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 (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).
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