Effects of NIR annealing on the characteristics of al-doped ZnO thin films prepared by RF sputtering
© Jun and Koh; licensee Springer. 2012
Received: 18 April 2012
Accepted: 23 May 2012
Published: 6 June 2012
Aluminum-doped zinc oxide (AZO) thin films have been deposited on glass substrates by employing radio frequency (RF) sputtering method for transparent conducting oxide applications. For the RF sputtering process, a ZnO:Al2O3 (2 wt.%) target was employed. In this paper, the effects of near infrared ray (NIR) annealing technique on the structural, optical, and electrical properties of the AZO thin films have been researched. Experimental results showed that NIR annealing affected the microstructure, electrical resistance, and optical transmittance of the AZO thin films. X-ray diffraction analysis revealed that all films have a hexagonal wurtzite crystal structure with the preferentially c-axis oriented normal to the substrate surface. Optical transmittance spectra of the AZO thin films exhibited transmittance higher than about 80% within the visible wavelength region, and the optical direct bandgap (Eg) of the AZO films was increased with increasing the NIR energy efficiency.
KeywordsAl-doped ZnO Transparent conducting oxide Thin films NIR RF sputtering
Transparent conducting oxide (TCO) has been widely applied for various optoelectronic devices, such as flat-panel and liquid crystal displays , organic light-emitting diodes , and thin-film solar cells . Most of the TCO materials are based on tin oxide (SnO2) and indium tin oxide (In2O3) with group III elements, such as boron, gallium, indium, or aluminum-doped zinc oxide (AZO) . In particular, Al-doped ZnO-based TCO have been extensively researched because of its low cost and useful properties, such as non-toxicity, excellent electrical and optical properties, and high thermal and chemical stability [4, 5].
To fabricate the AZO thin films, several deposition techniques, such as chemical vapor deposition, electron beam evaporation, thermal plasma, pulsed laser deposition, metal organic chemical vapor deposition, sol–gel method, and DC, radio frequency (RF) sputtering have been developed and studied [6–14]. RF sputtering technique is one of the most widely used because of its reproducibility, efficiency, and reliability. There have been lots of reports concerning substrate temperature, oxygen pressure, target to substrate distance, and post-deposition annealing on the AZO thin-film quality using RF sputtering methods [15–18]. Because one of the main factors affecting the deposited film is optical energy, we have employed advanced optical processing by near infrared ray (NIR) annealing method to get better quality AZO thin films. NIR curing method has some advantages. Heat transfer coefficients are high; the process time is short; and the cost of energy is low. Since air is primarily a mixture of oxygen and nitrogen, neither of which absorbs NIR radiation, energy is transferred from the heating source to the sample without heating the surrounding air .
In this paper, the NIR annealing effects on the microstructure and the electrical and optical properties of the AZO thin films are reported.
In this experiment, the AZO thin films were fabricated by RF sputtering on glass substrates from a ZnO:Al2O3 (98:2) target with 99.999% purity and a 2 in. diameter. The glass substrates (Corning 1737) were ultrasonically cleaned using acetone, methanol, and deionized water, and then dried by blowing nitrogen over them before being introduced into the sputtering chamber. The chamber was initially evacuated to a base pressure under 1 × 10−6 Torr, and the deposition was carried out at a working pressure of 2 × 10−3 Torr. The argon gas was used as the plasma source, and the gas flow rate was controlled at 40 sccm, using the mass flow controller. The RF power was fixed at 100 W. Prior to the film deposition, pre-sputtering was performed for 10 min to remove any contamination on the target surface. The films were annealed by RTA at 450°C for 10 min and then annealed by a NIR at 20%, 40%, 60%, and 80% energy efficiency for 10 min to investigate the NIR effect.
The thicknesses of the deposited films were investigated by using α-step, and the thickness of the final film was approximately 150 nm. The crystalline structures of the specimens were analyzed by X-ray diffraction (XRD) patterns. XRD 2θ scans were carried out by employing an X-ray diffractometer Rigaku with a Cu-Kα source (λ = 0.154056 nm). The surface microstructure was observed by a scanning electron microscope (SEM). The electrical properties were measured by a four-point probe method, and optical transmittance measurements were carried out using a UV–VIS spectrometer. Photoluminescence (PL) spectra were recorded using a PL spectrometer excited with a 325 nm He-Cd laser at room temperature.
Lattice parameter and strain and stress of AZO thin films evaluated from XRD patterns
Efficiency of NIR (%)
d hkl (Ǻ)
The inserted figure shows a magnified 2θ region from 33° to 35.4°. It also shows that the (002) diffraction peaks positions of the AZO thin films in the 2θ region shift to a higher diffraction angle as the efficiency of NIR increases. The increase of 2θ value of the (002) peak may be related to the decrease of lattice parameters that comes from the oxygen defect or the strain caused by crystallization during the NIR process [20, 21].
The elastic constants cij of single crystalline ZnO are as follow: c11 = 208.8; c33 = 213.8; c12 = 114.7 and c13 = 104.2 . Equation 2 can be simplified to: σfilm = -233 × ϵ (GPa); the negative sign means compressive stress. Table 1 shows that the calculated stress and strain for the AZO thin films relaxed with the increasing energy efficiency of the NIR process.
In this paper, the RF-sputtered AZO thin films deposited on glass substrates and post-annealed by the NIR process at the energy efficiency levels of 20%, 40%, 60%, and 80% have been discussed. The effects of the NIR process on the optical, structural, and electrical properties of the AZO thin films have been studied. The films were found to be c- axis oriented. The AZO thin film with the optimal crystal quality had the lowest sheet resistance at 47.3 Ω/□. We obtained a high transmittance in the visible range, and the optical bandgap energy Eopt obtained was Eopt = 3.26 to 3.29 eV for the films put through the NIR process.
In conclusion, it was observed that the structural, morphological, electrical, and optical characteristics of AZO thin films can be improved by the NIR process, and that the NIR process seems to be an effective annealing method for the TCO process.
This work was supported by the New & Renewable Energy Project of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government Ministry of Knowledge Economy (No.20103030010040) and internal research grant from Kwangwoon University 2012.
- Ohyama M, Kozuka H, Yoko T: Sol–gel preparation of transparent and conductive aluminum-doped zinc oxide films with highly preferential crystal orientation. J Am Ceram Soc 1998, 81: 1622.View Article
- Cheong KY, Muti N, Ramanan SR: Electrical and optical studies of ZnO:Ga thin films fabricated via the sol–gel technique. Thin Solid Films. 2002, 410: 142. 10.1016/S0040-6090(02)00286-9View Article
- Rech B, Wagner H: Potential of amorphous silicon for solar cells. Appl. Phys. A. Mater. Sci. Process 1999, 69: 155. 10.1007/s003390050986View Article
- Minami T, Miyata T, Yamamoto T: Stability of transparent conducting oxide films for use at high temperatures. J. Vac. Sci. Technol. A 1822, 1999: 17.
- Chang JF, Lin WC, Hon MH: Effects of post-annealing on the structure and properties of Al-doped zinc oxide films. Appl Surf Sci 2001, 183: 18. 10.1016/S0169-4332(01)00541-4View Article
- Nishino J, Kawarada T, Ohshio S, Saitoh H, Maruyama K, Kamata K: Conductive indium-doped zinc oxide films prepared by atmospheric-pressure chemical vapour deposition. J. Mater. Sci. Lett 1997, 16: 629. 10.1023/A:1018511131738View Article
- Cooray NF, Kushiya K, Fujimaki A, Sugiyama I, Miura T, Okumura D, Sato M, Ooshita M, Yamase O: Large area ZnO films optimized for graded band-gap Cu(InGa) Se2-based thin-film mini-modules. Sol. Energy Mater and Sol. Cells. 1997, 49: 291. 10.1016/S0927-0248(97)00055-XView Article
- Kluth O, Schöpe G, Rech B, Menner R, Oertel M, Orgassa K, Schock HW: Comparative material study on rf and dc magnetron sputtered ZnO:Al films. Thin Solid Films. 2006, 502: 311. 10.1016/j.tsf.2005.07.313View Article
- Kuroyanagi A: Properties of aluminum-doped ZnO thin films grown by electron beam evaporation. Jpn. J. Appl. Phys. 1989, 28: 219. 10.1143/JJAP.28.219View Article
- Groenen R, Linden JL, van Lierop HRM, Schram DC, Kuypers AD, van de Sanden MCM: An expanding thermal plasma for deposition of surface textured ZnO:Al with focus on thin film solar cell applications. Appl Surf Sci 2001, 173: 40. 10.1016/S0169-4332(00)00875-8View Article
- Sub ES, Kang HS, Kang JS, Kim JH, Lee SY: Effect of the variation of film thickness on the structural and optical properties of ZnO thin films deposited on sapphire substrate using PLD. Appl Surf Sci 2002, 186: 474. 10.1016/S0169-4332(01)00746-2View Article
- Fu Z, Lin B, Zu J: Photoluminescence and structure of ZnO films deposited on Si substrates by metal-organic chemical vapor deposition. Thin Solid Films. 2002, 402: 302. 10.1016/S0040-6090(01)01363-3View Article
- Nunes P, Fernandes B, Fortunato E, Vilarinho P, Martins R: Performances presented by zinc oxide thin films deposited by spray pyrolysis. Thin Solid Films. 1999, 337: 176. 10.1016/S0040-6090(98)01394-7View Article
- Tang W, Cameron DC: Aluminum-doped zinc oxide transparent conductors deposited by the sol–gel process. Thin Solid Films. 1994, 238: 83. 10.1016/0040-6090(94)90653-XView Article
- Nunes P, Fortunato E, Martins R: Influence of the post-treatment on the properties of ZnO thin films. Thin Solid Films. 2001, 383: 277. 10.1016/S0040-6090(00)01577-7View Article
- Su Shia Lin, Jow Lay Huang, Sajgalik P: The properties of heavily Al-doped ZnO films before and after annealing in the different atmosphere. Surf Coat Technol 2004, 185: 254. 10.1016/j.surfcoat.2003.12.007View Article
- Vigil O, Cruz F, Santa G, Vaillant L, Morales-Acevedo A, Contreras-Puente G: Influence of post-thermal annealing on the properties of sprayed cadmium–zinc oxide thin films. Appl Surf Sci 2000, 161: 27. 10.1016/S0169-4332(00)00117-3View Article
- Tahar Radhouane Bel Hadj, Tahar Noureddine Bel Hadj: Mechanism of carrier transport in aluminum-doped zinc oxide. J Appl Phys 2002, 92: 8.
- Jang YS: A study on the drying characteristics of NIR dryer. J. Kor. Sol. Energy. Soc. 2004, 24: 1.
- Kim H, Gilmore CM, Pique A, Horwitz JS, Mattoussi H, Murata H, Kafafi ZH, Chrisey DB: Electrical, optical and structural properties of indium–tin–oxide thin films for organic light-emitting devices. J Appl Phys 1999, 86: 6451. 10.1063/1.371708View Article
- Han MY, Jou JH: Determination of the mechanical properties of rf-magnetron-sputtered zinc oxide thin films on substrates. Thin Solid Films. 1995, 260: 58. 10.1016/0040-6090(94)06459-8View Article
- Fang GJ, Li DJ, Yao BL: Effect of vacuum annealing on the properties of transparent conductive AZO thin films prepared by dc magnetron sputtering. Phys. Status. Solidi A 2002, 193: 139. 10.1002/1521-396X(200209)193:1<139::AID-PSSA139>3.0.CO;2-DView Article
- Ohta H, Kawamura K, Orita M, Hirano M, Sarukura N, Hosono H: Current injection emission from a transparent p-n junction composed of p-SrCuO/n-ZnO. Appl Phys Lett 2000, 77: 475. 10.1063/1.127015View Article
- Cebulla R, Wendt R, Ellmer K: Al-doped zinc oxide films deposited by simultaneous rf and dc excitation of a magnetron plasma: relationships between plasma parameters and structural and electrical film properties. J Appl Phys 1998, 83: 1087. 10.1063/1.366798View Article
- Zhi ZZ, Liu YC, Li BS, Zhang XT, Lu YM, Shen DZ, Fan XW: Effects of thermal annealing on ZnO films grown by plasma enhanced chemical vapour deposition from Zn(C2H5)2 and CO2 gas mixtures. J. Phys. D: Appl. Phys. 2003, 36: 719. 10.1088/0022-3727/36/6/314View Article
- Cui ML, Wu XM, Zhuge LJ, Meng YD: Effects of annealing temperature on the structure and photoluminescence properties of ZnO films. Vacuum 2007, 81: 899. 10.1016/j.vacuum.2006.10.011View Article
- Lin SS, Huang JL, Sajgalik P: Effects of substrate temperature on the properties of heavily Al-doped ZnO films by simultaneous r.f. and d.c. magnetron sputtering. Surf Coat Technol 2005, 190: 40.
- Haacke G: New figure of merit for transparent conductors. J Appl Phys 1976, 47: 4086. 10.1063/1.323240View Article
- Fallah HR, Ghasemi M, Hassanzadeh A, Steki H: The effect of annealing on structural, electrical and optical properties of nanostructured ITO films prepared by e-beam evaporation. Mater. Res. Bulletin. 2007, 42: 487. 10.1016/j.materresbull.2006.06.024View Article
- Tauc J, Grigorovici R, Vancu A: Optical properties and electronic structure of amorphous germanium. Phys. Stat. Sol. 1966, 15: 627. 10.1002/pssb.19660150224View Article
- Shin JH, Choi DK: Effect of oxygen on the optical and the electrical properties of amorphous InGaZnO thin films prepared by rf magnetron sputtering. J. Kor. Phys. Soc. 2019, 2008: 53.
- Burstein E: Anomalous optical absorption limit in InSb. Phys Rev 1954, 93: 632. 10.1103/PhysRev.93.632View Article
- Moss TS: The interpretation of the properties of indium antimonide. Proc. Phys. Soc. Lond. B 1954, 67: 775. 10.1088/0370-1301/67/10/306View Article
- Johnson JC, Yan H, Yang P, Saykally RJ: Optical cavity effects in ZnO nanowire lasers and waveguides. J Phys Chem B 2003, 107: 8816. 10.1021/jp034482nView Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.