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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 In addition, we have systematically measured the spatial distribution of Ge islands as a function of the substrate vicinality, developing proper tools for the analysis of experimental data, successfully applied to the study of in-plane interactions among nanostructures [3]. 3 – Conclusion In conclusion, we have reported a complete picture which provides new insights into the microscopic growth mechanisms on vicinal surfaces. Moreover, we demonstrate that substrate misorientation strongly affects island distribution by modulating both the diffusion and the local strain fields, which are both responsible for the short-range ordering. We support our results by modelling elastic relaxation for different shapes and arrangements of islands with finite element calculations. [1] M. Bernardi, A. Sgarlata, M. Fanfoni, L. Persichetti, N. Motta, A. Balzarotti, Superlattices and Microstructures 46, 318 (2009). [2] L. Persichetti, A. Sgarlata, M. Fanfoni, and A. Balzarotti, Physical Review Letters (in press). [3] L. Persichetti, A. Sgarlata, M. Fanfoni, and A. Balzarotti, (submitted). 18H30-18H50 In situ and ex situ x-ray studies of the growth of Ge islands on nominal and nano-structured Si(001) substrates. M.-I. Richard (a), T.U. Schülli (b), G. Renaud (c), N.A. Katcho (d), M.G. Proietti (e), H. Renevier (f), V. Favre-Nicolin (c), Z. Zhong (g), G. Chen (g), J.J. Zhang (g), M. Stoffel (h), O. Schmidt (h) and G. Bauer (g). ((a) Univ. Paul Cézanne, IM2NP, Marseille (France); (b) ID01/ESRF, Grenoble (France); (c) CEA Grenoble, INAC (France; (d) Univ. Complutense Madrid (Spain); (e) Univ. Zaragoza, ICMA (Spain); (f) Grenoble INP Minatec, LMGP (France); (g) Johannes Kepler Univ. Linz (Austria); (h) IFW Dresden (Germany)). 1 – Introduction The knowledge of strain, chemical composition, interface quality, atomic intermixing and ordering, is of great importance to understand the growth mechanism as well as the electronic and optical properties of hetero and nanostructures [1]. X-ray techniques have been used to address such a general issue in the case of the molecular beam epitaxy growth of Ge islands on nominal and nano-structured Si(001) substrates. 2 – Abstract The kinetics of the Ge/Si island growth and of their shape transitions is still not completely understood. This calls for in situ studies to understand and investigate the whole dynamic growth process. On BM32 at the ESRF in Grenoble, we have developed in situ x-ray scattering methods giving access to the evolution of size, shape, faceting, strain relaxation and intermixing during the growth. The combination of Grazing Incidence Small Angle X-ray Scattering (GISAXS) [2], X-ray Diffraction (GIXD) and Anomalous Scattering using the MAD (Multiwavelength Anomalous Diffraction) [3] method, all performed in situ, during growth in an Ultra High Vacuum chamber, allowed for the characterization of the Ge nano-islands regarding their shape and organization (GISAXS) as well as their strain and composition. This made possible to track the facet size evolution for growing Ge superdomes [4] on nominal Si(001) surfaces using a low growth rate, as well as to determine the material transport from the wetting layer during the transition form two dimensional to island growth [5]. 103
A B S T R A C T S THURSDAY, JULY 1 N A N O S E A 2 0 1 0 To access to the Ge content inside the nano-islands, the MAD technique was combined ex situ with grazingincidence Diffraction Anomalous Fine Structure (DAFS) spectroscopy at beamline BM02, ESRF. DAFS gives information on the local environment of the Ge atoms (chemical sensitivity) located in an iso-strain volume selected by diffraction (spatial selectivity). This unique combination, together with recent atomistic simulations based on molecular dynamics (MD), has turned out to be a powerful approach to disentangle strain and composition (even in the case of sharp island/substrate interfaces [6,7]), to detect atomic ordering inside SiGe nano-islands and to quantify experimental spatial resolution issues. We will report recent results obtained on capped SiGe pyramids, free standing and capped domes. For small capped pyramids, DAFS is the unique non destructive method that allows to recover the actual Ge content and in-plane and out-of-plane strain. A challenge for the development of nano-electronics is also to elaborate semiconductor quantum dots that are homogeneous in shape, size, strain and composition, thus resulting in well-defined electronic and optical properties. Recently the growth of highly monodisperse Ge islands on prepatterned Si substrates has been obtained by a combination of lithography and self-assembly techniques [8]. To compare elastic relaxation and Si-Ge distribution in epitaxial islands grown on both pit-patterned and flat Si(001) substrates, ex situ studies were performed [9]. Anomalous x-ray diffraction yields that nucleation in the pits provides a higher relaxation. Using an innovative, model-free fitting procedure based on self-consistent solutions of the elastic problem, we provide real-space compositional and elastic-energy maps. Islands grown on flat substrates exhibit stronger composition gradients and do not show a monotonic decrease of elastic energy with height. Both phenomena are explained using both thermodynamic and kinetic arguments. 3 – Conclusion The combination of the GISAXS, GIXD, MAD and DAFS techniques provides detailed results on the evolution of the shape, composition and the strain of Ge islands during the whole growth process. It is demonstrated to be a useful, destruction-free tool to understand and control the self-organized growth of nanostructures. [1] J. Stangl et al, Rev. Mod. Phys. 76, 725 (2004). [2] G. Renaud et al, Science 300, 1416 (2003). [3] A. Létoublon, V. Favre-Nicolin, H. Renevier et al, Phys. Rev. Lett. 92, 186101 (2004). [4] M.-I. Richard, T.U. Schülli, G. Renaud et al, Phys. Rev. B. 80, 045313 (2009) [5] T.U. Schülli, M.-I. Richard et al, Appl. Phys. Lett. 89, 143114 (2006). [6] M.-I. Richard, N.A. Katcho et al, Eur. Phys. J. Special Topics 167, 3 (2009). [7] N.A. Katcho, M.I. Richard et al, Journal of Physics: Conference Series 190, 012129 (2009). [8] Z. Zhong and G. Bauer, Appl. Phys. Lett. 84, 1922 (2004). [9]T.U. Schülli, G. Vastola, M.-I. Richard et al, Phys. Rev. Lett. 102, 025502 (2009). 104
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