Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010, pp. 1092∼1095
Electrical and Optical Properties of Al-doped Zinc-oxide Thin Films Deposited at Room Temperature by Using the Continuous Composition Spread Method Keun Jung, Dong Wook Shin, Seok-Jin Yoon and Ji-Won Choi∗ Electronic Materials Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
Won-Kook Choi Opto-electronic Materials Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
Jong-Han Song Nano Material Analysis Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
Hyun Jae Kim School of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Republic of Korea (Received 15 January 2010, in final form 5 July 2010) Al-doped ZnO (AZO) thin films were deposited on glass substrates at room temperature by using the continuous composition spread (CCS) method. CCS is a thin-film growth method for various Alx Zn1−x O thin film compositions with a binary or ternary composition spread, and the critical properties can be evaluated as functions of position, which is directly related to material composition, by using an automated probe station. Various compositions of Al-doped ZnO thin films deposited at room temperature were explored to find excellent electrical and optical properties. The lowest resistivity of the AZO thin films deposited at room temperature was 2.8 × 10−3 Ω·cm, and the average transmittance in the 400-to-900-nm wavelength region was 93%. The optimized composition of the AZO thin film, which had the lowest resistivity and highest transmittance was Al0.05 Zn1 O1.05 (about 3.13-wt% Al2 O3 ). PACS numbers: 81.15.-z, 73.61.Ga, 78.66.Hf Keywords: Continuous composition spread, Thin films, Transparent conducting oxides, Al-doped ZnO DOI: 10.3938/jkps.57.1092
I. INTRODUCTION Transparent conducting oxide (TCO) films have been extensively investigated because of their important technological applications, such as flat panel displays (FPDs), thin film resistors, gas sensors and solar cells. Thus, many researchers are investigating TCO materials [1,2]. Mainly, indium tin oxides (ITOs) have been used as anodes of some devices such as FPDs and solar cells because of their high work function, high conductivity and high transparency over the visible range [3]. However, the price of indium is increasing due to the high demand for ITO in the rapid development of the FPD industry. In addition, the toxic nature and high cost due to the scarcity of indium have led researchers to seek an alternative candidate for ITO [4]. Recently, zinc-oxide ∗ E-mail:
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thin films have been considered as possible alternatives to ITO films because zinc-oxide thin films are more stable against a hydrogen plasma, more abundant, and less expensive in comparison with ITO films. Highly conductive ZnO films with high transmittance in the visible range have been used by doping with Al, B, Ga, and F. Especially, Al-doped ZnO films have some advantages, such a higher transmittance even at near infrared wavelengths than ITO. Thus, many researchers are investigating AZO thin films for different doping weight percentages of Al. However, there is a still controversy over the optimized Al-doping content in ZnO [5–8], so we investigated the full range of AZO composition to optimize the composition by using a continuous composition spread. The continuous composition spread (CCS) is a thinfilm growth method of various compositions on a substrate with a binary or ternary composition spread. The electrical properties of AZO thin films deposited by using CCS were measured by using an automatic probe station that could evaluate the critical properties as functions of
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Electrical and Optical Properties of Al-doped Zinc-oxide Thin Films · · · – Keun Jung et al.
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position. Also, the function of position is directly related to the material’s composition. Actually, there are various CCS systems such as plasma-laser-deposition(PLD) CCS, sputter CCS, molecular-beam-epitaxy (MBE) CCS and so on. Among of them, we chose the sputter CCS system because it deposits almost a full composition range easily and because controlling of system is simple [9–11]. In this study, a full range of compositions of Alx Zn1−x O were deposited on glass substrates at room temperature, and the electrical properties were evaluated to explore the optimized composition, which has excellent electrical and optical properties.
II. EXPERIMENTS AND DISCUSSION Al-doped ZnO thin films were deposited on 6 inch glass substrates (eagle 2000, corning) at room temperature by using off-axis sputter CCS. The off-axis sputter CCS has three independent radio frequency (RF) magnetron sputtering guns, which are located at 90◦ to the substrate. Zinc-oxide (purity: 99.99%, CERAC) and Al2 O3 (purity: 99.99%, CERAC) targets were used to explore the optimized composition of AZO. The sputtering chamber was pumped down to 2.66 × 10−4 Pa by using a rotary pump and turbo-molecular pump. The sputtering was performed at a pressure of 2.66 Pa in a pure argon atmosphere. The ZnO and the Al2 O3 targets were powered by using independent RF supplies (ZnO: 150 W and Al2 O3 : 300 W) to achieve the desired composition range on the substrate. The thicknesses of the thin films were examined through cross-section of observation by using filedemission scanning electron microscopy (FE-SEM, XL-30, FEG). The crystal structures of the AZO thin films were investigated by using X-ray diffraction (XRD, D/MAX2500, RIGAKU). The sheet resistances of the AZO thin films were measured using the four-point probe method (MCP-T600, Mitsubishi chemical) with an automatic probe station (19S, TEL). The optical transmittances were measured in the range of 200 - 900 nm by using an UV/VIS spectrometer (Lambda 18, Perkin Elmer). The optimized composition and thickness of the thin films were characterized by using Rutherford backscattering spectroscopy (RBS, 6SDH2, NEC). Figure 1 shows the thickness profiles of binary ZnOAl2 O3 , single ZnO, and single Al2 O3 thin films deposited at RF powers of 300 and 150 W for the Al2 O3 and the ZnO targets, respectively. The thickness profiles of each single Al2 O3 and ZnO thin films are directly related to the composition of binary the Al2 O3 -ZnO films as a function of position. The deposition rates and the thicknesses of single ZnO and Al2 O3 thin films were measured by using a cross-sectional SEM analysis. In the off-axis sputter-CCS system, the deposition rates and the thicknesses of the thin films are related to the distances from the target. When the distances from the target were in-
Fig. 1. (Color online) Thickness profile of ZnO-Al2 O3 thin films deposited at room temperature on glass substrates by using off-axis sputter CCS.
creased, the deposition rates and the thicknesses were decreased, as shown in Fig. 1. The thickness profile of the Al2 O3 -ZnO binary CCS film measured by using SEM agreed with the sum of the single Al2 O3 and the single ZnO thickness profiles, measured by using SEM. Figure 2(a) shows the electrical properties of AZOCCS thin films deposited at room temperature by using off-axis sputter CCS. Each position had a different sheet resistance horizontally because of different Alx Zn1−x O compositions. Especially, positions of AZO thin films deposited near the ZnO target had lower sheet resistance than those of the AZO thin films deposited near Al2 O3 target because the Al2 O3 has insulating properties. We focused on the positions of AZO thin films deposited near the ZnO target because the purpose of this study was to find an AZO composition with excellent electrical properties. Also, the horizontal line (white line) in Fig. 2(b) relates the resistivity to the substrate position. As shown in Fig. 2(b), the resistivities of positions from 10 to 90 mm (Al2 O3 -rich region) were higher than 1.0 × 10−2 Ω·cm or overloaded. However, the resistivities of positions from 90 to 100 mm (ZnO-rich region doped with Al2 O3 appropriately) decreased and increased again (100-140 nm). The lowest resistivity of AZO thin films deposited at room temperature by using CCS was 2.8 × 10−3 Ω·cm. This value is comparable to 4.8 × 10−3 Ω·cm, which is the resistivity of ZnO thin films doped with 3 wt% Al, and deposited at room temperature, are reported by Oh et al. [12]. We chose three positions (positions 1, 2, and 3) from the entire area of the AZO-CCS thin films for analysis. Figure 3 shows (a) the XRD patterns and (b) the transmittances of the three positions in the AZO thin films. The decrease in the intensities of the (002) peak for positions 1, 2, and 3 is due to the decrease in the film’s thickness, as shown in Fig. 2(b). Three positions of the
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Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010
Fig. 2. (Color online) (a) Sheet resistances for all positions and (b) resistivity and thickness profile as functions of substrate position (white line in (a)) for AZO thin films deposited by using off-axis sputter CCS.
AZO-CCS thin films exhibit a highly-preferred (002) orientation and small (004) peaks, which indicates that the films are highly oriented with their crystallographic caxes perpendicular to the substrate. Thus, one can infer that Al atoms substituted for Zn in the hexagonal lattice or probably segregated to the non-crystalline region in grain boundaries to form Al-O bonds. Much of the Al is believed to be able to be ionized into Al3+ and to substitute for Zn2+ , so a free electron from each Al atom [13]. The average transmittance of the three positions was over 90% in the 400-to-900-nm wavelength region. From these results, we confirmed that the electrical and the optical properties of ZnO thin films could be controlled by using the Al-doping level for transparent electrodes. There are many reports on the electrical and optical properties of Al-doped ZnO thin films. Many reports chose 2-wt% Al-doped ZnO targets, and some reports chose 1-, 3-, and 4-wt% Al-doped ZnO targets to obtain thin films that had excellent electrical and optical properties [14–18]. Figure 4 shows the compositional analysis performed by using RBS for the 740-nm-thick AZO film, which had
Fig. 3. (Color online) (a) XRD patterns and (b) optical transmittance of AZO thin films deposited by using off-axis sputter CCS.
Fig. 4. (Color online) Two(2)-MeV 4 He++ Rutherford backscattering spectrum of the 740-nm-thick AZO thin film, which had the lowest resistivity.
Electrical and Optical Properties of Al-doped Zinc-oxide Thin Films · · · – Keun Jung et al.
the best electrical and optical properties. RBS was carried out using two(2)-MeV 4 He++ particles at a scattering angle of 170◦ . The optimized composition of AZO film (position 3 in Figs. 2 and 3), Al0.05 Zn1 O1.05 , was obtained from a RUMP-code simulation. This composition corresponds to 3.13-wt% Al-doped ZnO, which is quite similar to the doping level reported by Oh et al. [12].
III. CONCLUSION Binary AZO thin films with various compositions were deposited on glass substrates at room temperature by using an off-axis sputter CCS. The lowest resistivity and the average transmittance in the 400-to-900-nm wavelength region for the AZO thin films deposited at room temperature were 2.8 × 10−3 Ω·cm and 93%, respectively. This AZO thin film exhibited (002) and (004) peaks, which indicated that the films were highly oriented with their crystallographic c-axes perpendicular to the substrates. By Rutherford backscattering spectroscopy (RBS), the AZO with an optimum composition was found to be Al0.05 Zn0 O1.05 (about 3.13 wt%). This technique holds promise for finding TCO materials with new optimized compositions more effectively than the time-consuming conventional growth and analysis processes.
ACKNOWLEDGMENTS This study was supported by “a grant-in-aid for R&D program (No. 10030068)” and “the R&D program for Core Converging Research Center Program (20090082023)” funded by the Ministry of Education, Science and Technology, Republic of Korea.
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