Optics and Photonics Journal, 2013, 3, 217-221
doi:10.4236/opj.2013.32B051 Published Online June 2013 (http://www.scirp.org/journal/opj)
Copyright © 2013 SciRes. OPJ
Photoluminescence of Por-Si with High-ordered Mosaic
Structure Received at Long Anodic Etching p-Si (100) in
the Electrolyte with an Internal Current Source
K. B. Tynyshtykbaev, V. B. Glazman, M. A. Yeleuov, A. T. Isova,
B. A. Rakymetov, D. Muratov, S.Z.Tokmoldin
Institute of Physics and Technology, Almaty, Kazakhstan
Email: kt011@sci.kz
Received 2013
Photoluminescence spectra and nature of light-emitting centers of a po rous silicon (por-Si) samples are given. The por-
Si samples had high-ordered mosaic structure, which was received under long anodic etching p-Si(100) in electrolyte
with an internal current source. The photoluminescence spectra were monitored at room temperature before and after
annealing in air and vacuum. Comparative analysis of photoluminescence spectra of the por-Si samples annealed at
different temperatures in air and vacuum shows that t he thermal annealing conditions has significant effect on the int e n-
sity and spectral content of the photoluminescence spectra. The nature of the luminescence emission centers at different
temperatures and annealing conditions was discussed.
Keywords: Photoluminescence Spectra; T hermal Annealing; Emitting Centershydrides; Oxides; Mosaic Structure;
Nanocr ystallite; P or-Si
1. Introduction
Today, a large number of works dedicated to the study of
photoluminescence (PL) properties of por-Si, promising
for practical applications, such as LEDs. The por-Si in-
cludes silicon nanocrystallites (Si-NCs) in the form of
nano wire s on t he s urfac e l aye r o f monocr ystalline silico n
with different phases of crystalline c-Si and amorphous
a-Si, covered with oxides (SiOy) and hydrides (SiHx).
The nature of light-e mitting c enters (LEC) P L is still not
a fully established and different models are offered for its
explanation [1]. One of the earliest and the most widely
used models is quantum model PL in which the lumine-
scence is determined by the recombination of excitons in
the Si-NCs. Another model suggests that the lumines-
cence related to hydride (Si-Hx) bonds on the surface of
Si-NCs po r-Si. T here is also a model of the PL related to
the presence of defect centers in oxides (SiOy) at the
interface of Si-NCs por-Si/SiO y. The most widely accepted
model for explaining the maximum intensity of PL at
λmax = 640 nm associated with the defective levels of
complexes hydrides and oxides on the surface of nanocry-
stallites (NCs), such as SiHx or SiOy (x, y = 1-4). Max-
imum PL intensity at λmax = 440 nm related to radiative
rec ombi natio n of excitons in Si-NCs por Si [2].
In our report, it considers the nature of the PL and
light- emitting ce nters in por-Si with high-ordered mosaic
struct ure (MS) , received at l ong ano dic e tching p -Si(100)
in electrolyte with an internal current source [3].
2. Experimental Results and Discussion
The PhL spectra of samples with MS of por-Si received
at room temperature before and after annealing on air and
in vacuum are presented. Samples Si (B ), ρ = 0.01 Ω∙cm
and with plane of crystallographic orientation (100), were
carried out etching in the electrolyte HF (49%): H2O2
(40 %) = 1:1. Ohmic In-contacts were created on the
back side of the samples by annealing at 300 during
30 min. Anode was p-Si, cathode - Ni. The densities of
anodic current were ja = 3 . 0 mA/ cm2. T he porous surfaces
of silicon were researched by the scanning electron
(JSM-6490LA) and atomic force (JSPM5200) micro-
scop es.
PhL excitatio n was carried out He -Cd laser at a wave-
length of λexc = 325 nm with output power 15 mW, fo-
cused on the sample surface by patch with diameter of
1.0 mm. Thermal annealing of freshly prepared samples
of por-Si was carried out both on air and under controlled
conditions of a vacuum 10-4 Torr (sputtering unit ARC
200 0) in the te mper ature r ange 50 - 500 at steps of
50o. It was fo und, that the P hL obser ved at room te mper-
ature as a red-orange glow in the place of incidence of
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the exciting laser radiation on the islets of NCs por-Si,
while the silico n led ge s and c ell- t he free sites fro m islets
no show light-emitting properties.
Islets o f NCs po r-Si is a n ens emble o f cluste rs o xi- di-
zed Si-NCs with size about 10 nm - 20 nm [4], and free
cell and ledges are the pure silicon [5]. Comparative
analysis of the PhL spectra of samples of por-Si, an-
nealed on air and in vacuum shows a significant the an-
nealing effect on parameters PhL. It is seen, that the PhL
spectra, received under these conditions have similarities
and im- portant differences. The PhL spectra of freshly
prepared samples of por-Si, received at room temperature,
repre- sent broad band and have two characteristic re-
gions of maximum intensity of the light emitting at wa-
velengths of λmax = 640 nm and λmax = 440 nm (Figure
The dominantly long-wavelength maximum of the
PhL spectrum λma x = at 640 nm usually associated with
the recombination of charge carriers at the defect centers
of the surface of hydrides and oxides coatings of Si-NCs
por-Si, while the short-wave maximum of the PL at λmax
= 440 nm with recombination of excitons in the itself of
Si-NCs [2].
In our case, in the long-wave part of spectra, received
both in air and vacuum, on the short-wave their wings i n
the region of λ = 600 nm, the inflection is observed. This
indicates on the presence of two different emitting cen-
ters. The superposition of their spectra and gives this
inflection. It can be assumed, that the emitting center
responsible for the inflection in the λ = 600 nm, due to
the presence of containing carbon-silicon complexes [6].
The presence of carbon in the samples of the por-Si ma y
be due to the adsorption of carbon-containing molecules
from the air, which is confirmed by our results of ele-
mental analysis [5]. The nature of short-wave part of the
PhL spectrum at λmax = 440 nm coincides with p ublished
data [2].
For a more complete understanding of the nature emit-
ting centers the PhL was performed thermal annealing of
Figure 1. The PhL spectrum of the samples por-Si, the re-
ceived in electrolyte HF (49%):H2O2 (40%), tetch.=4 h.
the samples por-Si on air and in vacuum, when at an-
nealing gradually removes the adsorbate, which is a reac-
tion product of etching and adsorption from the environ-
ment, and at this possibly transformation of their struc-
tures [6].
Thermal anneal ing of the samples freshly etchi ng por-
Si showed, that the maximum PhL intensity at a wave-
length of λmax = 640 nm observed at 100 annealing
both on air and in vacuum (F ig ures 2, 3). The increase
intensity of the Ph L at Tann = 100, is usually, attributed
to desorption of water molecules in this temperature
range [7,8].
In both cases are observed a broadening of the long-
wavelength wing of the PhL spectrum by shifting the
wavelength of maximum emitting and the significant
gro wth of the Ph L signal in t he highl y-energy part of the
spectrum. While its the short-wavelength wing of the
PhL spectrum is practically unchanged.
This experimental fact differs from the published re-
sults, when there is a broadening of the highly-energetic
wing the PL spectrum with shift of the maximum inten-
sity to highly-energetic region [9], or broadening of the
low-energy wing of the PL spectrum with shift of the
maximum intensity, but at this case is observed the de-
crease in PL intensity [10].
Figure 2. The PL spectrum as a function of annealing tem-
perature sample in vacuum 10 - 4 T orr.
Figure 3. Dependence of the maximum intensity of the PL
spectrum at λmax = 640 nm (a) and λ
max = 440 nm (b) as a
function of annealing temperature (-on the air, - in
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vacuum 10 -4 Torr).
Typically, according to quantum-size model the PL
maxi mum shifts to high-energy side as a result of reduc-
ing the sizes of Si-NCs. The broadening of the
low-energy wing of the PhL spectrum are explained in-
creasing the sizes Si-NCs at additional processes of hy-
drogenation and oxidation the surface Si-NCs at or after
the desorption of water. At the sa me time possible r econ-
struction of structure of hydride SiNy and oxide SiOx (x,
y = 1-3) complexes on the surface of Si-NCs in more
complex structures of various sizes. Low-energy maxi-
mum of the PhL spectrum of our samples is determined
by irradiative recombination in non-quantum-size NCs
por-Si [11], due to fluctuations potential in the highly
developed surface of por-Si, leading to a localization of
charge carriers. In our case, a significant increase in the
intensity of the low- energy maximum PL by compared
with the high-energy maximum of intensity of PhL spec-
trum is determined by the size of Si-NCs por-Si, which
are mostly of the order of 10 nm - 20 nm. It should be
noted the follo wing interes ting exp eri mental re sult, when
a small increase in vacuum of temperature annealing to
Tann. = 150, only on 50°, maximum intensity reduces to
the initial state and returns the original form of the
long-wavelength p art of the spectr um (Figure 3).
Recovery of intensity and line shape of the PhL maxi-
mum at λmax = 640 nm for small change in the annealing
temperature suggests, that the irreversibly structural
changes in the volume of Si-NCs does not occur, but
only changes the surface structure. That is, at this an-
nealing temperature take place the reconstruction of the
surf ace coating with out s ign ifica nt chan ges in thei r ch emic al
and structural state of the surface of Si-NCs por-Si. This
is confirmed by the fact that the decrease in PL intensity
occurs without a significant change in the shape of the
spectrum. All this only shows the transformation of
chemical bonding and structure the surface hydrides and
oxide complexes. That is, as a result of desorption of
water molecules from the surface of NCs por-Si take
place the transformation of complex systems at chemi-
sorptions of H and O, and thus decreases the PhL inten-
sity due to shielding of the emitting centers with these
complexes. In the case of annealing at Tann= 150 on
air maximum intensity decreases to lesser extent (Figure
3(a)) than during annealing in a vacuum, retaining the
original shape of the spectrum before annealing. The fact,
that at annealing on air the maximum intensity of the
PhL spectrum is reduced to lesser extent t han in the case
of annealing in a vacuum, can be explained the possible
formation of more complex systems, involving oxides. It
is po ssible , that they a re eit her give t he wea kly the shie l-
ding the emitting centers compared with complexes,
formed at vacuum, or in these complexes are formed
more defective emitting centers. Figure 4(a) shows, that
a further increases in air the temperature annealing to
Tann. = 2 50 the PhL i ntensit y mono tonicall y decreases,
and in range Tann. = 300 - 450, its intensity bec o mes
minimum and practically almost constant, and disappears
at Tann.= 500. This well-known experimental fact that
the decrease and disappearance of the PhL signal upon
annealing in the range Tann. = 250 - 500 due to the
formation of surface stable oxide layer, which can either
to shield emitting centers [6] or to replace hydrides on
oxid es [7]. I n addition, the PhL inte nsity can be reduced,
as we believe, due to annealing emitting defects centers
the surface complexes.
It is interesting to note, that the intensity of the PhL
signal samples por-Si, annealed at Tann= 250 - 350
in a vacuum, more higher (8 times), than signal PhL in-
tensity of the samples por-Si, annealed at the same tem-
peratures on air.
In samples annealed in a vacuum, the high PhL inten-
sity co ntinue s obser ved up to Tann.= 400, in contrast to
the samples annealed in air. Sufficiently intense PhL
signal, observed during annealing in vacuum at Tann. =
250 - 350 is explained the presence of hydride
bonds of SixHy, which were formed during etching of
λmax = 440 nm
0200 400600 8001000
Annealing temperature , oC
λmax = 640 nm
0500 1000
Figure 4. Dependence of the maximum intensity of the PL
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spectrum at λmax = 440 nm (a) and λ
max = 640 nm (b) as a
function of annealing temperat ure on the air.
crystalline silicon and is dominated by these annealing
conditions [6]. The sharp drop in PhL intensity at Tann.
above 350 is due to the loss of the hydrogen coverage
[12] as a result of dehydration of the surface emitting
NCs centers and growth the concentration of the centers
of non-emit- ting recombination due to the formation of
dangling bonds in silicon [13].
Previously, according to IR Fourier spectroscopy has
been installed complete correlation between the change
in signal PhL intensity and absorption intensity in all
types of bonds SiH [14]. About this is also evidenced by
recent results of the study the influence of aging on the
PhL of por-Si, obtained by ultrasoft x-ray emissi on spec-
troscopy USXES [1,15], which shows the influence of
phase composition of samples of the por-Si on the inte n-
sity and position of the PhL peak. In fresh samples of
por-Si dominated the amorphous hydrogenated phase of
silicon a-Si:H (48%) and crystalline phase c-Si (42%),
and only 10% are oxide phases SiO x + SiO 2.
Thus, the lo ng -wave PhL spectrum at λmax = 640 nm is
changed of due to formation the hydride complexes
(SixHy)n, in the process of porous formation.
For the PhL sig nal in t he shor t-wave range with λmax =
440 nm (Figure 3(b)) in samples annealed in air, there
are two maxima at Tann. = 100, and more intense at
Tann.= 200. Upon fur t her annealing them on air, ther e is
a monotonic decrease of PhL signal with a small splash
at Tann. = 350.
In samples annealed in a vacuum, apart from the peak
intensity o f the P hL signal at Tann = 100 has increase
in sig nal i n the r egio n 2 00 - 250, with access on the
plateau at Tann. = 250 - 300. At Tann. = 350 has a
sharp increase in the PhL signal, which at further at
Tann.= 400 rapidly decreases and disappears at 500.
This ambiguous behavior of photoluminescent proper-
ties of por-Si during annealing in air and vacuum in the
short-wave side of the PhL spectrum can be explained by
the manifestation of e mitting center s in Si-N Cs due to t he
change of the phase and elemental composition of a mul-
ticomponent, complex structure of the surface of NCs
por-Si. The increase the PhL signal at 100 both in air
and in vacuum due to the removal of water, which
screened emitting NCs centers. There is a second maxi-
mum PhL at Tann. = 200 in air, which is more intense,
than the first at 10 0. This maximum may be due to the
formation of siloxane compounds during a nnealin g in air
[8]. At this temperature annealing in vacuum formation
of siloxane complexes does not occur, therefore the PhL
signal don’t observed.
Decreasing the PhL signal at 200 and above on air
take place due to oxidation of the surface of Si-NCs. T his
may be as a result of the formation of stable oxides at
removing hydrogen from the complexes of the type
The growth of the signal PhL intensity at Tann. above
200 in vacuum is explained due to the formation of
different hydride complexes (SiyHx)n. At Tann. above
350 take place dehydrogenation surface emitting
centers Si-NCs and as is observed a sharp decrease of
PhL i ntensit y. T he ob se r ve s mal l inc r ea se o f t he intensi ty
of the PhL signal at Ta nn.= 350 on air has a similar
nature, as for por-Si samples a nneale d i n vac uum.
The intensity of the long-wave part of the PhL spec-
trum for 1 hour exposure of laser radiation decreases
more strongly (8 times), than in the short-wave part (2
times). A further increase of exposure time practically
does not change the intensity of the PhL spectra.
The change of PhL intensity is reversible, after stop
of illumination is observed the restoration of the original
PhL intensity in during several hours. Such a reversible
change signal PhL can be explained by changes in the
dielectric constant of the medium surrounding the Si-
NCs [6] through of photo-stimulated reactions on the
surface of por-Si at illumination [8]. At this forms of
dangling bonds and as a consequence non-emitting re-
combination centers. At exposed on air take place re-
versible “healing” of dangling bonds in the process of
natural oxid atio n of t he surface o f N C s por-Si.
Finally, the high-temperature annealing air at T =
800 leads to the appearance the PL signal (Figure 4
(a), (b)).
This fact we associate with the internal of oxidized of
silicon at these annealing temperatures on the air, when
the formation of light-emitting centers in the form of
oxides of silicon. Such is not observed during annealing
in a vacuu m.
3. Conclusions
Thus, 1) maximum intensity of PhL spectra of por-Si
samples a t Tann. = 100 on air and in vacuum is changed
of due to thermal desorption of H2O molecules from the
surface of light-emitting centers in the por-Si; 2) long-
wave part of the PhL spectrum of samples por-Si in the
field λmax = 640 nm for freshly prepared samples is due to
emitti ng rec ombinat ion of exc itons i n non-quan-tum-size
crystallites por-Si dimensions, mainly of 10 nm - 20 nm
and surface hydride complexes of the type (SixNy)n,
formed in the process a pore formation; 3) low intensity
short-wave part of the PhL spectrum of for freshly pre-
pared samples in the region λmax = 440 nm due to of
quantum-sized crystallites (1 nm to 1.5 nm); 4) thermal
variation of the maxima of the intensity of the short and
long-wave part of the PhL spectrum is due to the trans-
formation of hydride SiH-bonds in the porous surface
duri ng a n nea li n g in va c uu m a nd o n ai r i n t he tempe ra t ur e
range Tann. = 200 - 400; 5) d egradation the emitting
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centers at Tann.> 400 and complete absence of PhL at
Tann.> 500 are observed due to the delete of hydride
SiH-bonds on the porous surface; 6) long excitation of
PL leads to heating that transforms of hydride complexes.
This fact confirms, that the light emitting centers are
formed due to hydride coverages of surface NCs por-Si;
7) a nnealing at Tann = 8 00 of por-Si on air leads to the
formation of light-emitting centers in the form of oxides
of silicon.
4. Acknowledgements
This work was supported financially by the Ministry of
Science and Education of Republic of Kazakhstan.
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