Ion beam-induced modification of structural and ...

Radiation Effects and Defects in Solids Incorporating Plasma Science and Plasma Technology

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Ion beam-induced modification of structural and optical properties of MgTiO3 nanocrystalline thin films Arun Vinod, Mahendra Singh Rathore, T. Santhosh Kumar, D. Pamu, A. P. Pathak & N. Srinivasa Rao To cite this article: Arun Vinod, Mahendra Singh Rathore, T. Santhosh Kumar, D. Pamu, A. P. Pathak & N. Srinivasa Rao (2017) Ion beam-induced modification of structural and optical properties of MgTiO3 nanocrystalline thin films, Radiation Effects and Defects in Solids, 172:1-2, 81-89, DOI: 10.1080/10420150.2017.1286660 To link to this article: http://dx.doi.org/10.1080/10420150.2017.1286660

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Date: 25 April 2017, At: 02:39

RADIATION EFFECTS & DEFECTS IN SOLIDS, 2017 VOL. 172, NOS. 1–2, 81–89 http://dx.doi.org/10.1080/10420150.2017.1286660

Ion beam-induced modification of structural and optical properties of MgTiO3 nanocrystalline thin films Arun Vinoda , Mahendra Singh Rathorea , T. Santhosh Kumarb , D. Pamuc , A. P. Pathakd and N. Srinivasa Raoa a Department of Physics, Malaviya National Institute of Technology Jaipur, Jaipur, India; b Department of Applied Physics, Indian Institute of Technology (Indian School of Mines), Dhanbad, India; c Department of Physics, Indian Institute of Technology Guwahati, Guwahati, India; d School of Physics, University of Hyderabad, Central University (P.O), Hyderabad, India

ABSTRACT

ARTICLE HISTORY

MgTiO3 thin films were deposited on an amorphous silicon dioxide substrate by the RF magnetron sputtering method by varying the argon and oxygen mixing percentage. These pristine thin films were subjected to thermal annealing at a temperature of 700°C and subsequently two sets of crystalline thin films were irradiated using 100 MeV Au8+ ions at a fixed fluence of 2 × 1013 ions/cm2 . XRD spectra reveal that the crystallite size reduces from 21 to 15 nm and crystallinity also decreases drastically after irradiation. AFM images show significant variation in the surface morphology upon irradiation. Optical studies suggest that the band gap as well as the refractive index are both reduced due to SHI irradiation. Ion beam induced modification of the structural, morphological and optical properties of MgTiO3 thin films and the possible mechanisms responsible for variation of the properties of MgTiO3 due to swift heavy ion beam irradiation have been discussed in detail.

Received 26 November 2016 Accepted 17 January 2017 KEYWORDS

MgTiO3 thin films; sputtering; XRD; irradiation

1. Introduction During recent years, a growing interest in nanostructured thin films with high dielectric constants has attracted great attention due to their technological application in integrated optics and microwave devices (1–3). The use of dielectric materials is not only confined to optical switching, modulation and coupling in integrated optical communications but also in other integrated optical applications. The unprecedented growth of microwave communication has been made achievable due to materials of high dielectric constant, low loss and better thermodynamical and chemical stability (4). The use of nanostructured thin films plays a vital role in the miniaturization of dynamic random access memories which needs minimal power consumption in gadgets such as mobile phones, global positioning systems and satellite communications (5). Thus, a thin film material with high dielectric constant, low loss and good stability is in demand for numerous further miniaturizations of microwave integrated circuits (6–8). CONTACT A. P. Pathak [email protected], [email protected] Hyderabad, Central University (P.O), Hyderabad 500046, India © 2017 Informa UK Limited, trading as Taylor & Francis Group

School of Physics, University of

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Among various nanostructured dielectric thin films, MgTiO3 (MTO) is one of the most important materials for microwave system applications due to its excellent quality factor, dielectric constant and low dielectric loss (9). MTO qualifies due to its low dielectric loss resulting from its corundum structure, which is due to the isolation of the embedded layer of octahedral TiO6 between two layers of MO6 and of the cation vacancy that nevertheless happens in a perovksite structure. The growth of MTO thin films has been reported by using various methods such as sol-gel (10), electron beam (11), pulsed laser deposition (12) and RF sputtering (13). There are also reports on the growth of MTO nanostructured thick films using solar physical vapour deposition and electrophoretic deposition with a potential industrial electronic application as a possible replacement for bulk ceramic components (14); thus, the variation in pre- and post-deposition parameters and the methods of deposition result in modification of the structural, optical and morphological properties of the films (15). Among various methods, ion beam irradiation is one of the best techniques to engineer the properties of thin films due to an additional degree of freedom in choosing various input parameters such as ion species, energy and fluence. It also helps in basic and fundamental understanding of various changes in material properties due to high energy ion irradiation and harsh environment. Moreover, the swift heavy ion irradiation is one of the multifaceted tools for the modification of various properties of the materials by the producing controlled defects, structural disorder and phase transformations. Thus, ion beam irradiation using swift heavy ions (SHI) is one of the best and reliable techniques to tune and modify the properties of materials, nanocrystals and thin films by choosing appropriate ion energy and fluence (16). Hundred mega volts Au8+ ions were used to avoid the effects of ion implantation within the films by keeping the projected range of heavy ions fairly large compared to the film thickness and electronic energy loss is very much greater than nuclear energy loss. Hence, the present work aims at the tuning of structural and optical properties of MgTiO3 thin films using swift heavy ion beam irradiation.

2. Experimental details MgTiO3 sputtering targets were prepared by using a mechanochemical processing method. Powders of MgO and TiO2 with a purity of 99.99% were used as reagents and procured from Sigma-Aldrich. These powders were ball milled for 30 h using planetary ball milling, weighed according to the stoichiometry. The target was prepared using this powder after drying, sieving and pressing to achieve a target of 56 mm in diameter and 4 mm in thickness. The sputtering targets, then, were sintered at 1350°C for 3 h in air atmosphere. No trace of carbon contamination was observed, as the residue, after thermal treatment at 700°C, which was confirmed using EDX as reported in our earlier work (17). During deposition, the chamber was evacuated to high vacuum with the help of a diffusion pump which was backed by a rotary. Possible hydrocarbon contamination was restrained by using liquid nitrogen inlet traps connected to the diffusion pump. The base pressure was better than 1 × 10−5 mbar. Throughout the experiment, a constant working pressure 6 × 10−2 mbar was maintained. Thin films were deposited by changing the oxygen mixing percentage (OMP) at a constant power of 80 W at room temperature. These gases with high purity (99.99%) were introduced into the chamber using a mass flow controller. One

RADIATION EFFECTS & DEFECTS IN SOLIDS

83

Table 1. Summary of deposition conditions. Sl. no. 1 2

Sample code

RF Power (W)

OMP

Deposition Temp.

Annealed (°C)

MTO-1 MTO-2

80 80

30% 0%

Room Temp. Room Temp.

700 700

set of the MgTiO3 films was deposited at 30% oxygen (MTO-1) while the other at 0% oxygen (MTO-2) on amorphous silicon dioxide substrate. The thickness of the thin films was around 350 nm measured by a Digital Thickness Monitor. Subsequently, these pristine samples were heat-treated at 700°C for 1 h. The details of deposition parameters are shown in Table 1. Eventually, two sets of crystalline thin films were irradiated with swift heavy ions of 100 MeV Au with a fixed fluence of 2 × 1013 ions/cm2 , using a 15UD pelletron accelerator facility available at the Inter-University Accelerator Centre (IUAC), New Delhi, India. The XRD pattern along with the Rietveld refinement of the MTO sputtering target has been given in our earlier work (18). Crystal structures of MgTiO3 thin films were determined using Rigaku high power X-ray diffractometer (Rigaku,TTRAX) with Cu Kα (λ = 1.5406 Å) radiation. The XRD patterns of the thin films before and after irradiation were refined using the Rietveld refinement method with the help of the Fullprof program (19). Surface morphology of the samples were studied using Atomic Force Microscope (Agilent, Model 5500 series) which was used in non-contact mode. Optical spectra were recorded using UV–Vis-NIR spectrophotometer (UV 3101PC, Shimadzu) in the range of 200–1200 nm.

3. Results and discussions The as-deposited films exhibit amorphous nature while the subsequent annealing at 700°C resulted in crystallization. This is due to the high temperature annealing which helps in densification of the films and the orientation of grains in preferred directions. Films after annealing exhibited poly-crystalline trigonal structure. The XRD spectra of MTO-1 and MTO2 are shown in Figure 1. It is well understood from the spectra that the sample with less OMP are more crystalline in nature. The crystallite size was calculated using the Scherrer

Figure 1. XRD patterns of the MTO thin films (a) before and (b) after the ion beam irradiation.

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formula (20). D=

Kλ , β cos θB

where λ, β, θ B and K are, respectively, the wavelength of the X-ray used (Cu Kα – 1.5406 Å), full width at half maximum in radians, Bragg diffraction angle and a correction factor (which is a constant). A shrinkage of crystallite size from 24.7 to 20.1 nm was observed for the 30% OMP sample, whilst drastic changes in crystallite size before (29 nm) and after irradiation (15.1 nm) were observed for the sample without OMP. Crystallite size reduction may be due to irradiation of films with higher fluence, which results in multiple ion impacts and hence broadening of the peaks in XRD spectra was observed. It is evident from the reduction in intensity of the XRD spectra that the SHI induced partial amorphization (21). Furthermore, Rietveld analysis was performed through the Fullprof program by varying the position of atoms, cell and thermal parameters and the occupancy of Mg, Ti and O atoms (22). The refinement was performed by considering R3¯ space group for MgTiO3 . Figure 2 shows the Rietveld refinement results before and after irradiation of the samples. Rietveld refinement suggests that there is considerable difference in lattice constant and the volume of the cell expanded as a result of ion beam irradiation. The obtained values of crystallite size, lattice parameters and the volume of the cell have been listed in Table 2. AFM images show the signature of nanoparticles. The AFM images of MTO-2 before and after irradiation at different scales are shown in Figures 3 and 4. The roughness of the sample is found to decrease after ion beam irradiation. The surface roughness before and after irradiation is 5.99 and 2.85 nm. The decrease in roughness indicates that the surface is smoothened due to the rearrangement of the atoms by electronic excitation and ionization

Figure 2. Rietveld refinement of MTO-1 and MTO-2 samples (a) and (c) before and (b) and (d) after ion beam irradiation.


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Table 2. The crystallite size, lattice parameters and volume obtained from MTO thin films. Before Lattice parameter, a = b (Å) Lattice parameter, c (Å) Volume, V (Å)3

After

MTO-1

MTO-2

MTO-1

MTO-2

5.0443 (36) 13.8569 (54) 305.348 (407)

5.0403 (42) 13.8802 (50) 305.367 (451)

5.0477 (39) 13.8923 (122) 306.540 (423)

5.0573 (46) 13.8598 (118) 306.994 (412)

Figure 3. AFM image of MTO-2 before irradiation.

Figure 4. AFM image of MTO-2 after irradiation.

as a result of high energy ion irradiation (23). From the image, it is evident that the uniformity of the film increases with the irradiation. Thus, the increase in uniformity will be useful for applications involving less scattering of light (24, 25). The optical transmission spectra of MTO pristine as well as the SHI irradiated samples were recorded from 200 to 1200 nm. Figure 5 shows the variation of optical transmittance

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Figure 5. Optical transmittance spectra of MTO thin films before and after the ion beam irradiation.

for pristine and heavy ion irradiated samples. The occurrence of transmission fringes is clearly visible from the figure. This is due to the interference in air and substrate-film interface. At higher energy, transmission falls abruptly, and fringe presence disappears due to the presence of absorption edge at shorter wavelengths (17). It is also observed that the transmission of the film decreases after irradiation. The optical band gap (E g ) was calculated using the Tauc equation (26) for the samples before and after irradiation using the expression, (αhυ) = C(hυ − Eg )γ where C is a constant, hυ is the incident photon energy, α is the absorption coefficient and γ represents the allowed/forbidden electronic transitions such as allowed direct (0.5), forbidden direct (1.5), allowed indirect (2) and forbidden indirect (3). In this study, an assumption of γ = 2 was considered due to the indirect allowed electronic transitions exhibited by disordered titanates. Figure 6 shows the Tauc plot for the samples before and after irradiation. The bandgap of the annealed samples before irradiation are 4.07 and 4.14 eV for MTO-2 and

Figure 6. Tauc plot of MTO films (a) before and (b) after ion beam irradiation.


87

MTO-1, respectively. After irradiation, the bandgap of the films reduced to 3.21 and 3.50 eV, respectively. It is observed that samples with more OMP have a larger bandgap that may be attributed to the differences in density and defect states in the film (27, 28). Also, the decrease in bandgap after irradiation can be attributed to the appearance of defect levels in the films (29). By using the envelope method, the refractive index of the thin film is calculated from equation (30), n = [N + (N2 − n2S )0.5 ]0.5 Let T max and T min be the maximum and minimum transmittance at a certain wavelength λ and ns , the refractive index of the substrate used. Then, N can be calculated using N = 2nS

Tmax − Tmin Tmax Tmin

+

n2S + 1 . 2

If nf is the observed film refractive index and nb is the bulk refractive index of the thin film, then, the optical packing density (p) of the film is calculated by relation (31), n2f − 1 n2b + 2 p= . n2f + 2 n2b − 1 The porosity ratio, P, is defined as the volume of pores per unit volume of the film and is calculated by expression (32), n2f − 1 P =1− . n2b − 1 Table 3 represents the results of optical characterization before and after irradiation. It is found that the refractive index and the optical packing density of the thin films decrease while the porosity ratio of the film increases with ion beam irradiation. When the swift heavy ions pass through a material, they lose their energy mainly by two processes, namely nuclear energy loss and electronic energy loss. For SHI, the velocity of the incident ions is comparable to or greater than the Bohr velocity of electrons, which results in an inelastic scattering that leads to tracks creation, modifications, annealing effects, phase transitions, amorphization and damage creation. This is due to the sudden energy transfer resulting in localized thermal heating of the material. Furthermore, the energy is transferred to the sub-atomic system via electron-phonon coupling. The passage of 100 MeV Au ions in the material deposits the electronic energy (Se ) in the order of 20.42 KeV/nm and nuclear Table 3. Refractive index, packing density, porosity ratio and optical bandgap of the thin films before and after the ion beam irradiation. Before Refractive index, n (at 600 nm) Packing density Porosity ratio Optical bandgap, E g (eV)

After

MTO-1

MTO-2

MTO-1

MTO-2

2.11 0.910 0.194 4.14

2.14 0.924 0.166 4.07

1.91 0.793 0.389 3.50

1.90 0.787 0.398 3.21

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energy (Sn ) in the order of 0.36 KeV/nm. The projected range of the swift heavy ions is calculated, by SRIM software, as 9.61 µm, which is much higher than the film thickness (350 nm) and avoids ion implantation. The partial amorphization produced by high energy ion beams is evident from XRD results. SHI also induces smoothening of surface of thin films which is due to re-arrangement of atoms as indicated by AFM. The defect levels created by ion beams resulted in decrease in bandgap of MTO films as shown in UV–Vis spectra. Thus, ion beam irradiation turns out to be a versatile tool for the possible modification of properties of thin films.

4. Conclusion Ion beam induced modification and subsequent characterization of MgTiO3 films deposited on an amorphous silicon dioxide substrate by the RF magnetron sputtering method by varying the OMP have been investigated. The XRD Spectrum reveals the reduction in crystallinity and also a decrease in crystallite size. AFM image shows that the uniformity of the film is increased after irradiation and the surface roughness decreased. Samples before irradiation have better transmittance than samples after irradiation. A decrease in refractive index, band gap and packing density is observed after irradiation while the porosity ratio increases due to irradiation. The possible mechanism for tuning the properties of the films due to ion beam irradiation has been explained.

Acknowledgements Arun Vinod and NSR acknowledge the financial support from DST-SERB, New Delhi. We would like to thank Dr D.K. Avasthi, Inter-University Accelerator Centre (IUAC), New Delhi, for his help during ion irradiation experiments and fruitful discussions. APP thanks CSIR for the Emeritus Scientist award. TSK acknowledges the infrastructure facility of XRD provided by DST, New Delhi, through the FIST program [SR/FST/PSII-020/2009].

Disclosure Statement No potential conflict of interest was reported by the author(s).

References (1) Santhosh Kumar, T.; Gogoi, P.; Bhasaiah, S.; James Raju, K.C.; Pamu, D. Mater. Res. Express 2015, 2, 056403. (2) Gogoi, P.; Santhosh Kumar, T.; Sharma, P.; Pamu, D. J. Alloys Compd. 2015, 619, 527–537. (3) Tan, T.; Liu, Z., Lu, H., Liu, W.; Tian, H. Opt. Mater. 2010, 32, 432–435. (4) Robertson, J. Rep. Prog. Phys. 2006, 69, 327–396. (5) Chen, Y.-B.; Huang, C.-L. Surf. Coat. Technol. 2006, 201, 654–659. (6) Kingon, A.I.; Maria, J.-P.; Streiffer, S.K. Nature 2000, 406, 1032–1038. (7) Huang, C.-L.; Wang, S.-Y.; Chen, Y.-B.; Li, B.-J.; Lin, Y.-H. Curr. Appl. Phys. 2012, 12, 935–939. (8) Surendran, K.P.; Mohanan, P.; Sebastian, M.T. J. Solid State Chem. 2004, 177, 4031–4046. (9) Ferreira, V.M.; Baptista, J.L. Mater. Res. Bull. 1994, 29, 1017–1023. (10) Surendran, K.P.; Wu, A.; Vilarinho, P.M.; Ferreira, V.M. Chem. Mater. 2008, 20, 4260–4267. (11) Lee, C.-H.; Kim, S.-I. Integr. Ferroelectr. 2003, 57, 1265–1270. (12) Kang, S.; Lim, W.; Lee, J. Integr. Ferroelectr. 2000, 31, 94. (13) Santhosh Kumar, T.; Gogoi, P.; Thota, S.; Pamu, D. AIP Adv. 2014, 4, 067142. (14) Apostol, I.; Mahajan, A.; Monty, C.J.A.; Saravanan, K.V. Appl. Surf. Sci. 2015, 358, 641–646.


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(15) Khodier, S.A.; Sidki, H.M. J. Mater. Sci. 2001, 12, 107–109. (16) Avasthi, D.K., Mehta, G.K., Eds. In Swift Heavy Ions for Materials Engineering and Nanostructuring, Springer Series in Materials Science, 2011, 145, 1. (17) Santhosh Kumar, T.; Bhuyan, R.K.; Pamu, D. Appl. Surf. Sci. 2013, 264, 184–190. (18) Santhosh Kumar, T.; Bhuyan, R.K.; Perumal, A.; Pamu, D. Springer Proc. Phys. 2013, 143, 291–300. (19) Young, R.A. The Rietveld Method, International Union of Crystallography; Oxford University Press: London, 1996. (20) Cullity, B.D. Elements of X-Ray Diffraction; Addison-Wesley Publishing Company, Inc.: Massachusetts, 1956. (21) Saikiran, V.; Srinivasa Rao, N.; Devaraju, G.; Pathak, A.P. Nucl. Instrum. Methods Phys. Res. B 2014, 323, 14–18. (22) Rietveld, H.M. J. Appl. Crystallogr. 1969, 2, 65–71. (23) Hou, M.-d.; Klaumunzer, S.; Schumacher, G. Phys. Rev. B 1990, 41, 1144–1157. (24) Ebrahimiasl, S.; Yunus, W.Md.Z.W.; Kassim, A.; Zainal, Z. Solid State Sci. 2010, 12, 1323–1327. (25) Ko, H.; Tai, W.-P.; Kim, K.-C.; Kim, S.-H.; Suh, S.-J.; Kim, Y.-S. J. Cryst. Growth 2005, 277, 352–358. (26) Tauc, J.C. Optical Properties of Solids, F. Abeles, Ed.; North-Holland: Amsterdam, 1972. (27) Shin, J.-H.; Choi, D.K. J. Korean Phys. Soc. 2008, 53, 2019. (28) Subrahmanyam, A.; Karuppasamy, A. Solar Energy Mater. Solar Cells 2007, 91, 266–274. (29) Hu, B.; Yao, M.; Xiao, R.; Chen, J.; Yao, X. Ceramics Int. 2014, 40, 14133–14139. (30) Swanpoel, R. J. Phys. E 1983, 16, 1214–1222. (31) Jacobson, R. Phys. Thin Films 1975, 8, 51. (32) Ye, Q.; Liu, P.Y.; Tang, Z.F.; Zhai, L. Vacuum 2007, 81, 627–631.