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Abstract Porous silicon (PSi) is an excellent martial due to its highly mechanical and thermal properties; it’s obvious compatibility with silicon based microelectronic and its low cost. Also it has a large surface area within a small volume, controllable pore sizes and convenient surface chemistry. In this work PSi layers were prepared at three different electrolyte concentrations and two different etching times. The morphological and optical properties of the prepared PSi layers have been studied to determine the best conditions for reproducible PSi layers which can be used in solar cellsand in environmental sensing applications. The structural of the prepared PSi layers were characterized by using high resolution scanning electron microscope (JEOL 1200 EX П JAPANE) and X-ray device (Philips- mPW 1840) supplied from Philips Company. Photoluminescence spectra of the prepared PSi samples were carried out using (RF-530 SpectroFluorophotometer, Shimadzn). PSi surface reflectance, R, was recorded using a computer aided two beam spectrophotometer (JASCO V-670, UV/VIS/NIR, Japan). The FTIR spectrometer (Nicolet 6700 FT-IR, thermoscientific, class1) was used for the measuring FTIR spectra of prepared PSi samples. Summary 108 1- Study of morphological properties shows that the porosity, pore diameter and the PSi layer thickness increase by increasing etching time and decreasing electrolyte concentrations which is in a good agreement with the previous study (Gomezoet al., 2013). At HF: ethanol 1:2 concentration, the pore has maximum size and the surrounding solid cells are exhibiting the possibility of quantum wire structure. 2. XRD spectra of bulk silicon has showed a very sharp peak at 2θ = 32.95° that indicates the single crystalline nature of the silicon wafer. There is a significant crystallites size decrease trend which can be clearly noted on increasing etching time and dilution of HF: ethanol concentration. At HF: ethanol concentration 1:2, the porosity is maximum, the particle size is very small and almost all particles inside the etched surface have the same size. This can be taken as an evidence for quantum confinement effect. 3. The PL peak intensity increased and PL peak position shifts to shorter wavelength with increasing the porosity. This is achieved by controlling two variables: etching time and dilution of HF concentration. This result is in agreement with other publications(Li et al., 2014). PL spectra have shown that the highest intensity is produced for PSi sample prepared at HF: ethanol concentration 1:2 with 20 minute etching time because it has the most uniform pore size. Summary 109 3.1.PSi samples immersed in distilled water shows sudden decreases in PL peak intensity with no shift in peak position. This can be a good indicator for the using of PSi in sensing applications especially in vivo. These results can be used in preparation of porous silicon structures for high-sensitivity and selective bio- and gas sensors. 3.2. The PSi surface shows lower reflectance which is due to the very thin layer of PSi and changed refractive index profile (Duby and Gautam, 2011). It is clear that by increasing the etching time and dilution of HF concentration, the reflectance of PSi samples decreases and shifts to lower wavelengths. This can be explained due to an increase in surface roughness and porosity which was confirmed by SEM measurements. PSi samples showed red shift reflectance when they immersed in water. 3.3. The deduced absorption coefficient of PSi samples decreases with increasing the porosity and shifted to lower wavelengths as expected from decreasing silicon size that results in reducing the light scattering. 3.4. The deduced indirect band gaps of PSi samples have shown an increase from 2.06 eV to 2.1 eV by increasing the etching time. This result is in a good agreement with published results (Mortezaaliet al., 2009). The blue shift in absorption band edge has been claimed as a consequence of exiton confinement with increasing porosity (dilution of HFconcentration) and decreasing partial size in Si (the so-called quantum size effect). PSi samples have shown prompt and Summary 110 significant decrease in energy band gap to 2.9eV when they were immersed in water that confirms its ability in sensing applications. 4. In PSi, as-prepared samples, FTIR analysis shows that the peak around 1070 is from Si-O-Si which are depending on the oxidation degree of PSi. The dominant bonds are Si-Hx groups (x = 1, 2 or 3). The presence of hydrogen complexes on PSi surface has been suggested to explain the observed PL of PSi.It may be noted that the presence of hydrogen complexes on the PSi surface will not only passivate the dangling Si bonds but also widen the band gap which influences the PL of PSi(Jeyumranet al., 2007). |