Effect of thermal annealing on double anti reflection coating SiNx/SiO2

This work is a contribution towards the understanding of the optical and structural properties of graded refractive index layers SiN_x/SiO₂ to lead to antireflective coating and efficient passivation of multicrystalline silicon wafer. We investigate the influence of the thermal annealing on optical properties and chemical bonding of double anti-reflection coating (DARC) SiN_x/SiO₂ dielectric stack structure. Hydrogenated amorphous silicon nitride (SiN_x:H) coating is prepared by plasma enhanced chemical vapor deposition (PECVD) using a gas mixture of NH₃/SiH₄ on multicrystalline silicon wafer substrates. We have grown silicon oxide (SiO₂) thin films on the top of SiN_x by using thermal oxidation methods in the aim of creating graded-index refractive coating. The effects of annealing temperatures (500 to 1000°C) under nitrogen atmosphere are investigated. Based on Fourier Transform Infra-Red analysis (FTIR), it was observed that after annealing temperature, IR absorption of Si-N bonding decreased, while the integral intensity of Si-O bonding increased. This is due to a partial conversion of the phases Si-N and Si-O to Si-O-N phase. On the other hand, the change on the weighted reflectance in the range of 400-1100 nm (UV-Vis-IR) is observed. We have found a decrease of minimum reflectivity R= 0.77% (λ= 690 nm) and weighted reflectance R_w = 7.71%. Thus, the N₂ annealing method might have the tremendous potential for antireflective coating and surface passivation of solar cells applications.