Effects of high-energy ion-beam irradiation on ...

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

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Effects of high-energy ion-beam irradiation on structural and optical properties of (Mg0.95Co0.05)TiO3 thin films T. Santhosh Kumar, Arun Vinod, Mahendra Singh Rathore, A. P. Pathak, Fouran Singh, D. Pamu & N. Srinivasa Rao To cite this article: T. Santhosh Kumar, Arun Vinod, Mahendra Singh Rathore, A. P. Pathak, Fouran Singh, D. Pamu & N. Srinivasa Rao (2018) Effects of high-energy ion-beam irradiation on structural and optical properties of (Mg0.95Co0.05)TiO3 thin films, Radiation Effects and Defects in Solids, 173:1-2, 128-137, DOI: 10.1080/10420150.2018.1442452 To link to this article: https://doi.org/10.1080/10420150.2018.1442452

Published online: 13 Apr 2018.

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RADIATION EFFECTS & DEFECTS IN SOLIDS, 2018 VOL. 173, NOS. 1–2, 128–137 https://doi.org/10.1080/10420150.2018.1442452

Effects of high-energy ion-beam irradiation on structural and optical properties of (Mg0.95 Co0.05 )TiO3 thin films T. Santhosh Kumara , Arun Vinodb , Mahendra Singh Rathoreb , A. P. Pathakc , Fouran Singhd , D. Pamue and N. Srinivasa Raob a Department of Applied Physics, Indian Institute of Technology (Indian School of Mines) Dhanbad, Dhanbad,

India; b Department of Physics, Malaviya National Institute of Technology Jaipur, Jaipur, India; c School of Physics, University of Hyderabad, Central University (P.O), Hyderabad, India; d Materials Science Group, Inter-University Accelerator Centre, New Delhi, India; e Department of Physics, Indian Institute of Technology Guwahati, Guwahati, India ABSTRACT

ARTICLE HISTORY

We report the structural and optical properties of high-energy ionbeam irradiated Co-doped magnesium titanate thin films. (Mg0.95 Co0.05 )TiO3 (MCT) thin films were deposited on quartz substrates using radio frequency magnetron sputtering. Subsequently, the films were annealed for crystallinity and were irradiated with 100 MeV Ag ions by varying the ion fluence. The X-ray diffraction patterns of the films before and after the irradiation were refined using the Rietveld refinement and the variations in the lattice parameters were correlated with the ion fluence. Although, annealing of thin films results in an enhancement in refractive index and optical bandgap, the ion fluence induces significant changes in the refractive index and optical bandgap. Atomic force microscopy is employed to study the surface morphology of the films. The impact of ion fluence on structural and optical properties of MCT thin films has been investigated.

Received 23 November 2017 Accepted 29 January 2018 KEYWORDS

RF magnetron sputtering; MCT thin films; ion beam irradiation; bandgap

1. Introduction Recently, dielectric materials have attracted much attention in the size reduction of integrated circuits for optical and microwave communication applications. Various microwave dielectric devices require threefold characteristics such as high dielectric constant for the size reduction, the frequency selectivity and stability that depends on the high-quality factor and zero temperature coefficient of resonant frequency (1). MgTiO3 qualifies to possess low dielectric loss because of 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 perovskite structure (2). Many researchers have investigated the possibilities of attaining better-optimized dielectric characteristic by the introduction of various transition metals. A low percentage (0.5 at.%) doping of Nb results in the removal of defect levels produced due to the oxygen vacancies in MgTiO3 ceramics (3). The dielectric properties of CONTACT A. P. Pathak

[email protected], [email protected]

© 2018 Informa UK Limited, trading as Taylor & Francis Group

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MTO ceramics remain invariant even after the introduction of transition metals such as Mn and W (4). A partial substitution of Mg2+ by Co2+ , Ni2+ and Zn2+ significantly improves the microwave dielectric properties of MgTiO3 (5–6). The interaction between the TiO6 octahedra and MO6 octahedra layer will be minimized with doping, which also modifies the crystal structure (7). The doping effects of Cr in MgTiO3 have also been reported, where the Q value slightly increases for small doping concentration but decreases for higher concentration (8). An improvement in quality factor of around 17% with Ta doping has also been reported by Kuang et al. (9). The effects of B2 O3 on the structural and dielectric properties were investigated by Wang et al. (10) B2 O3 not only exhibited better dielectric properties but also improved the quality factor and temperature coefficient of resonant frequency. In addition, various oxides were used to modify the properties of MgTiO3 . Lee et al. (11) reported the MTO thin films as a buffer layer for high-purity LiNbO3 grown epitaxially on the c-axis oriented Al2 O3 substrates for the applications of integrated optical devices. The doped MTO thin films were synthesized by using various methods. Surendran et al. (12) studied the doping effects of transition metals like Ni and Zn on the temperature stability and dielectric constant of MgTiO3 thin films and found a decrease in dielectric constant with Ni and an increase with Zn due to the difference in ionic polarizabilities of these two dopants. A partial substitution of Ni and Co substitution in A-site improves the structural, dielectric and optical properties of MgTiO3 thin films (13–15). Thus, pre- and postdeposition parameters, doping element, annealing temperature and method of deposition greatly alter the optical, structural and morphological properties of the films (16). Swift heavy ion beam irradiation is one of the interesting tools to tune and modify the properties of materials, nanocrystals and thin films by choosing appropriate ion energy and fluence (17–18). In our previous works, (Mg0.95 Co0.05 ) TiO3 (MCT) thin films were prepared by radio frequency (RF) magnetron sputtering under different oxygen mixing percentage (OMP) on various substrates and the effects of post-annealing and OMP were studied. The structural, dielectric and impedance spectroscopy investigations at low frequencies were carried out on MCT films deposited on platinized silicon substrates (15), whereas the structural, morphology, optical and microwave dielectric properties have been studied for the MCT films deposited on amorphous silicon substrates (19). From the above studies, we found that the MCT films exhibit better structural, optical and dielectric properties than MTO thin films. Furthermore, the effects of fixed ion beam fluence on structural and optical properties of MgTiO3 thin films deposited under various OMPs have also been reported in our earlier work (20). Understanding various properties and behavior of these materials under high-energy ion beam irradiation is of great interest from a basic and applied research point of view. Hence, the present work aims at the ion-beam induced modification of the structural and optical properties of Co-doped MgTiO3 thin films at various ion fluences.

2. Experimental details The MCT thin films were deposited onto a quartz substrate at a deposition temperature of 300°C with a fixed RF power of 45 W from a ceramic (Mg0.95 Co0.05 )TiO3 sputtering target. The sputtering target was prepared from semi-alkoxide precursor method and the detailed preparation method is reported earlier (19, 21–22). Firstly, the deposition chamber

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T. SANTHOSH KUMAR ET AL.

Table 1. Sample details of films irradiated with 100 MeV Ag ions at various fluences. Irradiation fluence (ions/ cm2 )

Sample code

5 × 1012 1 × 1013 2 × 1013 3 × 1013

MCT1 MCT2 MCT3 MCT4

was evacuated up to a base pressure of 1.0 × 10−6 mbar prior to the deposition. The substrate to target distance was kept at 5 cm. High-purity argon (99.99%) gas was introduced into the chamber using a mass-flow controller. The sputtering target was pre-sputtered in argon ambient for 10 min to clean the surface and the sputtering pressure of 3 × 10−2 mbar was constantly maintained throughout the deposition. The deposition was carried out for 3 h and the thickness of deposited films was around 480 ± 10 nm, which was confirmed by a surface profilometer. Pristine films (MCTa) were annealed at 700°C for 1 h to achieve good crystallinity. Subsequently, these crystalline films (MCTc) were irradiated with swift heavy ions (SHI) of 100 MeV Ag7+ at different fluences ranging from 5 × 1012 to 3 × 1013 ions/cm2 , using Pelletron accelerator facility at Inter-University Accelerator Centre (IUAC), New Delhi, India. Details of films irradiated with 100 MeV Ag ions at various fluences are listed in Table 1. The ion beam was scanned over an area of 1 × 1 cm2 of the sample at a constant beam current of 1 pnA. The electronic energy loss (Se ), nuclear energy loss (Sn ) and projected range of 100 MeV Ag ions in MCTc thin films have been calculated using SRIM software and are given by 18.9 keV/nm, 0.094 keV/nm and 9.26 µm, respectively. Rigaku high-power X-ray diffraction (XRD) machine (Rigaku, TTRAX III) with CuKα radiation (λ = 1.5406 Å) operated at 9 kW (50 kV and 180 mA) power has been employed to study the structure of these films. The surface morphology of the thin films was studied using an Atomic Force Microscope (Agilent Technologies, Model 5250) under a constant force non-contact mode. The transmission spectra over the wavelength range 200–1000 nm were measured using a UV–VIS–NIR Spectrophotometer (UV 3101PC, Shimadzu).

3. Results and discussion The XRD patterns of annealed and ion-beam irradiated MCT thin films are shown in Figure 1. It is found that the annealed film at 700°C for 1 h exhibits a crystalline behavior with a rhombohedral crystal structure (JCPDS #060494). There is no evidence of any other secondary phase. After ion beam irradiation at different fluences, the intensity of crystalline planes was reduced and peaks were broadened up to 1 × 1013 ions/cm2 fluence and above this, all the peaks disappear. Also, the position of prominent (104) peak slightly shifted toward a higher angle with fluence. The crystallite size was calculated using Scherer’s formula (23): D=

Kλ , β cos θB

(1)

where λ, β, θ B and K are the wavelength of the X-ray used (Cu Kα – 1.54056 Å), full width at half maximum (FWHM) of the most dominant peak in radian, Bragg diffraction angle and a constant, respectively. The crystallite size is found to reduce to 11.3 nm from 19.8 nm, when


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Figure 1. XRD patterns of annealed and SHI-irradiated MCT thin films at different fluence.

irradiated with 1 × 1013 ions/cm2 fluence. The reduction in intensity of XRD peaks indicates that irradiation induces partial amorphization for fluence up to 1 × 1013 ions/cm2 . However, the films were completely amorphized when the ion fluence increases to 2 × 1013 ions/cm2 and above. The reduction of crystallite size could be due to the surface structure fragmentation resulting from multiple ion impacts (24). Rietveld analysis was performed through Fullprof program by varying the position of atoms, cell and thermal parameters and the occupancy of Mg, Co, Ti and O atoms. The Rietveld refinement method is a versatile technique for refining the crystal structures from neutron and X-ray powder diffraction data. This technique has the ability to extract detailed information about a crystal structure from powder diffraction data. It allows refinement of the selected parameters such as lattice parameters, atomic positions, occupancy and thermal parameters to minimize the difference between observed diffraction data and the theoretical pattern calculated, based on the hypothesized crystal structure and other instrumental parameters. It requires a high-quality diffraction pattern, structural model and suitable peak and background functions and provides the reliable crystallographic information of the experimental diffraction pattern (25). The refinement was performed by considering R3¯ space group for (Mg0.95 Co0.05 )TiO3 . Figure 2 shows the Rietveld refinement results of crystalline MCT films irradiated with 5 × 1012 and 1 × 1013 ions/cm2 . The typical lattice parameters and crystallite size values of MCT thin films are given in Table 2. Rietveld refinement suggests a considerable difference in lattice parameters with irradiation. The lattice parameters are found to decrease with ion fluence and finally, amorphization takes place. Similar kind of variation in lattice parameter and crystallite size has been reported by Singh et al. (26) in Au-irradiated ZnO thin films. The stress present in the irradiated films is responsible for this change. This is attributed to the electronic energy loss (Se ) induced by heavy ions which are liable to promote a significant amount of oxygen vacancies, resulting in variation of lattice constants and stress in the crystallites.

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Figure 2. Rietveld refinement results of (a) MCTc, (b) MCT1 and (c) MCT2 thin films.

Table 2. The crystallite size and lattice parameters of crystalline, MCT1 and MTC2 thin films. Sample name Parameters Lattice parameter, a = b (Å) Lattice parameter, c (Å) Crystallite size (nm)

MCTc

MCT1

MCT2

5.0525 (38) 13.8370 (76) 19.8

5.0477 (39) 13.8923 (122) 14.4

5.0403 (42) 13.8802 (50) 11.3

To study the effects of 100 MeV Ag7+ ion beam irradiation on the surface morphology of MCT thin films, atomic force microscopy (AFM) measurements were performed on the annealed and irradiated samples. The AFM images of crystalline and SHI-irradiated MCT thin films are depicted in Figure 3. Different surface morphologies and root mean square surface roughness have been noticed due to annealing and subsequent irradiation. The annealed films exhibit a uniform surface morphology with pronounced grain boundaries due to the significant diffusion process and the roughness is found to be 15.7 nm. A coarse morphology with a decrease in surface roughness of 1.6 nm is observed for the sample irradiated with 1 × 1013 ions/cm2 ion fluence. At higher fluence, films exhibit a rod-like structure due to ion beam irradiation. The decrease in roughness indicates that the surface is smoothened due to the rearrangement of the atoms by electronic excitation and ionization as a result of SHI irradiation (27).


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Figure 3. Atomic force microscopic images of MCT thin films at different fluence: (a) MCTc, (b) MCT2 and (c) MCT4.

Figure 4. (a) Optical transmittance spectra of MCT thin films before and after the SHI irradiation. (b) Envelope drawn for the MCT1 thin film.

The optical transmittance spectra were recorded for the pristine as well as SHI-irradiated MCT thin films in the range of 200 to 1000 nm. Figure 4(a) shows the variation of optical transmittance spectra of pristine and irradiated samples. The sharp fall in transmission and the disappearance of fringes at shorter wavelengths are due to the fundamental absorption of the films. At lower energies (i.e. above 300 nm), all the samples show transmittance fringes which originate due to the interference of light at the air and substrate–film interfaces. A sharp increase in transmittance has been observed at wavelengths of 280 nm for annealed samples, which is shifted to 300 nm after irradiation. The average transmittances of all the films were in the range of 75–98%, above 400 nm which indicates the nature of high quality in deposited films. It is also found that the ion-beam irradiated films exhibit high transmittance when compared to annealed films. The annealing causes an increase in average crystallite size, which scatters the light on grain boundaries. The packing density of the films is higher for annealed films, which do not allow the light to pass through the film and hence the transmittance is lower, which is also confirmed from the surface morphology of AFM images (Figure 3(a)). After ion irradiation, the surface of MCT films become porous, which causes an increase in the transmittance in the present samples. Envelope method is a well-known technique to determine the optical constants such as refractive index (n), extinction coefficient (k) and thickness (d) of the thin films deposited on transparent substrates (28). A wavelength-dependent transmittance (or reflectance)

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Figure 5. (a) Wavelength-dependent refractive index for the pristine and ion-beam-irradiated MCT thin films. (b) Variation in refractive index, packing density and porosity ratio as a funcion of sample conditions.

spectra is generally used to measure these parameters. The extremes of the interference transmittance fringes have been used to construct the envelopes using parabolic interpolation and derived the refractive index from the following equations: n = [N + (N2 − n2S )0.5 ]0.5 .

(2)

Let T max and T min be the maximum and minimum transmittance at a certain wavelength λ and the refractive index ns of the substrate used. Then, N can be calculated using 2 Tmax − Tmin ns + 1 N = 2ns + . (3) Tmax Tmin 2 The optical packing density (P) is the measure of the compactness of the deposited film and is directly related to the refractive index of the medium, i.e., speed of light in a medium. Higher optical packing density indicates the lesser light speed in a medium and vice versa. The optical packing density of the film is calculated by using the relation (29) n2f − 1 n2b + 2 P= , (4) n2f + 2 n2b − 1 where nf and nb are the thin film and bulk refractive indices of MgTiO3 (bulk refractive index = 2.31), respectively (30). Porosity ratio (Pr ) is defined as the volume of pores per unit volume of the film and is calculated by the following expression: n2f − 1 Pr = 1 − . (5) n2b − 1 The wavelength-dependent refractive index n of pristine and ion-beam irradiated MCT thin films have been shown in Figure 5(a). It is found that the refractive index exhibits dispersion behavior at the shorter wavelength below 600 nm and above this, it is constant irrespective of sample conditions. This indicates that 600 nm may be considered as the nondispersive region. Figure 5(b) shows the variation in refractive index, packing density and porosity ratio as a function of sample conditions. It is observed that the refractive index and packing density follow a similar trend and the porosity ratio exhibits opposite behavior with sample conditions. The refractive index shows the high value of 2.2 at 600 nm for


135

Figure 6. (a) Variation in optical bandgap as a function of ion beam fluence; (b) A plot of (αhv)1/2 vs. hv; (c) magnified view of optical transmittance spectra of pristine and ion-beam-irradiated MCT thin films.

the annealed film. After the ion beam irradiation, this gradually decreases to a minimum value of 2.05 at 2 × 1013 ions/cm2 fluence and at higher fluence it increases slightly. The refractive index and packing density values are in the range of 2.05–2.20 and 0.87–0.94, respectively, for pristine and irradiated samples. The higher value of refractive index for annealed films is attributed to the high packing density and low porosity ratio of the films. The post-deposition annealing causes a reduction in the inter-atomic spacing due to crystallization that leads to higher packing improvement in morphology and hence exhibits higher values of refractive index (31). The ion-beam irradiated films become amorphous above 1 × 1013 ions/cm2 , resulting in lower film density with porous morphology (Figure 3(c)), which leads to the lower refractive index. The optical band gap (Eg ) was calculated using Tauc equation (32) for the samples before and after irradiation and is given by (αhυ) = C(hυ − Eg )γ ,

(6)

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, γ = 2 was considered due to the indirect allowed electronic transitions exhibited by disordered titanates (33). Figure 6(a) shows the variation in optical bandgap as a function of ion beam

136


fluence. The bandgap is determined from an intercept on the energy axis of (αhv)1/2 vs. hv plot for MCT film with different conditions, as shown in Figure 6(b). The bandgap of annealed film is found to be 4.07 eV and is in good agreement with the earlier reports (34). A significant variation in Eg value has been observed with ion beam irradiation. With an increase in ion fluence, Eg reaches a minimum value of 3.63 eV at 2 × 1013 ions/cm2 . At higher fluence (3 × 1013 ions/cm2 ), a slight increase in Eg is found and is 3.74 eV. It is also confirmed from the optical transmittance spectra (Figure 6(c)) that absorption edge shifts towards the higher wavelength region with ion fluence. A similar behavior was reported by Thakur et al. (35) in their works on SnO2 thin films when irradiated with Ag15+ ion beam. There are various factors that contribute to the bandgap variation in oxide films with ion beam fluence. In the present case, the possible mechanism for variation in the Eg can be candidly related to the change in the particle size because we found a consistent fall in particle size up to 2 × 1013 ions/cm2 and then an increase at 3 × 1013 ions/cm2 fluence. Thus, the reduction of Eg with ion fluence is due to the creation of intermediate energy levels, which gives rise to a transition from the valence band to those levels instead of band-toband transitions (36). This decrease in the band gap gives an indication of the stoichiometric deviation and oxygen vacancies in the irradiated MCT films.

4. Conclusions MCT thin films were deposited onto quartz substrates by RF magnetron sputtering at 300°C. The as-prepared films were annealed at 700°C and subsequently, the crystalline films were irradiated with high energy ions by varying the fluence from 5 × 1012 to 3 × 1013 ions/cm2 . The ion beam irradiation induces amorphization of MCT thin films, which causes a gradual reduction in the lattice parameters, refractive index and optical bandgap. The uniform particle size, higher packing density and lower porosity ratio are the evidence for large refractive index after annealing, whereas a coarse morphology, less packing and higher porosity, as well as crystalline to amorphous transition significantly affect the optical properties as a result of ion beam irradiation. The results obtained from ion irradiation have been compared with those obtained by thermal annealing.

Disclosure statement No potential conflict of interest was reported by the authors.

Funding T. Santhosh Kumar and D. Pamu acknowledge the financial support from Board of Research in Fusion Science & Technology [NFP-RF-A12-01], the facilities provided by DRDO [ERIP/ER/0900371/M/01/1264], DST [SR/FTP/PS-109/2009], DAE-BRNS [37(1)/14/33/2015/BRNS], India and the infrastructure facility of XRD provided by DST, New Delhi, through the FIST program [SR/FST/PSII-020/2009]. T. Santhosh Kumar thanks Prof. A. K. Nirala for support and for mentoring his Post-doctoral Fellowship at IIT (ISM) Dhanbad. Arun Vinod and N. Srinivasa Rao acknowledge the support from Science and Engineering Research Board, New Delhi. A.P. Pathak thanks CSIR New Delhi for the Emeritus Scientist award.

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