3. Results and discussion
3.1. Structural analysis
The crystalline nature and preferred orientation of the AZO films were studied by means of X-ray diffraction. Fig. 2 presents the typical XRD patterns of AZO films deposited using unbalanced RF magnetron sputtering under various external magnetic field strengths (0–6.0 mT). Fig. 2 shows that all the AZO films were oriented along the (002) direction assigned to the ZnO with polycrystalline in nature grown along c-axis vertical to the substrate surface showing the formation of hexagonal wurtzite structure. The films formed at AZO-0 condition showed a weak ZnO (002) reflection near 34.11° 2θ, suggesting their lower crystalline nature. The intensity of the (002) reflection became higher with increase in magnetic field to 3.0 mT (AZO-3.0). In addition to the characteristic ZnO (002) reflection, a weak diffraction peak was observed at 62.3° 2θ, which represents the (103) reflection of ZnO. While the films formed at higher magnetic field of 6.0 mT (AZO- 6.0), the intensity of the (002) reflection became relatively stronger in comparison to that of AZO-0 and 3.0 films. Whereas, under the influence of additional magnetic field there was no noteworthy variation in the (103) reflection. Based on these results, it revealed that the additional magnetic field strongly influenced the intensity of preferential (002) reflection, suggesting that better crystalline films can be achieved by applying of more external magnetic field. It is to be noted that, as the magnetic field increased from 0 to 6.0 mT, the films growth were highly textured towards the c-axis with improved intensity of (002) reflection. The similar results of such an enhancement in the intensity of (002) has been reported by Ke et al. [13] for AlN films using unbalanced DC magnetron sputtering technique. The enhanced tendency of c-axis orientation for AZO films using unbalanced RF magnetron sputtering resembles here the effect of substrate temperature on the film growth [16]. With increasing of solenoid coil current, the magnetic field extends the plasma region towards the substrate and raises the induced ionization, which leads to the improvement in the film quality. The two major advantages behind over the applied external magnetic field are that the plasma density extends deeply towards the substrate, i.e., increases the yield of energetic ion density, which reduces the poor adhesion by impinging on the substrate, resulting in an improvement of crystal quality. The second is that it imparts more kinetic energy to the condensing particles, sequentially there is possibility to raise the temperature on the substrate surface would benefits for the film crystallization along preferential direction. Furthermore, more detailed crystal quality of the deposited films can be estimated from the full width at half maximum (FWHM). The results indicate that the applied magnetic field strongly influenced the FWHM of the (002) re- flection and crystallite size of the films. Fig. 3 shows the dependence of FWHM and crystallite size on applied magnetic field. The FWHM value decreased from 1.06° to 0.78° as the magnetic field increased from 0 to 6.0 mT respectively, indicating the significant improvement of crystal quality through the applying of additional magnetic field. This means that with increase of magnetic field more number of sputtered atoms gets deposited on the substrate surface through the formation of preferred (002) reflection. As a result of that increase in the degree of crystalline nature with applied magnetic field causes to the decrement in the FWHM of (002) peak along with variation in the crystallite size. In addition, the estimated crystallite size of the AZO films can be calculated using Scherrer's relation [17]. The calculated crystallite sizes were found to increase from 9.2 to 12 nm with the magnetic field strength increased from 0 to 6.0 mT respectively. The decreased FWHM accompanied by the improved crystallite size with applied magnetic field can be explained by the variation in microstructure through the formation of better crystalline films.
3. ผล และการอภิปราย 3.1. การวิเคราะห์ The crystalline nature and preferred orientation of the AZO films were studied by means of X-ray diffraction. Fig. 2 presents the typical XRD patterns of AZO films deposited using unbalanced RF magnetron sputtering under various external magnetic field strengths (0–6.0 mT). Fig. 2 shows that all the AZO films were oriented along the (002) direction assigned to the ZnO with polycrystalline in nature grown along c-axis vertical to the substrate surface showing the formation of hexagonal wurtzite structure. The films formed at AZO-0 condition showed a weak ZnO (002) reflection near 34.11° 2θ, suggesting their lower crystalline nature. The intensity of the (002) reflection became higher with increase in magnetic field to 3.0 mT (AZO-3.0). In addition to the characteristic ZnO (002) reflection, a weak diffraction peak was observed at 62.3° 2θ, which represents the (103) reflection of ZnO. While the films formed at higher magnetic field of 6.0 mT (AZO- 6.0), the intensity of the (002) reflection became relatively stronger in comparison to that of AZO-0 and 3.0 films. Whereas, under the influence of additional magnetic field there was no noteworthy variation in the (103) reflection. Based on these results, it revealed that the additional magnetic field strongly influenced the intensity of preferential (002) reflection, suggesting that better crystalline films can be achieved by applying of more external magnetic field. It is to be noted that, as the magnetic field increased from 0 to 6.0 mT, the films growth were highly textured towards the c-axis with improved intensity of (002) reflection. The similar results of such an enhancement in the intensity of (002) has been reported by Ke et al. [13] for AlN films using unbalanced DC magnetron sputtering technique. The enhanced tendency of c-axis orientation for AZO films using unbalanced RF magnetron sputtering resembles here the effect of substrate temperature on the film growth [16]. With increasing of solenoid coil current, the magnetic field extends the plasma region towards the substrate and raises the induced ionization, which leads to the improvement in the film quality. The two major advantages behind over the applied external magnetic field are that the plasma density extends deeply towards the substrate, i.e., increases the yield of energetic ion density, which reduces the poor adhesion by impinging on the substrate, resulting in an improvement of crystal quality. The second is that it imparts more kinetic energy to the condensing particles, sequentially there is possibility to raise the temperature on the substrate surface would benefits for the film crystallization along preferential direction. Furthermore, more detailed crystal quality of the deposited films can be estimated from the full width at half maximum (FWHM). The results indicate that the applied magnetic field strongly influenced the FWHM of the (002) re- flection and crystallite size of the films. Fig. 3 shows the dependence of FWHM and crystallite size on applied magnetic field. The FWHM value decreased from 1.06° to 0.78° as the magnetic field increased from 0 to 6.0 mT respectively, indicating the significant improvement of crystal quality through the applying of additional magnetic field. This means that with increase of magnetic field more number of sputtered atoms gets deposited on the substrate surface through the formation of preferred (002) reflection. As a result of that increase in the degree of crystalline nature with applied magnetic field causes to the decrement in the FWHM of (002) peak along with variation in the crystallite size. In addition, the estimated crystallite size of the AZO films can be calculated using Scherrer's relation [17]. The calculated crystallite sizes were found to increase from 9.2 to 12 nm with the magnetic field strength increased from 0 to 6.0 mT respectively. The decreased FWHM accompanied by the improved crystallite size with applied magnetic field can be explained by the variation in microstructure through the formation of better crystalline films.
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