Design and fabrication of infrared antireflecting bi-periodic micro-structured surfaces

Broadband antireflection properties of material surfaces are of primary interest for a wide variety of applications: to enhance the efficiency of photovoltaic cells, to increase the sensitivity of photodetectors, to improve the performance of light emitting diodes, etc... In the past, broadband antireflection multilayer coatings were widely used and recently very low refractive index materials in thin film form have been fabricated by several groups. The research work presented in this paper aims at modeling and fabricating bi-periodic micro-structured silicon surfaces exhibiting broadband antireflection properties in the infrared range. These structures of pyramidal shape, which typical dimensions are smaller than the wavelength, are not in the Effective Medium Theory (EMT) validity domain. The optimization of the optical properties of such patterned surfaces needs a fully Finite Difference Time Domain (FDTD) rigorous description of light propagation phenomena. The influence of various opto-geometrical parameters such as period, depth, shape of the pattern is examined. The antireflective properties of such bi-periodic patterned surfaces is then discussed using the photonic crystal theory and photonic band diagrams description. The structure is considered as a two dimensional periodic structure with a non-uniform third dimension. Correlations between the density of Bloch modes, flatness of dispersion curves and the surface reflectance are presented. The last part of this paper is devoted to the presentation of the fabrication and the characterization of the structures. Low cost and large surface processing techniques are proposed using wet anisotropic etching through a silica mask obtained by photolithography or nanoimprinting.