Formation mechanism of self-assembled gesige composite islands

Article abstract of DOI:10.1149/2.006111jes

This study investigated the formation mechanism of GeSiGe composite islands on Si(001) using a combination of selective wet chemical etching, atomic force microscopy and high-resolution transmission electron microscopy. After upper Ge-rich parts were removed by selective etching, the etched GeSiGe composite islands exhibited pyramid structures, which differed from the ring-like isocompositional profiles observed in conventional Ge islands. Experimental results demonstrated that the multilayered GeSiGe composite islands were almost defect-free and suffered from a serious SiGe intermixing during growth. The shape and size distribution of composite islands can be further controlled by tuning the island parameters. We elucidate these phenomena on the basis of low surface diffusivity and strain adjustment of the thin Si layer in composite islands. These composite islands were also found to exhibit an apparent enhancement and blueshift in photoluminescence emission. These results indicated that GeSiGe composite islands would be potentially useful as an optoelectronic material operating in the telecommunication range.

Full text of DOI:10.1149/2.006111jes

Journal of The Electrochemical Society, 158 (11) H1113-H1116 (2011)
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0013-4651/2011/158(11)/H1113/4/$28.00 © The Electrochemical Society
Formation Mechanism of Self-Assembled Ge/Si/Ge
Composite Islands
Sheng-Wei Lee,a,∗,z Hung-Tai Chang,a Jeng-Kuei Chang,a and Shao-Liang Chengb
aInstitute of Materials Science and Engineering, National Central University, Jhong-Li, 32001 Taiwan
bDepartment of Chemical and Materials Engineering, National Central University, Jhong-Li, 32001 Taiwan
This study investigated the formation mechanism of Ge/Si/Ge composite islands on Si(001) using a combination of selective wet
chemical etching, atomic force microscopy and high-resolution transmission electron microscopy. After upper Ge-rich parts were
removed by selective etching, the etched Ge/Si/Ge composite islands exhibited pyramid structures, which differed from the ring-like
isocompositional profiles observed in conventional Ge islands. Experimental results demonstrated that the multilayered Ge/Si/Ge
composite islands were almost defect-free and suffered from a serious SiGe intermixing during growth. The shape and size distribution
of composite islands can be further controlled by tuning the island parameters. We elucidate these phenomena on the basis of low
surface diffusivity and strain adjustment of the thin Si layer in composite islands. These composite islands were also found to exhibit
an apparent enhancement and blueshift in photoluminescence emission. These results indicated that Ge/Si/Ge composite islands
would be potentially useful as an optoelectronic material operating in the telecommunication range.
© 2011 The Electrochemical Society. [DOI: 10.1149/2.006111jes] All rights reserved.
Manuscript submitted June 14, 2011; revised manuscript received July 25, 2011. Published October 5, 2011.
Strain-induced self-organized semiconductor nanostructures have
attracted considerable interest for the future nanoscale applications
such as quantum computing and optoelectronic devices.1–3 With an
adjustable band structure and a moderate lattice mismatch, Ge/Si has
emerged as a model system for the fabrication and investigation of
nanoscale heteroepitaxy.4,5 The self-assembly of Ge islands on Si is
known to develop in Stranski–Krastanov growth mode; in which het-
eroepitaxial growth starts with the formation of a two-dimensional
wetting layer followed by the nucleation of three-dimensional Ge
islands.6 The physical properties of Ge islands are strongly influ-
enced by their sizes, shapes, composition and growth parameters,
which are interrelated.7–9 To achieve SiGe islands with relatively high
Ge content and sufficient uniformity in size still remains a critical
issue. To improve the size homogeneity, Tersoff et al. has proposed
the growth of multiple stacks of Ge-island layers separated by Si
spacers.10 When overgrowing the Ge island layer with a Si spacer,
the strain field of the underlying islands penetrates through the spacer
layer, creates a strain modulation at the surface, and induces stacks
of vertically aligned and more homogeneous islands. Nevertheless,
too thin a Si spacer or excessive Ge deposition still causes a rapid
decrease of island density, coarsening of islands and consequently,
the occurrence of conical shaped defects.11 In the previous study, we
demonstrated that the growth of composite islands using a Ge/Si/Ge
stacked structure can effectively avoids the formation of Ge super-
domes and prevents the occurrence of dislocations even with excess
Ge deposition.12 The composite islands were also found to exhibit
a stronger intensity in photoluminescence (PL) emission. However,
the exact formation mechanism and composition distribution in these
composite islands were not completely understood. Recently, it has
been shown that selective wet etching combined with atomic force
microscopy (AFM) can be used to obtain the atomic composition
profiles in the Ge islands.13–15 Many different isocompositional pro-
files have been discovered and explained by different island-formation
mechanisms.16–18 In this work, utilizing this technique, we observed
that the etched Ge/Si/Ge islands exhibited pyramid structures, which
differed from the ring-like isocompositional profiles observed in con-
ventional Ge islands. We attempt to elucidate the possible island-
formation mechanism on the basis of low surface diffusivity and strain
adjustment of the thin Si layer in composite islands. In addition, the
correlation between microstructural parameters and optical properties
was also discussed.
Experimental
10–25 ꢀ cm, 150 mm diameter p-type (001)-oriented Si wafers
were used in the present study. All investigated samples were grown
at 600◦C in a multi-wafer hot-wall ultra-high vacuum chemical vapor
deposition (UHV/CVD) system. Pure SiH4 and GeH4 were used as
gas precursors. Before epitaxial growth, the Si wafers were dipped in
a diluted HF solution to achieve the hydrogen-passivated surface, and
then transferred into an UHV/CVD system. The growth rates for Si
and Ge are 0.03 nm/s and 0.8 monolayer (ML)/s, respectively. A
50-nm-thick Si layer was first deposited. The Ge (13 MLs)/Si
(2 nm)/Ge (13 MLs) stacked structures were then deposited to form 1-
and 5-period self-assembled composite islands, where the Si spacer
and capping layer in 5-period multilayers were chosen to be 20 nm
and 100 nm, respectively. For comparison, a set of samples consisting
of Ge islands with an equivalent Ge deposition (26 MLs) was also pre-
pared as counterparts. As the deposition was terminated, the wafers
were kept in the deposition chamber for 3 min, unless otherwise spec-
ified, to evacuate the residual reactant and were then removed from
the chamber.
The selective chemical etching experiments were performed at
room temperature by dipping the samples for 100 min in a mixture of
NH4OH, H2O2, and deionized water, which selectively etches Ge over
Si at room temperature and stops etching SiGe alloy with Ge compo-
sition of about 30%.14 We checked that longer etching duration did
not change the morphology of the remaining structures any more. The
surface morphologies of the samples before and after etching were
analyzed ex situ by AFM in tapping mode. High-resolution trans-
mission electron microscopy (HRTEM) combined with fast fourier
transform (FFT) were employed to provide more detailed information
about the microstructures and local strain distribution. Micro-Raman
measurements were also carried out to characterize the alloying sta-
tus of the composite islands. The resolution of Raman spectra was
approximately 0.1 cm−1. PL measurements were performed at 10 K
to study the optical properties of islands using a 514.5 nm line of an
Ar+ laser. The PL spectra were recorded by a liquid-nitrogen-cooled
Ge photodetector with the standard lock-in technique.
Results and Discussion
Figure 1a shows the surface morphology of as-grown Ge islands
obtained by deposition of a 26-ML-thick Ge layer. Due to exces-
sive Ge deposition, a few large-size Ge superdomes appeared ac-
companying numerous smaller domes. This non-uniform distribution
of Ge islands may seriously deteriorate the device performance. As
shown in Fig. 1b, the etched Ge superdomes exhibited a ring-like (or
∗
Electrochemical Society Active Member.
z E-mail: swlee@ncu.edu.tw
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Journal of The Electrochemical Society, 158 (11) H1113-H1116 (2011)
Figure 2. (a) XTEM image of a typical composite island, which reveals a
clear Ge/Si/Ge stacked structure. (b) AFM images (1μm × 1μm) of as-grown
composite islands and after selective chemical etching (c).
Figure 1. AFM images (1μm × 1μm) of 26-MLs Ge islands grown at
600◦C (a) before and (b) after selective chemical etching. (c) XTEM image
of as-grown 26-MLs Ge islands consisting of superdomes and domes. Misfit
dislocations at the superdome/interface are marked as D.
lation, the Ge islands were found to change their shapes dramatically
from domes to pyramids, and eventually to nanorings.20 The dome to
pyramid shape transition can be explained in terms of the dependence
of equilibrium shape of Ge islands on their volume and composition
as a result of intermixing.21 In addition, due to the lower diffusion
coefficient of Si,13 the surface diffusion of the underlying Ge atoms
was suppressed during Si capping, resulting in a higher island density.
These Si-capped Ge islands (i.e., pyramids) thus acted as preferential
nucleation sites for the subsequent Ge deposition and thus, effectively
suppressed the formation of Ge superdomes. Eventually, defect-free
composite islands were formed with high size uniformity. Note that,
in this case of continuous growth, the nanorings should never appear
during growth since the formation of nanorings is a kinetic process.20
As shown in Fig. 3, the shape and size distribution of Ge/Si/Ge
composite islands can be further controlled by tuning the island
parameters. As the Ge deposition of sub-islands was decreased to
11 MLs, the highly uniform composite islands were transformed into
a bimodal size distribution (Fig. 3a), in which pyramids with shal-
low facets and domes with higher angle facets coexisted. Once we
further decreased the thickness of the intermediate Si layer to 1 nm,
the bimodal islands returned to be dominated by dome-shaped islands
(Fig. 3b). In addition, we can also increase the diameter or height
of composited islands by stacking up multifold Ge sub-islands into a
composite island. As shown in the AFM image of Fig. 3c, the compos-
ite islands using a Ge/Si/Ge/Si/Ge threefold structure exhibited larger
circle-like) structure, which indicated that they have a Ge-rich core
and a more Si-rich periphery. This can be explained by a thermo-
dynamic model that minimizes the Gibbs free energy. Because of
the difference in Si chemical potential between the substrate and Ge
superdomes, Si atoms are always attracted from the surrounding Si
substrate to intermix with the Ge superdome, resulting in Si enrich-
ment in the periphery. However, due to the limited diffusion length of
Si, the Ge-rich core was preserved in Ge superdomes. Noted that here
we only take surface diffusion into account since bulk interdiffusion
can be negligible at this growth temperature.19 In Fig. 1b, we also
found that the smaller Ge domes exhibit an almost symmetric (cross-
like or ring-like) structure rather than the half-moon-like one observed
in the previous studies.18,19 This transition of composition profiles in
Ge domes can be explained as a result of strain energy minimiza-
tion. As seen in Fig. 1c, Ge superdomes were supposed to be almost
strain-relaxed by the fact of numerous misfit dislocations formed at the
Ge/Si interface and conversely, the smaller Ge domes still remained
coherent. During island growth, there was a strong thermodynamic
driving force for Ge atoms migrating away from the highly strained
domes to add onto the strained-relief Ge superdomes. As a result, the
Ge domes shrunk in size during island growth. The repulsive strain
fields between neighbor domes were thus not strong enough to trig-
ger the initial island motion. Consequently, the lateral island motion
never occurred and the smaller domes remained an almost symmetric
composition profile.
Figure 2a shows an XTEM image of a typical composite island,
in which the Ge/Si/Ge stacked structure was clearly revealed and no
dislocation was observed. The AFM image in Fig. 2b also reveals the
Ge/Si/Ge composite islands with a high degree of size uniformity.
These composite islands had a height and diameter of 14
2 and
78 5 nm, respectively, with a density of 9 × 109 cm−2. In the mean-
while, no superdome was observed even though these composite is-
lands also had an equivalent Ge deposition of 26 MLs. Compared with
those results obtained from the 26-MLs conventional Ge islands, it is
speculated that the intermediate Si layer inserted in Ge/Si/Ge stacked
islands indeed played an important role in the Ge redistribution during
island growth. After upper Ge-rich parts removal by selective chem-
ical etching, as shown in Fig. 2c, these composite islands exhibited
pyramid structures, which were very distinct from the ring-like com-
position profiles commonly observed in conventional Ge islands. We
attempt to interpret this phenomenon on the basis of the difference of
surface diffusivity between Si and Ge. In our previous investigation of
the evolution of the Ge islands during the initial stages of Si encapsu-
Figure 3. AFM images (1μm × 1μm) of as-grown composite islands with
the (a) Ge (11 MLs)/Si (2 nm)/Ge (11 MLs), (b) Ge (11 MLs)/Si (1 nm)/Ge
(11 MLs), and (c) Ge (11 MLs)/Si (1 nm)/Ge (11 MLs)/Si (1 nm)/Ge (11 MLs)
stacked structure. The corresponding line profiles are also shown in the bottom
of the figure.
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Journal of The Electrochemical Society, 158 (11) H1113-H1116 (2011)
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Figure 6. Ge-Ge and Si-Ge peaks of multilayered 26-MLs Ge islands and
(b) Ge/Si/Ge composite islands in the high dispersion Raman spectra.
Figure 4. XTEM micrographs of (a) 5-period 26-MLs Ge islands and
(b) Ge/Si/Ge composite islands capped with a 100-nm-thick Si layer.
be SiGe-intermixed, we didn’t calculate the percentage of change in
lattice constants but solely characterized the lattice distortions caused
diameters and heights than those with a Ge/Si/Ge twofold structure
(Fig. 3b). These results indicated that the shape and size distribution
of composite islands can be well controlled by tuning the island pa-
rameters, including the Ge deposition of sub-islands, the thickness of
intermediate Si layer, and the number of sub-island stacks.
by strain. For a region that is not strained, the lattice constant ration
√
az/ax, i.e., d(220)/d(002), should correspond to 2. Strain will dis-
√
tort the crystal lattice and cause this ratio to deviate from 2. By
comparison of Fig. 5b and c, the intermediate Si layer in the compos-
ited islands suffered from a higher degree of tensile strain than the
Si spacer did. This result indicated that an intermediate Si layer in
composited islands indeed played a role in strain adjustment during
composite-island growth and thus avoids the occurrence of incoherent
Ge superdomes.
A widely used approach to increase the quantum efficiency of op-
toelectronic devices is to stack several Ge island layers on top of each
other with a moderate Si spacer.22 Figure 4 shows the XTEM micro-
graphs of 5-period island multilayers capped with a 100-nm-thick Si
capping layer. Conical shaped defects were found to form in the 26-
MLs conventional Ge-island multilayers with the island stacks heavily
distorted as shown in Fig. 4a. On the other hand, as shown in Fig. 4b,
no threading dislocation was seen for the composite-island stacks in
the observed region. This result indicated that Ge/Si/Ge composite
islands shall be a considerably feasible scheme for the possible po-
tential applications. To further understand the island formation mech-
anism, we analyzed the local strain of the intermediate Si layer sand-
wiched between two Ge sub-islands of a composited island. Fig. 5a
shows a high magnification view of a typical composite island in
the most top island stack. 3 nm × 3 nm regions of interest, as indi-
cated by the dotted square, were selected for FFT analysis to produce
the diffractogram in Fig. 5b and 5c, respectively. The separation d
between O and a spatial frequency peak is directly proportional to
the reciprocal of the atomic spacing a in the HRTEM images, i.e., d
= 2π/a. Therefore, the (002) and (220) reflections contain informa-
tion on the vertical atomic spacing az (in the [001] direction) and
the lateral atomic spacing ax (in the [110] direction), respectively.
Considering that the intermediate Si layer in composited islands may
Raman scattering was performed to analyze the alloying status of
Ge islands in these multilayered stacks. Reparaz et al. has accurately
determined the phonon deformation potentials and peak-splitting val-
ues of the Ge-like, Si-like, and mixed Si–Ge optical modes as a func-
tion of SiGe alloy composition.23 Here, it is reasonable to assume
that the Ge-related peaks in the Raman spectra are mainly contributed
from Ge superdomes or Ge/Si/Ge composite islands since both of
them occupy most Ge materials in Ge-island samples. As shown in
Fig. 6, the shifts of Ge-Ge and Ge-Si peaks to the lower wave num-
ber indicated that the Ge/Si/Ge composite islands suffered from a
higher degree of SiGe intermixing than conventional Ge islands did.
Figure 7 shows their corresponding PL spectra measured at 10 K. The
Ge/Si/Ge composite islands were found to exhibit a much stronger
PL emission located at about 0.84 eV with a narrower full width
at half-maximum (FWHM). This improvement of PL emission can
Figure 7. PL spectra measured at 10 K of multilayered 26-MLs Ge islands
and Ge/Si/Ge composite islands. The FWHM values of the PL emission were
indicated. The inset also shows an AFM image (5μm × 5μm) of 5-period
composite islands without the top Si capping layer.
Figure 5. (a) A high magnification HRTEM image of a composite island in
the most top island stack in Fig. 4b and (c) Reciprocal space diffractograms
obtained by FFT of selected regions in the HRTEM image of Fig. 5a.
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Journal of The Electrochemical Society, 158 (11) H1113-H1116 (2011)
be attributed to the absence of conical shape defects, which act as
nonradiative recombination centers. The PL peak located around 0.84
eV could be further deconvoluted into two Gaussian line-shaped peaks
at 852 and 814 meV, respectively. These two peaks might be assigned
to the NP transition and TO replica of composite islands.24 Compared
with conventional Ge islands, we observed an obvious blueshift in
PL spectra for the composite islands. Wan et al. has proposed type-II
and type-I band alignments for the Ge islands and the wetting layers,
respectively.25 In a type-II alignment, the indirect excitons are first
localized at the hetero-interfaces, and then they recombine. There-
fore, this pronounced PL blueshift can be mainly attributed to the
reduction of valance band offset, caused by the SiGe alloying effect
in composite islands. It also implied that the PL emission characteris-
tics can be tuned by adjusting the structural or growth parameters of
composite islands. These results indicated that the stacked Ge/Si/Ge
composite islands would be potentially useful as a photodetector ma-
terial operating in the telecommunication range. In addition, Ge/Si/Ge
composite islands would exhibit a larger Si/Ge interface density, lead-
ing to much higher scattering rates for phonons, which have a much
longer mean free path than that of charge carriers. Meanwhile, the
coupling between two sub-islands in a composite island would also
induce intermediate band formation, enhancing charge transport be-
tween two sub-islands.26 Such a stacked composite-island system may
open up many new thermoelectric applications.
100-2221-E-008-016-MY3, NSC 100-2120-M-008-003, and NSC
100-2622-E-008-009-CC3. The authors also thank National Nano
Device Laboratories and Center for Nano Science and Technology
at National Central University for the facility support.
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Conclusion
The formation mechanism of Ge/Si/Ge composite islands on
Si(001) have been investigated by using a combination of selec-
tive wet chemical etching, AFM, and HRTEM. After upper Ge-rich
parts removal by selective etching, the etched Ge/Si/Ge stacked is-
lands exhibited pyramid structures, which differed from the ring-
like isocompositional profiles commonly observed in conventional Ge
islands. The shape and size distribution of composite islands can be
further controlled by tuning the island parameters. Experimental re-
sults also demonstrated that the stacked Ge/Si/Ge composite islands
were almost defect-free and suffered from a serious Si-Ge intermix-
ing during island growth. We elucidate these phenomena on the basis
of low surface diffusivity and strain adjustment of the thin Si layer
in composite islands. These composite islands were also found to
exhibit an enhancement and blueshift in PL emission. Such a multi-
layered composite island system may open up many new applications
in optoelectronics and thermoelectrics.
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Acknowledgment
The research is supported by National Nano Device Laboratories
and National Science Council of Taiwan under Contracts No. NSC
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