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Nanopatterning of Si surfaces
by normal incident He plasma irradiation
Zhe Liu1, Long Li1, Zeshi Gao1, Ze Chen1, Chao Yin1, Shifeng Mao1, Shin Kajita2, Noriyasu Ohno3,
Minyou Ye1,a)
1School of Nuclear Sciences and Technology, University of Science and Technology of China, Hefei,
Anhui 230026, People’s Republic of China
2Graduate School of Frontier Sciences, The University of Tokyo, Chiba 277-8561, Japan
3Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
a)e-mail: yemy@ustc.edu.cn
Abstract:
This study reports on the formation of self-organized silicon (Si) nanostructures by 75 eV helium
(He) plasma irradiation at normal incidence without impurities participation. In contrast to the
featureless surface after normal incidence argon (Ar) ion beam irradiation without the co-deposition
of impurities, the Si surface exhibits the development of faceted nanostructures under 75 eV He
plasma irradiation. The faceted structures are interspersed with valleys that extend in two orthogonal
directions, imparting a mountain-like morphology to the surface. Our investigation verifies that the
He bubbles align themselves along the direction perpendicular to the surface, underneath these
valleys. Furthermore, the presence of He bubbles induces distortion in the surface layer and leads to
the formation of an amorphous Si layer. The underlying mechanism driving this surface evolution
could be attributed to the instability induced by the presence of He bubbles.
This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0186756
Low-energy ion beam irradiation (IB) is a promising technique for inducing self-organized
nanopatterns on solid material surfaces. Existing researches has affirmed the ability of the IB
method, employed with various experimental setups, to produce diverse nanostructures such as
nanoripples 1,2, nanodots 3–5, nanocones 6,7, and faceted structures 8. However, the predominant
choice of working gases in these experiments has been limited to argon (Ar), krypton (Kr), and
xenon (Xe), with relatively fewer investigations involving helium (He) ion beams. The dearth of
results concerning the interaction between He ions and silicon (Si) substrates presents an intriguing
gap in our understanding. This phenomenon has been attributed to the prevailing belief that when
incident ions are lighter than the substrate atoms, structural formations do not occur 9. Additionally,
another contributing factor may be the swelling phenomenon resulting from the accumulation of
several keV helium ions following implantation into the Si lattice 10. However, recent experiments
involving low-energy (< 100 eV) helium ion irradiation 11–13 have unequivocally demonstrated
nanostructure formation on the surface of Si irradiated by He ions. This compelling evidence
underscores the imperative need for further research in this domain.
In this work, we design He plasma irradiation experiments without impurity co-deposition and
demonstrate a faceted mountain-like nanostructure formation on Si (100) substrate. Commercially
available P-type (100) Si substrates (10 × 10 × 1 mm3) with an electrical resistivity of ~0.001 W·cm
are irradiated by He plasma at normal incidence in the high vacuum chamber of linear plasma device
CLIPS (Compact LInear Plasma-Surface interaction device). The base pressure in the vacuum
chamber is 5 × 10-5 Pa and the working pressure during plasma irradiation is maintained at 1.5 Pa.
The He ion flux to the Si samples range from 1.5 to 2.4 × 1022 m-2 s-1 in the experiments. The
incident ion energy, Ei, is determined by the potential difference between the negatively biased
sample and the plasma space potential, which is +5.0 V. The Si samples are heated by the plasma
and the sample temperature, Ts, is measured using a K-type thermocouple positioned at the central
back region of the samples. The temperature fluctuation within each experiment is approximately ±
10 K.
To ensure a non-impurity co-deposition environment, all the clamps in the sample holder are
covered by a tantalum (Ta) cover, as shown in Fig. 1. During the irradiation, the Ta cover floats in
He plasma with much less potential than the sputter threshold energy, which is more than 100 eV 14.
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The surface morphology analysis is carried out via Field Emission Scanning Electron Microscope
(FE-SEM, Hitachi SU8220), Atomic Force Microscope (AFM, Bruker Dimension Icon), and
Transmission Electron Microscope (TEM, JEM-2100PLUS). The compositional analysis has been
performed by X-ray Photoelectron Spectroscopy (XPS) and Energy Dispersive X-ray spectroscopy
(EDX) demonstrating the absence of impurity deposition on the sample surfaces.
FIG. 1. Schematic view of the experimental setup in the He plasma irradiation experiment.
FIG. 2(a)-(h) depict SEM images illustrating the He plasma irradiated Si (100) surface under
varying conditions of Ts, Ei, and fluences,
𝛷
He. When Ts increases from 700 K in FIG. 2(a) to 750 K
in FIG. 2(b), the Si surface exhibits faceted nanostructures interspersed with valleys aligned along
the crystallographic directions <110>, indicating a temperature-induced instability. The structure size
increases as Ts increase to 800 K in FIG. 2(c). However, the pattern disappears at 850 K leaving an
uneven surface in FIG. 2(d). The impact of Ei on the pattern formation is elucidated in FIG. 2(e)-(g),
with a minimum effective Ei of 75 eV. The lower
𝛷
He case is also presented in FIG. 2(h).
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FIG. 2. The top-view FE-SEM images of Si (100) surfaces irradiated by He plasma with different plasma parameters of
(a)-(d) 700 K, 750 K, 800 K, and 850 K at Ei of 75 eV and
𝛷
He of 5 × 1025 m-2, (e)-(g) 95 eV, 85 eV, and 65 eV at Ts of
800 K and
𝛷
He of 5 × 1025 m-2, (h) 2.5 × 1025 m-2 at Ei of 75 eV and Ts of 800 K.
For a visual representation of the surface morphology post-irradiation, we present 3D AFM
images corresponding to the SEM topography depicted in Fig. 2(h) and (c), as shown in Fig. 3 (a)-
(b). The scanned area covers dimensions of 20 × 20 µm2. The results elucidate that with increasing
fluence, both the height and the size of the surface nanostructures on Si increase, while the surface
density of nanostructures decreases. This growth pattern exhibits a resemblance to the coarsening
phenomenon observed under Ar ion beam irradiation 15, suggesting that, despite disparities in their
efficiency in inducing nanopatterning on the Si substrate, there exists a degree of similarity in the
underlying interaction processes.
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FIG. 3. 3D-AFM images of irradiated Si surface with mountain-like nanostructures growth. These micrographs
correspond to the two samples shown in Fig. 2(h) and (c).
In the ion beam bombardment regime, the irradiation of Si surfaces by Ar ions leads to damage.
At lower temperatures, the irradiated surfaces exhibit amorphous characteristics. The formation of
patterns is primarily elucidated by Bradley-Harper (BH) theory 16 and Carter-Vishnyakov (CV)
effect 17,18, which utilizes the interplay of surface instability due to sputtering, mass redistribution,
and surface relaxation as pattern evolution mechanisms. At elevated temperatures surpassing the
recrystallization point, the damage induced by ion beam bombardment is effectively repaired,
preserving the crystalline nature of the surface. The involvement of the Ehrlich-Schwoebel (ES)
barrier-induced surface instability is necessary unless the temperature is exceptionally high, allowing
for easy overcoming of the ES barrier.
The distinctive orientation of the patterns depicted in FIG. 2 indicates that the pattern formation
is influenced by the crystal orientation. This observation aligns with the characteristics of low-energy
He ions employed in our experiments, as they cannot damage the sample surface through the cascade
collision. Consequently, the applicability of the BH theory and CV effect is unsuitable for our case.
Xin Qu et al. 19 propose a reverse epitaxy mechanism for the evolution of crystalline surface patterns
at temperatures exceeding the recrystallization point. The identified inverse pyramids are attributed
to vacancy diffusion influenced by the Ehrlich-Schwoebel (ES) barrier. The temperature range
conducive to pattern formation in our study indicates a temperature-induced instability akin to
reverse epitaxy scenarios. However, the distinct growth directions observed in He plasma irradiations
suggest diverse mechanisms governing pattern formation compared to reverse epitaxy. Additionally,
the alignment of patterns with much bigger sizes in FIG. 2 differs from vacancy diffusion cases.
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Therefore, we posit that patterns induced by He plasma irradiation may be associated with adatom
diffusion 20, a current explanation for the growth of He-induced fiber-form nanostructures on
tungsten (W) surfaces 21. Subsequent analysis elucidates the plausible pathway for adatom
production in our low-energy (< 100 eV) He plasma irradiations.
The cross-sectional SEM images depicted in Fig. 4(a)-(d) provide compelling evidence of the
conspicuous presence of cavities related to He bubbles within the irradiated Si substrate. Fig. 4(a)
and (c) illustrate the growth of He bubbles in a downward direction, almost perpendicular to the
initial surface, which is located beneath each valley. To offer a more detailed view of these He
bubbles, enlarged micrographs of the bubbles are presented in Fig. 4(b) and (d), focusing on the
selected region delineated by a dashed box in Fig. 4(a) and (c).
During high fluence He plasma irradiation, He ions implant into the Si lattice and subsequently
form mobile He clusters upon interaction with other He atoms. These clusters can grow through the
absorption of additional clusters or individual He atoms, ultimately coalescing to form He bubbles 22–
24. During the He bubble evolution, Si atoms are expelled from the lattice, transforming into self-
interstitial atoms. The aligned He cavities may function as effective channels, allowing self-
interstitials to reach the free surface and transform into adatoms.
FIG. 4. Cross-sectional FE-SEM images of irradiated Si surface with aligned He bubbles. These micrographs correspond
to the two samples shown in Fig. 2(h) and (c).
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The distinct alignment of patterns along the <110> direction can also be explained by the
accumulation behavior of implanted He within the Si substrate. During the irradiation, the implanted
He atoms tend to migrate towards the tetrahedral interstitial site within the Si lattice 25,26 and exhibit
a capacity to diffuse deeper into the lattice through interstitial sites. Consequently, He atoms
preferentially accumulate on the (110) plane in the Si lattice during the diffusion process of He into
deeper depth. Ultimately, the distribution of He bubbles beneath the Si surface extends to depths of
several hundred nanometers, resulting in a pattern aligned along the <110> direction.
In comparison to the atomistic processes 19,27, the impact of He bubble aggregation is more
macroscopic, likely contributing to the larger size of the observed nanostructures. The observed
dependence of pattern formation on temperature and ion energy, particularly the pronounced
influence of temperature as evident in FIG. 2, is presumed to be associated with the He behavior in
Si lattice and the growth of He bubbles. This aligns with the prevailing explanation concerning the
parameter windows associated with the growth of He-induced nanostructures on metal surfaces 28.
Further insight into the surface layer is provided through TEM observations in Fig. 5, which
furnish detailed information on the structural changes and behavior of the surface layer. In Fig. 5(a),
we observe a terraced surface characterized by multiple layers. These terraces host aligned He
bubbles situated within the valleys of the faceted structures. These He bubbles have a notable impact
on the lattice, introducing a significant number of defects. The inset offers an expanded view of the
upper part of the region containing He bubbles, revealing that these bubbles effectively disrupt the
lattice, resulting in an alteration of the atomic arrangement in their immediate vicinity.
Fig. 5(b) provides a detailed examination of different components within this region, each
displaying distinct interfaces. Notably, the substrate lattice retains its original crystal structure. The
surface layer most profoundly affected by the presence of He bubbles appears to be an amorphous Si
layer, exhibiting a consistent thickness of approximately 45 nm. Above this layer, we discern the
presence of a Si-Pt mixed layer under EDX analysis. It is worth noting that this mixed layer appears
to have formed during the sample preparation process, in which Pt is deposited on the sample during
FIB.
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FIG. 5. Faceted Si surface irradiated by 75 eV He plasma at normal incidence with Ts = 800 K,
𝛷
He = 5 × 1025 m-2 [FIG.
4 (c)] (a) Cross-sectional TEM images of irradiated Si surface, the inset shows the He bubble region. (b) Details of the
inset in (a). The amorphous layer is about 45 nm thick.
The presence of amorphous layers on the facet surface introduces ambiguity regarding whether
adatom migration along the facet during pattern formation can be adequately explained by diffusion
controlled by the ES barrier. It is plausible that an alternative mechanism operates under He plasma
irradiation. The formation of He bubbles exhibiting specific orientations induces heightened lattice
stress 29, leading to the distortion of the surface layer, resulting in the formation of the amorphous
layer. The surface evolution is ascribed to the formation of He bubbles and the associated stress-
induced diffusion of adatoms. A similar amorphous layer, induced by low-energy He plasma
irradiation, has been observed in tungsten W 30, with ripple development influenced by crystal
orientation. Intriguingly, both crystal orientation dependence and amorphous layer formation on
facet surfaces manifest in He-Si and He-W cases. Further research is imperative to elucidate this
aspect.
In conclusion, we prove the nanopatterning ability of low-energy He plasma on Si substrate with
the experiments performed at normal incidence without impurity co-deposition. Distinct faceted
mountain-like nanostructures are induced under He plasma irradiation at specific temperatures and
energy ranges. We posit that the formation of self-organized faceted nanostructures is a direct
consequence of the instability introduced by the presence of He bubbles.
This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.
PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0186756
This work is supported by the Joint Funds of the National Natural Science Foundation of China
(Grant No. U2267208). We thank the USTC Center for Micro and Nanoscale Research and
Fabrication for supporting the surface morphology analysis.
DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author upon
reasonable request.
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