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Effect of Ar:N2 flow rate on morphology, optical and electrical properties of CCTO thin films deposited by RF magnetron sputtering
Ceramics International 45 (2019) 15077–15081
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of Ar:N2 flow rate on morphology, optical and electrical properties of CCTO thin films deposited by RF magnetron sputtering
T
Mohsen Ahmadipoura, Mohammad Arjmandc, Mohd Fadzil Ainb, Zainal Arifin Ahmada, Swee-Yong Punga,∗ a
School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300, Nibong Tebal, Penang, Malaysia School of Electrical and Electronic Engineering, Universiti Sains Malaysia, Engineering Campus, 14300, Nibong Tebal, Penang, Malaysia c School of Engineering, University of British Columbia, Kelowna, BC, V1V 1V7, Canada b
A R T I C LE I N FO
A B S T R A C T
Keywords: CCTO thin film Radio frequency magnetron sputtering Surface resistivity Optical energy bandgap
Calcium copper titanate (CCTO) thin films were deposited on indium tin oxide (ITO) substrates using radio frequency (RF) magnetron sputtering, at selected Ar:N2 flow rates (1:1, 1:2, 1:4, and 1:6 sccm) at ambient temperature. The effect of Ar:N2 flow rate on the morphology, optical and electrical properties of the CCTO thin films were investigated using FESEM, XRD, AFM, Hall effect measurement, and UV–Vis spectroscopy. It was confirmed by XRD analysis that the thin films were produced is CCTO with cubic crystal structure. As the flow rate of Ar:N2 increased up to 1:6 sccm, the thin film thickness reduced from 87 nm to 35 nm while the crystallite size of CCTO thin film decreased from 27 nm to 20 nm. Consequently, the surface roughness of thin film was halved from 8.74 nm to 4.02 nm. In addition, the CCTO thin films deposited at the highest Ar:N2 flow rate studied, at 1:6 sccm; are having the highest sheet resistivity (13.27 Ω/sq) and the largest optical energy bandgap (3.68 eV). The results articulate that Ar:N2 flow rate was one of the important process parameters in RF magnetron sputtering that could affect the morphology, electrical properties and optical properties of CCTO thin films.
1. Introduction Calcium copper titanate (CCTO) thin films have been attracting considerable attention due to their outstanding properties such as, wide band gap (3.37 eV) [1], low electrical sheet resistivity (8.22 Ω/sq) [2], and high dielectric permittivity (105 at the broad frequency range up to 106 Hz) at ambient temperature [3]. These unique properties of CCTO thin film are of great potential for optoelectronics, sensors, antenna, and solar cell applications [4–7]. Many techniques can be employed to deposit CCTO thin films on substrate, namely pulse laser deposition (PLD) [8], sol-gel [9], chemical vapor deposition [10] and radio frequency (RF) magnetron sputtering [11]. Amongst these techniques, RF magnetron sputtering is a better deposition technique as it offers several advantages such as high deposition rate at low temperature and good uniformity on a large surface area. Since the structures of CCTO thin films such as crystallite size, surface morphology, and porosity, could strongly influence the functional properties of CCTO thin films [12], it is crucial to understand the effects of the deposition parameters of RF magnetron sputtering on the properties of CCTO thin films to tailor for various applications.
∗
Many experimental works have been devoted to investigate the influence of process parameters of RF magnetron sputtering on the properties of CCTO thin films, for instants the deposition time and power at ambient temperature [2,11,12]. It is also noted that the argon/nitrogen (Ar:N2) flow rate has a significant influence on thin film properties due to its remarkable impact on deposition rate. For instance, Kumar et al. [13] and Tian et al. [14] studied the influence of Ar:N2 flow rate on the properties of sputtered-coated Ni–TiN and SrHfON thin films. Nevertheless, not many attempts have been made to investigate the effect of working gas flow rate on the properties of CCTO thin films. This study was devoted to investigate the impact of Ar:N2 flow rate on the deposition of CCTO thin films on ITO substrate at ambient temperature by RF magnetron sputtering. The effect of nitrogen-rich sputtering environment, i.e. by increasing the ratio of Ar:N2 flow rate up to 1:6 sccm; on the morphology, electrical properties and optical properties of CCTO thin films was systematically assessed. 2. Experimental details Deposition of CCTO thin films on ITO substrates was carried out by
Corresponding author. E-mail address: [email protected] (S.-Y. Pung).
https://doi.org/10.1016/j.ceramint.2019.04.245 Received 6 December 2018; Received in revised form 1 April 2019; Accepted 29 April 2019 Available online 02 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 15077–15081
M. Ahmadipour, et al.
Table 1 RF magnetron sputtering parameters of CCTO thin films. Deposition Parameters
Value
Target-substrate distance (mm) Operating pressure (mbar) Base pressure (mbar) Ar:N2 flow ratio (sccm) RF power (W) Target size (diameter × thickness) (mm) Duration of deposition (min) Deposition temperature Substrate size (mm)
200 30 × 10−5 2 × 10−5 1:1, 1:2, 1:4, and 1:6 150 75 × 5 60 Ambient temperature 10 × 10 × 1
RF magnetron sputtering system (HHVAuto 500). The CCTO ceramic target (Semiconductor Wafer, Inc., Taiwan – Purity: 99%) had a stoichiometry ratio of CaCu3Ti3.6O20.5. The ITO substrates were used so that the electrical property of CCTO thin films could be measured in subsequent stage. Prior to loading into the vacuum chamber, the ITO substrates (purchased from Magna Value Sdn Bhd, Malaysia) were sequentially cleaned with acetone, ethanol, and de-ionized water in an ultrasonic bath. The target surface was pre-sputtered for 5 min to remove possible contaminants. Subsequently, the deposition was carried out using different ratios of Ar:N2 flow rate i.e. 1:1, 1:2, 1:4, and 1:6 standard cubic centimeter per minute (sccm) at ambient temperature for 60 min. The purity of working gases (argon and nitrogen) used was 99% while the RF power was set at 150 W. The CCTO thin films were labelled as C1, C2, C4, and C6 corresponding to the ratio of Ar:N2 flow rates of 1:1, 1:2, 1:4, and 1:6 respectively. The ITO substrates were rotated at 10 rpm during the sputtering process in order to improve the thickness uniformity of CCTO thin films. The details of RF magnetron sputtering parameters are tabulated in Table 1. The crystalline structure of the films produced was analyzed by Xray diffraction (Bruker, D8 advance system, Cu-Kα radiation, λ = 1.54 Å). The XRD scans were recorded in the conventional 2θ configurations between 20° and 80°. The surface morphology and crosssection image of the CCTO thin films were analyzed by field emission scanning electron microscopy which coupled with energy dispersive spectroscopy (FESEM-EDAX, Zeiss Supra™ 35VP). Thin film surface roughness was measured by atomic force microscope (AFM, SPA 400), operated at non-contact mode. The AFM scanning area was 10 × 10 μm2. The electrical properties of thin film were studied via Hall Effect measurements (Lake Shore, 8404 Ac/Dc) using Silver paste (RS component, 186–3593) to establish electrical contacts with the thin
Fig. 2. Deposition rate of thin films using various ratios of Ar:N2 flow rate.
films. The optical energy bandgap of the thin films was determined using Tauc plot method [15] based on the data measured by UV–Vis spectrophotometer (Varian, Cary 50 conc). 3. Results and discussion 3.1. Nanostructure of CCTO thin films Fig. 1 shows the cross-sectional FESEM images of thin films deposited using different Ar:N2 flow rates. The thickness of thin films was reduced with increasing Ar:N2 flow rate, i.e. from 87 nm at 1:1 sccm to 37 nm at 1:6 sccm. Thus, the deposition rate of the CCTO thin films was found to be 1.45, 1.03, 0.75, and 0.61 nm/min for Ar:N2 flow rates of 1:1, 1:2, 1:4 and 1:6 sccm, respectively as shown in Fig. 2. The decrease of deposition rate was caused by the reduction of sputtering efficiency as a result of a larger amount of N2 molecules present in the vacuum chamber with the increase of Ar:N2 flow rate. In sputtering process, the particles of CCTO were mostly ejected from the target by Ar ions, which have a higher atomic mass than nitrogen ions [16,17]. By increasing the N2 flow rate, the collision of Ar ions with N2 was raised, causing the reduction of (i) mean free path and, (ii) kinetic energy of Ar ions. As a consequence, less Ar ions sputtered the CCTO target thus the sputtering yield deteriorated.
Fig. 1. FESEM images of the cross-section views of thin films deposited using various ratios of Ar:N2 flow rate, (a) C1, (b) C2, (3) C4, and (d) C6. 15078
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Fig. 3. (a) XRD pattern and (b) crystallite size estimation of the CCTO thin films deposited using various ratios of Ar:N2 flow rate.
Fig. 4. AFM images of CCTO thin films deposited using various ratio of Ar:N2 gas flow rates (a) C1, (b) C2, (c) C4, and (d) C6.
The XRD patterns of thin films deposited on the ITO substrates using various ratios of Ar:N2 flow rates is shown in Fig. 3a. The diffraction peaks of thin films matched well with the diffraction pattern of cubic phase CCTO [ICDD Card No. 98-000-8088]. The multiple diffraction peaks also indicates that these thin films have polycrystalline structure. No diffraction peaks assignable to nitrides were observed for all CCTO thin films although it was anticipated that formation of nitrides in the CCTO thin films could possibly occur particularly in a N2-rich sputtering environment such as one with Ar:N2 flow rate of 1:6 sccm. Furthermore, the diffraction intensities of CCTO thin films decreased with the increase of the ratio of Ar:N2 flow rate. This result suggests that N2rich sputtering environment would deteriorate the crystal quality of CCTO thin films. The crystallite size of CCTO thin films was estimated using Scherrer's formula [18]. The average crystallite size reduced from 27 nm (C1) to 20 nm (C6) as shown in Fig. 3b. The above observation could be explained below. The amount of argon ions and their kinetic energy were reduced when large amount of N2 molecules present in the sputtering chamber. Thus, there are lesser particles ejected from CCTO target and they also possess much lower energy. The probability for these particles to migrate effectively to their appropriate sites to form an ordered crystal structure of CCTO thin film was reduced. In addition, more crystal grains of CCTO with smaller size were formed as a result of less energetic sputtered particles. As a consequence, many crystal defects and grain boundaries were generated, affecting the electrical
performance of CCTO thin films that were deposited in a N2 rich environment. 3.2. Morphological analysis Fig. 4 displays the 3D AFM images of CCTO thin films deposited using various Ar:N2 flow rates. The surface roughness (root mean square, RMS) of CCTO thin films decreased with increasing ratio of Ar:N2 flow rate, i.e. C1 and C6 are 8.74 nm and 4.02 nm, respectively. This result reveals that higher Ar:N2 flow rate produced CCTO thin films with smoother surface (lower roughness). In accord with our results, Castotti et al. reported that a lower deposition rate resulted in a thinner film, smaller crystal grains and lower surface roughness [19]. Fig. 5 shows the EDAX spectrum of CCTO thin films deposited using different ratios of Ar:N2 flow rate. No N element was detected in all CCTO thin films. The small peaks around 1.4 keV and 2.1 keV belong to the ITO substrate (In) and gold coating (Au) respectively. 3.3. Electrical and optical properties The dependence of sheet surface resistivity, carrier concentration and mobility of the CCTO thin films on Ar:N2 flow rate are illustrated in Fig. 6a. The sheet surface resistivity of the thin films increased from 8.51 Ω/sq to 13.27 Ω/sq, whereas the carrier concentration and
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Fig. 5. EDAX spectrum of CCTO thin films deposited using various ratios of Ar:N2 gas flow rates (a) C1, (b) C2, (c) C4, and (d) C6.
Fig. 6. (a) Sheet resistivity, carrier concentration, and mobility and (b) Tauc plot of CCTO thin films deposited using various ratios of Ar:N2 flow rate. Table 2 The properties of CCTO thin film deposited using various ratios of Ar:N2 flow rate. Sample
Deposition rate (nm/min)
Thickness (nm)
Crystallite size (nm)
RMS (nm)
Resistivity (Ω/sq)
Optical energy bandgap (eV)
C1 C2 C4 C6
1.45 1.03 0.75 0.61
87 62 45 37
27 24 21 20
8.74 6.99 6.24 4.02
8.51 9.60 11.02 13.27
3.13 3.41 3.46 3.68
mobility reduced from 9.15 ( × 1015cm−2) to 2.17 ( × 1015cm−2) and from 7.78 (cm2/Vs) to 1.46 (cm2/Vs) by increasing the ratio of Ar:N2 flow rate. This observation could be attributed to the reduction of crystallite size. The decrease in crystallite size contributed to the increase of crystal grain boundaries. As a result, more charge carrier scattering centers were generated. The relatively poor crystal quality of CCTO thin film deposited at higher Ar:N2 flow rate as revealed by XRD analysis in Fig. 3a also increased the number of charge carrier scattering centers. Hence, the conductive performance, charge carrier concentration and mobility of CCTO thin films were reduced with
increasing ratio of Ar:N2 flow rate [20]. The optical energy bandgap (Eg) of CCTO thin films deposited using various ratios of Ar:N2 flow rate is shown in Fig. 6b. The Eg was found to be 3.13, 3.41, 3.46, and 3.68 eV for samples C1, C2, C4, and C6, respectively. Decrease in crystallite size i.e. from 27 nm to 20 nm with increasing ratio of Ar:N2 flow rate, leading to the increase of Eg as a result of quantum confinement effect. Such high Eg values are highly desirable for the applications in optoelectronic and supercapacitor industry. The calculated Eg at various ratios of Ar:N2 flow rate are summarized in Table 2.
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4. Conclusions CCTO thin films were deposited on ITO substrates using RF magnetron sputtering under various ratios of Ar:N2 flow rate of 1:1, 1:2, 1:4, and 1:6 sccm at ambient temperature. It was observed that the deposition rate of the films decreased gradually (1.45, 1.03, 0.75, and 0.61 nm/min) with increasing ratio of Ar:N2 flow rate. XRD analysis revealed that the crystal quality of thin films reduced with increasing ratio of Ar:N2 flow rate. The crystallite size and surface roughness of thin films were also found to decrease with increasing ratio of Ar:N2 flow rate. The resistivity and optical energy bandgap of the CCTO thin films were also affected by the Ar:N2 flow rate, where maximum values (13.27 Ω/sq and 3.68 eV) were measured at Ar:N2 flow rate of 1:6. In brief, the ratio of Ar:N2 flow rate should be precisely controlled during the CCTO thin film depositions in order to obtain high-quality CCTO thin films with desired electrical and optical properties for different applications.
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
Acknowledgments
[15]
The authors acknowledge the research funding of RUI 1001/ PBAHAN/8014095 from Universiti Sains Malaysia.
[16]
References [17] [1] A.O. Turky, M.M. Rashad, Z.I. Zaki, I.A. Ibrahim, M. Bechelany, Tuning the optical and dielectric properties of calcium copper titanate CaxCu3-xTi4O12 nanopowders, RSC Adv. 5 (2015) 18767. [2] M. Ahmadipour, M.F. Ain, S. Goutham, Z.A. Ahmad, Effects of deposition time on properties of CaCu3Ti4O12 thin film deposited on ITO substrate by RF magnetron sputtering at ambient temperature, Ceram. Int. 44 (2018) 18817–18820. [3] M.A. Subramanian, D. Li, N. Duan, B.A. Reisner, A. Sleight, High dielectric constant in ACu3Ti4O12 and ACu3Ti3FeO12 phases, J. Solid State Chem. 151 (2000) 323–325. [4] K. Pal, R. Jana, A. Dey, P.P. Ray, M.d.M. Seikh, A. Gayen, Application of CaCu3Ti4O12 based quadruple perovskites as a promising candidate for optoelectronic devices, Chem. Phys. Lett. 699 (2018) 229–233. [5] M. Ahmadipour, N.A. Rejab, M.F.A. Rahman, M.F. Ain, Z.A. Ahmad, High sensitivity and selectivity of CaCu3Ti4O12-ZnO composites towards acetone gas at room temperature, Ceram. Int. 44 (2018) 6904–6911. [6] L.C. Kretly, A.F.L. Almeida, R.S. de Oliveira, J.M. Sasaki, A.S.B. Sombra, Electrical
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and optical properties of CaCu3Ti4O12(CCTO) substrates for microwave devices and antennas, Microw. Opt. Technol. Lett. 39 (2003) 145–150. H.S. Kushwaha, N. A Madhar, B. Ilahi, P. Thomas, A. Halder, R. Vaish, Efficient solar energy conversion using CaCu3Ti4O12 photoanode for photocatalysis and photoelectrocatalysis, Sci. Rep.-Uk. 6 (2016) 18557. L. Fang, M.R. Shen, Deposition and dielectric properties of CaCu3Ti4O12 thin films on Pt/Ti/SiO2/Si substrates using pulsed-laser deposition, Thin Solid Films 440 (2003) 60–65. R. Jime´nez, M.L. Calzada, I. Bretos, J.C. Goes, A.S.B. Sombra, Dielectric properties of sol–gel derived CaCu3Ti4O12 thin films onto Pt/TiO2/Si(1 0 0) substrates, J. Eur. Ceram. Soc. 27 (2007) 3829–3833. R.L. Nigro, R.G. Toro, G. Malandrino, I.L. Fragala`, P. Fiorenza, V. Raineri, Effects of high temperature annealing on MOCVD grown CaCu3Ti4O12 films on LaAlO3 substrates, Surf. Coating. Technol. 201 (2007) 9243–9247. M. Ahmadipour, S.N. Ayub, M.F. Ain, Z.A. Ahmad, Structural, surface morphology and optical properties of sputter-coated CaCu3Ti4O12 thin film: influence of RF magnetron sputtering power, Mater. Sci. Semicond. Process. 66 (2017) 157–161. M. Ahmadipour, M.F. Ain, Z.A. Ahmad, Effect of thickness on surface morphology, optical and humidity sensing properties of RF magnetron sputtered CCTO thin films, Appl. Surf. Sci. 385 (2016) 182–190. M. Kumar, S. Mishra, R. Mitra, Effect of Ar:N2 ratio on structure and properties of Ni-TiN nanocomposite thin films processed by reactive RF/DC magnetron sputtering, Surf. Coating. Technol. 228 (2013) 100–114. H. Tian, L.P. Feng, Z.T. Liu, Effect of N2/Ar flow ratio on the structural and electrical properties of SrHfON thin films prepared by magnetron sputtering, Vacuum 109 (2014) 139–143. M. Ahmadipour, M.F. Ain, Z.A. Ahmad, Effects of annealing temperature on the structural, morphology, optical properties and resistivity of sputtered CCTO thin film, J. Mater. Sci. Mater. Electron. 28 (2017) 12458–12466. W.Y. Ding, J. Xu, W.Q. Lu, X.L. Deng, C. Dong, The effect of N2 flow rate on discharge characteristics of microwave electron cyclotron resonance plasma, Phys. Plasmas 16 (2009) 053502–053506. C. Zhang, W.Y. Ding, H.L. Wang, W.P. Chai, D.Y. Ju, Influences of working pressure on properties for TiO2 films deposited by DC pulse magnetron sputtering, J. Environ. Sci. 21 (2009) 741–744. A. Singh, P. Kuppusami, Shabana Khan, C. Sudha, R. Thirumurugesan, R. Ramaseshan, R. Divakar, E. Mohandas, S. Dash, Influence of nitrogen flow rate on microstructural and nanomechanical properties of Zr-N thin films prepared by pulsed DC magnetron sputtering, Appl. Surf. Sci. 280 (2013) 117–123. D. Casotti, V. Orsini, A. di Bona, S. Gardonio, M. Fanetti, M. Valant, S. Valeri, Ageing effects on electrical resistivity of Nb-doped TiO2 thin films deposited at a high rate by reactive DC magnetron sputtering, Appl. Surf. Sci. 455 (2018) 267–275. H. Zeng, Y. Wu, J. Zhang, C. Kuang, M. Yue, S. Zhou, Grain size-dependent electrical resistivity of bulk nanocrystalline Gd metals, Prog. Nat. Sci. Mater. 23 (2013) 18–22.
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of Ar:N2 flow rate on morphology, optical and electrical properties of CCTO thin films deposited by RF magnetron sputtering
T
Mohsen Ahmadipoura, Mohammad Arjmandc, Mohd Fadzil Ainb, Zainal Arifin Ahmada, Swee-Yong Punga,∗ a
School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, 14300, Nibong Tebal, Penang, Malaysia School of Electrical and Electronic Engineering, Universiti Sains Malaysia, Engineering Campus, 14300, Nibong Tebal, Penang, Malaysia c School of Engineering, University of British Columbia, Kelowna, BC, V1V 1V7, Canada b
A R T I C LE I N FO
A B S T R A C T
Keywords: CCTO thin film Radio frequency magnetron sputtering Surface resistivity Optical energy bandgap
Calcium copper titanate (CCTO) thin films were deposited on indium tin oxide (ITO) substrates using radio frequency (RF) magnetron sputtering, at selected Ar:N2 flow rates (1:1, 1:2, 1:4, and 1:6 sccm) at ambient temperature. The effect of Ar:N2 flow rate on the morphology, optical and electrical properties of the CCTO thin films were investigated using FESEM, XRD, AFM, Hall effect measurement, and UV–Vis spectroscopy. It was confirmed by XRD analysis that the thin films were produced is CCTO with cubic crystal structure. As the flow rate of Ar:N2 increased up to 1:6 sccm, the thin film thickness reduced from 87 nm to 35 nm while the crystallite size of CCTO thin film decreased from 27 nm to 20 nm. Consequently, the surface roughness of thin film was halved from 8.74 nm to 4.02 nm. In addition, the CCTO thin films deposited at the highest Ar:N2 flow rate studied, at 1:6 sccm; are having the highest sheet resistivity (13.27 Ω/sq) and the largest optical energy bandgap (3.68 eV). The results articulate that Ar:N2 flow rate was one of the important process parameters in RF magnetron sputtering that could affect the morphology, electrical properties and optical properties of CCTO thin films.
1. Introduction Calcium copper titanate (CCTO) thin films have been attracting considerable attention due to their outstanding properties such as, wide band gap (3.37 eV) [1], low electrical sheet resistivity (8.22 Ω/sq) [2], and high dielectric permittivity (105 at the broad frequency range up to 106 Hz) at ambient temperature [3]. These unique properties of CCTO thin film are of great potential for optoelectronics, sensors, antenna, and solar cell applications [4–7]. Many techniques can be employed to deposit CCTO thin films on substrate, namely pulse laser deposition (PLD) [8], sol-gel [9], chemical vapor deposition [10] and radio frequency (RF) magnetron sputtering [11]. Amongst these techniques, RF magnetron sputtering is a better deposition technique as it offers several advantages such as high deposition rate at low temperature and good uniformity on a large surface area. Since the structures of CCTO thin films such as crystallite size, surface morphology, and porosity, could strongly influence the functional properties of CCTO thin films [12], it is crucial to understand the effects of the deposition parameters of RF magnetron sputtering on the properties of CCTO thin films to tailor for various applications.
∗
Many experimental works have been devoted to investigate the influence of process parameters of RF magnetron sputtering on the properties of CCTO thin films, for instants the deposition time and power at ambient temperature [2,11,12]. It is also noted that the argon/nitrogen (Ar:N2) flow rate has a significant influence on thin film properties due to its remarkable impact on deposition rate. For instance, Kumar et al. [13] and Tian et al. [14] studied the influence of Ar:N2 flow rate on the properties of sputtered-coated Ni–TiN and SrHfON thin films. Nevertheless, not many attempts have been made to investigate the effect of working gas flow rate on the properties of CCTO thin films. This study was devoted to investigate the impact of Ar:N2 flow rate on the deposition of CCTO thin films on ITO substrate at ambient temperature by RF magnetron sputtering. The effect of nitrogen-rich sputtering environment, i.e. by increasing the ratio of Ar:N2 flow rate up to 1:6 sccm; on the morphology, electrical properties and optical properties of CCTO thin films was systematically assessed. 2. Experimental details Deposition of CCTO thin films on ITO substrates was carried out by
Corresponding author. E-mail address: [email protected] (S.-Y. Pung).
https://doi.org/10.1016/j.ceramint.2019.04.245 Received 6 December 2018; Received in revised form 1 April 2019; Accepted 29 April 2019 Available online 02 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 15077–15081
M. Ahmadipour, et al.
Table 1 RF magnetron sputtering parameters of CCTO thin films. Deposition Parameters
Value
Target-substrate distance (mm) Operating pressure (mbar) Base pressure (mbar) Ar:N2 flow ratio (sccm) RF power (W) Target size (diameter × thickness) (mm) Duration of deposition (min) Deposition temperature Substrate size (mm)
200 30 × 10−5 2 × 10−5 1:1, 1:2, 1:4, and 1:6 150 75 × 5 60 Ambient temperature 10 × 10 × 1
RF magnetron sputtering system (HHVAuto 500). The CCTO ceramic target (Semiconductor Wafer, Inc., Taiwan – Purity: 99%) had a stoichiometry ratio of CaCu3Ti3.6O20.5. The ITO substrates were used so that the electrical property of CCTO thin films could be measured in subsequent stage. Prior to loading into the vacuum chamber, the ITO substrates (purchased from Magna Value Sdn Bhd, Malaysia) were sequentially cleaned with acetone, ethanol, and de-ionized water in an ultrasonic bath. The target surface was pre-sputtered for 5 min to remove possible contaminants. Subsequently, the deposition was carried out using different ratios of Ar:N2 flow rate i.e. 1:1, 1:2, 1:4, and 1:6 standard cubic centimeter per minute (sccm) at ambient temperature for 60 min. The purity of working gases (argon and nitrogen) used was 99% while the RF power was set at 150 W. The CCTO thin films were labelled as C1, C2, C4, and C6 corresponding to the ratio of Ar:N2 flow rates of 1:1, 1:2, 1:4, and 1:6 respectively. The ITO substrates were rotated at 10 rpm during the sputtering process in order to improve the thickness uniformity of CCTO thin films. The details of RF magnetron sputtering parameters are tabulated in Table 1. The crystalline structure of the films produced was analyzed by Xray diffraction (Bruker, D8 advance system, Cu-Kα radiation, λ = 1.54 Å). The XRD scans were recorded in the conventional 2θ configurations between 20° and 80°. The surface morphology and crosssection image of the CCTO thin films were analyzed by field emission scanning electron microscopy which coupled with energy dispersive spectroscopy (FESEM-EDAX, Zeiss Supra™ 35VP). Thin film surface roughness was measured by atomic force microscope (AFM, SPA 400), operated at non-contact mode. The AFM scanning area was 10 × 10 μm2. The electrical properties of thin film were studied via Hall Effect measurements (Lake Shore, 8404 Ac/Dc) using Silver paste (RS component, 186–3593) to establish electrical contacts with the thin
Fig. 2. Deposition rate of thin films using various ratios of Ar:N2 flow rate.
films. The optical energy bandgap of the thin films was determined using Tauc plot method [15] based on the data measured by UV–Vis spectrophotometer (Varian, Cary 50 conc). 3. Results and discussion 3.1. Nanostructure of CCTO thin films Fig. 1 shows the cross-sectional FESEM images of thin films deposited using different Ar:N2 flow rates. The thickness of thin films was reduced with increasing Ar:N2 flow rate, i.e. from 87 nm at 1:1 sccm to 37 nm at 1:6 sccm. Thus, the deposition rate of the CCTO thin films was found to be 1.45, 1.03, 0.75, and 0.61 nm/min for Ar:N2 flow rates of 1:1, 1:2, 1:4 and 1:6 sccm, respectively as shown in Fig. 2. The decrease of deposition rate was caused by the reduction of sputtering efficiency as a result of a larger amount of N2 molecules present in the vacuum chamber with the increase of Ar:N2 flow rate. In sputtering process, the particles of CCTO were mostly ejected from the target by Ar ions, which have a higher atomic mass than nitrogen ions [16,17]. By increasing the N2 flow rate, the collision of Ar ions with N2 was raised, causing the reduction of (i) mean free path and, (ii) kinetic energy of Ar ions. As a consequence, less Ar ions sputtered the CCTO target thus the sputtering yield deteriorated.
Fig. 1. FESEM images of the cross-section views of thin films deposited using various ratios of Ar:N2 flow rate, (a) C1, (b) C2, (3) C4, and (d) C6. 15078
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Fig. 3. (a) XRD pattern and (b) crystallite size estimation of the CCTO thin films deposited using various ratios of Ar:N2 flow rate.
Fig. 4. AFM images of CCTO thin films deposited using various ratio of Ar:N2 gas flow rates (a) C1, (b) C2, (c) C4, and (d) C6.
The XRD patterns of thin films deposited on the ITO substrates using various ratios of Ar:N2 flow rates is shown in Fig. 3a. The diffraction peaks of thin films matched well with the diffraction pattern of cubic phase CCTO [ICDD Card No. 98-000-8088]. The multiple diffraction peaks also indicates that these thin films have polycrystalline structure. No diffraction peaks assignable to nitrides were observed for all CCTO thin films although it was anticipated that formation of nitrides in the CCTO thin films could possibly occur particularly in a N2-rich sputtering environment such as one with Ar:N2 flow rate of 1:6 sccm. Furthermore, the diffraction intensities of CCTO thin films decreased with the increase of the ratio of Ar:N2 flow rate. This result suggests that N2rich sputtering environment would deteriorate the crystal quality of CCTO thin films. The crystallite size of CCTO thin films was estimated using Scherrer's formula [18]. The average crystallite size reduced from 27 nm (C1) to 20 nm (C6) as shown in Fig. 3b. The above observation could be explained below. The amount of argon ions and their kinetic energy were reduced when large amount of N2 molecules present in the sputtering chamber. Thus, there are lesser particles ejected from CCTO target and they also possess much lower energy. The probability for these particles to migrate effectively to their appropriate sites to form an ordered crystal structure of CCTO thin film was reduced. In addition, more crystal grains of CCTO with smaller size were formed as a result of less energetic sputtered particles. As a consequence, many crystal defects and grain boundaries were generated, affecting the electrical
performance of CCTO thin films that were deposited in a N2 rich environment. 3.2. Morphological analysis Fig. 4 displays the 3D AFM images of CCTO thin films deposited using various Ar:N2 flow rates. The surface roughness (root mean square, RMS) of CCTO thin films decreased with increasing ratio of Ar:N2 flow rate, i.e. C1 and C6 are 8.74 nm and 4.02 nm, respectively. This result reveals that higher Ar:N2 flow rate produced CCTO thin films with smoother surface (lower roughness). In accord with our results, Castotti et al. reported that a lower deposition rate resulted in a thinner film, smaller crystal grains and lower surface roughness [19]. Fig. 5 shows the EDAX spectrum of CCTO thin films deposited using different ratios of Ar:N2 flow rate. No N element was detected in all CCTO thin films. The small peaks around 1.4 keV and 2.1 keV belong to the ITO substrate (In) and gold coating (Au) respectively. 3.3. Electrical and optical properties The dependence of sheet surface resistivity, carrier concentration and mobility of the CCTO thin films on Ar:N2 flow rate are illustrated in Fig. 6a. The sheet surface resistivity of the thin films increased from 8.51 Ω/sq to 13.27 Ω/sq, whereas the carrier concentration and
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Fig. 5. EDAX spectrum of CCTO thin films deposited using various ratios of Ar:N2 gas flow rates (a) C1, (b) C2, (c) C4, and (d) C6.
Fig. 6. (a) Sheet resistivity, carrier concentration, and mobility and (b) Tauc plot of CCTO thin films deposited using various ratios of Ar:N2 flow rate. Table 2 The properties of CCTO thin film deposited using various ratios of Ar:N2 flow rate. Sample
Deposition rate (nm/min)
Thickness (nm)
Crystallite size (nm)
RMS (nm)
Resistivity (Ω/sq)
Optical energy bandgap (eV)
C1 C2 C4 C6
1.45 1.03 0.75 0.61
87 62 45 37
27 24 21 20
8.74 6.99 6.24 4.02
8.51 9.60 11.02 13.27
3.13 3.41 3.46 3.68
mobility reduced from 9.15 ( × 1015cm−2) to 2.17 ( × 1015cm−2) and from 7.78 (cm2/Vs) to 1.46 (cm2/Vs) by increasing the ratio of Ar:N2 flow rate. This observation could be attributed to the reduction of crystallite size. The decrease in crystallite size contributed to the increase of crystal grain boundaries. As a result, more charge carrier scattering centers were generated. The relatively poor crystal quality of CCTO thin film deposited at higher Ar:N2 flow rate as revealed by XRD analysis in Fig. 3a also increased the number of charge carrier scattering centers. Hence, the conductive performance, charge carrier concentration and mobility of CCTO thin films were reduced with
increasing ratio of Ar:N2 flow rate [20]. The optical energy bandgap (Eg) of CCTO thin films deposited using various ratios of Ar:N2 flow rate is shown in Fig. 6b. The Eg was found to be 3.13, 3.41, 3.46, and 3.68 eV for samples C1, C2, C4, and C6, respectively. Decrease in crystallite size i.e. from 27 nm to 20 nm with increasing ratio of Ar:N2 flow rate, leading to the increase of Eg as a result of quantum confinement effect. Such high Eg values are highly desirable for the applications in optoelectronic and supercapacitor industry. The calculated Eg at various ratios of Ar:N2 flow rate are summarized in Table 2.
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4. Conclusions CCTO thin films were deposited on ITO substrates using RF magnetron sputtering under various ratios of Ar:N2 flow rate of 1:1, 1:2, 1:4, and 1:6 sccm at ambient temperature. It was observed that the deposition rate of the films decreased gradually (1.45, 1.03, 0.75, and 0.61 nm/min) with increasing ratio of Ar:N2 flow rate. XRD analysis revealed that the crystal quality of thin films reduced with increasing ratio of Ar:N2 flow rate. The crystallite size and surface roughness of thin films were also found to decrease with increasing ratio of Ar:N2 flow rate. The resistivity and optical energy bandgap of the CCTO thin films were also affected by the Ar:N2 flow rate, where maximum values (13.27 Ω/sq and 3.68 eV) were measured at Ar:N2 flow rate of 1:6. In brief, the ratio of Ar:N2 flow rate should be precisely controlled during the CCTO thin film depositions in order to obtain high-quality CCTO thin films with desired electrical and optical properties for different applications.
[7]
[8]
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Acknowledgments
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The authors acknowledge the research funding of RUI 1001/ PBAHAN/8014095 from Universiti Sains Malaysia.
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