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EasyChair Preprint Illuminations Condition Effect on Optical Properties of Nanocrystalline Porous Silicon Illuminations condition effect on optical properties of Nanocrystalline porous silicon

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  • Ministry of Education, Iraq
  • Mustansiriyah University
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Abstract and Figures

In this report, laser assisted in electrochemical etching technique PECE to prepare porous silicon layers under the constant conditions was used. To confirm the possibility of using different wavelengths of laser light to control the thickness of structures and porosity of silicon layers, a scanning electron microscope and weight measurements techniques were used to study of morphology of porous surface and its porosity. It was found that the porosity values according to the different wavelengths have same power density approximately and which ranged from 39 to 53% towards the shortest wavelength. The intensity of the integrated peak absorption of Si-H and O-H vibrations demonstrated in the FTIR spectra of these samples was calculated. In this work, optical properties dependence of porous silicon crystalline size is analyzed and correlated to photoluminescence spectra. It's clear that the Photoluminescence PL peak energy was decreasing from 2.37 to 2.10ev, and corresponding the increasing in refractive indexes. The peak PL intensity dropped from short wavelength (blue) toward long wavelength in visible region with large than 50% ratio.
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EasyChair Preprint
3284
Illuminations Condition Eect on Optical
Properties of Nanocrystalline Porous Silicon
Sarab T. Kasim and Hasan Hadi
EasyChair preprints are intended for rapid
dissemination of research results and are
integrated with the rest of EasyChair.
April 28, 2020
Illuminations condition effect on optical properties of
Nanocrystalline porous silicon
Sarab T. Kassim1 and Hasan A. Hadi2*
Department of Physics, College of Education, Mustansiryiah University, Baghdad, Iraq,
1Sarab.kasim@gmail.com
Department of Physics, College of Education, Mustansiryiah University, Baghdad, Iraq,
2*hasan.a.hadi@uomustansiriyah.edu.iq
Abstract
In this report, laser assisted in electrochemical etching technique PECE to prepare
porous silicon layers under the constant conditions was used. To confirm the possibility of
using different wavelengths of laser light to control the thickness of structures and porosity
of silicon layers, a scanning electron microscope and weight measurements techniques
were used to study of morphology of porous surface and its porosity. It was found that the
porosity values according to the different wavelengths have same power density
approximately and which ranged from 39 to 53% towards the shortest wavelength. The
intensity of the integrated peak absorption of Si-H and O-H vibrations demonstrated in the
FTIR spectra of these samples was calculated. In this work, optical properties dependence
of porous silicon crystalline size is analyzed and correlated to photoluminescence spectra.
It’s clear that the Photoluminescence PL peak energy was decreasing from 2.37 to 2.10ev,
and corresponding the increasing in refractive indexes. The peak PL intensity dropped from
short wavelength (blue) toward long wavelength in visible region with large than 50%
ratio.
Keyword:
Porous silicon; photoluminescence ;refractive index; porosity;
PECE;SEM;FTIR
Introduction
Porous silicon is remains very attractive to study because it has large photoluminescence
efficiency at room temperature in the visible region(Chao Y 2018) and its one of the most
important a direct band gap semiconductor material which obtained in easy and cheap
techniques (R Suryana et al 2018) . It has a high absorbability and low reflectivity due to
the spongy structure of porous layers (Olga Volovlikova et al 2020).To obtain a chemically
stable layer (Farid A.Harraz 2014) , mechanically strong (Chiang, C., Lee, B.T. 2019)
magneto-optic devices (Petra Granitzer and Klemens Rumpf 2010), anti-reflective coating
(Remachea Tahmid H. et al. 2019) , waveguide (Tahmid H. et al.2019) , Chemically and
biologically sensitive (Wei Li et al.2019 ; Jonathan Rassona et al. 2017) , photonic crystal
(Xiaxia Yue, et al. 2019) , photodetector (Hadi, H.A. et al. 2019) and suitable for existing
silicon technologies. The porous silicon layer is prepared in a number of different ways
such as (Junwen Xu1 et al. 2019).metal-assisted and stain etching (M. B. de la Mora et al
2013) which are giving this possibility to many applications. The control of Porosity and
thickness in order to study the optical properties of the porous silicon nanostructures open
up many fields for a wide range of applications in optoelectronic technology(Arvin
I.Mabilangana et al. 2016;YamanAfandi et al 2019;S.Praveenkumar et al2019).The photo
luminescence spectroscopy allows the determination of the energy gap, refractive index and
the change in the intensity beam value of semiconducting materials. Therefore, it is used to
study the optical emission properties of these materials and (F. S. Husairi et al 2018)
reported to explain the mechanism of photo emission such as quantum confinement, charge
accumulation, surface distribution etc. When preparing the porous silicon layers by
electrochemical methods, we note the surface of layers will be interaction with natural
atmosphere and the nature of the chemical surface will be changes. Also, may change their
optical properties and PL photoluminance (Laatar, F. et al 2020 ; Ramirez-Gutierrez, C.F.
et al. 2019) .the current etching density , etching time , the concentration and ratio of the
hydrofluoric solution, the type and orientation of the silicon substrate, the power density
and type of lighting used especially in n- type were usually the most commonly used
parameters to formed a nanostructure silicon. Also, others were study possibility of
modifying the photoluminescence characteristics of porous silicon layers by irradiation
with gamma ray (Bilenko, D.I. et al 2018) sample duration in air (Len’shin, A.S. et al
2013) and use the porosity to determines the refractive index(D. Estrada-Wiese 2018)
.Many of literature were focused on density of etching current and etching time to control
porosity and the thickness of the layer which will be prepared.We used PECE method to
preparing porous silicon layers and we have used different wavelengths lasers in order to
tunable morphology to understand the optical properties of porous silicon. The aim of this
article is to study the effect of the laser wavelength was used in preparing the porous silicon
layer, which plays an important role in its effect on photoluminance and refractive index of
layers, as they are important for optical properties and has consideration as an antireflective
feature.
Material and method
Preparation of the layers of porous silicon were using photo-electrochemical etching cell
PECE as shown in fig 1. Samples of porous silicon electrochemical etching were prepared
for wafers of type N mono crystalline silicon with resistance under constant conditions of
current density and etching time with different laser lighting conditions during the
electrochemical etching process. Firstly, the N-type silicon wafer with ordination (111)
was cut into a 0.6 x 0.6 cm2 and cleaned, respectively, with acetone, ethanol, and distilled
water ultrasonically for a few minutes to remove impurities. The porous silicon layer was
prepared with three different lasers (red 650nm, green 532nm, and blue 450nm) and they
have same power density approximately under the following constant conditions at room
temperature:
Etching current density (J) 7 mA/cm2,
Etching time (T) 120 sec,
Hydrofluoric acid concentration (HF) 40%,
Ethanol purity more than 99%
Volumetric ratio of 1: 1 for (HF: ethanol).
Figure 1: Schematic diagram of the electrochemical cell used for etching silicon and
image of porous silicon layers with a) blue laser, b) green laser, and c) red laser
Absorption FTIR spectra are performed over range between (400- 4000)cm1 in Nano
Center for porous silicon layers were measured using a Shimdzu-FTIR-IR Tracer 100
spectrometer .In order to study the nanostructures of porous silicon layers morphology,
scanning electron microscopy TeScan Mira III in Sharif center for laboratory services
(central Lab) Sharif University of technology was used. The PL spectra of all porous
silicon samples were measured by using a spectroscopy PL 2048 TEC in Sharif center for
laboratory services (central Lab) Sharif University of technology. In gravimetric method,
―Mettler AE-160 digital with accuracy of 104 gm‖ is used to weight the samples and then
to calculate the porous layer thickness and porosity.
Result and discussion
The structure and nature of the porous silicon layer is also related to the crystalline size
feature of those layers and thus the change in peak-photoluminance behavior is observed
specifically in the nano porous. Fig.2 displays a morphology image of porous silicon
layers surface was obtained using the scanning electron microscope at different
wavelengths a-blue laser ,b-green laser, and c- red laser which used in order to
morphology control of porous silicon formed by electrochemical etching. It’s clear that
the change of wavelengths for leads to changes Pore size ranged from (59-91 nm) , (126-
335 nm) and (29-51nm) for blue, green ,and red lasers respectively as shown in Fig.
2a,b,and c. As shown SEM images in Fig. 2, the highly porous layer yielded large cavities
(Chiang, C., &Lee, B.T. 2019).The sizes of pores at the surface are change when
illuminated the layer formed on front n-Si samples with variation wavelengths as shown in
Fig.2, and the change in pore size from the surface of porous silicon to n-Si bulk is due to
the transition from start pore formation to quick of pores growth.
Figure 2: shows the results of scanning electron microscopy image measurement, a) blue
laser, b) green laser, and c) red laser.
The porosity P% is given simply by the equation (Hasan A. Hadi 2018):
% = 12
13 (1)
where , and are the sample weight before and after anodization, while is the
sample weight after removing the PS layer with KOH solution for few minutes
From these measured masses, it is also possible to determine the thickness d of the layer
according to the equation (Hasan A. Hadi 2018):
d = m1m2
A (2)
Were A is the area of the etched surface and ρ is the density of silicon.density of porous
silicon can be calculated by following equation (Pavlo Lishchuk et al 2015):
. PSi =Si (1 P) (3)
Where PSi is the density of porous silicon and Si is the density of silicon.
The porosity and thickness of the porous layer were measured by two techniques, the first
one by using scanning electron microscopy for thickness layer prepared and which give
indicate of effect wavelength on thickness and also gravimetric method was confirmed
these effect. In the present work, a simple way was used to control of thickness and
porosity of porous silicon layer. The difference in thickness and porosity which expected in
the value as shown in table 1, because they have different procedure for calculates these
parameters.
Table 1: The porosity, thickness and density of the porous layer were measured by
gravimetric and SEM techniques for different wavelength lasers.
Samples
Illumination
SEM
P %
Weight
P%
Density 
sem
T (µm)
SEM
T(µm)
weight
Red laser
39
35
1.4213
1.2
2
Blue laser
53
56
1.0951
1.56
0.9
Green laser
50
54
1.165
1
1.4
Figure 3 Shows the FTIR spectra of porous silicon layers surfaces which were prepared
with different laser wavelengths of (450, 532, and 650 nm) at room temperatures. When the
red laser used in photo electrochemical etching PECE, the large vibration bond takes place
in (611.17cm-1 wave number) as wagging mode Si H. while, for blue and green laser
used in PECE, the large vibration bonds happened centered at (611.22 cm-1) Si H at the
same mode type. For Si-O-Si bond in red laser the large vibration bond takes place in
(1175 cm-1 wave number) as asymmetric stretching mode, but for blue and green laser
the large vibration bonds happened centered at (1200 cm-1 wave number ) as symmetric
stretching (M.A. Va´squez-A. et al 2007). In the O3-Si-H and Si-H3 bonds, the large
vibration for both of them appeared at (2360 cm-1 wave number ) as stretch mode for blue
and green laser, while they didn’t appear in red laser (Walter Jaimes et al 1997 ).
500 1000 1500 2000 2500 3000 3500
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
Abs.(%)
wavenumber(cm-1)
Si-O-Si
Si-H
Si-H3
red
blue
green
Si-O-H
O3-Si-H
Figure 3: FTIR absorption spectrum of porous silicon layers from500 to 4000 cm-1 for different
wavelength of red laser, blue laser, and green laser
Figures 4,5 and 6) show the photoluminescence spectra of the nanostructures porous
silicon layers prepared by electrochemical etching with three different illumination
condition (blue laser, green laser and red laser ) respectively at room temperature and
approximately the same power density. Also, at clear (see table 2) that the blue shift in
visible region from 2.01eV gap band (at red laser used) to 2.37eV band gap (at blue laser
used) because of nanostructure of the porous silicon layer which resulting quantum
confinement effect of the nano crystal porous silicon layer and also the increased happen in
the concentration of oxygen atoms in the formed pores and the substitution of hydrogen
atoms by the oxygen atoms.
429 462 495 528 561 594 627 660 693
0
22
44
66
88
110
132
154
176
198
220
lpeak = 528.85nm
Ipeak = 213.16
PL intensity (a.u.)
wavelength (nm)
Blue Laser
Figure 4: the photoluminance spectrum recorded under the influence of illumination blue
laser
From Figures 4, 5 and 6, it is clear that the relative intensity of peaks increases
about twice (213-99.59) when the wavelength decreases from red to blue in the visible
region. The peaks at wavelengths 528, 588, and 592 nm are most likely due to the direct
(intrinsic) transitions, so may be the transition due to the interface effect of impurity or
defects (extrinsic) is less than when using high-energy light effects.
406 435 464 493 522 551 580 609 638 667 696
0
18
36
54
72
90
108
126
144
162
lpeak = 588.55 nm
Ipeak = 155.43
PL intensity(a.u.)
wavelength(nm)
Green Laser
Figure 5: the photoluminance spectrum recorded under the influence of illumination green
laser
420 455 490 525 560 595 630 665 700 735 770
0
12
24
36
48
60
72
84
96
108
lpeak= 592.19nm
Ipeak= 99.59
PL intensity(a.u.)
wavelength(nm)
Red Laser
Figure 6: the photoluminance spectrum recorded under the influence of illumination red
laser
Table 2: The peak intensity, peak wavelength, and band gap energy of porous silicon
samples calculated by PL spectra at different wavelengths laser used
Illumination type
(nm)
PL peak
Intensity
PL- peak
wavelength
PL- peak
energies
EPsi(ev)
Nano-crystal
diameters
(L).nm
Blue laser
213
528
2.37
2.19
Green laser
155
588
2.11
2.58
Red laser
99
592
2.10
2.59
It was observed that the porosity, thickness, and photoluminance are modifiable through
illumination conditions, which makes change the peak energy gap, refractive index, and the
optical properties of porous silicon also modifiable.
According to the (S. Praveenkumar et al 2018) and by using the various mathematical
relationships between refractive and energy gap has been used (D. Bellet & G. Dolino,1996
;D.K. Ghosh et al 1984 ;P.J.L. Hervé& L.K.J.1995) the refractive index n1, n2,and n3 can
be determined using following equations:
n1= a + bEg ( 4)
where a and b are constant and it is given as 4.048 and -0.62 per eV respectively.
n2=1 + a
Eg+b2 ( 5 )
where a and b are constant and it is given as 13.6 eV and 3.4eV respectively
n3=1 + a
Eg+b (6 )
Where a and b can be modified as a= 25Eg + 212, b = 0.21Eg +4.25
Refractive index and energy gap data were calculated and listed as shown in table 3.The
refractive index value of the porous silicon layers were decreases with increasing of
wavelengths toward red laser in visible region which was used to formation layers of
porous silicon and enhance the rate of absorption, hence used for applications with high
absorption.
Table 3: refractive index calculated according energy gap of PL for three modules (D.
Bellet & G. Dolino,1996 ;D.K. Ghosh et al 1984 ;P.J.L. Hervé& L.K.J.1995)
Illumination type
(nm)
PL- peak
energies
EPsi(ev)
Nano-crystal
diameters
(L).nm
Blue laser
2.37
2.537
2.559
2.520
2.19
Green laser
2.11
2.735
2.660
2.590
2.58
Red laser
2.10
2.741
2.664
2.593
2.59
The PL of porous silicon layers reduces with the increase of wavelength. Fig.6a, b and c
show the image profile of photoluminescence spectra samples for blue, green and red laser
respectively. The peak photoluminescence intensity dropped from short wavelength (blue)
toward long wavelength in visible region with large than 50% ratio (from 213 to 99) as
shown in table 1.
Figure 7: Image profiles of PL spectra porous silicon prepared with a) blue laser) green
laser, and c) red laser
Conclusion
The photoluminescence in nano porous silicon layers can be explained in terms of nano
crystalline of porous layers quantum confinement, and also due to oxides or hydrogenated
surfaces of porous silicon layer, therefore we can interpreter why photo luminance spectra
PL peak shifted toward to blue shift of samples prepared under illuminated at blue laser
wavelength.
Acknowledgments
This study was completed with the help of by the Mustansiryiah University, College
Education, physics department.
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