## 3 Results and Discussion
### Microstructure Characterization  
The surface SEM images of Ti/(Mo-TiAlN)/(Mo-TiAlON)/Al2O3 films before and after baking are shown in Figure 7. The film surface is coarse in Figure 7a. After baking at 500 \({}^{\circ}\)C in atmosphere for 8 h, unreacted Al is oxidized and the surface Al2O3 layer becomes more pure. The surface layer becomes fine and compact, as seen in Figure 7b.  
The cross-sectional SEM image of the Ti/(Mo-TiAlN)/(Mo-TiAlON)/Al2O3 tandem absorber is shown in Figure 8. An obvious multi-layer structure that matches the theoretical design (as seen in Figure 1) is obtained, which indicates that the PEM-controlled reactive sputtering method is feasible to deposit the multi-layer compound film within the nanoscale. It clearly shows that the tandem absorber consists of four sub-layers. Above the substrate, the complex films include an infrared reflectance layer, interference absorptive layers with different metal volume ratios, and an anti-reflectance layer. The layer thicknesses of Ti, Mo-TiAlN, Mo-TiAlON, and Al2O3 were found to be approximately 100, 300, 200, and 80 nm, respectively. A compact Al2O3 layer is coated on the surface, which act as an antireflective layer and diffusion barrier of O atoms.  
Figure 8: Cross-sectional morphology of TiMoAlON selective absorbing films.  
Figure 7: Surface SEM images of TiMoAlON film (**a**) before and (**b**) after baking.  
Figure 9 shows TEM images of TiMoAlON film. Above the infrared reflectance layer (Ti layer), there are distinct columnar grains in the tandem absorber layers. Doping with Mo element, there are many dark accumulations aggregating in the columnar crystal, as shown in Figure 9b, the \(d\) value of which, calculated from HRTEM data in Figure 9c, is 0.2185 nm. It is a close match with body-centered cubic Mo(110), and its grain size is 3-5 nm, statistically. Mo nanocrystalline tends to aggregate in the center of TiN grain, and an undoped region exists between two neighboring columnar crystals, as seen in Figure 9c. This structure can form a light trap for certain wavelengths of light, and multiple reflections between Mo aggregating zones extend the optical travelling path and enhance the absorption.  
In columnar crystals, lattice fringes spacing is 0.1483 nm, which is a pretty good match with TiO\({}_{2}\) (200). Because \(d\) values of TiAlN and TiAlON are close to TiN, it is difficult to differentiate the dielectric materials, but substitution of solid solution (Ti,Al)(N,O) compound is quite possible. The selected area diffraction pattern of zone A in Figure 9d shows a spotty ring pattern, and some rings are undistinguished because TiN(220) and Mo(200), TiO(200), and Mo(110) have similar \(d\) values, respectively.  
When Mo content is less than 3 at.%, they do not aggregate, as mentioned before, instead, they form a nano-scale multilayer film. Metal Mo is interspersed in the Ti(Al,N) and (Ti,Al)(N,O) compounds matrix. From the bottom up, they are gradually decreased, as seen in Figure 10. Above zone C, oxides act as the main matrix materials. The gradient structure is obvious: it can fulfill photothermal conversion when exposed to light, which goes through the film in a top-down approach, just like a process of light filtration. Although increasing Mo content does not form a columnar structure in the dielectric matrix, the multi-layer structures also present a high absorption property due to continuous reflection among the neighboring layers.  
Figure 9: TEM images of TiMoAlON selective absorbing films. (**a**) Cross-sectional microscope, (**b**) HRTEM image of columnar grain, (**c**) HRTEM image of the zone between two neighboring columnar crystals, and (**d**) SAED pattern of zone A.