# Electrochemical Characteristics of Amorphous Ni-P Electroplated Thin Film
## 3 Results and Discussion  
### Morphology, Structure, and Composition of Ni-P Thin Films  
Figure 1 shows the FE-SEM images of the Ni-P thin films (P16, P21, and P28) presented in Table 1. The P16, P21, and P28 thin films contained phosphorus at 16, 21, and 28 at%, respectively, as shown in the composition analysis results (Figure 2a-c). A flat and uniform Ni-P thin film was formed on the substrate in all three cases. From the inset figure of Figure 1, representing the inclined view of the thin film, it was confirmed that all the thin films had a thickness of approximately 100 nm. It is noted that no interfacial cracks between the thin film and substrate were observed in the P28 thin film, while interfacial cracking became more severe as the phosphorus content decreased (see the cracks indicated by the arrow in the inset of Figure 1a,b). The plating metal (Ni) and substrate metal (Cu) have high crystallographic similarities, and plating proceeded at a relatively high temperature (60 \({}^{\circ}\)C). Therefore, the consistency and adhesion between the thin film and substrate were expected to be superior [49]. However, a higher reduction current density applied when a thin film with a lower phosphorus content was created resulted in more hydrogen being generated at the substrate surface. As this hydrogen was adsorbed, more voids were likely to form between the substrate and thin film [50]. These interface voids and contact inferiority could decrease the adhesion between the substrate and thin film.  
Figure 1: Scanning electron micrographs of the nickel phosphide thin films with different phosphorus (P) contents: (**a**) sample P16 (16 at% P), (**b**) sample P21 (21 at% P), and (**c**) sample P28 (28 at% P) in Table 1. Inset figures are the magnified inclined images. (**d**) represents the magnified image and composition of Ni-P electrodepeosits (spot 1) and Cu substrate (spot 2) in sample P28.  
The crystal structure analysis for the Ni-P thin films is shown in Figure 2d. In the specimens of P16 and P21, only the signal for the copper substrate was detected, indicating that they had an amorphous crystal structure, whereas the P28 thin film had an additional tiny signal for the Ni\({}_{3}\)P phase, strongly implying that the deposit was partially crystallized to form in the film crystallite phase adjacent to amorphous regions. According to the literature, when synthesizing Ni-P using electroplating, phosphorus atoms are included in the nickel lattice, preventing the growth of Ni-P crystals. As a result, the Ni-P plating layer gradually becomes a fine-grained or amorphous crystal structure. In particular, when the phosphorus content in the plating layer is 14-20 at% or more, the Ni-P plating layer becomes amorphous [23; 41]. The thin films fabricated in this study contained more than 16 at% phosphorus, and all of them had, overall, an amorphous crystal structure, consistent with the report in the literature. Nevertheless, a more in-depth analysis is needed to explore any presence of crystalline phase in Ni-P electro-deposits with high phosphorus content (e.g., around 26 at% and higher).  
Figure 2: EDS spectra together with chemical composition ratios of the nickel phosphide thin films: (**a**) sample P16, (**b**) sample P21, and (**c**) sample P28. (**d**) represents their XRD patterns.