We report nanopattern formation on CoSi sputtered thin film using ion beam.

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Current Applied Physics
journal homepage: www.elsevier.com/locate/cap
Morphological instabilities in argon ion sputtered CoSi binary mixtures
B.K. Paridaa
, M. Ranjanb
, S. Sarkara,∗
a
Department of Physics, Indian Institute of Technology Ropar, Nangal Road, Rupnagar, Punjab, 140001 India
b
FCIPT, Institute for Plasma Research, Gandhinagar, Gujarat, 382016 India
A R T I C L E I N F O
Keywords:
Nanopatterning
Atomic force microscopy
Binary mixture
A B S T R A C T
Nanopattern formation on CoxSi1−x ( < <x0 1) surfaces has been studied under a range of low energy Ar+
ion
bombardment at oblique incidence and also at varying ion fluences. Our study shows a clear morphological
change for the nanostructures depending upon the incident beam energies and binary mixture stoichiometries.
From an initial smoothening regime, well-ordered nanoscale ripples form at sub-keV energies at ion fluences
∼ 1018
ions cm−2
. These ripples further transform into micrometer-sized ellipsoidal structures upon increasing
the ion energy from 500 eV to 1200 eV. The wavelengths of the nanostructures increase with ion fluence obeying
a power law behaviour f 0.123
, where f is the fluence. For higher values of Co content, the rippled morphology is
gradually replaced by bug-like hierarchical structures.
1. Introduction
Ion beam sputtering (IBS) has been proven to be an easy tool to
create self-organized nanostructures over a wide variety of materials
e.g., semiconductors, metals and insulators [1–8]. This technique turns
out to be a low cost single step process to create wide area nanos-
tructures having various applications in the area of optoelectronic de-
vices [9], magnetic data storage [10] as well as surface templates for
further applications [11]. Such nanostructuring on elemental surfaces
have been studied both experimentally and theoretically by several
groups [12–16].
For the case of binary systems, like III-V semiconductors (GaSb,
GaAs, InP, InSb etc.), nanoripples were observed at oblique incidence
[17]. Eventually, this led to the study by Facsko et al. [18], where
regular hexagonally ordered GaSb nanodots were observed to form at
normal incidence. This was followed by the work of Frost et al. [19]
who noticed nanostructure evolution at oblique incidence but for ro-
tated InP substrates. Since then, a lot of interest has grown around the
study of binary systems subjected to ion bombardment. Studies on
binary systems have been carried out at different ion energy ranges
between 100 eV and 100 keV [20–23]. During the last few years, similar
binary systems have been considered where metal impurities [Fe, Ni,
Cu, Mo, W etc.] were added to the elemental surfaces during IBS
[24–26]. These impurity atoms modify the surface giving rise to na-
nodots, nanoholes, dots-on-ripple structures depending upon the type
and flux of impurity atoms. Concurrent impurity added ion irradiated
substrates have also been studied for different stoichiometric ratios
[24,25,27,28]. Michely and co-workers [29] found that silicide
formation was a necessary but not a sufficient condition for ripple
formation in such binary systems. However, a recent study by Moon
et al., found that silicide formation does not decide pattern formation as
reported earlier [30]. Norris and his group have recently demonstrated
the effect of phase separation in order to explain their experimental
findings. They found that phase separation can lead to a narrow band of
linearly unstable wavelengths [31].
Except for the case of impurity addition, ion beam patterning stu-
dies have not been carried out for metal-semiconductor systems to the
best of our knowledge. For the case of impurity addition via co-de-
position [25], the atoms having energies ∼ 50 eV essentially are a part
of the surface or near-surface layer having a penetration depth of sub-
nanometer dimensions [29]. Hence, bombardment induced diffusivity
is primarily confined to the surface. In contrast, for binary mixtures, the
incoming ions have a penetration depth ∼ few nanometers, thus fa-
cilitating stoichiometric rearrangements in the bulk as well, which may
eventually affect the surface concentration. Hence the study of such
binary mixture systems is of particular importance. In this work, we
have carried out low energy ( −500 1200 eV) ion beam patterning stu-
dies on CoxSi1−x( < <x0 1) binary samples having a wide range of
stoichiometries which are far away from the −50 50 ratio where the
coupling between the topography and the stoichiometry is believed to
be the strongest according to some theoretical models [32,33]. More-
over, the constituting elements in this case have widely different dif-
fusivities which is an important deciding factor for the resulting surface
atomic currents during irradiation. Experiments were performed with
varying ion beam energies and fluences. This study shows the extent of
ordering and the different morphological characteristics evolving out of
https://doi.org/10.1016/j.cap.2018.05.011
Received 17 January 2018; Received in revised form 7 May 2018; Accepted 9 May 2018
∗
Corresponding author.
E-mail address: sarkar@iitrpr.ac.in (S. Sarkar).
Current Applied Physics 18 (2018) 993–1000
Available online 12 May 2018
1567-1739/ © 2018 Korean Physical Society. Published by Elsevier B.V. All rights reserved.
T

2. the above experimental conditions. We found that with increase of
energy, ordering of the pattern formation gets poor above 500 eV. Best
achievable ordering can be found at low energies ∼ 500 eV. We have
also studied the effect of ordering and ripple wavelength behaviour
with increasing ion fluence at 700 eV. Experiments done on samples
with varying Co content revealed hierarchical structures for higher
concentration of Co. Enrichment of Co is found to occur at the hills of
the corrugated surfaces.
2. Experimental details
Commercially available Si (100) substrates were cut into ×1 1 cm2
pieces and ultrasonicated in ethanol for 30 min. CoxSi1−x( < <x0 1)
binary mixture thin films of different stoichiometric ratios were grown
over silicon substrates in a confocal magnetron sputtering chamber at a
base pressure of × −1 10 6 Torr and a working pressure of × −5 10 2 Torr
at room temperature. Sample rotation was performed during deposition
to ensure uniform growth. Argon with a flow rate of 50 sccm was used
as a sputter gas. Scanning electron microscopy-energy dispersive x-ray
analysis was used to get the surface composition of the pre-bombarded
samples from which the stoichiometries of the samples were de-
termined.
The thicknesses of the thin films (∼ 2 μm) were chosen such that
they would not be eroded away at specific energies of ion bombard-
ment. Fig. 1 shows the scanning electron microscopy (SEM) image of
the remnant film after irradiation at 1200 eV, which is the maximum
bombarding energy for our case. The thin films were characterized by x-
ray diffraction (XRD) and atomic force microscopy (AFM) before sub-
jecting them to ion bombardment. Ion irradiation was performed with a
Kaufman ion source in a specially designed vacuum chamber having a
base pressure of × −7.5 10 8 Torr and a working pressure of × −7.5 10 5
Torr. The beam flux for Ar+
ion was ×2.8 1015
ions cm−2
s−1
and the
energy was varied from 500 to 1200 eV. Keeping the angle of incidence
fixed at ∘67 with the sample surface normal, the fluence was varied from
× − ×2.5 10 1 1018 19 ions cm−2
corresponding to 15–60 min of irra-
diation. All the experiments were carried out at room temperature and
all samples were bombarded without rotation. Table 1 gives a summary
of the samples deposited and the bombardment experiments done
under different parameters. The samples have been named accordingly
for reference in the subsequent text. Ex-situ AFM (MultiMode 8, Bruker,
USA) characterization of the irradiated samples was performed in tap-
ping mode using a silicon tip having an approximate radius of 10 nm.
Data analyses of the acquired AFM images were performed using Na-
noScope Analysis software (v1.40).
3. Results
Fig. 2 shows AFM images of post-irradiated samples bombarded at
different ion energies between 500 eV and 1200 eV for the set of sam-
ples SE with a common Co content of 27%. The arrow in the picture
indicates the ion beam direction. Fig. 2(a) refers to the sample irra-
diated at 500 eV. Self-organized nanoscale ripples are observed for this
energy. With increase in the ion energy (700 eV), the ripple structures
gradually lose their distinctive character and defects start arising along
the ripple lengths. In contrast, with further increase of ion energy (1000
and 1200 eV), a distinctive change from nanoripple structures to el-
lipsoidal ones are clearly observed from the images. Thus a clear
morphology shift is observed in the present case. It must be noted here
that the images for 500 and 700 eV are on ×μm μm1 1 scale while those
for 1000 and 1200 eV are on a scale. Hence, the structures for the latter
two energies have large sizes as compared to the former ones. For all
the above cases, the structures are aligned perpendicular to the ion
beam direction i.e. the wave vector lies along the direction of the ion
beam.
Ion fluence variation studies were done on the set of samples SFs
with 16% Co composition keeping the energy at 700 eV and angle at
∘67 . Fig. 3 shows AFM images of these irradiated samples. Self-orga-
nized nanoscale ripples with defects are observed for these sets of
samples. Similar kind of ripples are observed on samples for fluences of
×2.5 1018 ions cm−2
and ×5 1018 ions cm−2
. Gradual change in mor-
phology was observed for a fluence of ×7.5 1018 ions cm−2
. Finally, for
a fluence of ×1 1019 ions cm−2
the ripples exhibit a better ordering. It is
evident from the images that the defect densities of the ripples for these
samples are much higher than the former ones (Fig. 2). However,
within the range of ion fluences studied, the defect density is seen to be
the lowest for the highest ion fluence.
For different stoichiometric set of samples (SE2, SF3, SA1) which
were irradiated with identical ion beam parameters (700 eV, ∘67 ,
×7.5 1018 ions cm−2
), AFM images are shown in Fig. 4. It is observed
that with increasing initial Co content the ordering of ripple patterns
change. For almost equal or high Co and Si contents rippled bug-like
hierarchical structures appear on the surface. The hierarchical nature is
evident from the line profile of one such bug-like structure.
Fig. 1. (a) SEM image of the sample irradiated at an energy of 1200 eV, incident
angle 67∘
and ion fluence ×7.5 1018 ions cm−2
. The arrow refers to the remnant
part of binary mixture thin film after erosion.
Table 1
Summary of the deposited samples along with their corresponding sputter
parameters.
Cobalt % Parameter Parameter
value
Other
parameters
Sample
name
27 Energy (eV) 500 eV Fluence = ×7.5 1018
ions cm−2
Angle = ∘67
SE-1
700 eV SE-2
1000 ev SE-3
1200 eV SE-4
16 Fluence (ions cm−2
) ×2.5 1018 Angle = ∘67
Energy = 700 eV
SF-1
×5.0 1018 SF-2
×7.5 1018 SF-3
×1.0 1019 SF-4
53 Angle (degree) 67∘
Energy = 700 eV
Fluence = ×7.5 1018 ions
cm−2
SA-1
B.K. Parida et al. Current Applied Physics 18 (2018) 993–1000
994

3. 4. Discussions
The deposited films were first analysed using x-ray diffraction to
check for possible silicide formation. No such evidence was found from
the data. Findings from the XRD results have been discussed towards
the end. The rms roughness of the as-deposited samples were ∼ 5 nm.
AFM results show that the roughness of the irradiated surfaces is less
than the as-grown one thereby implying smoothening during irradia-
tion. It was observed from Fig. 2 that there is a clear morphological
change of the irradiated surfaces from 500 eV to 1200 eV at an angle of
incidence ∘67 and ion fluence of ×7.5 1018 ions cm−2
. Ripple char-
acteristics obtained for the different energies are plotted in Fig. 5.
The ripple structures for 500 eV are self-organized with approxi-
mately −25 30 defects per μm2 (Fig. 2 (a) shows three defects as re-
presentative cases). The average wavelength and amplitude of these
ripples are found to be 41 nm and 1.7 nm respectively. The average
length of the ripples is about 344 nm. The average wavelength of the
ripples for the 700 eV case is about 47 nm. When compared with the
previous image for 500 eV, the ripple structure for this case gradually
starts disappearing as evident from the AFM images (Fig. 2) thereby
resulting in a lower roughness (Fig. 5). In addition, several defects arise
in the ripple morphology for this case. Above this energy, the roughness
again starts increasing with ion energy. Thus the morphology under-
goes a smoothening process up to 700 eV and roughens thereafter. A
similar trend is noticed for the ripple amplitudes which increases after
an initial decreasing trend up to 700 eV. At 1000 eV, the ripples com-
pletely disappear giving rise to ellipsoidal mound-like structures
aligned in the direction perpendicular to the ion beam. The sizes of
these structures are order of magnitude larger than the ripple structures
which are clearly evident from the AFM scan sizes. Similar structures
are also observed for the 1200 eV case as well. The ellipsoidal mounds
are regularly arranged with number densities of approximately 0.6 and
0.5 −μm 2 for 1000 and 1200 eV respectively. These structures become
more elongated and possess a higher amplitude when the energy of the
ion beam increases to 1200 eV (Fig. 5). Also comparing Fig. 2(c) and
(d), it is clear that these ellipsoidal structures are larger than the pre-
vious ones and the inter-ellipsoidal distance increases with energy. Fi-
nally, the aspect ratio (width/length) of the nanostructures is noticed to
decrease almost linearly with energy up to 1000 eV after which the
decrease rate slows down to a considerable extent. It is important to
note in this context that majority of the studies done at bombarding
energies around 1 keV or less on binary materials have been performed
on III-V semiconductors for fluences less than 1018 ions cm−2
[34–38].
In all of the above, nanodot formation was observed on the surfaces.
Fig. 2. AFM images of bombarded samples with Ar ion energies of (a) 500, (b) 700, (c) 1000 and (d) 1200 eV. White arrow indicates the ion beam direction. The z
value indicates the maximum height for each image. Representative defects are pointed out in (a) by white circles. Inset of (c): Schematic side view of an ellipsoidal
mound like structure, where the lower part is Si wafer containing the white colored patterned thin film.
B.K. Parida et al. Current Applied Physics 18 (2018) 993–1000
995

4. Again, experiments performed at lower fluences as compared to the
present case, on oxide surfaces have yielded hexagonally arranged na-
nodots and ripples [39–43]. In comparison, studies on impurity sput-
tering show ripple formation at energies comparable to the present case
for fluences below 1018 ions cm−2
[28,44,45]. A recent study by Gago
et al. showed both ripple and dot formation at higher fluences when Fe
and Mo impurities are added to Si surfaces during Ar ion bombardment
[46]. Majority of the experiments have thus been performed at fluences
less than ∼ 1018 ions cm−2
. To the best of our knowledge, ripple for-
mation at sub-keV energies and at fluences higher than ∼1018
ions
cm−2
for bulk metal-semiconductor binary systems have not been re-
ported till date.
The power spectral density (PSD) curves for the above cases are
plotted in Fig. 6. The prominent peak for the 500 eV case corresponds to
a wavelength of 41.6 nm which tallies well with our calculated value.
The additional peak refers to high lateral ordering of the ripple struc-
tures [5,47]. The PSD for the 700 eV case shows a broader peak shifted
slightly towards lower spatial frequency or higher ripple wavelength.
The broadness signifies loss of ripple regularity when compared with
those for the 500 eV case. The inter-ellipsoidal distance as obtained
from the PSDs are about ∼540 nm and ∼ 1 μm for 1000 eV and
1200 eV respectively. The variation of wavelength of nanostructures for
binary systems formed during ion bombardment has been studied by
few groups [18,34]. According to their results, the characteristic wa-
velengths of the ripples increase with ion energy which is in contra-
diction to the linear instability mechanism where thermal diffusion is
the primary relaxation process. According to this mechanism, the wa-
velength decreases with energy following a power law ∼λ Ep
, where p
is negative [6]. Several other studies have also pointed out that p could
also be positive thereby indicating a different relaxation mechanism
during the bombardment process [6] which corroborated very well
with the findings of Facsko et al. [18,34]. Under such a scenario, ion
induced surface diffusion (IISD) process dominates and the wavelength
of the structures are found to increase with energy. IISD corresponds to
the diffusion due to the direct transfer of momentum and energy from
ion to individual atoms of the binary mixture in ion-atom collisions,
which depends upon difference between the activation energy of sur-
face diffusion and ion-to atom energy transfer [48]. The increase in
wavelength with energy was taken care of by the fact that lateral
straggling of the incoming ions depends upon the energy deposited as a
power law ∼λ E m2 , where m is positive (∼ 0.25) and is dependent on
the interatomic potential within the low energy limits [34]. Thus, the
Fig. 3. AFM images of irradiated samples under varying ion fluence but identical beam energy of 700 eV and angle of incidence 67∘
. Arrows indicate the ion beam
direction. The z values indicate the maximum height for each image.
B.K. Parida et al. Current Applied Physics 18 (2018) 993–1000
996

5. surface diffusion comprises of contributions from thermally activated
diffusion (TD) and IISD [48]. The thermal diffusion term dominates at
higher temperatures while the effective ion induced diffusion term
dominates at higher energies. p can also be positive under beam scan-
ning over the sample wherein ion-induced effects eventually dominate
over thermal effects [49]. In the present case, the variation in
wavelength with the ion energy cannot be mapped or compared ac-
cording to this power law since the surface structures altogether suffer a
change in their shape within the energy range studied.
The wavelength and roughness variations of the ripple structures
with respect to ion fluence variation are shown in Fig. 7 for the set of
samples SFs. The wavelength is seen to increase monotonically with the
Fig. 4. (a)–(c) AFM images of samples having varied Co content irradiated under identical ion beam parameters (700eV, ×7.5 1018 ions cm−2
, ∘67 ), The Co/Si content
is indicated at the top left corner of each image. (Inset of (a) shows an enlarged area). (d) Typical line profile of a bug-like structure as observed in (c). (e) Roughness
variation as a function of Co/Si composition ratio. White arrow indicates the ion beam direction w.r.t. the surface. Schematic of bug like structure is shown beside fig
(e).
B.K. Parida et al. Current Applied Physics 18 (2018) 993–1000
997

6. ion fluence until the range studied following a power law given by d0.123
where d is the fluence. The average wavelength of the ripples is about
42 nm for a fluence of ×2.5 1018 ions cm−2
. Interestingly, this pertains
to the surface where the defects are more but the overall roughness is
smaller than the other cases as observed from Fig. 3. The roughness of
the films increases exponentially in the ion fluence range studied as
evident from the graph (Fig. 7). Bobek et al. had found that the
roughness increases quite abruptly and saturates thereafter for the case
of GaSb binary system [36]. Compared to their low fluence regime, the
present case pertains to a scenario of higher fluence where a consistent
exponential increase of the roughness is observed within the conditions
investigated. In general, the ripples are found to coarsen with time as
evident from Figs. 3 and 7. Nanostructure coarsening has been ad-
dressed by several groups in the literature for elemental as well as
binary compounds [13,18]. For binary systems, the individual sput-
tering rates, diffusivities, and surface viscous flow can together play a
significant role in the coarsening phenomenon. However, such
considerations have not been considered in any of the experimental
studies on such systems [18,36]. In both of the above cases, experi-
ments were performed under normal incidence and nanostructure or-
dering was found to improve with ion fluence. In the present one,
bombardment was carried out under oblique incidence. Still we observe
a progressively better ordering of the ripple structures within the ion
fluence range under consideration. We have also performed the ex-
periment at varying fluence for samples having 27% Co. Ripples are not
found to be well-formed at lower fluences as compared to higher ones
as evident from Fig. 7 (c).
Experiments done with varying Co content reveal that the roughness
increases linearly with the Co concentration in the sputtered binary
mixture samples (Fig. 4). Ripple structures are noticed to exist for Co
concentrations of 16% and 27% but with an increasing roughness. For
an increased Co concentration of 53%, the simple ripple structure is
seen to be replaced with rippled bug-like structures having higher
amplitudes as compared to the earlier ones. The bug-like hierarchical
structures are seen to be oriented along the ion beam direction. They
seem to arise in groups due to a pile up of a few ripple structures along
the beam direction.
We observe that with increase in Co concentration sputtering yield
becomes comparable for both the constituents as evident from Table 2.
Under this condition, the extent of phase separation tends to be less
than that for low concentrations of Co.
Macko et al. [25] had reported growth of similar structures resulting
out of shadowing and silicide formation but with co-deposition of Fe
under Kr+
bombardment. On the other hand, Zhang et al. [26] ob-
served evolution of ripples and dots for Fe surfactant atoms with 5 keV
Xe ion at normal incidence over silicon substrate. The formation of
these structures depends upon the Fe concentration on the surface. For
a higher iron composition, dots and ripple patterns are observed and
surfaces remain flat for lower composition. The above studies suggested
that silicide formation appeared to be a necessary but not a sufficient
condition for pattern formation. However, a recent study by Moon et al.
[30] has suggested that instability can set in for different stoichiome-
tries even if silicide formation is absent. Fig. 8 compares the XRD
profiles for different energies, fluences and compositions with that of
the pristine film. Silicide phases appear for different energies and
Fig. 5. Plots showing variations of (a) wavelength, (b) roughness, (c) amplitude and (d) aspect ratio of the surface patterns as a function of energy of the ion beam.
For amplitude and wavelength, the line profile is shown as inset in (c). The as-grown sample roughness is indicated in (b).
Fig. 6. Power spectral densities of the surface heights for samples irradiated at
different energies.
B.K. Parida et al. Current Applied Physics 18 (2018) 993–1000
998

7. fluences of irradiation that takes over the place of the pristine film.
Silicide formation seems to dominate for films having higher Co com-
position. It has been reported that silicides form at elevated tempera-
tures on clean Si surfaces with no native oxide [50]. For the present
case, the sample temperature is expected to rise with increase in ion
energy and fluence. The native oxide layer would be eroded upon ion
irradiation thereby leading to the formation of silicides. This is corro-
borated from the XRD data.
Finally, Fig. 9 summarizes the fate of the nanostructures formed via
Ar+
bombardment on various CoSi binary systems. The different kinds
of structures formed are shown as a function of ion energy in the dia-
gram. This suggests, focussing future research on systematic studies
covering the entire parameter space in such binary mixture systems
which would unravel promising routes to nanotechnology.
5. Conclusion
In conclusion, we have studied the morphology evolution of dif-
ferent stoichiometric CoxSi1−x ( < <x0 1) binary mixture thin films
under low energy (500–1200 eV) ion irradiation at oblique incidence.
The stoichiometric ratios of the samples investigated were far from the
theoretically assumed maximum coupling regime (i.e., 50:50) of topo-
graphy and stoichiometric instabilities. We have reported ripple for-
mation at sub-keV energies and at fluences higher than ∼ 1018 ions
cm−2
for such bulk metal-semiconductor binary systems which have
not been reported till date. There is a clear morphological change of the
irradiated binary surfaces when the incident energy varies from 500 eV
to 1200 eV. The surface undergoes a smoothening process up to 700 eV
and roughens thereafter. Complete disappearance of well-ordered na-
noscale ripples and appearance of large scale semi-ellipsoidal mound
like structures occur at 1000 eV which are aligned perpendicular to the
direction of ion beam. Inter structural distances also increase with
bombarding energy. The aspect ratio (width/length) of the nanos-
tructures decreases linearly up to 1000 eV after which the rate slows
down to a considerable extent. From the fluence study, it is found that
the wavelength of the nanostructures increases with fluence (d) fol-
lowing a power law d0.123, whereas the roughness increase is ex-
ponential. Substantial morphological changes are also observed as one
goes to higher Co content when the ripple structures are replaced by
hierarchical bug-like structures.
Acknowledgement
The work has been supported by Department of Science and
Technology, India by the proposal grant no. SR/S2/CMP-112/2012.
The authors also acknowledge the experimental assistance of Mr. H.
Singh, Mr. A. Kaushal, and Mr. A. K. Kar of IIT Ropar and Mr A. Sharma
of FCIPT/IPR, Gandhinagar.
Fig. 7. Plots showing variations of (a) wavelength and (b) roughness of surface structures for different fluences (Energy = 700 eV, Incident angle = 67∘
) (c) AFM
image of Ar ion irradiated Co27Si73 sample at an incident angle of 67∘
and fluence of ×2.5 1018 ions cm−2
.
Table 2
Sputtering yields of Co and Si for films of different stoichiometries obtained
from TRIM for 700eV Ar at 67∘
incidence.
Co content (%) 16 27 53
YCo 0.51 0.87 1.78
YSi 2.61 2.31 1.48
B.K. Parida et al. Current Applied Physics 18 (2018) 993–1000
999

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Fig. 8. Comparision of XRD profiles for different fluences, energies, and com-
position with pristine film. The description of the samples is as given in Table 1.
Fluences of ×1.0 1019, ×7.5 1018, ×5.0 1018 and ×2.5 1018 shown in the middle
panel are in units of ions cm−2
for 60, 45, 30 and 15 min of ion irradiation
respectively.
Fig. 9. Phase diagram for pattern formation in the case of CoSi systems at
different energies.
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