book of abstracts - IM2NP

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.
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[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).
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