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Influence of target to substrate spacing on the - saranya - 08-16-2017 Influence of target to substrate spacing on the properties of ITO thin films Aldrin Antony, M. Nisha, R. Manoj, M.K. Jayaraj * Optoelectronics Device Laboratory, Department of Physics, Cochin University of Science & Technology, Kochi 682022, Kerala, India Received 3 October 2003; received in revised form 3 October 2003; accepted 16 October 2003 Abstract Indium tin oxide thin films were deposited at room temperature on glass substrates by RF magnetron sputtering. The structural, electrical and optical properties of the films showed a dependence on target to substrate spacing and annealing temperature. Films deposited with a target to substrate spacing of 4 cm showed the lowest resistivity of 3:07 10 3 O cm and maximum band gap of 3.89 eV on annealing at a temperature of 250 8C under high vacuum for 1 h. # 2003 Elsevier B.V. All rights reserved. PACS: 81.40.Ef; 81.40-z; 78.66 Keywords: Indium tin oxide; RF magnetron sputtering; Annealing 1. Introduction Thin films of indium oxide doped with tin (ITO) are commonly used in various applications such as dis- plays, image sensors and solar cells owing to their outstanding properties [1]. These films show high transmission in the visible region and a low electrical resistivity. Their high conductivity results from the non-stoichiometry produced by oxygen deficiency and the introduction of tin as dopant [2]. Because of its well-matched work function, it is the most commonly used material acting as hole injector and transparent conducting anode in organic light emitting diodes [3]. ITO films can be prepared by a wide variety of techniques such as plasma enhanced metallorganic chemical vapor deposition (PEMOCVD) [4], ion assisted deposition [5], sputtering [6], pulsed laser deposition (PLD) [7], dip coating [8] etc. Sputtering is one of the effective methods to obtain good quality ITO thin films. It is superior in both its controllability and resultant uniformity of the films deposited on a large area substrate [9]. Reports also show that good quality polycrystalline ITO films can be grown at room temperature by adopting PLD technique coupled with laser irradiation of substrate [10]. Production of low resistivity films at room temperature is of impor- tance in high performance flat panel displays (FPDs) which use heat sensitive substrates such as polymers. In the present study, ITO films were prepared at room temperature by RF magnetron sputtering of ITO target. The deposition was carried out for target to substrate spacing (T-S spacing) of 4, 6 and 8 cm. The films were then annealed under high vacuum (2 10 5 mbar) for 1 h. The structural, electrical Applied Surface Science 225 (2004) 294 301 * Corresponding author. Tel.: 91-484-2577404; fax: 91-484-2577595. E-mail address: [email protected] (M.K. Jayaraj). 0169-4332/$ see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.10.017Page 2 and optical properties were investigated as a function of T-S spacing and annealing temperature. 2. Experimental ITO films were deposited on to glass substrates by RF magnetron sputtering at room temperature using an ITO target (5 cm diameter) containing 95 wt.% of In 2 O 3 and 5 wt.% of SnO 2 . Deposition rate was found to decrease almost linearly from 35 to 12 A/min as target to substrate distance was increased from 4 to 8 cm. The sputtering time was adjusted in such a way that all the films studied had the same thickness irrespective of the substrate to target distance. The base pressure in the chamber was 5 10 6 mbar. Sputtering was carried out in argon atmosphere at a pressure of 0.01 mbar with RF power of 20 w. The films prepared at room temperature were then annealed at various temperatures ranging from 100 to 300 8C in high vacuum (2 10 5 mbar) for 1 h. The thickness of the films was determined by Tolansky interference technique. In the present study all the measurements were performed on the films having the thickness of 2500 A. Electrical measurements were carried out using a Hall measure- ment system (Model MMR technology H-50) which employs four-probe in Vanderpauw configuration. Transmission spectra of the samples were recorded using a UV-Vis-NIR spectrophotometer (Hitachi U 3410). The crystallinity of the films were analysed using an X-ray diffractometer using the Cu Ka radia- tion (1.5414 A). 3. Results and discussion Fig. 1 shows the XRD pattern of the ITO films deposited on glass substrates at various target to substrate spacings (T-S spacing 4, 6 and 8 cm). The substrates were not preheated intentionally. How- ever, the substrate temperatures rouse upto 55 8C during deposition when the T-S spacing was 4 cm, 45 8C when the T-S spacing was 6 cm and 40 8C when the T-S spacing was 8 cm. From the XRD pattern it is evident that the as deposited films are polycrystalline even though the crystallization temperature of ITO is 150 8C [11]. All of them showed a peak at 2y 30 , which correspond to (2 2 2) plane of In 2 O 3 [12]. This is because of the greater kinetic energy of the sputtered Fig. 1. XRD patterns of as deposited ITO films at various target to substrate spacings. A. Antony et al. / Applied Surface Science 225 (2004) 294 301 295Page 3 particles reaching the substrate surface. Generally, sputtered particles have kinetic energies of several electron volts. This kinetic energy enhances the sur- face migration of sputtered particles arriving at the substrate surface and the crystallinity of the films are greatly affected by them. Thus it is possible to deposit Fig. 2. Variation of (2 2 2) peak intensity and grain size with target to substrate spacing. Fig. 3. XRD pattern of ITO films prepared with a target to substrate spacing of 4 cm at room temperature and annealed at various temperatures. 296 A. Antony et al. / Applied Surface Science 225 (2004) 294 301Page 4 polycrystalline films even at room temperature by sputtering [13]. The crystallinity of ITO films showed a dependence on T-S spacing. With increase in T-S spacing the kinetic energy of the sputtered particles reaching the substrate surface decreases. This retards the surface migration of sputtered particles and hence reduces the crystallinity of the films. The grain size of the films as calculated from Scherrer s formula [14] is in agreement with the above result. The variation of grain size and the (2 2 2) peak intensity with T-S spacing is shown in Fig. 2. The decrease in grain size with increase in T-S spacing confirms the degradation in crystallinity with T-S spacing. Fig. 3 shows the XRD pattern of the ITO films deposited at a T-S spacing of 4 cm as a function of annealing temperature. The films showed a peak at 2y 30 corresponding to (2 2 2) plane and 2y 51 which corresponds to (4 4 0) plane of In 2 O 3 . With increase in annealing temperature the intensity of the (2 2 2) peak increased which is in agreement with the literature [15]. The crystallinity, transparency, electrical resistivity and mobility of the films prepared at T-S spacing of 6 and 8 cm showed similar variations on annealing as that of the variation in these properties for films deposited at a T-S spacing of 4 cm. However, better film properties viz. lower resistivity, higher crystal- linity and better transparency were observed for the films deposited at T-S spacing of 4 cm in comparison with the other two T-S spacing. So the discussion regarding the variation of film properties on annealing is limited to the case of T-S spacing of 4 cm only. The transmission spectra of the ITO films for various T-S spacings are shown in Fig. 4. All the films irrespective of T-S spacing were highly transparent. Fig. 4. Transmission spectra of ITO thin films prepared at various target to substrate spacings. Inset shows the variation of bandgap with target to substrate spacing. A. Antony et al. / Applied Surface Science 225 (2004) 294 301 297Page 5 The average transmission in the visible region of the electromagnetic spectrum was >85%. The bandgap of the ITO films were calculated from the transmission spectra. By assuming a parabolic band structure for the material, the absorption coeffi- cient and bandgap can be related by the expression ahn A hn-E g 1=N where E g is the band gap energy and a is the absorption coefficient corresponding to frequency n [16]. The constant N depends on the nature of electronic transition. In the case of ITO films N is equal to 2, for direct allowed transition. The bandgap of ITO films were determined from the plot of (ahn) 2 versus hn by extrapolating the linear portion of the curve to ahn equal to 0. In the present study it was found that the bandgap increases with increase in target to substrate spacing (inset of Fig. 4). The effect of annealing on the optical properties of ITO films was also studied. The annealed films exhib- ited high transmission in the visible region with long tail in the IR region. It was seen that the reflecting edge shift towards the lower wavelength region on anneal- ing the film at high temperatures in vacuum (Fig. 5). The shift in reflecting edge is due to increase in carrier concentration introduced by the oxygen deficiencies created during annealing. The band gap of ITO films increased with annealing temperature, showed a maximum value at 250 8C (3.89 eV) and then decreased. The variation of band gap with annealing temperature is shown in the inset of Fig. 5. The increase in band gap can be explained on the basis of Burstein-Moss effect [17]. Burstein-Moss shift is proportional to carrier concentration. Increase in carrier concentration with increase in annealing temperature results in band gap widening. The electrical characteristics of the films also showed dependence on target to substrate spacing. Fig. 6 gives the variation of resistivity ® and mobility (m) with T-S spacing. The decrease in mobility (m) and Fig. 5. Transmission spectra of ITO films annealed at various temperatures. Inset shows the variation of bandgap as a function of annealing temperature. 298 A. Antony et al. / Applied Surface Science 225 (2004) 294 301Page 6 the increase in resistivity ® with T-S spacing is related to the degradation in crystallinity of the films with T-S spacing. At greater T-S spacing, the smaller grains increase thegrain boundary scattering of carriers and hence reduce the mobility of the films, which result in higher resistivity. The resistivity and sheet resistance of the ITO films were found to decrease with increase of annealing Fig. 6. Mobility (m) and resistivity ® of ITO films deposited at various target to substrate spacings. Fig. 7. Sheet resistance (R s ) and resistivity ® of ITO films annealed at various temperatures. A. Antony et al. / Applied Surface Science 225 (2004) 294 301 299Page 7 temperature (Fig. 7). The lowest resistivity of 3:07 10 3 O cm and sheet resistance of 110 O/sq. was obtained for the film prepared with a target to substrate distance of 4 cm and annealed at 250 8C. The mobility of the ITO films increased with the increase of annealing temperature whereas carrier concentra- tion was maximum for ITO films annealed at 250 8C (Fig. 8). In ITO, oxygen deficiency is one of the reasons for high conductivity. Oxygen deficiencies induce free electrons as conduction carriers [18]. Vacuum anneal- ing creates oxygen deficiency and this reduces the resistivity of the ITO films. The increase in carrier concentration, mobility and crystallinity of the films is also responsible for the decrease in resistivity. The films annealed at 250 8C showed a preferred orienta- tion in the (2 2 2) plane and showed the minimum resistivity. 4. Conclusion Tin doped indium oxide films were prepared at room temperature by RF magnetron sputtering. The effect of target to substrate spacing and annealing temperature on the structural, electrical and optical properties was investigated. The films are found to show a preferential orientation in the (2 2 2) plane. Highly transparent and conducting films were obtained for a target to substrate spacing of 4 cm. The X-ray diffraction pattern of the ITO films annealed at 250 8C shows the maximum peak intensity for the (2 2 2) plane. The resistivity of the film decreased with the annealing temperature and resistivity is minimum at 250 8C. The carrier concen- tration and band gap of the film increased with anneal- ing temperature. Acknowledgements The authors wish to thank Department of Science and Technology for the financial support. References [1] J.F. Nierengarten, G. Hadziioannou, N. Armaroli, Mater. Today 4 (3) (2001) 6. [2] C. Nunes de Carvalho, A. Luis, O. Conde, E. Fortunato, G. Lavareda, A. Amaral, J. Non-Cryst. Solids 299 302 (2002) 1208. [3] D. Vaufrey, M. Ben Khalifa, J. Tardy, C. Ghica, M.G. Blanchin, C. Sandu, J.A. Roger, Semicond. Sci. Technol. 18 (2003) 253. Fig. 8. Mobility (m) and carrier concentration (n) of ITO films annealed at various temperatures. 300 A. Antony et al. / Applied Surface Science 225 (2004) 294 301Page 8 [4] Y.-C. Park, Y.-S. Kim, H.-K. Seo, S.G. Ansari, H.-S. Shin, Surf. Coat. Technol. 161 (2002) 2. [5] Dale E. Mortan, Andrea Dinca, Vacuum Technol. Coat. (2000) 53. [6] Y. 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