Annealing Temperature Effect of ZnO Thin Film

In recent years, low temperature techniques have attracted a great deal of attention because of flexible substrate [177] and several other materials developing practical optical devices for the huge future market needs. Deposition and annealing temperatures are both affected by TCO thin film characteristics, depending upon their applications [178-179]. For instance, liquid crystal display (LCD) applications should be lower than 140℃ or 250℃, because the substrate is plastic or glass. Meanwhile, the deposition temperature should be lower than 400℃ for plasma display panels (PDP) applications.

There are more and more researchers interested in chemical solution techniques because it is cheaper, easier, requires low temperature processes, etc. For this reason, annealing (thermal) treatment is necessary to achieve high-performance devices after the fabrication processes of several kinds of optoelectronic devices [180].

Annealing treatment is an important and easy way to improve the material properties [181-182]. The annealing time, temperature and ambient are both critical conditions for annealing treatment. In fact, the annealing temperature is one of the most sensitive factors for effectively qualifying the crystal quality, defect and local structure of semiconductor material during the annealing process [183]. Consequently, we investigated annealing effect by adjusting the annealing temperature range up to 500℃

to better understand the thermal energy effect on the structural and optical characteristics of ZnO thin films in this study.

For testing the annealing effect, we chose the best condition of sputtering, as mentioned above, to prepare ZnO thin film. The details of this condition are: RF 400 W, 15 mtorr, O2/(Ar+O2) ratio of 0.30 and 10 minutes for deposition time, respectively. To comprehend the difference of various annealing temperature effects, the film thickness is fixed at 200 nm ± 5% for approximation in practical applications. The specimens are subsequently annealed in a high-temperature oven for 1 hour at 200-500℃ with 3℃/min. of heating rate and allowed to return to RT slowly. The detail of the test conditions for annealing temperature effect is listed in Table 4.11.

4.7.1. Structural Characteristics

Figure 4.41 exhibits the XRD pattern of the ZnO thin films taken at different annealing temperatures. All of these specimens are poly-crystalline, with its c-axis

normal to the substrate surface as shown in Fig. 4.41(a). Results demonstrated that the (0002) diffraction peak possesses a remarkable improvement at the intensity and the angle of diffraction peak, respectively. In addition, we zoomed in on the (0002) diffraction peak to clearly indicate this point, as illustrated in Fig. 4.41(b). There are two main phenomena that we observed. For one thing, the intensity of the diffraction peak increases with increasing the annealing temperature [182, 184]. This is because the thermal energy promotes the surface mobility of adatoms and reduces surface energy to occupy the correct site or re-grow in the crystal structure by higher annealing temperature [185]. Therefore, the structure’s internal atoms rearrange its lattice constant, causing it to produce the period arrangement/diffraction patterns to enhance the intensity of the diffraction peak. But the raise rate of intensity gradually slows as the annealing temperature increases. This means that higher annealing temperature can be effectively improved to high crystalline quality, but the temperature is not without limits.

Figure 4.42 displays the ZnO (0002) diffraction peak shift as a function of

annealing temperature. The (0002) diffraction peaks obviously shift to higher 2θ angles below 300℃ of annealing temperature, and then maintain a constant 2θ angle however much the annealing temperature increased. Based on this information, we imagined that a moderate annealing temperature is necessary for obtaining the optimum material

properties. Liu et al. [183] argues that ZnO films experience compressive in-plain strain attributed to lattice misfit [59] when the thickness is below 200 nm, which is consistent with this study. Hong et al. argue that the residual stress in c-axis direction is partly relaxed due to the gradual increase of the 2θ angles with increasing annealing temperature [186]. Likewise, the lattice constant (interplanar spacing) decreases with increasing annealing temperature, also consistent with the stress relaxation as shown in Table 4.12 [185]. Incidentally, the lattice constant is calculated by Bragg’s law and is

closed to the bulk ZnO lattice constant (about 5.2069 Ǻ) [70] after annealing treatment.

However, the as-grown ZnO thin film was transited from the compressive to tensile one to raise the 2θ angle and reduce the lattice constant by using annealing treatment.

The influence of grain size and FWHM of ZnO thin films as a function of annealing temperature is illustrated in Fig. 4.43. The film quality evaluated from the FWHM of (0002) diffraction peak is enhanced by annealing treatment [182, 184-185, 187]. The minimum value of FWHM was observed above 300℃ of annealing

temperature. All of these specimens indicated that the value of FWHM is very close when the annealing temperature over 300℃. It is consistent with the result of (0002) diffraction peak shift, as mentioned above. Additionally, the annealing temperature provides thermal energy to obtain the larger grain size by the re-grow mechanism better

than as-grown ZnO thin film. However, the largest grain size appeared at 400℃ of annealing temperature.

In conclusion, the smallest value of FWHM, largest grain size and best relaxation stress were simultaneously found by annealing treatment. But the moderation annealing temperature is an important key to improve the film quality, defects, surface roughness, etc. It shows significant improvement below 300℃, whereas it has no obvious variance over 400℃ [179]. Finally, we deduced that the best annealing temperature is 400℃.

4.7.2. Optical Characteristics

Figure 4.44 shows transmittance of pure ZnO thin film as a function of annealing

temperature. Regardless of as-grown or annealed specimens, the highly transparent (~90%) and UV-shielding characteristics (below 16%) can be observed in Fig. 4.44(a).

Actually, the average transmittance of ZnO thin film slightly increases by increasing the annealing temperature. This improvement of transmittance may be attributed to the crystalline and surface roughness [179]. Defects and surface roughness induced various non-radiative centers and reduced light emission from the ZnO thin films. In addition to H. Li et al. report [185], the grain growth and the reduction of grain boundary density induced decreasing optical scattering. In Fig. 4.44(b), a blueshift phenomenon occurred near the UV edge when the annealing temperature increased. It is believed to originate from the residual stress along the c-axis due to lattice distortion [186]. Moreover, the

UV-shielding characteristics seem to improve at higher annealing temperatures. But these variances of optical characteristics make only a little difference for annealing treatment.

Ultimately, the annealing treatment is a critical method for improving the material properties of thin film structures, such as film quality, stress, grain size, transmittance and UV-shielding characteristics. Consequently, we determined that the best annealing temperature is about 400℃ for the glass substrate.