## 4 Sensing Material Characterization  
### Analysis of Sensing Material  
The sensing material should be defect-free, surface-defect-free, and exhibit high mobility to achieve high sensitivity and fast response times. Additionally, the sensing material should reach an immediate steady state in carrier perturbation during UV-C light exposure. The sensing material is analyzed using XRD, SEM, and EDX. The presented figure provides a comprehensive analysis of HfO\({}_{2}\) thin films deposited using ALD under varying annealing conditions. Figure 7a displays XRD patterns for films annealed at 400 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, and 600 \({}^{\circ}\)C, as well as an unannealed sample. The XRD patterns show distinct peaks at 600 \({}^{\circ}\)C, indicating pronounced crystallinity with reflections from various crystallographic planes, such as (111), (200), (002), (220), and (311). This confirms the formation of a well-defined polycrystalline structure at higher temperatures. In contrast, the unannealed film exhibits no distinct peaks, consistent with an amorphous phase. The films annealed at 400 \({}^{\circ}\)C and 500 \({}^{\circ}\)C show intermediate characteristics, with the 500 \({}^{\circ}\)C annealed sample exhibiting faint but noticeable peaks, particularly around the (111) and (200) planes. While the crystallinity at 500 \({}^{\circ}\)C is not as prominent as at 600 \({}^{\circ}\)C, these peaks suggest the initiation of partial crystallization. The SEM images further illustrate the surface morphology of the HfO\({}_{2}\) thin films deposited via ALD. At 400 \({}^{\circ}\)C in Figure 7b, the film exhibits a uniform, smooth, and dense morphology with minimal grain growth, reflecting its predominantly amorphous structure--features typical of ALD films formed at lower temperatures. By comparison, the film annealed at 500 \({}^{\circ}\)C (Figure 7c) shows a slightly rougher surface with discernible contrast variations, indicating the onset of grain boundaries and partial crystallization. This morphological change at higher temperatures highlights the increased crystallinity and grain development, which may improve properties such as the dielectric constant but could also influence leakage current behavior. In this study, 500 \({}^{\circ}\)C was identified as the optimal deposition temperature, balancing partial crystallinity with the piezoelectric properties of the lithium niobate (LN) substrate. The LN substrate is highly sensitive to elevated temperatures, losing its piezoelectric properties beyond 500 \({}^{\circ}\)C [23]. Therefore, 500 \({}^{\circ}\)C was selectively chosen as the most suitable annealing temperature for this investigation. Figure 7d presents EDS data for the sample annealed at 500 \({}^{\circ}\)C, confirming its elemental composition. The analysis reveals significant peaks for Hafnium (Hf), Oxygen (O), and Silicon (Si), consistent with a stoichiometric HfO\({}_{2}\) composition. Elemental quantification indicates the presence of Hafnium at approximately 30.65% by weight, supporting the high purity and controlled composition of the thin films.  
Figure 7: HfO\({}_{2}\) thin film characterization results. (**a**) XRD, (**b**,**c**) SEM images at 400 \({}^{\circ}\)C and 500 \({}^{\circ}\)C, and (**d**) EDS spectrum analysis.