Michael J. Aziz Harvard University
Jerrold A. Floro University of Virginia
Stefan G. Mayr Georg-August-Universitaet Goettingen
Christopher C. Umbach Cornell University
KK1: Ion Irradiation of Metals and Insulators
Eric H. Chason
Monday PM, November 26, 2007
Room 304 (Hynes)
9:30 AM - **KK1.1
Surface Structuring by Ion Beam Erosion.
Bernd Rauschenbach 1 , Bashkim Ziberi 1 , Frank Frost 1 , Thomas Hoeche 1 Show Abstract
1 , Leibniz-Institute of Surface Modification, Leipzig Germany
Low-energy ion sputtering has recently attracted increasing interest as an effective method for generation of self-organized nanostructures on the surface of solids with a wide range of possible technological applications. It is well known that under certain conditions, sputtering can produce well-ordered patterns, like ripples or dots on different materials [1-6]. The formation, ordering and size of nanostructures depends on different process conditions. Usually this pattern formation process is considered as a result of the interplay between curvature dependent sputtering that roughness the surface, and different surface relaxation mechanisms that act to smooth the surface.In this contribution the dot and ripple surface topography emerging on Si, Ge and compound semiconductor surfaces during low-energy (≤ 2000 eV) noble gas ion beam erosion at oblique ion incidence is studied. The results show that there is a much more complex behavior of the surface topography with ion energy, ion fluence, angle of incidence, etc.For example, the experimental results show that at certain ion sputtering conditions, at oblique ion incidence ripple patterns can form on Si and Ge surfaces with size below 100 nm. By varying the ion incidence angle a morphological transition from ripple to dot patterns could be observed. Due to self-organization processes and also influenced by the previous existence of ripples, the dot patterns show a long range ordering that covers the whole sample area. For Ge the dots show a hexagonal ordering while for Si the dots show a quadratic ordering. Experimental results reveal that the wavelength of ripples remains constant with increasing ion fluence, while the ordering increases, leading to ripple patterns with a very high degree of ordering. Moreover, the influence of different ion species on pattern formation is investigated. Atomic force microscopy and high-resolution transmission electron microscopy are used to characterize the evolving nanostructures. S. Facsko, T. Dekorsy, C. Koerdt, C. Trappe, H. Kurz, A. Vogt, and H. L. Hartnagel, Science 285, 1551 (1999). F. Frost, A. Schindler, and F. Bigl, Phys. Rev. Lett. 85, 4116 (2000). R. Gago, L. Vazquez, R. Cuerno, M. Varela, C. Ballesteros, and J. M. Albella, Appl. Phys. Lett. 78, 3316 (2001). B. Ziberi, F. Frost, B. Rauschenbach, and T. Hoche, Appl. Phys. Lett. 87 (2005). B. Ziberi, F. Frost, Th. Hoche, and B. Rauschenbach, Phys. Rev. B 72, 235310 (2005). B. Ziberi, F. Frost, and B. Rauschenbach, Appl. Phys. Lett. 88 (2006).
10:00 AM - KK1.2
Controlling Ion Beam Induced Self-organization on Si Surfaces by Lithographic Pre-patterning.
Bashkim Ziberi 1 , Theresa Lutz 1 , Renate Fechner 1 , Dietmar Hirsch 1 , Klaus Zimmer 1 , Frank Frost 1 , Bernd Rauschenbach 1 Show Abstract
1 , Leibniz-Institut für Oberflächenmodifizierung, Leipzig Germany
Pattern formation on the surface of different materials due to low-energy ion beam erosion is a versatile tool for large scale nanostructuring. The evolving patterns are arranged in domains of well ordered nanostructures showing hexagonal ordering. However, usually this self-organization process lacks long-range order due to the formation of domains and defects in the pattern. Also there is a little positional control of the evolving structures. One possibility to influence the ordering of structures is by using pre-patterned substrates . In this way due to spatial limitations and guided by the lateral ordering of the pre-patterned templates the evolving topography shows an improved ordering, a fabrication principle also known as guided self-organization. The method allows also an exact positioning of nanostructures on the surface. In this contribution results on the ripple and dot pattern formation on pre-patterned Si surfaces during low energy (≤ 2000 eV) Kr+ and Xe+ ion beam erosion are presented. The pre-patterned substrates are fabricated by various lithographic techniques in combination with etching techniques for structure transfer. Depending on the shape of the pre-patterned structure (binary gratings with different periods, square arrays of cylinders, gratings with V-grooves) different results are obtained. Examples are: i) formation of curved ripples on the surface, where the curvature is caused by a continuous change in the local topography within pre-patterned regions; ii) perfectly square ordered dots on exact positions on the surface; iii) enhanced ordering of ripples and the formation of ripples with different orientation in accordance with the local surface orientation. Another interesting effect is the continuous and controlled change in orientation of ripples independent of the direction of the incident ion beam. In general the main parameters determining the pattern formation with this method are the local incidence of ions, orientation of the local surface with respect to the ion beam direction, and the local surface curvature.A. Cuenat, H. B. George, K. C. Chang, J. M. Blakely, and M. J. Aziz, Adv. Mat. 17, 2845 (2005).
10:15 AM - KK1.3
Studies of Si Surface Morphology Evolution during Ar+ Ion Bombardment.
Gozde Ozaydin 2 1 , Karl Ludwig Jr. 1 Show Abstract
2 Aerospace and Mechanical Engineering Department, Boston University, Boston, Massachusetts, United States, 1 Physics Department, Boston University, Boston , Massachusetts, United States
Systematic studies of Si surface morphology evolution during low energy Ar+ ion bombardment are reported. Real-time grazing incidence small-angle x-ray scattering (GISAXS) measurements are performed at the National Synchrotron Light Source of Brookhaven National Laboratory. The stress state of the surface is investigated during ion bombardment using real-time wafer curvature measurements. Ex-situ atomic force microscopy is also used to provide real-space information.Si (100) samples are bombarded with 300 eV Ar+ ions at normal incidence in the presence and the absence of seed atoms at room temperature. Although Si surfaces remain smooth during bombardment at room temperature, in the presence of seed atoms formation of correlated nanodots is observed. The results of the stress measurements are discussed in detail and the effects of surface stress on the nanodot formation with seeding are investigated.Separate studies on the effects of substrate temperature during ion bombardment at 500 eV and 1000 eV in the presence and absence of seed atoms are also presented. For both cases a transition region from amorphous to crystalline structure is observed around 400°C and above 600°C the surface remains crystalline. The effects of substrate temperature and crystal structure on the nanodots are also discussed.This work is partially supported by NSF DMR-0507351 and DOE DE-FG02-03ER46037.
10:30 AM - KK1.4
Evolution of Two Lengthscales During Argon Ion Irradiation of Silicon.
H. George 1 , Michael Aziz 1 Show Abstract
1 , Harvard School of Engineering & Applied Sciences, Cambridge, Massachusetts, United States
During 150 to 425 eV argon ion irradiation of Si(001) at temperatures below ~ 300 degrees C and incidence angles less than ~ 20 degrees from normal, we observe a variety of sputter ripple morphologies with two prevailing length scales: long-wavelength anisotropic ripples along the projected beam direction at off-normal incidence or, at normal incidence, isotropic "rings"; short wavelength isotropic dots; or combinations of features at both length scales. We present an experimental phase diagram that maps the transitions between the different morphological regions. We discuss possible mechanisms leading to the various features.
10:45 AM - KK1.5
Stabilization and Bifurcation Points in the Dynamics of Ion Sputtered Surfaces.
Benny Davidovitch 1 2 , H. Bola George 1 , Michael Brenner 1 , Michael Aziz 1 Show Abstract
1 School of Engineering and applied sciences , Harvard , cambridge, Massachusetts, United States, 2 Physics, UMass , Amherst, Massachusetts, United States
The classical linear theory of ion beam sputtering predicts the instabilityof a flat surface to uniform ion irradiation at any temperature, beam energy or incidence angle. Thus, this linear dynamics lacks bifurcation points, where the homogenous solution becomes unstable upon smooth variation of a control parameter. This fact in turn makes the nonlinear problem of pattern selection a baffling task, because the general tools of pattern formation theory, e.g. amplitude equation analysis, which are perturbatively derived in the proximity of bifurcation points, are not available. In this contribution we point out that, in contradiction to this prediction, various ion sputtering experiments do exhibit bifurcation points, at which a flat surface becomes stable upon smooth variation of beam angle or energy. Analyzing central physical observables such as pattern wavelength and amplitude near these points, we discuss the possible nature of linear and nonlinear processes underlying this phenomenon. In particular, we show how the existence of nonlocal mechanism in the surface dynamics can be discerned, and we explain how important information on approximate symmetries of the dynamics can be extracted from such an analysis.
11:30 AM - **KK1.6
Interplay between Morphology and Surface Transport in Nanopatterns Produced by Ion-Beam Sputtering.
Rodolfo Cuerno 1 , Javier Munoz-Garcia 2 , Mario Castro 3 , Raul Gago 4 , Luis Vazquez 5 Show Abstract
1 Departamento de Matemáticas and Grupo Interdisciplinar de Sistemas Complejos (GISC), Universidad Carlos III de Madrid, Leganés, Madrid, Spain, 2 Departamento de Matemáticas and GISC, Universidad de Castilla la Mancha, Ciudad Real Spain, 3 Escuela Técnica Superior de Ingeniería (ICAI) and GISC, Universidad Pontificia Comillas de Madrid, Madrid Spain, 4 Centro de Microanálisis de Materiales, Universidad Autónoma de Madrid, Madrid Spain, 5 Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, Madrid Spain
Recently, a "hydrodynamic" model has been proposed to describe nanopattern formation and dynamics on amorphous surfaces eroded by ion-beam sputtering (IBS)  that relates to descriptions of pattern formation in macroscopic systems such as aeolian sand dunes [2,3]. In contrast to previous continuum models of the morphology of ion-sputtered surfaces, in which the only field considered is the height of the bombarded surface , an additional field is here coupled with the former, describing the material density that diffuses onto the surface. We describe a multiple scales analysis of the model, in which we consider normal ion incidence and oblique incidence, for both fixed and rotating targets. Closed equations for the surface height evolution can be obtained in the vicinity of the morphological instability threshold, that are generalizations of the anisotropic Kuramoto-Sivashinshy equation containing additional conserved Kardar-Parisi-Zhang type nonlinearities. In general dot or ripple patterns form, that later evolve exhibiting complex nonlinear dynamics. Thus, we observe interrupted coarsening behavior  in such a way that for normal incidence and in appropriate parameter regions, domains of hexagonally ordered structures appear, that compare favorably with those obtained in many experimentsof nanodot formation by IBS . Meanwhile, in other parameter regions, this short-range ordered pattern coexists with long range disorder and kinetic roughening. In the case of oblique incidence, a ripple pattern is generically obtained . In our model, these ripples also show interrupted coarsening and feature additional nonlinear features, such as non-uniform transverse motion, that again compare well with experimental observations on nanoripples. In order to discuss the applicability and limitations of our model we will consider alternative approaches as well as further experimental results in terms of the occurrence of lateral order and pattern coarsening properties.
 M. Castro, R. Cuerno, L. Vázquez, R. Gago, Phys. Rev. Lett. 94, 016102 (2005); J. Muñoz-García, M. Castro, R. Cuerno, ibid. 96, 086101 (2006).
 T. Aste, U. Valbusa, Physica A 332, 548 (2004).
 See Z. Csahók, C. Misbah, F. Rioual, A. Valance, Eur. Phys. J. E 3, 71 (2000) and refs. therein.
 See e.g. J. Muñoz-García, L. Vázquez, R. Cuerno, J. A. Sánchez-García, M. Castro, R. Gago, to appear in Lecture Notes on Nanoscale Science and Technology, ed. Z. M. Wang, Springer. arXiv:0706.2625.
 P. Politi, C. Misbah, Phys. Rev. Lett. 92, 090601 (2004).
 R. Gago, L. Vázquez, O. Plantevin, T. H. Metzger, J. Muñoz-García, R. Cuerno, M. Castro, Appl. Phys. Lett. 89 233101 (2006).
12:00 PM - KK1.7
Ion Beam Induced Surface Modulations from Nano to Pico: Optimizing Deposition During Erosion and Erosion During Deposition.
Warren MoberlyChan 1 , Richard Schalek 2 Show Abstract
1 , LLNL, Livermore, California, United States, 2 , Harvard University, Cambridge, Massachusetts, United States
Ion beams of sufficient energy to erode a surface can lead to surface modulations that depend on the ion beam, the material surface it impinges, and extrinsic parameters such as temperature and geometric boundary conditions. Focused Ion Beam technology both enables site-specific placement of these modulations and expedites research through fast, high dose and small efficient use of material. The DualBeam (FIB/SEM) enables in situ metrology, with movies observing ripple formation, wave motion, and the influence of surface defects. Nanostructures (ripples of >400nm wavelength to dots spaced <40nm) naturally grow from an atomically flat surface during erosion, however, a steady state size may or may not be achieved as a consequence of numerous controlled parameters: temperature, angle, energy, crystallography. Geometric factors, which can be easily invoked using a FIB, enable a controlled component of deposition (and/or redeposition) to occur during erosion, and conversely allow a component of etching to be incurred during (ion-beam assisted) deposition. High angles of ion beam inclination commonly lead to "rougher" surfaces, however, the extreme case of 90.0° etching enables deposition of picostructures (ripples of atomic spacings). The orientation and position of these naturally quantized picostructures may be controlled by the same parameters as for nanostructures (e.g. ion inclination and imposed boundary conditions, which are flexibly regulated by FIB). Judicious control of angles during FIB-CVD growth stimulates erosion with directionality that can produce surface modulations akin to those observed for sputtering. Just as the surface of diamond roughens from 1-D ripples to 2-D steps with increasing angle of ion sputtering, so do ripples and steps appear on carbon-grown surfaces with increase in angle of FIB-CVD.This work was performed under the auspices of the United States Department of Energy by the University of California, Lawrence Livermore National Laboratories under contract of No. W-7405-Eng-48. UCRL-ABS-IM348387.
12:15 PM - KK1.8
Quantitative Determination of Smoothing Mechanisms of Self-organized Sputter Ripple Patterning on Sapphire.
Hua Zhou 1 , Lan Zhou 1 , Randall Headrick 1 , Gozde Ozaydin 2 , Karl Ludwig Jr. 2 Show Abstract
1 Physics, University of Vermont, Burlington, Vermont, United States, 2 Physics, Boston University, Boston, Massachusetts, United States
Ion beam sputtering of solid surfaces is known to produce self-organized patterns composed of highly correlated arrays of dots or ripples. Those surface morphologies have demonstrated the potential to tailor related surface properties for optoelectronic and spintronic applications. On the other hand, one considerable practical importance of ion beam erosion is that of surface smoothing of nanometer features, during etching or film deposition coincident with energetic species. In this work, systematic investigations of ripple pattern formation and smoothing during low energy ion erosion of sapphire surfaces using in-situ synchrotron grazing incidence small angle x-ray scattering and ex-situ atomic force microscopy are demonstrated. It is found in the pattern formation that the wavelength of ripples can be varied over a remarkably wide range by tuning the experimental parameters, such as ion energy, temperature and ion incidence angle. The kinetics of ripple formation is discussed quantitatively within the linear theory regime for different surface smoothing mechanisms. An ion bombardment-induced effective downhill current model is proposed to explain the strong smoothing observed near normal incidence. Quantitative agreement is obtained using ion-collision simulations to compute the magnitude of the surface current. The results lead to predictions for the surface morphology phase diagram as a function of ion beam energy and incidence angle that substantially agree with experimental observations.
12:30 PM - KK1.9
Self-ordering of Surface Nanofacets on Vicinal 4H-SiC(0001).
Satoru Tanaka 1 , Masahiro Fujii 1 Show Abstract
1 Applied Quantum Physics, Kyushu Univ., Fukuoka Japan
Vicinal solid surfaces of single crystals often show regularly spaced step/terrace and facet structures and thus have been considered as a template of nanostructures in hetero-systems. For future device applications, using the surface nanostructures physics behind self-ordering phenomenon is of great importance. In particular, equilibrium faceting, including step bunching on vicinal surfaces, is an important phenomenon that contributes to periodic surface morphology in a mesoscopic scale. This can be applicable to low-dimensional confinement systems such as quantum wires and dots in semiconductors.Silicon carbide is a unique compound semiconductor that possesses polymorphism and will be used for high power and high frequency electronic devices in the next generation. We have studied vicinal SiC(0001) surfaces after high-temperature H2 etching and found self-organized nano-facet formation . Energy considerations were provided, but the ordering mechanisms were not clear. In this work, we shed light on the ordering mechanisms of nano-facets on vicinal 4H-SiC having a series of vicinal angles. Nano-facet structures, ordering distances, and fluctuation in ordering at several vicinal angles are examined, and we provide a possible reason for the ordering using surface energetics .4H-SiC(0001) substrates with several vicinal angles of 4.2 - 7.8° were provided. Samples were etched by H2 gas and were examined by AFM and HRTEM. A typical AFM image (Fig. 1) of the nano-facet surface at the 5.7° off sample, indicating ordered nano-facet features. The ordering distance observed in the AFM image is the length of a pair of (0001) and (11-2n) nano-facet. Statistical analysis of the ordering distance L using AFM images over the length of 10 microns (~1000 pairs of nano-facets) was performed. We notice two important features from the results (Fig. 2): First, the ordering distance peaks at ~10 nm, which is the so-called “a characteristic ordering distance L0”, and is independent of vicinal angles possibly due to elastic effects on the surface . Second, the degree of fluctuation of the ordering distance is at a minimum on the 5.7° off surface. The fact that there is a unique vicinal angle (5.7°) of 4H-SiC substrate reveals that highly ordered surface structures, could in turn be a characteristic of polytypes of SiC. Moreover, the degree of ordering fluctuation differed in each vicinal angle - the dependence of the vicinal angle on periodicity. The elastic theory could predict a constant characteristic ordering distance but could not explain the fluctuation behavior. This is unique in SiC, which possesses polytypes; for example, 4H-SiC has a stacking sequence of ABCB(A) along its c-axis, which results in quantized step bunching due to periodic surface energy.References: H. Nakagawa et al., Phys. Rev. Lett. 91, 226107 (2003). M. Fujii and S. Tanaka, Phys. Rev. Lett., in press. V. I. Marchenko and A. Y. Parshin, Sov. Phys. JETP 52, 1 (1980).
12:45 PM - KK1.10
Ripples on Polyimide Induced by Focused Ion Beam.
Myoung-Woon Moon 1 , Jun Hyun Han 2 , Kwang-Ryeol Lee 1 , Kyu Hwan Oh 3 , Ashkan Vaziri 4 , John Hutchinson 4 Show Abstract
1 Future Fusion Technology Laboratory, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 Division of Materials Science and Engineering, KIST, Seoul Korea (the Republic of), 3 Department of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 4 School of Engineering and Applied Sciences, Harvard University, Seoul, Massachusetts, United States
Surface modification at micron and submicron scale has drawn much attention in variety of technological applications that range from semi-conductor industry and flexible electronics to tissue engineering. Specifically for modification of polymeric surfaces, focused ion beam (FIB) irradiation can be used effectively to create variety of structural features such dots and wrinkle patterns . In this study, we demonstrate the possibility of fabricating surface ripples on polymeric surface. Ripples at submicron scales were created on a polyimide substrate with thickness of about 100 microns using Ga+ FIB. By varying the beam incident angle between 0 to 90 degree and the ion fluence, different morphologies of ripples of straight, step-like shapes and hierarchical structures, were fabricated. Moreover, the current and acceleration voltage of the focused ion beam were varied between 1 to 20nA and 5 to 30keV, leading to ripples with characteristic wavelength of 200-500nm and amplitude of 10-100nm. We examined the characterization of the polyimide surface exposed to ion beam along depth using HRTEM analysis. The depth of the induced new surface layer was found to be non-uniform and gradually varied in the range of 20-50 nm depending on the relative orientation of the surface with ion beam incident angle. M.-W. Moon, S. H. Lee, J.-Y. Sun, K. H. Oh, A. Vaziri, J. W. Hutchinson, Proc. Natl Acad. Sci. USA 104 (2007) 1130.
KK2: Ion Irradiation of Metals and Alloys
Monday PM, November 26, 2007
Room 304 (Hynes)
2:30 PM - KK2.1
Self Organization in Irradiated Cu Alloys.
See Wee Chee 1 , Charles Enloe 1 , Brad Stumphy 1 , Pascal Bellon 1 , Robert Averback 1 Show Abstract
1 Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
2:45 PM - KK2.2
Compositionally Modulated Ripples Induced by Sputtering of Alloy Surfaces.
Vivek Shenoy 1 Show Abstract
1 , Brown University, Providence, Rhode Island, United States
Sputtering of an amorphous or crystalline material by an ion beam often results in the formation of periodic nanoscale ripple patterns on the surface. In this letter, we show that in the case of alloy surfaces, the differences in the sputter yields and surface diffusivities of the alloy components will also lead to spontaneous modulations in composition, that can be in or out of phase with the ripple topography. The degree of this kinetic alloy decomposition can be altered by varying the flux of the ion beam. In the high-temperature and low-flux regime, the degree of decomposition scales linearly with the ion flux, but it scales inversely with the ion flux in the low-temperature, high-flux regime. This work has been accepted for publication in Physical Review Letters.
3:00 PM - **KK2.3
Ion Patterning: Kinetic Mechanisms and Persistent Puzzles.
Eric Chason 1 , Wai Lun Chan 2 , Nikhil Medhekar 1 , Vivek Shenoy 1 Show Abstract
1 Division of Engineering, Brown University, Providence, Rhode Island, United States, 2 , University of Illinois, Urbana, Illinois, United States
It is well known that low energy ion beams can induce spontaneous patterning on surfaces. However, many questions still remain about how they form and what controls their growth. We will describe some of the different mechanisms that have been proposed to control pattern formation and how they can lead to a linear instability in the early stages of pattern formation. Results of experiments and kinetic Monte Carlo simulations will be used to show how the competition between different kinetic mechanisms leads to pattern formation. A kinetic phase diagram will be presented as a way to organize the dependence on processing parameters into a coherent framework. We will also focus on deviations between the models and experiments to consider additional roughening mechanisms that may be active.
3:30 PM - KK2.4
Engineering Morphology of Surfaces by Oblique Angle Etching.
Mehmet Cansizoglu 1 , Tansel Karabacak 1 Show Abstract
1 Applied Science, University of Arkansas at Little Rock, Little Rock, Arkansas, United States
During a typical chemical etching process growth front morphology generally generates an isotropic rough surface. In this work, we show that it is possible to form a rippled surface morphology through a geometrical self-assembly process using a chemical oblique angle etching technique. We observe in our Monte Carlo simulations that obliquely incident reactive species preferentially etch the hills that are exposed to the beam direction due to the shadowing effect. In addition, species with non-unity sticking (etching) coefficients can be re-emitted from the side walls of the hills and etch the valleys, which at the end can lead to the formation of ripples along the direction of the beam. This mechanism is quite different than the previously reported ripple formation during ion-beam bombarded surfaces where the particles have much higher energies, lower incidence angle and ripple formation is due to physical deformation of the surface. We investigate the ripple formation process in our simulated surfaces for a wide range of etching angle and sticking coefficient values.
KK3: Growth and Annealing
Monday PM, November 26, 2007
Room 304 (Hynes)
4:15 PM - **KK3.1
Pattern Formation by Step Edge Barriers: The Growth of Spirals.and wedding cakes
Joachim Krug 1 Show Abstract
1 Institute for Theoretical Physics, University of Cologne, Koeln Germany
Frank's ingenious suggestion of the spiral growth mode in 1949 resolved the enormous discrepancy between experimentally observed crystal growth rates and two-dimensional nucleation theory, and defined a pivotal moment in the history of crystal growth studies . Recent decades have seen a resurrection of two-dimensional nucleation theory in the context of growth experiments on defect-free homoepitaxial metal films . In particular, the key role of interlayer transport controlled by step edge barriers of the Ehrlich-Schwoebel (ES) type in shaping the morphology of multilayer films has been increasingly recognized. Very recently similar concepts are beginning to be developed for organic thin film growth. I will briefly describe a microscopic calculation using empirical potentials that provides the first direct evidence for the relevance of the ES effect in an organic thin film system (PTCDA) .In the main part of the talk I will then report on a combined experimental, computational and theoretical study of spiral growth in the presence of step edge barriers. Using a phase field model that incorporates the ES effect , it is shown that step edge barriers lead to unconventionally shaped spiral hillocks that display the same characteristic ever-steepening height profiles as wedding cakes formed during growth by two-dimensional nucleation . By inducing screw dislocations through ion bombardment of the Pt(111) surface, a homoepitaxial growth system has been created experimentally on which spiral hillocks and wedding cakes coexist. The observed height profiles of both type of structures are consistent with the most simple analytic model of wedding cake growth , but due to the different physical processes at the top of the structures the spiral hillocks are consistently higher than the wedding cakes. The height difference grows as the square root of the total coverage, and can be formally quantified as an increase in the apparent step edge barrier governing the (fictitious) nucleation of new layers . The talk is based on joint work with M. Fendrich, P. Kuhn, T. Michely, A. Redinger, O. Ricken, A. Rätz and A. Voigt.  W. Burton, N. Cabrera, F.C. Frank, Nature 163 (1949) 398. T. Michely, J. Krug, Islands, Mounds and Atoms: Patterns and Processes in Crystal Growth Far from Equilibrium (Springer, 2004). M. Fendrich, J. Krug, preprint arXiv:0706.1723v2. F. Otto, P. Penzler, A. Rätz, T. Rump, A. Voigt, Nonlinearity 17 (2004) 477. J. Krug, J. Stat. Phys. 87 (1997) 505. J. Krug, Physica A 313 (2002) 47. J. Krug, P. Politi, T. Michely, Phys. Rev. B 61 (2000) 14037.
4:45 PM - **KK3.2
Pattern Formation in Inorganic and Organic Semiconductor Film Growth.
Christian Teichert 1 Show Abstract
1 Institute of Physics, University of Leoben, Austria, Leoben Austria
Spontaneous pattern formation in strain-induced inorganic heteroepitaxy [1-3] opens an elegant and efficient route towards fabrication of large-scale arrays of uniform semiconductor nanostructures. After a brief review on this type of pattern formation, it will be shown for the model molecule parasexiphenyl (6P) that organic semiconductor growth may result in a variety of self-organized crystallite arrays depending on substrate type and growth conditions . Here, atomic-force microscopy (AFM) – applying super-sharp tips – was used to study the growth of 6P on mica (001) and TiO2(110). For 6P grown by hot-wall epitaxy on mica, the initial growth stage is characterized by a coexistence of individual crystallites (typical 100 nm x 50 nm x 20 nm in size) and self-organized crystallite chains with micrometer length. In both cases, the molecules are almost parallel to the sample surface. The chains run parallel to each other and their orientation is mediated by the surface geometry of the substrate. These straight chains form spontaneously as soon as a critical density of crystallites on the surface is present. Analysis of the chain length for different coverages reveals very narrow length distributions and the existence of a minimum chain length. The observation of denuded zones around the chains and the interior chain structure suggests that this self-alignment process is based on the rearrangement of the crystallites – containing about 140,000 molecules – as entities. This spontaneous rearrangement is driven by the formation of a one-dimensional defect array which evolves within the monomolecular para-sexiphenyl wetting layer when a critical crystallite density is reached . For organic molecular beam epitaxy of 6P on TiO2(110), either needle-like growth at room temperature  or a striped terraces composed of upright standing molecules  is observed. The latter case has been explained by an interplay of sticking and diffusion anisotropy . C. Teichert, Phys. Rep. 365 (2002) 335. C. Teichert, et al., Thin Solid Films 380 (2000) 25. J. Werner, Surf. Sci. 2007 in print. G. Hlawacek, et al., phys. stat. sol. a 202 (2005) 2376. C. Teichert, et al., Appl. Phys. A 82 (2006) 665. G. Koller, et al., Adv. Mater. 16 (2004) 2159. S. Berkebile, et al., Surf. Sci. 600 (2006) L313.This research is supported in the framework of the Austrian Science Fund (FWF) within the National Research Network “Interface controlled and functionalized organic films” (S9707-N08) and has been performed in collaboration with C. Hofer, G. Hlawacek, A. Andreev, (Leoben), K. Lyutovich and E. Kasper (Stuttgart), H. Sitter, N.S. Sariciftci (Linz), and A. Winkler, P. Frank, R. Resel, S. Berkebile, G. Koller, M. Ramsey (Graz).
5:15 PM - KK3.3
Self-assembly of Metallic Quantum Dots on Semiconductor Substrates.
Wei Lu 1 , David Salac 1 Show Abstract
1 Mechanical Engineering, University of Michigan, Ann Arbor, Ann Arbor, Michigan, United States
Nanometer scale metallic dots or clusters grown on a semiconductor substrate have wide applications in optical, electronic and magnetic devices. Production of these structures over a large area using techniques such as lithography and etching can be expensive and difficult. Recent experiments showed that uniform metallic dots may form spontaneously. Examples include Cu on TiO2 , Au on Si(111), and CoSi2 on Si(111). Despite their large collective surface area, the densely packed dots did not coalesce, but maintained small distances from one another. The observations call for a repulsive force when they are close. In a traditional quantum dot system, such as Ge dots on a Si substrate, the repulsion is achieved by elastic interaction. Both Ge and Si have the same cubic lattice structure, but their lattice constants differ by about 4%. The Ge dots deform laterally to match the Si lattice. Each Ge dot, due to its larger lattice constant, induces below it a tensile stress region in the substrate. The repulsion between these tensile stress regions keeps the dots separated.We propose a self-assembly mechanism for metallic dots. These systems may not involve coherent lattice or lattice mismatch, so that the elastic effect cannot explain the phenomena. A qualitative understanding is in the following. The metallic dots and the substrate have different Fermi levels. When they are brought in contact, charge transfer occurs. Take n-doped semiconductor as an example. Negative charges accumulate at the metal interface while a cloud of positive charges form in the substrate. This configuration is known as electric double layer. Thus, underneath a metallic dot, there is a charge cloud in the semiconductor. The exact shape and density of this cloud depends on factors such as the strength of contact potential and contact geometry. When the dot moves, the charge cloud moves with it. When two dots approach each other, the accompanying charge clouds overlap, leading to a repulsive force that prevents them to coalesce. This possibility is exciting since the repulsive force may lead to the self-assembly of a lattice of dots like colloidal crystals.Electric double layers form the basis of p-n junctions and metal-semiconductor contacts. However, the attention of existing work focuses on electronic properties. Little work has been done to investigate the self-assembly phenomena. We aim to investigate the role of electric double layers of semiconductors in controlling morphology. We show that the electrostatic and van der Waals energies lead to two regimes separated by an energy barrier. These two energies determine the size and spatial ordering. The energy barrier depends on contact potential. We demonstrate the possibility of materials selection or application of a bias voltage to the substrate to change the contact potential and thus engineer surface feature sizes. Our simulations reveal rich dynamics during self-assembly and pattern formation.
5:30 PM - KK3.4
Self-Organization and Ordering of Metallic Nanoclusters in Non-stoichiometric GaAs by Isovalent Impurity Doping.
Vladimir Chaldyshev 1 , Nikolay Bert 1 , Anton Boitsov 1 , Maria Yagovkina 1 , Valerii Preobrazhenskii 2 , Boris Semyagin 2 , Michail P