Alloying and Strain Relaxation in SiGe Islands Grown on Pit-Patterned Si(001) Substrates Probed by Nanotomography
F. Pezzoli
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M. Stoffel
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T. Merdzhanova
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A. Rastelli
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O. G. Schmidt
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Present Address: M. Stoffel Institut fur Halbleitertechnik
, Pfaffenwaldring 47, 70569 Stuttgart,
Germany
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F. Pezzoli (&) M. Stoffel A. Rastelli O. G. Schmidt Institute for Integrative Nanosciences
, IFW Dresden, Helmholtzstrae 20,
01069 Dresden, Germany
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T. Merdzhanova Max-Planck-Institut fur Festkorperforschung
, Heisenbergstrae 1, 70569 Stuttgart,
Germany
The three-dimensional composition profiles of individual SiGe/Si(001) islands grown on planar and pit-patterned substrates are determined by atomic force microscopy (AFM)-based nanotomography. The observed differences in lateral and vertical composition gradients are correlated with the island morphology. This approach allowed us to employ AFM to simultaneously gather information on the composition and strain of SiGe islands. Our quantitative analysis demonstrates that for islands with a fixed aspect ratio, a modified geometry of the substrate provides an enhancement of the relaxation, finally leading to a reduced intermixing.
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The lattice mismatch between Si and Ge drives the
formation of SiGe quantum dots (QD) during strained layer
heteroepitaxy [1, 2]. For large-scale integration
technologies [3], the position of such islands needs to be accurately
controlled on the substrate surface [4]. A viable process
relies on the fabrication of lithographically defined pits,
which act as a sink for the deposited adatoms, allowing the
exact positioning and addressability of individual QDs. In
addition, a precise control of the chemical composition of
the SiGe islands is required, since the three-dimensional
(3D) composition profile ultimately determines their
electronic behavior and optical properties. However, little work
has been done on this topic, and the different intermixing
mechanisms sustaining the growth and evolution of Ge
islands in presence of a surface with an extrinsic
morphology are still debated [57]. It has been shown that SiGe
islands grown on patterned areas have larger volumes than
those on the surrounding planar surfaces [8, 9]. These
observations are corroborated by a recent comparison of
X-ray measurements and finite element calculations, which
suggests a different compositional state with a larger
intermixing and relaxation on the patterned substrates [7].
However, the compositional differences at the single dot
level were not yet considered.
In this letter we address the issue of the impact of
substrate patterning on shape, composition, and strain
relaxation at the single dot level by using atomic force
microscopy (AFM)-based nanotomography (NT-AFM).
Following Ref. [10], we have recently extended the
capabilities of NT-AFM to quantitatively determine the full 3D
composition profiles of strained SiGe islands [11]. In this
study, we compare lateral and vertical composition
gradients of individual SiGe islands grown on pit-pattern and
planar Si(001) substrates. Above all, by combining
structural data with the average island compositions as obtained
by NT-AFM, we are able to determine island strain only by
means of an AFM analysis. The experimental ability to
map the chemical composition at the nanoscale helps
indeed to shed new light on the driving forces governing
alloying. Our findings provide direct experimental
evidence that a nanostructured surface plays a major role in
determining strain relaxation and therefore in defining the
compositional profiles of the islands.
Experimental Procedure
The sample considered here consists of 8.5 monolayer of Ge
deposited by molecular beam epitaxy at 700 C on a
patterned Si(001) substrate [12]. A 500 9 500 lm2 mesh of
pits aligned along the h110i directions was realized by
electron beam lithography followed by reactive ion etching.
The distance between nearby pits is 450 nm and their depth
and width about 25 and 85 nm, respectively. The surface
morphology of the sample was analyzed by AFM operating
in tapping mode with a super sharp silicon tip (nominal
radius of curvature of 2 nm). Fig. 1a shows a 30 9 30 lm2
AFM image of the surface morphology in proximity of a
corner of the patterned area. The observed material
depletion region is due to a directional diffusion of Ge from
the unpatterned, flat surface toward the patterned area,
which results in a gradient of the Ge amount available for
island formation in the patterned area [9]. Islands close to
the pattern edge are therefore larger than those a few
microns away from it and some of them exceed the critical
size for dislocation introduction [13]. Here we focus on the
two areas marked in Fig. 1a, in order to exclude most of the
large, dislocated islands at the boundaries of the patterned
field. The two island ensembles consist mainly of
barnshaped islands [14], as corroborated by a facet analysis
(not shown). As reported in Fig. 1b, the mean height of
coherent islands is similar, being (49 6) nm and (54 3)
nm on the flat and patterned surfaces, (...truncated)