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
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].
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