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Evaluation of the effect of low fluence ion beam pre-damage with sequential high fluence ion beam exposure on the characteristics of the resultant surface
Surfaces and Interfaces 18 (2020) 100425
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Surfaces and Interfaces journal homepage: www.elsevier.com/locate/surfin
Evaluation of the effect of low fluence ion beam pre-damage with sequential high fluence ion beam exposure on the characteristics of the resultant surface
T
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Vandana Panchala, Tanuj Kumarb, , B. Satpatic, Sunil Ojhad, Shyam Kumara a
Department of Physics, Kurukshetra University, Kurukshetra 136119, India Department of Nanosciences & Materials, Central University of Jammu, Jammu 180011, India c Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700064, India d Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nano-patterning Ion Beam Interface Sputtering
In this work, we present the ripple patterns formation on silicon for different energy predamage and sequential Ar+ ion irradiation. Firstly, the three set of samples namely set-A, set-B and set-C were prepared, which differ in depth locations of amorphous/crystalline (a/c) interface from the free surface, using 100 keV, 200 keV and 300 keV Ar+ oblique (60o) irradiation of silicon (Si) at a fluence of 1 × 1015 ions/cm2. The depth locations were estimated using Rutherford backscattering spectroscopy in channeling mode (RBS-C) and the cross-sectional transmission electron microscopy (X-TEM). The depth locations were found to be ~147 nm, 212 nm, and 302 nm for set-A, set-B and set-C samples using TEM measurement, respectively. Further, the sequential second stage irradiation for ripples growth was performed using 100 keV Ar+ beams for all the sets. After the 2nd stage of irradiation, the dimensions of ripple patterns were estimated using the atomic force microscopy (AFM). Similar ripple patterns formation on all the three sets after sequential irradiation shows the insignificant role of different energy predamage in surface patterning of Si (100) surface. An approach of re-utilization of predamaged silicon for nanopatterns growth is proposed.
1. Introduction In the field of surface nanotechnology, some of the most interesting device proposals require the well-ordered nanostructures over longer distance and control of their shape and size. For this purpose, various physical and chemical routes are available with some advantages and disadvantages. In this direction, the physical route of ion-beam sputtering (IBS) has gained a remarkable attention for fabrication of selforganized nano-patterns over solid surfaces in last couple of decades [1]. In principle IBS is found to be a versatile, fast and low-cost bottomup technique for surface nanopatterning [1]. The mass production of nano-patterns and large process areas makes this technique sometime more preferable over top-down approaches like lithographic techniques, where the multistage masking is needed, or techniques based on scanning probe microscopy (SPM) [2]. For intermediate energies (102–104 eV) of ion beam the surface topography may evolve in the form of ripples, holes or dots etc [3–7]. A great control over the characteristic length and size distribution of these patterns can be done
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easily by varying the ion beam parameters (ion species, energy, fluence, flux and angle of irradiation etc). In 1988s, the first analytical approach towards the origin of nanopatterns on solid surface under ion bombardment was proposed by Bradley and Harper (BH) [8] on the basis of two competing processes: the curvature dependent sputtering induced surface de-stabilization and the diffusion mediated surface stabilization. With the preceding of time, so many theoretical refinements have been proposed in the BH's model based on the advanced experimental observations s.t. the secondary effect of local curvature dependent sputtering, ion beam induced smoothing and hydro-dynamical contribution [9,10]. These modifications have been successful in explaining the nanoscale patterning in correlation with the surface and ion-beam parameters [11]. However, BH's linear and its extended non-linear models could not rectify certain experimental observations [12–14]. Ozaydin et al. [15] pointed out the influence of metallic contaminants in patterning. For 1 keV Ar irradiation at the normal incidence on Si(100) surfaces, the characteristic length of patterns was found to be sensitive to the ‘Mo’
Corresponding author. E-mail address: [email protected] (T. Kumar).
https://doi.org/10.1016/j.surfin.2019.100425 Received 30 July 2019; Received in revised form 21 October 2019; Accepted 16 December 2019 Available online 16 December 2019 2468-0230/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic diagram guiding the mechanism of ripples formation.
atoms incorporation, whereas the surface smoothing was observed in the absence of metal [15]. In recent years, Madi et al. [14] and Norris et al. [16] proposed that the ion impact induced mass redistribution is the prominent factor over the sputtering that's only lead the surface patterning and smoothening for high and low angles, respectively. In all these approaches the prominent role of free surface has been considered in surface patterning. In recent investigations, a hydrodynamic model of surface patterning has been proposed in which the importance of ion beam stress induced solid flow in the near surface damage layer was considered [17,18]. Castro et al. [17,18] found that the surface evolution is an intrinsic property of the dynamics of amorphous surface layer [19]. Recently, kumar et al. [20] observed the important role of amorphous/crystalline (a/c) interface in surface patterning. Consequently, Fig. 1 shows a schematic diagram guiding the possible mechanisms of ripples formation under ion bombardment where the contribution of sputtering, surface diffusion and the hydrodynamical stress induced mass flow in near surface have been presented. In the field of IBS induced nano-patterning, the material “Silicon” is being widely studied due to its large accessibility and availability of extreme flatness of surface. Silicon is a material which has a prominent interest in nanotechnology due to its well-known applications in electronics, opto-electronics, electrochemical devices, sensing and laser devices [21]. Further, the IBS induced nanopatterning on silicon surfaces is found to be reproducible to a large extent for various irradiation conditions. Besides all these, being a mono-elemental material, the role of a/c interface in patterning can be easily investigated in silicon which is hard to understand in compound materials s.t. InP, GaAs etc. where the preferential sputtering of one element over the other can make the system complex. All these reasons motivating our choice for the IBS induced nanopatterning of silicon material. In this work, we studied the different energy induced pre-damage dependent surface patterning of Si (100) for 100 keV Ar+ irradiation using two stage irradiation approach.
Fig. 2. Schematic view of the experimental plan. In first stage of irradiation, three different depth locations of a/c interface are created using Ar+ beam irradiations for the fluence of 1 × 1015 ions/cm2 at an angle of 60° corresponding to three different energies: (a) 100 keV (set-A), (b) 200 keV (set-B) and (c) 300 keV (set-C). In the 2nd stage, for fabrication of ripples, sequential irradiation of these sets of samples is performed using 100 keV Ar+ at an angle of 60°. Schematics of ripples formation on pre-irradiated silicon surface with 100 keV Ar+ ions is shown in Fig. 1 (g).
Cross-sectional transmission electron microscopy (XTEM) was performed using a FEI TF30, S-TWIN microscope operating at 300 kV equipped with a GATAN Orius CCD camera. EDAX SUTW (super ultra thin window) detector unit with 0.13 srad EDS solid angle has been used for compositional study of samples. To measure the damage and Ar incorporation in near surface of silicon, Rutherford Backscattering Spectroscopy (RBS) has been performed in random and channeling mode using ion beam of 2 MeV He2+ (1.7 MV 5SDH-2 Pelletron (Tandem) accelerator) at IUAC, New Delhi. The depth locations of a/c interfaces were estimated from the RBS-C data through the DICADA simulation code [23].
2. Experimental In order to study the role of different depth locations of amorphous/ crystalline (a/c) interface in surface patterning of Si (100), firstly the three different sets of a/c interfaces were prepared using three different energies of 100 keV, 200 keV and 300 keV Ar+ beam at an angle of 60o as shown in schematic view of Fig. 2. The ion beam current was kept stabilized at 5 μA. The ion beam fluence was chosen 1 × 1015 ions/cm2, which is just above the full amorphization of Si in near surface region [22]. Subsequently, 2nd stage irradiation has been performed for all these three sets of samples using 100 keV Ar+ at an angle of 60o w.r.t. surface normal as shown in Fig. 2 (d, e and f). The ion beam current density was chosen 15 µA/cm2 for all irradiations. The fluence has been varied from 3 × 1017 ions/cm2 to 9 × 1017 ions/cm2 for each set of samples. Base pressure of chamber during the irradiation was kept around ~10−7 mbar. Uniform scanning of samples over an area of 10 mm × 10 mm was done using electromagnetic beam scanner. Offline surface feature study was performed through Nano Scope IIIa atomic force microscope (AFM) in tapping mode at IUAC, New Delhi.
3. Results and discussion Fig. 3(a), (b) and (c) shows the XTEM images after the 1st stage of irradiation for 100 keV, 200 keV and 300 keV Ar+ beam corresponding to set-A, set-B and set-C samples, respectively. The ion beam fluence has been fixed at 1 × 1015 ions/cm2. From the corresponding XTEM images, the depth locations of a/c interface from the free surface were estimated using ImageJ software [24] and these were found to be ~147 nm, 212 nm and 302 nm for the set-A, set-B and set-C samples, respectively. Further, the structural idea of crystallinity has been investigated on both sides of a/c-interface through the Selected Area 2
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set of samples in the 2nd stage of irradiation. Scan profile analysis show that the amplitude as well as wavelength of ripple patterns are of the same order of magnitude for all the three set of samples at the corresponding fluences of irradiation. A plot of the RMS roughness and average wavelength of ripples are shown in Fig. 5 (a) and (b), respectively. Fig. 6 shows the X-TEM images of subsequently 2nd stage irradiated sample of set-A at the fluence of 3 × 1018 ions/cm2. Fig. 6(a) confirms the formation of ripples at the free surface as well as at the a/c interface which are also identical in nature. Non-uniformity in the thickness of near surface amorphous layer can be visualized in the X-TEM image. A magnified image in fig 6(b) shows the average thickness of amorphous layer is about ~240 nm in the front slope and ~ 73–84 nm in the rear slope of ripples. This kind of non-uniformity in the amorphous layer thicknesses is well known [25,26]. Also, there is an appearance of Ar bubbles in the front slope of ripples which are missing in the rear slope. A magnified image of bubbles in Fig. 6 (c) shows the size distribution of selected Ar bubbles, which is found to be decreasing from ~19 nm to few nano-meters towards the interface. The argon bubble distribution is more appreciable towards the free surface. Further, a magnification of a/c interface is shown in Fig. 6(d) which shows that the interface is not smooth at the nanoscale level. Rutherford Backscattering Spectroscopy in channeling mode (RBSC) has also been performed on the number of samples of all three sets as shown in Fig. 7. Fig 7 (a) shows the RBS-random and RBS-C spectra of the pristine Si sample as well as the RBS-C data for the set-A samples at the fluences of 1 × 1017 ions/cm2 and 3 × 1018 ions/cm2 after the 2nd stage of irradiation. To convert the channel number into depth in micrometer, a depth conversion has been done using RBS-random and channeling spectra. The depth conversion has also been shown in the inset of Fig. 7 (a). This unit conversion has been performed from the software DECADA [23] with consideration of energy loss of 2MeV αparticle in the silicon. This depth conversion has been utilized to measure the average values of depth locations of a/c interfaces in terms of micro-meter. The Damage Profile (DP) for the 1st stage and the subsequently 2nd stage irradiated samples for all the three sets; Set-A, Set-B and Set-C, are shown in Fig. 7 (b), (c) and (d), respectively. The average value of the depth locations of a/c interface is actually the FWHM of the damage profile and which were estimated for all the three set of samples as shown in Table 1. From the table, it could be seen that
Fig. 3. Cross-sectional transmission electron microscopy (X-TEM) images of pre-damaged samples corresponding to the fluence of 1 × 1015 ions/cm2: (a) set-A, (b) set-B and (c) set-C. x-TEM shows the three different depth locations (thickness of amorphous layer) of a/c interface s.t 147 nm, 212 nm and 302 nm for the set-A, set-B and set-C, respectively. Selected Area diffraction (SAD) pattern for set-A sample: (d) bulk crystalline, and near surface amorphous layer.
Diffraction (SAED) pattern. In Fig 3 (d), the dot pattern confirms the pure single crystalline nature of Si (100) below the a/c interface in the target and the full amorphization of near surface amorphous layer for the set-A sample, which has also been observed for all the used three set of samples. Fig. 4 (a,b, and c) shows the AFM images of all the three set of samples after irradiation. Each AFM image has been taken in the size of 20 µm × 20 µm. The AFM images in left column of Fig. 4 corresponds to the 1st stage irradiation for 100 keV, 200 keV and 300 keV Ar+ beam for the set-A, set-B and set-C samples, respectively, at the fluence of 1 × 1015 ions/cm2. After the 1st stage of irradiation, the average values of RMS roughness were found to be 1.3 nm, 0.9 and 2.3 nm for set-A, set-B and set-C samples, respectively. Subsequently, the 2nd stage irradiation of these samples has been carried out at 100 keV Ar+ beam at 60° for fluence range 1 × 1017 ions/cm2 to 3 × 1018 ions/cm2. The corresponding AFM images are shown in the right column of Fig. 4. From the AFM images it could be seen that the ripples formation of appreciable amplitude and wavelength were observed for all the three
Fig. 4. Surface topographic images of Si surfaces after 1st stage of irradiation at 1 × 1015 ions/cm2 and the sequential stage irradiation at the fluences 1 × 1017 ions/ cm2, 9 × 1017 ions/cm2 and 3 × 1018 ions/cm2: (a) set-A; (b) set-B and (c) set-C. For each AFM images z-axis scale is given in ‘nm’. 3
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Fig. 5. (a and b) Variation in the RMS-Roughness values and the wavelength of ripples for all the set of samples as a function of ion beam fluence after 2nd stage of irradiation.
fluence 3 × 1018 ions/cm2 as compared to the fluence 1 × 1015 ions/ cm2. This damage profile has been correlated with the real damage observed in the corresponding XTEM images as shown in 8 (b) and (c). It could be clearly seen that a steep Damage profile for fluence 1 × 1015 ions/cm2 shows the existence of uniform thickness of amorphous layer as could be seen in the corresponding X-TEM image of Fig. 8 (b). However, for the fluence 3 × 1018 ions/cm2, the observed damage recovery in the peak region corresponds to the contribution coming in RBS channeling from the rear slope of ripples while the enhancement in the damage profile in the tail region show the dominant contribution coming from the thicker (front) slope of ripple as shown in Fig 8 (c). It could be emphasized here that the damage recovery in the ripples formation case is not real rather a structural effect of ripples. This effect has also been observed for all the three sets viz. Set-A, Set-B and Set-C, respectively, with increase in fluence. To confirm the sample composition and the concentration distribution, the energy dispersive x-ray (EDX) study and high angle annular dark field (HAADF) measurements has been carried out along with the XTEM characterization on the selected samples. Fig. 9 (a) and (b) shows the EDX, HAADF and the concentration variation of silicon and argon across the amorphous layer for the set-A and set-C samples, respectively. The counts are collected for the K-line of Si and Ar along the orange line shown in the HAADF image of samples. For both the sets, the ion irradiation at 1 × 1015 ions/cm2 shows the presence of Silicon and Ar in the near surface damage region. The concentration gradient of silicon and Ar in the region of interest of near surface has been measured separately which shows that silicon concentration is more prominent towards the a/c interface. For set-A sample, the Ar is found to be distributed in the near surface amorphous layer upto the thickness of ~150 nm, approximately. While for the set-C sample, the Ar distribution is more broadened as compared to set-A sample up to thickness of ~300 nm. This Si and Ar distribution could be understood considering the sputtering of Si and Ar incorporation in the near surface damage layer. The broadened distribution of Ar in set-C as compared to set-A sample is due to higher penetration range for 300 keV as compared to 100 keV Ar+. To physically explain the observed similar patterns after 2nd stage of irradiation for all the three sets despite of having different depth locations of a/c interfaces could be understood by considering the sputtering phenomenon. Due to higher sputtering of Si for 100 keV Ar+ irradiation at oblique angle, it is expected that for the set-A samples in which the sequential 2nd stage irradiated beam directly reaching at the interface leads the surface etching and down movement of a/c interface. While, a prior etching of free surface is expected for reaching the beam at the a/c interface of set-B and set-C samples. However, this duration is expected to be much smaller (or a small fluence is needed) as compared to the time zone of ripples formation which occurred
Fig. 6. X-TEM images of set-A sample after 2nd stage of irradiation at the fluence 3 × 1018 ions/cm2; (a) formation of ripples at the surface and a/c interface; (b) amorphous layer thickness ~240 nm in the front slope and 73–84 nm on rear slope; (c) Magnified amorphous layer shows the Ar bubbles formation of variable sizes; (d) Magnified image of a/c interface shows the non-uniformity of the interface.
the average values of the thicknesses of amorphous layers for set-A, setB, and Set-C samples after first stage of irradiation are found to be ~147 nm, ~215 nm and ~298 nm which are very consistent with the XTEM observations as discussed earlier. For set-A samples, no remarkable change has been observed in the thicknesses of amorphous layers with irradiation, as the beam energy used for both stage of irradiations are same to 100 keV Ar+. Whereas, for set-B and Set-C, the subsequent 2nd stage irradiation leads to a remarkable decrease in thickness from ~215 nm to ~153 nm for set-B and ~298 nm to ~210 nm for set-C samples at fluence of 1 × 1017 ions/cm2. Further a reduction in the values of thickness was observed upto ~128 nm, ~128 nm and ~122 nm for the set-A, set-B and set-C samples, respectively, at higher irradiation fluence of 3 × 1018 ions/cm2. This reduction in thickness at higher fluences is not real rather a feature of averaging of thicknesses of the front and rear slope of ripples. A reduction in thickness of amorphous layer on the rear side at higher fluences as well as the angle contribution with respect to the RBS depth analysis is making a sloppy behavior of damage profile for dominant ripple surface as shown in Fig. 8. Fig. 8 (a) shows the simulated disorder concentration (damage) profile extracted from software DICADA23 for 100 keV Ar+ irradiated samples corresponding to the fluence of 1 × 1015 and 3 × 1018 ions/cm2 (for set A samples). Fig. 8 (a) shows that in these samples, damage recovery has been observed in the peak region while the damage enhancement on the tail regions for the 4
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Fig. 7. (a) RBS-C data for set-A samples at selected fluences. In inset, depth conversion from channel number to nanometer. Damage profile (extracted from DICADA) as a function of ion beam fluence shown in; (b) set-A; (c) set-B and (d) set-C samples, respectively. The average values of the thicknesses of amorphous layer are calculated from the FWHM of the damage graphs.
bombardment. On the other hand, Kumar et al. [20] observed the sensitivity of a/c interface in surface patterning of silicon for the preamorphization with fluence of 5 × 1016 ions/cm2. In a similar study, the fractal behavior of sequential irradiation approach has also been investigated and which was found to be sensitive to the position of a/c interface [27]. On the other side, in one of the report, Barrado et al. [28] investigated the role of predamage of silicon in surface patterning and the results were found to be insensitive to pre-damage [20,29]. In the present study, it is observed that a pre-damage of Si with low fluence of Ar+ does not influence the ripples growth in sequential patterns formation. Here we propose the re-utilization of pre-damage/amorphous silicon samples for the growth of nano ripple patterns.
Table 1 Average thicknesses of amorphous layers for the three set of samples viz. set-A, set-B and set-C after the 1st and 2nd stage of irradiations at different fluences estimated from Rutherford Backscattering spectroscopy in channeling mode. Average thickness of amorphous layer (nm) from RBS-C 1st stage 2nd stage Fluence (ions/ cm2) Set-A Set-B Set-C
1 × 1015
1 × 1017
9 × 1017
3 × 1018
~147 ~215 ~298
~144 ~153 ~210
~153 ~157 -
~128 ~128 ~122
comparatively at very high fluences. So, it is found that the initial predamaged different depth location of a/c interface for the low fluence value of 1 × 1015 ions/cm2 does not affect the surface patterning in sequential stage of ripple formation. In literature, Castro et al. [17,18] proposed the role of stress induced solid flow inside the amorphous layer in surface rippling. It was proposed that the fast flow in the front slope in the comparison of slow flow in the rear slope results in the formation of ripples on the free surface as well as the a/c interface of the solid targets under ion
4. Conclusion In conclusion, the role of pre-damage silicon in surface pattering has been investigated using the two stage irradiation approach. First stage irradiation has been used in creating different positions of a/c interface and the sequential 2nd stage irradiation has been used to grow the patterns. Three different depth locations of a/c interfaces were grown for set-A, set-B and set-C samples, which were confirmed using the Fig. 8. A correlation between RBS-C graphs of selected set-A samples and the corresponding x-TEM images. RBS-C damage profile for the Ist stage irradiated set-A sample at 1 × 1015 ions/cm2 and the sequentially 2nd stage irradiated sample for the fluence 3 × 1018 ions/cm2. A steep slope of damage profile in RBS-C corresponds to the uniform a/c interface for the 1st stage irradiated set-A sample as shown in X-TEM image of (b). Damage recovery in the peak region and enhancement in damage in the tail region are correlated with the variation of thicknesses of amorphous layers of front and rear slope of ripples as correlated with the corresponding x-TEM image of ripple as shown in Fig (d).
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Fig. 9. EDX and HAADF images of pre-damaged (after 1st stage of irradiation) samples: (a) set-A and (b) set-C. The corresponding concentration variations of Si and Ar across the amorphous layer are also plotted.
References
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Declaration of Competing Interest Authors don't have conflict of interest with any other work.
Acknowledgments One of the authors (Vandana) is thankful to Dr. Indra Sulania for the help received in AFM characterization. Author (TK) also acknowledge that this work is partially supported by Science Engineering and Research Board (SERB), New Delhi, through Fast Track-Young Scientist Award (FT-YSA), File no. YSS/2015/001982 and University Grant Commission, Delhi (Inter University Accelerator Centre, Project no. UFR-59311).
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Surfaces and Interfaces journal homepage: www.elsevier.com/locate/surfin
Evaluation of the effect of low fluence ion beam pre-damage with sequential high fluence ion beam exposure on the characteristics of the resultant surface
T
⁎
Vandana Panchala, Tanuj Kumarb, , B. Satpatic, Sunil Ojhad, Shyam Kumara a
Department of Physics, Kurukshetra University, Kurukshetra 136119, India Department of Nanosciences & Materials, Central University of Jammu, Jammu 180011, India c Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Kolkata 700064, India d Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nano-patterning Ion Beam Interface Sputtering
In this work, we present the ripple patterns formation on silicon for different energy predamage and sequential Ar+ ion irradiation. Firstly, the three set of samples namely set-A, set-B and set-C were prepared, which differ in depth locations of amorphous/crystalline (a/c) interface from the free surface, using 100 keV, 200 keV and 300 keV Ar+ oblique (60o) irradiation of silicon (Si) at a fluence of 1 × 1015 ions/cm2. The depth locations were estimated using Rutherford backscattering spectroscopy in channeling mode (RBS-C) and the cross-sectional transmission electron microscopy (X-TEM). The depth locations were found to be ~147 nm, 212 nm, and 302 nm for set-A, set-B and set-C samples using TEM measurement, respectively. Further, the sequential second stage irradiation for ripples growth was performed using 100 keV Ar+ beams for all the sets. After the 2nd stage of irradiation, the dimensions of ripple patterns were estimated using the atomic force microscopy (AFM). Similar ripple patterns formation on all the three sets after sequential irradiation shows the insignificant role of different energy predamage in surface patterning of Si (100) surface. An approach of re-utilization of predamaged silicon for nanopatterns growth is proposed.
1. Introduction In the field of surface nanotechnology, some of the most interesting device proposals require the well-ordered nanostructures over longer distance and control of their shape and size. For this purpose, various physical and chemical routes are available with some advantages and disadvantages. In this direction, the physical route of ion-beam sputtering (IBS) has gained a remarkable attention for fabrication of selforganized nano-patterns over solid surfaces in last couple of decades [1]. In principle IBS is found to be a versatile, fast and low-cost bottomup technique for surface nanopatterning [1]. The mass production of nano-patterns and large process areas makes this technique sometime more preferable over top-down approaches like lithographic techniques, where the multistage masking is needed, or techniques based on scanning probe microscopy (SPM) [2]. For intermediate energies (102–104 eV) of ion beam the surface topography may evolve in the form of ripples, holes or dots etc [3–7]. A great control over the characteristic length and size distribution of these patterns can be done
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easily by varying the ion beam parameters (ion species, energy, fluence, flux and angle of irradiation etc). In 1988s, the first analytical approach towards the origin of nanopatterns on solid surface under ion bombardment was proposed by Bradley and Harper (BH) [8] on the basis of two competing processes: the curvature dependent sputtering induced surface de-stabilization and the diffusion mediated surface stabilization. With the preceding of time, so many theoretical refinements have been proposed in the BH's model based on the advanced experimental observations s.t. the secondary effect of local curvature dependent sputtering, ion beam induced smoothing and hydro-dynamical contribution [9,10]. These modifications have been successful in explaining the nanoscale patterning in correlation with the surface and ion-beam parameters [11]. However, BH's linear and its extended non-linear models could not rectify certain experimental observations [12–14]. Ozaydin et al. [15] pointed out the influence of metallic contaminants in patterning. For 1 keV Ar irradiation at the normal incidence on Si(100) surfaces, the characteristic length of patterns was found to be sensitive to the ‘Mo’
Corresponding author. E-mail address: [email protected] (T. Kumar).
https://doi.org/10.1016/j.surfin.2019.100425 Received 30 July 2019; Received in revised form 21 October 2019; Accepted 16 December 2019 Available online 16 December 2019 2468-0230/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic diagram guiding the mechanism of ripples formation.
atoms incorporation, whereas the surface smoothing was observed in the absence of metal [15]. In recent years, Madi et al. [14] and Norris et al. [16] proposed that the ion impact induced mass redistribution is the prominent factor over the sputtering that's only lead the surface patterning and smoothening for high and low angles, respectively. In all these approaches the prominent role of free surface has been considered in surface patterning. In recent investigations, a hydrodynamic model of surface patterning has been proposed in which the importance of ion beam stress induced solid flow in the near surface damage layer was considered [17,18]. Castro et al. [17,18] found that the surface evolution is an intrinsic property of the dynamics of amorphous surface layer [19]. Recently, kumar et al. [20] observed the important role of amorphous/crystalline (a/c) interface in surface patterning. Consequently, Fig. 1 shows a schematic diagram guiding the possible mechanisms of ripples formation under ion bombardment where the contribution of sputtering, surface diffusion and the hydrodynamical stress induced mass flow in near surface have been presented. In the field of IBS induced nano-patterning, the material “Silicon” is being widely studied due to its large accessibility and availability of extreme flatness of surface. Silicon is a material which has a prominent interest in nanotechnology due to its well-known applications in electronics, opto-electronics, electrochemical devices, sensing and laser devices [21]. Further, the IBS induced nanopatterning on silicon surfaces is found to be reproducible to a large extent for various irradiation conditions. Besides all these, being a mono-elemental material, the role of a/c interface in patterning can be easily investigated in silicon which is hard to understand in compound materials s.t. InP, GaAs etc. where the preferential sputtering of one element over the other can make the system complex. All these reasons motivating our choice for the IBS induced nanopatterning of silicon material. In this work, we studied the different energy induced pre-damage dependent surface patterning of Si (100) for 100 keV Ar+ irradiation using two stage irradiation approach.
Fig. 2. Schematic view of the experimental plan. In first stage of irradiation, three different depth locations of a/c interface are created using Ar+ beam irradiations for the fluence of 1 × 1015 ions/cm2 at an angle of 60° corresponding to three different energies: (a) 100 keV (set-A), (b) 200 keV (set-B) and (c) 300 keV (set-C). In the 2nd stage, for fabrication of ripples, sequential irradiation of these sets of samples is performed using 100 keV Ar+ at an angle of 60°. Schematics of ripples formation on pre-irradiated silicon surface with 100 keV Ar+ ions is shown in Fig. 1 (g).
Cross-sectional transmission electron microscopy (XTEM) was performed using a FEI TF30, S-TWIN microscope operating at 300 kV equipped with a GATAN Orius CCD camera. EDAX SUTW (super ultra thin window) detector unit with 0.13 srad EDS solid angle has been used for compositional study of samples. To measure the damage and Ar incorporation in near surface of silicon, Rutherford Backscattering Spectroscopy (RBS) has been performed in random and channeling mode using ion beam of 2 MeV He2+ (1.7 MV 5SDH-2 Pelletron (Tandem) accelerator) at IUAC, New Delhi. The depth locations of a/c interfaces were estimated from the RBS-C data through the DICADA simulation code [23].
2. Experimental In order to study the role of different depth locations of amorphous/ crystalline (a/c) interface in surface patterning of Si (100), firstly the three different sets of a/c interfaces were prepared using three different energies of 100 keV, 200 keV and 300 keV Ar+ beam at an angle of 60o as shown in schematic view of Fig. 2. The ion beam current was kept stabilized at 5 μA. The ion beam fluence was chosen 1 × 1015 ions/cm2, which is just above the full amorphization of Si in near surface region [22]. Subsequently, 2nd stage irradiation has been performed for all these three sets of samples using 100 keV Ar+ at an angle of 60o w.r.t. surface normal as shown in Fig. 2 (d, e and f). The ion beam current density was chosen 15 µA/cm2 for all irradiations. The fluence has been varied from 3 × 1017 ions/cm2 to 9 × 1017 ions/cm2 for each set of samples. Base pressure of chamber during the irradiation was kept around ~10−7 mbar. Uniform scanning of samples over an area of 10 mm × 10 mm was done using electromagnetic beam scanner. Offline surface feature study was performed through Nano Scope IIIa atomic force microscope (AFM) in tapping mode at IUAC, New Delhi.
3. Results and discussion Fig. 3(a), (b) and (c) shows the XTEM images after the 1st stage of irradiation for 100 keV, 200 keV and 300 keV Ar+ beam corresponding to set-A, set-B and set-C samples, respectively. The ion beam fluence has been fixed at 1 × 1015 ions/cm2. From the corresponding XTEM images, the depth locations of a/c interface from the free surface were estimated using ImageJ software [24] and these were found to be ~147 nm, 212 nm and 302 nm for the set-A, set-B and set-C samples, respectively. Further, the structural idea of crystallinity has been investigated on both sides of a/c-interface through the Selected Area 2
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set of samples in the 2nd stage of irradiation. Scan profile analysis show that the amplitude as well as wavelength of ripple patterns are of the same order of magnitude for all the three set of samples at the corresponding fluences of irradiation. A plot of the RMS roughness and average wavelength of ripples are shown in Fig. 5 (a) and (b), respectively. Fig. 6 shows the X-TEM images of subsequently 2nd stage irradiated sample of set-A at the fluence of 3 × 1018 ions/cm2. Fig. 6(a) confirms the formation of ripples at the free surface as well as at the a/c interface which are also identical in nature. Non-uniformity in the thickness of near surface amorphous layer can be visualized in the X-TEM image. A magnified image in fig 6(b) shows the average thickness of amorphous layer is about ~240 nm in the front slope and ~ 73–84 nm in the rear slope of ripples. This kind of non-uniformity in the amorphous layer thicknesses is well known [25,26]. Also, there is an appearance of Ar bubbles in the front slope of ripples which are missing in the rear slope. A magnified image of bubbles in Fig. 6 (c) shows the size distribution of selected Ar bubbles, which is found to be decreasing from ~19 nm to few nano-meters towards the interface. The argon bubble distribution is more appreciable towards the free surface. Further, a magnification of a/c interface is shown in Fig. 6(d) which shows that the interface is not smooth at the nanoscale level. Rutherford Backscattering Spectroscopy in channeling mode (RBSC) has also been performed on the number of samples of all three sets as shown in Fig. 7. Fig 7 (a) shows the RBS-random and RBS-C spectra of the pristine Si sample as well as the RBS-C data for the set-A samples at the fluences of 1 × 1017 ions/cm2 and 3 × 1018 ions/cm2 after the 2nd stage of irradiation. To convert the channel number into depth in micrometer, a depth conversion has been done using RBS-random and channeling spectra. The depth conversion has also been shown in the inset of Fig. 7 (a). This unit conversion has been performed from the software DECADA [23] with consideration of energy loss of 2MeV αparticle in the silicon. This depth conversion has been utilized to measure the average values of depth locations of a/c interfaces in terms of micro-meter. The Damage Profile (DP) for the 1st stage and the subsequently 2nd stage irradiated samples for all the three sets; Set-A, Set-B and Set-C, are shown in Fig. 7 (b), (c) and (d), respectively. The average value of the depth locations of a/c interface is actually the FWHM of the damage profile and which were estimated for all the three set of samples as shown in Table 1. From the table, it could be seen that
Fig. 3. Cross-sectional transmission electron microscopy (X-TEM) images of pre-damaged samples corresponding to the fluence of 1 × 1015 ions/cm2: (a) set-A, (b) set-B and (c) set-C. x-TEM shows the three different depth locations (thickness of amorphous layer) of a/c interface s.t 147 nm, 212 nm and 302 nm for the set-A, set-B and set-C, respectively. Selected Area diffraction (SAD) pattern for set-A sample: (d) bulk crystalline, and near surface amorphous layer.
Diffraction (SAED) pattern. In Fig 3 (d), the dot pattern confirms the pure single crystalline nature of Si (100) below the a/c interface in the target and the full amorphization of near surface amorphous layer for the set-A sample, which has also been observed for all the used three set of samples. Fig. 4 (a,b, and c) shows the AFM images of all the three set of samples after irradiation. Each AFM image has been taken in the size of 20 µm × 20 µm. The AFM images in left column of Fig. 4 corresponds to the 1st stage irradiation for 100 keV, 200 keV and 300 keV Ar+ beam for the set-A, set-B and set-C samples, respectively, at the fluence of 1 × 1015 ions/cm2. After the 1st stage of irradiation, the average values of RMS roughness were found to be 1.3 nm, 0.9 and 2.3 nm for set-A, set-B and set-C samples, respectively. Subsequently, the 2nd stage irradiation of these samples has been carried out at 100 keV Ar+ beam at 60° for fluence range 1 × 1017 ions/cm2 to 3 × 1018 ions/cm2. The corresponding AFM images are shown in the right column of Fig. 4. From the AFM images it could be seen that the ripples formation of appreciable amplitude and wavelength were observed for all the three
Fig. 4. Surface topographic images of Si surfaces after 1st stage of irradiation at 1 × 1015 ions/cm2 and the sequential stage irradiation at the fluences 1 × 1017 ions/ cm2, 9 × 1017 ions/cm2 and 3 × 1018 ions/cm2: (a) set-A; (b) set-B and (c) set-C. For each AFM images z-axis scale is given in ‘nm’. 3
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Fig. 5. (a and b) Variation in the RMS-Roughness values and the wavelength of ripples for all the set of samples as a function of ion beam fluence after 2nd stage of irradiation.
fluence 3 × 1018 ions/cm2 as compared to the fluence 1 × 1015 ions/ cm2. This damage profile has been correlated with the real damage observed in the corresponding XTEM images as shown in 8 (b) and (c). It could be clearly seen that a steep Damage profile for fluence 1 × 1015 ions/cm2 shows the existence of uniform thickness of amorphous layer as could be seen in the corresponding X-TEM image of Fig. 8 (b). However, for the fluence 3 × 1018 ions/cm2, the observed damage recovery in the peak region corresponds to the contribution coming in RBS channeling from the rear slope of ripples while the enhancement in the damage profile in the tail region show the dominant contribution coming from the thicker (front) slope of ripple as shown in Fig 8 (c). It could be emphasized here that the damage recovery in the ripples formation case is not real rather a structural effect of ripples. This effect has also been observed for all the three sets viz. Set-A, Set-B and Set-C, respectively, with increase in fluence. To confirm the sample composition and the concentration distribution, the energy dispersive x-ray (EDX) study and high angle annular dark field (HAADF) measurements has been carried out along with the XTEM characterization on the selected samples. Fig. 9 (a) and (b) shows the EDX, HAADF and the concentration variation of silicon and argon across the amorphous layer for the set-A and set-C samples, respectively. The counts are collected for the K-line of Si and Ar along the orange line shown in the HAADF image of samples. For both the sets, the ion irradiation at 1 × 1015 ions/cm2 shows the presence of Silicon and Ar in the near surface damage region. The concentration gradient of silicon and Ar in the region of interest of near surface has been measured separately which shows that silicon concentration is more prominent towards the a/c interface. For set-A sample, the Ar is found to be distributed in the near surface amorphous layer upto the thickness of ~150 nm, approximately. While for the set-C sample, the Ar distribution is more broadened as compared to set-A sample up to thickness of ~300 nm. This Si and Ar distribution could be understood considering the sputtering of Si and Ar incorporation in the near surface damage layer. The broadened distribution of Ar in set-C as compared to set-A sample is due to higher penetration range for 300 keV as compared to 100 keV Ar+. To physically explain the observed similar patterns after 2nd stage of irradiation for all the three sets despite of having different depth locations of a/c interfaces could be understood by considering the sputtering phenomenon. Due to higher sputtering of Si for 100 keV Ar+ irradiation at oblique angle, it is expected that for the set-A samples in which the sequential 2nd stage irradiated beam directly reaching at the interface leads the surface etching and down movement of a/c interface. While, a prior etching of free surface is expected for reaching the beam at the a/c interface of set-B and set-C samples. However, this duration is expected to be much smaller (or a small fluence is needed) as compared to the time zone of ripples formation which occurred
Fig. 6. X-TEM images of set-A sample after 2nd stage of irradiation at the fluence 3 × 1018 ions/cm2; (a) formation of ripples at the surface and a/c interface; (b) amorphous layer thickness ~240 nm in the front slope and 73–84 nm on rear slope; (c) Magnified amorphous layer shows the Ar bubbles formation of variable sizes; (d) Magnified image of a/c interface shows the non-uniformity of the interface.
the average values of the thicknesses of amorphous layers for set-A, setB, and Set-C samples after first stage of irradiation are found to be ~147 nm, ~215 nm and ~298 nm which are very consistent with the XTEM observations as discussed earlier. For set-A samples, no remarkable change has been observed in the thicknesses of amorphous layers with irradiation, as the beam energy used for both stage of irradiations are same to 100 keV Ar+. Whereas, for set-B and Set-C, the subsequent 2nd stage irradiation leads to a remarkable decrease in thickness from ~215 nm to ~153 nm for set-B and ~298 nm to ~210 nm for set-C samples at fluence of 1 × 1017 ions/cm2. Further a reduction in the values of thickness was observed upto ~128 nm, ~128 nm and ~122 nm for the set-A, set-B and set-C samples, respectively, at higher irradiation fluence of 3 × 1018 ions/cm2. This reduction in thickness at higher fluences is not real rather a feature of averaging of thicknesses of the front and rear slope of ripples. A reduction in thickness of amorphous layer on the rear side at higher fluences as well as the angle contribution with respect to the RBS depth analysis is making a sloppy behavior of damage profile for dominant ripple surface as shown in Fig. 8. Fig. 8 (a) shows the simulated disorder concentration (damage) profile extracted from software DICADA23 for 100 keV Ar+ irradiated samples corresponding to the fluence of 1 × 1015 and 3 × 1018 ions/cm2 (for set A samples). Fig. 8 (a) shows that in these samples, damage recovery has been observed in the peak region while the damage enhancement on the tail regions for the 4
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Fig. 7. (a) RBS-C data for set-A samples at selected fluences. In inset, depth conversion from channel number to nanometer. Damage profile (extracted from DICADA) as a function of ion beam fluence shown in; (b) set-A; (c) set-B and (d) set-C samples, respectively. The average values of the thicknesses of amorphous layer are calculated from the FWHM of the damage graphs.
bombardment. On the other hand, Kumar et al. [20] observed the sensitivity of a/c interface in surface patterning of silicon for the preamorphization with fluence of 5 × 1016 ions/cm2. In a similar study, the fractal behavior of sequential irradiation approach has also been investigated and which was found to be sensitive to the position of a/c interface [27]. On the other side, in one of the report, Barrado et al. [28] investigated the role of predamage of silicon in surface patterning and the results were found to be insensitive to pre-damage [20,29]. In the present study, it is observed that a pre-damage of Si with low fluence of Ar+ does not influence the ripples growth in sequential patterns formation. Here we propose the re-utilization of pre-damage/amorphous silicon samples for the growth of nano ripple patterns.
Table 1 Average thicknesses of amorphous layers for the three set of samples viz. set-A, set-B and set-C after the 1st and 2nd stage of irradiations at different fluences estimated from Rutherford Backscattering spectroscopy in channeling mode. Average thickness of amorphous layer (nm) from RBS-C 1st stage 2nd stage Fluence (ions/ cm2) Set-A Set-B Set-C
1 × 1015
1 × 1017
9 × 1017
3 × 1018
~147 ~215 ~298
~144 ~153 ~210
~153 ~157 -
~128 ~128 ~122
comparatively at very high fluences. So, it is found that the initial predamaged different depth location of a/c interface for the low fluence value of 1 × 1015 ions/cm2 does not affect the surface patterning in sequential stage of ripple formation. In literature, Castro et al. [17,18] proposed the role of stress induced solid flow inside the amorphous layer in surface rippling. It was proposed that the fast flow in the front slope in the comparison of slow flow in the rear slope results in the formation of ripples on the free surface as well as the a/c interface of the solid targets under ion
4. Conclusion In conclusion, the role of pre-damage silicon in surface pattering has been investigated using the two stage irradiation approach. First stage irradiation has been used in creating different positions of a/c interface and the sequential 2nd stage irradiation has been used to grow the patterns. Three different depth locations of a/c interfaces were grown for set-A, set-B and set-C samples, which were confirmed using the Fig. 8. A correlation between RBS-C graphs of selected set-A samples and the corresponding x-TEM images. RBS-C damage profile for the Ist stage irradiated set-A sample at 1 × 1015 ions/cm2 and the sequentially 2nd stage irradiated sample for the fluence 3 × 1018 ions/cm2. A steep slope of damage profile in RBS-C corresponds to the uniform a/c interface for the 1st stage irradiated set-A sample as shown in X-TEM image of (b). Damage recovery in the peak region and enhancement in damage in the tail region are correlated with the variation of thicknesses of amorphous layers of front and rear slope of ripples as correlated with the corresponding x-TEM image of ripple as shown in Fig (d).
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Fig. 9. EDX and HAADF images of pre-damaged (after 1st stage of irradiation) samples: (a) set-A and (b) set-C. The corresponding concentration variations of Si and Ar across the amorphous layer are also plotted.
References
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Declaration of Competing Interest Authors don't have conflict of interest with any other work.
Acknowledgments One of the authors (Vandana) is thankful to Dr. Indra Sulania for the help received in AFM characterization. Author (TK) also acknowledge that this work is partially supported by Science Engineering and Research Board (SERB), New Delhi, through Fast Track-Young Scientist Award (FT-YSA), File no. YSS/2015/001982 and University Grant Commission, Delhi (Inter University Accelerator Centre, Project no. UFR-59311).
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