The document summarizes research on the morphological properties of porous silicon prepared from p-type bulk silicon using electrochemical etching. Porosity and layer thickness were determined using gravimetric and SEM imaging methods. Results showed that porosity initially increased then decreased with etching time, while layer thickness first increased then decreased and later increased again. SEM images visually showed the porous silicon formation and removal of layers over time. In conclusion, the reaction rate between acid and silicon affected properties like porosity and layer thickness due to quantum confinement effects.

1. RESEARCH ARTICLE
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Morphological Properties of Nanocrystalline Silicon from
p-Type Bulk Silicon
Ahmed K. Al-Kadumi* and M. AL-Baghdadi
In this paper, the topography properties as layer thickness, porosity, and
shape are studied for porous silicon prepared by using electro-chemical
etching method from p-type bulk silicon like one side mirror with hydrofluoric
acid (39%–43%) and ethanol (99.9%) (2:1), the etching time is (2, 4, 6, 8, and
10) min, with potential (6 V DC), the porosity and layer thickness can
determine by using gravimetric method and SEM image to study the
comparison between the image for different etching time.
1. Introduction
In the last years, the searchers interested in the quantum con-
finement effect, this is because of that effect gives new mate-
rials with deferent properties which have a wide application in
technology,[1,2]
porous silicon is one of this branches which make
nanosilicon from crystalline bulk silicon, this causes a new mate-
rial deferent from bulk silicon as a new electrical properties and
new optical properties, one can note the deferent solar cell from
bulk silicon and solar cell from porous silicon,[3,4]
the properties
of porous silicon related with the prepare conditions of porous
silicon as the etching time or electrical current, electrolyte so-
lution concentration, etc., layer thickness one of properties of
porous silicon which causes variables applications in technology
like gas sensor, ideal electrical diode,[5,6]
one of the important
properties of the porous silicon is the porosity, which is mean
the number of air pores with respect the area which make the
walls of nanocrystalline silicon where the quantum confinement
effect appear, this effect can be a good base for multi-application
and make nanocomposite by mixture between two or more mat-
ters to improve the characteristics of this new matters,[7,8]
in the
present work the porous silicon prepared from bulk silicon and
the porosity and layer thickness were determine according to the
conditions prepared and one can see the effect of the concentra-
tion and the etching time on the morphology porous silicon.
A. K. Al-Kadumi, M. AL-Baghdadi
Department of Physics
College of Education for Pure Sciences
University of Kerbala
Karbala 56001, Iraq
E-mail: ahmedkalkadumi@gmail.com
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/masy.202100306
DOI: 10.1002/masy.202100306
2. Experimental Section
In this work, the samples were prepared by
using electrochemical etching for bulk sili-
con p-type, one like a mirror with resistiv-
ity (1-10 Ω cm) and orientation (100) made
in England, cutting the bulk silicon into
pieces with (1.3 × 1.6) cm and every sam-
ple dealing with HF and ethanol (1:10) to re-
move any oxidation and dirty, by using elec-
trochemical etching method to prepare the
samples with different etching time (2, 4, 6,
8, and 10) min, Figure 1 shows the system-
atic which assisted prepared the samples.
After making porous silicon, the sample rinsed with deionized
water and put in the plastic container with ethanol.
2.1. The Measurements
The morphology of porous silicon can be discovered by two
branches:
(a) scanning electron microscope: the topography for porous
silicon can be discovered by taking SEM image for the samples,
in the present work the SEM took for the samples in the Tehran
University, from these images one can know about the behaviour
of the nanocrystalline for silicon and the shape for the porosity
and layer thickness for porous silicon.
(b) porosity and layer thickness: the porosity P of porous silicon
can be deduced by the relation[9]
:
P =
m1 − m2
m1 − m3
(1)
where m1 is the weight of the sample before electrochemical
etching, m2 the weight after making the porous silicon layer on
the sample, and m3 is the weight after removing the porous sili-
con layer.
The layer thickness L can be determined by using the
relation[7]
:
L =
m1 − m3
𝜌∗S
(2)
where 𝜌 is the density of silicon (2.3 g cm−3
), and S is the etched
surface area in the present work (0.7546 cm2
).
The ratio between layer thickness and etching time which
called the etching rate can be calculated by[10]
C =
L
t
(3)
where C is etching rate and t is etching time.
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Power
supply
The sample as the anode
electrode
HF and ethanol between
platen and sample
Platen electrode
Teflon container
Figure 1. The systematic for samples preparation.
Table 1. The etching rate during prepare the samples.
Etching time [min] Etching rate [cm s−1]
2 6.6355*10−7
4 4.7396*10−7
6 0.12323*10−7
8 9.9664*10−7
10 9.4793*10−7
3. Result and Discussion
Many researchers deal with porous silicon properties as an im-
portant new materials speciality in the last years, which improve
many devices related to these properties’ solar cells gas sensor,
etc.
The etching rate for the electrochemical etching process can
be shown in Table 1, from this table one can see the fluctuation
for the values because of the unstable reaction between the acid
and the silicon, one can see in the time etching 6 min, there is a
polishing occur which makes shot down in the reaction.
Because of that may be the acid has a few reaction charges due
to make shot down in etching rate, this fluctuation can be seen
in many papers because it is the black box for the properties of
porous silicon.[11]
The relation between porosity and etching time is shown in
Figure 2, which shows the increase in the porosity for 2 and 4 min
because the formation of porous silicon and then the bulk sili-
con become contains from air pores when the time increases the
porosity decreases gradually, this occurs because of the air pores
will be opened between them and make tranches, this causes start
to remove the porous silicon layer and start to make another new
layer, the porosity of porous silicon is very important, and many
researchers study these properties as new materials.[12–14]
Figure 3 shows the layer thickness of porous silicon as a func-
tion of etching time, from this figure one can recognize the for-
mations layer thickness and after that can see the polishing of
that layer the short time between these two states may be because
the high concentration of the acid HF/ethanol (2:1), which make
strong reaction between the acid and bulk silicon which make the
layer thickness of porous silicon and gradually decreases because
of the polishing occurs, after that the reaction between the acid
and bulk silicon become slow because the depletion in the reac-
tion charge and this cause increased in the new layer thickness
bigger than the first, the layer thickness and this result can be
shown in many types of research.[15–17]
Figure 2. The relation between the porosity and etching time.
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Figure 3. The relation between layer thickness and etching time.
Figure 4. a) SEM image for the sample at etching time 2 min. b) SEM image for the sample at etching time 6 min. c) SEM image for the sample at
etching time 10 min.
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Due to the importance of the SEM images in the determina-
tion of the morphology,[18–27]
therefore the SEM for the samples
can be seen in Figure 4a–c, these figures show the porous silicon
as preparation with etching time 2, 6, and 10 min, respectively,
from Figure 4a one can see the porous silicon formation which
can show the air pores and nanocrystalline silicon that is in the
beginning, (Figure 4b) sample under etching time 6 min shows
the removing porous silicon layer and the shot down on the reac-
tion charges, which affect on the etching rate and layer thickness
and then on the porosity, after removing layer thickness the layer
thickness makes a new porous silicon layer but with new reac-
tion, which makes new energy levels between the acid and bulk
silicon due to quantum confinement effect, this results can be
shown in many papers.[28–30]
4. Conclusion
By this paper, the increase in the HF rate concerning ethanol
makes the change in the reaction function, and this makes the
formation of porous silicon fast and the change in etching time
effect the morphological properties like porosity and layer thick-
ness due to the reaction between the bulk silicon and the acid,
from that this case effect on the many application on the technol-
ogy, and from that one can conclude this case make new energy
levels effective from the quantum confinement effect.
Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
The data that support the findings of this study are available from the cor-
responding author upon reasonable request.
Keywords
layer thickness, nanocrystalline bulk silicon, nanosilicon, porous silicon
Received: August 16, 2021
Revised: September 22, 2021
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