Si/SiO2 multilayer -: a 1-dimensional photonic crystal with a polaritonic gap

The silicon-silicondioxide system is used to illustrate the effect of interaction between a photonic gap in a periodic structure and a polaritonic gap originating from one of the constituent materials. Si is a near ideal dielectric material in the infrared region with a high refractive index and modest dispersion for lambda>4 m. Amorphous SiO2 has lattice absorption in the infrared, with a strong Reststrahlen band covering the wavelengths 8-9.3 mum. Optical multilayer calculations of reflectance spectra for Si/SiO2 double- and multilayers have been made. The results illustrate the effect of the metal-like optical properties of SiO2 in the Reststrahlen region. The high reflectance band persists in thin double layers and combines with conventional interference in the dielectric Si-film. From conventional optical coating technology it is known since long that a dielectric coating can be used to broaden and strengthen a Reststrahlen band, but this has not previously been applied to photonic crystals. For the experimental part, the Si/SiO2-system was prepared using standard microelectronic fabrication technology. Polycrystalline Si (poly-Si) and amorphous SiO2 (a-SiO2) were both deposited by CVD processes. Si from silane, and SiO2 from decomposition of tetra-ethoxy-silane (TEOS). a-SiO2 is also gown by wet- and dry oxidation of a Si wafer. The calculated and the measured reflectance spectra for Si/SiO2 double-layers are compared, and the overall agreement is very satisfactory. In particular, we can observe the Reststrahlen band of high reflectance and the interaction between this material stop band and the designed stop band, defined by the layer thicknesses.