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.
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.
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