book of abstracts - IM2NP

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A B S T R A C T S THURSDAY, JULY 1 N A N O S E A 2 0 1 0 3. S. Cereda, F. Montalenti and L. Miglio, Surface Sci. 591, 23-31 (2005) 4. S. Cereda and F. Montalenti, Phys. Rev B 75, 195321 (2007) 5. T. Miyazaki, D.R. Bowler, M.J. Gillan and T. Ohno, J. Phys. Soc. Japan 77, 123706 (2008); D.R. Bowler and T. Miyazaki, J. Phys. Condensed Matter (2009) in press, arXiv:0911.3584v1 [cond-mat.mtrl-sci] 17H30-17H50 Epitaxial growth on a dynamically patterned substrate: a theoretical study. C. Taillan, N. Combe, J. Morillo (CNRS, CEMES (Centre d'Elaboration des Matériaux et d'Etudes Structurales), BP 94347, 29 rue J. Marvig, 31055 Toulouse,France - Université de Toulouse, UPS, 31055 Toulouse, France). Experimentally, the epitaxial growth consists in a low energy deposition of atoms or molecules on a substrate and is generally used to create nanostructures (for instance quantum wells, wires or dots for semi-conductors or magnetic nanoparticles for metals...) that can be organized using their self- assembling properties. Unfortunately, the control of the spatial or size distribution of these structures remains indirect and difficult: for instance, experimentalists take advantages of the elastic properties of materials (Stranski-Krastanov growth mode, use of buried dislocations network ... etc). Our study aims to explore an alternative approach that could potentially improve this control. We study the diffusion of adatoms on a substrate submitted to a standing surface acoustic wave. The standing surface acoustic wave induces an effective potential for diffusing atoms and thus dynamically patterns the substrate. Minima of this potential form preferential sites for atomic diffusion and thus for island nucleation. Our study both implies molecular dynamic simulations of adatoms diffusion on crystalline surface, and analytical calculations using a Langevin approach. Using this last approach, we derive the expression of the effective potential under few hypothesis, and analyze the stability of its minima in the general case. The relation between numerical simulations and analytical calculations is given and reveals the coherence between both approaches results. The standing surface acoustic wave thus affects the adatoms diffusion on the substrate by creating preferential sites of nanostructures nucleation. The use of these waves would potentially allow a precise control of the spatial distribution of the nanostructures during the epitaxial growth 17H50-18H10 Sorting thermodynamic and kinetic paths in the transition from 2D to 3D dot growth M Brehm1, M Grydlik1, T Fromherz1, G Vastola2, F Montalenti2, L Miglio2, F Schäffler1 and G Bauer1 (1 Institute of Semiconductor Physics, Johannes Kepler University, Linz, Austria, 2 L-NESS and Materials Science Department, University of Milano-Bicocca, I-20125 Milano, Italy). moritz.brehm@jku.at 1 – The well established sequence of appearance of SiGe dots (from mounds to superdomes) grown in the Stranski-Krastanow growth mode on planar Si(001) substrates with increasing Ge deposition was straightforwardly understood in terms of increasing aspect ratio, providing a larger volumetric strain relaxation. The thermodynamic stability of the island types is still a matter of discussion. 101
A B S T R A C T S THURSDAY, JULY 1 N A N O S E A 2 0 1 0 2 – Here we present clear experimental evidence for the formation of larger domes prior to smaller pyramids at growth temperatures (Tg) smaller than 675°C and for an "overcritical" behavior of the wetting layer (WL).These findings allow us to sort out thermodynamic and kinetic paths in the early stages of quantum dot formation. The results are obtained from systematic investigations of atomic force microscopy (AFM) and photoluminescence (PL) on samples grown by molecular beam epitaxy. By implementing a Ge gradient over 4” wafers we achieved an extremely high deposition resolution of 0.025 monolayers (ML). At Tg =700°C we observe dome formation at Θ = 4.2 ML, while pyramids appear later, i.e. at Θ > 4.38 ML. The dome formation is initiated by a Ge transfer of 1 ML from the WL to the islands, as determined from the abrupt shift of the WL PL at the onset of the island PL. At Tg = 625°C the common dot formation sequence is observed. Mounds and small pyramids form already at about 2 ML followed by the nucleation of domes at 4.9 ML. In order to understand whether the domes are actually stable prior to pyramids, accurate density functional theory calculations of surface energies and finite element method simulations of the elastic energies were performed, taking particular care in determining the total energy variations in the first few MLs of the WL. By a morphological phase diagram we both confirmed the early stability of the Ge-rich domes and the depletion of the WL at their onset, also addressing the appearance first of pyramids at 625°C to be a kinetic effect, leading to metastable islands. 3 – An experimental proof for this prediction is obtained from extensive annealing experiments. After annealing at 700°C pyramids grown with Θ < 3.2 ML at 625°C form back to a completely flat WL, while for Θ > 3.2 ML domes are formed. Thus, we conclude that the critical WL thickness (tc) for stable island nucleation is about 3.2 ML. At the same tc domes are formed if we anneal an overcritical WL grown at 700°C. Finally, we would especially like to highlight the strong similarities between dot formation in SiGe and III/V systems. 18H10-18H30 Formation and Localization of GeSi Quantum Dots on vicinal surfaces: a Scanning Tunneling Microscopy Characterization. L. PERSICHETTI, A. SGARLATA, M. FANFONI, A. BALZAROTTI (Department of Physics, University of Roma “Tor Vergata” - Via della Ricerca Scientifica, 1 Roma, Italy). persichetti@roma2.infn.it 1 – Introduction Due to the potential applications in novel nanostructured devices, the Stranski-Krastanov growth of ordered self-assembled islands in strained heteroepitaxial systems is a topic attracting ever-increasing interest. A natural model system is Ge on vicinal Si(001), which allows us to tune both the energetic and the kinetic factors governing the growth of individual nanostructures by changing the substrate miscut [1]. 2 – Abstract A complete description of Ge growth and ordering processes on vicinal Si(001) surfaces in the angular miscut range 0°-8° is presented. The key role of substrate vicinality is clarified from the very early stages of Ge deposition up to the nucleation of 3D islands [2]. By a systematic scanning tunnelling microscopy investigation we are able to explain the competition between step-flow growth and 2D nucleation and the progressive elongation of the 3D islands along the miscut direction [110]. 102
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