Effect of Topological Morphology on Optical Filtering Properties of Porous Silicon
Macroporous silicon samples of differing topological porous properties were manufactured by way of electrochemical etching. Different etching parameters (etching current, time, electrolyte concentration) were used on four different (in terms of crystal orientation and resistivity) types of samples in order to obtain a series of samples of differing pore topology. It is known that macroporous silicon acts as a high-wavelength pass filter in the infrared regime. FTIR spectroscopy was performed on each of these samples in order to obtain an optical cutoff wavenumber for each sample. Furthermore, SEM analysis was performed in order to determine the number of pores per unit area on the surface as well as the percentage of the surface that was covered in pores for each sample. Furthermore, the average linear dimension per pore was determined using these values. Finally, the average pore-to-pore distance was also estimated on each sample. These four sets of measurements were performed in order to find a relationship between the optical and topological properties of macroporous silicon. It was found that there is a relationship between pore number density and cutoff; the cutoff wavenumber increases as the pore number density is increased. Additionally, a correlation between the pore spacing and the cutoff was also determined; the cutoff wavelength increases as the pore spacing increases. It was expected that there would be a correlation between the average linear dimension per scattering element as is seen in other types of scattering filters; however such a trend was only observed for one of the sample types. This suggests that the scattering mechanism by which porous silicon filters operate differs between samples of significantly differing surface topology. In addition to this, the temperature-dependence of the cutoff was investigated. Through low-temperature optical analysis using FTIR spectroscopy and liquid helium as a coolant, it was determined that the cutoff wavenumber exhibits no temperature-dependence below 100 K. For higher temperatures, the measurements performed were inconclusive. This was due to the thermal expansion at higher temperatures of the copper sample holder coupled with the inhomogeneity of the surface structure of each of the silicon filters.