1. Introduction

ZnO thin film has attracted a great deal of attention as a potential material because of its merits of a direct band gap of 3.37 eV at room temperature and a large excitation binding energy^[1-4]. Due to its excellent electronic performance and high transparency of ZnO to visible light, it can be widely used from the active region of the transparent electronics which is established as one of the most promising technologies for leading the next generation of flat panel displays to the buffer layers of solar cell^[5-7]. The lattice mismatch and different thermal expansion coefficients between ZnO films and substrates result in a residual stress existing in the deposited ZnO films. Usually, conventional rapid thermal annealing is used to reinforce the stability of the films and to reduce the possible undesirable influence of the surface^{[8, 9]}. However, the thermal budget of semiconductors on a low cost substrate, such as glass, is much smaller than the conventional semiconductor, like a silicon device. Therefore, the key issues here are to choose a low temperature process which can fabricate ZnO thin film with highly smooth surfaces and to develop a low temperature annealing process that can further improve the characteristic of the ZnO thin film.

We used atomic layer deposition (ALD) to deposit high crystal quality ZnO films, because compared with the other method^[10], it can precisely control the deposited film thickness at the monolayer level, fabricate the ZnO film at a relatively low process temperature and manipulate the crystal growth direction by changing the deposition temperature^[11]. Further, for effective improvement of the crystallinity of ZnO films, numerous research works have indicated an optimum rapid thermal annealing temperature range of 600-1000 ℃, which limits the use of RTA when using low cost substrates ^{[8, 11-16]}. There is a nonclassical heating method using microwaves that has attracted a lot of attention for its ability to fabricate a silicon device under the 22 nm node. Compared with conventional RTA, the new process shows a clear advantage for its features of a low process temperature and a high crystallization rate^[17-19]. Due to the characteristics of this method, it is attractive and suitable to be involved in the fabrication of ZnO-based semiconductors.

In this work, we study the influence of microwave annealing (MWA) on the structural and optical properties of ZnO films by the X-ray diffraction (XRD) and photoluminescence (PL) techniques. The mechanism of MWA will be briefly studied by analyzing the observed XRD patterns and PL spectra.

2. Experimental

After surface cleaning, ZnO thin films were grown on glass substrates using the PE-ALD technique by KE-MICRO T-ALD-150A. DEZn and deionized water, which were used as precursors for zinc and the oxidant, were respectively fed into the chamber through separate pipelines. One ALD cycle of ZnO contains four steps sequentially: (1) DEZn pulsed into the chamber, (2) N$_{2}$ purge (to bring the redundant DEZn out), (3) H$_{2}$O pulsed into the chamber, and (4) N$_{2}$ purge (to bring the redundant H$_{2}$O out). The precursors were pulsed into the chamber through transporting nitrogen of high purity (99.999%), and the chamber pressure is 0.15 Torr. The pulse time for DEZn and DI water were 0.02 s and 0.015 s, respectively. The evacuation time between the precursors is 25 s for the purpose of removing excess precursors. The substrate temperature of 150 ℃ was the temperature at which the growth rate of ZnO was about 0.14 nm per cycle. The detailed process information is listed in Table 1. After the growth of 400 cycle, microwave annealing was used in an ambience of air at microwave powers of 1000 W and 2000 W. For comparison, a reference sample was annealed by rapid thermal annealing (RTA) at 500 ℃ in an ambience of air. The temperature of the samples was monitored by an infrared pyrometer at the top of the MWA chamber. Figure 1 shows the plot of the annealing temperature, with a schematic of the MWA equipment in the inset. As previous reports have shown^[18], the positions of the quartz and the susceptor are essential to keep the annealing temperature uniform.

Then X-ray diffraction (XRD) and photoluminescence (PL) analyses were performed to investigate the crystallinity and optical properties of the ZnO thin film.

3. Results and discussion

The crystal structures of as-deposited films are analyzed by XRD. Figure 2 displays the XRD patterns of the as-deposited and the annealed ZnO films with different annealing conditions from 30° to 40°. It is clear that all samples deposited by PEALD in the substrate temperature of 150 ℃ show two dominant XRD peaks, i.e. (100) and (002).

The (002) peak position of ZnO films using different post annealing methods is listed in Table 2 and plotted in Fig. 3(a). Since the (00) peak position of strain-free ZnO films is 2$\theta$ $=$ 34.43°^[20], the fluctuation of the (002) peak position indicates the existence of residual stress between the ZnO film and the glass substrate, which is the combined effect of thermal stress and intrinsic stress^{[13-15, 20-22]}. Because the thermal expansion coefficient of ZnO film is bigger than that of glass substrate, there is a tensile stress in the ZnO films as the substrate cools down from high temperature to room temperature, while in Refs. [13, 14, 22], it has shown that the intrinsic stress of the as-deposited ZnO films is compressive. The (002) positions of the samples annealed at RTA 500 ℃, MWA 1000 W and MWA 2000 W are located at 2$\theta$ $=$ 34.423°, 2$\theta$ $=$ 34.398° and 2$\theta$ $=$ 34.446°, respectively. The shift of the (002) position indicates both RTA and MWA reduce the residual stress and MWA 2000 W exhibits an even a bigger deviation from the powder value than RTA 500 ℃. As the previous reports have shown, the (002) peak position increases with the annealing temperature^{[8, 12, 20]}, so MWA 2000 W with the maximum temperature of 434 ℃ shows the same annealing effect with an RTA temperature of greater than 500 ℃.

The estimated values of the stress in ZnO films are reported in Table 2 and plotted in Fig. 3(b). The biaxial film stress $\sigma$ can be calculated by the formula^[14]:

$\begin{equation} \sigma=[2C_{13}-(C_{11}+C_{12})C_{33}/C_{13}](c-c_{\rm o})/c_{\rm o}, \end{equation}$

(1)

where $c_{\rm o}$ (0.5215 nm) is the strain-free lattice constant, and $C_{ij}$ is the elastic stiffness constant. For ZnO, $C_{13}$ $=$ 106.1 GPa, $C_{11}$ $=$ 207.0 GPa, $C_{12}$ $=$ 117.7 GPa and $C_{33}$ $=$ 209.5 GPa. Obviously, the residual stress of the as-deposited ZnO film is compressive. By annealing at RTA 500 ℃ and MWA 1000 W for 100 s, the residual stress of the ZnO films was relaxed. While increasing the MWA power to 2000 W, the tensile stress becomes stronger than the compressive stress and changes the direction of stress. Comparing with RTA 500 ℃ and the previous reports^{[14, 20, 23]}, MWA shows a more effective stress relaxation than RTA at the lower temperature.

Assuming a homogeneous strain across the ZnO films, the average grain size can be estimated from the full-width at half-maximum (FWHM) of the (002) peak by Sherrer's relation^[22]:

$\begin{equation} D=0.9 \lambda/B \cos \theta, \end{equation}$

(2)

where $D$, $\lambda$, $\theta$ and $B$ are the grain size, the X-ray wavelength of 0.15406 nm, the Bragg diffraction angle and the FWHM of the diffraction peak of the (002) direction for ZnO films, respectively. The FWHM of the diffraction peak of the (002) and (100) direction is plotted in Fig. 4(a). The calculated grain size is listed in Table 2 and plotted in Fig. 4(b). It is obvious that the grain size at the lower temperature of MWA 2000 W is bigger than RTA. As has already been reported^{[20, 23]}, the grain size of ZnO thin film increases with the RTA post annealing temperatures. It is reasonable to say that MWA with a lower temperature (2000 W, max temperature: 434 ℃) has the better structure improvement than RTA (max temperature 500 ℃).

Figure 5 shows the PL spectra of the as-deposited and the ZnO thin films annealed for a fixed 100 s at different annealing conditions in the wavelength from 340 to 630 nm. What should be noted is that the PL spectra have been normalized with the UV peak amplitude to facilitate a comparison. It is clear that the green luminescence (about 490-530 nm) of ZnO thin film annealed by MWA 2000 W is smaller than that by RTA 500 ℃. It is consistent with the previous reports^{[12, 23, 24]} that the intensity of the green luminescence of ZnO thin film increases as the annealing temperature increases in the same ambient and further, that the green luminescence is strongly related with the intrinsic defects, such as oxygen vacancies^[23], and the antisite defect^[24]. Therefore, it could be considered that the green luminescence of ZnO films annealed by MWA is mainly affected by the thermal effect of microwave heating.

Next, we discuss briefly the different annealing effect between MWA and RTA, whichis possibly related to the different heating mechanism. The energy transmission of MWA is produced by energy loss mechanisms of the electric vector, so the microwave heating is volumetric, while that of RTA are conduction and convection processes. The absorption of microwave power by solid materials is generally governed by their magnetic, dielectric, and conductive properties. For the as-deposited undoped ZnO film here, the magnetic susceptibility and conductivity are negligible. The significant microwave loss mechanisms in ZnO thin films are polarized. Microwave volumetric heating occurs in ZnO thin films as a result of interfacial (space charge) and reorientation polarization. The defects such as oxygen vacancy, zinc vacancy, and interstitial zinc can act as the dipole under the microwave irradiation, enhancing the absorption of microwaves, while glass cannot be heated directly by microwaves. Consequently, the temperature gradient results that the glass endures a relatively small temperature and ZnO thin films have a higher local temperature. Therefore, microwave annealing, a kind of volumetric and selective heating method, could improve the crystal quality of ZnO films at an overall lower temperature.

4. Conclusion

In conclusion, microwave annealing can be used to improve the characterization of ZnO films on glass. Annealing for 100 s at a microwave power of 2000 W (a maximum temperature of 434 ℃) shows a strong intensity and narrow FWHM of the (002) XRD peak. Compared with the as-deposited sample, the MWA can relax the residual compressive stress and improve the crystal quality of the ZnO film. MWA 2000 W shows a lower intensity of green luminescence than RTA 500 ℃. We can say that the MWA shows a better crystal quality improvement and a lower defect introduction in air. The presented samples indicate that the microwave annealing process provides a broad scope in the future development of devices, which is built on the substrate with a relatively low heat resistance.

Acknowledgements: This work was helped by Master Feng Jiaheng.