- Nano Commentary
- Open Access
Comparative studies of Al-doped ZnO and Ga-doped ZnO transparent conducting oxide thin films
© Jun et al.; licensee Springer. 2012
- Received: 5 October 2012
- Accepted: 16 November 2012
- Published: 22 November 2012
We have investigated the influences of aluminum and gallium dopants (0 to 2.0 mol%) on zinc oxide (ZnO) thin films regarding crystallization and electrical and optical properties for application in transparent conducting oxide devices. Al- and Ga-doped ZnO thin films were deposited on glass substrates (corning 1737) by sol–gel spin-coating process. As a starting material, AlCl3⋅6H2O, Ga(NO3)2, and Zn(CH3COO)2⋅2H2O were used. A lowest sheet resistance of 3.3 × 103 Ω/□ was obtained for the GZO thin film doped with 1.5 mol% of Ga after post-annealing at 650°C for 60 min in air. All the films showed more than 85% transparency in the visible region. We have studied the structural and microstructural properties as a function of Al and Ga concentrations through X-ray diffraction and scanning electron microscopy analysis. In addition, the optical bandgap and photoluminescence were estimated.
- Transparent conducting oxide
- Thin films
Transparent conducting oxide (TCO) films have been intensively investigated for optical and electrical applications, such as flat-panel displays, liquid crystal displays, organic light-emitting diodes, thin-film transistors, and thin-film solar cells[1–4]. TCO thin films should have low resistivity, high transmittance in the visible region (400 to 800 nm), and high thermal/chemical stability[5, 6]. In most cases, indium tin oxide (ITO) has been widely employed as a TCO material because of its superb electrical and optical properties. However, ITO has low stability, high toxicity, and high cost and is a rare material, motivating efforts to develop alternatives.
Recently, zinc oxide (ZnO) has been regarded as a promising candidate to replace ITO due to its low cost and excellent properties as compared with ITO. For the purpose of improving the electrical conductivity and optical transmittance of ZnO thin films, group III elements such as boron, aluminum, gallium, and indium are usually introduced to ZnO. Undoped and doped ZnO thin films have been prepared by a variety of thin film deposition techniques, such as chemical vapor deposition, DC and RF magnetron sputtering, electron beam evaporation, thermal plasma, pulsed laser deposition, metal organic chemical vapor deposition, spray pyrolysis, and sol–gel method[9–17].
In this study, Al-doped ZnO (hereafter AZO) and Ga-doped ZnO (hereafter GZO) thin films were prepared by sol–gel spin-coating method since this particular technique offers several advantages, such as large deposition area, simple equipment, low fabrication cost, and high homogeneity of the precursor. We compare the effects of Al and Ga dopants on the microstructure, electrical, and optical properties of the AZO and GZO thin films as a function of doping concentration.
Thin films were prepared by sol–gel spin-coating method. As starting materials, Ga(NO3)2, AlCl3⋅6H2O, and Zn(CH3COO)2⋅2H2O were used. As solvent and stabilizer, 2-methoxyethanol and monoethanolamine (MEA) were used, respectively. Zinc acetate dihydrate was first dissolved in a mixture of 2-methoxyethanol and MEA solution at room temperature. The molar ratio of MEA to zinc acetate dihydrate was maintained at 1.0, and the concentration of zinc acetate dihydrate was 0.7 mol/L. In order to study the influence of Al and Ga dopant concentrations on the properties of Al-doped and Ga-doped ZnO thin films, the concentrations were varied at 0, 0.5, 1.0, 1.5, and 2.0 mol% with respect to Zn. The solutions were stirred at 60°C for 2 h to yield a clear and homogeneous solution. Thereafter, Corning 1737 glass (Corning Inc., Corning, NY, USA) was ultrasonically cleaned in acetone, methanol and DI water for 5 min, respectively. AZO and GZO films were then deposited on glass substrates (Corning 1737) by sol–gel spin-coating method. Spin coating was performed at room temperature, with a rate of 3,000 rpm for 20 s. After being deposited by sol–gel spin coating, the films were preheated at 300°C for 10 min on a hot plate to evaporate the solvent and remove organic residuals. The procedures from coating to drying were repeated six times. The films were then placed in a furnace and post-heated in air at 650°C for 1.5 h.
The crystalline structures of the specimens were analyzed by X-ray diffraction (XRD) patterns. XRD 2θ scans were carried out by employing a Rigaku X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) with a Cu-Kα source (λ = 0.154056 nm). The surface microstructure was observed by SEM (Hitachi S-4300, Hitachi High-Tech, Minato-ku, Tokyo, Japan). Electrical resistance was measured using four-point probe method and Hall measurement system. Optical transmittance measurements were carried out using a UV–vis spectrophotometer. Photoluminescence (PL) spectra were recorded using a PL spectrometer excited with a 325-nm He-Cd laser at room temperature.
Aluminum- or gallium-doped ZnO thin films were prepared by sol–gel spin-coating method for TCO applications. All films had a hexagonal wurtzite crystal structure, and a minimum sheet resistance of 3.3 × 103 Ω/□ was obtained for 1.5-mol% Ga-doped ZnO thin film. We also found that Al and Ga dopants acted as electrical dopants at the initial doping concentration but as impurities at greater doping concentration. The transmittance of the AZO and GZO thin films was higher than 85% in the visible region, and the optical bandgap of the AZO and GZO thin films became broader with increasing Al or Ga dopant concentration because of the Burstein-Moss effect. In conclusion, the structural, morphological, electrical, and optical characteristics of AZO and GZO thin films were observed, and Ga doping seems to be more effective than Al doping.
This work was supported by the New and Renewable Energy Project of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government's Ministry of Knowledge Economy (no. 20103030010040).
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- Lan JH, Kanicki J, Catalano A, Keane J, Boer WD, Gu TJ: Patterning of transparent conducting oxide thin films by wet etching for a-Si:H TFT-LCDs. Electron Mater 1996, 25: 1806. 10.1007/BF02657158View ArticleGoogle Scholar
- Rech B, Wagner H: Potential of amorphous silicon for solar cells. Appl. Phys. A. Mater Sci Process 1999, 69: 155. 10.1007/s003390050986View ArticleGoogle Scholar
- Gordillo G, Calderon C: Properties of ZnO thin films prepared by reactive evaporation. Solar Energy Mater. Solar Cells 2001, 69: 251. 10.1016/S0927-0248(00)00394-9View ArticleGoogle Scholar
- Oztas M, Bedir M: Thickness dependence of structural, electrical and optical properties of sprayed ZnO:Cu films. Thin Solid Films 2008, 516: 1703. 10.1016/j.tsf.2007.05.018View ArticleGoogle Scholar
- Chen M, Pei ZL, Sun C, Gong J, Huang RF, Wen LS: ZAO: an attractive potential substitute for ITO in flat display panels. Mater Sci and Eng B 2001, 85: 212. 10.1016/S0921-5107(01)00584-0View ArticleGoogle Scholar
- Gordon RG: Criteria for choosing transparent conductors. MRS Bull 2000, 25: 52–57.View ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- Kuo SY, Chen WC, Lai FI, Cheng CP, Kuo HC, Wang SC, Hsieh WF: Effects of doping concentration and annealing temperature on properties of highly-oriented Al-doped ZnO films. J Cryst Growth 2006, 287: 78. 10.1016/j.jcrysgro.2005.10.047View ArticleGoogle Scholar
- Kim KH, Park KC, Ma DY: Structural, electrical and optical properties of aluminum doped zinc oxide films prepared by radio frequency magnetron sputtering. J Appl Phys 1997, 81: 7764. 10.1063/1.365556View ArticleGoogle Scholar
- Asmar RA, Juillaguet S, Ramonda M, Giani A, Combette P, Khoury A, Foucaran A: Fabrication and characterization of high quality undoped and Ga2O3-doped ZnO thin films by reactive electron beam co-evaporation technique. J Cryst Growth 2005, 275: 512. 10.1016/j.jcrysgro.2004.12.034View ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- Hu J, Gordon RG: Textured aluminum-doped zinc oxide thin films from atmospheric pressure chemical‐vapor deposition. J Appl Phys 1992, 72: 5381. 10.1063/1.351977View ArticleGoogle Scholar
- Sanon G, Rup R, Mansingh A: Growth and characterization of tin oxide films prepared by chemical vapour deposition. Thin Solid Films 1990, 190: 287.View ArticleGoogle Scholar
- Burstein E: Anomalous optical absorption limit in InSb. Phys Rev 1954, 93: 632. 10.1103/PhysRev.93.632View ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- Tauc J, Grigorovici R, Vancu A: Optical properties and electronic structure of amorphous germanium. Phys Stat Sol 1966, 15: 627. 10.1002/pssb.19660150224View ArticleGoogle Scholar
- 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 2008, 53: 2019.Google Scholar
- Bagnall DM, Chen YF, Zhu Z, Yao T, Shen MY, Goto T: High temperature excitonic stimulated emission from ZnO epitaxial layers. Appl Phys Lett 1998, 73: 1038. 10.1063/1.122077View ArticleGoogle Scholar
- Lin B, Fu Z: Green luminescent center in undoped zinc oxide films deposited on silicon substrates. Appl Phys Lett 2001, 79: 943. 10.1063/1.1394173View ArticleGoogle Scholar
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