## 4 Results and Discussion
It is seen that the intensity of the (002) peak increases as the annealing temperature is increased to 500 \({}^{\circ}\)C, but then decreases as the temperature is further increased to 600 \({}^{\circ}\)C.  
Figure 8 presents top-view SEM micrographs of the ZnO films shown in Figure 7. As shown in Figure 8a, the non-annealed sample has a flaky surface morphology with a small crystal size. For the sample annealed at 300 \({}^{\circ}\)C, the crystal size increases as a result of grain growth (Figure 8b). As the annealing temperature is increased to 400 \({}^{\circ}\)C, the ZnO grain size continues to increase. However, some pores are evident in the crystal structure (Figure 8c). For the sample annealed at 500 \({}^{\circ}\)C, the ZnO grain size increases once again. Moreover, the crystal structure is more dense, and fewer voids are observed (Figure 8d). Finally, at an annealing temperature of 600 \({}^{\circ}\)C, the ZnO grain size continues to increase and the crystal structure becomes even more dense (Figure 8e). Overall, the results presented in Figure 8 show that both the grain size and the densification of the ZnO film increase with an increasing annealing temperature. As a result, the thermal conduction performance of the ZnO sensing layer improves, leading to an enhanced sensing performance. However, despite the better grain growth at 600 \({}^{\circ}\)C (see Figure 8e), the XRD results presented in Figure 7 show that the intensity of the (002) peak at 600 \({}^{\circ}\)C is lower than that at 500 \({}^{\circ}\)C. Thus, the optimal annealing temperature was determined to be 500 \({}^{\circ}\)C.  
Figure 7: XRD patterns of as-deposited ZnO film and ZnO films annealed at temperatures in the range of 300–600 \({}^{\circ}\)C.  
The element composition of the ZnO film annealed at 500 \({}^{\circ}\)C was examined by energy-dispersive X-ray spectroscopy (EDS). The corresponding results are presented in Figure 9 and show that the film consists of 22.39 wt. % O and 77.61 wt. % Zn. Notably, the stoichiometry ratio of Zn to O is not 1:1 since the annealing environment is not oxygen free and hence the content of O atoms is slightly increased. Figure 8: SEM micrographs showing sub-micrometer grain size of ZnO thin films annealed at different temperatures: (**a**) non-annealed, (**b**) 300 \({}^{\circ}\)C, (**c**) 400 \({}^{\circ}\)C, (**d**) 500 \({}^{\circ}\)C and (**e**) 600 \({}^{\circ}\)C.  
Figure 9: EDS analysis results of sputtered ZnO. Figure 10 shows the experimental results obtained for the voltage responsivities of the ZnO pyroelectric sensors with different substrate thicknesses of 1, 100, 200, 300, 400, and 500 \(\upmu\)m, respectively, at cutoff frequencies in the range of 0-3500 Hz. For comparison purposes, the responsivity of a packaged sensor is also shown. Note that the voltage responsivity is defined here as the ratio of the output voltage of the sensor to the input power of the incident heat source. As expected, the voltage responsivity increases with a reducing substrate thickness as a result of the greater temperature variation within the ZnO film. From inspection, the maximum voltage sensitivity is equal to 3800 mV/mW and is obtained for a substrate thickness of 1 \(\upmu\)m and a cutoff frequency of 400 Hz. It is noted that the packaged sensor has a low voltage responsivity of just 3100 mV/mW due to connection losses and internal resistance losses, respectively. Figure 11 shows the output voltages of the various sensors at a cutoff frequency of 400 Hz for various distances of the sensor from the infrared power source in the range of 0-200 cm. For distances of less than 100 cm, the packaged sensor provides the highest voltage output among all the sensors. However, the sensor with a ZnO thickness of 1 \(\upmu\)m also provides a reasonable response at sensing distances less than approximately 25 cm. For all of the sensors, the output voltage reduces to zero as the sensing distance increases beyond 100 cm.