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Magnetron sputtered Dy2O3 with chromium and copper contents for antireflective thin films with enhanced absorption
Journal of Rare Earths 37 (2019) 989e994
Contents lists available at ScienceDirect
Journal of Rare Earths journal homepage: http://www.journals.elsevier.com/journal-of-rare-earths
Magnetron sputtered Dy2O3 with chromium and copper contents for antireflective thin films with enhanced absorption Shahid M. Ramay a, Asif Mahmood b, Hamid M. Ghaithan c, Nasser S. Al-Zayed a, Adnan Aslam c, Abdullah Murtaza c, Nisar Ahmad c, Saadat A. Siddiqi d, Murtaza Saleem e, * a
Physics and Astronomy Department, College of Science, King Saud University Riyadh, Saudi Arabia Chemical Engineering Department, College of Engineering, King Saud University Riyadh, Saudi Arabia Department of Physics, The University of Lahore, 1 - KM Raiwind Road, Lahore, Pakistan d Interdisciplinary Research Centre in Biomedical Materials, COMSATS Institute of Information Technology, Defence Road, Off Raiwind Road, Lahore, Pakistan e Department of Physics, SBASSE, Lahore University of Management Sciences (LUMS), Lahore 54792, Pakistan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2018 Received in revised form 9 December 2018 Accepted 10 December 2018 Available online 30 January 2019
Dy2O3 is a rare earth oxide having a number of advanced applications in various fields including protective or antireflective coatings. Main objective of this novel research work is to check the effect of Cr and Cu addition on different properties of Dy2O3 and achievement of antireflective thin films with enhanced absorption. Thin films of these materials were deposited using DC magnetron with reactive cosputtering. XRD studies reveals the crystalline nature of thin films having Dy2O3 (222) reflection in all samples with Cr2O3 (116) and CuO (111) reflections in Cr and Cu containing compositions. Field emission scanning electron microscopy demonstrates the homogeneous deposition of thin films with uniform shape, size and distribution of grains. Refractive index, extinction coefficient and absorption coefficient significantly increase while optical reflectance decreases with Cr and Cu mediation corroborating an improved antireflective mechanism. The imaginary part of dielectric constant is found to increase slightly with low tangent loss for Cr containing composition considered favorable for energy storage applications. © 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Keywords: Thin films Dysprosium oxide X-ray diffraction FESEM Optical properties Anti-reflective
1. Introduction Dysprosium oxide (Dy2O3) is a rare earth oxide having good chemical and thermal stability with broad band gap (z4.26 eV) and high dielectric constant (z13). Dy2O3 is frequently exercised as a constituent of optical fibers in the areas of optics, as well as filters, switches, modulators, corrosion resistive and antireflection coatings with enlarged spectral transparency over the region of ultraviolet to infrared and high refractive index.1e7 The refractory nature and thermo-dynamic stability of Dy2O3 present it as an appropriate material for corrosion resistive coatings of high temperature stainless steel.8,9 It has been studied as a constituent of
* Corresponding author. E-mail address: [email protected] (M. Saleem).
protective optical layers,10 as an optical coating for envelops of ceramic lamps,11 as superconductors, as a solid oxide fuel cell anode material,12 as resistive switching in microelectronic devices.13e15 Dy2O3 thin films have been fabricated by various techniques, including electron-beam and thermal evaporation,16,17 atomic layer deposition,18 and reactive magnetron sputtering.14,15 It seems that physical properties of thin films are strongly dependent upon fabrication technique and deposition parameters. The optical properties including refractive index (n), absorption coefficient (∞), extinction coefficient (k), and band gap (Eg) of Dy2O3 have been extracted and studied using small prism technique.4 The value of refractive index 1.85 was reported by Goswami and Varma17 in vacuum evaporated dysprosium oxide thin films. Wiktorczyk16 fabricated Dy2O3 thin film using electron beam evaporation for refractive index analysis to find its dispersion relationship. Among rare earth oxides, 4.26 and 4.8 eV band gaps of Dy2O3 have been achieved with relatively high dielectric constant.19,20 High thermal
https://doi.org/10.1016/j.jre.2018.12.002 1002-0721/© 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
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S.M. Ramay et al. / Journal of Rare Earths 37 (2019) 989e994
stability and low leakage current were ultimately observed in these thin films. Dielectric properties in metal/oxide/metal structure of these thin films were also reported by Taylor and Gomez.21 Horoz et al. reported the band gap value of Dy2O3 as 3.90 eV using ab initio calculations.22 Recently Cherif et al. reported the effect of temperature on electrical properties of Dy2O3 thin films grown on Si substrate.23 We have found a little experimental work in literature on Dy2O3 related thin films. In this novel research work we focus on the optical properties of Dy2O3 thin films for improved antireflecting mechanism. Cr and Cu are considered as good absorbers for anti-reflective or protective coating layers.24e26 The effects of Cr and Cu addition on Dy2O3 thin films regarding structural, morphological and optical properties are briefly explored. Enhanced absorption in Dy2O3 thin films with addition of Cr and Cu contents reveals it an interesting candidate for modern micro and opto-electronic devices. 2. Experimental Dy2O3, Cr/Dy2O3 and Cu/Dy2O3 thin films were fabricated using DAON 1000 S magnetron sputtering system. Dy and Cu were cosputtered in reactive argon and oxygen atmosphere and same for Dy and Cr case. Silicon (100) substrates were ultrasonically cleaned before deposition. Initial vacuum of 1.33104 Pa was achieved with turbo pumping station. Argon and oxygen flow were kept with ratio of 70:30, respectively. Flow rate was adjusted at 6.65 Pa during deposition process. Sputter rates for Dy, Cu and Cr were calibrated and controlled using sycon thickness monitor (STM). The contents of Cu and Cr were controlled as 10 wt% during deposition. Substrate temperature was kept at 500 C during deposition for crystalline growth of thin films as reported by Barreca.27 DC magnetron sputtering with voltage of 300 V and 100 mA current for Dy, 250 V and 30 mA for Cu and Cr was employed. Thin films with 100 ± 5 nm thickness of Dy2O3, Cu/Dy2O3 and Cr/Dy2O3 were achieved to explore its various properties. X-ray diffraction (XRD) analysis was
carried out using a Panalytical Xpert Pro Multipurpose X-ray diffractometer to explore the crystal structure and phase identification. XRD was operated at 30 kV and 10 mA using Cu Ka1 radiation (l ¼ 0.154 nm) with a step scan size of 0.05 . The presence of required elemental contents was confirmed with an Oxford instrument INCA X-Act energy dispersive X-ray spectroscope (EDS). EDS spectra were taken with 20 kV electron beam and spot size of 6 mm to gain maximum counts for elemental composition analysis. Surface morphology of thin films was observed with a FEI Nova Nano-SEM-450 field emission scanning electron microscope (FESEM). The micrographs were acquired using electron beam energy of 10 kV at 200 K magnification with through lens detector (TLD) under secondary electron (SE) mode. Brief optical properties containing refractive index, extinction coefficient, absorption coefficient, band gap, high dielectric constant and percentage reflectance were determined using spectroscopic ellipsometry and UV-Vis spectrophotometry. Dielectric measurements were acquired using a Quadtech Precision 1910 LCR meter. A separate set of silicon substrate were deposited with platinum (Pt) before deposition of thin films by leaving some area un-deposited for contact formation. Silver paste was then used to make contacts on both sides of thin films. 3. Results and discussion 3.1. X-ray diffraction (XRD) analysis Fig. 1(a) shows the XRD spectra of Dy2O3 thin film deposited with magnetron sputtering system. Main XRD peak was observed approximately at 29.11 which is exactly matched with (222) reflection of Dy2O3 given in standard ICDD pattern # 00-009-0197. Dy2O3 cubic phase with space group la3-206 is usually developed under these conditions and was already reported in some other published work as well.28 Fig. 1(b) presents the Cr/Dy2O3 pattern of magnetron sputtered thin films. It also contains another peak
Fig. 1. XRD patterns of Dy2O3 (a), Cr/Dy2O3 (b), Cu/Dy2O3 (c) thin films and crystallite size, lattice constant variation (d).
S.M. Ramay et al. / Journal of Rare Earths 37 (2019) 989e994 Table 1 Structural and optical properties of magnetron sputtered Dy2O3, Cr/Dy2O3, and Cu/Dy2O3 thin films. Property
Dy2O3
Cr/Dy2O3
Cu/Dy2O3
Crystallite size (nm) Lattice constant (nm) Dislocation density (nm2) Refractive index Extinction coefficient Absorption coefficient (cm1) Band gap (eV)
43.25 0.987 5.34 104 1.71 0.140 2.65 104 3.83
31.92 0.943 9.81 104 1.89 0.039 0.28 104 3.53
26.77 0.931 1.39 103 2.01 0.212 4.28 104 3.36
addition to the Dy2O3 which is corresponding to the Cr2O3 associated with most intense (116) reflection as given in standard ICDD pattern # 00-001-1294 of Cr2O3 and reported by Larbi et al.29 Similarly, Fig. 1(c) presents Cu/Dy2O3 XRD spectra of magnetron sputtered thin films. It contains some other peaks in addition to the Dy2O3 which correspond to the CuO associated with (101) phase as given in standard ICDD pattern #00-001-1117 of CuO.30 This shows that in case of DyeCueO and DyeCreO deposition, Cu and Cr not only substitute the Dy sites but also form the CuO and Cr2O3 phases as well, respectively. Crystallite size and dislocation density were kl calculated using well known Scherrer's formula D ¼ FWHM and cosq d ¼ 1/D, respectively.31 Average crystallite size and lattice parameter determined and plotted in Fig. 1(d) exhibit variation with addition of Cr and Cu due to difference in ionic radii and lattice distortion. Crystallite size, lattice constant and dislocation density of Dy2O3 thin films were found to decease with contents of Cr and Cu as given in Table 1. 3.2. Field emission scanning electron microscopy (FESEM) FESEM micrographs were taken using 10 kV electron beam at 200 k magnification with TLD detector in secondary electron (SE) mode, working distance was kept at 5 mm. Fig. 2(aec) show
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the FESEM micrographs of Dy2O3, Cu/Dy2O3 and Cr/Dy2O3 thin films, respectively. All the micrographs reveal homogeneous deposition of thin films with approximately uniform shape, size and distribution of grains as can also be seen from surface plots given in insets of the micrographs. It is observed that Cr/Dy2O3, and Cu/Dy2O3 thin films contains smaller size of grains than Dy2O3 thin film due to incorporation of Cr and Cu content exactly in accordance with the XRD analysis. Overall average grain size in all samples was observed under less than 50 nm in range. SEM histogram profiles in Fig. 2(d) showing the same trend of surface morphology depicts that larger particles or grains produces more intense number of pixels as briefly described in our previously published work.32 3.3. Energy dispersive X-ray spectroscopy (EDS) EDS is a state-of-the-art technique associated with qualitative and quantitative elemental analysis of material compositions. EDS spectra were taken with 20 kV electron beam and spot size of 6 mm to gain maximum counts for elemental composition analysis. Fig. 3 shows the EDS spectra of magnetron sputtered Dy2O3, Cr/Dy2O3 and Cu/Dy2O3 thin films. All EDS spectra confirm the presence of expected elemental contents in corresponding thin film compositions. Si peak is associated with silicon substrate. Elemental compositions acquired for each thin film are presented in Table 2. 3.4. Optical properties Optical properties were measured using spectroscopic ellipsometry for magnetron sputtered Dy2O3, Cr/Dy2O3 and Cu/Dy2O3 thin films. Mean square error (MSE) of the instrument was recorded before starting of analysis. Spectroscopic ellipsometry data were acquired by adjusting the source and detector at angle of 70 . Cauchy model was employed for thin films with silicon substrate to extract the psi (J) and delta (D) parameters. Thickness of Dy2O3, Cr/Dy2O3 and Cu/Dy2O3 thin films were observed as approximately
Fig. 2. SEM micrograph of Dy2O3 (a), Cr/Dy2O3 (b), and Cu/Dy2O3 (c) thin films. with inset surface plots, and SEM histograms (d).
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S.M. Ramay et al. / Journal of Rare Earths 37 (2019) 989e994
Fig. 3. EDS spectra of Dy2O3 (1), Cr/Dy2O3 (2), and Cu/Dy2O3 (3) thin films.
Table 2 Elemental compositions of samples calculated using EDX analysis.
Fig. 5. Extinction coefficient variation with wavelength of Dy2O3 (1), Cr/Dy2O3 (2) and Cu/Dy2O3 (3) thin films.
100 ± 5 nm. The variations of J and D along wavelength for all samples were fitted with best model curves. Refractive index (n), extinction coefficient (k) and absorption coefficient (∞) for all thin films were measured and plotted regarding the corresponding wavelength values. Fig. 4 shows the trend of refractive indices with wavelength for Dy2O3, Cr/Dy2O3 and Cu/Dy2O3 thin films. Refractive index for Dy2O3 was recorded to be 1.71 at 632.8 nm and significantly enhanced to 1.89 with addition of Cr and 2.01 for Cu content. Extinction coefficient is basically defined as the strength of material to absorb the light. Extinction coefficient of Dy2O3 is found to decrease first from 0.140 to 0.039 with addition of Cr content while increase with Cu content up to 0.212 as can be seen in Fig. 5. Similar trend was observed for absorption coefficient (∞) as shown in Fig. 6, initially, absorption coefficient reduces with the increase of
wavelength in all samples then a slightly increase is noticeable at elevated wavelength values. Overall, it is observed that refractive index, extinction coefficient and absorption coefficient significantly change with addition of Cu and Cr in magnetron sputtered Dy2O3 thin films which can be useful for applications of these thin films with high antireflection. Optical band gap values for each thin film were extracted from the relation between refractive index and band gap defined by Ravindera and Gupta (n ¼ 4:084 0:62Eg).33 The value of optical band gap was found to decrease from 3.83 to 3.53 eV for Cr/Dy2O3 and 3.36 eV for Cu/Dy2O3 thin films. This experimentally obtained band gap value for Dy2O3 is in close consistence with the recently reported band gap value of 3.90 eV using ab-initio calculations.22 High frequency dielectric constant was also evaluated using the relation (Ɛ∞ ¼ n2)31 and depicts a significant decrease with addition of Cr and Cu contents in Dy2O3 thin films. All extracted optical parameters from spectroscopic Ellipsometry are given in Table 1. Percentage reflection was measured using UV-Vis spectrophotometry as shown in Fig. 7, which depicts a significant decrease in reflection in thin film with Cr and Cu contents. Structure morphology, elemental and optical properties present significant changes with addition of Cr and Cu in Dy2O3 thin films. The variation in optical parameters of Dy2O3 dealt it as improved protective coating layer. Thin films with Cr and Cu addition in Dy2O3 will definitely present improved antireflective mechanism.
Fig. 4. Refractive index variation with wavelength of Dy2O3 (1), Cr/Dy2O3 (2) and Cu/ Dy2O3 (3) thin films.
Fig. 6. Absorption coefficient variation with wavelength of Dy2O3 (1), Cr/Dy2O3 (2) and Cu/Dy2O3 (3) thin films.
Dy2O3 Element O(K) Dy(L) Si(K) Cr(K) Cu(K) Total
wt% 9.35 4.88 85.77 e e 100
Cr/Dy2O3 at% 4.68 20.16 75.16 e e 100
wt% 11.28 3.86 83.66 1.20 e 100
Cu/Dy2O3 at% 5.73 16.31 74.41 1.82 e 100
wt% 9.45 3.97 84.61 e 1.97 100
at% 4.79 16.59 75.01 e 3.61 100
S.M. Ramay et al. / Journal of Rare Earths 37 (2019) 989e994
Fig. 7. Reflectance spectra showing variation with wavelength of Dy2O3 (1), Cr/Dy2O3 (2) and Cu/Dy2O3 (3) thin films.
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Fig. 10. Tangent loss (tand) of Dy2O3 (1), Cr/Dy2O3 (2) and Cu/Dy2O3 (3) thin films.
permittivity of free space Ɛ0 ¼ 8.85 1012 F/m, while d and A represent thickness and effective area of thin film, respectively. The real dielectric constant as a function of frequency is given in Fig. 8, showing the decreasing trend with frequency. The value of dielectric constant was decreased with addition of Cu and Cr contents in Dy2O3 thin films. Imaginary part of dielectric constant was 00 0 also extracted from real part using the relation, ε ¼ ε tand, presented in Fig. 9, demonstrating the similar trend like real part. Dissipation factor or tangent loss as a function of frequency as presented in Fig. 10 reveals a considerable decrease in tangent loss especially Cu containing Dy2O3 thin films. 4. Conclusions
Fig. 8. Real dielectric constant of Dy2O3 (1), Cr/Dy2O3 (2) and Cu/Dy2O3 (3) thin films.
The uniform and homogeneous thin films of Dy2O3, Cr/Dy2O3 and Cu/Dy2O3 were successfully deposited on Si substrate in thickness range of 100 ± 5 nm using reactive magnetron cosputtering. Structural analysis exhibits the crystalline nature with cubic structure of these thin films with significant variation in crystallite size, lattice parameters and dislocation density. Surface morphology shows the smooth, even, flat, plane and uniform distribution of nano-sized grains with expected elemental contents in corresponding compositions. Brief optical properties reveal a significant variation in refractive index, extinction coefficient, absorption coefficient, optical band gap and reflectance. Dielectric properties reveal a low tangent loss in Dy2O3 thin films having Cu and Cr contents. Enhanced absorption of Dy2O3 thin films with Cu and Cr contents dealt it with high antireflective mechanism and improved protective coating layer. Acknowledgement The authors would like to extend his sincere appreciation to the Deanship of Scientific Research at King Saud University for funding under Research Group (No. RG 1435-004).
Fig. 9. Imaginary dielectric constant of Dy2O3 (1), Cr/Dy2O3 (2) and Cu/Dy2O3 (3) thin films.
3.5. Dielectric measurements The parallel capacitance Cp and dissipation factor Df or tand as a function of frequency were acquired through precise LCR meter. Real part of dielectric constant was calculated using the relation, ε'' ¼ Cpd /ε0 A, where Cp denotes the parallel capacitance,
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4. Medenbach O, Dettmar D, Shannon RD, Fischer RX, Yen WM. Refractive index and optical dispersion of rare earth oxides using a small-prism technique. J Opt Pure Appl Opt. 2001;3:174. 5. Zahra S, Sahar ZA, Masoud SN. Dysprosium cerate nanostructures: facile synthesis, characterization, optical and photocatalytic properties. J Rare Earths. 2017;35:805. 6. Grosse F, Watahiki T, Braun W. Rare earth oxide alloys and stacked layers: an ab initio study. Thin Solid Films. 2010;518:4747. 7. Abu-Zied BM, Asiri AM. Synthesis of Dy2O3 nanoparticles via hydroxide precipitation: effect of calcination temperature. J Rare Earths. 2014;32:259. 8. Bonnet G, Lachkar M, Colson JC, Larpin JP. Characterization of thin solid films of rare earth oxides formed by the metallo-organic chemical vapour deposition technique for high temperature corrosion applications. Thin Solid Films. 1995;261:31. 9. Bonnet G, Lachkar M, Larpin JP, Colson JC. Organometallic chemical vapour deposition of rare earth oxide thin films. Application for steel protection against high temperature oxidation. Solid State Ionics. 1994;72:344. 10. Dong X, Cheng XL, Zhang XF, Sui LL, Xu YM, Gao S, et al. A novel coral-shaped Dy2O3 gas sensor for high sensitivity NH3 detection at room temperature. Sens Actuators, B. 2018;255:1308. 11. Wei GC. Transparent ceramics for lighting. J Eur Ceram Soc. 2009;29:237. 12. He BB, Zhao L, Wang WD, Chen FL, Xia CR. Electro-catalytic activity of Dy2O3 as a solid oxide fuel cell anode material. Electrochem Commun. 2011;13:194. 13. Zhao HB, Tu HL, Wei F, Zhang XQ, Xiong YH, Du J. Resistive switching characteristics of Dy2O3 film with a Pt nanocrystal embedding layer formed by pulsed laser deposition. Rare Met. 2014;33:75. 14. Pan TM, Lu CH. Forming-free resistive switching behavior in Nd2O3, Dy2O3, and Er2O3 films fabricated in full room temperature. Appl Phys Lett. 2011;99: 113509. 15. Pan TM, Chang WT, Chiu FC. Structural properties and electrical characteristics of high-k Dy2O3 gate dielectrics. Appl Surf Sci. 2011;257:3964. 16. Wiktorczyk T. Dysprosium oxide thin films: preparation and characterization. Eur J Solid State Inorg Chem. 1991;28:581. 17. Goswami A, Varma R. Dielectric behaviour of dysprosium oxide films. Thin Solid Films. 1975;28:157. 18. Xu K, Ranjith R, Laha A, Parala H, Milanov AP, Fischer RA, et al. Atomic layer deposition of Gd2O3 and Dy2O3: a study of the ald characteristics and structural and electrical properties. Chem Mater. 2012;24:651. 19. Chandar NK, Jayavel R. Wet chemical synthesis and characterization of pure and cerium doped Dy2O3 nanoparticles. J Phys Chem Solid. 2012;73:1164.
20. Ohmi SI, Yamamoto H, Taguchi J, Tsutsui K, Iwai H. Effects of post dielectric deposition and post metallization annealing processes on metal/Dy2O3/Si(100) diode characteristics. Jpn J Appl Phys. 2004;43:1873. 21. Taylor DM, Gomez HL. Electrical characterization of the rectifying contact between aluminium and electrodeposited poly(3-methylthiophene). J Phys D Appl Phys. 1995;28:2554. 22. Horoz S, Simsek S, Palaz S, Mamedov AM, Ozbay E. Electronic and Optical Properties of Dy2O3: ab initio calculation. Int J Sci Technol Res. 2015;1:36. 23. Cherif A, Jomni S, Belgacem W, Elghoul N, Khirouni K, Beji L. The temperature dependence on the electrical properties of dysprosium oxide deposited on p-Si substrate. Mater Sci Semicond Process. 2015;29:143. 24. Sabine K, David K, Rafael SP, Claudio A, Miguel ALM, Silvana B. Optical properties of Cu-chalcogenide photovoltaic absorbers from self-consistent GW and the Bethe-Salpeter equation. Phys Rev B. 2015;91, 075134. 25. Hu D, Wang HY, Zhu QF, Zhang XW, Tang ZJ. Investigation of a broadband and polarization-insensitive optical absorber based on closed-ring resonator. J Nonlinear Opt Phys Mater. 2016;25:1650032. 26. Teixeira V, Sousa E, Costa MF, Nunes C, Rosa L, Carvalho MJ, et al. Spectrally selective composite coatings of CreCr2O3 and MoeAl2O3 for solar energy applications. Thin Solid Films. 2001;392:320. 27. Barreca D, Milanov A, Tondello E, Devi A, Fischer RA. Nanostructured Dy2O3 films: an XPS investigation. Surf Sci Spectra. 2007;14:52. 28. Al-Kuhaili MF, Durrani SMA. Structural and optical properties of dysprosium oxide thin films. J Alloys Compd. 2014;591:234. 29. Larbi T, Ouni B, Gantassi A, Doll K, Amlouk M, Manoubi T. Structural, optical and vibrational properties of Cr2O3 with ferromagnetic and antiferromagnetic order: a combined experimental and density functional theory study. J Magn Magn Mater. 2017;444:16. 30. Akaltun Y. Effect of thickness on the structural and optical properties of CuO thin films grown by successive ionic layer adsorption and reaction. Thin Solid Films. 2015;594:30. 31. Akaltun Y, Yıldırım MA, Ates A, Yıldırım M. The relationship between refractive index-energy gap and the film thickness effect on the characteristic parameters of CdSe thin films. Opt Commun. 2011;284:2307. 32. Saleem M, Manzoor A, Zaffar M, Hussain SZ, Anwar MS. Tailoring of ZnO with selected group-II elements for LED materials. Appl Phys A. 2016;12:589. 33. Gupta VP, Ravindra NM. Comments on the Moss Formula. Phys Stat Sol (b). 1980;100:715.
Contents lists available at ScienceDirect
Journal of Rare Earths journal homepage: http://www.journals.elsevier.com/journal-of-rare-earths
Magnetron sputtered Dy2O3 with chromium and copper contents for antireflective thin films with enhanced absorption Shahid M. Ramay a, Asif Mahmood b, Hamid M. Ghaithan c, Nasser S. Al-Zayed a, Adnan Aslam c, Abdullah Murtaza c, Nisar Ahmad c, Saadat A. Siddiqi d, Murtaza Saleem e, * a
Physics and Astronomy Department, College of Science, King Saud University Riyadh, Saudi Arabia Chemical Engineering Department, College of Engineering, King Saud University Riyadh, Saudi Arabia Department of Physics, The University of Lahore, 1 - KM Raiwind Road, Lahore, Pakistan d Interdisciplinary Research Centre in Biomedical Materials, COMSATS Institute of Information Technology, Defence Road, Off Raiwind Road, Lahore, Pakistan e Department of Physics, SBASSE, Lahore University of Management Sciences (LUMS), Lahore 54792, Pakistan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2018 Received in revised form 9 December 2018 Accepted 10 December 2018 Available online 30 January 2019
Dy2O3 is a rare earth oxide having a number of advanced applications in various fields including protective or antireflective coatings. Main objective of this novel research work is to check the effect of Cr and Cu addition on different properties of Dy2O3 and achievement of antireflective thin films with enhanced absorption. Thin films of these materials were deposited using DC magnetron with reactive cosputtering. XRD studies reveals the crystalline nature of thin films having Dy2O3 (222) reflection in all samples with Cr2O3 (116) and CuO (111) reflections in Cr and Cu containing compositions. Field emission scanning electron microscopy demonstrates the homogeneous deposition of thin films with uniform shape, size and distribution of grains. Refractive index, extinction coefficient and absorption coefficient significantly increase while optical reflectance decreases with Cr and Cu mediation corroborating an improved antireflective mechanism. The imaginary part of dielectric constant is found to increase slightly with low tangent loss for Cr containing composition considered favorable for energy storage applications. © 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Keywords: Thin films Dysprosium oxide X-ray diffraction FESEM Optical properties Anti-reflective
1. Introduction Dysprosium oxide (Dy2O3) is a rare earth oxide having good chemical and thermal stability with broad band gap (z4.26 eV) and high dielectric constant (z13). Dy2O3 is frequently exercised as a constituent of optical fibers in the areas of optics, as well as filters, switches, modulators, corrosion resistive and antireflection coatings with enlarged spectral transparency over the region of ultraviolet to infrared and high refractive index.1e7 The refractory nature and thermo-dynamic stability of Dy2O3 present it as an appropriate material for corrosion resistive coatings of high temperature stainless steel.8,9 It has been studied as a constituent of
* Corresponding author. E-mail address: [email protected] (M. Saleem).
protective optical layers,10 as an optical coating for envelops of ceramic lamps,11 as superconductors, as a solid oxide fuel cell anode material,12 as resistive switching in microelectronic devices.13e15 Dy2O3 thin films have been fabricated by various techniques, including electron-beam and thermal evaporation,16,17 atomic layer deposition,18 and reactive magnetron sputtering.14,15 It seems that physical properties of thin films are strongly dependent upon fabrication technique and deposition parameters. The optical properties including refractive index (n), absorption coefficient (∞), extinction coefficient (k), and band gap (Eg) of Dy2O3 have been extracted and studied using small prism technique.4 The value of refractive index 1.85 was reported by Goswami and Varma17 in vacuum evaporated dysprosium oxide thin films. Wiktorczyk16 fabricated Dy2O3 thin film using electron beam evaporation for refractive index analysis to find its dispersion relationship. Among rare earth oxides, 4.26 and 4.8 eV band gaps of Dy2O3 have been achieved with relatively high dielectric constant.19,20 High thermal
https://doi.org/10.1016/j.jre.2018.12.002 1002-0721/© 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
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S.M. Ramay et al. / Journal of Rare Earths 37 (2019) 989e994
stability and low leakage current were ultimately observed in these thin films. Dielectric properties in metal/oxide/metal structure of these thin films were also reported by Taylor and Gomez.21 Horoz et al. reported the band gap value of Dy2O3 as 3.90 eV using ab initio calculations.22 Recently Cherif et al. reported the effect of temperature on electrical properties of Dy2O3 thin films grown on Si substrate.23 We have found a little experimental work in literature on Dy2O3 related thin films. In this novel research work we focus on the optical properties of Dy2O3 thin films for improved antireflecting mechanism. Cr and Cu are considered as good absorbers for anti-reflective or protective coating layers.24e26 The effects of Cr and Cu addition on Dy2O3 thin films regarding structural, morphological and optical properties are briefly explored. Enhanced absorption in Dy2O3 thin films with addition of Cr and Cu contents reveals it an interesting candidate for modern micro and opto-electronic devices. 2. Experimental Dy2O3, Cr/Dy2O3 and Cu/Dy2O3 thin films were fabricated using DAON 1000 S magnetron sputtering system. Dy and Cu were cosputtered in reactive argon and oxygen atmosphere and same for Dy and Cr case. Silicon (100) substrates were ultrasonically cleaned before deposition. Initial vacuum of 1.33104 Pa was achieved with turbo pumping station. Argon and oxygen flow were kept with ratio of 70:30, respectively. Flow rate was adjusted at 6.65 Pa during deposition process. Sputter rates for Dy, Cu and Cr were calibrated and controlled using sycon thickness monitor (STM). The contents of Cu and Cr were controlled as 10 wt% during deposition. Substrate temperature was kept at 500 C during deposition for crystalline growth of thin films as reported by Barreca.27 DC magnetron sputtering with voltage of 300 V and 100 mA current for Dy, 250 V and 30 mA for Cu and Cr was employed. Thin films with 100 ± 5 nm thickness of Dy2O3, Cu/Dy2O3 and Cr/Dy2O3 were achieved to explore its various properties. X-ray diffraction (XRD) analysis was
carried out using a Panalytical Xpert Pro Multipurpose X-ray diffractometer to explore the crystal structure and phase identification. XRD was operated at 30 kV and 10 mA using Cu Ka1 radiation (l ¼ 0.154 nm) with a step scan size of 0.05 . The presence of required elemental contents was confirmed with an Oxford instrument INCA X-Act energy dispersive X-ray spectroscope (EDS). EDS spectra were taken with 20 kV electron beam and spot size of 6 mm to gain maximum counts for elemental composition analysis. Surface morphology of thin films was observed with a FEI Nova Nano-SEM-450 field emission scanning electron microscope (FESEM). The micrographs were acquired using electron beam energy of 10 kV at 200 K magnification with through lens detector (TLD) under secondary electron (SE) mode. Brief optical properties containing refractive index, extinction coefficient, absorption coefficient, band gap, high dielectric constant and percentage reflectance were determined using spectroscopic ellipsometry and UV-Vis spectrophotometry. Dielectric measurements were acquired using a Quadtech Precision 1910 LCR meter. A separate set of silicon substrate were deposited with platinum (Pt) before deposition of thin films by leaving some area un-deposited for contact formation. Silver paste was then used to make contacts on both sides of thin films. 3. Results and discussion 3.1. X-ray diffraction (XRD) analysis Fig. 1(a) shows the XRD spectra of Dy2O3 thin film deposited with magnetron sputtering system. Main XRD peak was observed approximately at 29.11 which is exactly matched with (222) reflection of Dy2O3 given in standard ICDD pattern # 00-009-0197. Dy2O3 cubic phase with space group la3-206 is usually developed under these conditions and was already reported in some other published work as well.28 Fig. 1(b) presents the Cr/Dy2O3 pattern of magnetron sputtered thin films. It also contains another peak
Fig. 1. XRD patterns of Dy2O3 (a), Cr/Dy2O3 (b), Cu/Dy2O3 (c) thin films and crystallite size, lattice constant variation (d).
S.M. Ramay et al. / Journal of Rare Earths 37 (2019) 989e994 Table 1 Structural and optical properties of magnetron sputtered Dy2O3, Cr/Dy2O3, and Cu/Dy2O3 thin films. Property
Dy2O3
Cr/Dy2O3
Cu/Dy2O3
Crystallite size (nm) Lattice constant (nm) Dislocation density (nm2) Refractive index Extinction coefficient Absorption coefficient (cm1) Band gap (eV)
43.25 0.987 5.34 104 1.71 0.140 2.65 104 3.83
31.92 0.943 9.81 104 1.89 0.039 0.28 104 3.53
26.77 0.931 1.39 103 2.01 0.212 4.28 104 3.36
addition to the Dy2O3 which is corresponding to the Cr2O3 associated with most intense (116) reflection as given in standard ICDD pattern # 00-001-1294 of Cr2O3 and reported by Larbi et al.29 Similarly, Fig. 1(c) presents Cu/Dy2O3 XRD spectra of magnetron sputtered thin films. It contains some other peaks in addition to the Dy2O3 which correspond to the CuO associated with (101) phase as given in standard ICDD pattern #00-001-1117 of CuO.30 This shows that in case of DyeCueO and DyeCreO deposition, Cu and Cr not only substitute the Dy sites but also form the CuO and Cr2O3 phases as well, respectively. Crystallite size and dislocation density were kl calculated using well known Scherrer's formula D ¼ FWHM and cosq d ¼ 1/D, respectively.31 Average crystallite size and lattice parameter determined and plotted in Fig. 1(d) exhibit variation with addition of Cr and Cu due to difference in ionic radii and lattice distortion. Crystallite size, lattice constant and dislocation density of Dy2O3 thin films were found to decease with contents of Cr and Cu as given in Table 1. 3.2. Field emission scanning electron microscopy (FESEM) FESEM micrographs were taken using 10 kV electron beam at 200 k magnification with TLD detector in secondary electron (SE) mode, working distance was kept at 5 mm. Fig. 2(aec) show
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the FESEM micrographs of Dy2O3, Cu/Dy2O3 and Cr/Dy2O3 thin films, respectively. All the micrographs reveal homogeneous deposition of thin films with approximately uniform shape, size and distribution of grains as can also be seen from surface plots given in insets of the micrographs. It is observed that Cr/Dy2O3, and Cu/Dy2O3 thin films contains smaller size of grains than Dy2O3 thin film due to incorporation of Cr and Cu content exactly in accordance with the XRD analysis. Overall average grain size in all samples was observed under less than 50 nm in range. SEM histogram profiles in Fig. 2(d) showing the same trend of surface morphology depicts that larger particles or grains produces more intense number of pixels as briefly described in our previously published work.32 3.3. Energy dispersive X-ray spectroscopy (EDS) EDS is a state-of-the-art technique associated with qualitative and quantitative elemental analysis of material compositions. EDS spectra were taken with 20 kV electron beam and spot size of 6 mm to gain maximum counts for elemental composition analysis. Fig. 3 shows the EDS spectra of magnetron sputtered Dy2O3, Cr/Dy2O3 and Cu/Dy2O3 thin films. All EDS spectra confirm the presence of expected elemental contents in corresponding thin film compositions. Si peak is associated with silicon substrate. Elemental compositions acquired for each thin film are presented in Table 2. 3.4. Optical properties Optical properties were measured using spectroscopic ellipsometry for magnetron sputtered Dy2O3, Cr/Dy2O3 and Cu/Dy2O3 thin films. Mean square error (MSE) of the instrument was recorded before starting of analysis. Spectroscopic ellipsometry data were acquired by adjusting the source and detector at angle of 70 . Cauchy model was employed for thin films with silicon substrate to extract the psi (J) and delta (D) parameters. Thickness of Dy2O3, Cr/Dy2O3 and Cu/Dy2O3 thin films were observed as approximately
Fig. 2. SEM micrograph of Dy2O3 (a), Cr/Dy2O3 (b), and Cu/Dy2O3 (c) thin films. with inset surface plots, and SEM histograms (d).
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Fig. 3. EDS spectra of Dy2O3 (1), Cr/Dy2O3 (2), and Cu/Dy2O3 (3) thin films.
Table 2 Elemental compositions of samples calculated using EDX analysis.
Fig. 5. Extinction coefficient variation with wavelength of Dy2O3 (1), Cr/Dy2O3 (2) and Cu/Dy2O3 (3) thin films.
100 ± 5 nm. The variations of J and D along wavelength for all samples were fitted with best model curves. Refractive index (n), extinction coefficient (k) and absorption coefficient (∞) for all thin films were measured and plotted regarding the corresponding wavelength values. Fig. 4 shows the trend of refractive indices with wavelength for Dy2O3, Cr/Dy2O3 and Cu/Dy2O3 thin films. Refractive index for Dy2O3 was recorded to be 1.71 at 632.8 nm and significantly enhanced to 1.89 with addition of Cr and 2.01 for Cu content. Extinction coefficient is basically defined as the strength of material to absorb the light. Extinction coefficient of Dy2O3 is found to decrease first from 0.140 to 0.039 with addition of Cr content while increase with Cu content up to 0.212 as can be seen in Fig. 5. Similar trend was observed for absorption coefficient (∞) as shown in Fig. 6, initially, absorption coefficient reduces with the increase of
wavelength in all samples then a slightly increase is noticeable at elevated wavelength values. Overall, it is observed that refractive index, extinction coefficient and absorption coefficient significantly change with addition of Cu and Cr in magnetron sputtered Dy2O3 thin films which can be useful for applications of these thin films with high antireflection. Optical band gap values for each thin film were extracted from the relation between refractive index and band gap defined by Ravindera and Gupta (n ¼ 4:084 0:62Eg).33 The value of optical band gap was found to decrease from 3.83 to 3.53 eV for Cr/Dy2O3 and 3.36 eV for Cu/Dy2O3 thin films. This experimentally obtained band gap value for Dy2O3 is in close consistence with the recently reported band gap value of 3.90 eV using ab-initio calculations.22 High frequency dielectric constant was also evaluated using the relation (Ɛ∞ ¼ n2)31 and depicts a significant decrease with addition of Cr and Cu contents in Dy2O3 thin films. All extracted optical parameters from spectroscopic Ellipsometry are given in Table 1. Percentage reflection was measured using UV-Vis spectrophotometry as shown in Fig. 7, which depicts a significant decrease in reflection in thin film with Cr and Cu contents. Structure morphology, elemental and optical properties present significant changes with addition of Cr and Cu in Dy2O3 thin films. The variation in optical parameters of Dy2O3 dealt it as improved protective coating layer. Thin films with Cr and Cu addition in Dy2O3 will definitely present improved antireflective mechanism.
Fig. 4. Refractive index variation with wavelength of Dy2O3 (1), Cr/Dy2O3 (2) and Cu/ Dy2O3 (3) thin films.
Fig. 6. Absorption coefficient variation with wavelength of Dy2O3 (1), Cr/Dy2O3 (2) and Cu/Dy2O3 (3) thin films.
Dy2O3 Element O(K) Dy(L) Si(K) Cr(K) Cu(K) Total
wt% 9.35 4.88 85.77 e e 100
Cr/Dy2O3 at% 4.68 20.16 75.16 e e 100
wt% 11.28 3.86 83.66 1.20 e 100
Cu/Dy2O3 at% 5.73 16.31 74.41 1.82 e 100
wt% 9.45 3.97 84.61 e 1.97 100
at% 4.79 16.59 75.01 e 3.61 100
S.M. Ramay et al. / Journal of Rare Earths 37 (2019) 989e994
Fig. 7. Reflectance spectra showing variation with wavelength of Dy2O3 (1), Cr/Dy2O3 (2) and Cu/Dy2O3 (3) thin films.
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Fig. 10. Tangent loss (tand) of Dy2O3 (1), Cr/Dy2O3 (2) and Cu/Dy2O3 (3) thin films.
permittivity of free space Ɛ0 ¼ 8.85 1012 F/m, while d and A represent thickness and effective area of thin film, respectively. The real dielectric constant as a function of frequency is given in Fig. 8, showing the decreasing trend with frequency. The value of dielectric constant was decreased with addition of Cu and Cr contents in Dy2O3 thin films. Imaginary part of dielectric constant was 00 0 also extracted from real part using the relation, ε ¼ ε tand, presented in Fig. 9, demonstrating the similar trend like real part. Dissipation factor or tangent loss as a function of frequency as presented in Fig. 10 reveals a considerable decrease in tangent loss especially Cu containing Dy2O3 thin films. 4. Conclusions
Fig. 8. Real dielectric constant of Dy2O3 (1), Cr/Dy2O3 (2) and Cu/Dy2O3 (3) thin films.
The uniform and homogeneous thin films of Dy2O3, Cr/Dy2O3 and Cu/Dy2O3 were successfully deposited on Si substrate in thickness range of 100 ± 5 nm using reactive magnetron cosputtering. Structural analysis exhibits the crystalline nature with cubic structure of these thin films with significant variation in crystallite size, lattice parameters and dislocation density. Surface morphology shows the smooth, even, flat, plane and uniform distribution of nano-sized grains with expected elemental contents in corresponding compositions. Brief optical properties reveal a significant variation in refractive index, extinction coefficient, absorption coefficient, optical band gap and reflectance. Dielectric properties reveal a low tangent loss in Dy2O3 thin films having Cu and Cr contents. Enhanced absorption of Dy2O3 thin films with Cu and Cr contents dealt it with high antireflective mechanism and improved protective coating layer. Acknowledgement The authors would like to extend his sincere appreciation to the Deanship of Scientific Research at King Saud University for funding under Research Group (No. RG 1435-004).
Fig. 9. Imaginary dielectric constant of Dy2O3 (1), Cr/Dy2O3 (2) and Cu/Dy2O3 (3) thin films.
3.5. Dielectric measurements The parallel capacitance Cp and dissipation factor Df or tand as a function of frequency were acquired through precise LCR meter. Real part of dielectric constant was calculated using the relation, ε'' ¼ Cpd /ε0 A, where Cp denotes the parallel capacitance,
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