## 3 Results and Discussion  
### Composition and Phase Analysis of the Film  
It has been reported that the absorptivity of single-layer TiAlN film is about 0.8 [12], and TiAlON film has low reflectivity (below 16%) and high absorptivity in the visible-ultraviolet spectrum. Photothermal conversion performance of titanium nitride coatings can be tailored by controlling the stoichiometric ratio of Ti and N, which affects the free electrons density of the Ti d-band [13]. So, the overall proportion of N and O elements, that is compounds in the film, play a key role in determining the performance of photothermal conversion. Depositing the TiMoAlON film controlled by the N\({}_{2}\)-O\({}_{2}\) dual-gas reactive sputtering system, the ratio of N/O is varied adaptively with sputtering yield of Al (or Ti), so by changing the target current of Al (or Ti), different metallic compounds are finally obtained. Doping N or O elements in TiN\({}_{\rm x}\) matrix can change the electronic band structure. When the proportion of O atoms comes up to a certain ratio in TiON matrix, the nitride dielectric layer will turn into oxide. For that, each kind of compound (nitride, oxide) has a different absorption edge, and if any nonstoichiometric compounds can be regulated and deposited, an absorption plateau will appear in the absorption curve. In view of the above, Al content is set as variable to deposit the film. The ratio of N/O changes with Al content, as shown in Figure 4. Increased Al target current tends to form metallic oxide. Al content increases to 12 at %, and the ratio of N/O is nearly 0.1. Nitride is continuously changed to oxide. The Al atom can capture the O atom to establish a new equilibrium point for the feedback system. So, different proportions of nitride and oxide in each sub-layer can be achieved; accordingly, the refractive index (n) and extinction coefficient (k) of the whole film can be changed gradually due to the type of gradient material (nitride, oxynitride, oxide) [13]. The refractive index decreases from the bottom to the surface. Doping O, N, and Al elements into TiN\({}_{\rm x}\) matrix, the k value is continuously adjusted; that is, absorption peaks will shift towards short wave with the increase in doping elements. So, overlapped impurity energy level can broaden the absorption peak. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & \multicolumn{8}{c}{**Deposition Parameters**} \\ \cline{3-8}  & & \multicolumn{2}{c}{**Duration/s**} & \multicolumn{2}{c}{**Ion Current of Targets/A**} & \multicolumn{2}{c}{**PEM Set-**} & \multicolumn{2}{c}{**O\({}_{2}\) Flow**} \\ \cline{3-8}  & & & **Mo** & **Al** & **Ti** & **Point/\%** & **Rate/Sccm** \\ \cline{2-8} Sub-layer & infrared layer (Ti) & 300 & 0 & 0 & 4.0 & 100 & 0 \\  & main absorber (Mo-TiAlN) & 900 & 1.0 & 0\(\rightarrow\)0.6/0.9/1.2 & 4.0 & 50 & 0 \\  & secondary absorber (Mo-TiAlON) & 360 & 0.6/1.0/1.2\(\rightarrow\)0 & 0.9 & 4.0 & 50 & 0\(\rightarrow\)8.5 \\  & antireflective layer (Al\({}_{2}\)O\({}_{3}\)) & 180 & 0 & 3.0 & 0 & OFF & 8.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Process parameters for depositing Ti/Mo-TiAlN/Mo-TiAlON/Al\({}_{2}\)O\({}_{3}\) film. Figure 4: Effect of target current on Al content and the ratio of N/O. The film is designed to use in non-vacuum and high-temperature environment, and slow oxidation at high temperatures is inevitable. Besides that, the oxygen atom will diffuse among each sub-layer. For long-time application, composition homogenization will narrow the intrinsic absorption limit. So, doping Mo element in the film to form a ceramic-metal composite can offset the absorption reduction caused by oxidation. As seen in Figure 4, as Mo content increases in the film, the ratio of metallic elements (Ti,Al) and non-metallic elements (N,O) changes slightly, as shown in Figure 5. That implies that Mo does not participate in the reactive sputtering process. They possibly exist in each layer in the form of metal Mo, as demonstrated by XRD results.