4. Ijapbcr - Preparation and Physical Characterizayion of Porous

www.tjprc.org [email protected]

PREPARATION AND PHYSICAL CHARACTERIZAYION OF POROUS SILICON LAYERS FOR SENSING APPLICATIONS

G .M. YOUSSEF , S.Y. EL- ZAIAT2, M. EL-MALKY 3 & H.A. NAWAR 4 1, 2Faculty of science, AinShamsUniversity, Egypt

3, 4 Institutes of Environmental Studies & Research, AinShamsUniversity, Egypt

ABSTRACT

Porous silicon (PSi) has emerged as a potential sensing applications because of its high surface area to volume ratio, convenient surface chemistry, and very low toxicity. Porous silicon layers have been prepared from non-polished p- type silicon wafers of (100) orientation using electrochemical etching with different electrolyte concentrations and etching times. Scanning electron microscopy (SEM), photolumincensce (PL) spectroscopy and spectrophotometer measurement have been used to characterize the morphological and optical properties of porous silicon. The influence of fabrication parameters (hydrofluoric acid (HF) solution, the anodizing current density and anodizing time) on the morphological and optical properties of porous silicon has been investigated. SEM micrographs showed that by changing HF: ethanol concentration ratios, in the electrochemical process two peculiar surface morphologies were obtained. The surface morphology in the central region of the sample consists of solid cells delimited by trenches and the trenches bottom was covered by polyhedral pores. The PL spectrum peak at the anodizing time ranged from 590 to 610nm. PSI samples showed lower reflectance measurements and the optical energy band gap increases with increasing etching time.It could be clearly seen that porosity of PSi layers increases with increasing etching time and dilution of the electrolyte concentration which reflect on both increasing of PL intensity and decreasing reflectance with increasing porosity that gives the ability for using PSi in solar cells and bio sensing applications.

KEYWORDS: Porous Silicon, SEM, PL

Received: Oct 15, 2015; Accepted: Oct 28, 2015; Published: Nov 04, 2015; Paper Id.: IJAPBCRDEC20154

INTRODUCTION

Today, photovoltaic industry is dominated by silicon solar cells technology because of the reduced cost.

Due to wide use of solar energy, there is the need of creation of new technologies and materials hence; porous

silicon is expected to be promising one. The crystalline silicon is an important and dominant material over several years due to its well-known properties and established infrastructure for photovoltaic manufacturing. It is the basic

material for the production of solar cell and about 90% of fabricated solar modules are made of crystalline silicon[1].

Recently, porous silicon (PS) was intensely studied by researchers since it was reported for visible photoluminescence by Canham in 1990. According to Canham, porous silicon has potential application in optoelectronic and sensor [2]. When crystalline silicon (C-Si) wafers are electrochemically etched in hydrofluoric acid (HF) at specific current densities, pores are formed, which is known as a porous silicon (PS) layer. This is an interesting material due to its unique and unusual optical and electrical properties compared to bulk S33i

substrate. Structurally, PS is very complicated [3]. Nevertheless, the properties of PS, such as porosity, thickness,

Orig

ina

l Article

International Journal of Applied, Physical and Bio-Chemistry Research (IJAPBCR) ISSN(P): 2277-4793; ISSN(E): 2319-4448 Vol. 5, Issue 3, Dec 2015, 33-48 TJPRC Pvt. Ltd.

34 G .M. Youssef, S.Y. El- Zaiat, M. El-Malky & H.A. Nawar

Impact Factor (JCC): 1.9028 Index Copernicus Value (ICV): 3.0

pore diameter and, microstructure of silicon, have been reported to depend on iodization conditions, including the

electrolyte, current density, wafer type and resistivity, etching time, and temperature [4].

The discovery of visible photoluminescence (PL) from porous silicon an intensive research effort has been taken towards the study of nanostructured silicon. Several models have been proposed to explain the observed luminescence

from porous silicon such as quantum confinement in silicon nanocrystals, luminescence from siloxene, luminescence from

silicon hydride complexes and combinations of above models [5].

Morphology, which is determined by the distribution of matter in space, is the least quantifiable aspect of a

material. It is thus very difficult to systematically characterize morphology of PS, which has extremely rich details with

respect to the range of variations in pore size, shape, orientation, branching, interconnection, and distribution [6]. The reduction of size to a few nanometers is required to observe efficient light emission as it modifies in the electronic, optical and vibration properties [7].

Properties of porous silicon (PS) that make it very attractive for solar cell applications include band gap broadening, wide absorption spectrum, wide optical transmission range (7001000 nm), and good antireflection (AR) coating for solar cells [8]. In the following, we report on our systematic investigation of the relationship linking PS total porosity, morphology, PL properties and fabrication conditions [9].

Experimental

The silicon samples used in this study were using [100] oriented P-type CZ wafers with resistivity ranging from 2 up to 5 .cm with a thickness of 450 m [10]. Before electrochemical etching of silicon, silicon substrates were rinsed in de-ionized water and dried in the presence of nitrogen gas after heating in trichloroethylene (isopropyl) for 5 minutes [11]. Porous silicon technology operations were carried out in the teflon electro-chemical cell. As the anode the lower silicon

substrates contact was used, cathode contacts of the electrolyte made of platinum. This design allows using the cell for electrochemical etching. Etching only one side of the silicon substrates has been considered in this design, while the

backside is completely separated from the etching acid, thereby, the metallization of the backside, which is coated by

aluminum, will be protected [12]. Effect of HF concentration and etching time were examined. Different etching time 5 and 20 minutes under constant current densities 40 mA/cm2 were used for the electrochemical etching and to study effect of one parameter, while the others were kept constant.

The dependence of porosity of samples prepared at various HF concentrations, where the anodizing solution

contains Hydrofluoric acid HF (40 %) and Ethanol (95 %). Six samples are prepared at three HF: Eth concentrations ratio (by their volumes)[2:1(S1), 1:1(S2) and 1:2 (S3) under anodizing times five minutes in each concentration] and [2:1(S4), 1:1(S5) and 1:2 (S6) under anodizing times twenty minutes in each concentration]. The anodizing current density for electrochemical etching process is constant at 40 mA / cm. All growths are carried out under room light illumination and

at room temperature. The electrochemically etched area for all samples is about 1cm. The mixing of ethanol in electrolyte

solution is helpful to improve the lateral homogeneity and the uniformity of the porous silicon layer by promoting the hydrogen bubble removal. After etching, the samples were rinsed in acetone and after that in ethanol and dried in the

presence of nitrogen gas [5].

Surface morphology and structural properties of the samples under treatment were characterized by using high

resolution scanning electron microscope (JEOL 1200 EX JAPANE).

Preparation and Physical Characterizayion of Porous Silicon Layers for Sensing Applications 35


PL measurements were carried out using (RF-530 Spectro Fluorophotometer, Shimadzn). The PL is produced by using Xe excitation lamp with wavelengths in the range of about 200-999 nm, and incident angle was (45) degree. The excitation wavelength used is =299 nm and emitted florescence is in the range (300-600 nm). The photoluminescence of the PS was characterized in the range of 500 nm to 800 then, from the PL spectrum, the energy gap was determined by equation (1):

Egap = hc/ (1)

where Egap is energy gap of the PS, h is Planck constant, c is the speed of light and is the peak wavelength of the

photoluminescence[2].

X-ray diffraction (XRD) measurements were carried out using a high-resolution X-ray diffractometer system (Philips-mpw1840 supplied from Philips company) to determine the PS crystallite structure using Cu K a radiation with a wavelength of 0.15406 nm [13]. PS optical reflectance was obtained by using an optical reflectometer (JASCO V-670, UV/VIS/NIR, and Japan).The measurements were carried out at room temperature for the entire spectral range 190 2500 nm.

RESULTS AND DISCUSSIONS Photoluminescence

The PL Spectrum, was obtained when the excitation wavelength is shorter than the emission wavelength. This is

because shorter wavelength has higher energy to cause photoluminescence. The PS produced is able to illuminate visible light, which fall in the range (596-599) nm as can be seen in Figure 1 and Figure 2. Hence, orange red luminescence was observed from the PS.

Figure 1: PL Spectra of PS Samples Prepared at HF: Eth 2:1, 1:1, 1:2, Anodizing Time= 5 Minutes and Fixed Current Density of 40mA/cm

Figure 2: PL Spectra of PS Samples Prepared at HF: Eth 2:1, 1:1, 1:2, Anodizing Time= 20 Minutes and Fixed Current Density of 40mA/cm.

The PL intensity is proportional to the number of emitted photons on the PS surface. In addition, we can see that

PL maximum shifts to lower wavelengths substantially with decreasing HF concentration in etching solution (Figure 2).



The PL blue shift caused by decrease of HF concentration. We can conclude that total porosity of porous layer is not

directly linked with the mean size of silicon nanostructures that are responsible for visible photoluminescence. Therefore, it is proved that the porous silicon is better to be used for optoelectronics devices more than bulk silicon. Furthermore, the

photoluminescence properties of porous silicon used to convert Infrared into visible region light, so that it improves the

efficiency of solar cell. In addition, porous silicon has highly textured nature. Hence, it will enhance light trapping and

reduces reflectance losses.

The PL intensity of the p-type PS layers increased with increasing etching time and obtained the maximum

intensity at 20 min because of the increasing porosity that resulted from the complete etching of the PS layer. The peak

intensities fell because of the further increase in the etching time of over 20 min caused by the increased and widened pore

walls, which led to the decrease in the number of pores on the PS surface and there by the porosity was also decreased. The PL intensity is proportional to the number of emitted photons on the PS surface.

Therefore, according to the PL measurements, an etching time of 20 min is the optimum for c-Si p-type (100) to achieve the maximum intensity. This can be attributed to increase in total volume of the nanocrystallites on PS surface

according to the increasing PL intensity[14].

The PS is able to illuminate light because of the surface states and the quantum confinement that developed on the

PS after the etching process. [1, 8] The PL intensity is affected by porosity or the total volume of crystallites on the surface of PS [9].

The energy gap was then calculated using equation (1) and is recorded in Table (1) and Table (2).

Table 1: PL Intensity and Energy Gap for Different PS Samples at HF: Ethanol 1:1, 2:1, 1:2 for Etching Time 5 Min and Current Density = 40 mA/cm

Time HF: Ethanol concentrations (nm)

Intensity (a.u) Energy(e.v)

FWHM(nm)

5 min 2:1 597 500 2.077 3.2 1:1 596.5 450 2.078 3.7 1:2 598 400 2.073 4.1

Table 2: PL Intensity and Energy Gap for Different PS Samples at HF: Ethanol 1:1,2:1, 1:2 for Etching Time 20 Min and Current Density = 40 MA/cm

Time HF: Ethanol Concentrations (nm) Intensity (a.u.) Energy (e.v) FWHM (nm)

20 min 2:1 599 180 2.070 3.9 1:1 597 800 2.077 4 1:2 596.6 980 2.079 4.2

The PS energy gap is definitely having higher energy gap compare to Silicon (1.11eV) and it increases from (2.07 2.079) eV as the HF concentration decreases, etching time increase and porosity increased.

Surface Morphology

The observed dependence of PL spectra from PS on preparation conditions was correlated with PS surface

morphology determined by SEM.

On the other hand, PS samples prepared at different HF concentrations exhibit very different morphology

resulting in a large blue shift of PL maximum. Similar unconventional behavior of porosityPL shift was reported by



Bessas et al. [15] who explained their observation by correlation of PL shift with PS morphology and optical absorption.

PS samples prepared at the same HF concentration revealed similar morphology in electron microscope (not shown) for a broad range of current densities, whereas substantial differences were observed for samples prepared at different HF concentrations. Figure 3(a,b,c, d, e, f, g, h, I, j, .k and l) shows morphology of three samples prepared at HF concentrations 2:1, 1:1 and 1:2 (current density 4040 mA/cm and anodizing time five minutes).

(a) 2:1 5 min (500) (b) 2:1 5 min (2000)

(g)2:1 20 min (500) (h)2:1 20 min (2000)

(c) 1:1 5 min (500) (d)1:1 5 min (2000)

(i) 1:1 20 min (500) (j)1;1 20 min. 2000



(e)1:2 5 min 500 (f)1:2 5 min (800)

(k) 1:2 20 min 500 (l)1:2 20 min 600 Figure 3: SEM Micrograph of PS Samplesat the Current Density= 40mA/cm

It seems that the porosity of porous silicon samples is increased by decreasing the HF concentration as the other etching parameters as the time and current density are kept constant.

Figure (3a) shows the SEM micrograph of PS sample prepared at HF: ethanol 2:1. no visible pores is observed but small aggregates appear in some parts of prepared sample. By increasing the magnification of the image, as shown in

(Figure 3. b), the image shows characteristics of double-deck, the surface is slightly crack and crooked but it still attaches to porous layer and the surface seems to be sponge like pore channels [16].At HF 1:1, the pores can be clearly visible as black spot in dark grey background [17]. The pores are aligned in random directions as shown in Figure (3.c,d).By decreasing electrolyte concentration ratio, inhomogeneous trenches are formed (Figure 3. f) the trench formation starts when ethanonic solution infiltrates depression or defects on the Si wafer surface and reacts leaving small pores. As the reaction continues,the pores become larger and larger forming the trench (Figure 3.e).Then the trenches is surround by a region which called solid cells [18].

In case of etching time 20 min. and at HF: ethanol concentration 2:1,it can be see that the surface seriously cracked and flake off. The pore structure has characteristics of web like pore channels which is responsible for the shelling

layer figure (3.g). The small aggregates are disappeared. The channels become narrower, clearer and deeper as shown in Figure (3.h). The layer becomes thicker which means that dissolution of the thin pore walls is complete and next layer has just open [16, 19]. At HF: ethanol concentration 1:1, the pores are smaller with larger number than that in Figure (3. c). The pores distribution is random with highly interconnected meshwork of pores (figure 3. i). The thickness and the depth of the layers are increased as it can be seen in figure (3. j).

Figure (3. K, l) shows the SEM micrograph of PS samples prepared at HF:ethanol 1:2 and etching time 20min. it is clear in Figure (3.k) that the surface morohology of the central region in the sample which is very similar to that in Figure (3.e) except that the trench look more homogeneous, deeper and wider. Solid cells were in uniform shape and the pore has maximum size. These results are in agreement with published articles[18].



X-ray Characteristics of PS

XRD studies showed distinct variations between the bulk silicon surface and the porous silicon surfaces formed at different electrolyte concentrations and different anodizing time.XRD spectra of bulk silicon showed a very sharp peak at

2 = 32.95, showing the single crystalline nature of the wafer Figure (5a). the narrow peaks with a small value of Full Width at Half Maximum (FWHM) can be taken to indicate good crystallinity and large grain size of silicon particles. This peak becomes very broad with varying full- width at half maximum (FWHM) for prepared PS samples using different HF: ethanol concentration ratios of 2:1, 1:1 and 1:2 for the anodization time 5 and 20 min. as shown in Figure (5 b,c,d,e).

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(d) (e) Figure 5: X-ray Diffraction of (a) Silicon Substrate, (b) Prepared PS at HF: ethanol:

1:1,t= 5Min. (c) 2:1, t 5Min,(d) 1:2, t= 5Min, (e), 1:2, t= 20 Min.

This confirms the formation of pores on the crystalline silicon surface, the presence of this peak in all the PS structures confirms that the cubic structure of the crystalline. The crystallites size obtained for crystalline silicon and



porous silicon samples are shown in table (3). The size is estimated by the Scherrers equation:

=

(2)

Where B is the FWHM in radians, K is the Scherrer constant (1 >K > 0.89), is the wavelength in nanometers, in radians is the diffraction angle and L isthe mean crystallite size [20].

Table 3: Average Pore Size and Thickness for Different PS samples

HF: ethanol Concentration

T (Min)

FWHM

crystalline size

(Nm) T

(min) FWHM crystalline Size(nm)

Crystalline Si silicon silicon 5

0.027 308

20

1:1 2:1 1:2

0.08 103 0.086 96 0.046 180 0.044 170 0.06 130 0.12 78

A significant decrease in the crystallites size can be clearly noted on increasing etching time and dilution of HF:

ethanol concentration. Silicon is retained even after the pore formation.

It should be noted that as the HF:ethanol concentration ratio is decreased to 1:2, the SEM studies show that pores

of maximum sizes are formed in the PS layer that indicates high porosity of the sample. This is supported by the XRD spectrum as shown in figure (5e). The drop in the peak height of the main peak and the considerable increase in its FWHM (i.e., decrease of the grain size) see table 4. The presence of wide single peak indicates small and uniform size for almost all particles. At this HF concentration, when the porosity is maximum, the particle size is very small and almostall particles

inside and all over the etched surface have nearly the same size, which can be taken as evidence for quantum confinement

eects.

Reflectance Measurements

Figures (6a, 7a and 8 a) show the reflectance of the PS layer. The PS surface shows lower reflectance which is due to the formation of very thin layer of PS and change in refractive index profile[21].

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(a) (b) Figure 6: Reflection Spectra of PS Layer Etched at HF: Ethanol 1:1, Anodizing Time 5 min, for

Fixed Current Density of 40mA/cm, (a) in Air and (b) Immersed in Water



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(a) (b) Figure (7): Reflection Spectra of PS Layer Etched at HF: Ethanol 1:1, Anodizing Time 20 Min. for


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(a) (b) Figure 8: Reflection Spectra of PS Layer Etched at HF: Ethanol 2:1, Anodizing Time 20 min. for


The reduction in reflectance can be attributed to the light scattering and trapping of weakly absorbed photons.

The total internal reflection within the porous structure produces a change in the direction which ultimately increases the

optical path length. The scattering light may be due to the surfaceroughness at the PS/Si interface. It is noted that by increasing the etching time, the reflectance of PS samples decreases as shown in Figure(6a, 7a). Also there is a shift to lower wavelengths (higher energies PL intensity) due to the increase in surface roughness and porosity which was confirmed by SEM results. Figure (8a) shows that the reflectance of the PS layer decreases and shifts to higher wavelengths (small energies) which were confirmed by SEM and PL measurements. It is concluded that the reflectance decreases and shifts to lower wavelengths by increasing etching time and decreasing of HF concentrations. This implies the

possibility of using the PS layer as an antireflection coating for solar cells because the PS surface reduces the light

reflection.

Absorption Coefficient

The absorption coefficient can be determined from the reflectance spectra as [22](Doliaet al., 2006):

2t = ln [( / )] (3)



Where t is the thickness of PS layer and R is the reflectance for any intermediate photon energy. A sudden fall of

reflectance from R to R is due to the absorption of light by the material.

Absorption Coefficient

Figures 9, 10 shows the calculated absorption coefficient of PS sample prepared at HF: ethanol 1:1, anodizing time 5, 20 min. for fixed current density of 40mA/cm.

(a) (b)

Figure 9: Calculated Absorption Coefficient of PS Sample Prepared at HF: ethanol 1:1, Anodizing Time 5min, for Fixed Current Density of 40mA/cm, (a) in Air, (b) Immersed in Water

Figure 10: Calculated Absorption Coefficient of PS Sample Prepared at HF: ethanol 1:1, Anodizing Time 20min. for Fixed Current Density of 40mA/cm, (a) in air, (b) Immersed in Water

It is noted, from these figures, that the absorption coefficient of PS decreases with increasing porosity and shifted to lower wavelengths (higher energies). This is expected from decreasing silicon size that results in reduced scattering. The smaller silicon crystals obtained with lower HF concentration and higher etching time 20 min. enhances the quantum

confinement effects that widen the band gap and blue shift of the absorption edge and further reducing absorption losses in the visible range [23, 24].

Optical Energy Band Gap

The most convenient method for determining the band gap energy is from transmittance spectroscopy. But it is

not possible to measure the transmitted spectra for thin films grown on top of an opaque substrate. Hence, for studying the optical band gap, optical reflectance have been used.



According to Tauc relation, the absorption coefficient for energy band gap material is[22]:

= A ( ) (3)

Where A is the edge width parameter representing the film quality, h is the incident photon energy, E" is optical

gap and n is constant determines the type of transition. When (h#/) = 0 for certainh, thenE"=h. Parameter n has the value 1/2 for the direct allowed transition and the value 2 for the indirect allowed transition. From equations 3 and 4 the

value of the energy gap E" is determined from the intercept of extrapolation to zero absorption with the photon energy axis.

An indirect allowed transition is considered for PS samples with different porosity.

Figure (11, 12) shows the plotof [ ' ( / )])/*versus which is used to calculate the energy band gap for PS samples prepared at HF:ethanol 1:1 with etching times 5 and 20 min. at current density of 40mA/cm.

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(a) (b) Figure 11: Calculated Energy Band Gap of PS Sample Prepared at HF: ethanol 1:1, Anodizing

Time 5min for fixed Current Density of 40mA/cm, (a) in air, (b) Immersed in Water

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(a) (b) Figure 12: Calculated Energy Gap of PS Sample Prepared at HF: Ethanol 1:1, Anodizing Time 20 min for Fixed Current Density of 40mA/cm, (a) in Air, (b) Immersed in Water

Table 4 shows that the indirect band gap increases from 2.06 eV to 2.2 eV when the etching time is increased to 20 min. Thus the gap increases with increasing the film porosity which is in good agreement with the published results[25].



Table 4: Deduced Optical Band Gaps of PS Sample Prepared at HF: ethanol 1:1, Anodizing Time 20 min for Fixed Current Density of 40 mA/ cm

HF: ethanol (Concentrations)

Etching time (min)

Eg (air) (eV)

Eg (water) (eV)

1:1 5 2.06 2.91 1:1 20 2.15 2.03

The blue shift in absorption band edge has been claimed as a consequence of exiton confinement with increasing

porosity and decreasing partial size in PS (the so-called quantum size effect).

PL Spectra for PS Samples Dispersed in Water

It would be more interesting to prepare and characterize luminescent emitted from PS immersed in water since most of biological applications occur in aqueous environment. Figure (13a, b) shows PL spectra of PS samples at HF:ethanol 2:1 and 1:2 with etching time 20 min, at fixed current density of 40mA/cm respectively. The samples were

immersed in distilled water for 15 min at room temperature.

(a) (b) Figure 13: PL Spectra of PS Samples Immersed in Distilled Water at (a) HF: ethanol=2:1,

(b) HF: ethanol=1:2, Anodizing Time 20 min and Fixed Current Density of 40 mA/cm

The PL intensities have a great rapid decrease when PS samples are immersed in water for period of time referring

that there is not a shift in PL peak position. This means that the bulk silicon does not change at low concentration of HF,

while there is a small decrease and little shift in PL intensity and position at high concentration ratio of HF. Thus the PS

samples that prepared at low concentration of HF are to be good material for uses in wide sensing applications. These results can be used in preparation of porous silicon structures for high-sensitivity and selective bio- and gas sensors.

Reflectance Measurements for PS Samples Dispersed in Water

Figures (6.b), (7.b) and (8.b) show the variations in the reflectance spectra of the PS layer samples immersed in water. The reflectance spectra promptly shifted toward the longer wavelengths. This phenomenon can be ascribed to the absorption of the water within the PS layer pores. In other words, during the immersion of a PS layer in water, the

substitution of the air in the pores by the water leads to a shift in the reflectance spectrum due to the change in the optical

thickness [26]. Since the geometrical thickness of each PS layer is fixed, the shift in the reflectance spectra can only be due to changes in the average refractive indices of the PS layer. It could be concluded that the reflectance spectra of PS layer

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samples showed a prompt and significant red shift when they were immersed in water. The PS layers showed an excellent

sensing ability to water. We believe that our experimental results will help in development of PS-based sensors for gas sensing, chemical sensing and biosensing.

Absorption coefficient PS samples dispersed in water

Figures (9. b, 10. b) shows the calculated absorption coefficient of PS sample prepared at HF: ethanol 1:1, anodizing time 5, 20 min and then immersed in distilled water for 15 min. It is noted, from these figures that PS samples showed a prompt and significant red shift when they were immersed in water.

Optical Energy Band Gap for PS Samples Dispersed in Water

Figures (11.b) and (12.b) show the deduced energy band gap of PS samples prepared at HF:ethanol 1:1with different etching times 5 and 20 min. and then immersed in distilled water. It is noted from these figures and table 5 that the energy gap decreases in the two samples which mean that PS samples showed a prompt and significant red shift when

they were immersed in water.

CONCLUSIONS

In this work porous silicon layers are prepared by electrochemical etching for three different HF concentration

ratios and two different etching times for fixed current density. It can be concluded that:

The SEM investigation shows that both the pore diameter and layer porosity increase with increasing etching time

and dilution of the electrolyte.

The PL spectra shows that as the mean size of silicon nanoparticle is expect to get smaller with increasing

porosity which is responsible for the PL spectra blue shift of PSi with higher porosity. The results show that PL peaks shifted to the shorter wavelength with increasing etching time and dilution of the electrolyte.

PSi samples immersed in water shows sudden decreases in PL peak intensity with no shift in peak position, which

can be a good indicator for the using of PSi in sensing applications especially in vivo.

PSi samples showed a very low reflectance that decreases with increasing etching time and shifted to lower wave lengths with dilution of HFconcentration. This can make the feasibility to use PSi in heterojunction thin film solar cells with promising antireflective performance. Moreover, the important specific surface of these nanostructures

can also be potentially useful as antireflection layers for photovoltaic applications and as optical sensors.

PSi samples immersed in water showed a red shift in reflectance spectra which confirmed the ability of using PSi

in biosensing applications.

From these results we can introduce PSi as a physical sensing element, that has been successfully extended too

many sensing applications especially in biological world.It offers a number of properties for controlling in many applications. Firstly nanostructured materials based on silicon are promising platforms for pharmaceutical applications

because they provide low toxicity. Their ability to degrade in the body presents fewer challenges for chronic use. Secondly,

the electrochemical means of fabrication allows one to dial in the properties of surface area, free volume and pore size.

Pores can be generated anywhere from a few nanometers to several hundreds ofnanometers in diameter. Thirdly, the surface of freshly prepared porous Si is easily modified via convenient chemistry with a large range of organic or



biological molecules (drugs, peptides, antibodies, proteins, etc.). Fourthly, the optical properties of porous Si provide a useful dimension for in-vivo sensing or therapeutics. Porous Si can display fluorescence deriving from Si quantum dot structures that are produced during the etching and it can be prepared with unique optical reflectivity spectra. These

features allow porous Si to exhibit a signal that is affected in a predictable way when exposed to environmental changes,

presenting possibilities for the development of advanced functional systems that incorporate sensors for diagnostic or

therapeutic functions.

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