DEPOSITED INDIUM TIN OXIDE (ITO) THIN FILMS BY DC

DEPOSITED INDIUM TIN OXIDE (ITO) THIN FILMS BY DC- MAGNETRON SPUTTERING ON POLYETHYLENE TEREPHTHALATE SUBSTRATE (PET) M.K.M. ALI 1, K. IBRAHIM 1, OSAMA S HAMAD 3, M.H. EISA1, 2, M.G. FARAJ 1, and F. AZHARI 1 1

School of Physics, University Sains Malaysia, Penang 11800, Malaysia Department of Physics, College of Science, Sudan University of Science and Technology, Khartoum 11113, Sudan 3 School of Electrical and Electronic Engineering, University Sains Malaysia, Penang 11800, Malaysia E-mail: [email protected], [email protected], [email protected], [email protected] 2

Received January 12, 2010 This paper is presenting the Indium Tin Oxide (ITO) thin films of low resistivity with different thicknesses. The thin films of ITO are deposited on polyethylene terephthalate (PET) substrate by means of DC-sputtering. The properties of the deposited films samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmittance electron microscopy (TEM), atomic force microscopy (AFM) and Raman spectroscopy. The obtained results for the surface morphology, the sheet resistance, the resistivity, the carrier concentration, Hall mobility, optical transmittance and the transmittance ratio of the ITO films deposed on PET substrate are reported. Details on the sample preparations and experimental details will be presented. Key words: Indium Tin Oxide; Polyethylene Terephthalate; DC-Magnetron Sputtering.

1. INTRODUCTION

Indium tin oxide (ITO) thin films are wide gap semiconductors with a relatively low resistivity and widely used as transparent in the visible range of the spectrum [1]. Due to these characteristics, ITO films are widely used for many applications [2-4]. Many works have been reported on the material ITO; however, most of them were using glass substrates to deposit the films, while few reports treated the polymer substrates [6-7]. In order to deposit the ITO films on these polymer substrates with certain properties and good adhesion, detailed study on structural and electrical properties of the films is important. In addition, target utilization efficiency becomes increasingly critical when employing expensive target materials such as ITO [8]. Therefore, the improvement of target utilization efficiency reduces the cost of the film formation processes, in addition to eliminating downtime that result from exchanging targets. Recently, the growth of Rom. Journ. Phys., Vol. 56, Nos. 5–6, P. 730–741, Bucharest, 2011

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ITO films on plastic substrates by sputtering has been reported [9, 10]. Today there is strong need for the high-quality ITO films for different applications should be deposited at substrate such as polyethylene terephthalate (PET) [11]. The ITO thin films commonly fabricated by different techniques such as electron beam (EB), RF and DC magnetron sputtering [12]. However, most of the films are deposited at high temperature. In this study ITO thin film deposited on plastic substrate (PET) at lower deposition temperature to prevent the PET substrate from damage. In this study, different thicknesses of ITO films deposed on PET substrates have been prepared by DC- sputtering method using low deposition temperature with different deposition conditions. The structural, optical and electrical properties of the obtained films depending on deposition parameters, such as sputtering power and working pressure, have been investigated. 2. MATERIALS AND METHODS

Thin Film Preparation: ITO films, having thickness in the 25.00–114.50 nm range, were deposited on PET substrates by DC magnetron sputtering technique (Model AUTO306). The sputtering system utilized a sintered ITO target having an In2O3:SnO2 composition of 90:10 wt. % were used as targets after slightly compressing them into copper holders, 3 inches in diameter. The substrate temperature during deposition was maintained at low value. The sputtering deposition was carried out in a pure argon atmosphere at a pressure from 5x10-5 to 2x10 -3 torr and the sputtering power at 145 W. The substrate was PET sheet, which were degreased ultrasonically in a dilute detergent solution, rinsed in demonized water, and blown dry in N2 gas before they were introduced into the chamber. The substrate was fixed directly above the target with a target-to-substrate distance of 6 cm. Before the sputtering deposition, presputtering was carried out under the same condition as the deposition. A parallel experiment on ITO films deposited onto glass substrate was also carried out for comparison. The thickness of the ITO film was determined for each time by using optical reflectometer (Model: Filmetric F20), with a lift-off type pattern marked on the substrate. The deposition times stared from 10 mints with equivalence varying to 15 and 20 mints. The different ITO film thicknesses of 25.0 nm, 87.0 nm and 114.5 nm were used in this work. The surface morphology for each thickness of the ITO films was studied by Atomic Force Microscope (AFM, model: Ultra Objective) and scanning electron microscope (SEM, model: JSM–6460 LV). X-ray diffraction (XRD) measurements were performed using high resolution X-ray diffractometer system (model: PANalytical X’ Pert PRO MRD PW3040). Structural changes of the ITO films were evaluated by X-ray diffraction (XRD) measurements in the 2θ mode. Carrier

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concentration and Hall mobility were obtained by Hall-effect measurement system (Model: Accent/HL 5500 PC). The sheet resistance of the samples was measured with a four-point probe (Model: Changmin Tech CMT–SR2000N) and the resistivity of the film was calculated. The sheet resistance was calculated by simple relation RS = ρ/t, where, RS is sheet resistant, ρ is the density and (t) is the thickness. The optical transmittance of the films was measured as the transmittance ratio of a film coated substrate relative to an uncoated substrate by UV Spectrophotometer (Model: U-2000 HITACHI). For very high levels of transmission, Fourier Transform Infrared Spectrometer System (FTIR, model: PERKEN ELMER SPECTRUM GX) infrared (IR) region and Raman and Photoluminescence spectroscopy system (model: Jobin Yvon HR 800 UV), has been used to characterize the optical transmittance. 3. RESULTS AND DISCUSSION 3.1. FILM THICKNESS

In this work, a variety of thin film deposition techniques have been employed to obtain ITO films. Numerous studies have been carried out on the preparation of highly conducting transparent Indium Tin Oxide (ITO) films. The results reported in these studies vary significantly from one another and suggest that the film properties greatly depend on deposition conditions and demand a careful optimization of growth parameters as well as post-deposition treatments for reproducible results. Hence some of the more influential factors determining the electrical properties and optical of the deposited ITO film including the pressures of the reactive gases, the substrate temperature, target oxidation as well as other growth related effects are discussed here. Table 3.1 shows the variation of the film thickness with deposition time. It is found that the thickness increases nearly linearly from 25.0 to 114.5 nm when the deposition time increases from 10 to 20 min. 3.2. STRUCTURAL CHARACTERIZATION

XRD Results: Fig. 3.1(a) and (b) shows the X-ray diffraction spectra of ITO films deposited on PET substrates. X-ray diffraction analysis indicated that the deposited films were polycrystalline. It can be seen that the ITO films deposited on PET substrate exhibit a preferred orientation. It may be pointed out that the preferred orientation for ITO films not only depends on the deposition conditions but also depends on the substrates materials. Compared ITO with the films deposited on PET substrate, the intensity of the diffraction peak of the films deposited on PET substrate is higher and the half width is smaller.

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a

b Fig. 3.1. – XRD spectrum of (a) of PET substrates; (b) ITO on PET at different Thicknesses.

SEM Results: Fig. 3.2 shows the SEM morphologies for ITO films deposited on PET substrates. During the deposition, the substrate temperature was maintained with the pressure. It can be observed that the crystallite size of the film deposited on PET substrate is larger, uniform and clean than that not deposited on PET substrate. This result is consistent with the X-ray observations. The surface is found to be studded with spots of size ∼ 1 µm. The spots increased in density in a sample subjected to time treatment (SEM picture not shown).

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Fig. 3.2. – SEM observations of the surfaces of ITO films on PET.

Indium Tin Oxide is essentially formed by substitutional doping of In2O3 with Sn which replaces the In3+ atoms from the cubic bixbyte structure of indium oxide [3]. However, in ITO, both substitutional tin and oxygen vacancies contribute to the high conductivity. In these ITO films, only In203 phase was detected from the X-ray patterns, which imply that tin, replace indium substitutional in the lattice as shown in Fig. 3.3. The preferred orientation for ITO films depends on the deposition conditions and the opto-electrical properties of the ITO films are independent of their orientations. The obtained results of SEM for ITO films grown on PET substrates showed very smooth surface morphology.

Fig. 3.3. – Show EDX measurements of ITO coated films on PET substrates.

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The surface morphology by AFM: The surface morphology for each thickness of the ITO films was studied by AFM. Figs. 3.4 (a), (b) and (c) show the three 3D-AFM images of the ITO/PET samples deposited under different thicknesses by D C sputtering. Figs. 3.4 (a), (b) and (c) show ITO/PET samples deposited at thicknesses 25.0 nm, 87.2 nm and 114.5 nm, respectively. The average roughness (Ra) of ITO/PET samples increased with a corresponding increase in thickness, while a relatively small Ra was obtained for the ITO films that were deposited at thickness of 25.0 nm. Fig. 3.4: shows the three 3D-AFM images of the ITO/PET samples deposited under different thicknesses by D C sputtering.

Fig. 3.4. – (a): 3D-AFM images of the ITO/PET samples deposited at thicknesses of (a) 25.0 nm; (b) 87.20 nm; (c) 114.50 nm by D C sputtering.

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Table 3.1 Shows AFM Parameters for ITO/PET samples deposited under different thicknesses No.

Parameters

Sample1

Sample2

Sample3

1

Thickness (nm)

25

87.0

114.5

2

Max Height Diff. (nm)

30.53

63.49

15.87

3

Mean (nm)

12.00

15.08

06.68

4

Root Mean Square nm

03.85

06.99

02.26

5

Average Deviation nm

03.07

05.20

01.89

6

Skewness

00.29

01.45

-02.e -002

7

Kurtosis

00.23

05.40

-00.49

3.3. ELECTRICAL PROPERTIES

The sheet resistance of ITO coated films on PET substrates is listed in Table 3.2. It is found that the thickness increases nearly linearly from 25 to 114.5 nm when the deposition time increases from 10 to 20 min. The sheet resistance of ITO film decreases with deposition time as the thickness of the film increases. This result indicates that, thickness of the film is directly proportional to the deposition time. Table 3.2 A summary of electrical and optical properties of typical ITO films deposited using DC sputtering technique No.

Time of deposition (min)

Film thickness (nm)

Sheet resistant Rs (Ω / )

Resistivity ρ (Ω-cm )

Carrier concentration N (cm-3)

Hall mobility µH (cm2/vs)

1

10

25.00

18.85

2.08x 10-4

1.09 x 1017

4.84

-4

19

5.32

7.94 x 1021

6.20

2

15

87.20

18.72

1.82 x 10

3

20

114.50

17.75

1.44 x 10-4

3.01x 10

Table 3.2 shows the dependence of the electrical resistivity, Hall mobility and carrier concentration on the film thickness of ITO films deposited on PET substrates. The electrical resistivity variation in of the ITO films is from 1.44 x 10-4 to 2.08 x 10-4 Ω - cm. It is clear that the electrical resistivity of the ITO films dramatically changes with the deposition time and sheet resistance. This is because the fact that, to get more uniform ITO film, thickness should increases to have less impact of the roughness due to the substrate. One can conclude that the lower resistivity (1.44 x 10-4 Ω-cm) at thickness (114.5 nm) is the result of an improved

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crystallinity, high oxygen vacancy concentration, and grain growth, which reduces grain boundary scattering and surface roughness. The values of µH and N are found to be (4.84 to 6.20) cm2 V− 1 S− 1 and (1.09 x 1017 to 7.94 × 1021) cm− 3, respectively. The origin of increasing resistivity of the ITO film (2.08 × 10-4 Ω - cm) at (25 nm) has a couple of possibilities. First, if many free electrons exist in ITO films, the mobility is rapidly decreased by scattering with carriers or with crystal defects. Second, if a change in chemical composition or micro-crystal structure of an ITO film occurs, resistivity can increase. Furthermore, the electrical resistivity of the ITO on PET is observed to be 1.44 × 10-4 Ω-cm, which is the lowest ever reported for films deposed on flexible substrates at room temperature by any method. Mobility is said to increase due to enhanced crystallinity of films deposited at higher substrate temperatures. 3.4. OPTICAL PROPERTIES

Fig. 3.5 also shows that the optical transmittance in the UV-VIS region of the ITO films changes with different thicknesses and a strong correlation with electrical resistivity. In addition, the optical transmission of the ITO films on PET is greater than ∼85% in the visible range (400-700 nm). As resistivities increase, the optical transmittance of ITO thin films increase with oxygen pressure. Therefore, optical transmittance is also closely related to oxygen vacancies, which are associated with free electrons in ITO films. We conclude that many electrons or disordered lattice structures in ITO films are scattered by incident light, thus reducing the transparency of the ITO thin film.

Fig.3. 5. – Optical transmittance, T, as a function of the light wavelength, λ, for ITO thin films with various thicknesses deposited on PET substrates.

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Fig. 3.6. – The Raman spectroscopy result of ITO/PET samples deposited by DC sputtering.

Raman spectroscopy reveals the structural changes related to the strain with the unique properties of thin film and demonstrate the changes of the band gap in the concept of the strain associated with the nanometer structure of ITO thin films with the correlation between the increased band gap and decreased lattice constants of deferent thickness of ITO thin films. All the films showed a very high transmittance in the visible light region Highly orientated ITO thin films were studied with Raman and photoluminescence spectroscopy. The high efficiency of the phonon and electron coupling enable us to observe up to 3rd order of the Raman scattering. The sample of measured no resonant Raman spectra of ITO thin film are shown in Figure 3.4. The peak at 860.7 cm−1 corresponded to E1_TO_, 1292.97 cm-1 the feature of two–phonon processes, the mode appearing at 1726.97cm−1 in the whole spectrum can be attributed to LO phonons of ITO [13].

Fig. 3.7. – Pl for ITO on PET.

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Figure 3.7 shows the typical room-temperature photoluminescence (PL) spectra of the ITO thin films grown at different time. Clearly, three bands have been observed from the room-temperature PL spectrum of the ITO nanostructures, i.e., UV emission, blue emission and green emission at 243, 404.7 and 513.6 nm, respectively. Interestingly, it was observed that, in both cases, the UV emission is stronger as compared to the blue and green emission bands. The UV emission is also called near band edge (NBE) emission, and it originates due to the recombination of free exciton through an exciton–exciton collision process [14].

Fig. 3.8. – Fourier-transform infrared (FTIR) spectra of ITO on PET substrate.

A typical FT-IR transmittance spectrum, acquired from a 170 -1200 cm-1) film deposited on PET, is shown in Fig. 3.8. Fig. 3.8 was obtained by FTS IR spectrometer equipped with a detector and beam splitter, in the absorbance mode and in the 170–1200 cm−1 wave number range. Generally, the relative intensity change of these FTIR vibrational peaks implies that there may be some predominant structure-orientation change when it is sublimated into solid films, or that the predominant orientations are different for different deposition conditions. This can be seen clearly from the XRD patterns. CONCLUSION

In conclusion, the surface morphology, the sheet resistance, the resistivity, the carrier concentration, the Hall mobility, the optical transmittance and the

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transmittance ratio of the ITO films deposed on PET substrate are reported. The reaction takes place at room temperature in the presence of water and weak electric field and results in metallic indium being reduced from the original ITO. Fast and dramatic changes in the optical transmission have been observed in ITO thin films subjected to such treatments. The deposition conditions such as sputter power and pressure played an important role in film properties. High quality films on PET substrate give the possibility as alternative substrates to the conventional glasses. It was confirmed that the electrical and optical property of target–substrate strongly depended on deposition conditions. However, a very accurate control of process parameters is necessary. Acknowledgments. This work was supported by the Nano-optoelectronics Research Laboratory, School of Physics; University Sains Malaysia under grant (No.305/PFIZIK/613321). The authors would also like to express their thanks for support from the Department of Physics, College of Science, Sudan University of Science and Technology, Khartoum, Sudan and Third World Academy of Science and Technology (TWAS).

REFERENCES 1. Gessert T.A. et al., (1991), “Properties of ITO thin films deposited by RF magnetron sputtering at elevated substrate temperature”, Proc. of 3rd Intl. Conf. on InP and Related Materials, Wales, UK, p. 32. 2. Wang L.M. et al., (2008), “Characterization of indium–tin oxide thin films grown on flexible plastic substrates at room temperature” Journal of Physics and Chemistry of Solids 69, p. 527–530. 3. Donghwan Kim et al., (2000), “Low temperature deposition of ITO thin films by ion beam sputtering”, Thin Solid Films, 377–378, p. 81–864. Mohamed S H and et al., (2009), “Properties of Indium Tin Oxide Thin Films Deposited on Polymer Substrates”, ACTA PHYSICA POLONICA A 115, 3, p. 704–708. 5. Jung Rak Lee et al., (2008), “Characteristics of ITO Films Deposited on a PET Substrate Under Various Deposition Conditions”, METALS AND MATERIALS International, 14, 6, pp. 745–751. 6. Terzini E., Thilakan P. and Minarini C., (2000), “Properties of ITO thin films deposited by RF magnetron sputtering at elevated substrate temperature” Materials Science and Engineering B77, p. 110–114. 7. Kulkarni A.K., Schulz K.H., Lim T.-S. and Khan M., (1997), “Electrical, optical and structural characteristics of indium-tin-oxide thin films deposited on glass and polymer substrates”, Thin Solid Films 308–309, p. 1–7. 8. Ederth J. et al., (2003), “Electrical and optical properties of thin films consisting of tin-doped indium oxide nanoparticles”, PHYSICAL REVIEW B 68, 155410. 9. Keran Zhang et al., (2000), “Indium tin oxide films prepared by radio frequency magnetron sputtering method at a low processing temperature”, Thin Solid Films, 376, 1–2, p. 255–263. 10. Annop Klamchuen, Nisaporn Pornteerapha and Jiti Nukeaw, ((2005), “Characterization of ITO thin films on PET substrates prepared by gas-timing RF magnetron sputtering”, e-Journal of Surface Science and Nanotechnology, 3, ISSN: 1348-0391, p. 272–275.

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11. Jaehyeong Lee et al., (2008), “Growth and characterization of indium tin oxide thin films deposited on PET substrates”, Thin Solid Films, 516, 7, p. 1634–1639. 12. Čyžiūte B. et al., (2006), “Metallization of poly(ethylene terephthalate) in the wide range of substrate temperatures”, Surface & Coatings Technology 200, p. 6490–6494. 13. R. Cusco, E. Alarcon-Llado, J. Ibanez, L. Artus, J. Jimenez, B. Wang, M. J. Callahan, 2007, Phys. Rev. B, 75. 165202. 14. Umar A, Karunagaran B, Suh E K and Hahn Y B 2006 Nanotechnology 17 4072.

DEPOSITED INDIUM TIN OXIDE (ITO) THIN FILMS BY DC

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