Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954
Structural and Optical Properties of DC Magnetron Sputtered Zirconium Titanate Thin Films of Varied Film Thickness1 D. Jhansi Rani1,a, A. Guru Sampath Kumar1, T. Subba Rao1 1 – Materials research laboratory, Dept. of Physics, Sri Krishnadevaraya University, Anantapuramu, India a – jhansiranidvr@gmail.com DOI 10.2412/mmse.82.12.44 provided by Seo4U.link
Keywords: zirconium titanate thin films, film thickness, DC magnetron reactive sputtering, wave guides.
ABSTRACT. Zirconium titanate thin films with thickness in the range of 245 to 715 nm were deposited by employing direct current magnetron reactive sputtering technique and the film properties have been studied as a function of film thickness. The films exhibited high transmittance of 80-91% and the band gap energy decreased from 3.4 to 3.1 eV with increase in thickness while the packing density of the films increased with film thickness. The crystallinity of the films improved with increase in thickness. The X-ray diffractograms showed a predominant peak in (111) orientation corresponding to the scattering angle of 30o. The surface morphology demonstrated that the denser is the film the smoother is the surface.
Introduction. The high-k dielectric materials, ZrO2, TiO2, Ta2O5, ZrTiO4 and Zr (Sn, Ti) O4 act as potential candidates for gate dielectrics, dynamic random access memory (DRAM) and microwave communications. Besides high dielectric constant, high quality factor, good thermal, chemical stabilities [1] and high permittivity zirconium titanate (ZTO) exhibits good optical properties. It has high transmittance over a wide range of wave length and high refractive index [2] as well. Hence it finds application as wave guides in microwave frequency regions. In this paper, we demonstrated the fabrication, characterization and the effect of thickness on the properties of nano crystalline ZTO thin films deposited on glass substrates by DC magnetron reactive sputtering technique. Experimental details. Initially, the vacuum chamber was evacuated to a base pressure of 1 X 10 -5 mbar by the combination of diffusion and rotary pumps. The sputtering powers of 155 watt and 175 watt were applied to Zr and Ti targets respectively. Thickness has been measured by Bruker’s α step profilometer as 245, 325, 545, 620 and 715nm. Results and discussion: XRD. Thickness had a pronounced effect on structural properties and is illustrated by the diffractograms shown in Fig.1.The films with thickness of 245 and 325 nm were not well crystallized but, for the films with thickness from 545nm onwards crystallinity improved gradually. The denser film (715 nm) is characterized by more crystallization and orientation, with a high intense peak in (111) direction at 30.15o and minor peaks in (120), (311) directions appeared with less intensities at 50.35o and 59.82o respectively. The grain sizes were estimated from Debye Scherrer’s formula [3]:
D
k cos b
where D – is size of the crystallite; 1
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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954
k – is the Scherer’s constant; o
λ – is the X-ray wave length 1.54 A ; β – is the full width at half maximum; θb – is the Bragg’s angle. The crystallite size varied from 1.71 nm to 13.14 nm, confirming the nano structure of the deposited films. The thickness vs. crystallite size is shown in Fig. 1.
Fig. 1. (a) X-ray diffractograms and (b) crystallite size. SEM. The surface morphology has been studied from SEM micrographs obtained by using FESEMSUPRA 55. Fig. 2 (a)-(e) illustrates that the films were pin hole and crack free. The images revealed the evolution and thickness dependence of the grain size.
Fig. 2. SEM micrographs of the films with thickness (a) 245 nm, (b) 325 nm, (c) 545 nm, (d) 620 nm and (e) 715 nm.
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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954
EDXS. The spectra shown in Fig. 4 also conveyed the presence of only the elements Zr, Ti and O at the corresponding binding energies of 0.7 keV (O), 2 keV (Zr) and 4.5 and 5 keV (Ti). This ensures the purity of the films.
Fig. 3. EDAS spectra of the films with thickness (a) 245 nm, (b) 325 nm, (c) 545 nm, (d) 620 nm and (e) 715 nm. Optical properties. The transmittance spectra of ZTO thin films have been recorded in the ultra violet visible near IR (UV-VIS-NIR) region by Hitachi U-3400 spectrophotometer within a wave length range from 200 to 900 nm. From the spectra, transmittance varied as 91, 87, 86, 83 and 80% for films with thickness 245, 325, 545, 620 and 715 nm respectively.
Fig. 4. Transmittance spectra of films deposited at distinct thicknesses. The optical packing density of the deposited films could be obtained by [4]: n f 1 nb 2 2 % p 2 2 n f 2 nb 1 2
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Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954
where nb – is the bulk refractive index; nf – is the refractive index of the thin films. The packing density increased from 81 to 98% with thickness.
Fig. 5. Variation of packing density with film thickness. Summary Nano crystalline zirconium titanate thin films were deposited on to glass substrates using DC magnetron sputtering, by varying the deposition time, which in turn varies film thickness. Thickness has a considerable effect on the film properties. The films showed higher transmittance. The films have high optical packing density of 81 to 98%. Optical transmittance spectra revealed that all the films have high transmittance in the visible region above 300nm of wavelength. Acknowledgements. The author, D. Jhansi Rani, gratefully acknowledges Department of Science and Technology (DST), New Delhi for financial aid under INSPIRE Fellowship (IF120615). References [1] Y. Kim, Jeongmin oh, T.G. Kim, B.W. Park, Jpn. J. Appl. Phys 40, 4599-4603 (2001). [2] A.P. Huang, P.K. Chu, H. Yan, M.K. Zhu, J. Vac. Sci.Technol. B 23, 566 (2005). [3] B.D. Cullity, Elements of XRD,Addison Wesley publishing company, Massachusetts, 170, (1967). [4] D. Pamu, K. Sudheendran, M.G. Krishna, K.C.J. Raju, J. Mat. Sci. Engg. B 168, 208-213, (2010).
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