THE EFFECT OF ANNEALING ATMOSPHERES ON STRUCTURAL, ELECTRICAL AND OPTICAL PROPERTIES OF THE ATO FILMS PREPARED BY RF MAGNETRON SPUTTERING

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1 Functional Materials Letters Vol. 3, No. 2 (2010) World Scientific Publishing Company DOI: /S THE EFFECT OF ANNEALING ATMOSPHERES ON STRUCTURAL, ELECTRICAL AND OPTICAL PROPERTIES OF THE ATO FILMS PREPARED BY RF MAGNETRON SPUTTERING SUNG UK LEE and BYUNGYOU HONG Department of Electrical and Computer Engineering, Sungkyunkwan University Chunchun-dong, Jangan-gu Suwon , Republic of Korea byhong@skku.edu JIN-HYO BOO Department of Chemistry, Sungkyunkwan University Chunchun-dong, Jangan-gu, Suwon , Republic of Korea Received 8 March 2010; Revised 30 March 2010 Antimony-doped tin oxide (SnO 2 :Sb, ATO) films were deposited on 7059 corning glass substrate by radio frequency magnetron sputtering method using a commercial ceramic target with a mixture of SnO 2 and Sb (6 wt.%) for application to transparent electrodes. The ATO film was deposited at working pressure of 5 mtorr and RF power of 175 W without substrate heating. The thickness of the deposited ATO films was about 150 nm using a surface profiler (alpha-step). The films were annealed at temperatures ranging from to C in step of 100 C using RTA equipment in vacuum and oxygen atmosphere, respectively. We investigated the effects of the post-annealing atmospheres on structural, electrical and optical properties of the ATO films. The results show that the increase of the annealing temperature improved the crystallinity of the ATO films, increased the grain size and improved electrical and optical properties, regardless of annealing atmospheres. The resistivity of the ATO films decreased significantly with higher annealing temperatures in vacuum and oxygen atmosphere. In the visible range from to 800 nm, the optical transmittance of the ATO films increased over 90% at higher annealing temperature. Keywords: Antimony-doped tin oxide; transparent conductive oxide; RF magnetron sputtering; resistivity; transmittance; annealing atmosphere. Transparent conducting oxide (TCO) has been widely used as a transparent conducting thin film material for application in various fields such as solar cells, optoelectronic devices, heat mirrors and gas sensors, etc. 1 4 The increased utilization of many transparent electrodes has accelerated the development of inexpensive TCO materials. Indium-doped tin oxide (ITO) film is well known for TCO materials because of its excellent electrical and optical properties. 5 However, its high processing cost is a disadvantage. ZnO film is cheaper than ITO, but it shows poor thermal stability. In contrast, SnO 2 film shows the best thermal and chemical stability. Also, it is inexpensive to manufacture and has good mechanical durability, but a high resistivity. SnO 2 is an n-type semiconductor with a wide band gap of approximately 3.7 ev. Pure SnO 2 films are poor electrical conductors that are highly transparent in the visible range. However, their poor electrical conductivity can be improved by controlling stoichiometry or doping with impurities. The electrical conductive antimony-doped tin oxide films are prepared by various methods such as chemical vapor deposition (CVD), 6 spray pyrolysis, 7 sputtering and evaporation Among these methods, RF magnetron sputtering is the most advantageous because high quality film deposition can be carried out at low temperatures while yielding the preferred orientation and uniform properties. 13 It has been found that doping SnO 2 with Sb increases both conductivity and transmittance. This is attributed to the increase in the density of free charge carriers. In this work, antimony (6 wt.%) doped tin oxide (ATO) films were deposited on 7059 corning glass substrate at room temperature by RF magnetron sputtering method. The films were annealed at temperatures ranging from to Cin steps of 100 C using rapid thermal annealing (RTA) equipment in vacuum and oxygen atmospheres, respectively. We investigated the effects of the post-annealing atmosphere on structural, electrical and optical properties of the ATO films. Corresponding author. 119

2 120 S. U. Lee, B. Hong & J.-H. Boo ATO films were prepared on glass substrate (7059 Corning glass) with dimensions of 1 2 cm by a conventional RF magnetron sputtering method. The sputtered target was a mixture of SnO 2 (99.99%) and Sb 2 O 3 (99.99%), pressed on a copper saucer with a diameter of 4 inch. The content of Sb 2 O 3 added in the SnO 2 target was 6 wt.%. The sputtering chamber was evacuated to a base pressure of Torr by a turbomolecular pump before the generation of plasma, activated by RF power at MHz. The two electrodes were placed in parallel and the distance between the target and the glass substrate was 6 cm. The flow rate of Ar (99.999%) gas was fixed to 100 sccm by a mass flow controller. The ATO film was deposited at a working pressure of 5 mtorr and RF power of 175 W at room temperature. The thickness of ATO films was kept constant at 150 nm. The films were annealed at temperature ranging from to C in step of 100 CusingRTA equipment in vacuum and oxygen atmosphere, respectively. The thickness of the ATO film was measured using a TENCOR surface profiler (alpha-step). The surface morphologies of the ATO films were measured using a Seiko, SPA- atomic force microscope (AFM). The crystalline structure of the film was characterized by a Bruker, AXS D8 Discover X-ray diffractometer (XRD). The Hall mobility and the carrier concentration in the films were examined from Hall measurement (ECOPIA, HMS-0). The optical transmittance of the film was observed using a Hitachi, U 0 UV-spectrophotometer in the visible wavelength range of 800 nm. XRD patterns of the ATO films as-deposited and annealed at C in different annealing atmosphere are shown in Fig. 1. The increase of annealing temperature resulted in crystallization of the films. This means that the annealing leads to an improvement in the crystallinity of the films, and annealing treatment could increase the activation energy of crystallization. 14 The (110), (101), (200) and (211) diffraction peaks were observed for the annealed ATO films. All annealed samples were found to be polycrystalline with tetragonal rutile structure. The preferred orientation of the (101) direction peak become more intense and sharper with increasing annealing temperature, and the full-width at half-maximum (FWHM) of the (101) peak decreased. This indicates that the crystallinity of the film was improved and the grain size became larger at high annealing temperature. Figure 2 shows the grain size and FWHM of the ATO films annealed at different annealing temperature in different annealing atmosphere, supporting the XRD spectra results. Grain size was calculated by using the Scherrer equation 15 : λ D = 0.9. (1) β cos B In the Scherrer s equation, D is the grain size, λ is the wavelength of the X-rays ( Å), β is the broadening of the diffraction line measured at half its maximum intensity in radians (FWHM), and θ is the diffractive angle. The grain size of the ATO film increased with increasing annealing temperature regardless of the annealing atmosphere. In general, the change in FWHM results from the variation in the grain size of crystallites. In vacuum atmosphere, as the annealing temperature increased, the grain size gets (101) (110) (200) (211) Vacuum at C Intensity Oxygen at C sited Degree (2θ) Fig. 1. X-ray diffraction (XRD) patterns of the ATO films as-deposited and annealed at C in different annealing atmospheres. Color online. Fig. 2. Grain size and FWHM of the ATO films annealed at different temperature in different annealing atmosphere: in vacuum and in oxygen. Color online.

3 Effect of Annealing Atmospheres on ATO Films Prepared by RF Magnetron Sputtering 121 bigger with the increase in the annealing temperature, because enhanced crystallinity of the film decreased FWHM of the (101) peak. Results indicated that annealing treatment could increase the activation energy of diffusion, and increased the activation energy of crystallization. However, in oxygen atmosphere, the grain size increased with increasing annealing temperature up to C. It then decreased because decreased FWHM of the (101) peak due to the chemisorption of oxygen on the surface of the film annealed in oxygen atmosphere. 16 In order to confirm the effect of annealing temperature on the surface morphology of the ATO films, the AFM images and the dependence of the surface roughness of the ATO films as-deposited and annealed at C in different annealing atmosphere are shown in Fig. 3, respectively. From the AFM images, the surface roughness of the as-deposited film was higher than that annealed film. As the annealing temperature increased up to C, the surface roughness of the film decreased regardless of the annealing atmospheres. This behavior could be attributed to the increased annealing temperature improving ATO film crystallinity. The higher the annealing temperature, the better the grains grew and the crystallinity of the deposited ATO films improved. As a result, surface roughness was observed to be improved because the radical s mobility at the film surface was enhanced with the increase of annealing temperature as a smother surface causes less surface scattering of charge carriers. Figure 4 shows the resistivity of the ATO films annealed at different annealing temperature in different annealing atmospheres. It can be seen that the resistivity of the ATO films is affected slightly by the annealing temperature and annealing atmosphere. With the annealing temperature increasing, the resistivity of the ATO films annealed in vacuum decreased. However, as the annealing temperature increased from to C, the resistivity of the ATO films slightly increased when annealed in oxygen. For the analysis of the conduction mechanism, the Hall mobility and carrier concentration were measured, and the Resistivity (Ω-cm) vacuum oxygen Annealing temperature( C) Fig. 4. Resistivity of the ATO films annealed at different annealing temperature in different annealing atmospheres. Color online. Carrier concentration (cm -3 ) Carrier concentration (cm -3 ) Carrier concentration Hall mobility Annealing temperature ( C) Carrier concentration Hall mobility Annealing temperature ( C) Mobility (cm 2 V -1 s -1 ) Mobility (cm 2 V -1 s -1 ) Fig. 5. Hall mobility and carrier concentration of the ATO films annealed at different annealing temperature in different annealing atmospheres: in vacuum and in oxygen. Color online. Fig. 3. AFM images of the ATO films as-deposited and annealed at C in different annealing atmospheres: sited, annealed in vacuum and (c) annealed in oxygen. Color online. results are shown in Fig. 5. Annealed in vacuum, Hall mobility and carrier concentration increased from 5.22 to 6.05 cm 2 V 1 s 1 and from 1.38 to cm 3 with increasing annealing temperature up to C. It is well known from many previous studies that higher annealing temperature in vacuum leads to lower resistivity. The decrease in resistivity

4 122 S. U. Lee, B. Hong & J.-H. Boo with increasing annealing temperature can be attributed to the improved crystallinity of the films. This results in a decreased resistivity, due to the oxygen vacancies and interstitial tin atoms that act as donors, giving rise to higher carrier concentration and Hall mobility. Moreover, the surface mobility of sputtered atoms (Sn, O, Sb) from ATO target at the sample surface was enhanced with increasing annealing temperature and thus the grain size became larger. This resulted in carrier scattering due to reduced grain boundaries and enhanced mobility. The annealing effect of the film annealed in vacuum was consistent with the other studies. 9,17 However, the resistivity of the film annealed at high temperature ( C) in oxygen atmosphere is slightly increased because of the number of oxygen vacancies is substantially reduced, resulting in a decrease in carrier concentration. Therefore, the conduction mechanism of ATO film can be explained by the variation of carrier concentration and Hall mobility. The optical transmittance in the visible wavelength range from to 800 nm of the ATO films annealed at different annealing temperature in different annealing atmospheres is shown in Fig. 6. The optical transmittance of the ATO films increased over 90% at higher annealing temperature. This implies that the transmittance in the visible wavelength range of film is closely related to the film structure. Moreover, surface morphology also influences the transmittance of films. When grain size increases, light is less scattered by decreasing grain-boundaries. This results in high transmittance in the visible wavelength range. ATO films have been prepared on a 7059 Corning glass substrate by radio frequency magnetron sputtering method using a commercial ceramic target (SnO 2 :Sb, 94:6 wt.%) without substrate heating. We investigated the effects of annealing temperature on structural, electrical and optical properties of the ATO films. The films were annealed at temperatures ranging from to C, using RTA equipment in vacuum and oxygen atmospheres, respectively. Increasing annealing temperature improves the crystallinity of the films, and decreases surface roughness. XRD measurements showed the ATO films with a strong (101) preferred orientation as the annealing temperature increased. The spectra revealed that the annealed films were polycrystalline, retaining the tetragonal rutile structure. The electrical resistivity of the films decreased significantly with annealing temperature up to C. The higher annealing temperature improved both charge carrier concentration and Hall mobility and thus enhanced the conductivities of the films. The lowest resistivity was cm at annealing temperature of C in vacuum. In the visible wavelength range, the optical transmittance of the ATO films without annealing is about 86.5%; it increases up to 90.8% after annealing at C in vacuum. This implies that the transmittance in the visible wavelength range of film is closely related to film structure. Compared with the results of the ATO Transmittance (%) Transmittance (%) C 60 C C C Wavelength (nm) 70 C 60 C C C Wavelength (nm) Fig. 6. Optical transmittance in the visible wavelength range of the ATO films annealed at different annealing temperature in different annealing atmospheres: in vacuum, and in oxygen. Color online. films annealed in oxygen, the structural, electrical and optical properties of the films annealed in vacuum are better. Acknowledgments This study was supported by the Korea Science and Engineering Foundation (KOSEF) grant and funded by the Korea government (MOST), No. R , the Brain Korea 21, and No. R References 1. S. M. Park et al., Curr. Appl. Phys. 7, 474 (2007). 2. C. M. Lampert, Solid Energy Mater. 6, 1 (1981). 3. N. S. Murty and S. R. Jawalekar, Thin Solid Films 108, 277 (1983). 4. A. Chaturvedi et al., Microelectron. J. 31, 283 (2000). 5. E. Terzini et al., Mater. Sci. Eng. B. 77, 110 (2000). 6. D. Liu et al., J. Mater. Res. 10, 1516 (1995). 7. E. Elangovan et al., J. Cryst. Growth. 276, 215 (2005).

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