hong Lu, N.-Y. Ma, Feng Liu, Martin Cuma Ge growth on Si(001) is characterized by layer-by-layer growth, followed by 3D island formation with a distinct initial island shape of ``hut'' bounded by (105) facets. Despite of extensive studies over the last decade, our understanding of hut formation remains qualitative that it forms by strain relaxation overcoming the cost of increased surface energy but the values of strain energies and surface energies are unknown. We have carried out extensive first-principles calculations to determine, respectively, the surface energies and stresses of Ge/Si (001) and (105) surfaces and their strain dependence as a function of Ge coverage. Using these as inputs to continuum theory, we provide a quantitative analysis of Ge hut formation on Si(001). [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N40.00012: Time Dependent Surface Morphology During Pulsed Laser Deposition of SrTiO$_{3}$ J.Z. Tischler, B.C. Larson, Gyula Eres, C.M. Rouleau, P. Zschack The time evolution of the SrTiO$_{3}$ surface morphology has been studied using time-resolved surface x-ray diffraction during pulsed laser deposition. Measurements made at the $(00\frac{1}{2})$ anti-Bragg position on the crystal truncation rod (CTR) show the intensity oscillations associated with layer by layer growth, and the high count rate available provides information on surface kinetics at the sub-millisecond time scale. Diffuse scattering from the islands measured around the CTR oscillates out of phase with the intensity on the rod demonstrating the growth and filling in of islands through the layer by layer growth cycle. Measurements of the diffuse intensity peak position, which provide a measure of the island size, show the island size varying with both coverage and substrate temperature. The variation of island size with substrate temperature will be discussed in relation to surface mobility and the short and long time evolution of surface morphology. [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 10:36AM |
N40.00013: Surface roughness systematics in manganite thin films grown by pulsed laser deposition C. Cardoso, S.B. Ogale, S. Dhar, S.R. Shinde, T. Venkatesan Surface roughness is a critical quality factor in high quality heterointerface-based devices for oxide electronics. Given the significance of manganite films in the potential spin valve and magnetic tunnel junction type spintronics devices, we have examined the roughness systematics of the corresponding films grown by pulsed laser deposition (PLD) on key substrates such as (001) LaAlO$_{3}$ (LAO) and (001) SrTiO$_{3}$ (STO). Thus films of La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ were grown by PLD at different substrate temperatures in the range 600-800 $^{\circ}$C at oxygen pressures in the range 20-200 mTorr. Films with thickness in the range between a few nanometers up to several hundred nanometers were examined for surface roughness (by using Atomic Force Microscopy) as well as various other properties such as crystallinity (x-ray diffraction and Rutherford backscattering channeling), four probe resistivity and magnetization (squid magnetometry). Films thinner than about 20 nm were found to be extremely flat on both the substrates. The surface roughness increase with thickness showed a two step transition-like structure in the case of LAO while a single step structure in the case of the STO substrate. The transport and magnetization properties exhibited a monotonic change with thickness in the case of STO, but a non-monotonic behavior in the case of LAO substrate. [Preview Abstract] |
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