Properties of Indium-Zinc-Oxide Thin Films Prepared by Facing

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Journal of the Korean Physical Society, Vol. 54, No. 3, March 2009, pp. 1267 1272 

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 461-701

(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 (In2 O3 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 O2 ). 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-e ect measurement system, an X-ray di ractometer (XRD), a UV/VIS spectrometer in the visible range and an atomic4force microscope (AFM), respectively. We obtained IZO thin lms with a resistivity of 4.2 10 -cm, a carrier concentration of 3.1 1020 cm 3 , a electron mobility of 36.4 cm2 /vs and a transmittance of over 85 %, at a P O2 of 3 %. PACS numbers: 42.79.Kr, 68.55.Jk, 81.15.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 re ectors [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 (4 nm) [13]. Figure 5 shows the electrical properties of the deposited IZO thin lms as a function of the P O2 . The lowest resistivity (4.2  10 4 -cm) was observed when the lm was deposited at a P O2 of 3 %. We found that increasing the P O2 between 3 % and 5 % increased the resistivity of the lms. This increase in resistivity with

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 O2 .

increasing P O2 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 3.9  1020 cm 3 to 1.9  1020 cm 3 with increasing P O2 . We observed that the electron mobility increased with increasing P O2 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 O2 . Adding a small amount of oxygen to the gas used in the FTS apparatus signi cantly 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.

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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 O2 .

Figure 7 shows AFM images of the IZO lms deposited for a range of P O2 . The Rrms roughness and the peakto-valley (Rrpv ) values of the IZO lms rapidly decreased with increasing P O2 . The lowest Rrms roughness and Rrpv values were 0.419 nm and 4.151 nm, respectively, obtained when the IZO lm was deposited at a P O2 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 O2 . 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 O2 . The optical absorption coecient ( ) and the optical energy band gap (Eg ) are related by [16] h = C (h

Eg )1=2 ;

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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 Eg 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 O2 . These results demonstrate that the optical band gap decreases with increasing input current and P O2 , 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 O2 was increased, the resulting thin lms showed both lower resistivity and higher transmittance due to oxygen de ciency compensation. The lowest resistivity (4.2  10 4

-cm) was observed at a P O2 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 Inno-

vation Center) at Kyungwon University.

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