Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V40: Focus Session: Morphology and Evolution at Surfaces: Phase-field and Ge/Si |
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Sponsoring Units: DMP DCMP Chair: Jacques Amar, University of Toledo Room: LACC 408A |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V40.00001: Phase-Field Simulations of Morphological Development During Heteroepitaxy Katsuyo Thornton, Steven Wise, Nirand Pisutha-Arnond, John Lowengrub, P.W. Voorhees In heteroepitaxial growth, a lattice mismatch between the substrate and the film materials drives morphological instabilities, resulting in surface roughening and dot or island formation on the surface. Controlling the patterns formed by these instabilities at the nanoscale is of technological importance since uniform dot size and spatial distribution are needed to provide highly desired electronic and optical characteristics. Large-scale three-dimensional simulations have recently become possible using a multigrid code. With this model, we explore options for directed self-assembly of quantum dots. In particular, we will discuss patterning of the substrate surface, patterning of the film surface and subsequent annealing, and multistep deposition. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V40.00002: Phase field model for growth of adatom islands Yan-Mei Yu, Bang-Gui Liu We developed a phase-field model for epitaxial growth of 2D/3D adatom islands and self-organized formation of regular nanostripes. A local phase-field variable is introduced to describe adatom islands. The evolution of this phase field is determined by a time-dependent equation coupled to a diffusive transport equation of local adatom density. The limited interlayer diffusion and atomic detachment at steps are included in the model. Applied to real submonolayer epitaxial systems, we reproduce not only the scaling law of the island density but also the experimental size and spatial distribution of the islands. With large coverages of adatoms we obtain not only the 3D mounding islands but also their coarsening and roughening exponents. We explored the self-organized formation of regular arrays of Fe nanostripes on W(110) by the hybrid growth of islands and step flows during the post-deposition annealing. Compared with atomic models and mean-field models, this phase-field model can not only span larger space and time scales while containing the elemental atomic kinetic of epitaxy, but also provide a fine visualized morphology of epitaxial features in 2+1 dimensions. Y. M. Yu and B.-G. Liu, Phys. Rev. E 69, 021601 (2004); Phys. Rev. B 70, 051444 (2004). [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V40.00003: Phase field modeling of the morphology evolution of metal silicide thin films Mathieu Bouville, Dongzhi Chi, Shenyang Hu, Long-Qing Chen, David J. Srolovitz The formation and evolution of thin polycrystalline films formed from the reaction of metals on silicon (metal silicides) or germanium (germanides) is key to the performance in semiconductor devices. NiSi is a candidate for replacing CoSi$_{2}$ because of its lower resistivity and its lower Si consumption, but is less stable at high T. As the NiSi grains agglomerate into islands \textit{via} grain boundary grooving the film may lose connectivity. At elevated temperature a higher resistivity disilicide phase, NiSi$_{2}$, will form. We use phase field simulations to predict the evolution of the microstructure and morphology of the silicide film to determine the controlling mechanisms and key physical parameters to aid in metal silicide optimization. Diffusion, phase transformation, elastic energy, and interfacial energy all play important roles in the evolution. Experiments suggest that alloying Ni with Pt or implanting BF$_{2}$ delays NiSi agglomeration and the formation of NiSi$_{2}$. We test two possible mechanisms: additives modify (i) the relative stability of the phases or (ii) the interface energies (which changes the wetting angles and hence the driving force for boundary grooving). [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:27PM |
V40.00004: Properties of Ge/Si Nanostructures: Alloying, Stability and Positioning Invited Speaker: The self--assembly of Ge(Si) nanostructures on Si surfaces is a model system in crystal growth [1]. This process follows the Stranski--Krastanov (SK) mode, and may be exploited as a parallel approach to engineer dense low--cost arrays of quantum dots (QDs). This special opportunity has recently driven a wealth of theoretical and experimental efforts. Nonetheless, several critical issues remain to be addressed. These range over the uniform size, shape and positioning of the nanoislands, their thermal stability and the Ge/Si intermixing that occurs during growth [2]. Low energy electron / X--ray photoelectron microscopies (LEEM / XPEEM) proved very successful in addressing such problems. By dynamically monitoring evolution phenomena (by acquiring LEEM movies) and developing spectromicroscopic analytical tools [3], we have correlated the above phenomena to the underlying atomistic processes. This approach represents an effective means towards the bottom-up control over the system features. Our investigation led to a picture where entropy and kinetic factors play an overwhelming role in determining the overall properties of QDs. [1] F. Rosei, \textit{J. Phys. Cond. Matt.} \textbf{16}, S1373 (2004). [2] F. Ratto et al. \textit{Appl. Phys. Lett.} \textbf{84}, 4526 (2004). [3] F. Ratto et al. \textit{J. Appl. Phys}. \textbf{97}, in press (Jan. 2005). [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V40.00005: Local distribution of segregated Si on the hydrogen-treated Ge/Si(001) studied by STM Yasunori Fujikawa, Atsushi Kuwano, Yukiko Yamada-Takamura, Tadaaki Nagao, Toshio Sakurai Intermixing effect between Si and Ge is the key issue to achieve precise composition control of the SiGe nanodevices. It has been known that hydrogen-rich condition at $\sim $300 °C induces segregation of Si to the surface layer of the Ge covered Si(001) surface, while Ge prefers to stay at the surface layer without hydrogen$^{1}$. The local distribution of Si and Ge on the Ge/Si(001) surface was investigated by STM to study the atomic process of this intermixing effect. We found that Si-H and Ge-H sites on the H-terminated Ge/Si(001) can be resolved using the empty-state imaging condition, which is similar to the recently-reported case of Cl-terminated Ge/Si(001)$^{2}$. Furthermore, it was found that the segregation of Si induced by the H-annealing produces significantly bright features beside the dimer-row vacancies (DVLs) in the empty state image of H-Ge/Si(001), in addition to the features coming from normal Ge-H and Si-H sites. The statistic analysis of these features revealed that the segregated Si atoms are mostly located at the brightest feature beside the DVLs, supporting the previously suggested model of intermixing process$^{1}$. $^{1}$ Rudkevich \textit{et al}., Phys. Rev. Lett. \textbf{81}, 3467 (1998). $^{2}$ Lin \textit{et al}., Phys. Rev. Lett. \textbf{90}, 046102 (2003). [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V40.00006: X-ray diffraction analysis of Ge islands on patterned Si(001) D.A. Walko, D.A. Arms, J.T. Robinson, O.D. Dubon, J.A. Liddle, D.S. Tinberg, P.G. Evans We have used x-ray diffraction to study the structure of Ge islands grown on patterned Si(001) substrates. Square arrays of submicron Au dots were deposited by electron-beam evaporation through a stencil mask followed by molecular-beam epitaxy growth of Ge. This approach produces Ge islands in the shape of truncated pyramids assembled on a two-dimensional square lattice that extends over thousands of square microns. We have used x-ray diffraction to analyze the structure of the Ge islands, with the x rays focused to illuminate only the patterned regions of the substrate. Structure of the Ge islands was measured as a function of Au dot size, probing the evolution of Si/Ge composition, strain, and orientation with island size. We show that the structure as well as the morphology of the truncated-pyramidal islands is qualitatively distinct from that of dome-shaped islands on the unpatterned regions of the substrate. Supported by US Department of Energy and National Science Foundation. [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V40.00007: Metal-mediated assembly of Ge island arrays on Si Jeremy Robinson, J. Alexander Liddle, Andrew Minor, Velimir Radmilovic, Don Walko, Dohn Arms, Oscar Dubon A new approach for the directed assembly of Ge islands via molecular beam epitaxy at 873 K on Au-patterned Si surfaces has been realized. For a square Au-patterned array, a square lattice of sub 100nm Ge islands extending over thousands of square microns can be formed. Ge islands grow at the (1/2,1/2) positions of the original Au-pattern and have distinct shapes strongly influenced by substrate orientation; square-based truncated pyramidal islands grow on Si(100) while long rod-shaped islands grow on Si(110). These islands are strikingly different from huts and domes that grow on unpatterned Si. Island density, size, and degree of ordering are all controlled by the characteristics of the patterned Au. Transmission electron microscopy and x-ray diffraction show that islands relax by both the introduction of dislocations and intermixing with Si. The approach presented here is an effective and versatile method to manipulate surface kinetics and thereby control island-array assembly processes. Supported by US Department of Energy [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:39PM |
V40.00008: Simulations of Stress and Composition in Ge/Si(100) Quantum Dots Invited Speaker: In this talk, I will review research, carried out in my group in recent years, on the problem of intermixing in semiconductor quantum dots (QD). The work is based on constrained-equilibrium Monte Carlo (MC) atomistic simulations, which assume that QD's are in a quasi-equilibrium state, due to the enhanced diffusion in the surface region, and which are able to address the issues of stress evolution, intermixing, and the resulting island composition. The method is applied to Ge islands grown on Si(100). Site-by-site analyses reveal inhomogeneous stress and composition profiles at typical growth temperatures, which are directly linked to each other. Comparison is made with recent experimental and other theoretical investigations. Similarities, differences, and their origin are pointed out. I present on-going efforts to include appropriate diffusion barriers in the MC algorithms, in order to examine whether some subtle differences in the theoretical and experimental profiles are of kinetic origin. I also discuss the stress evolution in the heteroepitaxial system as the islands are growing, and its association to trench formation. [Preview Abstract] |
Thursday, March 24, 2005 1:39PM - 1:51PM |
V40.00009: Comparison of Morphology Evolution of Ge(001) Homoepitaxial Films Grown by Pulsed Laser Deposition and Molecular Beam Epitaxy B. Shin, J.P. Leonard, J.W. McCamy, M.J. Aziz Using a dual MBE-PLD UHV chamber, we have conducted the first experiments under identical thermal, background, and surface preparation conditions to compare homoepitaxial growth morphology in Pulsed Laser Deposition (PLD) and Molecular Beam Epitaxy (MBE). We have studied Ge(001) homoepitaxy as a model system to compare these deposition techniques. In PLD, the laser fluence is varied to generate depositing species with high and low kinetic energy. We find that in PLD with low kinetic energy and in MBE, the film morphology evolves in a similar fashion: initially round-based mounds form, followed by pyramid-like mounds with edges of the square base along $<$100$>$ directions; the film roughness and mound separation increase with film thickness. In PLD with high kinetic energy, well-defined pyramid-like mounds are not observed and the morphology rather resembles that of an ion-etched Ge(001) surface. The areal mound density is higher for PLD films than for MBE films grown at the same average growth rate and temperature. Furthermore, the dependence upon film thickness of roughness and mound separation differ for PLD and MBE. We discuss these results in terms of the similarities and differences between kinetic mechanisms expected to be operating in PLD and MBE. [Preview Abstract] |
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V40.00010: Ge islands ordering on top of multilayered Ge/Si structures: from atomistic data to AFM measurements Francesco Montalenti, Riccardo Marchetti, Leo Miglio, Giovanni Capellini, Monica De Seta, Florestano Evangelisti The strain field induced by a buried Ge island in a Si capping layer (CL) is investigated by Tersoff-potential molecular dynamics simulations. It is shown that the tensile region above the island changes both quantitatively and qualitatively by increasing the thickness of the CL. A simple model based on thermodynamic arguments and on the local energetics predicted by the simulations shows that small Ge islands directly grown over the capping layer surface tend to arrange in flower-like shapes for a thin CL, while they cluster in a more close-packed way over a thick CL. A direct comparison with recent CVD-grown multilayered SiGe structures is carried out. The simulated minimum-energy shapes are shown to agree rather well with AFM images of the Ge-island distribution on top of the outermost layer. [Preview Abstract] |
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