International audienceThis paper reports the AFM and HREM study of the Sb surfactant mediated growth of Ge on Si(0 0 1). We show that very dense self-organised Ge dots of small lateral dimensions can be grown by using a sub-monolayer coverage of Sb on Si(0 0 1) in the transient growth regime between 2D nucleation and step flow. The dramatic Ge growth change induced by Sb is attributed to both kinetic and thermodynamic effects. Indeed, HREM observations evidence mainly two phenomena: the close-packing of ultra-small Ge islands indicating a lower surface diffusion in presence of Sb and a mono-modal island shape and size that strongly differs from the bimodal islands 'huts' and 'domes' commonly observed without Sb. Morphological and microstructural features of Ge islands formed with and without Sb are studied and the differences between facets and aspect ratio are exhibited. Moreover, at lower growth temperature (in the 2D nucleation regime, T g 0 350°C), a delay to 3D island nucleation is observed and defect free 2D flat layers can be grown up to thicknesses of 18 A ,. At higher growth temperature, (in pure step flow at T g 750°C) large, well separated 'dome' islands partially relaxed by dislocation nucleation on their edges are obtained. Such islands are very similar to those obtained without Sb coverage. The complete desorption of Sb on Ge rich surface at T\ 720°C explains this result. This study which improves the understanding on the formation of ultra-small dense islands is very promising for the fabrication of quantum devices that require highly homogeneous islands of small lateral sizes and of MOSFET heterostructures with strained SiGe n-channel which require flat Ge rich layers with abrupt interfaces

Berbezier, I.

Portavoce, A.

Ronda, A.

English

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Sb-surfactant mediated growth of Ge nanostructures
A. Portavoce, A. Ronda, I. Berbezier
To cite this version:
A. Portavoce, A. Ronda, I. Berbezier. Sb-surfactant mediated growth of Ge nanostructures. Materials
Science and Engineering, Elsevier, 2002, 89, pp.205 - 210. ￿hal-02393607￿
Sb-surfactant mediated growth of Ge nanostructures
A. Portavoce a,b, A. Ronda a, I. Berbezier a,*
a CRMC2 – CNRS Case 913, Campus de Luminy, 13288 Marseille Cedex 9, France
b L2MP Faculte´ des Sciences de St Je´roˆme, Case 511, 13397 Marseille, France
Abstract
This paper reports the AFM and HREM study of the Sb surfactant mediated growth of Ge on Si(0 0 1). We show that very
dense self-organised Ge dots of small lateral dimensions can be grown by using a sub-monolayer coverage of Sb on Si(0 0 1) in
the transient growth regime between 2D nucleation and step flow. The dramatic Ge growth change induced by Sb is attributed
to both kinetic and thermodynamic effects. Indeed, HREM observations evidence mainly two phenomena: the close-packing of
ultra-small Ge islands indicating a lower surface diffusion in presence of Sb and a mono-modal island shape and size that strongly
differs from the bimodal islands ‘huts’ and ‘domes’ commonly observed without Sb. Morphological and microstructural features
of Ge islands formed with and without Sb are studied and the differences between facets and aspect ratio are exhibited. Moreover,
at lower growth temperature (in the 2D nucleation regime, Tg350 °C), a delay to 3D island nucleation is observed and defect
free 2D flat layers can be grown up to thicknesses of 18 A . At higher growth temperature, (in pure step flow at Tg750 °C)
large, well separated ‘dome’ islands partially relaxed by dislocation nucleation on their edges are obtained. Such islands are very
similar to those obtained without Sb coverage. The complete desorption of Sb on Ge rich surface at T720 °C explains this
result. This study which improves the understanding on the formation of ultra-small dense islands is very promising for the
fabrication of quantum devices that require highly homogeneous islands of small lateral sizes and of MOSFET heterostructures
with strained SiGe n-channel which require flat Ge rich layers with abrupt interfaces. © 2002 Elsevier Science B.V. All rights
reserved.
Keywords: Sb; Surfactant mediated growth; Quantum dots; Ge; Si(0 0 1); Molecular beam epitaxy
1. Introduction
Si(0 0 1) surface is expected to remain the most
widely used surface for homo- and hetero-epitaxy for
microelectronic device applications in the next 20 years
with the SiGe heterostructures being increasing their
market place. The latter are already fabricated by most
of the largest companies in hetero bipolar transistors
(HBT) and present great interest for future microelec-
tronic (MODFET) and nanoelectronic (single electron
transistor and nanocrystal memories) devices.
Two ways to realise heterostructures have to be
distinguished depending if devices are based on
Si1−XGeX films (strained or relaxed) or on Ge quantum
dots. In the first case, the aim is to produce flat layers
with abrupt interfaces and low defect concentrations (in
bulk and at interfaces). Difficulties to realise this type
of structure come first from the stress induced by the
lattice parameter mismatch (4%) and second from the
lower surface energy of Ge as compared to Si. These
two phenomena lead to a Stranski–Krastanov growth
mode of Ge on Si. Schematically, two critical thick-
nesses which limit Si1−XGeX layers epitaxy on Si have
to be considered. One corresponds to the critical thick-
ness beyond which 3D islands begin to form. For sake
of clarity, all the Si1−XGeX metastable states induced
by the interplay of kinetics and of thermodynamics on
Si(0 0 1) will not be described here. This thickness is
directly related to the elastic strain energy. The other
corresponds to the critical thickness beyond which dis-
locations nucleate. It is related to the balance between
stress relaxation and stress gradient induced by the
dislocation. Moreover, other difficulties arise from the
Ge surface segregation (induced by growth kinetics,
stress and surface energy) and the significant Si/Ge
intermixing, which leads to broad interfaces. But the
* Corresponding author. Tel.: +33-6-6036-2813; fax: +33-4-9141-
8916.
E-mail address: berbezier@crmc2.univ-mrs.fr (I. Berbezier).
Fig. 1. Surface AFM images after the growth of 18 A of Ge on 1 ML
Sb/Si(0 0 1): (a) Tg=350 °C, (b) Tg=550 °C and (c) Tg=750 °C.
to grow thicker smooth Si1−XGeX layers with higher
Ge concentration. The use of a surfactant is also re-
ported to decrease the Ge segregation [1,6,11,15,16] and
the interface Si/Ge intermixing [4–7,9,11,15,16,20–23].
In addition, the use of a surfactant seems to be an
interesting process to realise SiGe or Ge relaxed layers
grown on Si substrates [13,14] with lower threading
dislocation density and lower surface roughness.
On another side, the realisation of quantum devices
based on Ge quantum dots meets different difficulties
more related to the mechanisms of nucleation, evolu-
tion and self-assembling of islands. Lithographic and
complex technological processes will not be envisaged
here because of the huge additional cost induced by the
fabrication. Indeed, after the growth of 3–4 ML of Ge
on Si(0 0 1), 3D ‘hut’ islands with very low aspect ratio
(large lateral sizes and low height) begin to form. They
rapidly elongate into rectangular flat islands and then
transform into large rounded ‘domes’. Such first-degree
shape transformation is related to both surface kinetics
(coarsening process) and energetics (stress relaxation).
But the lateral sizes of these islands are much too large
to obtain quantum confinement effect. The use of a
predeposited impurity is a relevant way to reduce the
lateral size of Ge dots [18] and Sb (element of group V)
which is the most commonly n-type dopant used in
MBE seems to be a good candidate for that. Indeed, in
order to reduce the lateral size of islands, different ways
can be explored: the reduction of the ad-atom surface
diffusion (and this has been shown for group V ele-
ments [1,10,11,20]); the increasing of stress (by decreas-
ing the Si/Ge intermixing for instance); the increasing
of islands nuclei density [3,24].
However, in spite of encouraging results, the physical
mechanism by which a surfactant modifies the Ge/Si
growth mode remains unsolved, notably about the Sb
effect since contradictory results (both theoretical and
experimental) are reported in the literature. Indeed,
while some results reveal crystallinity damages induced
by Sb mediated growth [2,8] others show a decrease of
the defect density [9]. In addition, Sb has also been
shown to increase the surface and interface roughness
[2,6,8,10,12] instead of promoting Ge epitaxy on Si
[4,9,12,15,17,23].
In order to improve our basic understanding of the
Ge/Si(0 0 1) growth mechanisms in presence of a sub
monolayer Sb coverage, this paper presents an AFM
and HREM study of the nanostructures formed. The
influence of Sb coverage, growth temperature (Tg) and
deposited thickness are investigated. Peculiar attention
is paid to the transient growth regime between 2D
nucleation and step flow. We show that it is possible to
grow ultra-small mono-modal dense Ge islands in this
regime. This is explained by the thermodynamics (de-
crease of the surface energy, increase of strain energy)
and kinetics (decrease of the surface diffusion) changes
induced by Sb.
use of a group V surfactant element can solve several of
these problems. Indeed, it was reported [4,5,9,11,
12,15,17,22,23] that surfactant mediated growth could
suppress or at least delay islands formation and permit
2. Experimental
All the structures were grown in a Riber MBE system
with a base pressure typically 10−11 Torr. Floating
zone silicon was evaporated thermally from an electron
beam evaporator. Ge and Sb were evaporated from
effusion Knudsen cells. Boron doped Si(0 0 1) wafers of
nominal orientation (miscut0.2°) were ex situ cleaned
and protected by an oxide layer as a final step. Subse-
quent in situ cleaning consisting in thermal desorption
of the oxide layer at temperature about 900 °C was
realised. A 50 nm thick Si buffer layer was systemati-
cally grown to achieve reproducible surface whose
cleanness is qualitatively checked by the (2×1) recon-
struction streaks intensity. Two types of Ge nanostruc-
tures were realised either deposited on Sb
sub-monolayer or directly deposited on Si(0 0 1). The
effect of temperature (and consequently also growth
mode), deposited thickness and Sb coverage on the
shape and stress relaxation of Ge nanostructures were
studied. Morphology and microstructure of samples
were observed by AFM and HREM (JEOL 4000 EX),
respectively.
3. Results
Fig. 1 shows the evolution of the Ge surface mor-
phology (thickness 18 A ) with the growth tempera-
ture. First, the different growth mode at 350 and
550 °C or 750 °C are clearly seen. At 350 °C (Fig. 1a
and Fig. 2) the surface is flat with a root mean square
surface roughness about 5 A suggesting the onset of 3D
island nucleation (L20–30 nm). It is only at 550 °C
that small dense Ge islands can be formed. At higher
temperature, large isolated islands (Fig. 4b) grow over
flat surfaces. Such islands are very similar to those
obtained for pure Ge growth (large dislocated domes).
This result confirms the desorption of Sb at this tem-
perature and indicates that Sb partial desorption during
Ge growth at lower temperatures should also be consid-
ered as a possible way to modify growth (increase of
the density of nucleation centres). Since it was shown
that Sb desorbs from SiGe surface even at 550 °C but
not at 350 °C [24], it is a relevant difference between
growth at 550 °C and at 350 °C to take into account.
For comparison, Fig. 3 shows the surface morphol-
ogy of Ge/Si without Sb at Tg=550 °C. The com-
monly observed bimodal ‘huts’ (Fig. 4a) and ‘domes’
(Fig. 4c) islands are visible. Lateral sizes, height and
densities of islands formed with and without Sb cover-
age are summarised in Table 1. The increase of islands
density (×3.75) and decrease of lateral islands size
(/2.5) induced by Sb is clearly evidenced.
Fig. 2. Cross-section TEM image of the 1 ML Sb mediated growth of
18 A of Ge with Tg=350 °C.
HREM observations of Ge/Si (Fig. 4) samples
confirm the presence of the expected [1 0 5] facets in
huts islands (at 11.3° of (0 0 1) plane) [10,11]. Concern-
ing domes two different facets can easily be observed
with angles about 25°1 and 55°1. They correspond
to {1 1 3} and {1 1 1} (at 25.2° and 54.7° of (0 0 1)
surface, respectively) that are low energy planes com-
monly observed in equilibrium shape of Si. Even if
some smaller facets can also be distinguished their
angles cannot be accurately determined (an angle about
13° was also measured for the ‘meniscus’ at the base of
the island which could correspond to (1 1 6) facets),
neither the orientation of the top of the island which
presents a rounded shape.
Fig. 1b shows the morphology of the surface for the
growth of Ge in same conditions but with the predepo-
sition of 1 ML of Sb. In this case, only one type of
small islands is obtained (Fig. 5). Due to the small
extension of facets (small lateral size of islands), it is
not possible to determine their exact orientations. How-
ever, these islands do not have a shape similar to huts
neither to domes. TEM images also evidence the small
distance separating islands bases. The formation of
smaller dots with a higher density can be interpreted as
a diminution of the surface diffusion (in these condi-
tions of growth, islands do not touch each other).
Moreover, high-resolution images show that such is-
lands are dislocation free and consequently fully
strained, which is the case for huts but not for most of
the domes islands. This is in good agreement with our
previous results [25]: the existence of two types of Ge
islands (hut and domes) is related to the stress relax-
ation, hut allows relaxation about 0.25% and domes
about 0.85%. However, in presence of Sb, which is
known to reduce the surface energy of both Si and Ge
[4,19], island shape changes induced by surface energy
changes [10] could be expected.
Fig. 3. Surface AFM image after the growth of 18 A of Ge on Si(0 0 1).
Fig. 4. Cross-section TEM images of islands formed after the growth of 18 A of Ge on Si(0 0 1): (a) hut Tg=550 °C, (b) dome Tg=750 °C and
(c) dome Tg=550 °C.
Fig. 5. Cross-section TEM images of ultra-small islands formed after the growth of 18 A of Ge on 1 ML Sb/Si(0 0 1) at Tg=550 °C.
Table 1
Islands lateral size, height and surface density versus Ge thickness
and predeposited Sb coverage
9 2718Ge thickness (A ) 12
Later size (A )
Without Sb 400×700, 800a350
250 300 300 660With 1 ML Sb
460With 1/2 ML Sb
Height (A )
20 20, 100aWithout Sb
3010With 1 ML Sb 8 10
20With 1/2 ML Sb
Density (dots/cm2)
109 4×1010, 2×109aWithout Sb
108 6×1010 1.5×1011With 1 ML Sb 2×1010
With 1/2 ML Sb 5×1010
a Hut, dome.
4. Conclusion
AFM and HREM study of the Sb surfactant effect
on the Ge/Si(0 0 1) growth was performed. First Sb
effect was found to depend on the temperature: flat
surface was obtained at low temperature (350 °C) and
small island (30 nm) with high surface density
(1.5×1011/cm2) and homogeneous surface distribution
at higher temperature (550 °C). Sb was found to delay
the island nucleation (even at 550 °C), to decrease the
ad-atoms surface diffusion, to change the equilibrium
shape of islands, and to increase the number of island
nucleation sites. Then playing a role on the kinetic of
growth, on thermodynamic parameters and on the
stored up strain of the film, Sb mediated growth was
found to be an interesting tool to realise both very
small Ge islands and Ge smooth surfaces.
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Sb-surfactant mediated growth of Ge nanostructures

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Sb-surfactant mediated growth of Ge nanostructures

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