Design and Fabrication of Si/ZnO Multilayer Films Based on Energy Band Structure Characteristics

Objective Infrared high reflective materials are widely employed to reduce surface emissivity. According to Kirchhoff's law, increasing the reflectivity of a material in the atmospheric window of mid-and far-infrared wavelengths can reduce the thermal radiation intensity of an object, thus decreasing the radiation difference between this object and surrounding environments. As a periodic structured functional material, photonic crystal ( PC) has been extensively studied due to its extremely high infrared reflectivity and spectral compatibility. Various schemes have been designed in terms of PC film thickness and periodic structure to improve its forbidden band width and reflectance. However, there is a challenge to designing one-dimensional PCs for achieving the infrared high reflectance in 3-5 mu m and 8-14 mu m while minimizing the number of layers as much as possible. Therefore, this paper hopes to broaden the photonic forbidden band by constructing PC energy bands and adopting new material systems. Methods Due to the action of the periodic potential field in semiconductor materials, electrons will form band structure and energy gaps exist between bands. However, photons in the periodic arrangement of dielectric materials will change their propagation properties and form a similar band structure. Based on Maxwell's equation, the propagation characteristics of electromagnetic waves in one-dimensional PCs are equivalent to superposition in multiple monolayer media. Since the wave vector k outside the Brillouin region is repeated, when the light wave reaches the boundary of the region, it is reflected back to the Brillouin region. After repeated reflections, a standing wave is formed, which constitutes the photonic band gap region. The upper and lower frequency regions are completely separated by the standing wave to form a photonic band gap. The light waves in the band gap cannot propagate, so the band gap in PC means high reflectance. Based on this, the transmission matrix of light waves is derived, and the PC band structure and band gap reflectance are calculated by MATLAB and CST software. According to the calculated results, the parameter is optimized and the new material system is adopted to design the one-dimensional PC model with better performance. The sample is prepared by the magnetron sputtering method for experimental verification. Results and Discussions Firstly, the optical properties of monolayer SiO2, ZnO, and Si films at room temperature are compared and analyzed (Fig. 3). SiO2 has a low refractive index at 3-5 mu m, it is suitable as a dielectric layer in PCs. However, when the refractive index and extinction coefficient increase sharply at 8-14 mu m, the PC reflectance with SiO2 as the low refractive index layer decreases greatly. The refractive index and extinction coefficient of ZnO vary less in the band of 2-14 mu m, and it has a smoother reflectivity in 8-14 mu m when employed as a low refractive index layer in PC (Fig. 7). In addition, the combination of one-dimensional PCs with different center wavelength structures can achieve 3-5 mu m and 812 mu m band infrared compatible high reflection. Based on this, 9-17 layers of PCs are designed and their infrared reflectances are compared ( Table 1). Considering the performance of PCs and the process complexity and cost of multilayer film preparation, a 13-layer Si/ZnO one-dimensional PC is designed. The photonic band gap can be adjusted by changing the thickness of the film layer. Comparing the calculation results, it is found that the bandwidth range of each layer is optimal at one-quarter wavelength optical thickness. The structure is optimized and the final designed PC structure is shown in Fig. 7(a). The relations of the reflection spectrum with incident angle (Fig. 8) and the electric field intensity distribution of incident electromagnetic wave in PC (Fig. 9) are calculated, indicating that the structure possesses a very high infrared reflectance while being stable to the incident angle. Conclusions In this paper, a new one-dimensional PC for infrared high reflectance is designed based on the energy band theory. According to Maxwell's theory, the reflectances of the 3-5 mu m and 8-12 mu m forbidden bands of PC under the dispersion conditions are derived and calculated. A comparison of two material systems, Si/ZnO and Si/ SiO2, reveals that the material with smaller dispersion can form more stable photonic forbidden bands. The selection of Si/ ZnO is beneficial to achieve high infrared reflectivity in the 3-5 mu m and 8-12 mu m forbidden bands. Finally, a 13-layer Si/ZnO onedimensional PC is designed and prepared. The results show that the reflectance is greater than 91. 3% in the infrared bands of 3-5 mu m and 8-12 mu m. The experimental results are in good agreement with the simulation results, which verifies the high reliability of the model and theory.