Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G12: Self-Assembled Nanostructures: Growth and Patterning |
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Sponsoring Units: DCMP DMP Chair: Christian Ratsch, University of California, Los Angeles Room: Baltimore Convention Center 304 |
Tuesday, March 14, 2006 8:00AM - 8:12AM |
G12.00001: A Level-Set Method for Epitaxial Growth and Self-Organization of Quantum Dots Christian Ratsch, Young-Ju Lee, Xiaobin Niu, Russel Caflisch We have developed an island dynamics model that employs the level-set technique to describe epitaxial growth. One virtue of this method is that the typical simulation timestep can be chosen much larger than in an atomistic simulation, even when several microscopic processes with vastly different rates are relevant. This makes it feasible to solve the elastic equations and obtain the entire strain field at every timestep of the simulation. The strain field modifies the potential energy surface, and different limits will be discussed. We will present simulation results where we self-consistently modify the strain dependent microscopic rates for surface diffusion and detachment of adatoms from island edges. Our results for the island size distributions indicate that such strain dependent kinetic rates lead to the regularization of island sizes, and ultimately the formation and self-organization of quantum dots. [Preview Abstract] |
Tuesday, March 14, 2006 8:12AM - 8:24AM |
G12.00002: Multiscale Theory of Fluctuating Interfaces: From Self-Affine to Unstable Growth Christoph Haselwandter, Dimitri Vvedensky We describe a framework for the multiscale analysis of atomistic surface processes which we apply to the Wolf-Villain model for epitaxial growth. Coarse-graining is accomplished by calculating the renormalization-group trajectories from initial conditions determined by the regularized atomistic theory. All of the intermediate scaling regimes known from computer simulations are obtained, but we also find that the asymptotic behavior of two-dimensional substrates is determined by a hitherto unknown fixed point. This provides an explanation of recent experiments on Ge(001) with the intriguing conclusion that the relaxation mechanism responsible for ordered structures early in the growth process produces an instability at longer times that leads to epitaxial breakdown. [Preview Abstract] |
Tuesday, March 14, 2006 8:24AM - 8:36AM |
G12.00003: The Structure and Evolution of Ge Nanoscale Structures on Si(111) -- Observations and Theory Sanwu Wang, H.F. Ma, Z.H. Qin, D.X. Shi, Y.L. Wang, H.M. Guo, H.-J. Gao, S.T. Pantelides We report scanning tunneling microscopy (STM) observations and first-principles calculations for the evolution of self-organized Ge nanostructures formed on Si(111)-$7{\times}7$ for Ge coverages up to 0.5~ML and operating temperatures from room temperature to 300$^{\circ}$C. STM measurements show that, depending on coverage and temperature, Ge atoms form various structures ranging from single-atom correlated patterns, 2-D ordered nanoscale domains, and 3-D disordered and ordered nanoclusters. First-principles theory focuses on the single-atom patterns and 2-D ordered nanostructures. We show that Ge atoms replace the Si adatoms in the initial adsorption stage. We also show that annealing of the Ge/Si(111) surface results in a partial transformation of ($7{\times}7$) reconstructed unit cells to unreconstructed Si(111) configurations on which the Ge adatoms reside at the $T_{4}$ sites and form a $({\sqrt{3}}{\times}{\sqrt{3}})R30^{\circ}$ reconstruction. [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 8:48AM |
G12.00004: Evolution of Si nanostructures on the Ge(001) surface Yasunori Fujikawa, Toshio Sakurai Growth of compressively strained thin films has been studied extensively for the applications in quantum devices composed of 2D quantum well and/or 1D quantum dots. Recent progress in the strain-controlled Si and SiGe high-speed devices brings increasing interest in the growth control of the oppositely strained thin films and nanostructures. In this work, we report on the morphology evolution of the Si layer formed on the Ge(001) substrate to investigate the effect of tensile strain in the growth of Si. Initially the Si layer forms a wetting layer characterized by the formation of missing dimer rows with spacing of 3-4 rows. The direction of the defect row is 90-degree rotated from the case of the Ge wetting layer on Si(001), being explained by the anisotropic strain of the dimer-row structure. Successive growth of Si results in the formation of Si islands, whose surface is composed of four {\{}113{\}} facets with a small (001) terrace on top. Its differences from the Ge dome structure on Si(001) composed of {\{}113{\}} facets and {\{}105{\}} facets on top can be understood by considering the tensile strain of the {\{}105{\}} facets [1]. [1] Y. Fujikawa \textit{et al.}, Phys. Rev. Lett. \textbf{88}, 176101 (2002). [Preview Abstract] |
Tuesday, March 14, 2006 8:48AM - 9:00AM |
G12.00005: Atomic structure and strain of Ge wetting layer on silicon Hao Chen, Boquan Li, Jianguo Zheng, Jian-Min Zuo The atomic structure of Ge wetting layer grown on Si(001) surfaces by chemical vapor deposition at several substrate temperatures under UHV was characterized by low energy electron diffraction and cross sectional high-resolution scanning transmission microscopy. The Ge film is atomically flat with sharp interface with silicon. The surface lattice constant changes with Ge coverage. At high Ge coverage on stepped Si surface, surface relaxation is observed near step edges. The ability to modify Si surface lattice with Ge has many applications. [Preview Abstract] |
Tuesday, March 14, 2006 9:00AM - 9:12AM |
G12.00006: Quantitative Correlation of Local Stress Field and Surface Morphology in Undulated Si1-xGex/Si(100) Thin Films Chi-Chin Wu, Robert Hull We have performed in-situ transmission electron microscope measurements of misfit dislocation propagation velocities to quantitatively characterize the stress field variations for undulated epitaxial Si1-xGex/Si(100) thin films. For a strained Si0.7Ge0.3 film with a 30 nm mean thickness and annealed at $\sim$480$^{\circ}$C, the dislocation velocity varies from 50 to 200 nm/s as the threading arm of a propagating dislocation line traverses a period of the surface morphology. Using detailed correlations we have previously developed between misfit dislocation velocities and local driving stresses, we can translate this velocity range into an effective stress on the dislocation that varies from $\sim$ 0.3 to 1.2 GPa. These results show great promise for correlating local stresses with morphology in these materials. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:24AM |
G12.00007: Surface Mobility Difference between Si and Ge and its Effect on Growth of SiGe Alloy Films and Islands Li Huang, Guanghong Lu, Xingao Gong, Feng Liu Based on first-principles calculations of surface diffusion barriers, we show that on a compressive Ge(001) surface, the diffusivity of Ge is $10^2-10^3$ times higher than that of Si in the temperature range of 300 to 900 K; while on a tensile surface, the two diffusivities are comparable. Consequently, growth of a compressive SiGe film is rather different from that of a tensile film. The diffusion disparity between Si and Ge is also greatly enhanced on the strained Ge islands compared to that on the Ge wetting layer on Si(001), explaining the experimental observation of Si enrichment in the wetting layer relative to that in the islands. [Preview Abstract] |
Tuesday, March 14, 2006 9:24AM - 9:36AM |
G12.00008: Modeling the Self-Assembly of Quantum Dots in Thin Solid Films Margo Levine, Alexander Golovin, Stephen Davis, Peter Voorhees The self-assembly of quantum dots in a thin solid film caused by epitaxial stress and wetting interactions with the substrate is studied. It is shown that wetting interactions change the instability spectrum from long wave to short wave which can lead to spatially regular arrays of quantum dots. A nonlocal, nonlinear evolution equation for the film shape is derived, and the stability of dot arrays with different symmetries is studied. Regions in the parameter space are determined where spatially regular surface structures can be observed in experiments. [Preview Abstract] |
Tuesday, March 14, 2006 9:36AM - 9:48AM |
G12.00009: Island Alignment on Patterned Substrates Demitris Kouris, Robert Kukta A recent approach to fabricating self-assembled epitaxial nanostructures involves the use of topographically patterned substrates to control the formation of material deposits during growth. This article investigates the energetic and kinetic mechanisms that guide the positioning of strained epitaxial islands in these systems. A continuum-level model is used wherein the free energy of system consists of surface energy and strain energy. The substrate shape is represented by small amplitude sinusoidal features. An energetic phase diagram is calculated to determine which sites are most favorable---peaks, valleys, or side walls---depending on factors as substrate shape, lattice mismatch, surface energy, and the amount of material deposited. The kinetic evolution through the processes of deposition and surface diffusion is then simulated. It is found that the resulting morphology depends largely on the rate of deposition relative to surface mobility. Relatively slow deposition rates produce configurations of minimum free energy while higher deposition rates give rise to novel metastable states. Mechanisms by which various configurations arise are discussed in detail. Results are found to be in excellent agreement with published experiments. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:00AM |
G12.00010: Utilizing Growth Kinetics to Self-Assemble Complex SiGe Nanostructures Jennifer Gray, Robert Hull, Jerrold Floro Heteroepitaxial growth of Si$_{x}$Ge$_{1-x}$ on Si (001) substrates under conditions of reduced adatom mobility results in limited formation of stress-relieving islands occurring only at energetically favorable sites. These favorable sites are at the edges of shallow pyramidal pits that form at random locations on the film surface under these conditions. However, by using a focused ion beam (FIB) to pattern the substrate before growth, it is possible to controllably create pits at predefined locations. This results in the formation of four self-assembled islands at the edges of each pit. In order to obtain uniform, well ordered islands, the FIB milling parameters and Si buffer thickness must be tailored to produce pits in the strained film with dimensions equal to or smaller than the compositionally-dependent natural length scale of the islands. The ability to produce islands at specific locations is important for applications such as quantum computing where quantum dots must be arranged into logic structures. Exploiting this constraint on islanding that occurs under kinetically limited growth may therefore provide a new route to hierarchical assembly of nanostructures when combined with substrate patterning techniques. [Preview Abstract] |
Tuesday, March 14, 2006 10:00AM - 10:12AM |
G12.00011: Shape and Composition Map of a Prepyramid Quantum Dot Brian Spencer We present a theory for the shape, size, and nonuniform composition profile of a small prepyramid island in an alloy epitaxial film when surface diffusion is much faster than deposition and bulk diffusion. The predicted composition profile has segregation of the larger misfit component to the island peak, with segregation enhanced by misfit strain and solute strain but retarded by alloy solution thermodynamics. Vertical composition gradients through the center of the island due to this mechanism are on the order of 2\%/nm for Ge$_X$Si$_{1-X}$/Si and 10 - 15\%/nm for In$_X$GaAs$_{1-X}$/GaAs [PRL 95, 206101 (2005)]. [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G12.00012: Temperature and Flux Dependence of Unstable Growth Mode on Patterned GaAs(001) Tabassom Tadayyon-Eslami, Hung-Chih Kan, Subramaniam Kanakaraju, Chris Richardson, Ray Phaneuf We report on the temperature and flux dependence of the growth mode of GaAs(001) during molecular beam epitaxy (MBE). Our previous results [1] for growth on patterned GaAs(001) surfaces showed that growth under typical conditions produces a transient instability. Lowering the temperature beneath $\sim $530 $^{o}$C at a growth rate of $\sim $ 0.3 nm/s produces a qualitative change in the way that the corrugations evolve. This temperature is close to that of a preroughening transition in the absence of growth [2], however on increasing the As2 flux the onset temperature for preroughening moves \textit{up}, while the onset temperature of the growth mode change moves \textit{down}. We correlate the change in growth mode with a change in the surface reconstruction. [1] H. C. Kan, S. Shah, Tadayyon-Eslami, and R. J. Phaneuf,. Phys. Rev. Lett, \textbf{92}, 146101, (2004). [2] V. P. Labella et al. Phys. Rev. Lett. \textbf{84}, 4152, (2003). [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G12.00013: Self-organized Evolution on Patterned GaAs(001) Surfaces during Homo-Epitaxial Growth Hung-Chih Kan, Erin Flanagan, Tabassom Tadayyon-Eslami, Subramaniam Kanakaraju, Chris Richardson, Raymond Phaneuf We report on experimental characterization of morphological evolution of patterned GaAs(001) surfaces during homo-epitaxial growth. We lithographically patterned our GaAs(001) substrates with arrays of cylindrical pillars whose size and spacing are systematically varied. The growth of GaAs layers on the patterned substrate were performed in a MBE chamber at a substrate temperature of 580 \r{ }C. The growth rate is $\sim $ 2.7 {\AA}/s. After each growth step, the surface topography is characterized with atomic force microscopy (AFM). AFM profiles along [-110] show that flat-topped pillars evolve into concave downward parabola whose curvature is nearly independent of the initial pillar diameter. We also perform both physically based and phenomenological numerical calculations to simulate the growth and compare the results with our experimental observations. [Preview Abstract] |
Tuesday, March 14, 2006 10:36AM - 10:48AM |
G12.00014: Real-Time Studies of Ga Droplet Formation for the Directed Seeding of Semiconductor Nanopillars W. Ye, B.L. Cardozo, X. Weng, J.F. Mansfield, R.S. Goldman The directed self-assembly of low-dimensional semiconductor structures has been achieved using a variety of approaches to producing topographical patterns. However, an approach for producing highly ordered arrangements of nanostructures with well-controlled shapes and size distributions has yet to be developed. Therefore, we are exploring the seeded-assembly of semiconductor nanocrystal and nanopillars on substrates topographically patterned using a focused-ion-beam (FIB). For nanopillar formation, we have seeded ordered arrays of holes with controlled concentrations of Ga droplets using FIB implantation. These holes have nearly uniform sizes and shapes. By controlling the ion beam energy, current, and size, hole arrays with various sizes, depths, and periodicities may be produced. Interestingly, after scanning the ion beam over the patterned area, Ga atoms diffuse to the holes and agglomerate, leading to the formation of ordered arrays of nearly uniform sized Ga dots. We will discuss the mechanisms of Ga droplet formation and present real-time studies of Ga droplet dynamics and their interaction with various gases. We will also discuss the use of these ordered arrays of Ga dots as catalysts for vapor-liquid-solid growth of semiconductor nanopillars. [Preview Abstract] |
Tuesday, March 14, 2006 10:48AM - 11:00AM |
G12.00015: Real-Time UV-FEL PEEM of Silicide Nanostructure Coarsening Dynamics on Si(100) Matthew Zeman, Woochul Yang, Robert Nemanich The coarsening dynamics of nanoscale silicide (TiSi$_{2}$, ZrSi$_{2}$, HfSi$_{2}$) islands on Si(100) are observed in ultra high vacuum using tunable ultra-violet free electron laser (UV-FEL) excitation for photo electron emission microscopy (PEEM). The UV-FEL PEEM is employed for real-time, in situ observation of the nanostructure dynamics and evolution during continuous annealing at high temperatures (1200$^{\circ} $C). In situ movies will be presented which detail the coarsening processes where the silicide nanostructures grow via Ostwald ripening and attractive migration and coalescence (AMC). AMC is a coarsening process where nearby islands are observed to migrate attractively towards each other and subsequently coalesce. This process has been attributed to the growth-decay flow of the island edges driven by a non-uniform chemical potential around the islands. This non-uniform chemical potential is predicted to result in an island shape distortion due to the local flux of adatoms from/onto the inner- facing edges of the coalescing islands. This non-uniform chemical potential is observed as an increase in the island radius along the migration direction as well as a decrease in the radius of curvature of the inner facing (growing) edges of the coalescing islands. [Preview Abstract] |
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