Properties of Indium-Zinc-Oxide Thin Films Prepared by Facing Targets Sputtering at Room Temperature

Transcription

1 Journal of the Korean Physical Society, Vol. 54, No. 3, March 2009, pp Properties of Indium-Zinc-Oxide Thin Films Prepared by Facing Targets Sputtering at Room Temperature You Seung Rim, Sang Mo Kim and Kyung Hwan Kim Department of Electrical Engineering, Kyungwon University, Seongnam (Received 16 June 2008, in nal form 22 August 2008) Indium zinc oxide (IZO) thin lms were deposited on glass substrates by using the facing targets sputtering (FTS) method at room temperature. Two IZO (In 2O 3 90 wt%, ZnO 10 wt%, 5 N, 2 inch) targets in a FTS apparatus were used for deposition. The IZO lms were deposited at various input currents and oxygen partial pressures (P O 2). As the input current was increased from 0.04 A to 0.08 A and then to 0.14 A, the surface morphology of the IZO lms became rough. IZO thin lms deposited in the presence of oxygen exhibited an amorphous structure, with higher transmittance and lower resistivity, than those deposited without oxygen. The electrical, optical and structural characteristics of IZO thin lms were evaluated using a Hall-eect measurement system, an X-ray diractometer (XRD), a UV/VIS spectrometer in the visible range and an atomic force microscope (AFM), respectively. We obtained IZO thin lms with a resistivity of cm, a carrier concentration of cm 3, a electron mobility of 36.4 cm 2 /vs and a transmittance of over 85 %, at a P O 2 of 3 %. PACS numbers: Kr, Jk, Cd Keywords: TCO, Indium zinc oxide, FTS, Low-temperature process I. INTRODUCTION Transparent conductive oxide (TCO) thin lms are used in a wide variety of applications, including the transparent electrodes of at panel displays (FPDs) and solar cells, optoelectronic devices, touch panels and IR reectors [1]. One TCO lm, tin-doped indium oxide (ITO) lm, has been widely used in FPDs, including liquid crystal displays (LCDs), plasma displays panels (PDPs) and organic light-emitting devices (OLEDs). ITO thin lms have the lowest available resistivity (<10 4 -cm) when they are deposited at elevated temperatures (250 { 350 C), yet for specic applications like exible displays and OLEDs, the TCO lm must be deposited at temperatures below 200 C [2, 3]. However, problems, including reduced resistance to moist heat, increased resistivity and chemical instability, arise when an ITO thin lm is deposited at a low temperature. Furthermore, ITO lms are easily transformed from an amorphous to a crystalline form at about 150 C and the internal stress of the crystalline form leads to severe cracking [4]. Surface defects, such as spikes or pinholes, in TCO lms cause performance degradation in OLEDs, showing up as dark-spots and resulting in shorter lifetimes. For improved their performance, the surface morphology of TCO lms must be extremely at [5]. khkim@kyungwon.ac.kr; Fax: Therefore, the need to develop a substitute for ITO lm has become increasingly urgent. Amorphous indium zinc oxide (IZO) lms with low resistivity, high transmittance, high etching rate and excellent surface smoothness can easily be prepared at a low deposition temperature [6,7]. A smooth surface also oers the possibility of improved electron transport performance, without interface roughness scattering or grain boundaries. Therefore, an amorphous IZO lm is the best candidate for the production of high-quality transparent conducting electrodes for OLEDs and exible displays. In this study, we prepared IZO lms at room temperature by using a facing targets sputtering (FTS) system. A FTS system consists of two targets facing each other and a substrate located to one side of the center line between the two targets. Magnetic elds are applied perpendicular to the surfaces of the two targets. By using a low voltage and high current, a FTS system can suppress the bombardment of the substrate by hot particles. Thus, a FTS system can deposit thin lms without stress at a low working gas pressure and a low temperature [8,9]. We investigated the electrical, optical and crystallographic properties of IZO thin lms prepared using a FTS system. II. EXPERIMENTS AND DISCUSSION We prepared IZO thin lms on glass substrates at room temperature by using a FTS system. Two IZO (In 2 O 3 90 wt%, ZnO 10 wt%, 5 N, 2 inch) targets in

2 Journal of the Korean Physical Society, Vol. 54, No. 3, March 2009 Fig. 1. Diagram of the facing targets sputtering system. Table 1. Sputtering Conditions. Deposition parameters Conditions Targets IZO (In 2O 3: 90 wt.%, ZnO: 10 wt.%) Substrate Slide Glass Sputtering Gas Ar, O 2 Base Pressure Pa Working Pressure 0.13 Pa Substrate Temperature Room Temperature Input Current 0.04 { 0.12 A, 0.08 A Oxygen Partial Pressure Ratio 0 { 5 % Film Thickness 100 nm a FTS apparatus were used for deposition. Before lm deposition, the glass substrate was cleaned by ultrasonic cleansing with distilled water for 20 min and with isopropyl alcohol (IPA) for 20 min. It was then dried in a stream of N 2 gas. The FTS apparatus used in the deposition process is illustrated in Figure 1. The apparatus is designed to deposit thin lms from two facing targets onto the substrate. Inside the chamber, two IZO targets were installed facing each other. The distance between the substrate and the targets was xed at 90 mm and the distance between the facing targets was xed at 50 mm. The chamber was evacuated to Pa before the lm deposition began and the pressure was maintained at 0.13 Pa throughout the process. The IZO lms were deposited at various input currents and oxygen partial pressures. The oxygen partial pressure (PO 2 ) is dened as P O 2 = P (O 2 )/[P (O 2 ) + P (Ar)]. The input current was adjusted from 0.04 A to 0.14 A and the P O 2 was adjusted from 0 % to 5 %. More details of the sputtering conditions used are given in Table 1. The electrical, optical and crystallographic properties of each of the thin lms deposited on the glass substrates were evaluated using a Hall-eect measurement system (ECOPIA), a UV/VIS spectrometer (HP) with a spectral range of 300 { 800 nm, an X-ray diractometer (XRD, Rigaku) and an atomic force microscope (AFM, PSIA). Figure 2 shows the electrical properties of the deposited IZO thin lms as a function of the input current. When the input current was increased from 0.04 A to 0.14 A, the resistivity increased from to cm. The electron mobility values for the lms also increased slightly with an increasing input current. As the input current was increased from 0.04 A to 0.08 A, the sputtering particles achieved a more adequate surface diusion with an increased in electron mobility and an improved crystallinity [10]. However, when the input current was increased above 0.10 A, the electron mobility gradually decreased. The resistivity of the lms increased with input current due to the decrease in the carrier concentration. We believe that the kinetic energy of the negatively charged O ions increases with input current, thus causing a higher input current to increase lattice damage by high-energy ions. This might deteriorate the crystallinity of the lm and reduce the concentration of electrically active donor sites, thus causing the decrease in the electron mobility that we observed [11].

3 Properties of Indium-Zinc-Oxide Thin Films Prepared { You Seung Rim et al Fig. 2. Electrical properties of the deposited IZO thin lms as a function of the input current. Fig. 3. Optical properties of the deposited IZO thin lms as a function of the input current. Fig. 4. AFM images of the deposited IZO thin lms at input currents of (a) 0.04 A, (b) 0.08 A and (c) 0.12 A. Figure 3 shows the optical transmittance of the IZO thin lms as a function of the input current. Average transmittances of about 80 { 90 % were obtained in this study. The lm deposited using an input current of 0.14 A had a lower transmittance than that deposited at 0.04 A. It is evident that the input current inuences the optical transmittance of IZO thin lms. Figure 4 shows AFM images of lms produced using a range of input currents. The AFM images show 2 2 m areas. Two-dimensional plane views were observed for IZO lms that were deposited using dierent input currents at a P O 2 of 3 %. As the input current was increased from 0.04 A to 0.08 A and then the 0.12 A, the grain size of the IZO lms increased and their surface morphology became rough, as shown in Figure 4. Their root-mean-square roughness (R rms ) values increased from nm to nm and then to nm. This increase in the R rms roughness of the IZO lms with an increasing input current explains the reduction in the optical transmittance for lms of the same thickness, as shown in Figure 3. This seems to be related to the fact that a smoother lm scatters less light from its surface.

4 Journal of the Korean Physical Society, Vol. 54, No. 3, March 2009 Fig. 5. Electrical properties of the deposited IZO thin lms as a function of the P O 2. Fig. 6. Optical properties of the deposited IZO thin lms as a function of the P O 2. Fig. 7. AFM images of the deposited IZO thin lms at a P O 2 of (a) 0 % (b) 3 % and (c) 5 %. It is clear that the grain size and the surface morphology of an IZO lm are directly related to its transmittance. We believe that the changes in the microstructures of the IZO thin lms are the result of bombardment of the growth surface by high-kinetic-energy particles, such as charged O ions, -electrons and Ar particles. The increase in surface roughness of lms with increasing input current has the eect of reducing the optical transmittance, as shown in Figure 3. This seems to be related to the fact that a lm with low roughness has little light scattered from its surface. The phenomenon of surface roughness increasing with increasing sputtering power is also observed in ITO lms [12]. The R rms roughness values of the IZO lms produced in this study were superior to those of commercially available ITO lms (>4 nm) [13]. Figure 5 shows the electrical properties of the deposited IZO thin lms as a function of the P O 2. The lowest resistivity ( cm) was observed when the lm was deposited at a P O 2 of 3 %. We found that increasing the P O 2 between 3 % and 5 % increased the resistivity of the lms. This increase in resistivity with

5 Properties of Indium-Zinc-Oxide Thin Films Prepared { You Seung Rim et al Fig. 8. XRD patterns of the deposited IZO thin lms as functions of (a) the input current and (b) the P O 2. increasing P O 2 can be explained by the density of oxygen vacancies in the IZO lm. In an IZO lm, each oxygen vacancy generates two free electrons [14]. A decrease in the density of oxygen vacancies thus leads to a decrease in the carrier concentration and an increase in the resistivity of the lm. As expected, the carrier concentration of the deposited IZO lms decreased from cm 3 to cm 3 with increasing P O 2. We observed that the electron mobility increased with increasing P O 2 between 0 % and 5 %, which is probably due to a decrease in the ionized scattering centers of carrier sites, such as electrically active oxygen vacancies [15]. Figure 6 shows the optical properties of the deposited IZO thin lms as a function of the P O 2. Adding a small amount of oxygen to the gas used in the FTS apparatus signicantly improves the transmittance of the resulting IZO lms. We produced IZO thin lms with transmittances higher than 85 % for wavelengths in the visible range. This improvement in transmittance can be attributed to oxygen vacancy compensation in the IZO lm. Fig. 9. Square of the absorption coecient ( 2 ) of the deposited IZO thin lms as functions of (a) the input current and (b) the P O 2. Figure 7 shows AFM images of the IZO lms deposited for a range of P O 2. The R rms roughness and the peakto-valley (R rpv ) values of the IZO lms rapidly decreased with increasing P O 2. The lowest R rms roughness and R rpv values were nm and nm, respectively, obtained when the IZO lm was deposited at a P O 2 of 5 %. The low surface roughness of the IZO thin lms we produced indicates that IZO may be a promising substitute for ITO in OLEDs and exible display applications, which require a very smooth surface and amorphous structure. Figure 8 shows the XRD patterns for IZO thin lms deposited at various values of the input current and the P O 2. The lms did not show any crystalline peaks, regardless of the sputtering conditions. Figures 9(a) and (b) show plots of (h) 2 versus h for the IZO lms deposited at various values of the input current and P O 2. The optical absorption coecient () and the optical energy band gap (E g ) are related by [16] h = C(h E g ) 1=2 ;

6 Journal of the Korean Physical Society, Vol. 54, No. 3, March 2009 where h is Planck's constant, is the frequency of the incident photon, C is a constant for a direct transition and is the optical absorption coecient. The optical energy gap E g can then be obtained from the intercept of (h) 2 versus h for possible direct transitions [17]. The direct band gaps of the IZO thin lms were observed to range from 3.60 ev to 3.52 ev with increasing input current from 0.04 A to 0.14 A. Figure 9(b) shows band gaps ranging from 3.57 ev to 3.51 ev with increasing P O 2. These results demonstrate that the optical band gap decreases with increasing input current and P O 2, which is indicative of a narrowing of the optical band gap at low carrier concentration. III. CONCLUSION IZO thin lms were deposited at various input currents and oxygen partial pressures in a FTS system. As the input current was increased, the resistivity of the resulting lms also increased, but their carrier concentrations decreased slightly. The surfaces of the IZO thin lms deposited using a higher input current were rougher than those deposited using a lower input current. This reduces the optical transmittance of lms deposited using a higher input current. We observed that as the P O 2 was increased, the resulting thin lms showed both lower resistivity and higher transmittance due to oxygen deciency compensation. The lowest resistivity ( cm) was observed at a P O 2 of 3 %. An average transmittance of over 85 % was achieved in the visible range. The deposited lms did not show any crystalline peaks, regardless of sputtering conditions. Thus, we have con- rmed that IZO thin lms prepared at room temperature by using the FTS method have excellent electrical and optical properties and a very smooth surface morphology. ACKNOWLEDGMENTS This work was supported by the RIC (Regional Innovation Center) at Kyungwon University. REFERENCES [1] Z. L. Pei, C. Sun, M. H. Tan, J. Q. Xiao, R. F. Huang and L. S. Wen, J. Appl. Phys. 90, 3432 (2001). [2] J. S. Hong, B. R. Rhee, H. M. Kim, K. C. Je, Y. J. Kang and J. S. Ahn, Thin Solid Films 467, 158 (2004). [3] T. Minami, T. Kakumu and S. Takata, J. Vac. Sci. Technol. A 14, 1704 (1996). [4] Y. Shigesato, D. C. Paine and T. E. Haynes, Adv. Mater. 4, 503 (1994). [5] A. I. Ali, C. H. Kim, J. H. Cho and B. G. Kim, J. Korean Phys. Soc. 49, S652 (2006). [6] K. E. Cheon, D. Y. Lee, Y. R. Cho, G. H. Lee and P. K. Song, J. Korean Phys. Soc. 53, 1 (2008). [7] A. Wang, J. Dai, J. Cheng, M. P. Chudzik, T. J. Marks, R. P. H. Chang and C. R. Kannewurf, Appl. Phys. Lett. 73, 327 (1998). [8] S. M. Kim, M. J. Keum and, K. H. Kim, J. Korean Phys. Soc. 51, 1023 (2007). [9] Y. S. Kim, W. H. Park, S. H. Kong, S. Nakagawa and K. H. Kim, Surface & Coatings Technology 532, 169 (2003). [10] W. W. Wang, X. G. Diao, Z. Wang, M. Yang, T. M. Wang and Z. Wu, Thin Solid Films 491, 54 (2005). [11] Y. S. Jung, J. Y. Seo, D. W. Lee and D. Y. Jeon, Thin Solid Films 445, 63 (2003). [12] D. Vaufrey, M. B. Khalifa, M. P. Besland, J. Tardy, C. Sandu, M. G. Blanchin and J. A. Roger, Mater. Sci. Eng. C 21, 265 (2002). [13] W. J. Lee, Y. K. Fang, J. J. Ho, C. Y. Chen, L. H. Chiou, S. J. Wang, F. Dai, T. Hsieh, R.Y. Tsai, D. Huang and F. C. Ho, Solid-State Electronics 46, 477 (2002). [14] M. Bendera, W. Seelig, C. Daube, H. Frankenberger, B. Ocker and J. Stollenwerk, Thin Solid Films 326, 72 (1998) [15] A. Yonesu, S. Watashi, K. Kagawa and Y. Yamashiro, Vacuum 74, 521 (2004). [16] N. Serpone, D. Lawless and R. Khairutdinov, J. Phys. Chem. 99, (1995). [17] I. K. E. Zawawi and R. A. A. Alla, Thin Solid Films 339, 314 (1999).