We report pattern formation on CoSi alloy. ion beam nanopatterning of binary compound

1. Contents lists available at ScienceDirect
Physica B: Condensed Matter
journal homepage: www.elsevier.com/locate/physb
Influence of obliquely incident primary ion species on patterning of CoSi
binary mixtures: An experimental study
Basanta 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:
Binary mixture
Nanopatterning
Surface
A B S T R A C T
Nanostructure evolution on binary material surfaces using low energy ion beams is being intensively studied
recently owing to the varied forms of structures obtained as well as its immense potential applicability in various
fields of research. The present work aims to investigate ion induced pattern formation at different angles of
incidence on binary systems. We report nanostructure formation during low energy ion erosion of composi-
tionally varied CoxSi1−x (in the ratios of 43:57 and 64:36) binary mixtures with Ar+
(700 eV) and Xe+
(500 eV)
ions respectively at a constant fluence of 7.5 × 1018
ions cm−2
. Pre- and post-irradiated surfaces have been
studied using Atomic Force Microscopy (AFM). Specifically, in these measurements we have demonstrated the
role of incidence angle in surface nanostructuring. For near normal incidence of Ar+
ions, smoothening is ob-
served for the irradiated samples. As we go to higher incidence angles, ripple formation is observed which
transforms into spherical hump-like structures at grazing incidences. In contrast, for Xe+
ion irradiation,
emergence of periodic structures is hardly observed for the range of incidence angles studied. It is observed that
the development of roughness with incident angle is notably prominent for Xe+
than Ar+
ions. Our results could
demonstrate that low energy oblique incidence ions having lighter mass can generate varied nanostructures on
binary materials which could potentially be used for further applications.
1. Introduction
Among different top-down and bottom-up approaches of nano-
pattern development techniques, low energy (0.1–10 keV) ion beam
sputtering (IBS) has the advantage of creating self-organized nano-
patterns (ripples, dots etc.) up to a scale of even a few cm2
in a cost
effective and maskless way within a few minutes of irradiation [1–3].
These can be achieved under different conditions of ion beam para-
meters like ion energy, incident angle, ion flux, ion dose, substrate
temperature, etc. The first theory of temporal surface evolution was
proposed by Bradley and Harper (BH) considering the non-equilibrium
physical processes due to interplay of roughening and smoothing of
surface [4].
Semiconductor surfaces of Si, Ge (monoelemental) and GaSb, InP
(III-V binary) exhibit formation of self-organized nanoripples and dots
when bombarded obliquely and normally by low energy ion beams
respectively [1,3,5,6]. In addition to the studies on III-V binary systems,
metal-semiconductor systems i.e. metal impurity co-deposition or sur-
factant inclusion to surfaces have shown formation of different isotropic
nanostructures [7–9]. Studies have shown that pattern formation is
influenced by metal impurities like Fe, Cu, Ni, etc. which can form si-
licides upon sputtering [10–13]. However, Moon et al. has delineated
that silicide formation is not a sufficient condition for patterning [14].
In addition, ion beam nanopatterning has been rarely studied for bulk
binary mixtures. The above findings have thus triggered various groups
to work extensively on binary systems.
In case of impurity addition via co-deposition or surfactant driven
sputtering, 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. 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. As discussed above and has
been done later in this paper, most experimental studies on III-V
semiconductors accompanied with Ar ion bombardment has been done
by several groups at energies ≳1 keV. Different ion species owing re-
spectively to their different momentum transfer to the elementary or
binary compound systems influence the morphology evolution ac-
cordingly [15,16]. Ziberi et al. has reported the ripple formation on Si
https://doi.org/10.1016/j.physb.2018.05.039
Received 19 April 2018; Received in revised form 22 May 2018; Accepted 28 May 2018
∗
Corresponding author.
E-mail address: sarkar@iitrpr.ac.in (S. Sarkar).
Physica B: Condensed Matter 545 (2018) 34–39
Available online 30 May 2018
0921-4526/ © 2018 Elsevier B.V. All rights reserved.
T

2. for higher mass ions like Ar+
, Kr+
, Xe+
and no such patterns for lower
mass ion Ne+
. Ion mass, which plays a key role for the patterns to be
developed, confirms no regular patterns for Ne+
ion at low-energy
range from their experiments. Ziberi et al. had reported nanostructuring
for bombarding ions having higher masses [15]. For Xe+
ion bom-
bardment, El-Atwani et al. have reported various nanopatterns on
GaSb, GaP, GaAs surfaces at 500 eV [16]. In a similar work by Saeed
[17], patterning on InSb has been studied for higher energies in the
range of 2–5 keV. In contrast, it is difficult to find low energy ion beam
patterning with Xe+
ions for binary materials other than III-V semi-
conductors or for binary mixtures having stoichiometric ratios far from
50: 50 proportion. Moreover, none of the studies show the dependence
of incidence angle on pattern formation. The occurrence of nanos-
tructures as well as smoothening phenomenon have been observed
more specifically for III-V systems (stoichiometric ratio 50: 50) at ob-
lique incidences [18,19]. Such experiments can provide greater insight
into the theoretical understanding of the pattern formation process for
other binary mixtures subjected to ion bombardment which are rather
complex in explaining the morphology changes.
In the present study, we have investigated nanostructuring effect on
low energy ion bombarded CoxSi1−x systems for different incidence
angles and ion species. The samples having different stoichiometric
ratios were irradiated at angles of low (normal) as well as high in-
cidences. We have employed two ion species for our studies - Ar+
(700 eV) and Xe+
(500 eV). Smoothening is observed for the irradiated
samples for normal and near normal incidences of Ar+
ions. These
surfaces deviate from self-affine behaviour at low incidences. As we go
to higher incidence angles, ripple formation is observed which trans-
forms into spherical hump-like structures at grazing incidences. In
contrast, for Xe+
ion irradiation, emergence of periodic structures is
hardly observed for the range of incidence angles studied. The evolu-
tion of surface roughness with incidence angle is higher for Xe+
than
the Ar+
ions.
2. Experimental details
Commercially available Si(100) wafers were cut into 1 × 1 cm2
pieces. They were ultrasonicated in ethanol for 30 min followed by
cleaning with deionised water for 1 min. Cobalt and silicon were co-
sputtered in a confocal magnetron sputtering chamber over these wafer
pieces at a base pressure of 1 × 10−6
Torr and a working pressure of
5 × 10−2
Torr. Argon was used as the sputtering gas with 50 sccm flow
into the chamber and all depositions were done at room temperature.
Binary mixture thin films having different stoichiometries were grown
with thickness such that they will not get completely eroded after ion
irradiation. These films were irradiated by Ar+
and Xe+
ions with
different ion beam parameters as summarised in Table 1. The remnant
of the deposited layer after erosion was confirmed by cross-sectional
Scanning Electron Microscopy (SEM) (Fig. 1 (b)). Cross-section SEM
was performed by putting a scratch on the sample and the removed
portion was observed in the SEM with 5 keV beam energy and magni-
fication of 75000×. For the surface composition, elemental analysis
was done using SEM-Energy Dispersive X-Ray (SEM-EDX) spectroscopy
as shown in Fig. 2. The EDX spectra was acquired for the detected
nanostructures on the surface. The configured SEM system is used to
detect and analyze structures with an equivalent diameter of the pro-
jection area higher than 2 μm. Magnification of 5000× was used. X-ray
spectra was acquired for 10s of live time over a 0–20 kV energy range
where the X-ray collection time was kept in between 180 and 200s.
Sample crystallinities were checked by XRD analysis which showed
amorphous nature for the films. Initial root mean square (rms) rough-
ness of the thin film was ∼5 nm. For irradiation studies, the base
pressure of the ion source chamber was 9 × 10−8
Torr and the working
pressure was 7.5 × 10−5
Torr. Kaufman-type broad beam ion source
with a diameter of 4 cm was used in our experiments with a current
density of 450 μA cm−2
. The corresponding beam flux was 2.8 × 1015
ions cm−2
s−1
. All the experiments were carried out at room tem-
perature and all samples were bombarded without rotation. Ex-situ
AFM (MultiMode 8, Bruker, USA) characterization of the irradiated
samples was performed in tapping mode using a silicon cantilever
having an approximate tip radius of 10 nm. Data analyses of the ac-
quired AFM images were performed using NanoScope Analysis software
(version 1.40). RMS roughness of the sample surfaces were calculated
from the surface height fluctuations h(r) around its mean value ⟨ ⟩h
using the relation ⟨ − ⟨ ⟩ ⟩h r h( ( ) ) r
2 1/2
, where ⟨⟩r denotes average over all r
in a system of size L and r ≤ L.
3. Results and discussions
Fig. 3 depicts the AFM images of SA samples for the ion beam
parameters as tabulated in Table 1. For the samples irradiated at 0° and
Table 1
Summary of the deposited samples and their corresponding sputter parameters.
Sample name Co:Si % Ion type Parameter values Other parameters
SA1 43:57 Ar+
0° Energy = 700 eV
Fluence = 7.5 × 1018
ions cm−2
SA2 30°
SA3 67°
SA4 80°
SX1 64:36 Xe+
0° Energy = 500 eV
Fluence = 7.5 × 1018
ions cm−2
SX2 30°
SX3 50°
SX4 67°
Fig. 1. (a) AFM image of unirradiated (as-grown) film (b) Cross-sectional SEM image of the post irradiated sample. The arrow refers to the remnant part of binary
mixture thin film after erosion.
B.K. Parida et al. Physica B: Condensed Matter 545 (2018) 34–39
35

3. Fig. 2. SEM-EDX mapping of unirradiated CoxSi1−x surfaces.
Fig. 3. 5 μm × 5 μm AFM topographic images of the Ar+
ion (E = 700 eV, fluence = 7.5 × 1018
ions cm−2
) irradiated thin films for different angles of incidence.
Arrows indicate the direction of the ion beam. Inset (c): magnified view of the same surface. Inset (d): 3D view of the same surface.
Fig. 4. (a) Roughness variation for Ar+
(E = 700 eV) ion irradiated Co43Si57 thin films at different angles of incidence (b) PSD curves for the same set of samples.
B.K. Parida et al. Physica B: Condensed Matter 545 (2018) 34–39
36

4. 30° (Fig. 3 (a) and (b)) no distinctive patterns were observed and the
surfaces appeared to be smooth even after irradiation. Samples bom-
barded at 67° and 80° show nanostructures evolving on them. At 67°,
the sample surface reveals well ordered nanoripples oriented perpen-
dicular to the direction of ion beam (inset). The average wavelength of
these ripples are found to be ∼55 nm. At 80° incidence, the surface
shows (Fig. 3(d)) large spherical protrusions on the surface with an
approximate diameter ∼500 nm and an aspect ratio (height:diameter)
∼0.07. A three dimensional AFM image of these structures is presented
as inset in Fig. 3 (d).
Fig. 4 refers to the roughness variation and the power spectral
density (PSD) plots with angles of incidence for the above. Surfaces
irradiated at lower incident angles have lower roughnesses as compared
to the as grown CoSi surface thereby indicating that the surface
smoothens at lower angles of sputtering. Smoothening has been re-
ported for elemental samples like Si, Ge, and III-V semiconductor sur-
faces for low energy ions at incidence angles between 35° and 60°
[18,20,21].
In the case of binary compounds like InP, InAs, InSb, etc., when the
surfaces are eroded by Ar+
or reactive +
N2 ions, the surfaces undergo
smoothening owing to surface gradient dependent sputtering [21]. The
sputtering causes initial surface protrusions to erode downwards
thereby creating trenches around it. With continuous irradiation, the
reflected ions from these protrusions diminishes roughness thereby
creating further depressions on the surface. Subsequent erosion leads to
smoothening of the surface up to a large extent. Moreover, at normal
Fig. 5. 5 μm × 5 μm AFM topographic images of the Xe+
ion (E = 500 eV, fluence = 7.5 × 1018
ions cm−2
) irradiated thin films at different angles of incidence.
Arrows indicate the direction of the ion beam.
Fig. 6. (a) Roughness variation for Xe+
(E = 500 eV) ion irradiated Co64Si36 thin films at different angles of incidence (b) PSD plots for the same set of samples.
B.K. Parida et al. Physica B: Condensed Matter 545 (2018) 34–39
37

5. incidence these local surface regions which are inclined towards the ion
beam gets eroded faster than a smooth surface leading to smoothening
[21,22]. On the other hand, roughening over binary compounds (GaAs)
have been studied and reported by Chowdhury et al. [23]. They had
shown that starting from normal incidence to higher incident angles,
there is an increasing trend in ripple wavelength upto 55°, which again
decreases towards grazing incidence angles. Simultaneously, the
roughness also follows a similar behaviour as the wavelength. For
monoelemental cases of Si and Ge, the surface remains smooth in be-
tween 35° and 60° independent of ion energy and type of ion used [20].
This large scale smoothening instability is owing to surface relaxation
effects of ballistic transport which is an additional directed flux of
surface atoms parallel to the surface, viscous flow and surface diffusion
mechanisms [24,25]. In contrast to the above findings, we observe a
smoothening effect at much lower angles of incidence. Moreover, the
occurrence of hump-like structures on the surface at high incidence
angle is in contrast to what has generally been found by other groups.
Nanodots and needle shaped structures have been reported for low
energy ion irradiated elemental silicon surfaces at grazing incidences
[26] and also for sample rotation during irradiation [27]. PSD plots for
the irradiated samples corroborate the findings obtained from AFM.
The increase in roughness of the samples is signified by the upward shift
of the PSD curves. The saturation of the PSD plots for 0° and 30° at low
k values justify similar roughness for these surfaces at large length
scales. The nature of these curves exhibit a deviation from self-affine
behaviour similar to those found in other surface topographies [28,29].
This could be due to the broad humps on these surfaces effectively
acting as overlayers on a smooth substrate as modelled in the above
studies.
Fig. 5 refers to AFM images of the Xe+
ion bombarded surfaces
(SXs) as described in Table 1. For the measured incidence angles, very
little regularity is observed on the surfaces. For normal incidence, dot
like structures are formed on the surface. At 30°, the surface appears to
be more random without any prominent periodicity. Semi-ellipsoidal
hillocks of different sizes are observed for 50° whose major axes are
aligned perpendicular to the ion beam direction. Albeit these structures
are not of equal height, they have an uni-directional arrangement.
Triangular nano-hillocks were noticed for 67° incidence having lateral
sides of ∼280 nm. These structures do not have a well defined
orientation but the sides of the triangular shapes face the ion beam
direction. To the best of our knowledge, patterning with Xe+
ions have
not been done for binary mixtures. Low energy Xe+
ion irradiation on
Si and Ge has been studied by Ziberi et al. [15]. It was observed that at
normal incidence, Si and Ge exhibited smooth surfaces and dot patterns
respectively. Upon increasing the angle of incidence to 5°, both Si and
Ge exhibited a rippled morphology.
Fig. 6 shows the roughness variation and corresponding PSD plots
for the Xe+
ion irradiated surfaces. A smoothening is observed at 30°
which is followed by a sharp increase in roughness. Coarsening of these
structures could be due to the large momentum transfer from Xe+
ions
(greater than 1.5 times) to constituent elements during bombardment
as compared to Ar+
. PSD plots (Fig. 6(b)) confirm the increase of
roughness with incidence angle and also the saturation of roughness
that is seen for 50° and 67°. The distinctive structures observed for Xe+
ion irradiated surfaces are different from that of Ar+
. It might be due to
heavier projectile mass, which drives the binary mixture surface atoms
to an unstable state thereby resulting in unusual structures [14].
The line profiles of Ar+
and Xe+
ion irradiated surfaces are com-
pared in Fig. 7. For the Ar+
case, the heights of the lines reveal
smoothening of the surface for normal and 30° incidence as compared
to the 67° and 80° incidence case. For 67° and 80° incidence, the profiles
are clearly distinct with large ripple heights and amplitudes. In con-
trast, for the case of Xe+
ions, roughening starts at lower incidence
angles and the coarsening behaviour is faster with incidence angle for
this case. Inspite of the higher jaggedness of surface morphology for the
Xe+
case, the final rms roughness for both Ar+
and Xe+
achieve almost
similar values. For a better visualization of Fig. 7, Table 2 furnishes
maximum and minimum heights of the structures for different ion beam
parameters. This suggests that the small scale roughness of surfaces
bombarded with Xe+
would be higher than that for the Ar+
bombarded
ones.
4. Conclusion
We have reported low energy ion irradiation of Ar+
(E = 700 eV)
and Xe+
(E = 500 eV) ions over compositionally varied CoSi binary
mixtures. Smoothening is discerned for near normal angle of incidence
in case of Ar+
ions. The surfaces deviate from a self-affine behaviour at
these angles as revealed from PSD data. As we go to higher incidence
angles, ripple formation is observed which transforms into spherical
hump-like structures at grazing incidences. These hump-like protru-
sions are statistically distributed having aspect ratio 0.07. In contrast,
for Xe+
ion irradiation, emergence of periodic structures is hardly ob-
served for the range of incidence angles studied. For Xe+
ion irradia-
tion, the rate of increase of roughness with incident angle is higher than
the case of Ar+
ions. Our results could demonstrate that low energy
oblique incidence ions having lighter mass can generate varied nanos-
tructures on binary materials which could potentially be used for fur-
ther applications. Further, these findings can help in validating
Fig. 7. Representative line profiles from AFM images of (a) Ar+
(b) Xe+
ion irradiated samples.
Table 2
Maximum and minimum heights of the structures for different ion bombarded
parameters.
Ion type Ar+
Xe+
Angle (degree) 0° 30° 67° 80° 0° 30° 50° 67°
Max. height
(nm)
2.2 2.2 7.1 26.3 4.3 2.6 11.3 13.4
Min. height
(nm)
−2.2 −2.4 −10.7 −23.3 −4.4 −2.8 −7.1 −8.41
B.K. Parida et al. Physica B: Condensed Matter 545 (2018) 34–39
38

6. theoretical models concerning binary mixture systems subjected to ion
bombardment.
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 acknowledge the experimental assistance of Mr. Ashok
Sharma of FCIPT/IPR, Gandhinagar.
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