Preparation and study of the porous Si surfaces obtained by electrochemical method
A review of original results concerning the electrochemical formation of porous Si layers and an investigation of properties inherent to the formed layers have been presented. The results related to observation of changes in pores’ morphology depending on the etching conditions, correlation of morph...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2017
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| Cite this: | Preparation and study of the porous Si surfaces obtained by electrochemical method / V. Lytovchenko, T. Gorbanyuk, V. Kladko, A. Sarikov, N. Safriuk, L. Fedorenko, Steponas Asmontas, Jonas Gradauskas, Edmundas Sirmulis, Ovidijus Zalys // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2017. — Т. 20, № 4. — С. 385-395. — Бібліогр.: 28 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1860275454490968064 |
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| author | Lytovchenko, V. Gorbanyuk, T. Kladko, V. Sarikov, A. Safriuk, N. Fedorenko, L. Asmontas, Steponas Gradauskas, Jonas Sirmulis, Edmundas Zalys, Ovidijus |
| author_facet | Lytovchenko, V. Gorbanyuk, T. Kladko, V. Sarikov, A. Safriuk, N. Fedorenko, L. Asmontas, Steponas Gradauskas, Jonas Sirmulis, Edmundas Zalys, Ovidijus |
| citation_txt | Preparation and study of the porous Si surfaces obtained by electrochemical method / V. Lytovchenko, T. Gorbanyuk, V. Kladko, A. Sarikov, N. Safriuk, L. Fedorenko, Steponas Asmontas, Jonas Gradauskas, Edmundas Sirmulis, Ovidijus Zalys // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2017. — Т. 20, № 4. — С. 385-395. — Бібліогр.: 28 назв. — англ. |
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| container_title | Semiconductor Physics Quantum Electronics & Optoelectronics |
| description | A review of original results concerning the electrochemical formation of porous Si layers and an investigation of properties inherent to the formed layers have been presented. The results related to observation of changes in pores’ morphology depending on the etching conditions, correlation of morphology of the porous layers with their surface composition, photoluminescence and structural characteristics, catalytic activity of porous Si-based MIS structures, as well as theoretical modeling of the kinetics and mechanisms of the porous Si growth have been described.
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| first_indexed | 2026-03-21T12:42:02Z |
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 4. P. 385-395.
doi: https://doi.org/10.15407/spqeo20.04.385
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
385
PACS 61.72.Dd, 78.55.Mb, 81.05.Rm
Preparation and study of porous Si surfaces
obtained using the electrochemical method
V.G. Lytovchenko1*, T.I. Gorbanyuk1, V.P. Kladko1, A.V. Sarikov1, N.V. Safriuk1, L.L. Fedorenko1,
S. Ašmontas2, J. Gradauskas2, E. Širmulis2*, O. Žalys2
1V. Lashkaryov Institute of Semiconductor Physics,
National Academy of Sciences of Ukraine
45, prospect Nauky, 03680 Kyiv, Ukraine
2Center for Physical Sciences and Technology,
Saulėtekio av., 3, LT-10257, Vilnius, Lithuania
*Corresponding authors e-mail: lvg@isp.kiev.ua1; sirmulis@gmail.com2
Abstract. Review of original results concerning electrochemical formation of porous Si
layers and investigation of properties inherent to the formed layers has been presented.
The results related with observation of changes in pores’ morphology depending on the
etching conditions, correlation of morphology of the porous layers with their surface
composition, photoluminescence and structural characteristics, catalytic activity of
porous Si based MIS structures as well as theoretical modeling of the kinetics and
mechanisms of the porous Si growth have been described.
Keywords: porous Si, electrochemical etching, gas sensing, MIS structure, photolumi-
nescence, X-ray diffraction, kinetic modeling.
Manuscript received 03.10.17; revised version received 02.11.17; accepted for
publication 07.12.17; published online 07.12.17.
1. Introduction
First demonstrated in 1990, porous silicon (PS) layers
were formed on crystalline silicon (c-Si) wafers using
electrochemical and chemical etching in concentrated
solution of HF and C2H5OH [1]. These layers exhibited
photoluminescent properties as a semiconductor with the
direct energy band gap [2]. Due to IUPAC guidelines,
porous silicon has been classified depending on pore size
as microporous, mesoporous and macroporous. Micro-
pores have width smaller than 2 nm. The width of
mesopores ranges between 2 and 50 nm. Macropores are
wider than 50 nm. Many works demonstrate that the
surface of freshly prepared PS is passivated by Si
hydrides and fluorides. However, long time exposure of
the porous Si layers to air leads to slow oxidation of the
PS surface. The presence of silicon oxide on the PS
surface stimulates the shift of PL spectra to the shorter
(blue) wavelength range. The structural data for PS are
very complicated. Some published works indicate that
highly porous crystalline silicon layers consist of Si
columns and pores or of isolated nanocrystallites [3]. On
the other hand, PS may be considered as a system of
interconnected quantum wells, the so-called “quantum
sponge” [4-6]. It is now well established that the
electropolishing of silicon begins for the current density
j > jcrit [7-9]. At the same time, anodic dissolution of p-
type c-Si in fluoric acid solution leads to fabrication of
mesoporous silicon. In this work, the structure and
composition are studied in detail depending on HF
concentration in the electrochemical solution. It is shown
that decrease in HF concentration at a constant current
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 4. P. 385-395.
doi: https://doi.org/10.15407/spqeo20.04.385
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
386
density leads to the conversion of the structures from
meso-PS to macro-PS. Thus, the composition of PS
surface is significantly changed (polymerized hydride
silicon coverage on meso-PS surface transforms to
silicon oxide on the macro-PS surface). Also, the shift of
PL peak has been observed.
2. Chemical reactions governing the electrochemical
dissolution of crystalline Si
Electrochemical formation of porous silicon layers
involves reactions of Si–Si, Si–H, Si–O, and Si–F bonds
at the surface of crystalline silicon wafers. As a rule, Si–
H species passivate the silicon surface in aqueous
solutions, while the Si–F bond is highly reactive
[10, 11]. Electronegative elements such as O and F form
more polar Si–X bonds. The surface of freshly prepared
PS is covered with a passivating layer of Si–H bonds and
small quantities of Si–F and Si–O species. Silicon is
thermodynamically unstable in water and/or air, and it
reacts spontaneously to form an oxide layer:
Si + O2 → SiO2 . (1)
SiO2 is an insulator that forms passivating films on
c-Si. Preparation of porous silicon thus requires an
additive into the solution to dissolve silicon oxide and
allow electrochemical corrosion to continue. The Si–F
bond is stronger than Si–O, and the Si–F bond enthalpy
drives the main chemical dissolution reaction preparing
porous silicon. In the presence of aqueous HF, SiO2
spontaneously dissolves in the form of SiF6
2–:
SiO2 + 6HF → SiF6
2– + 2H+ + 2H2O. (2)
The interaction reaction of SiO2 with HF is a
common industrial reaction. It is used to remove SiO2
masking layers in the processing of silicon integrated
circuits in microelectronics. The silicon hexafluoride ion
(SiF6
2–) is a stable dianion that is highly soluble in water.
Thus, fluoride is the most important additive used in
preparation of porous silicon layers, dissolving the
insulating silicon oxide that would otherwise cut down
the reaction of electrochemical corrosion [12, 13]. The
freshly etched surface of porous silicon is covered
primarily with hydride species. Residual oxides and/or
fluorides are removed by the HF electrolyte. Three types
of silicon hydrides are on the porous silicon surface:
SiH, SiH2, and SiH3. When silicon is electrochemically
etched in HF solution, Si surface becomes terminated
with H atoms. The surface Si–H species are not readily
removed in HF acid, and these must be chemically
oxidized to continue the silicon dissolution reaction.
A two-electrode electrochemical cell is usually
used to prepare porous silicon. In this case, silicon anode
is one of the working electrodes, and the oxidation
reaction takes place on its surface. The cathode counter-
electrode is typically platinum. The main oxidation and
reduction half-reactions occurring during formation of
porous silicon are as follows:
Si + 6F– + 2H+ +2h+ → SiF6
2– + H2 , (3)
2H + 2e– → H2. (4)
The electrochemical reaction (4) on the platinum
electrode is reduction of protons to hydrogen gas
molecules. The reaction (3) is the main half-reaction
responsible for porous Si formation. The most important
factor during pores formation is the availability of
valence band holes. This is primarily determined by the
doping level of silicon substrate, but it can be also
influenced by illumination, HF concentration, and the
applied electric field. As a rule, crystallographically
oriented pores are generally formed at low current
densities and can appear as facet-like structures in cross-
sectional scanning electron microscope (SEM) images,
while “current-line-oriented-pores” or “current pores”
are formed at higher current densities. These pores are
usually oriented along the normal to the surface plane of
Si wafer. There is a high concentration of valence band
holes in p-type silicon, and the etching is not limited by
their availability. In n-type silicon, however, the
deficiency of valence band holes limits the density of
pores. In this case, the surface of n-type silicon should
be illuminated for additional generation of holes in the
valence band.
3. Formation of mesopores and macropores in p-type
Si wafers
A part of the investigated PS layers was formed on (100)
oriented boron doped Si wafers (NA = 4×1015 cm–3) by
electrochemical etching using the solutions of
HF (48%):C2H5OH with the ratios 4:1 and 1:4 at the cur-
rent density jPS = 30 mA/cm2 [14-17]. Ethanol was added
to HF acid to minimize hydrogen bubble formation du-
ring the anodization process. The thickness of the for-
med PS layers (in the range of 5 to 20 nm) was measured
by Dektak 3030 auto II profilometer followed by strip-
ping. The PS porosity (~60%) was calculated using the
gravimetric method. The stripping of PS films for thick-
ness and porosity measurements was carried out in
1 M·KOH aqueous solutions. The mean pore size
(2…8 nm) and specific area (~500 m2/g) of free standing
meso-PS films were obtained by the low-temperature
nitrogen adsorption (BET) method using the ASAP 2405
microanalyser. Chemical composition of the PS layers
was studied using IR-spectroscopy in a broad range of
wavenumbers (400…4000 cm–1) by using the Fourier
transform spectrometer (Bruker). SEM and AFM inves-
tigations were carried out to study the PS layer micro-
structure. Morphology of porous Si on the n-type Si wa-
fers is characterized by the presence of macropores. PS
layers prepared on the highly doped p+- or n+-Si wafers
are mesoporous, and p-type Si yields meso- and/or
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 4. P. 385-395.
doi: https://doi.org/10.15407/spqeo20.04.385
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
387
microporous silicon. It should be noted that different
morphologies of the porous Si layers result from dif-
ferent pores formation mechanisms. The overall structu-
re of the PS layers very strongly depends on the HF con-
centration in the electrolyte. The AFM image of PS layer
can be seen in Fig. 1a. It appears that increasing the con-
centration of HF causes meso-PS on p-type of silicon
substrate with the level of doping ~1015…1016 cm–3. The
prepared PS samples were rinsed in H2O or C2H5OH
after the electrochemical etching. In the case of rinsing
in water, PS films possess the two-layer structure
(Fig. 1b) caused by its interaction with H2O during
washing [14, 15]. For the meso-PS layers, the porosity is
about 60%, and the average size of the pores determined
by BET method is in the range of 3 to 8 nm.
(a)
(b)
Fig. 1. AFM and SEM images of mesoporous silicon: (a)
surface, (b) cross-section.
Macro-PS layers fabricated in diluted HF solution
exhibit mat surface (Fig. 2) and high porosity
(~80…90%). The process of the macropores formation
during the electrochemical etching of Si may be
presented as follows. The disproportionation reaction
first induces formation of supersaturation of the
electrolyte with Si atoms during an incubation period
[17]. Then, deposition of Si atoms on the c-Si surface
starts, and microcrystalline silicon columns form
simultaneously with the substrate etching. Both these
processes take place simultaneously due to high electric
resistance of the microcrystalline Si columns. These
experimentally observed phenomena were also
confirmed by the fact that after the macro-PS layer was
removed in the diluted KOH, the pores remained on the
c-Si surface (Fig. 3).
(a)
(b)
Fig. 2. (a), (b) SEM images of macroporous surface silicon
formed on p-type silicon wafer.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 4. P. 385-395.
doi: https://doi.org/10.15407/spqeo20.04.385
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
388
Fig. 3. AFM images of surface silicon wafer after removal of
macro-PS layer.
Formation of polymerized meso-PS films may be
possible due to a chain reaction. This mechanism of the
meso-PS formation occurs only in electrolyte with high
HF concentrations (60 to 80%) and at sufficiently low
current densities (5 to 30 mA/cm2) [15, 17, 18]. In
particular, siloxane polymer has been formed on the
meso-PS surface [16, 17]. It is also important to note that
definition of pore sizes as well as measurements of the
refraction index and specific area of PS surface by BET
method need free standing PS films. The meso-PS films
easily peel off from the c-Si surface at the current
density j >> jPS (jPS = 30 mA/cm2). However, in the case
of macro-PS layers the electropolishing is initiated under
this condition. Furthermore, meso-PS interacts with
water during washing, which causes hydrogen emission.
In contrast, the macro-PS layers rarely interact with
water but are easily dissolved in 5% KOH solution.
Different origins of the meso-PS and macro-PS have
also been confirmed by the following facts: in the first
case, it is possible to obtain polymerized meso-PS films,
while in the second case microcrystalline Si columns are
coupled with pores in the silicon substrate. Comparison
of IR spectra of c-Si and macro-PS has demonstrated
(Fig. 4) that in both cases the Si–H valence vibrations is
absent. However, Si oxide bonds are present in macro-
PS layers in a large amount, so that their surfaces are
strongly oxidized. The IR absorption between 2300 and
2000 cm–1 of the meso-PS films appears due to Si–H
valence vibrations. The initial SiH band around
2100 cm–1 can be decomposed into three peaks near
2080, 2110 and 2140 cm–1, which are generally ascribed
to SiH, SiH2 and SiH3 modes, respectively. Appearance
of the OxSiH band indicates formation of SiH groups
connected to one, two or three oxygen atom with typical
positions at 2140, 2210 and 2270 cm–1, respectively. The
analysis of the low wavenumber region of meso-PS
spectra (<1000 cm–1) is more difficult since the valence
vibrations (SiFy) of (SiHxFy) molecules occur in the
same frequency range as for the deformation vibrations
(SiHx) in these molecules. We usually assign the peak at
910 cm–1 to the SiF valence vibrations [18]. Indeed,
SIMS spectra confirm the presence of the fluorinated
hydride surface of freshly etched meso-PS layers
[15, 17]. The Si–O valence vibration in meso-PS layers
does not need to be taken into account. However, the
progressive oxidation takes place.
4. Adsorption properties of mesoporous silicon
Gas sensitive structures based on meso-PS were formed
with top catalytically active electrodes (Pd, WO3) on
MIS structures shown in Fig. 5. In this case, the I–V
characteristics shown in Fig. 6 were measured for the
registration of response signal under the action of H2S in
N2 atmosphere.
We consider briefly a simplified model of the PS
based MIS-structure using the energy band diagram of
the diode structures. In fact, it is known that the metal –
porous Si structure is a very complicated Schottky-like
structure consisting of numerous connected in parallel
metal – PS junctions [14, 15, 18, 19].
400 800 1200 1600 2000 2400 2800
5
6
7
8
9
10
11
3
2
1
A
bs
or
ba
nc
e,
a
rb
. u
ni
ts
Wavenumber, cm-1
Si-H2
Si-H
Si-Si
Si
-O
O-Si-O
SiHn
Fig. 4. IR-spectra of c-Si, meso-PS and macro-PS prepared on
p-type Si wafer.
Fig. 5. Schematic representation of MIS-structure based on PS
with catalytically active top electrode.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 4. P. 385-395.
doi: https://doi.org/10.15407/spqeo20.04.385
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
389
-6 -4 -2 0 2 4
0
100
200
300
400
500
Voltage, V
Current, mA
1
2
3
4
5
6
7
1 - Nitrogen
2 - H2S 10 ppm
3 - H2S 20 ppm
4 - H2S 30 ppm
5 - H2S 40 ppm
6 - H2S 50 ppm
7 - H2S 75 ppm
Fig. 6. Effect of H2S on I–V characteristics of MIS-structures
based on meso-PS on p-type silicon substrate.
The forward current-voltage (I–V) characteristic of
a non-ideal Schottky diode behavior in the case of
thermionic emission model is:
⎥
⎦
⎤
⎢
⎣
⎡
−⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
= 1exp
nkT
qU
II D
S , (5)
where q is the elementary charge, UD – voltage applied
across the Schottky diode, k denotes the Boltzmann
constant, n stands for the ideality factor, and T is the
absolute temperature.
Is can be expressed as
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−×= ∗∗
kT
q
TAAI B
effS
φ
exp2 , (6)
where Aeff is the effective area of the diode, A**
(~8.6 сm–2K–2A) stands for the Richardson constant, φB
denots the Schottky barrier height.
We want to point out that all the responses
observed in this study may be characterized by the
Schottky barrier height φB (see Eq. (6)), which depends
on the gas environment.
Gas sensitivity of Pd/PS/Si Schottky diode based
on the PS under H2 adsorption has been studied with and
without polymer film on the porous surface for p-type
silicon substrates. Without polymer film and with
negative bias on the metal electrode with respect to
semiconductor, the current direction is assumed as
forward. In this regime, the Schottky diode is not
sensitive to the test gas adsorption. In this case, the
reverse branch of the I–V characteristic shifts under the
action of 500 ppm H2, H2O and/or O2 in nitrogen, and
this shift can reach a substantial magnitude at the applied
bias voltages 0.9 to 1.2 V. One explanation of the
observed H2 and H2O sensitivity suggests that hydrogen
or water molecules dissociate rapidly due to catalytic
action of Pd producing free hydrogen atoms. Then, the
hydrogen atoms penetrate through the Pd film with high
diffusivity and form a dipole layer at the metal-PS
interface causing an additional increase of the energy
barrier height at the metal-PS interface. As a result, the
reverse current decreases. On the other hand, adsorption
of oxygen decreases the energy barrier due to the
influence of O2
– or O– ions.
Consider now the effect of inorganic polymer film
located on the PS surface on the I–V characteristics of
the MIS structures based on p-type Si wafers. In
presence of the film, the increase of barrier height
improves the gas adsorption sensitivity in the forward
current mode. External field applied to the diode
structure leads to the rearrangement of charges inside the
polymer film. It can be assigned to migration of F– ions.
Hence, the additional charge of the F species at the
metal-polymer film interface influences the barrier
height and, thus, the current level. The uptake of
hydrogen on Pd–Cu/Pd top electrodes of the MIS
structures was followed by the fluorine species transport
to the interface of the metal-PS with polymer film on its
top. In this case, the internal electrical field within the
polymer film decreases, and an increase of the forward
current is observed. In contrast, adsorption of oxygen
gas leads to the decrease of the forward current (for p-
type Si substrate) [16].
On the other hand, the response signal from MIS
structure based on mesoporous Si with pores filled with
copper clusters was investigated by means of C–V
method. In this case, the influence of adsorption of the
gas mixtures of H2S in nitrogen on the flat-band voltage
shift (ΔVFB) of the MIS C–V curves was measured by the
high-frequency C–V method. These structures demon-
strate high sensitivity to H2S gas (Fig. 7). Measurement
of the shift ΔVFB under the action of adsorption of gas
molecules (H2, H2S) allow to analyze the adsorption
isotherms (Fig. 8).
0 100 200 300 400 500
200
210
220
230
240
250
50 ppm
40 ppm
30 ppm
20 ppm
10 ppmC
ap
ac
ita
nc
e,
p
F
Time, s
5 ppm
H2S in Air
Fig. 7. Kinetic responses of gas sensitive MIS-structures based
on meso-PS with pores filled with copper clusters under H2S
action.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 4. P. 385-395.
doi: https://doi.org/10.15407/spqeo20.04.385
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
390
0 20 40 60 80 100 120 140 160
0
50
100
150
200
250
300 1 - WO3/Pd
2 - Pd
m=1
m=0,5
m=0,5
V
ol
ta
ge
s
hi
ft,
м
В
Concentration, ppm
m=1
1
2
Fig. 8. Isotherm adsorption of H2S on gas sensitive MIS-
structure with pores filled with Cu clusters.
The isotherms were analyzed in the framework of
the Freundlich theory. The latter gives different initial
slopes and index “m” of the dependence for the ordinary
molecular (m = 1) and decomposition (m = 0.5)
processes in the case of homogeneous surface:
( )m
m
a
pp
p
N
n
∗+
=
1max
, (7)
where na is the number of molecules adsorbed by
adsorption centers, Nmax – total number of different
adsorption centers, p – gas pressure in the chamber,
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=∗
kTA
Bp aε
exp – characteristic pressure at which the
adsorption magnitude is equal to half of the possible
maximum, and εa – adsorption heat [20-23],
respectively. Fig. 8 shows the adsorption isotherms for
hydrogen sulfide dissolved in air. They have been
obtained using the gas-sensitive structures with various
electrode types: WO3–Pd–PS (curve 1) and Pd–PS
(curve 2). Approximations of these isotherms by
Freundlich Eq. (7) at different values of m (= 1 or 0.5)
demonstrated good agreement at relatively high
pressures in the case of m = 0.5 and at low pressures in
the case of m = 1. This fact testifies that the hydrogen
sulfide molecule undergoes decomposition at high H2S
pressures (p > 80 ppm), whereas hydrogen sulfide is
absorbed in the molecular form at low pressures (p <
25 ppm). The sensitivity of the structures with WO3–Pd–
PS composites with respect to H2S in air is higher than
that of the structures with Pd–PS. The voltage shift at the
ΔVFB level for H2S concentration of 30 ppm in air
amounts to 220 mV for the sensitive WO3–Pd–PS layer
(Fig. 8, curve 1), which is higher than for the Pd–PS
structure (≤180 mV; Fig. 8, curve 2). This is the result of
decomposition of H2S molecules adsorbed on the surface
of WO3, which gives rise to the strong enhancement of
the H2S sensitivity.
5. Photoluminescence (PL) spectra of porous Si
Fig. 9 shows the PL spectra of the meso-PS and macro-
PS layers. In the case of macro-PS, the PL intensity
decreases by the factor of ~2 and the peak position
shifts to the shorter wavelength range. It may be
associated either with the surface composition or
possibly with the surface microstructure and the
porosity of the PS layers. The red photoluminescence
observed in the meso-PS samples may be related with
polymerized hydride. At the same time, the green-blue
PL has been found in oxidized microcrystalline silicon
in the case of macro-PS layers that is in good
accordance with IR-measurements.
The adsorption of glycine (aminoacid) leads to the
increase of PL intensity almost two times as well as to
the shift of the PL peak to the long-wave region (from
660 to 700 nm) caused by the increase in the average
size of nc-Si as a result of the recovery process on the PS
surface by glycine molecules (Fig. 10) [22].
525 600 675 750 825 900
0
20
40
60
2
1
hνex=3.7 eV
T=300 K
P
L
In
te
ns
ity
, a
rb
. u
ni
ts
Wavelength, nm
Fig. 9. PL spectra of mesoporous and macroporous silicon on
p-type Si wafers.
550 600 650 700 750 800 850 900
0,0
0,2
0,4
0,6
0,8
1,0
2
Sipor/c-Si p-type (1)
1 -blue-glycine
2 - green-initial
λ, nm
P
L,
a
rb
. u
ni
ts
1
Fig. 10. Effect of glycine adsorption on PL spectra of meso-
PS.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 4. P. 385-395.
doi: https://doi.org/10.15407/spqeo20.04.385
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
391
6. Study of the structure of the porous Si layer by
using the Х-гау scanning method
In the case of structural studies, the PS layers were
formed on silicon substrates of p-type, B-doped with
crystallographic orientation of the surface (100) and the
resistivity of 7.5 Ω·cm. To fill the pores with copper, Cu
layers deposited on PS surface using the magnetron
deposition technique at room temperature followed by
annealing at 450 °C in Ar. The current densities during
formation of porous silicon layers were jPS = 3 mA/cm2
and jPS = 30 mA/cm2.
The deformation state, structure parameters and
dislocation density of the porous silicon were examined
using high-resolution X-ray diffraction (HRXRD) with
PANalytical X'Pert Pro MRD XL (X’Pert, PANalytical
B.V., Almelo) equipped with a CuKα1 source of
radiation (λ = 0.15406 nm), standard four-bounce
Ge(220) monochromator and three-bounce (220) chan-
nel-cut Ge analyzer. The ω/2θ XDP of the symmetrical
004 and 111, 333 reflections were measured. The po-
rosity of PS films and their roughness were determined
using the XRR method. Diffraction curves for porous si-
licon using a magnetron method for filling pores with
copper, dependences on a current density of 3 and
30 mA/cm2 are shown in Figs. 11a and 11b, respectively.
As can be seen from Fig. 11a, with formation current of
3 mA/cm2 the broken layer consists of two layers, with
parameters of the lattice larger than the Si lattice pa-
rameter, indicating the expansion of the lattice of porous
silicon after the incorporation of copper into the pores.
While at 30 mA/cm2 porous layer consists of one layer
with less stretching deformation (Fig. 11b). Furthermore,
after annealing the halfwidth of peaks from the PS layers
both for current density 3 mA/cm2 and for 30 mA/cm2 is
slightly decreased, indicating to the improvement in
defect structure of the porous layer. The porosity of PS
films ρ and their roughness were determined by the XRR
method. In this case, the densities of the PS films and its
porosity in % were calculated (Table 1). As can be seen
from Table 1, a PS layers with smaller pores and surface
roughness are formed at a lower current density (jPS =
3 mA/cm2) as compared to the samples with the higher
current density (jPS = 30 mA/cm2) during PS formation.
68,8 69,0 69,2
100
101
102
103
104
105
3
2
In
te
ns
ity
, a
rb
. u
ni
ts
2θ−ω, deg
1
2
3
1
68,8 69,0 69,2
100
101
102
103
104
105
65
4
In
te
ns
ity
, a
rb
. u
ni
ts
2θ−ω, deg
4
5
6
(a) (b)
Fig. 11. X-ray spectra of PS layers obtained using electrochemical etching of p-type Si (100) at current densities (a) jPS =
3 mA/cm2; curves: #1 – initial curve (without embedded Cu clusters), no annealed, #2 – with embedded Cu clusters, no annealed;
#3 – with embedded Cu clusters, annealed (b) jPS = 30 mA/cm2, curves: # 4 – initial curve (without embedded Cu clusters), no
annealed, #5 – with embedded Cu clusters, no annealed; #6 – with embedded Cu clusters, annealed, Cu clusters was embedded by
magnetron method to fill the pores with copper.
Table 1. The porosity of PS layers (ρ) and their roughness (with Cu clusters embedded using the magnetron method).
Samples jPS, mA/cm2 ρ, % Roughness, nm
# 1 (p-type B-doped (100), without embedded Cu
clusters) no annealed 3 46.7 1.5
# 2 (p-type B-doped (100), with Cu clusters) no annealed 3 16.3 1.2
# 3 (p-type B-doped (100), with Cu clusters) annealed 3 14.1 0.5
# 4 (p-type B-doped (100), without embedded Cu
clusters) no annealed 30 60.5 2.1
# 5 (p-type B-doped (100), with Cu clusters) no annealed 30 34.7 1.7
# 6 (p-type B-doped (100),with Cu clusters) annealed 30 27.0 2
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 4. P. 385-395.
doi: https://doi.org/10.15407/spqeo20.04.385
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
392
7. Modeling of pores formation in Si during electro-
chemical etching
The model of the pore formation during electrochemical
etching of Si wafers was developed in our previous
publications [24]. This model enables studying evolution
of pore morphology through all the PS layer thickness in
the approximation of an array of cylindrical pores etched
along the normal to the wafer surface, by taking into
account the reagent and by–products of Si dissolution
transport within the pores [25, 26]. The generalized
model reaction of Si dissolution includes the interaction
of holes in Si wafer with hydrofluoric acid molecules
and may be presented in the following form:
2.5h+ + HF + Si → BY–PRODUCTS. (8)
The valence of the reaction (8) equal to 2.5 is chosen,
on the one hand, being based on the averaged
experimental data, and on the other hand, to make
calculations easier.
The system of differential equations describing the
kinetics of pore etching in Si is shown in Table 2, while
the respective explanations to the definitions of the
parameters of the equations are presented in Table 3
[27]. The concentration of holes in Si wafer prior to the
onset of the etching process is set equal to their
equilibrium concentration corresponding to the doping
level of the Si wafer. During the etching process, the
holes were injected from the x = 0 coordinate into the
wafer depth. The holes’ current is fixed at this point in
agreement with the galvanostatic etching regime. The
first boundary condition for holes in Table 1 is written
assuming that the flux of holes Jp at the point x = 0 is
equal to their diffusion flux to the reaction zone.
Table 2. Dynamic differential equations of the pores formation process.
Holes transport and balance:
freep CNkp
x
pD
t
p
HF
5.2
2
2
5.2−
∂
∂
=
∂
∂
Boundary conditions:
⎪⎩
⎪
⎨
⎧
=
=−
=
∂
∂
0,0
0,
Lx
x
D
J
x
p
p
p
Initial conditions:
( ) 00, pxp =
Hydrofluoric acid transport and balance:
freeCNkp
x
ND
t
N
HF
5.2
2
HF
2
NF
HF −
∂
∂
=
∂
∂
0
0
HF =
∂
∂
=xx
N
( ) 0
HF0HF , NtLN =
( )
( )0
0
HF
HF 0,
LxN
xN
−δ=
=
Bound by–products balance:
boundfree
bound aNCNkp
t
N
−=
∂
∂
HF
5.2
Free by–products transport and balance:
bound
free
free
free aN
x
N
D
t
N
−
∂
∂
=
∂
∂
2
2
Pore radius evolution:
a
free
RN
CNkp
t
R
π
Ω
=
∂
∂
2
HF
5.2
0
0
=
∂
∂
=x
free
x
N
( ) 0,0 =tLN free
Reaction places evolution:
t
RNn
t
N
at
t
∂
∂
π=
∂
∂ 2
Free places evolution:
t
bound
free N
NC −= 1
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 4. P. 385-395.
doi: https://doi.org/10.15407/spqeo20.04.385
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
393
Table 3. Definitions for Si parameters.
Symbol Parameter description
p
NHF, Nbound, Nfree
nt, Nt, Cfree
R
Na
Ω
k
a
Dp, DHF, Dfree
p0
0
HFN
holes concentration in the wafer
HF, bound by–products and free by–products concentration, respectively
surface density, total concentration and free places on pore walls portion where Si dissolution can
take place
pore radius
surface density of the pores on wafer surface
Si atomic volume
reaction constant
desorption coefficient of bound by–products of Si dissolution
hole, HF molecules and free by–products of Si dissolution diffusitivities
majority carriers (holes) concentration in Si wafer
bulk electrolyte HF concentration
The supply of HF molecules in the reaction zone
takes place by diffusion from the bulk of electrolyte,
where the concentration of HF molecules is set to a fixed
value during the whole anodization process. The
dissolution rate of Si and, hence, the pore radius kinetics
depends on the local HF concentration as well as is
additionally determined by the pores’ surface density,
Na, and the degree of passivation of internal pore surface
by the bound by–products, Cfree.
The modeled lengths of pores versus time of
electrochemical etching for different anodization current
densities are shown in Fig. 12. The results demonstrate
linear dependence of the pore length on the etching time
during the initial process stages with subsequent slowing
down at prolonged etching. Higher current densities lead
to earlier transition to the sub-linear dependence due to
the diffusion–limited supply of HF molecules to the
reaction zones deep inside the pores. In addition, linear
dependences of pore growth rate on the concentration of
HF in the electrolyte as well as on the anodization
current density are obtained.
Comparison of the results of the kinetic modeling
of pore growth process during the electrochemical
etching of Si wafers with respective experimental data
show their full consistency at a qualitative level. The
thickness of the porous Si film is known to increase
linearly with the increase of anodization time at the first
stage of etching, while the pore growth slows down at
later stages due to the decrease of electrolyte concen-
tration inside the pores [28]. Moreover, the rate of pore
growth also linearly depends on the electrolyte
concentration, and for p-type silicon it is proportional to
the anodization current density.
Analysis of the pore profiles obtained by the kinetic
modeling (see Fig. 13) reveals formation of cylindrical
pores at smaller current densities (up to ~20 mA/cm2 in
our calculations). Formation of cylindrical pores corres-
ponds to Si dissolution reaction limited by the supply of
holes to the reaction zone. At higher values of Jp, the sup-
ply of HF molecules to the reaction zone by means of dif-
fusion begins limiting the electrochemical reaction, which
leads to the preferential etching of Si closer to the surface
and to formation of “bottle-like” pores. Finally, the
merging of the neighbour pores starts, which corresponds
to the experimentally observed electropolishing stage.
Fig. 12. Dependence of the pore length on etching time. HF
concentration in the electrolyte – 40%, Si wafer resistivity –
5 Ω·cm. Anodization current density, mA/cm2: 1 – 10, 2 – 20,
3 – 40.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 4. P. 385-395.
doi: https://doi.org/10.15407/spqeo20.04.385
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
394
Fig. 13. Simulated profiles of pores etched at various current
densities. HF concentration in the electrolyte – 30%, Si wafer
resistivity – 14 Ω·cm. The anodization current density,
mA/cm2: 1 – 10, 2 – 20, 3 – 30, 4 – 40, 5 – 50.
8. Conclusion
In conclusion, we have presented the review of the
results of porous silicon properties such as morphology
and microstructure, chemical composition, photolu-
minescence and adsorption. In this study, we
demonstrated the effect of the electrolyte composition
(concentration values of HF) and density current on
porous silicon formation (the structure and chemical
composition) for p-type silicon substrates. PS layers
were formed on crystalline silicon wafers of (100)
crystallographic orientation using electrochemical
etching in the mixture of hydrofluoric acid and ethanol.
It was found that the polymerized mesoporous silicon
films were formed at high HF concentrations, whereas
the macroporous Si layers with oxidized microcrystalline
silicon have been formed at low HF concentrations on p-
type silicon substrates. FTIR spectrum measured within
the range of wavenumbers from 400 up to 4000 cm–1
exhibit the polymerized hydride films (SiH)n on the
meso-PS surface. At the same time, silicon oxide was
detected on the macro-PS surface. Photoluminescence
studies demonstrate the red photoluminescence in the
case of the polymerized (SiHn, n = 1…3) mesoporous
silicon films (passivation effect). Whereas, the oxidized
macroporous silicon layers with microcrystallites exhibit
the green-blue photoluminescence (quantum-confine-
ment effect).
Basic parameters of the PS layers such as average
pore size (2 to 8 nm for meso-PS and 500 to 3000 nm for
the macro-PS), porosity (60% for meso-PS and 86% for
macro-PS), refraction index (n = 1.75 for meso-PS), PS
layer thickness (5 to 20 μm) have been found, too.
Gas sensitive MIS structures based on mesoporous
silicon layers with top catalytically active (Pd, Cu, WO3)
electrodes have been formed to study the adsorption
properties. Gas sensitivity of these structures with top
catalytic active electrodes on PS and with pores doped
with Cu/WO3 clusters have been studied by the analysis
of I–V characteristics and by high-frequency C–V
method under the action of H2S and H2.
The theoretical model describing the kinetics of
pores formation in Si during the electrochemical etching
has been presented. The model takes into account a
simplified way of transport of reagent and by–products
of Si dissolution inside the pores, making it possible to
model the morphology evolution through all the porous
layer thickness as well as to explain the mechanisms of a
number of experimentally observed effects.
Acknowledgments
This investigation was carried out under financial
support of National Academy of Sciences of Ukraine
(Projects No. 23-2017, No. ІІІ-5-16, No. ІІІ-10-15) and
Research Council of Lithuania (Grant No. TAP-LU-5-
2016).
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|
| id | nasplib_isofts_kiev_ua-123456789-215003 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1560-8034 |
| language | English |
| last_indexed | 2026-03-21T12:42:02Z |
| publishDate | 2017 |
| publisher | Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| record_format | dspace |
| spelling | Lytovchenko, V. Gorbanyuk, T. Kladko, V. Sarikov, A. Safriuk, N. Fedorenko, L. Asmontas, Steponas Gradauskas, Jonas Sirmulis, Edmundas Zalys, Ovidijus 2026-03-06T09:56:25Z 2017 Preparation and study of the porous Si surfaces obtained by electrochemical method / V. Lytovchenko, T. Gorbanyuk, V. Kladko, A. Sarikov, N. Safriuk, L. Fedorenko, Steponas Asmontas, Jonas Gradauskas, Edmundas Sirmulis, Ovidijus Zalys // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2017. — Т. 20, № 4. — С. 385-395. — Бібліогр.: 28 назв. — англ. 1560-8034 PACS: 61.72.Dd, 78.55.Mb, 81.05.Rm https://nasplib.isofts.kiev.ua/handle/123456789/215003 https://doi.org/10.15407/spqeo20.04.385 A review of original results concerning the electrochemical formation of porous Si layers and an investigation of properties inherent to the formed layers have been presented. The results related to observation of changes in pores’ morphology depending on the etching conditions, correlation of morphology of the porous layers with their surface composition, photoluminescence and structural characteristics, catalytic activity of porous Si-based MIS structures, as well as theoretical modeling of the kinetics and mechanisms of the porous Si growth have been described. This investigation was carried out under the financial support of the National Academy of Sciences of Ukraine (Projects No. 23-2017, No. ІІІ-5-16, No. ІІІ-10-15) and Research Council of Lithuania (Grant No. TAP-LU-5-2016). en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics Preparation and study of the porous Si surfaces obtained by electrochemical method Article published earlier |
| spellingShingle | Preparation and study of the porous Si surfaces obtained by electrochemical method Lytovchenko, V. Gorbanyuk, T. Kladko, V. Sarikov, A. Safriuk, N. Fedorenko, L. Asmontas, Steponas Gradauskas, Jonas Sirmulis, Edmundas Zalys, Ovidijus |
| title | Preparation and study of the porous Si surfaces obtained by electrochemical method |
| title_full | Preparation and study of the porous Si surfaces obtained by electrochemical method |
| title_fullStr | Preparation and study of the porous Si surfaces obtained by electrochemical method |
| title_full_unstemmed | Preparation and study of the porous Si surfaces obtained by electrochemical method |
| title_short | Preparation and study of the porous Si surfaces obtained by electrochemical method |
| title_sort | preparation and study of the porous si surfaces obtained by electrochemical method |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/215003 |
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