Thin Photonic Crystals for Optical Characterization of Ultrathin Hafnium Dioxide in Mesoporous Layers

Thin porous silicon (PSi)-based photonic crystals as optical sensor for characterization of ultrathin film deposition in mesoporous silicon is presented. Layer deposition on the mesoporous layer was performed by ultrathin hafnium dioxide (HfO
₂
) fabricated by thermal atomic layer deposition (ALD). Photonic crystals were electrochemically etched in the form of a rugate filters and served as optical sensor elements for the process characterization. A HfO
₂
atomic layer deposited in the pores causes change of the refractive index in the mesoporous layer and leads to red peak shift of the rugate filter spectrum. Effective medium approximation was used to develop a material specific model. Optical properties of thin and thick rugate filters are compared. Simulation and measurement results show that penetration depth and deposition thickness have a strong influence on the peak shift. Large peak shift is achieved by application of thin photonic crystals. This optical characterization method allows investigation of the ALD process steps and the pore filling in complex mesoporous materials in the pore size range of 4-8 nm.