- Email: [email protected]
The effect of annealing on the structural, electrical, optical and electrochromic properties of indium-tin-oxide films deposited by RF magnetron sputtering technique
Optik 142 (2017) 320–326
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
The effect of annealing on the structural, electrical, optical and electrochromic properties of indium-tin-oxide films deposited by RF magnetron sputtering technique Gizem Durak Yüzüak a,∗ , Özlem Duyar Cos¸kun b a b
Ankara University, Department of Physics Engineering, Ankara, 06100, Turkey Hacettepe University, Department of Physics Engineering, Ankara, 06100, Turkey
a r t i c l e
i n f o
Article history: Received 3 March 2017 Accepted 2 June 2017 Keywords: Indium-tin-oxide RF magnetron sputtering Thermal annealing Optical properties Electrochromic properties
a b s t r a c t Indium-tin-oxide (ITO) thin films with 200 nm thicknesses were deposited on high temperature glass substrates by RF magnetron sputtering at 200 ◦ C substrate temperature. The effect of annealing temperature on physical properties of the ITO films including crystalline, sheet resistance, optical transparency and electrochromic behavior was investigated. The identification of intrinsic changes in the ITO film properties is crucially important because the ITO films need further treatment by thermal annealing for a wide range of applications. These treatments could alter the initially optimized film properties. By annealing, a red-shift occurred in the optical transmittance of the ITO films. A decrease in the free electron charge density caused by the growth of ITO crystal grains resulted from narrow band gap energies in the range of 3.94 eV–4.05 eV. That is attributed to the decrease of the free electron charge density. The ITO film was annealed at 300 ◦ C in ambient conditions showed a cubic structure (space group: I a-3) and had the optical transmittance of 90% in the visible range. The results indicated that the electrical conductivity of the ITO films was more greatly influenced by the decrease of the electrically active tin dopant concentration in the film structure. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction The ITO films are the transparent conducting oxides that have been extensively utilized in the electronic and optoelectronic industries due to their high electrical conductivity combined with a higher transmission in the visible and near-IR regions of the electromagnetic spectrum [1]. These superior properties make them the most used material in applications such as flat panel displays, solar cells, and electrochromic (EC) applications. There have been some studies that show the EC behavior of ITO films as much. The mechanism is not understood in detail. ITO films have been produced by a variety of deposition techniques, such as magnetron sputtering [2,3], electron beam evaporation [4], chemical vapor deposition [5], pulsed laser deposition [6] and reactive thermal evaporation [7]. Among these deposition techniques, magnetron sputtering is the most widely used method for preparing conductive and transparent thin films with good film adhesion, reproducibility, and large area preparation with higher uniformity. Magnetron sputtering deposition can also provide a high deposition rate at room temperature [8].
∗ Corresponding author. E-mail address: [email protected] (G.D. Yüzüak). http://dx.doi.org/10.1016/j.ijleo.2017.06.016 0030-4026/© 2017 Elsevier GmbH. All rights reserved.
G.D. Yüzüak, Ö.D. Cos¸kun / Optik 142 (2017) 320–326
321
Fig. 1. (a) The XRD patterns of the as-deposited and annealed ITO thin films at different temperatures, (b) the XRD pattern of the observed, calculated, the difference between observed-calculated spectra and Bragg positions of the annealed ITO film at 300 ◦ C. The green bars show the Bragg positions of the cubic phase of the ITO film. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The optimization of the film properties during its fabrication process does not ensure that the film will retain its improved properties when as-deposited ITO films are integrated into electrochromic, opto-electronic and sensor devices which involve even more heat treatment cycles [9–12]. These additional cycles could affect the film properties and ultimately the device performance. Thus, it is of great importance to investigate how additional treatments to the ITO film could affect its initially optimized properties so as to ensure that ITO films used in a wide variety of devices should have appropriate optical, electrical and structural properties. To date, the influences of these further low and high temperature heat treatments on the ITO films have not been sufficiently investigated. Although EC properties of ITO films were weak, it would be useful to know how ITO acts as an EC material under these heat treatment conditions to achieve a well performed EC device. So, the effects of annealing on a wide range of temperatures on the electrical, optical and EC properties of the ITO thin film were systematically investigated. 2. Experimental details The ITO films were deposited on the high temperature glass substrates using RF magnetron sputtering technique with an ITO target (purity: 99.99%, diameter: 2 , thickness: 0.25 , In2 O3 : SnO2 = 90: 10 wt.%) at room temperature. Prior to the deposition process, the substrates were ultrasonically cleaned and degreased in the acetic acid, acetone and propanol then dried with an air. The sputtering chamber was initially pumped down to 2 × 10−6 Torr, and then pure argon gas was introduced into the chamber through a mass flow controller to achieve the required sputtering pressure. During the sputtering, the deposition pressure was maintained at 9 × 10−3 Torr, and the RF power was kept at 60 W. The distance between the substrate and the target was 7.5 cm. The film thicknesses were about 200 nm. After the deposition, the ITO films were annealed in 100 ◦ C increment from 300 to 800 ◦ C for an hour using the thermal annealing system. The crystal structure of the film was analyzed by X-ray diffraction (XRD) using a Rigaku D/MAX 200 V/PC diffractometer with a Cu-K␣1 radiation source. Scanning Electron Microscope images were taken by using an FEI Quanta 200 FEG microscope to investigate the surface topography of the samples. The optical transmittance and reflectance measurements were performed with an Aquila nkd 8000e spectrophotometer in the wavelength range of 350–1100 nm at 30◦ angle of incidence. The optical absorbance of the film was measured with a Hitachi U8000 spectrophotometer in the wavelength range of 190–1100 nm. The thicknesses of the films were calculated by simultaneous fitting of the transmission and reflection spectra using an appropriate optical model. The detailed fitting procedure was given in our previous paper [13]. EC properties were measured using a CH Instruments electrochemical analyzer with a standard three electrode arrangement. The Pt, the Ag/AgCl and the ITO were used as working electrode, a reference electrode, a counter electrode as an EC film, respectively. The 0.1 M LiClO4 in propylene carbonate (PC) was used as an electrolyte solution. 3. Results and discussion Fig. 1(a) shows the XRD patterns of the as-deposited and annealed ITO films for the various annealing temperatures. All the diffraction peaks could be indexed by assuming the C-type rare-earth oxide structure (JCPDS card #6-0416) had the characteristic diffraction peaks of ITO thin films in the temperature range of 300–800 ◦ C. As the elementary peaks shift to the right, the volume of the unit cell decreases with the increase in the annealing temperature regarding the formation energy
322
G.D. Yüzüak, Ö.D. Cos¸kun / Optik 142 (2017) 320–326
Fig. 2. Surface morphology image of ITO (a) as deposited (b) annealed at 600 ◦ C. Table 1 The values of the sheet resistances and transmittances at the wavelength of 550 nm of the as-deposited and annealed films. Sample
Rs (/sq) (300 K)
T = 550 nm
as-deposited annealed at 300 ◦ C annealed at 400 ◦ C annealed at 500 ◦ C annealed at 600 ◦ C annealed at 700 ◦ C annealed at 800 ◦ C
2000 300 240 200 180 120 120
91.79 90.30 90.21 90.51 90.81 89.53 88.05
of ITO. Fig. 1(b) shows the XRD pattern for the annealed film at 300 ◦ C as an example of the XRD analysis. The pattern is refined using the Fullprof package based on the Rietveld method. According to the Rietveld refinement, the as-deposited films crystallize in the cubic structure (space group: Ia-3). The unit cell parameters of the cubic structure are a = 10.104 (3) Å. This agrees with the reported values for the ITO thin films [14]. These patterns have good reliability factors from the refinement (RP = 5.05, RWP = 6.42 REXP = 6.08 and Chi2 = 1.12). By using the XRD results, the particle size distribution of the ITO films was estimated using the Scherrer equation [15] D=
B cos
where D is the grain size distribution, is the Scherrer constant (0.94), is the X-ray wavelength (1.5418 Å), B is full-width at half-maximum (FWHM) of the diffraction peak (in radian) and is the diffraction angle (in degree). The (222) diffraction peak with the highest intensity was selected for the calculation of the size distribution of the film. The calculated grain sizes are in the range of 40–45 nm for the annealed films. So grain size distributions of thin films were comparatively homogenous for the all annealing temperatures. The surface topographies of the as-deposited and the annealed ITO film reveal a significant difference as seen in Fig. 2(a–b), respectively. A large amount granules were observed on the surface of as-deposited ITO film in Fig. 2(a). In Fig. 2(b), the numbers of granules significantly decreased by annealing at 600 ◦ C and the grain boundaries became ambiguous with the annealing procedure. According to the backscattered SEM image with element mapping analysis didn’t show any phase separation of ITO and these results were harmony in BSD − electron microscopy observations of structures. Table 1 lists the sheet resistance values of the as-deposited and annealed films. The sheet resistances of the films were varied from 2000 to 120 /sq by annealing. The sheet resistance of the as deposited film decreased distinctly from 2000 to 300 /sq with the annealing up to 300 ◦ C. The sheet resistance of the films decreased slightly with the increasing annealing temperature up to 700 ◦ C. Then the sheet resistance of the films maintained at the value of 120 /sq with the further increase of the annealing temperature. The films with higher sheet resistance were pale yellow in color, where as lower resistance films were colorless. The variations at the transmittances of the films at the wavelength of 550 nm were also given in Table 1. The transmittance of the as-deposited film was relatively higher when compared with the annealed ones. However, the transmittances of the annealed samples are enough high for the variety of optoelectronic and EC applications. The optical transmission spectra of the as-deposited and annealed ITO films are presented in Fig. 3. The shift in the transmission and reflection maxima to the smaller wavelengths with the annealing of the film is originated from the thickness variation of the film [16]. The as-deposited ITO films are in the amorphous structure. The ITO films crystallize and their
G.D. Yüzüak, Ö.D. Cos¸kun / Optik 142 (2017) 320–326
323
Fig. 3. Optical spectrum of ITO.
structure changes into the cubic form by annealing. The annealing treatment can increase the homogeneity and crystallinity of the films and decrease the defect density at the edge of the energy band gap as indicated in the previous researches [17–20]. The change in a crystalline structure of the film has a little effect on the optical transmission in the visible range up to the annealing temperature of 400 ◦ C as shown in Fig. 3. The optical transmission of the film gets worse with annealing to 500 ◦ C. During annealing from 500 ◦ C to 600 ◦ C, the optical transmission of the sample increases with the oxidation of indium and tin and with the decrease of the number of defects such as vacancies and interstitial impurities in the structure. This meant that the film structure gets closed packed structure. At 600 ◦ C, the optical transmission at a wavelength of 550 nm can all reach up to 90.81%. The defect density of the films gradually decreased with the increasing annealing temperature, and the crystalline of the films also improved. Based on previous research results [17–20], In the vicinity of the fundamental gap, in the near UV range, a shift was observed, at higher annealing temperatures. This effect is the well-known Burstein–Moss shift [21]. After 700 ◦ C film started to deteriorate and optical transmission decrease with further annealing. As discussed above, the crystallization of the ITO films strongly depends on the annealing temperature. The increase in optical transmission after annealing can be attributed to the improvement of crystalline [22]. The optical absorption coefficient ␣ is determined by the band edge transmittance and film thickness by using the following relation: T = A1 exp (˛d) where T and d are the transmittance and thickness of the film, respectively, and A1 is constant. The absorption coefficient data are used to determine the energy band gap Eg . Since ITO has a direct band gap semiconductor, thus the band gap can be determined by the following formula: 2
(hv˛) = A hv − Eg
where A is the constant, and hv is the photon energy. The optical band gap Eg could be obtained by extrapolating the linear part of the curve to hv˛ = 0. The calculated optical band gap for the as-deposited ITO film is 3.64 eV, and the band gap for the film after annealing at 600 ◦ C is 4.05 eV. The difference of the carrier concentration between the as-deposited and annealing films caused the band gap. The band gap widens with the increase of the carrier concentration and can be explained by Burstein–Moss effect [23]. Fig. 4(a–b) shows EC behavior of the ITO films which were investigated with cyclic voltammetry (CV) measurements. In Fig. 4(a) data shows the first, fifth, tenth, fifteenth and twentieth potential cycles. It shows the twentieth potential scan, the shape of which is typical of stable behavior. Increasing the cycle numbers affected the charge density. The calculated total charge values for every reported cycle 1., 5., 10., 15., 20th were 1.6, 1.65, 2.37, 2.45, 2.70 mC/cm2 , respectively. These values were well matched with the chronoamperometric measurements. While intercalation and deintercalation process continued, the deposited charge on the ITO films didn’t discharge completely and increased the stored charge on the ITO
324
G.D. Yüzüak, Ö.D. Cos¸kun / Optik 142 (2017) 320–326
Fig. 4. CV measurements of the as-deposited (a) for 1., 5., 10., 15., 20th cycles (b) and annealed ITO films.
films. In Fig. 4(b), at potentials more negative than 0.5 V reduction current peak was obtained. As-deposited and annealed ITO films at 300 ◦ C and 600 ◦ C showed the cathodic peak at the same range around −0.92 V. These curves measured at a scan rate at 50 mV/s, are compatible with others [24,25]. The coloration-bleaching kinetics of the ITO films was followed by putting the ITO electrode in the prepared electrolyte, and then applying different coloring voltages to them. The potentiodynamic current-potential characterization characteristics were recorded in 0.1 M LiClO4 in PC solution. By applying the voltages to the electrodes at t = 20s, the transmittance of the films continuously decreased and they were colored. For all the samples, the considered coloring voltages were disconnected at t = 80 s then the polarity of the applied voltages was inverted. By changing the polarity, the transmittance of the films was increased and they were bleached with a similar rate the coloration process. The coloration and bleaching processes of ITO films were associated with the intercalation and deintercalation of Li+ ions and electrons in the films according to the following reaction [26–28]. The voltammogram of the as-deposited film (Fig. 4b) displays a high cathodic current intensity and well-defined oxidation peak in comparison to the annealed ones that show a good CV response. The value of cathodic charge density for the as-deposited film was similar irrespective of the number of cycles, indicating a good electrochemical stability and a fully reversible insertion–extraction process. Therefore, the film obtained by as-deposited gave a better electrochemical response than the films obtained from annealing. Besides the morphological difference between both types of films, the main difference that marks the electrochemical response was the presence of a crystalline structure. Fig. 5(a) shows the chronoamperometric (CA) measurements performed by applying alternating potentials of −2.0 and 2.0 V for 200 s (colored and bleached state, respectively) for all ITO samples. The comparison between the anodic and cathodic current intensities indicates a faster kinetic extraction than the insertion process, with an approximate bleaching time of 6 s in comparison with the colored time of 20s. On the other hand, the i–t curves during the coloration closely follow a t1/2 dependence, indicative of a Li+ ion diffusion process across the film. The Li+ injection at the film/electrolyte interface was the rate-determining step [29]. This fact was associated with the lower inserted charge during the colored state for the film as-deposited in comparison to that with annealed ones. The estimated values for films were close to 3.85 mA/cm2 for film annealed at 600 ◦ C and higher than 1.50, 1.14 mA/cm2 for as-deposited and annealed at 300 ◦ C. Fig. 5(b) shows the chronocoulometric (CC) measurements performed by applying alternating potentials of −2.0 and 2.0 V for 200 s for all ITO films. For CC measurements, there are differences between first 20th second and 200th second. In the beginning, ITO samples didn’t show the expected efficiency because intercalation was associated with charge transfer at the electrolyte interface. The obtained values for films, 5.42 mC/cm2 for annealed at 600 ◦ C, 0.67 and 1.72 mC/cm2 for annealed at 300 ◦ C and as-deposited one, respectively. Large changes in the CC measurements accompany the intercalation process rendering the 600 ◦ C heat treated ITO film attractive for transparent conductive for EC devices. The annealing process only affects the surface morphology, increasing the roughness and decreasing the thickness of the film, with a contribution to the electrochemical response. For the 1st electrochemical cycle, the highest value of cathodic charge density corresponds to the
G.D. Yüzüak, Ö.D. Cos¸kun / Optik 142 (2017) 320–326
325
Fig. 5. (a) CA (b) CC measurements of the as deposited and annealed ITO films.
annealed at 600 ◦ C film, showing a higher capacity for Li+ ion insertion. Also, this value of charge density increased with the number of cycles indicating that the films retained the inserted lithium ions and that the insertion/extraction process was reversible. However, the film obtained by annealed at 300 ◦ C did not present a stabilized CV response after the 20th cycle because of the lowest charge density [29]. 4. Conclusion This work provides fundamental insights of the effects of low and high temperature heat treatments on the physical properties of ITO films. Indium tin oxide thin films have been prepared on high temperature glass substrates by RF magnetron sputtering at room temperature. The as-deposited ITO film is in the amorphous structure. As the annealing temperature increases, the films show a polycrystalline cubic structure and the resistivity of ITO films decreases drastically. The lowest resistivity of 120 /sq is obtained by annealing at 800 ◦ C. The enlargement of the ITO crystallites is mainly caused by the incorporation of oxygen into the structures, forming more electrically inactive tin oxide complexes and decreasing the number of oxygen vacancies. Therefore the electrical resistivity of ITO film tends to increase after multiple heat treatment cycles. The transmission in the visible region of ITO films increases by increasing the annealing temperature. High transmittance (around 90%) is exhibited by the films annealed at 300 ◦ C and above. These results indicate that these annealing processes produce high quality ITO films which will extend their applications. However, the electrochromic measurements of asdeposited and annealed films show different behavior. Charge density increases with annealing process above crystalline temperature and reaches the maximum value of 5.42 mC/cm2 for annealed at 600 ◦ C ITO film. So, the annealing process not only increased the transmission but also charge density of the ITO films. References [1] S. Elhalawaty, K. Sivaramakrishnan, N.D. Theodore, T.L. Alford, The effect of sputtering pressure on electrical, optical and structure properties of indium tin oxide on glass, Thin Solid Films 518 (2010) 3326–3331. [2] K.J. Kumar, N.R.C. Raju, A. Subrahmanyam, Thickness dependent physical and photocatalytic properties of ITO thin films prepared by reactive DC magnetron sputtering, Appl. Surf. Sci. 257 (2011) 3075–3080. [3] C.H. Yang, S.C. Lee, T.C. Lin, S.C. Chen, Electrical and optical properties of indium tin oxide films prepared on plastic substrates by radio frequency magnetron sputtering, Thin Solid Films 516 (2008) 1984–1991. [4] N. Wan, T. Wang, H.C. Sun, G. Chen, L. Geng, X.H. Gan, S.H. Guo, J. Xu, L. Xu, K.J. Chen, Indium tin oxide thin films for silicon-based electro-luminescence devices prepared by electron beam evaporation method, J. Non-Cryst. Solids 356 (2010) 911–916. [5] T. Kondo, Y. Sawada, H. Funakubo, K. Akiyama, T. Kiguchi, M.H. Wang, T. Uchida, Good conformability of indium-tin-oxide thin films prepared by spray chemical vapor deposition, Electrochem. Solid-State Lett. 12 (2009) D42–D44. [6] K.A. Sierros, D.R. Cairns, J.S. Abell, S.N. Kukureka, Pulsed laser deposition of indium tin oxide films on flexible polyethylene naphthalate display substrates at room temperature, Thin Solid Films 518 (2010) 2623–2627. [7] G.S. Belo, B.J.P. da Silva, E.A. de Vasconcelos, W.M. de Azevedo, E.F. da Silva, A simplified reactive thermal evaporation method for indium tin oxide electrodes, Appl. Surf. Sci. 255 (2008) 755–757. [8] H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadsik, Semiconducting transparent thin films, Institute of Physics, Philadelphia, 1995.
326
G.D. Yüzüak, Ö.D. Cos¸kun / Optik 142 (2017) 320–326
´ Z.C. Orel, Electrical and optical properties of Zn/(PEO)4ZnCl2/[CeO2 or CeO2-SnO2(17%)] ITO thin film galvanic cells, Solid State Ionics 89 [9] A. Turkovic, (1996) 255–261. [10] L.P. Martin, A.-Q. Pham, R.S. Glass, Electrochemical hydrogen sensor for safety monitoring, Solid State Ionics 175 (2004) 527–530. [11] C.-Y. Li, L.-M. Huang, T.-C. Wen, A. Gopalanb, Superior performance characteristics for the poly(2,5 dimethoxyaniline)-poly(styrene sulfonic acid)based electrochromic device, Solid State Ionics 177 (2006) 795–802. [12] T. Sako, A. Ohmi, H. Yumoto, K. Nishiyama, ITO-film gas sensor for measuring photodecomposition of NO2 gas, Surf. Coat. Technol. 142–144 (2001) 781. [13] O.D. Coskun, S. Demirel, The optical and structural properties of amorphous Nb2O5 thin films prepared by RF magnetron sputtering, Appl. Surf. Sci. 277 (2013) 35–39. [14] R.N. Joshi, V.P. Singh, J.C. McClure, Characteristics of indium tin oxide films deposited by r.f. magnetron sputtering, Thin Solid Films 257 (1995) 32–35. [15] A.L. Patterson, The Scherrer formula for X-ray particle size determination, Phys. Rev. 56 (1939) 978–982. [16] E. Terzini, P. Thilakan, C. Minarini, Properties of ITO thin films deposited by RF magnetron sputtering at elevated substrate temperature, Mater. Sci. Eng. B 77 (2000) 110–114. [17] H.N. Cui, S.-Q. Xi, The fabrication of dipped CdS and sputtered ITO thin films for photovoltaic solar cells, Thin Solid Films 288 (1996) 325–329. [18] M.J. Alam, D.C. Cameron, Optical and electrical properties of transparent conductive ITO thin films deposited by sol–gel process, Thin Solid Films 377–378 (2000) 455–459. [19] D.V. Morgan, Y.H. Aliyu, R.W. Bunce, A. Salehi, Annealing effects on opto-electronic properties of sputtered and thermally evaporated indium-tin-oxide films, Thin Solid Films 312 (1998) 268–272. [20] D.V. Morgan, A. Salehi, Y.H. Aliyu, R.W. Bunce, Electro-optical characteristics of indium tin oxide (ITO) films: effect of thermal annealing, Renew. Energy 7 (1996) 119–222. [21] E. Burstein, Anomalous optical absorption limit in InSb, Phys. Rev. 93 (1954) 632. [22] Y. Djaoued, V.H. Phong, S. Badilescu, P.V. Ashrit, F.E. Girouard, V.V. Truong, Sol-gel-prepared ITO films for electrochromic systems, Thin Solid Films 293 (1997) 108–112. [23] H. Han, J.W. Mayer, T.L. Alford, Band gap shift in the indium-tin-oxide films on polyethylene napthalate after thermal annealing in air, J. Appl. Phys. 100 (2006) 123711. [24] A.K. Sytchkova, M.I. Grilli, S. Boycheva, A. Piegari, Optical and electrical characterization of r.f. sputtered ITO films developed as art protection coatings, Appl. Phys. A 89 (2007) 63–72. [25] T.C. Gorjanca, D. Leonga, C. Py, D. Rotha, Room temperature deposition of ITO using r.f. magnetron sputtering, Thin Solid Films 413 (2002) 181–185. [26] P.M.S. Monk, R.J. Mortimer, D.R. Rosseinsky, Electrochromism: Fundamentals and Applications, VCH, Weinheim, 1995. [27] P.R. Somani, S. Radhakrishnan, Electrochromic materials and devices: present and future, Mater. Chem. Phys. 77 (2002) 117–133. [28] R. Azimirad, O. Akhavan, A.Z. Moshfeghz, Influence of coloring voltage and thickness on electrochromical properties of e-beam evaporated WO3 thin films, J. Electrochem. Soc. 153 (2) (2006) E11–E16. [29] R. Romero, E.A. Dalchiele, F. Martin, D. Leinen, J.R. Ramos-Barrado, Electrochromic behaviour of Nb2O5 thin films with different morphologies obtained by spray pyrolysis, Sol. Energy Mater. Sol. Cells 93 (2009) 222–229.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
The effect of annealing on the structural, electrical, optical and electrochromic properties of indium-tin-oxide films deposited by RF magnetron sputtering technique Gizem Durak Yüzüak a,∗ , Özlem Duyar Cos¸kun b a b
Ankara University, Department of Physics Engineering, Ankara, 06100, Turkey Hacettepe University, Department of Physics Engineering, Ankara, 06100, Turkey
a r t i c l e
i n f o
Article history: Received 3 March 2017 Accepted 2 June 2017 Keywords: Indium-tin-oxide RF magnetron sputtering Thermal annealing Optical properties Electrochromic properties
a b s t r a c t Indium-tin-oxide (ITO) thin films with 200 nm thicknesses were deposited on high temperature glass substrates by RF magnetron sputtering at 200 ◦ C substrate temperature. The effect of annealing temperature on physical properties of the ITO films including crystalline, sheet resistance, optical transparency and electrochromic behavior was investigated. The identification of intrinsic changes in the ITO film properties is crucially important because the ITO films need further treatment by thermal annealing for a wide range of applications. These treatments could alter the initially optimized film properties. By annealing, a red-shift occurred in the optical transmittance of the ITO films. A decrease in the free electron charge density caused by the growth of ITO crystal grains resulted from narrow band gap energies in the range of 3.94 eV–4.05 eV. That is attributed to the decrease of the free electron charge density. The ITO film was annealed at 300 ◦ C in ambient conditions showed a cubic structure (space group: I a-3) and had the optical transmittance of 90% in the visible range. The results indicated that the electrical conductivity of the ITO films was more greatly influenced by the decrease of the electrically active tin dopant concentration in the film structure. © 2017 Elsevier GmbH. All rights reserved.
1. Introduction The ITO films are the transparent conducting oxides that have been extensively utilized in the electronic and optoelectronic industries due to their high electrical conductivity combined with a higher transmission in the visible and near-IR regions of the electromagnetic spectrum [1]. These superior properties make them the most used material in applications such as flat panel displays, solar cells, and electrochromic (EC) applications. There have been some studies that show the EC behavior of ITO films as much. The mechanism is not understood in detail. ITO films have been produced by a variety of deposition techniques, such as magnetron sputtering [2,3], electron beam evaporation [4], chemical vapor deposition [5], pulsed laser deposition [6] and reactive thermal evaporation [7]. Among these deposition techniques, magnetron sputtering is the most widely used method for preparing conductive and transparent thin films with good film adhesion, reproducibility, and large area preparation with higher uniformity. Magnetron sputtering deposition can also provide a high deposition rate at room temperature [8].
∗ Corresponding author. E-mail address: [email protected] (G.D. Yüzüak). http://dx.doi.org/10.1016/j.ijleo.2017.06.016 0030-4026/© 2017 Elsevier GmbH. All rights reserved.
G.D. Yüzüak, Ö.D. Cos¸kun / Optik 142 (2017) 320–326
321
Fig. 1. (a) The XRD patterns of the as-deposited and annealed ITO thin films at different temperatures, (b) the XRD pattern of the observed, calculated, the difference between observed-calculated spectra and Bragg positions of the annealed ITO film at 300 ◦ C. The green bars show the Bragg positions of the cubic phase of the ITO film. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The optimization of the film properties during its fabrication process does not ensure that the film will retain its improved properties when as-deposited ITO films are integrated into electrochromic, opto-electronic and sensor devices which involve even more heat treatment cycles [9–12]. These additional cycles could affect the film properties and ultimately the device performance. Thus, it is of great importance to investigate how additional treatments to the ITO film could affect its initially optimized properties so as to ensure that ITO films used in a wide variety of devices should have appropriate optical, electrical and structural properties. To date, the influences of these further low and high temperature heat treatments on the ITO films have not been sufficiently investigated. Although EC properties of ITO films were weak, it would be useful to know how ITO acts as an EC material under these heat treatment conditions to achieve a well performed EC device. So, the effects of annealing on a wide range of temperatures on the electrical, optical and EC properties of the ITO thin film were systematically investigated. 2. Experimental details The ITO films were deposited on the high temperature glass substrates using RF magnetron sputtering technique with an ITO target (purity: 99.99%, diameter: 2 , thickness: 0.25 , In2 O3 : SnO2 = 90: 10 wt.%) at room temperature. Prior to the deposition process, the substrates were ultrasonically cleaned and degreased in the acetic acid, acetone and propanol then dried with an air. The sputtering chamber was initially pumped down to 2 × 10−6 Torr, and then pure argon gas was introduced into the chamber through a mass flow controller to achieve the required sputtering pressure. During the sputtering, the deposition pressure was maintained at 9 × 10−3 Torr, and the RF power was kept at 60 W. The distance between the substrate and the target was 7.5 cm. The film thicknesses were about 200 nm. After the deposition, the ITO films were annealed in 100 ◦ C increment from 300 to 800 ◦ C for an hour using the thermal annealing system. The crystal structure of the film was analyzed by X-ray diffraction (XRD) using a Rigaku D/MAX 200 V/PC diffractometer with a Cu-K␣1 radiation source. Scanning Electron Microscope images were taken by using an FEI Quanta 200 FEG microscope to investigate the surface topography of the samples. The optical transmittance and reflectance measurements were performed with an Aquila nkd 8000e spectrophotometer in the wavelength range of 350–1100 nm at 30◦ angle of incidence. The optical absorbance of the film was measured with a Hitachi U8000 spectrophotometer in the wavelength range of 190–1100 nm. The thicknesses of the films were calculated by simultaneous fitting of the transmission and reflection spectra using an appropriate optical model. The detailed fitting procedure was given in our previous paper [13]. EC properties were measured using a CH Instruments electrochemical analyzer with a standard three electrode arrangement. The Pt, the Ag/AgCl and the ITO were used as working electrode, a reference electrode, a counter electrode as an EC film, respectively. The 0.1 M LiClO4 in propylene carbonate (PC) was used as an electrolyte solution. 3. Results and discussion Fig. 1(a) shows the XRD patterns of the as-deposited and annealed ITO films for the various annealing temperatures. All the diffraction peaks could be indexed by assuming the C-type rare-earth oxide structure (JCPDS card #6-0416) had the characteristic diffraction peaks of ITO thin films in the temperature range of 300–800 ◦ C. As the elementary peaks shift to the right, the volume of the unit cell decreases with the increase in the annealing temperature regarding the formation energy
322
G.D. Yüzüak, Ö.D. Cos¸kun / Optik 142 (2017) 320–326
Fig. 2. Surface morphology image of ITO (a) as deposited (b) annealed at 600 ◦ C. Table 1 The values of the sheet resistances and transmittances at the wavelength of 550 nm of the as-deposited and annealed films. Sample
Rs (/sq) (300 K)
T = 550 nm
as-deposited annealed at 300 ◦ C annealed at 400 ◦ C annealed at 500 ◦ C annealed at 600 ◦ C annealed at 700 ◦ C annealed at 800 ◦ C
2000 300 240 200 180 120 120
91.79 90.30 90.21 90.51 90.81 89.53 88.05
of ITO. Fig. 1(b) shows the XRD pattern for the annealed film at 300 ◦ C as an example of the XRD analysis. The pattern is refined using the Fullprof package based on the Rietveld method. According to the Rietveld refinement, the as-deposited films crystallize in the cubic structure (space group: Ia-3). The unit cell parameters of the cubic structure are a = 10.104 (3) Å. This agrees with the reported values for the ITO thin films [14]. These patterns have good reliability factors from the refinement (RP = 5.05, RWP = 6.42 REXP = 6.08 and Chi2 = 1.12). By using the XRD results, the particle size distribution of the ITO films was estimated using the Scherrer equation [15] D=
B cos
where D is the grain size distribution, is the Scherrer constant (0.94), is the X-ray wavelength (1.5418 Å), B is full-width at half-maximum (FWHM) of the diffraction peak (in radian) and is the diffraction angle (in degree). The (222) diffraction peak with the highest intensity was selected for the calculation of the size distribution of the film. The calculated grain sizes are in the range of 40–45 nm for the annealed films. So grain size distributions of thin films were comparatively homogenous for the all annealing temperatures. The surface topographies of the as-deposited and the annealed ITO film reveal a significant difference as seen in Fig. 2(a–b), respectively. A large amount granules were observed on the surface of as-deposited ITO film in Fig. 2(a). In Fig. 2(b), the numbers of granules significantly decreased by annealing at 600 ◦ C and the grain boundaries became ambiguous with the annealing procedure. According to the backscattered SEM image with element mapping analysis didn’t show any phase separation of ITO and these results were harmony in BSD − electron microscopy observations of structures. Table 1 lists the sheet resistance values of the as-deposited and annealed films. The sheet resistances of the films were varied from 2000 to 120 /sq by annealing. The sheet resistance of the as deposited film decreased distinctly from 2000 to 300 /sq with the annealing up to 300 ◦ C. The sheet resistance of the films decreased slightly with the increasing annealing temperature up to 700 ◦ C. Then the sheet resistance of the films maintained at the value of 120 /sq with the further increase of the annealing temperature. The films with higher sheet resistance were pale yellow in color, where as lower resistance films were colorless. The variations at the transmittances of the films at the wavelength of 550 nm were also given in Table 1. The transmittance of the as-deposited film was relatively higher when compared with the annealed ones. However, the transmittances of the annealed samples are enough high for the variety of optoelectronic and EC applications. The optical transmission spectra of the as-deposited and annealed ITO films are presented in Fig. 3. The shift in the transmission and reflection maxima to the smaller wavelengths with the annealing of the film is originated from the thickness variation of the film [16]. The as-deposited ITO films are in the amorphous structure. The ITO films crystallize and their
G.D. Yüzüak, Ö.D. Cos¸kun / Optik 142 (2017) 320–326
323
Fig. 3. Optical spectrum of ITO.
structure changes into the cubic form by annealing. The annealing treatment can increase the homogeneity and crystallinity of the films and decrease the defect density at the edge of the energy band gap as indicated in the previous researches [17–20]. The change in a crystalline structure of the film has a little effect on the optical transmission in the visible range up to the annealing temperature of 400 ◦ C as shown in Fig. 3. The optical transmission of the film gets worse with annealing to 500 ◦ C. During annealing from 500 ◦ C to 600 ◦ C, the optical transmission of the sample increases with the oxidation of indium and tin and with the decrease of the number of defects such as vacancies and interstitial impurities in the structure. This meant that the film structure gets closed packed structure. At 600 ◦ C, the optical transmission at a wavelength of 550 nm can all reach up to 90.81%. The defect density of the films gradually decreased with the increasing annealing temperature, and the crystalline of the films also improved. Based on previous research results [17–20], In the vicinity of the fundamental gap, in the near UV range, a shift was observed, at higher annealing temperatures. This effect is the well-known Burstein–Moss shift [21]. After 700 ◦ C film started to deteriorate and optical transmission decrease with further annealing. As discussed above, the crystallization of the ITO films strongly depends on the annealing temperature. The increase in optical transmission after annealing can be attributed to the improvement of crystalline [22]. The optical absorption coefficient ␣ is determined by the band edge transmittance and film thickness by using the following relation: T = A1 exp (˛d) where T and d are the transmittance and thickness of the film, respectively, and A1 is constant. The absorption coefficient data are used to determine the energy band gap Eg . Since ITO has a direct band gap semiconductor, thus the band gap can be determined by the following formula: 2
(hv˛) = A hv − Eg
where A is the constant, and hv is the photon energy. The optical band gap Eg could be obtained by extrapolating the linear part of the curve to hv˛ = 0. The calculated optical band gap for the as-deposited ITO film is 3.64 eV, and the band gap for the film after annealing at 600 ◦ C is 4.05 eV. The difference of the carrier concentration between the as-deposited and annealing films caused the band gap. The band gap widens with the increase of the carrier concentration and can be explained by Burstein–Moss effect [23]. Fig. 4(a–b) shows EC behavior of the ITO films which were investigated with cyclic voltammetry (CV) measurements. In Fig. 4(a) data shows the first, fifth, tenth, fifteenth and twentieth potential cycles. It shows the twentieth potential scan, the shape of which is typical of stable behavior. Increasing the cycle numbers affected the charge density. The calculated total charge values for every reported cycle 1., 5., 10., 15., 20th were 1.6, 1.65, 2.37, 2.45, 2.70 mC/cm2 , respectively. These values were well matched with the chronoamperometric measurements. While intercalation and deintercalation process continued, the deposited charge on the ITO films didn’t discharge completely and increased the stored charge on the ITO
324
G.D. Yüzüak, Ö.D. Cos¸kun / Optik 142 (2017) 320–326
Fig. 4. CV measurements of the as-deposited (a) for 1., 5., 10., 15., 20th cycles (b) and annealed ITO films.
films. In Fig. 4(b), at potentials more negative than 0.5 V reduction current peak was obtained. As-deposited and annealed ITO films at 300 ◦ C and 600 ◦ C showed the cathodic peak at the same range around −0.92 V. These curves measured at a scan rate at 50 mV/s, are compatible with others [24,25]. The coloration-bleaching kinetics of the ITO films was followed by putting the ITO electrode in the prepared electrolyte, and then applying different coloring voltages to them. The potentiodynamic current-potential characterization characteristics were recorded in 0.1 M LiClO4 in PC solution. By applying the voltages to the electrodes at t = 20s, the transmittance of the films continuously decreased and they were colored. For all the samples, the considered coloring voltages were disconnected at t = 80 s then the polarity of the applied voltages was inverted. By changing the polarity, the transmittance of the films was increased and they were bleached with a similar rate the coloration process. The coloration and bleaching processes of ITO films were associated with the intercalation and deintercalation of Li+ ions and electrons in the films according to the following reaction [26–28]. The voltammogram of the as-deposited film (Fig. 4b) displays a high cathodic current intensity and well-defined oxidation peak in comparison to the annealed ones that show a good CV response. The value of cathodic charge density for the as-deposited film was similar irrespective of the number of cycles, indicating a good electrochemical stability and a fully reversible insertion–extraction process. Therefore, the film obtained by as-deposited gave a better electrochemical response than the films obtained from annealing. Besides the morphological difference between both types of films, the main difference that marks the electrochemical response was the presence of a crystalline structure. Fig. 5(a) shows the chronoamperometric (CA) measurements performed by applying alternating potentials of −2.0 and 2.0 V for 200 s (colored and bleached state, respectively) for all ITO samples. The comparison between the anodic and cathodic current intensities indicates a faster kinetic extraction than the insertion process, with an approximate bleaching time of 6 s in comparison with the colored time of 20s. On the other hand, the i–t curves during the coloration closely follow a t1/2 dependence, indicative of a Li+ ion diffusion process across the film. The Li+ injection at the film/electrolyte interface was the rate-determining step [29]. This fact was associated with the lower inserted charge during the colored state for the film as-deposited in comparison to that with annealed ones. The estimated values for films were close to 3.85 mA/cm2 for film annealed at 600 ◦ C and higher than 1.50, 1.14 mA/cm2 for as-deposited and annealed at 300 ◦ C. Fig. 5(b) shows the chronocoulometric (CC) measurements performed by applying alternating potentials of −2.0 and 2.0 V for 200 s for all ITO films. For CC measurements, there are differences between first 20th second and 200th second. In the beginning, ITO samples didn’t show the expected efficiency because intercalation was associated with charge transfer at the electrolyte interface. The obtained values for films, 5.42 mC/cm2 for annealed at 600 ◦ C, 0.67 and 1.72 mC/cm2 for annealed at 300 ◦ C and as-deposited one, respectively. Large changes in the CC measurements accompany the intercalation process rendering the 600 ◦ C heat treated ITO film attractive for transparent conductive for EC devices. The annealing process only affects the surface morphology, increasing the roughness and decreasing the thickness of the film, with a contribution to the electrochemical response. For the 1st electrochemical cycle, the highest value of cathodic charge density corresponds to the
G.D. Yüzüak, Ö.D. Cos¸kun / Optik 142 (2017) 320–326
325
Fig. 5. (a) CA (b) CC measurements of the as deposited and annealed ITO films.
annealed at 600 ◦ C film, showing a higher capacity for Li+ ion insertion. Also, this value of charge density increased with the number of cycles indicating that the films retained the inserted lithium ions and that the insertion/extraction process was reversible. However, the film obtained by annealed at 300 ◦ C did not present a stabilized CV response after the 20th cycle because of the lowest charge density [29]. 4. Conclusion This work provides fundamental insights of the effects of low and high temperature heat treatments on the physical properties of ITO films. Indium tin oxide thin films have been prepared on high temperature glass substrates by RF magnetron sputtering at room temperature. The as-deposited ITO film is in the amorphous structure. As the annealing temperature increases, the films show a polycrystalline cubic structure and the resistivity of ITO films decreases drastically. The lowest resistivity of 120 /sq is obtained by annealing at 800 ◦ C. The enlargement of the ITO crystallites is mainly caused by the incorporation of oxygen into the structures, forming more electrically inactive tin oxide complexes and decreasing the number of oxygen vacancies. Therefore the electrical resistivity of ITO film tends to increase after multiple heat treatment cycles. The transmission in the visible region of ITO films increases by increasing the annealing temperature. High transmittance (around 90%) is exhibited by the films annealed at 300 ◦ C and above. These results indicate that these annealing processes produce high quality ITO films which will extend their applications. However, the electrochromic measurements of asdeposited and annealed films show different behavior. Charge density increases with annealing process above crystalline temperature and reaches the maximum value of 5.42 mC/cm2 for annealed at 600 ◦ C ITO film. So, the annealing process not only increased the transmission but also charge density of the ITO films. References [1] S. Elhalawaty, K. Sivaramakrishnan, N.D. Theodore, T.L. Alford, The effect of sputtering pressure on electrical, optical and structure properties of indium tin oxide on glass, Thin Solid Films 518 (2010) 3326–3331. [2] K.J. Kumar, N.R.C. Raju, A. Subrahmanyam, Thickness dependent physical and photocatalytic properties of ITO thin films prepared by reactive DC magnetron sputtering, Appl. Surf. Sci. 257 (2011) 3075–3080. [3] C.H. Yang, S.C. Lee, T.C. Lin, S.C. Chen, Electrical and optical properties of indium tin oxide films prepared on plastic substrates by radio frequency magnetron sputtering, Thin Solid Films 516 (2008) 1984–1991. [4] N. Wan, T. Wang, H.C. Sun, G. Chen, L. Geng, X.H. Gan, S.H. Guo, J. Xu, L. Xu, K.J. Chen, Indium tin oxide thin films for silicon-based electro-luminescence devices prepared by electron beam evaporation method, J. Non-Cryst. Solids 356 (2010) 911–916. [5] T. Kondo, Y. Sawada, H. Funakubo, K. Akiyama, T. Kiguchi, M.H. Wang, T. Uchida, Good conformability of indium-tin-oxide thin films prepared by spray chemical vapor deposition, Electrochem. Solid-State Lett. 12 (2009) D42–D44. [6] K.A. Sierros, D.R. Cairns, J.S. Abell, S.N. Kukureka, Pulsed laser deposition of indium tin oxide films on flexible polyethylene naphthalate display substrates at room temperature, Thin Solid Films 518 (2010) 2623–2627. [7] G.S. Belo, B.J.P. da Silva, E.A. de Vasconcelos, W.M. de Azevedo, E.F. da Silva, A simplified reactive thermal evaporation method for indium tin oxide electrodes, Appl. Surf. Sci. 255 (2008) 755–757. [8] H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadsik, Semiconducting transparent thin films, Institute of Physics, Philadelphia, 1995.
326
G.D. Yüzüak, Ö.D. Cos¸kun / Optik 142 (2017) 320–326
´ Z.C. Orel, Electrical and optical properties of Zn/(PEO)4ZnCl2/[CeO2 or CeO2-SnO2(17%)] ITO thin film galvanic cells, Solid State Ionics 89 [9] A. Turkovic, (1996) 255–261. [10] L.P. Martin, A.-Q. Pham, R.S. Glass, Electrochemical hydrogen sensor for safety monitoring, Solid State Ionics 175 (2004) 527–530. [11] C.-Y. Li, L.-M. Huang, T.-C. Wen, A. Gopalanb, Superior performance characteristics for the poly(2,5 dimethoxyaniline)-poly(styrene sulfonic acid)based electrochromic device, Solid State Ionics 177 (2006) 795–802. [12] T. Sako, A. Ohmi, H. Yumoto, K. Nishiyama, ITO-film gas sensor for measuring photodecomposition of NO2 gas, Surf. Coat. Technol. 142–144 (2001) 781. [13] O.D. Coskun, S. Demirel, The optical and structural properties of amorphous Nb2O5 thin films prepared by RF magnetron sputtering, Appl. Surf. Sci. 277 (2013) 35–39. [14] R.N. Joshi, V.P. Singh, J.C. McClure, Characteristics of indium tin oxide films deposited by r.f. magnetron sputtering, Thin Solid Films 257 (1995) 32–35. [15] A.L. Patterson, The Scherrer formula for X-ray particle size determination, Phys. Rev. 56 (1939) 978–982. [16] E. Terzini, P. Thilakan, C. Minarini, Properties of ITO thin films deposited by RF magnetron sputtering at elevated substrate temperature, Mater. Sci. Eng. B 77 (2000) 110–114. [17] H.N. Cui, S.-Q. Xi, The fabrication of dipped CdS and sputtered ITO thin films for photovoltaic solar cells, Thin Solid Films 288 (1996) 325–329. [18] M.J. Alam, D.C. Cameron, Optical and electrical properties of transparent conductive ITO thin films deposited by sol–gel process, Thin Solid Films 377–378 (2000) 455–459. [19] D.V. Morgan, Y.H. Aliyu, R.W. Bunce, A. Salehi, Annealing effects on opto-electronic properties of sputtered and thermally evaporated indium-tin-oxide films, Thin Solid Films 312 (1998) 268–272. [20] D.V. Morgan, A. Salehi, Y.H. Aliyu, R.W. Bunce, Electro-optical characteristics of indium tin oxide (ITO) films: effect of thermal annealing, Renew. Energy 7 (1996) 119–222. [21] E. Burstein, Anomalous optical absorption limit in InSb, Phys. Rev. 93 (1954) 632. [22] Y. Djaoued, V.H. Phong, S. Badilescu, P.V. Ashrit, F.E. Girouard, V.V. Truong, Sol-gel-prepared ITO films for electrochromic systems, Thin Solid Films 293 (1997) 108–112. [23] H. Han, J.W. Mayer, T.L. Alford, Band gap shift in the indium-tin-oxide films on polyethylene napthalate after thermal annealing in air, J. Appl. Phys. 100 (2006) 123711. [24] A.K. Sytchkova, M.I. Grilli, S. Boycheva, A. Piegari, Optical and electrical characterization of r.f. sputtered ITO films developed as art protection coatings, Appl. Phys. A 89 (2007) 63–72. [25] T.C. Gorjanca, D. Leonga, C. Py, D. Rotha, Room temperature deposition of ITO using r.f. magnetron sputtering, Thin Solid Films 413 (2002) 181–185. [26] P.M.S. Monk, R.J. Mortimer, D.R. Rosseinsky, Electrochromism: Fundamentals and Applications, VCH, Weinheim, 1995. [27] P.R. Somani, S. Radhakrishnan, Electrochromic materials and devices: present and future, Mater. Chem. Phys. 77 (2002) 117–133. [28] R. Azimirad, O. Akhavan, A.Z. Moshfeghz, Influence of coloring voltage and thickness on electrochromical properties of e-beam evaporated WO3 thin films, J. Electrochem. Soc. 153 (2) (2006) E11–E16. [29] R. Romero, E.A. Dalchiele, F. Martin, D. Leinen, J.R. Ramos-Barrado, Electrochromic behaviour of Nb2O5 thin films with different morphologies obtained by spray pyrolysis, Sol. Energy Mater. Sol. Cells 93 (2009) 222–229.