xxiii πανελληνιο ÏƒÏ Î½ÎµÎ´ÏÎ¹Î¿ Ï†Ï ÏƒÎ¹ÎºÎ·Ï‚ στερεας καταστασης & επιστημης ...

Facet-Stress-Driven Ordering in SiGe Nanoislands
G.E. Vantarakis, 1,2 I.N. Remediakis 1 and P.C. Kelires 2,3*
1
Department of Materials Science and Technology, University of Crete, P.O. Box 2208, 71003 Heraclion, Crete, Greece
2 Department of Mechanical Engineering & Materials Science and Technology, Cyprus University of Technology,
P.O. Box 50329, 3036 Limassol, Cyprus
3 Department of Physics, University of Crete, P.O. Box 2208, 71003 Heraklion, Crete, Greece
* kelires@physics.uoc.gr
SiGe nanoislands, which appear during Ge on Si(100) heteroepitaxy, have attracted considerable attention because of
potential applications in optoelectronics, such as in light-emitting devices. One of the crucial factors which dictate the optical
properties of these nanoislands is their chemical composition. In particular, it is essential to know and, if possible, to have
control over the distribution of species in the interior, in order to optimize the optical efficiency. It is expected that the
strength of optical transitions would be higher in case of ordered domains in the islands (i.e. with Si and Ge atoms distributed
in ordered phases), rather than in domains with random distributions.
There could be two possible factors which influence the composition profiles of SiGe nanoislands. (a) The profile is
driven by bulk equilibrium thermodynamics, which would give rise to a random distribution (SiGe is an ideal random alloy in
bulk form). (b) The profile is driven by surface equilibrium. Diffusion is fast at the near-surface region, equilibrium
distribution is established and maintained when surface layers are buried under further material deposition. However, we
could have a combination of these two generic mechanisms.
Driven by these concepts, we explore in this work the effect of reconstructed island surfaces (facets) on the equilibrium
distribution of species. Reconstructions, in general, generate huge stress fields in the subsurface region, which influence the
site-specific occupancy. We examine the three most important island facet orientations (see “Fig. 1”): {105}, {113},
{15 3 23}, each having its own characteristic reconstruction. For the equilibrium structure, we use novel Monte Carlo
simulations [1] within the empirical potential approach. We run the simulations at rather low temperatures to get the
maximum of the effect.
(a)
(b)
Fig. 1: A multifaceted dome structure showing
the three facet orientations.
(a) top view
(b) side view
Fig. 2: Local ordered geometries
in Si 0.5 Ge 0.5 {15 3 23} surface.
White represents either non-four-fold atoms or sites that
do not show preference for a particular local geometry.
Light, dark and darkest gray represents atoms with a
preference for having one, two or three atoms of the
same kind, respectively.
We find that indeed the stress field of all three facets, especially of the {15 3 23} facet, is quite large, approaching in
some subsurface sites the values of 7-8 GPa. The strong stress fields drive preferential occupation of certain sites near the
surface. Seen at a larger scale, we observe a tendency of Si and Ge atoms to order, mostly in rhombohedral-type structures, as
shown in “Fig. 2”. These ordered structures appear within a thin layer of ~ 0.7 nm just below the surface. This is the range
within which the reconstruction strain field is significant, showing the link between stress field and compositional ordering.
We propose that subsequent deposition buries the ordered structures which are frozen-in, while the deposited material is
driven to ordering by the newly formed surface. These results might explain recent experimental observations [2] of ordering
in SiGe nanoislands. Although not shown directly in this work, the experimental ordered profiles could be the result of a
cooperative effect between the above mentioned three facet subsurface structures.
[1] G. Hadjisavvas and P.C. Kelires, Phys. Rev. B 72 (2005) 075334.
[2] A. Malachias et al., Phys. Rev. B 72 (2005) 165315.
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