130 MeV Au ion irradiation induced dewetting on In2Te3 thin film

Applied Surface Science 258 (2012) 8558–8563

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130 MeV Au ion irradiation induced dewetting on In2 Te3 thin film P. Matheswaran a , K.M. Abhirami a , B. Gokul a , R. Sathyamoorthy a,∗ , Jai Prakash b , K. Asokan c , D. Kanjilal c a

Department of Physics, Kongunadu Arts and Science College, Coimbatore 641029, India Department of Chemistry, M.M.H. College, Ghaziabad 201001, India c Materials Science Division, Inter University Accelerator Centre, New Delhi 110067, India b

a r t i c l e

i n f o

Article history: Received 29 February 2012 Received in revised form 27 April 2012 Accepted 9 May 2012 Available online 22 May 2012 Keywords: SHI irradiation Bilayer mixing Dewetting Indium chalcogenide

a b s t r a c t In/Te bilayer thin films were prepared by sequential thermal evaporation and subsequently irradiated by 130 MeV Au ions. The pristine and irradiated samples were characterized by X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) techniques. RBS spectra reveal the sputtering of Te film and interface mixing, with increasing fluence. The surface morphology showed the beginning of dewetting of Te thin film and formation of the partially connected with the mixed zones at the fluence of 1 × 1013 ions/cm2 . At the higher fluence of 3 × 1013 ions/cm2 , dewetted structures were isolated at the surface. Above results are explained based on the formation of craters, sputtering and dewetting followed by inter-diffusion at the interface of molten zones due to thermal spike induced by Au ions. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Ion beam irradiation is a unique tool for the synthesis and engineering of thin films. Interface mixing by ion beam irradiation is the topic of interest, not only for the technological point of view, but also in the scientific point of view, for understanding the ion beam interaction of surfaces and interfaces [1,2]. In the case of bilayer thin films, swift heavy ion (SHI) irradiation may induce mixing with nanostructuring at the surface embedded in the matrix and forming composite materials [3,4]. In recent years, there have been efforts to synthesis the material with right stoichiometry by SHI irradiation because of their importance in microelectronics and fabrication of nanoelectronic devices. Indium (In)–chalcogenide (S, Se, Te) systems have been widely studied for contact material and as dopant, and are of considerable interest in thin-film device applications. In particular, the In–Te thin films are of significant interest because of their switching applications [5,6]. Surface/interface modification in thin film systems can be achieved using low-energy [7,8] and high-energy ions [9,10] as well. Low-energy ions up to a few hundred keV undergo mostly nuclear stopping and the energy loss in this process is called as the nuclear energy loss (Sn). The energy transfer is used to displace atoms from its lattice site, which in turn may cause other atoms to recoil resulting in a collision cascade. Swift heavy ions (SHI) of few hundred MeV undergoes inelastic collisions resulting

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (R. Sathyamoorthy). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.05.048

in the excitation or ionization of the atoms by electronic stopping. From application point of view, synthesis of stoichiometric thin films by ion beam is an expensive process. On the other hand, producing desired stoichiometric films is not possible by other conventional methods [11,12]. Moreover, there are reports of diffusion of metal films leading to compound formation by other methods [13–15]. Present study reports synthesis of In2 Te3 thin film from In/Te bilayer by SHI irradiation with the emphasis on SHI induced mixing using 130 MeV Au ions. The motivation of present work is to (i) synthesize the material with desired stoichiometry, (ii) study and control the surface morphology/microstructure by controlling ion fluence and (iii) understand the phenomena of ion-induced mixing. It have been observed that ion beam irradiation of bilayer systems results in chemical modification at the interface, forming desired chemical compounds resulting in improved adhesion between constituting elements [16,17]. We have chosen Au for ion beam irradiation because of its nonreactive stable nature and therefore expected to drive interface mixing.

2. Experimental details In/Te bilayer thin films of thickness 600 nm were prepared by sequential thermal evaporation in an ultra-high-vacuum chamber (UHV) with the deposition rate 10 A˚ s−1 (Te)/2 A˚ s−1 (In). The total film thickness was chosen as 600 nm by depositing In/Te as bottom/top layer in atomic proportions. SHI irradiation was carried out with 130 MeV Au ions at the fluencies varying from 1 × 1012 to 3 × 1013 ions/cm2 at a current of 0.5 pnA (particle nano Ampre) using the Pelletron accelerator at Inter University Accelerator

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Centre (IUAC), New Delhi. Using SRIM-08 program, the electronic energy loss (Se ) and nuclear energy loss (Sn ) for 130 MeV Au ion in In/Te were calculated as 2.018 keV/Å and 3.742 keV/Å respectively. Range of 130 MeV Au ion was found to be 1.44 ␮m, which was greater than the film thickness implying that these Au ions will be deposited on the substrate. Structural analysis was carried out by glancing incidence XRD (GI-XRD) with minimum diffraction angle of 2◦ at the wavelength of CuK␣ (0.154 nm) using RIGAKU ULTIMA-3 instrument. Rutherford backscattering spectrometry (RBS) was used to analyze the interface mixing and sputtering with 2 MeV He+ ions at IUAC, New Delhi. He+ ions were bombarded perpendicularly to the samples and backscattered ions were detected at the angle 170◦ to the incident beam direction. Surface morphology of these samples was characterized by scanning electron microscope (MIRA-TESCAN and S4300-Hitachi). Energy dispersive X-ray (EDX) analysis was performed along with the SEM imaging technique using INCA Penta FETx3.

fluence of 3 × 1013 ions/cm2 is required to synthesis single-phase In2 Te3 phase from In/Te bilayer by 130 MeV Au ion irradiation. When SHI passes through this layered system, interacts with the lattice ions via inelastic collision, and favors the thermal spike diffusion at interfaces [18]. In the electronic stopping regime, defect creation and amorphous track formation are caused by SHI irradiation and the electronically deposited energy is transferred to the lattice atoms. Two theoretical models, namely the coulomb explosion (CE) and the thermal spike model (TSM) are under discussion to explain the interface mixing phenomena [19,20]. In the present context, conduction electrons in metals (In) and semi-metal (Te) would spread out the excitation and ionization of atoms rapidly, so that coulomb explosion method may not suitable to explain ion beam mixing phenomena. According to TSM, the kinetic energy of excited electrons was transferred to the lattice via electron–phonon coupling. The passage of SHI might increase the local temperature, thereby creating a transient molten state within short duration [21]. Thus mixing in In/Te is the consequence of inter-diffusion during the transient molten state and forms the In2 Te3 compound.

3. Results and discussion

3.2. Surface morphological and composition analysis

3.1. Structural analysis

Surface morphology and elemental composition of the samples were investigated before and after irradiation by SEM and EDX respectively. Fig. 2 shows the surface morphology of In/Te thin films. Surface of the pristine sample seems to have flake like morphology which corresponds to the top layer (Te) of the In/Te system. This flake like morphology of Te may be due to the higher chalcogenide rate of deposition [22]. Morphology of the sample irradiated with the fluence 1 × 1012 ions/cm2 also shows the presence of flake like structure but smaller in size, which may be due to annihilation of Te flakes at the surface. Further increase in the fluence 3 × 1012 ions/cm2 , flake like structure disappears and lead to the formation of tiny spherical particles. The film irradiated at the fluence of 1 × 1013 ions/cm2 shows the annihilation of tiny spherical particles. An intermediate stage of dewetting occurs, when the instabilities are comparable with the initial film thickness, which is characterized by the nucleation of holes whose size and shape evolve with time [23]. At higher fluence (3 × 1013 ions/cm2 ), complete dewetting structure is clearly visible and scattered throughout the surface. When the height of the instabilities reaches the substrate, the number of holes through surface layer is increased. The number of holes and their diameters increase considerably thereby creating coalesce with neighbors forming larger vacancies, which is referred as final stage of dewetting.

XRD pattern of pristine and irradiated In/Te samples at different fluencies (1 × 1012 , 3 × 1012 , 1 × 1013 , 3 × 1013 ions/cm2 ) are shown in Fig. 1. Pristine sample shows the crystallographic peaks correspond to the constituent elements (In & Te) [JCPDS Card No. 85-1409 & 86-2269]. The sample irradiated with the fluence of 1 × 1012 ions/cm2 shows the decrease in intensity of the elemental peaks, implies that the sample tends to form near amorphous phase. When fluence increases to 3 × 1012 ions/cm2 , the new peaks are evident in the XRD spectra which corresponds to In2 Te3 phase at 2 = 24.94, 41.39 and 49.0 along with the elemental peaks (Te (1 0 0), Te (1 0 1) and In (1 0 1)) with weak intensity [JCPDS Card No. 89-3978]. The sample irradiated with the fluence of 1 × 1013 ions/cm2 shows the formation of In2 Te3 phase. Further, the sample irradiated with the fluence of 3 × 1013 ions/cm2 shows the increase in intensity of In2 Te3 phase, which implies that the improvement in crystallinity of In2 Te3 phase. It shows that the

3.3. Elemental composition analysis Elemental composition of pristine and irradiated In/Te bilayer thin films are observed at two distinct parts of the film. It is denoted as white spot and dark spot in Fig. 2(a–e), where the EDX analysis was performed. In general, scattering of electrons from heavier element (Te) is more, hence it form bright field image, where as the scattered electron from light element (In) results dark field image [24]. In all these films, Te content in white spot is relatively greater than that in black spot, which also corroborate the scattering/inter-diffusion of In on to Te. The remaining wt.% of the elements corresponds to Si, O that arises from the substrate and Au from conductive surface coating (Table 1). 3.4. Optical analysis Fig. 1. XRD pattern of In/Te thin films [(a) pristine, (b) irradiated with 1 × 1012 ions/cm2 , (c) 3 × 1012 ions/cm2 , (d) 1 × 1013 ions/cm2 and (e) 3 × 1013 ions/cm2 ].

The band gap energy of In/Te system irradiated with Au 130 MeV for different fluence is shown in Fig. 3. As fluence increases, the band gap value increases progressively which implies that the phase

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Fig. 2. Surface morphology of In/Te thin films [(a) pristine, (b) irradiated with 1 × 1012 ions/cm2 , (c) 3 × 1012 ions/cm2 , (d) 1 × 1013 ions/cm2 and (e) 3 × 1013 ions/cm2 ].

formed. At lower ion fluence (1 × 1012 ions/cm2 ), the band gap of the irradiated system (0.72 eV) is found to be comparable with the band gap of Te phase (0.61 eV). In general, presence of high composition of Te may induce lower band gap of the system. As fluence increases to 3 × 1012 ions/cm2 , the Te rich phase starts diminish, whereas the expected In2 Te3 phase increases, which results Table 1 Elemental composition of pristine and irradiated Inx Tey thin films. Sample code

Pristine 1 × 1012 3 × 1012 1 × 1013 3 × 1013

3.5. Electrical analysis

Elemental Composition (wt.%) White spot

increase in band gap. At moderate fluence (1 × 1013 ions/cm2 ), the band gap of the irradiated system (1.05 eV) is found to be comparable with the band gap of In2 Te3 phase (1.10 eV). At higher fluence (3 × 1013 ions/cm2 ), the deduced band gap energy (1.13 eV) is approximately same as the band gap of In2 Te3 phase. Hence, the effect of SHI induced mixing confirms the presence of synthesized material by optical measurements and in line with XRD results.

Black spot

In

Te

In

Te

13.01 13.24 13.36 12.19 10.31

67.17 64.37 60.14 69.51 67.92

30.24 38.59 45.50 29.79 33.62

55.60 27.71 29.20 37.13 41.34

Arrhenius plot of In/Te system irradiated with Au 130 MeV for different fluence and temperature is shown in Fig. 4. The resistivity decreases with the increase in the temperature, indicating the semiconducting nature of the Inx Tey film. The activation energy is calculated in the temperature range from 348 K to 473 K. As fluence increases, the resistivity increases steadily which entail that ion induced phase transition. At lower fluence, presence of unreacted Te along with In4 Te3 results in lower resistivity, which in turn

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Fig. 3. (˛h)2 vs photon energy plot of Au irradiated Inx Tey thin films of different fluence.

shows higher conduction. As evident from XRD spectra, when fluence increases (3 × 1012 , 1 × 1013 ions/cm2 ) the ion induced phase transformation occurs from Te-rich phases to In2 Te3 phase by solid state reaction which implies increase in resistivity. This may be accredited to higher resistivity of In2 Te3 phase than In4 Te3 and Te phase. At higher fluence (3 × 1013 ) the system demonstrates the maximum resistivity similar to In2 Te3 phase. The activation energy value progressively increases (0.317–0.582 eV) with fluence, which is in good accord with the earlier reports for In2 Te3 phase [25]. The obtained value of activation energy (0.582 eV) is half that obtained for the value of optical band gap (1.13 eV). The low value of activation energy may be due to the presence of high concentration of localized states in the band structure [26]. As ion fluence increases, the unsaturated defects are gradually annealed out due to ion induced phase transition, producing a large number of saturated bonds. The reduction in the unsaturated defects decreases the density of localized states in the band structure, consequently increases the optical band gap, and hence increases the activation energy.

Fig. 4. Arrhenius plot of Au irradiated Inx Tey thin films of different fluence.

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Fig. 5. RBS spectra of pristine and Au irradiated Inx Tey bilayer thin films.

3.6. Rutherford back scattering spectra analysis RBS spectra were recorded at room temperature for pristine and irradiated In/Te bilayer thin films in an evacuated chamber at 1 × 10−9 m bar (shown in Fig. 5). In the case of pristine sample, the two separate peaks with different peak width appears implying the bilayer nature of In/Te thin film, where Te peak width (higher channel side) is greater than that of In peak, which indicates that the thickness of the Te layer is greater than that of In layer [27]. RBS spectra of Au ion irradiated (1 × 1012 ions/cm2 ) samples shows the decrease in intensity of Te and In peak may be due to the loss of In and Te atoms. Moreover, tailing in low energy edge of In peak indicates the inter diffusion of In on to Te layer. Further, the sample irradiated with the fluence of 3 × 1012 ions/cm2 shows the inward diffusion of In atoms on to Te layer. Complete tailing nature of In atom on to Te is observed in the case of 1 × 1013 ions/cm2 fluence which depicts the formation of In2 Te3 compound. Further increase in fluence results in sputtering of In2 Te3 compound. Rutherford Universal Manipulation Program (X-RUMP) was used to study the depth profile of In and Te. The simulated RUMP code was fitted over experimental RBS data [28]. The thickness of the pristine sample is found to be 600 nm. As fluence increases, the inward diffusion also increases. At the fluence of 3 × 1013 ions/cm2 , the loss of thickness (100 nm) was observed, which might be due to sputtering of the Te species. Above results of ion beam induced mixing and dewetting phenomena in In/Te bilayer thin films is schematically shown in Fig. 6. When an energetic ion penetrates a target lattice, the incoming ion loses its energy by interacting with both the electronic system and the nuclei of the lattice; this process results in local heating and atomic displacements [29]. The amount of energy dissipated in the lattice is expected to be very high in the case of heavy elements. This results in local melting along the ion track and results in thermal spike that could persist for tens of picoseconds [30,31]. Local melting at surfaces can lead to dramatic changes in the film morphology since large gradients in the temperature lead to gradients in the surface tension and local pressure, and these drive mass flow [32]. Craters or even holes can then form as a result of atom depletion in the original track of the ion. This ion induced formation of craters and holes results the dewetting pattern. One possible explanation is that local melting creates craters and holes. These dry patches (instabilities) act as nuclei for dewetting. The molten zone size thus sets a limit to the size of the nuclei, and defines the lateral

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Fig. 6. Schematic illustration of Au ion bean induced mixing and dewetting in Inx Tey thin films.

scale of the patterns. However, the SEM images show the presence of fissures in the as-deposited films due to grain boundary grooving or noncoalesced islands. The fissures expose dry patches of the substrate, which are more likely to be the nuclei for dewetting. The network of the dry patches produces the embryo of the pattern. The predefined density of the nuclei thus defines the lateral scale of the initial pattern formation [33]. Pristine sample illustrates the bilayer nature of the un-irradiated film. As fluence increases, the amount of mixing increases across the interface up to 1 × 1013 ions/cm2 . At high fluence (i.e., 3 × 1013 ions/cm2 ), the dewetting pattern is formed on the surface of the film. 4. Conclusion Thin films of single phase of In2 Te3 were prepared from In/Te bilayer by SHI irradiation. Structural and RBS analysis showed the formation of In2 Te3 phase as ion fluence increases to 1 × 1013 ions/cm2 . Further increase of ion fluence (3 × 1013 ions/cm2 ) results in the sputtering of In2 Te3 thin film. Surface morphology analysis gives raise to a clear picture of dewetting. The initial, intermediate and final stages of dewetting pattern were characterized by different ion fluence. The mechanism responsible for interface diffusion is explained on the basis of thermal spike model by forming molten zones. The SHI irradiation results in better atomic mixing across the In/Te interface and leads to a single phase In2 Te3 . Acknowledgments The author (PM) is thankful to Inter University Accelerator Centre (IUAC), New Delhi for supporting this research work through thesis proposal AUC-49222. He also thanks Dr. Ambuj Tripathi and Mr. Sunil Ojha for supporting respectively SEM and RBS analysis at IUAC, New Delhi. References [1] A.C. Sosa, P. Schaaf, W. Bolse, K.P. Lieb, Irradiation effects in Ag–Fe bilayers: ion-beam mixing, recrystallization, and surface roughening, Physical Review B 53 (1996) 14795–14805. [2] A. Hashibon, A.Y. Lozovoi, Y. Mishin, C. Elsässer, P. Gumbsch, Interatomic potential for the Cu–Ta system and its application to surface wetting and dewetting, Physical Review B 77 (2008) 0941311–0941319. [3] P. Süle, M. Menyhárd, Strong mass effect on ion-beam mixing in metal bilayers: a ballistic picture, Physical Review B 71 (2005) 113413–1134134. [4] J. Prakash, A. Tripathi, V. Rigato, J.C. Pivin, J. Tripathi, K.H. Chae, S. Gautam, P. Kumar, K. Asokan, D.K. Avasthi, Synthesis of Au nanoparticles at the surface and embedded in carbonaceous matrix by 150 keV Ar ion irradiation, Journal of Physics D: Applied Physics 44 (2011) 125302–125310.

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