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Characterization of ion beam induced nanostructures
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 244 (2006) 45–51 www.elsevier.com/locate/nimb
Characterization of ion beam induced nanostructures J. Ghatak a, B. Satpati a, M. Umananda a, D. Kabiraj b, T. Som a, B.N. Dev a, K. Akimoto c, K. Ito c, T. Emoto d, P.V. Satyam a,* a Institute of Physics, Sachivalaya Marg, Bhubaneswar 751 005, India Nuclear Science Center, Aruna Asaf Ali Marg, New Delhi 110 067, India Department of Quantum Engineering, Nagoya University, Nagoya 464-8603, Japan d Toyota National College of Technology, 2-1, Toyota, Aichi 471-8525, Japan b
c
Available online 9 January 2006
Abstract Tailoring of nanostructures with energetic ion beams has become an active area of research leading to the fundamental understanding of ion–solid interactions at nanoscale regime and with possible applications in the near future. Rutherford backscattering spectrometry (RBS), high resolution transmission electron microscopy (HRTEM) and asymmetric X-ray Bragg-rocking curve experimental methods have been used to characterize ion-induced effects in nanostructures. The possibility of surface and sub-surface/interface alloying at nano-scale regime, ion-beam induced embedding, crater formation, sputtering yield variations for systems with isolated nanoislands, semi-continuous and continuous films of noble metals (Au, Ag) deposited on single crystalline silicon will be reviewed. MeV-ion induced changes in specified Au-nanoislands on silicon substrate are tracked as a function of ion fluence using ex situ TEM. Strain induced in the bulk silicon substrate surface due to 1.5 MeV Au2+ and C2+ ion beam irradiation is determined by using HRTEM and asymmetric Bragg X-ray rocking curve methods. Preliminary results on 1.5 MeV Au2+ ion-induced effects in nanoislands of Co deposited on silicon substrate will be discussed. 2005 Published by Elsevier B.V. PACS: 61.82.Rx; 79.20.Rf; 61.80.Jh Keywords: Ion irradiation; Nanoislands; Strain; HRTEM; Asymmetric X-ray bragg reflection
1. Introduction Understanding structure–property relationship of nanostructures can facilitate many important applications in various fields of advanced technology due to their unique physical properties that are governed by their structure [1]. Ion beams ranging from a few eV to MeV have been used to modify the thin film structures, synthesizing new structures, coating thin films, mixing immiscible layers, and for characterizing materials. Heinig et al. recently reviewed synthesis of nanostructures and their modifications using ion beams [2]. When an energetic ion impinges *
Corresponding author. Tel.: +91 674 2301058; fax: +91 674 2300142. E-mail addresses: [email protected], [email protected] (P.V. Satyam). 0168-583X/$ - see front matter 2005 Published by Elsevier B.V. doi:10.1016/j.nimb.2005.11.147
on a solid material, many interesting phenomena can occur, like sputtering of target material, surface and interface morphological changes, etc. Until now, not much effort has been given on involved mechanisms to understand various processes during an energetic ion interaction with nanoparticles or embedded nanostructures. All the spike models involve a confined region where the energy or pressure or temperature arising because of ion impact is confined to nanometer zones. It requires a systematic study on ion irradiation with a control on separating nuclear and electronic energy loss regimes and under various irradiation parameters like fluence, dose-rate, substrate temperature, and geometry. The process of ion-irradiation is an athermal or nonequilibrium thermal process and hence the properties of nanostructures could be tailored [3,4], which are otherwise
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difficult or not feasible by conventional methods. It has been shown that energetic ion irradiation induces (i) anisotropic plastic deformation turning spherical colloids into ellipsoidal shape [3], (ii) burrowing of the nanostructures into the substrates [4,5] and (iii) sub-surface mixing at nanoscale regime for Au nanoislands deposited on silicon substrate forming a metastable mixed phase for Au–Si [5]. Previously, we have carried out systematic studies on ion-induced effects (for Au ions at the energies ranging from keV to MeV) in the systems consisting of nanoislands deposited on silicon substrates and these will be briefly reviewed in the following sections [5–12]. There have been several efforts to understand ion–solid interactions for many decades [13]. Birtcher and Donnelly reported that spike effects produce surface craters in materials in which cascades occur in the surface region with energy densities sufficient to cause melting. These authors reported that the mechanism for material ejection to form a crater involves explosive outflow of material from the hot molten core of the spike and deduced that the duration of the spike was sufficient for major mass transport to occur [14–17]. Following this, Rehn et al. [17] reported the ejection of atomic clusters during ion sputtering; this is associated with the shock waves generated by subsurface displacement cascades, as predicted in the model of Bitensky and Parilis [18]. The evidence for both the molten core in the thermal spike and shock wave propagation together, where a non-linear cascading dominates is intriguing [14,17]. While the phenomena at surface and interfaces are yet to be properly understood, the ion–solid interaction for the case of isolated nanoislands has become an important topic. For example, in the case of ion–nanosolid interaction, sputtering [8], burrowing [4] and wetting [4] could be competitive mechanisms for nanoparticles that are immiscible with the substrate to undergo smoothing reactions. Ion-induced viscous flow and the effect of surface energies lead to establish ion irradiation as another process of burrowing nanoparticles besides thermal processes [4,5]. One of these studies rule out the significant effect of extended thermal spikes [4], while the other showed the need of thermal spike leading to reaction component [5]. Kissel and Urbassek performed molecular dynamics (MD) simulation in order to understand sputtering from clusters [19]. For nanostructures grown on substrates, the simulations by Kissel and Urbassek predict the possibility of embedding a part of the nanostructure into the substrate causing mixing over a length scale dictated by the nanostructure dimension [19]. We briefly review our previous results in case of ion-nanosolid interaction process [5–12]. In our previous studies, nanoislands of Au, Ag and Ge were deposited on single crystal silicon substrate. The thin films were deposited either using thermal evaporation or using e-beam evaporation under high vacuum conditions. Irradiation was performed with 32 keV, 1.5 MeV and 100 MeV gold ions. Upon irradiation with 1.5 MeV Au2+ ions on isolated Au nanoislands on silicon substrate, it was found that (i) higher probability of crater formation, (ii) larger particle
size and its coverage and (iii) enhanced sputtering yield when compared with continuous films of Au on Si substrate [6–8]. The average sputtered particle size and areal coverage were determined from TEM measurements where as the amount of gold on the substrate has been found by RBS. The size distribution of larger particles (n P 1,000) shows an inverse power-law (i.e. /n1) in broad agreement with a molecular dynamics simulation of ion impact on cluster targets [11]. Two other important aspects of ioninduced effects on nanostructures: (i) effect of energy/thermal spike confinement causing nano-scale ion beam mixing and formation of nanoscale gold silicide (this might provide a route to fabricate embedded nanostructures) (ii) absence of this behavior in case of continuous Au film on silicon [5,9]. In these studies, the possibility of having sub-surface nano-alloy-mixing is demonstrated with nano AuxSiy formation. We have also shown about the possibility of surface alloying for nano Au–Ge systems [12]. It is to be noted that the projected range of 1.5 MeV gold ions in silicon is very large compared to the thickness of the nano-islands and the substrate is found to be amorphized at a fluence of 1 · 1014 ions cm2 [20,21]. Lattice strain may be caused by lattice defects or heteroepitaxy or other kind of stresses. Lattice strain at surface and interfaces plays an important role in the structural evolution of many systems. Band gap engineering of electro-optical systems is made possible with the strainedlayer-superlattice systems. The strain measurements, in general, are carried out by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Rutherford backscattering spectrometry/channeling (RBS/C) methods. These experimental techniques measure the strain values averaged over length scales from sub-microns to few hundreds of microns. It is possible to measure strain from smaller length scales (in nanometer scales) by using special geometrical conditions in the above experimental characterizations. For example, it is possible to limit the penetration of X-rays by using grazing incidence methods or using RBS/ C in glancing angle geometry. It is also possible to get strain in specific locations using nanobeam diffraction (NBD in TEM) or convergent beam electron diffraction (CBED in TEM) or lattice imaging (HRTEM). More details on the strain determination are presented elsewhere [10]. Selected area electron diffraction (SAED) and lattice imaging (using TEM) has been used to determine the strain at surface and interfaces on virgin systems and on irradiated system. The TEM results directly indicate a contraction in the silicon lattice due to ion-induced effects. For 1.5 MeV Au ion irradiation on gold nanostructures, it has been demonstrated that the nanoislands have shadowed the ion beam resulting in lesser strain beneath the island structures in silicon substrates [10]. High-resolution lattice imaging has also been used to determine the strain in and around amorphized zones caused by the ion irradiation [10]. In this paper, we will discuss: (i) tracking of individual (or specific) gold nanoislands on silicon substrate under the MeV-ion irradiation as a function of ion fluence
J. Ghatak et al. / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 45–51
using ex situ TEM, (ii) determination of strain induced in the silicon single crystal surface and interfaces with 1.5 MeV Au2+ and C2+ ions by using HRTEM and asymmetric Bragg X-ray rocking curve method, and (iii) preliminary results on 1.5 MeV Au2+ ion-induced effects in nanoislands of Co deposited on silicon substrate will be discussed.
2. Experimental For the work reported here, Au films of about 2.0 nm thickness and Ag films of 2.3 nm thickness were deposited at room temperature by evaporation under high-vacuum conditions (5 · 105 mbar) onto Si(1 0 0) substrates. A 2.1 nm thick native oxide layer was present on the surface. For the study of ion-induced effects in cobalt nanosislands, thin films of cobalt were evaporated by electron beam evaporation method in an evaporator pumped by cryo and turbo molecular pump leading to 2 · 108 mbar prior evaporation. The pressure during evaporation was 7 · 108 mbar and the rate of evaporation 0.01 nm/s. The irradiation has been carried out with 1.5 MeV Au2+ ions and C2+ ions (at Ion Beam Laboratory, Institute of Physics, Bhubaneswar) using a raster scanning facility for uniform irradiation at various impact angles. During the irradiation the incident beam current was kept around a value of about 20–30 nA. The Rutherford backscattering spectrometry measurements were carried out with 2.0 MeV He2+ ions using the 3.0 MV tandem accelerator facility and TEM measurements were carried out with JEOL 2010 (UHR) electron microscope. For the tracking of nanoislands, initially, the cross-section specimen of asdeposited gold on silicon substrate was made and the structures at the perforation edge were identified. It was relatively easy to keep a tag on the nanostructures at the perforation edge as the two edges of cross-section sample have different morphologies. The irradiation on the cross-section sample was carried out up to a fluence of 1 · 1014 ions cm2 and by tilting the specimen about 60 towards the surface normal. Surface sensitive asymmetric X-ray diffraction experiments were carried out in atmosphere at room temperature at beam line BL-15C at the Photon Factory of High Energy Accelerator Research Organization in Tsukuba, Japan [21]. For the observation of strain field near the substrate surface, the rocking curves of the (1 1 3) reflection of the substrate were measured. Since (1 1 3) planes are oriented at 29.5 to (1 1 1) surface, the detector (NaI) was positioned at 59 with respect to surface. The incident X-ray energy was tuned such that the measurements were done near the critical angle of total reflection (i.e. grazing incidence geometry). Under the grazing incidence conditions, X-rays could penetrate up to a depth of few nanometers The penetration depth (or projected range) of the 1.5 MeV gold ions in Si at an impact angle of 60 is found out to be 330 nm using SRIM2003 [22].
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3. Results and discussions 3.1. Tracking the ion-induced effects in individual gold nanoislands on silicon substrates Fig. 1(a) and (b) show the planar and the cross-section (XTEM) bright field (BF) images of as-deposited gold nanoislands, respectively. These nanoislands have been formed due to the difference in surface energies of gold and SiO2 surface. The average diameter of gold nanoislands range from 4 to 20 nm with a 28% surface coverage. The average thickness of the gold layer was determined by RBS measurements leading to be about 2 nm assuming the bulk gold density. The sample whose bright field image is shown in Fig. 1(b) was used as the specimen under study for tracking the nanoislands. As shown in Fig. 1(b), we have concentrated on the nanoislands shown by rectangular boxes. The islands that are enclosed in the depicted boxes are located at one edge of the perforation in the XTEM specimen. Fig. 1(c) shows a bright field XTEM micrograph after the ion irradiation with 1.5 MeV gold ions at a fluence of 5 · 1012 ions cm2, and Fig. 1(d) at a fluence of 1 · 1014 ions cm2. For 5 · 1012 ions cm2, it appears from Fig. 1(c), that neither embedding nor ionbeam-mixing is a prominent feature, while Fig. 1(d) shows embedding effect at a fluence of 1 · 1014 ions cm2. This embedding phenomenon was characterized using RBS and TEM. This kind of studies could yield direct observation of individual islands and their modification process. 3.2. Strain induced in silicon with 1.5 MeV Au2+ and C2+ ions: X-ray and TEM study High resolution diffraction experiments using asymmetric Bragg reflection were carried out at the Photon Factory beamline BL-15C. The experimental setup at BL-15C is shown in Fig. 2(a). This beamline is equipped with a fixedexit double-crystal Si(1 1 1) monochromator. In this figure, D1 is ionization chamber to detect monitor counts after the monochromator, D2 is NaI detector for detecting scattered X-rays. S1, S2 and S3 are slits to minimize the background. The surface-sensitive asymmetric rocking curves measurements was made using the (1 1 3) reflection of the substrate. The incident angle of X-rays could be varied by tuning the wavelength of the incident X-rays. In all the cases the incident angle is close to the critical angle for total reflection for silicon substrate. The {1 1 3} plane is oriented at 29.5, and hence the X-ray wavelength around a ˚ has been used to make the technique surface value of 1.61 A sensitive. More details on the instrumentation can be found elsewhere [23]. The experimental rocking curve obtained under asymmetric Bragg condition is shown in Fig. 2(b) for the Si(1 1 1) irradiated with 1.5 MeV Au2+ ions at 0 and 30 angles towards surface normal at a fluence of 5 · 1013 ions cm2. The secondary oscillations become stronger for the sample that was irradiated at 30 impact angle.
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Fig. 1. (a) Bright field (BF) planar-TEM micrograph of as-deposited 2 nm thick gold deposited on silicon substrate, (b) BF cross-section (XTEM) micrograph near one of the perforation edge. Inset rectangular boxes represents the islands that are tracked as function of ion fluence, (c) XTEM micrograph of the same sample at a fluence of 5 · 1012 ions cm2 and (d) XTEM micrograph of the same sample at a fluence of 1 · 1014 ions cm2. The impact angle was kept at 60.
Fig. 2. (a) Schematic setup for asymmetric Bragg reflection at Photon Factory (KEK, Tsukuba, Japan) at BL-15C, (b) experimental rocking curve for irradiated silicon substrate with 1.5 MeV Au2+ ions at a fluence of 5 · 1013 ions cm2 and at two impact angles: 0 and 30 and (c) experimental rocking curve for irradiated silicon substrate with 1.5 MeV Au2+ and C2+ ions normal to the surface at fluence of 5 · 1013 ions cm2.
If it is assumed that these secondary peaks arise due to strain, then the maximum strain for the peak at extreme right show a maximum of 0.1% lattice contraction in {1 1 3} inter-planar spacing. The appearance of oscillations (or satellite peaks) arises either due to strain or due to the presence of a very thin layer of different electron density from the substrate matrix. Fig. 2(c) shows the rocking
curves from the irradiated specimen with 1.5 MeV Au2+ and C2+ ions at a fluence of 5 · 1013 ions cm2 normal to the surface. It is interesting to note the absence of secondary satellite peaks for the case of 1.5 MeV C2+ irradiated specimen. For C irradiation, XTEM measurements showed the presence of strain comparable to the gold ion irradiated specimen. It is not very clear whether the variations
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observed in the rocking curves shown in Fig. 2(c) are due to the minute changes in electron density. Simulations using modified Darwin theory and dynamical diffraction theory indicate broadening of rocking curve and appearance of satellite peak due to presence of compressive strain in the system [10]. The simulations data using dynamical theory of diffraction are shown in Fig. 3(a). It is clear from this figure that small changes in the strain can be seen. These simulation mechanism needs to be modified to account for the grazing incidence geometry (asymmetric Bragg case) [23,24]. More simulation work is being carried out using the modified Darwin theory for asymmetric Bragg condition. Fig. 3(b) shows the variation of strain obtained from the TEM studies using selected area diffraction method [10]. As predicted, the strain is found to be maximum at the projected range of gold in silicon substrate.
Fig. 3. (a) Simulation using X-ray dynamical diffraction theory as a function of strain. Inset shows two layers system with top 500 nm as a strained layer. The strain has been assumed to be constant in top layer. The bottom layer is assumed to be bulk lattice constant and (b) distribution of strain values at various depth in the irradiated silicon (with 1.5 MeV gold ions at a fluence of 5 · 1013 ions cm2). The strain values are obtained using selected area diffraction in TEM measurements (details are given in [10]).
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3.3. MeV ion induced effects in nanoislands of Co on silicon Formation of cobalt silicide using ion mixing for thick and continuous films was carried out by many groups [25,26]. More recently, a self-assembly nanopatterning method for epitaxial-CoSi2 layers has been developed based on anisotropic diffusion of Co/Si atoms in a stress field during rapid thermal oxidation [27]. CoSi2-nanostructures were also fabricated using a self-assembly process involving local oxidation of silicides [28]. Mantl reviewed the work on ion beam synthesis of silicide fabrication and characterization [29]. In this work, we report the MeV ion-induced effects in cobalt nanoislands deposited on crystalline silicon substrate and possibility of cobalt silicide formation using nano-scale ion beam mixing process. In our previous report, nanoscale ion beam mixing leading to formation of surface and sub-surface nano-gold silicide was discussed [5]. Fig. 4(a) shows a bright field planarTEM micrograph for thin cobalt film deposited on silicon substrate, while Fig. 4(b) shows RBS spectrum obtained from this specimen. The effective cobalt thickness found to be 2.3 nm. To obtain effective film thickness, bulk cobalt
Fig. 4. (a) BF planar TEM micrograph of 2.8 nm thick as-deposited cobalt film on silicon substrate and (b) RBS spectra for the above system used to determine the effective thickness.
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Fig. 5. (a) BF planar-TEM micrograph at lower magnification for the irradiated nano-cobalt film on silicon with 1.5 MeV Au2+ at a fluence of 1 · 1014 ions cm2, (b) high resolution TEM micrograph taken at high contrast area of the same system shown in part (a), (c) XTEM micrograph at lower magnification for the irradiated nano-cobalt film on silicon with 1.5 MeV Au2+ at a fluence of 1 · 1014 ions cm2, and (d) high resolution XTEM micrograph taken at high contrast area of the same system shown in part (c).
density has been used. From Fig. 4(a), the cobalt islands were found to form islands in a network fashion. From planar TEM micrograph, it is clear that no isolated nanoislands of cobalt were found in as-deposited case (Fig. 4(a)). Fig. 5(a) and (b) show BF planar-TEM micrograph for a system irradiated at a fluence of 1 · 1014 ions cm2 at an impact angle of 0 with respect to the surface normal. Planar micrograph taken at a lower magnification (Fig. 5(a)) shows high contrast zones from where a lattice image is taken and shown in Fig. 5(b). The lattice spacing is found to be 0.201 ± 0.005 nm from the high contrast (darker) areas. A low magnified BF image in cross-section mode is shown in Fig. 5(c). Few interesting observations can be made from this figure: (a) after the irradiation at a fluence of 1 · 1014 ions cm2, there appears to still a continuous layer present on the silicon substrate, (b) there are periodically spaced higher contrast areas from where a lattice image could be collected (Fig. 5(d)) and lattice spacing corresponds to 0.201 ± 0.005 nm. But lower contrast areas are found to be amorphous in nature, and (c) underneath the higher contrast islands, there are some modulation at SiO2–Si interface (i.e. change in the contrast in a wavy pattern at the interface). Bulk cobalt is found to have a structure either in the fcc phase (b-Co) (with lattice constant, ˚ ) or a close packed hcp (with a = 0.251 nm a = 3.55 A and c = 0.406 nm). For b-Co phase, the inter-planar spacing for {1 1 1} planes is (d111) 0.205 nm and is closed to the observed value of 0.201 nm. The closest to observed spacing for CoSi2 (diamond lattice) is that correspond to d220 (=0.188 nm). There are also other phases of CoxSiy where it exists lattice spacing closer to the observed lattice value (for example: CoSi-d100 = 0.198 nm; Co2Si3-d212 = 0.205 nm, d220 = 0.185 nm; Co2S-d021 = 0.205 nm, d220 =
2.02 nm, d12 = 1.97 nm). It could also be possible that the observed lattice spacing corresponds to the pure bulk Co and no silicide is formed. One requires additional methods to determine exact phases that are formed. To understand, energy confinement effects in these cobalt structures, we plan to perform more controlled experiments. 4. Conclusions Ion induced effects on nano-islands of gold and silver deposited silicon substrates have been reviewed. Enhanced sputtering yield and ion beam mixing leading to surface and subsurface nano-alloying found be interesting outcome of these studies. Tracking of MeV-ion induced changes in specified Au-nanoislands on silicon substrate as a function of ion fluence lead to determination of the threshold fluence for embedding and nano-scale mixing effects. Strain induced in the bulk silicon in the presence of nano-islands on the surface due to ion beam irradiation is determined by using HRTEM and asymmetric Bragg X-ray rocking curve method. Preliminary results on 1.5 MeV Au2+ ion-induced effects in nanoislands of Co deposited on silicon substrate have been discussed. Acknowledgements We thank Mr. A.K. Dash for his help in TEM measurements and the staff at IBL, Institute of Physics for ion implantations. We thank Dr. K. Hirano of the Photon Factory for help with X-ray measurements. P.V. Satyam would like to thank JSPS, Japan for funding his visit to KEK, and Nagoya University, Nagoya, Japan under invitation fellowship. We also thank Dr. D.K. Goswami for fruitful
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discussions. X-ray measurements (using synchrotron radiation) was performed under approval of the Photon Factory (PF-PAC No. 2002G197 and 2004G218). References [1] O.G. Schmidt, Ch. Deneke, Y. Nakamura, R. Zapf-Gottwick, C. Muller, N.Y. Jin-Phillipp, Adv. Solid State Phys. 42 (2002) 231, and references therein (B. Kramer (Ed.)). [2] K.-H. Heinig, T. Mu¨ller, B. Schmidt, M. Strobel, W. Mo¨ller, Appl. Phys. A 77 (2003) 17. [3] T. van Dillen, A. Polman, W. Fukarek, A. van Blaaderen, Appl. Phys. Lett. 78 (2001) 910. [4] Xiaoyuan Hu, David G. Cahill, R.S. Averback, J. Appl. Phys. 92 (2002) 3995. [5] B. Satpati, P.V. Satyam, T. Som, B.N. Dev, Appl. Phys. A: Mater. Sci. Process. A 79 (2004) 447. [6] P.V. Satyam, J. Kamila, S. Mohapatra, B. Satpati, D.K. Goswami, B.N. Dev, R.E. Cook, Lahsen Assoufid, J. Wang, N.C. Mishra, J. Appl. Phys. 93 (2003) 6399. [7] B. Satpati, D.K. Goswami, U.D. Vaishnav, T. Som, B.N. Dev, P.V. Satyam, Nucl. Instr. and Meth. B 212 (2003) 157. [8] B. Satpati, D.K. Goswami, S. Roy, T. Som, B.N. Dev, P.V. Satyam, Nucl. Instr. and Meth. B 212 (2003) 332; I. Baranov et al., Nucl. Instr. and Meth. B 193 (2002) 798. [9] B. Satpati, P.V. Satyam, T. Som, B.N. Dev, J. Appl. Phys. 96 (2004) 5212. [10] J. Ghatak, B. Satpati, M. Umananda, P.V. Satyam, K. Akimoto, K. Ito, T. Emoto, Nucl. Instr. and Meth. B (2005), this Volume, doi:10.1016/j.nimb.2005.11.016.
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[11] B. Satpati, J. Ghatak, P.V. Satyam, B.N. Dev, J. Appl. Phys. 98 (2005) 064904. [12] T. Som, B. Satpati, P.V. Satyam, D. Kabiraj, J. Appl. Phys. 97 (2005) 014305. [13] J.A. Brinkman, Am. J. Phys. 24 (1956) 246. [14] R.C. Birtcher, S.E. Donnelly, S. Schlutig, Phys. Rev. Lett. 85 (2000) 4968. [15] R.C. Birtcher, S.E. Donnelly, Phys. Rev. Lett. 77 (1996) 4374. [16] S.E. Donnelly, R.C. Birtcher, Phil. Mag. A 79 (1999) 133. [17] L.E. Rehn, R.C. Birtcher, S.E. Donnelly, P.M. Baldo, L. Funk, Phys. Rev. Lett. 87 (2003) 207601. [18] I.S. Bitensky, E.S. Parilis, Nucl. Instr. and Meth. B 21 (1987) 26. [19] Rolf Kissel, Herbert M. Urbassek, Nucl. Instr. and Meth. B 180 (2001) 293. [20] J. Kamila, B. Satpati, D.K. Goswami, M. Rundhe, B.N. Dev, P.V. Satyam, Nucl. Instr. and Meth. B 207 (2003) 291. [21] T. Emoto, K. Akimoto, Y. Ishikawa, A. Ichimiya, A. Tanikawa, Thin Solid Films 369 (2000) 281. [22] J.P. Biersack, L. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. Available from:
Nuclear Instruments and Methods in Physics Research B 244 (2006) 45–51 www.elsevier.com/locate/nimb
Characterization of ion beam induced nanostructures J. Ghatak a, B. Satpati a, M. Umananda a, D. Kabiraj b, T. Som a, B.N. Dev a, K. Akimoto c, K. Ito c, T. Emoto d, P.V. Satyam a,* a Institute of Physics, Sachivalaya Marg, Bhubaneswar 751 005, India Nuclear Science Center, Aruna Asaf Ali Marg, New Delhi 110 067, India Department of Quantum Engineering, Nagoya University, Nagoya 464-8603, Japan d Toyota National College of Technology, 2-1, Toyota, Aichi 471-8525, Japan b
c
Available online 9 January 2006
Abstract Tailoring of nanostructures with energetic ion beams has become an active area of research leading to the fundamental understanding of ion–solid interactions at nanoscale regime and with possible applications in the near future. Rutherford backscattering spectrometry (RBS), high resolution transmission electron microscopy (HRTEM) and asymmetric X-ray Bragg-rocking curve experimental methods have been used to characterize ion-induced effects in nanostructures. The possibility of surface and sub-surface/interface alloying at nano-scale regime, ion-beam induced embedding, crater formation, sputtering yield variations for systems with isolated nanoislands, semi-continuous and continuous films of noble metals (Au, Ag) deposited on single crystalline silicon will be reviewed. MeV-ion induced changes in specified Au-nanoislands on silicon substrate are tracked as a function of ion fluence using ex situ TEM. Strain induced in the bulk silicon substrate surface due to 1.5 MeV Au2+ and C2+ ion beam irradiation is determined by using HRTEM and asymmetric Bragg X-ray rocking curve methods. Preliminary results on 1.5 MeV Au2+ ion-induced effects in nanoislands of Co deposited on silicon substrate will be discussed. 2005 Published by Elsevier B.V. PACS: 61.82.Rx; 79.20.Rf; 61.80.Jh Keywords: Ion irradiation; Nanoislands; Strain; HRTEM; Asymmetric X-ray bragg reflection
1. Introduction Understanding structure–property relationship of nanostructures can facilitate many important applications in various fields of advanced technology due to their unique physical properties that are governed by their structure [1]. Ion beams ranging from a few eV to MeV have been used to modify the thin film structures, synthesizing new structures, coating thin films, mixing immiscible layers, and for characterizing materials. Heinig et al. recently reviewed synthesis of nanostructures and their modifications using ion beams [2]. When an energetic ion impinges *
Corresponding author. Tel.: +91 674 2301058; fax: +91 674 2300142. E-mail addresses: [email protected], [email protected] (P.V. Satyam). 0168-583X/$ - see front matter 2005 Published by Elsevier B.V. doi:10.1016/j.nimb.2005.11.147
on a solid material, many interesting phenomena can occur, like sputtering of target material, surface and interface morphological changes, etc. Until now, not much effort has been given on involved mechanisms to understand various processes during an energetic ion interaction with nanoparticles or embedded nanostructures. All the spike models involve a confined region where the energy or pressure or temperature arising because of ion impact is confined to nanometer zones. It requires a systematic study on ion irradiation with a control on separating nuclear and electronic energy loss regimes and under various irradiation parameters like fluence, dose-rate, substrate temperature, and geometry. The process of ion-irradiation is an athermal or nonequilibrium thermal process and hence the properties of nanostructures could be tailored [3,4], which are otherwise
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difficult or not feasible by conventional methods. It has been shown that energetic ion irradiation induces (i) anisotropic plastic deformation turning spherical colloids into ellipsoidal shape [3], (ii) burrowing of the nanostructures into the substrates [4,5] and (iii) sub-surface mixing at nanoscale regime for Au nanoislands deposited on silicon substrate forming a metastable mixed phase for Au–Si [5]. Previously, we have carried out systematic studies on ion-induced effects (for Au ions at the energies ranging from keV to MeV) in the systems consisting of nanoislands deposited on silicon substrates and these will be briefly reviewed in the following sections [5–12]. There have been several efforts to understand ion–solid interactions for many decades [13]. Birtcher and Donnelly reported that spike effects produce surface craters in materials in which cascades occur in the surface region with energy densities sufficient to cause melting. These authors reported that the mechanism for material ejection to form a crater involves explosive outflow of material from the hot molten core of the spike and deduced that the duration of the spike was sufficient for major mass transport to occur [14–17]. Following this, Rehn et al. [17] reported the ejection of atomic clusters during ion sputtering; this is associated with the shock waves generated by subsurface displacement cascades, as predicted in the model of Bitensky and Parilis [18]. The evidence for both the molten core in the thermal spike and shock wave propagation together, where a non-linear cascading dominates is intriguing [14,17]. While the phenomena at surface and interfaces are yet to be properly understood, the ion–solid interaction for the case of isolated nanoislands has become an important topic. For example, in the case of ion–nanosolid interaction, sputtering [8], burrowing [4] and wetting [4] could be competitive mechanisms for nanoparticles that are immiscible with the substrate to undergo smoothing reactions. Ion-induced viscous flow and the effect of surface energies lead to establish ion irradiation as another process of burrowing nanoparticles besides thermal processes [4,5]. One of these studies rule out the significant effect of extended thermal spikes [4], while the other showed the need of thermal spike leading to reaction component [5]. Kissel and Urbassek performed molecular dynamics (MD) simulation in order to understand sputtering from clusters [19]. For nanostructures grown on substrates, the simulations by Kissel and Urbassek predict the possibility of embedding a part of the nanostructure into the substrate causing mixing over a length scale dictated by the nanostructure dimension [19]. We briefly review our previous results in case of ion-nanosolid interaction process [5–12]. In our previous studies, nanoislands of Au, Ag and Ge were deposited on single crystal silicon substrate. The thin films were deposited either using thermal evaporation or using e-beam evaporation under high vacuum conditions. Irradiation was performed with 32 keV, 1.5 MeV and 100 MeV gold ions. Upon irradiation with 1.5 MeV Au2+ ions on isolated Au nanoislands on silicon substrate, it was found that (i) higher probability of crater formation, (ii) larger particle
size and its coverage and (iii) enhanced sputtering yield when compared with continuous films of Au on Si substrate [6–8]. The average sputtered particle size and areal coverage were determined from TEM measurements where as the amount of gold on the substrate has been found by RBS. The size distribution of larger particles (n P 1,000) shows an inverse power-law (i.e. /n1) in broad agreement with a molecular dynamics simulation of ion impact on cluster targets [11]. Two other important aspects of ioninduced effects on nanostructures: (i) effect of energy/thermal spike confinement causing nano-scale ion beam mixing and formation of nanoscale gold silicide (this might provide a route to fabricate embedded nanostructures) (ii) absence of this behavior in case of continuous Au film on silicon [5,9]. In these studies, the possibility of having sub-surface nano-alloy-mixing is demonstrated with nano AuxSiy formation. We have also shown about the possibility of surface alloying for nano Au–Ge systems [12]. It is to be noted that the projected range of 1.5 MeV gold ions in silicon is very large compared to the thickness of the nano-islands and the substrate is found to be amorphized at a fluence of 1 · 1014 ions cm2 [20,21]. Lattice strain may be caused by lattice defects or heteroepitaxy or other kind of stresses. Lattice strain at surface and interfaces plays an important role in the structural evolution of many systems. Band gap engineering of electro-optical systems is made possible with the strainedlayer-superlattice systems. The strain measurements, in general, are carried out by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Rutherford backscattering spectrometry/channeling (RBS/C) methods. These experimental techniques measure the strain values averaged over length scales from sub-microns to few hundreds of microns. It is possible to measure strain from smaller length scales (in nanometer scales) by using special geometrical conditions in the above experimental characterizations. For example, it is possible to limit the penetration of X-rays by using grazing incidence methods or using RBS/ C in glancing angle geometry. It is also possible to get strain in specific locations using nanobeam diffraction (NBD in TEM) or convergent beam electron diffraction (CBED in TEM) or lattice imaging (HRTEM). More details on the strain determination are presented elsewhere [10]. Selected area electron diffraction (SAED) and lattice imaging (using TEM) has been used to determine the strain at surface and interfaces on virgin systems and on irradiated system. The TEM results directly indicate a contraction in the silicon lattice due to ion-induced effects. For 1.5 MeV Au ion irradiation on gold nanostructures, it has been demonstrated that the nanoislands have shadowed the ion beam resulting in lesser strain beneath the island structures in silicon substrates [10]. High-resolution lattice imaging has also been used to determine the strain in and around amorphized zones caused by the ion irradiation [10]. In this paper, we will discuss: (i) tracking of individual (or specific) gold nanoislands on silicon substrate under the MeV-ion irradiation as a function of ion fluence
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using ex situ TEM, (ii) determination of strain induced in the silicon single crystal surface and interfaces with 1.5 MeV Au2+ and C2+ ions by using HRTEM and asymmetric Bragg X-ray rocking curve method, and (iii) preliminary results on 1.5 MeV Au2+ ion-induced effects in nanoislands of Co deposited on silicon substrate will be discussed.
2. Experimental For the work reported here, Au films of about 2.0 nm thickness and Ag films of 2.3 nm thickness were deposited at room temperature by evaporation under high-vacuum conditions (5 · 105 mbar) onto Si(1 0 0) substrates. A 2.1 nm thick native oxide layer was present on the surface. For the study of ion-induced effects in cobalt nanosislands, thin films of cobalt were evaporated by electron beam evaporation method in an evaporator pumped by cryo and turbo molecular pump leading to 2 · 108 mbar prior evaporation. The pressure during evaporation was 7 · 108 mbar and the rate of evaporation 0.01 nm/s. The irradiation has been carried out with 1.5 MeV Au2+ ions and C2+ ions (at Ion Beam Laboratory, Institute of Physics, Bhubaneswar) using a raster scanning facility for uniform irradiation at various impact angles. During the irradiation the incident beam current was kept around a value of about 20–30 nA. The Rutherford backscattering spectrometry measurements were carried out with 2.0 MeV He2+ ions using the 3.0 MV tandem accelerator facility and TEM measurements were carried out with JEOL 2010 (UHR) electron microscope. For the tracking of nanoislands, initially, the cross-section specimen of asdeposited gold on silicon substrate was made and the structures at the perforation edge were identified. It was relatively easy to keep a tag on the nanostructures at the perforation edge as the two edges of cross-section sample have different morphologies. The irradiation on the cross-section sample was carried out up to a fluence of 1 · 1014 ions cm2 and by tilting the specimen about 60 towards the surface normal. Surface sensitive asymmetric X-ray diffraction experiments were carried out in atmosphere at room temperature at beam line BL-15C at the Photon Factory of High Energy Accelerator Research Organization in Tsukuba, Japan [21]. For the observation of strain field near the substrate surface, the rocking curves of the (1 1 3) reflection of the substrate were measured. Since (1 1 3) planes are oriented at 29.5 to (1 1 1) surface, the detector (NaI) was positioned at 59 with respect to surface. The incident X-ray energy was tuned such that the measurements were done near the critical angle of total reflection (i.e. grazing incidence geometry). Under the grazing incidence conditions, X-rays could penetrate up to a depth of few nanometers The penetration depth (or projected range) of the 1.5 MeV gold ions in Si at an impact angle of 60 is found out to be 330 nm using SRIM2003 [22].
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3. Results and discussions 3.1. Tracking the ion-induced effects in individual gold nanoislands on silicon substrates Fig. 1(a) and (b) show the planar and the cross-section (XTEM) bright field (BF) images of as-deposited gold nanoislands, respectively. These nanoislands have been formed due to the difference in surface energies of gold and SiO2 surface. The average diameter of gold nanoislands range from 4 to 20 nm with a 28% surface coverage. The average thickness of the gold layer was determined by RBS measurements leading to be about 2 nm assuming the bulk gold density. The sample whose bright field image is shown in Fig. 1(b) was used as the specimen under study for tracking the nanoislands. As shown in Fig. 1(b), we have concentrated on the nanoislands shown by rectangular boxes. The islands that are enclosed in the depicted boxes are located at one edge of the perforation in the XTEM specimen. Fig. 1(c) shows a bright field XTEM micrograph after the ion irradiation with 1.5 MeV gold ions at a fluence of 5 · 1012 ions cm2, and Fig. 1(d) at a fluence of 1 · 1014 ions cm2. For 5 · 1012 ions cm2, it appears from Fig. 1(c), that neither embedding nor ionbeam-mixing is a prominent feature, while Fig. 1(d) shows embedding effect at a fluence of 1 · 1014 ions cm2. This embedding phenomenon was characterized using RBS and TEM. This kind of studies could yield direct observation of individual islands and their modification process. 3.2. Strain induced in silicon with 1.5 MeV Au2+ and C2+ ions: X-ray and TEM study High resolution diffraction experiments using asymmetric Bragg reflection were carried out at the Photon Factory beamline BL-15C. The experimental setup at BL-15C is shown in Fig. 2(a). This beamline is equipped with a fixedexit double-crystal Si(1 1 1) monochromator. In this figure, D1 is ionization chamber to detect monitor counts after the monochromator, D2 is NaI detector for detecting scattered X-rays. S1, S2 and S3 are slits to minimize the background. The surface-sensitive asymmetric rocking curves measurements was made using the (1 1 3) reflection of the substrate. The incident angle of X-rays could be varied by tuning the wavelength of the incident X-rays. In all the cases the incident angle is close to the critical angle for total reflection for silicon substrate. The {1 1 3} plane is oriented at 29.5, and hence the X-ray wavelength around a ˚ has been used to make the technique surface value of 1.61 A sensitive. More details on the instrumentation can be found elsewhere [23]. The experimental rocking curve obtained under asymmetric Bragg condition is shown in Fig. 2(b) for the Si(1 1 1) irradiated with 1.5 MeV Au2+ ions at 0 and 30 angles towards surface normal at a fluence of 5 · 1013 ions cm2. The secondary oscillations become stronger for the sample that was irradiated at 30 impact angle.
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Fig. 1. (a) Bright field (BF) planar-TEM micrograph of as-deposited 2 nm thick gold deposited on silicon substrate, (b) BF cross-section (XTEM) micrograph near one of the perforation edge. Inset rectangular boxes represents the islands that are tracked as function of ion fluence, (c) XTEM micrograph of the same sample at a fluence of 5 · 1012 ions cm2 and (d) XTEM micrograph of the same sample at a fluence of 1 · 1014 ions cm2. The impact angle was kept at 60.
Fig. 2. (a) Schematic setup for asymmetric Bragg reflection at Photon Factory (KEK, Tsukuba, Japan) at BL-15C, (b) experimental rocking curve for irradiated silicon substrate with 1.5 MeV Au2+ ions at a fluence of 5 · 1013 ions cm2 and at two impact angles: 0 and 30 and (c) experimental rocking curve for irradiated silicon substrate with 1.5 MeV Au2+ and C2+ ions normal to the surface at fluence of 5 · 1013 ions cm2.
If it is assumed that these secondary peaks arise due to strain, then the maximum strain for the peak at extreme right show a maximum of 0.1% lattice contraction in {1 1 3} inter-planar spacing. The appearance of oscillations (or satellite peaks) arises either due to strain or due to the presence of a very thin layer of different electron density from the substrate matrix. Fig. 2(c) shows the rocking
curves from the irradiated specimen with 1.5 MeV Au2+ and C2+ ions at a fluence of 5 · 1013 ions cm2 normal to the surface. It is interesting to note the absence of secondary satellite peaks for the case of 1.5 MeV C2+ irradiated specimen. For C irradiation, XTEM measurements showed the presence of strain comparable to the gold ion irradiated specimen. It is not very clear whether the variations
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observed in the rocking curves shown in Fig. 2(c) are due to the minute changes in electron density. Simulations using modified Darwin theory and dynamical diffraction theory indicate broadening of rocking curve and appearance of satellite peak due to presence of compressive strain in the system [10]. The simulations data using dynamical theory of diffraction are shown in Fig. 3(a). It is clear from this figure that small changes in the strain can be seen. These simulation mechanism needs to be modified to account for the grazing incidence geometry (asymmetric Bragg case) [23,24]. More simulation work is being carried out using the modified Darwin theory for asymmetric Bragg condition. Fig. 3(b) shows the variation of strain obtained from the TEM studies using selected area diffraction method [10]. As predicted, the strain is found to be maximum at the projected range of gold in silicon substrate.
Fig. 3. (a) Simulation using X-ray dynamical diffraction theory as a function of strain. Inset shows two layers system with top 500 nm as a strained layer. The strain has been assumed to be constant in top layer. The bottom layer is assumed to be bulk lattice constant and (b) distribution of strain values at various depth in the irradiated silicon (with 1.5 MeV gold ions at a fluence of 5 · 1013 ions cm2). The strain values are obtained using selected area diffraction in TEM measurements (details are given in [10]).
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3.3. MeV ion induced effects in nanoislands of Co on silicon Formation of cobalt silicide using ion mixing for thick and continuous films was carried out by many groups [25,26]. More recently, a self-assembly nanopatterning method for epitaxial-CoSi2 layers has been developed based on anisotropic diffusion of Co/Si atoms in a stress field during rapid thermal oxidation [27]. CoSi2-nanostructures were also fabricated using a self-assembly process involving local oxidation of silicides [28]. Mantl reviewed the work on ion beam synthesis of silicide fabrication and characterization [29]. In this work, we report the MeV ion-induced effects in cobalt nanoislands deposited on crystalline silicon substrate and possibility of cobalt silicide formation using nano-scale ion beam mixing process. In our previous report, nanoscale ion beam mixing leading to formation of surface and sub-surface nano-gold silicide was discussed [5]. Fig. 4(a) shows a bright field planarTEM micrograph for thin cobalt film deposited on silicon substrate, while Fig. 4(b) shows RBS spectrum obtained from this specimen. The effective cobalt thickness found to be 2.3 nm. To obtain effective film thickness, bulk cobalt
Fig. 4. (a) BF planar TEM micrograph of 2.8 nm thick as-deposited cobalt film on silicon substrate and (b) RBS spectra for the above system used to determine the effective thickness.
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Fig. 5. (a) BF planar-TEM micrograph at lower magnification for the irradiated nano-cobalt film on silicon with 1.5 MeV Au2+ at a fluence of 1 · 1014 ions cm2, (b) high resolution TEM micrograph taken at high contrast area of the same system shown in part (a), (c) XTEM micrograph at lower magnification for the irradiated nano-cobalt film on silicon with 1.5 MeV Au2+ at a fluence of 1 · 1014 ions cm2, and (d) high resolution XTEM micrograph taken at high contrast area of the same system shown in part (c).
density has been used. From Fig. 4(a), the cobalt islands were found to form islands in a network fashion. From planar TEM micrograph, it is clear that no isolated nanoislands of cobalt were found in as-deposited case (Fig. 4(a)). Fig. 5(a) and (b) show BF planar-TEM micrograph for a system irradiated at a fluence of 1 · 1014 ions cm2 at an impact angle of 0 with respect to the surface normal. Planar micrograph taken at a lower magnification (Fig. 5(a)) shows high contrast zones from where a lattice image is taken and shown in Fig. 5(b). The lattice spacing is found to be 0.201 ± 0.005 nm from the high contrast (darker) areas. A low magnified BF image in cross-section mode is shown in Fig. 5(c). Few interesting observations can be made from this figure: (a) after the irradiation at a fluence of 1 · 1014 ions cm2, there appears to still a continuous layer present on the silicon substrate, (b) there are periodically spaced higher contrast areas from where a lattice image could be collected (Fig. 5(d)) and lattice spacing corresponds to 0.201 ± 0.005 nm. But lower contrast areas are found to be amorphous in nature, and (c) underneath the higher contrast islands, there are some modulation at SiO2–Si interface (i.e. change in the contrast in a wavy pattern at the interface). Bulk cobalt is found to have a structure either in the fcc phase (b-Co) (with lattice constant, ˚ ) or a close packed hcp (with a = 0.251 nm a = 3.55 A and c = 0.406 nm). For b-Co phase, the inter-planar spacing for {1 1 1} planes is (d111) 0.205 nm and is closed to the observed value of 0.201 nm. The closest to observed spacing for CoSi2 (diamond lattice) is that correspond to d220 (=0.188 nm). There are also other phases of CoxSiy where it exists lattice spacing closer to the observed lattice value (for example: CoSi-d100 = 0.198 nm; Co2Si3-d212 = 0.205 nm, d220 = 0.185 nm; Co2S-d021 = 0.205 nm, d220 =
2.02 nm, d12 = 1.97 nm). It could also be possible that the observed lattice spacing corresponds to the pure bulk Co and no silicide is formed. One requires additional methods to determine exact phases that are formed. To understand, energy confinement effects in these cobalt structures, we plan to perform more controlled experiments. 4. Conclusions Ion induced effects on nano-islands of gold and silver deposited silicon substrates have been reviewed. Enhanced sputtering yield and ion beam mixing leading to surface and subsurface nano-alloying found be interesting outcome of these studies. Tracking of MeV-ion induced changes in specified Au-nanoislands on silicon substrate as a function of ion fluence lead to determination of the threshold fluence for embedding and nano-scale mixing effects. Strain induced in the bulk silicon in the presence of nano-islands on the surface due to ion beam irradiation is determined by using HRTEM and asymmetric Bragg X-ray rocking curve method. Preliminary results on 1.5 MeV Au2+ ion-induced effects in nanoislands of Co deposited on silicon substrate have been discussed. Acknowledgements We thank Mr. A.K. Dash for his help in TEM measurements and the staff at IBL, Institute of Physics for ion implantations. We thank Dr. K. Hirano of the Photon Factory for help with X-ray measurements. P.V. Satyam would like to thank JSPS, Japan for funding his visit to KEK, and Nagoya University, Nagoya, Japan under invitation fellowship. We also thank Dr. D.K. Goswami for fruitful
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