الفهرس 
Studies on environmental applications of PSiOnly 14 pages are availabe for public view
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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).