Preparation and characterization of multilayer porous silicon nanostructure for Bragg grating waveguide

Materials characterization for Bragg grating application was presented in this thesis. Multilayer porous silicon is the basic material for Bragg grating structure and was directly etched on silicon wafer. Multilayer porous silicon with alternating values of thickness and refractive index were characterized and verified. The optimization of parameters during electrochemical etching process of silicon wafer has leads to the realization of multilayer properties of porous silicon. The optical properties of Bragg grating structure depends highly on the thickness and periods of multilayer porous structure, meanwhile electrical properties relies on surface homogeneity and internal structure of porous silicon. It is well known from previous studies that the modulation of multilayer porous silicon was much affected by the hydrofluoric acid concentration of electrolyte, etching time, and current density during the etching process. Surface morphology investigation from atomic force microscopy shows that surface roughness was decreased when higher etching time applied to the samples. FESEM shows the multilayer porous silicon structure with alternating contrast of high and low porosity layer. The average refractive index, n for 15.38 mA/cm2etching current density is from 1.530 to 1.850 and for 46.15 mA/cm2current density is from 1.350 to 1.450. Average alternating thickness for multilayer structure is 163-200 nm for 10 seconds etching time, 255-300 nm for 20 seconds etching time and 341-387 nm for 30 seconds etching. Reflectance measured from multilayer porous silicon structure has produced a significant reflectivity of approaching 40% within the telecom C-band regions. Low reflectivity of around 2-10% within the visible region is corresponding to high light absorption from photoluminescence effect on the porous silicon surface. Photoluminescence study also revealed that higher etching time will produce higher electronic band gap energy. Nitrogen gas isotherm shows a good pore size distribution and high specific surface area also obtained from samples. This fabricated multilayer porous silicon with Bragg grating waveguide ability has a big potential as optical and gas sensor in many areas such as aerospace, safety and industry.