## 3 Results and Discussion
### Morphologies and Thickness of the Aerosol-Deposited ZnO Films  
The effects of deposition cycles on the morphology and thickness of ZnO films were characterized using FESEM and the results are shown in Figure 5. From the SEM image shown in Figure 5a, a rather smooth, compact, and dense surface is observed with some remnant particles on the surface. The cross-sectional view in Figure 5b shows that the deposited ZnO film has a rather uniform thickness with an average of 271 \(\pm\) 44 nm (\(n=10\)). In the initial stage of AD, formation of a stable anchor layer at the substrate interface is important prior to subsequent deposition of dense film. During the anchor layer formation, a sufficient amount of independent particles plays a vital role in forming the initial anchoring layer [39]. More remnant agglomerated particles were also observed on the surface of AZ-II as highlighted in circles in Figure 5c. The thickness of ZnO film obtained was slightly reduced to 249 \(\pm\) 51 nm as shown in Figure 5d. Further increase in the deposition cycle to three cycles led to the formation of large particle agglomerates as shown in Figure 5e. The surface appearance of the AZ-III films was less dense, with rough surfaces observed. The thickness of ZnO film was 258 \(\pm\) 51 nm as shown in Figure 5f. From the cross-sectional SEM images shown in Figure 5f, the thickness was less uniform as compared to AZ-I and AZ-II. It is thought that the abrasion of the deposited ZnO films by the subsequent deposition cycle caused the reduction in thickness. During the aerosolization process in the rotating aerosol chamber, agglomeration of the ZnO particles occurred forming submicron-sized agglomerates due to Van der Waals interaction. The injection of agglomerated ZnO particles could cause film abrasion. Therefore, the thickness of the AZ films decreased during the second deposition cycle and increased slightly after the third deposition cycle as shown in Figure 6. Nevertheless, the slight increase in thickness of AZ-III could be due to process variation, or the formation of many agglomerated particles that are observed on the surface of AZ-III as shown in Figure 5e,f.  
Figure 4: Crystallize size of ZnO powder and the aerosol-deposited ZnO films.  
The reduction in thickness and the generation of rough surface due to particles' agglomeration of the surface of the deposited ZnO films could be elucidated as the schematic shown in Figure 7. The subsequent bombardment of ZnO particles onto the film could cause etching and abrasion of the outermost layer during the 2 nd cycle or 3 rd cycle. With a prolonged cycle, ZnO particle agglomeration is thought to occur during the aerosolization process. These agglomerates would cause higher momentum transfer resulting in surface abrasion or etching. The rough surface and loose particles found particularly on the surface of AZ films which are shown in Figure 5c,e, are illustrated and shown as Figure 7b,c. The abrasion of the surface by ZnO particles or agglomerates caused the generation of particle agglomerates and coarse surface after repetitive cycles. This observation is aligned with the finding of [PERSON] et. al. as they mentioned that the film thickness is influenced by factors such as the transport of the aerosol droplets to the substrate, concentration of particles used and aggregation of the aerosol droplet [40].  
Figure 5: FESEM images showing the surface morphology of (**a**) AZ-I, (**c**) AZ-II, (**e**) AZ-III films, and cross-sectional view of (**b**) AZ-I, (**d**) AZ-II and (**f**) AZ-III films produced under different deposition cycles.