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Effect of substrate temperature on the growth of ITO thin films - yasar123 - 08-16-2017 Effect of substrate temperature on the growth of ITO thin films M. Nisha, S. Anusha, Aldrin Antony, R. Manoj, M.K. Jayaraj * Optoelectronics Device Laboratory, Department of Physics, Cochin University of Science and Technology, Kochi 682 022, Kerala, India Received 2 November 2004; received in revised form 20 February 2005; accepted 20 February 2005 Abstract Indium tin oxide (ITO) thin films were deposited onto glass substrates by rf magnetron sputtering of ITO target and the influence of substrate temperature on the properties of the films were investigated. The structural characteristics showed a dependence on the oxygen partial pressure during sputtering. Oxygen deficient films showed (4 0 0) plane texturing while oxygen-incorporated films were preferentially oriented in the [1 1 1] direction. ITO films with low resistivity of 2.05 10 3 V cm were deposited at relatively low substrate temperature (150 8C) which shows highest figure of merit of 2.84 10 3 square/V # 2005 Published by Elsevier B.V. PACS: 81.40. z; 78.66. w; 81.40.Ef Keywords: Transparent conducting oxides; Indium tin oxide; Rf magnetron sputtering 1. Introduction Transparent conducting oxides (TCOs) have been widely used as transparent electrode for flat panel displays including liquid crystal displays, organic light emitting diodes and plasma displays. Among the various TCOs tin doped indium oxide (ITO) is widely used due to its low electrical resistivity and compat- ibility with fine patterning processes [1]. ITO is an n- type degenerate wide bandgap semiconductor. The degeneracy is caused by both oxygen vacancy and substitutional tin created during deposition [2]. As a degenerate semiconductor, ITO can be used as the window layer in n + p heterojunctions [3]. Because ITO films have good efficiency for hole injection into organic materials, they have been widely utilized as the anode contact for organic light emitting diodes (OLEDs) [4]. ITO thin films can be prepared by a wide variety of techniques like plasma enhanced metal organic chemical vapour deposition (PEMOCVD) [5], ion assisted deposition [6], pulsed laser deposition (PLD) [7], dip coating [8], ion beam sputtering [9], rf magnetron sputtering [10], reactive thermal evapora- tion, [11] etc. Most of the preparation methods involve elsevierlocate/apsusc Applied Surface Science xx (2005) xx xx 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 * Corresponding author. Tel.: +91 484 2577404; fax: +91 484 2577595. E-mail address: [email protected] (M.K. Jayaraj). 0169-4332/$ see front matter # 2005 Published by Elsevier B.V. doi:10.1016/j.apsusc.2005.02.115 APSUSC 12683 1 6Page 2 UNCORRECTED PROOF a relatively high substrate temperature in order to obtain thin films with a reasonably high conductivity and transmittance. However, some ITO based devices such as amorphous silicon photovoltaic devices and flexible electro optical devices demand the deposition of ITO at low substrate temperature (<200 8C) [12]. Magnetron sputtering offers the possibility to prepare ITO thin films at low processing temperature and on large areas [13]. In this paper, we report the influence of substrate temperature, oxygen partial pressure and fluorine doping on the properties of sputtered ITO thin films. 2. Experimental In the present study the Indium tin oxide films were deposited by rf magnetron sputtering of ITO target containing 95 wt.% of In 2 O 3 and 5 wt.% of SnO 2 . The target used for sputtering was prepared from In 2 O 3 (99.99% pure) and SnO 2 (99.999% pure) powders. The powders were mixed in a mechanical shaker for 1 h pressed into a pellet of two-inch diameter and then sintered at 1300 8C for 6 h in air. The rf sputtering was carried out in a vacuum chamber in which high vacuum of the order of 2 10 5 mbarwas createdbymeansofanoildiffusion pump backed by a rotary pump. The rf power was delivered to the target material by an rf generator (13.56 MHz) through an impedance matching network. Glass slides of dimension 2.5 cm 1 cm was used as thesubstrates.Thesubstrateswere keptabovethe target at a distance of 4 cm, which was found to be the optimumforthegrowthofgoodqualitycrystallinefilms [14]. Initially the substrate was heated to the required temperature. After attaining the required substrate temperature high purity argon gas was allowed to flow into the chamber and it was so adjusted by a mass flow controller maintaining the argon pressure at 0.01 mbar. The films were deposited at an rf power of 30 W. The target was pre-sputtered for 10 m before each deposi- tion in order to remove any contaminants and to eliminateanydifferentialsputteringeffects.Bykeeping allotherparametersthesame,thesputteringwascarried out for various substrate temperatures ranging from room temperature to 300 8C. For films deposited at room temperature, the temperature of the substrate increasedfrom20 to50 8C duringdeposition.But when the deposition was carried out onto preheated substrates, the temperature of the substrate was maintained at the specified value by controlling power into the heating coil. The sputtering time was adjusted suchthatalltheresultingfilmsusedinthisstudywereof thickness 220 nm. The structural characterisation of the films were carried out using a Rigaku X-ray diffractometer with Cu Ka radiation (l = 1.5414 A). The optical transmis- sion was taken in the wavelength range of 200 2500 nm using a Hitachi U-3410 model UV VIS NIR spectrophotometer. The electrical characterisation was done by measuring the resistivity using Vander Pauw four-probe method and the carrier type and concentration was determined using the Hall mea- surement system (H-50, MMR Technologies Inc.). 3. Results and discussion Fig. 1 shows the X-ray diffraction (XRD) pattern of the ITO thin films deposited at various substrate temperatures. All the films are polycrystalline. They crystallized in the cubic bixbyite structure of indium oxide. The growth of the films showed preferred orientation depending on the substrate temperature. The films deposited onto unheated substrates showed reflection corresponding to (2 2 2) and (4 4 0) planes [15]. A substrate temperature of 100 8C resulted in films with a prominent (4 0 0) peak indicating a M. Nisha et al. / Applied Surface Science xx (2005) xx xx 2 DTD 5 APSUSC 12683 1 6 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 Fig. 1. XRD pattern of ITO thin films deposited at various substrate temperatures.Page 3 UNCORRECTED PROOF preferred orientation along [1 0 0] direction. The films grown at all other substrate temperatures do not show any preferential orientation as seen from the XRD pattern. It was also observed that all the films deposited onto heated substrates showed (4 0 0) plane texturing. With an increase in substrate temperature above 100 8C, a decrease in intensity of (4 0 0) peak and an increase in intensity of (2 2 2) peak was observed (Fig. 2). On the other hand, post deposition annealing of ITO films deposited at room temperature resulted in films showing (2 2 2) and (4 4 0) diffrac- tion peaks [14]. The film annealed at 250 8C was preferentially oriented in the [1 1 1] direction. None of the films showed (4 0 0) plane texturing on annealing. The change in orientation of the films from (2 2 2) to (4 0 0) plane is related to the deposition conditions. The variation of orientation from (1 1 1) to (1 0 0) at higher substrate temperatures is related to the energy of the sputtered particles reaching the substrate surface [16]. Energy of the sputtered particles on the substrate surface should attain a certain value in order to form thin film with (1 0 0) orientation. Sputtering at higher substrate temperature satisfies this criterion and it result in the (1 0 0) orientation. The influence of oxygen incorporation on the structural properties of the films were studied by depositing the films at various oxygen partial pressures. The deposition was carried at a substrate temperature of 150 8C and an rf power of 30 W.The XRD pattern (Fig. 3) shows that in the absence of oxygen, the films orient randomly. Oxygen incorpora- tion leads to films oriented in the (1 1 1) direction. According to Kim et al. (1 0 0) orientation is related to oxygen deficiency [17]. The change in orientation of the films from (1 0 0) to (1 1 1) texture results from the incorporation of oxygen into the films. Fluorine doping in ITO thin films enhanced the crystallinity (Fig. 3). Doping was carried out by placing indium fluoride (InF 3 ) pellets on the erosion area of the target. The deposition was carried out at an rf power of 30 W and a substrate temperature of 150 8C. Incorporation of fluorine resulted in films with (1 0 0) preferred orientation. This may be due to the substitutional incorporation of fluorine in the place of oxygen which arises because their radii are compar- able. The resulting oxygen deficiency leads to (1 0 0) oriented films. The lattice parameter of the films were calculated using the equation n 2 d 2 h 2 k 2 l 2 a 2 An increase in lattice parameter of ITO thin films with substrate temperature was observed (Fig. 4). The increase in lattice parameter is attributed to the increase in repulsive forces arising from the extra positive charge of the tin cations. Tin is incorporated into In 2 O 3 lattice as Sn 4+ . In the oxidised state, the interstitial oxygen anion charge compensate the mate- rial [18]. As those oxygen anions are removed, which M. Nisha et al. / Applied Surface Science xx (2005) xx xx 3 DTD 5 APSUSC 12683 1 6 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 Fig. 2. Variation of the ratio of peak intensity of (2 2 2) and (4 0 0) planes with substrate temperature. Fig. 3. XRD pattern of fluorine doped ITO thin films and those deposited at various oxygen pressures. Substrate tempera- ture = 150 8C and rf power = 30 W.Page 4 UNCORRECTED PROOF is the case with higher substrate temperatures the repulsive forces increase, leading to an enlargement of the unit cell. In the present case, the increase in substrate temperature resulted in oxygen deficient films. This was confirmed by the carrier density measurements, which will be discussed along with the electrical properties of the films. Similar effect was observed in thin films prepared at various oxygen concentrations (inset of Fig. 4). The lattice constant decreased with increase in oxygen concentration. The transmission spectra (Fig. 5) of the films deposited at various substrate temperatures shows that the films are highly transparent in the visible region of the electromagnetic spectrum. The average transmis- sion in the visible range was greater than 80%. The transmission in the higher wavelength region decreased with increase in substrate temperature. This is because at high deposition temperature carrier concentration increases because of oxygen deficiency. Higher carrier concentration results in higher reflection in long wavelength region. Inset of Fig. 5 shows the absorption and cut off for glass substrate. The optical bandgap of the films were determined by extrapolating the linear portion of the hn versus (ahn) 2 curve to (ahn) = 0 (inset of Fig. 6). The absorption coefficient, a, was determined from the relation, I = I 0 exp( at) where t is the thickness of the sample, I the transmitted intensity at a particular wavelength and I 0 the maximum transmitted intensity which is taken to be 100%. This relation gives a = (1/ t)ln(I 0 /I). Inthepresentstudyithasbeenfoundthatbandgapof ITO films increased with increase in substrate temperature (Fig. 8). The increase in bandgap may be due to an increase in carrier concentration with substratetemperatureasaresultofwhichtheabsorption edge shifts towards the near UV range [19]. The increase in bandgap with carrier concentration can be explained on the basis of Burstein Moss effect. Assuming that the conduction band and valence band are parabolic in nature and that B M shift is the M. Nisha et al. / Applied Surface Science xx (2005) xx xx 4 DTD 5 APSUSC 12683 1 6 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 Fig. 4. Variation of lattice parameter of ITO thin films with sub- strate temperature. Inset shows the variation of lattice parameter of ITO thin films deposited at a substrate temperature of 150 8C with oxygen concentration. Fig. 5. Transmission spectra of ITO thin films deposited at various substrate temperatures. Inset shows the transmission and cut off for glass substrate. Fig. 6. Variation of bandgap of ITO thin films with substrate temperature. Inset shows a typical plot of hn vs. (ahn) 2 for ITO film deposited at a substrate temperature 150 8C.Page 5 UNCORRECTED PROOF predominant effect, we can write E g = E g0 + DE B M g where E g0 istheintrinsicbandgap and DE B M g theB M shift due to filling of low lying levels in the conduction band [20]. An expression for B M shift is given by DE B M g = (h 2 /8p 2 m vc )(3p 2 n) 2/3 where n is the carrier concentration and m vc the reduced effective mass of the carriers.FromthisexpressionitisclearthatB Mshiftis directly proportional to n 2/3 . However, at very high carrier concentrations it is seen that there is bandgap narrowing due to electron-electron scattering and electron impurity scattering. The electrical properties of the ITO thin film depend on the film composition and deposition parameters such as substrate temperature, oxygen pressure, etc. There is a trade off between the carrier density and carrier mobility for achieving low resistivity [21]. In the present study it was found that the resistivity and sheet resistance decreased with increase in substrate temperature and became a minimum at a temperature of 150 8C and then increased on further increase of substrate temperature (Fig. 7). The carrier mobility and carrier density increased with increase of substrate temperature and showed their maximum value around 150 8C and then decreased (Fig. 8a). The increase in mobility may be due to better crystallinity of the film, which increases with the increase in substrate temperature. The decrease in resistivity with increase in substrate temperature can also be explained by the fact that the crystallite grain size increases significantly with the increase in deposition temperature, thus reducing grain boundary scattering and increasing conductivity. The decrease in resistivity was also associated with the observed increase in carrier mobility. For the film grown at higher substrate temperatures >200 8C the resistivity was found to increase again. This increase may be due to the contamination of the films by alkali ions from glass substrates [22]. The increase in carrier concentration may be due to an increase in diffusion of Sn atoms from interstitial locations and grain boundaries into the In cation sites. Since the Sn atom has a valency of 4 and In is trivalent, the Sn atoms act as donors in ITO films. Hence the increase in diffusion with substrate temperature results in higher electron concentration. The figure of merit (F) proposed by Haake [23] for transparent conductors for photovoltaic applications is given by F = T 10 a /R s where T a is the average transmittance in the visible range and R s the sheet resistance of the film. The highest value of figure of merit was observed for the film deposited at a substrate temperature of 150 8C(Fig. 8b). The figure of merit for commercial ITO thin film was 5.9 10 2 square/V. 4. Conclusion Indiumtinoxidethinfilmswere depositedontoglass substratebyrfmagnetronsputteringatvarioussubstrate temperatures. The film orientation showed a depen- dence on the processing parameters such as substrate M. Nisha et al. / Applied Surface Science xx (2005) xx xx 5 DTD 5 APSUSC 12683 1 6 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 Fig. 7. Variation of resistivity and sheet resistance of ITO thin films with substrate temperature. Fig. 8. Variation of (a) carrier mobility, carrier density and (b) figure of merit of ITO thin films with substrate temperature. The figure of merit for commercial ITO film was 5.9 10 2 square/V.Page 6 UNCORRECTED PROOF temperature and oxygen pressure during sputtering. The films deposited onto preheated substrates showed a (4 0 0) plane texturing. Oxygen incorporation led to orientationofthefilmsinthe(2 2 2)planeparalleltothe substrate surface. Minimum resistivity of 2 10 3 V cmwithhighestfigureofmeritof2.8 10 3 square/ V was obtained for the films deposited at a sufficiently lower substrate temperature of 150 8C using pure argon as sputtering gas. Acknowledgements This work was supported by Department of Science and Technology, Government of India. MKJ wish to thank Kerala State Council for Science, Technology and Environment for the financial assistance under SARD programme. 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