Citation: Suchikova, Y.; Kovachov, S.;
Bohdanov, I.; Popova, E.; Moskina,
A.; Popov, A. Characterization of
CdxTeyOz/CdS/ZnO
Heterostructures Synthesized by the
SILAR Method. Coatings 2023, 13, 639.
https://doi.org/10.3390/
coatings13030639
Academic Editor: Panos Poulopoulos
Received: 19 February 2023
Revised: 11 March 2023
Accepted: 15 March 2023
Published: 17 March 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
coatings
Communication
Characterization of CdxTeyOz/CdS/ZnO Heterostructures
Synthesized by the SILAR Method
Yana Suchikova 1,* , Sergii Kovachov 1, Ihor Bohdanov 1, Elena Popova 2, Aleksandra Moskina 3
and Anatoli Popov 3
1 The Department of Physics and Methods of Teaching Physics, Berdyansk State Pedagogical University,
71100 Berdyansk, Ukraine
2 Centro de Investigación en Astronomía, Universidad Bernardo O’Higgins, Santiago 8370854, Chile
3 Institute of Solid State Physics, University of Latvia, 8 Kengaraga, 1063 Riga, Latvia
* Correspondence: yanasuchikova@gmail.com
Abstract: CdxTeyOz/CdS/ZnO heterostructures were obtained by the SILAR method using ionic
electrolytes. A CdS film was formed as a buffer layer for better adhesion of the cadmium-tellurium
oxides to the substrate surface. In turn, the ZnO substrate was previously prepared by electrochemical
etching to form a rough textured surface. In addition, an annealing mode was used in an oxygen
stream to complete the oxidation process of the heterostructure surface. The resulting nanocomposite
was investigated using RAMAN, XRD, SEM, and EDX methods. We assume that the oxides CdO and
TeO4 initially form on the surface and later evolve into TeO2 and TeO3 when saturated with oxygen.
These oxides, in turn, are the components of the ternary oxides CdTeO3 and CdTe3O8. It should be
noted that this mechanism has not been fully studied and requires further research. However, the
results presented in this article make it possible to systematize the data and experimental observations
regarding the formation of cadmium-tellurium films.
Keywords: heterostructures; films; oxides; SILAR method; electrolytes; annealing
1. Introduction
The development of modern electronics and new technologies requires new materials
with different properties. Significant progress in this area has been achieved by nanos-
tructuring the surface of semiconductors which is already a well-known technique and
has been studied in detail. In particular, quantum dots [1,2], nanoneedles [3,4], porous
surfaces [5,6], thin films [7,8], etc., have received considerable attention. In recent years,
there has been a trend toward the design of multilayer structures due to the prospect of
their application in laser technology [9,10], such as optical filters [11,12], photocatalysts [13],
sensors [14], and materials for solar cells [15,16].
The main technological tasks facing researchers are the search for inexpensive starting
materials and simple methods of synthesis. This will make it possible to bring nanotechnol-
ogy products from scientific laboratories into the industrial sector.
In this respect, semiconductors such as silicon Si [17], zinc oxide ZnO [18], cadmium
sulfide CdS [19], cadmium telluride CdTe [20], etc., have received considerable attention.
In particular, ZnO has been the subject of significant research for many years due to
its successful synthesis technology and its promise for numerous applications [21,22].
Although ZnO has a low surface nanostructuring capacity, this material is now widely used
as a substrate for the synthesis of multilayer heterostructures [23,24]. It is important to note
here that, in this aspect, a wide-gap CdS semiconductor, widely used in optoelectronics and
as a phosphor, can be a promising and inexpensive material [25,26]. It is more amenable to
nanostructuring, which is why quantum dots [27], nanowires [28], and nanotubes [29,30]
have been synthesized on its basis and studied in detail. This material is also interesting for
the creation of heterostructures as visible-light-driven photocatalysts for efficient hydrogen
Coatings 2023, 13, 639. https://doi.org/10.3390/coatings13030639 https://www.mdpi.com/journal/coatings
Coatings 2023, 13, 639 2 of 13
generation [31]. In addition, lead-free halide perovskite–COF nanocomposites are also
currently being actively considered as photocatalysts [32].
CdTe films became the basis for the creation of terrestrial solar cells due to the avail-
ability of the material and its high stability. The main problem remains the search for ohmic
contacts with cadmium telluride and the increase in the efficiency of cadmium-telluride
solar cells [33].
In recent years, oxide semiconductors, such as Ga2O3 [34–36], NiO [37], and In2O5 [38],
have gained more and more importance and interest. They exhibit quite good optoelectronic
properties and high stability due to surface self-passivation [39]. Cadmium telluride
oxidation technologies make it possible to obtain materials with a controlled band gap from
1.5 eV (for CdTe) to 3.8 eV, depending on the oxygen concentration [40].
It should be noted that CdTexOy materials have already been partially described and
studied, particularly in the case of CdO (x = 0), CdTe (y = 0), and various tellurates: CdTeO,
Cd2TeO4, Cd3TeO6, Cd3TeO6, CdTe3O8, CdTe2O5, and CdTeO3 [41–43].
The interest in these materials is due, first of all, to the search for cheap technologies
for the mass production of solar cells that do not require high-quality single crystals [44].
Additionally, the fact that the main absorption gap of CdTe is in the region of the maximum
intensity of solar radiation makes it an important material for solar energy conversion [45].
Solar cells based on CdTe/CdS heterojunctions are also a valuable option [46,47]. In
addition, it has been shown that O-enriched CdTe can form complex structures of CdxTeyOz.
This highly conductive partially amorphized material can be useful in devices exposed
to powerful radiation, for example, in extraterrestrial applications, where further lattice
disorder caused by radiation damage would not significantly affect the performance of the
device. That is, the presence of several phases in this material does not affect the efficiency
of the solar cells, but, on the contrary, it positively affects its resistance to radiation [48–51].
Another article [52] emphasized that amorphous and polycrystalline CdTeOx films can be
effectively used in devices that require insulating layers. Furthermore, complex CdxTeyOz
nanocomposites have been actively used for the manufacture of phosphors, dosimeters,
and sensors [53–56].
In this work, we report the synthesis of a complex CdxTeyOz/CdS/ZnO multilayer
heterostructure by a combination of simple methods, namely, electrochemical deposition
and the SILAR method. These methods are characterized by speed, simplicity, and low
cost. In addition, they allow for the controlled synthesis of structures of high quality on a
large surface area. Furthermore, we investigated the chemical and structural composition
of the formed heterostructure, as well as the morphological characteristics of the surface.
2. Materials and Methods
2.1. Materials and Samples for the Experiment
Preparation of the substrate. Single crystals of high-crystalline hexagonal ZnO were
used as the substrate. ZnO single crystals were polished on both sides and cut into
1 cm × 1 cm × 0.5 cm plates. Before the experiment, the samples were degreased with
vinegar and washed in a water-alcohol solution. To better adhere the deposited substrates
to the substrate surface, texturing of the ZnO surface was carried out. For this, an anodic
electrochemical reaction was carried out using electrochemical etching in a hydrochloric
acid solution (HCl:H2O:C2H5OH = 2:1:1) at constant voltage U = 5 V for 10 min in illumi-
nation mode with a 250 W Osrm XBO xenon lamp (Osram, Regensburg, Germany) at a
distance of 10 cm from the semiconductor surfaces. As a result, the microrelief containing
terraces, steps, and pores was formed on the ZnO surface.
Preparation of the solution for CdS deposition (solution 1). An aqueous solution
of cadmium chloride (0.1 M CdCl2) was used to form a layer of cadmium sulfide (CdS).
Thiourea (CH4N2S) and ammonia (NH3) were added to the solution in the proportion
of components:
CdCl2:CH4N2S:NH3 = 0.1 M:0.1 M:5 M
Coatings 2023, 13, 639 3 of 13
The solution was heated to 80 ◦C with constant stirring with a magnetic stirrer for 20 min.
Solutions for the formation of CdxTeyOz nanoparticles. An aqueous solution of
sodium telluride (0.01 M Na2TeO3) was used as a tellurium source—solution 2; cadmium
sources—alcoholic solution of cadmium nitrate (0.01 M Cd(NO3)2)—solution 3.
2.2. Synthesis Methods
The successive ionic layer adsorption and reaction method (SILAR) was used to
synthesize the CdxTeyOz/CdS/ZnO heterostructure. This method includes the following
necessary steps:
• Preparation of solutions containing ions of precipitated substances;
• Immersion of the substrate in the prepared solution for the purpose of adsorption of ions;
• Washing the substrate to remove excess (non-adsorbed and weakly bound) ions from
its surface;
• Drying of samples.
The sample was dipped alternately into the prepared solutions. The peculiarity of
the method is that between each stage of deposition (immersion in ionic solutions), it is
necessary to wash the samples to remove excess reaction products. Cycles can be repeated
several times to achieve the desired result. This achieves layer-by-layer adsorption which
allows for the formation of multilayer heterostructures. The stages of the experiment and
their durations are shown in Figure 1 and Table 1.
Coatings 2023, 13, 639 4 of 14 
 
 
 
Figure 1. Scheme of the SILAR method for obtaining heterostructure CdxTeyOz/CdS/ZnO. 
One cycle of CdxTeyOz formation consisted of (3–6) stages. Five such cycles were con-
ducted. 
After the experiment, to complete the process of crystallization and oxidation, the 
samples were placed in the JetFirst (ECM Lab Solutions, Grenoble, France) high-speed 
annealing diffusion furnace in a flow of oxygen. The treatment temperature was 150 °C 
and the duration was 20 min. 
2.3. Characterization of Synthesised Structures 
The morphology of the obtained structures was studied using an SEO-SEM Inspect 
S50-B (Novations LLC, Kyiv, Ukraine) scanning electron microscope. The chemical com-
position of the surface layers was studied using the EDX method on an AZtecOne spec-
trometer with an X-MaxN20 (Oxford Instruments Group, Oxford, UK) detector. X-ray dif-
fraction was performed on a Dron-3 M diffractometer with unfiltered Cu Ka-radiation in 
the range of angles 2ϑ 10°–80° with a step of 0.01°. Raman measurements were performed 
at room temperature in a RENISHAW inVia Reflex system (Renishaw Technology Group, 
Wotton-under-Edge, UK) with an excitation wavelength of 532 nm at an intensity of 5%. 
To describe the morphological and structural characteristics, ImageJ (version 1.53q, 
2022, Wayne Rasband and contributors, Bethesda, MD, USA), Vesta (version 3.5.7, 2021, 
Koichi Momma and Fujio Izumi, Tsukuba-shi, Japan), and databases of crystallographic 
structures Crystallography Open Database (COD) software packages and The Cambridge 
Crystallographic Data Centre (CCDC) software packages were used. 
  
Figure 1. Scheme of the SILAR method for obtaining heterostructure CdxTeyOz/CdS/ZnO.
One cycle of CdxTeyOz formation consisted of (3–6) stages. Five such cycles were conducted.
After the experiment, to complete the process of crystallization and oxidation, the
samples were placed in the JetFirst (ECM Lab Solutions, Grenoble, France) high-speed
annealing diffusion fur i flo of oxygen. The treatment temperature was 150 ◦C
and the duration was 20
Coatings 2023, 13, 639 4 of 13
Table 1. Stages of the layer-by-layer formation of the CdxTeyOz/CdS/ZnO heterostructure.
Stage№ Name, Purpose Solution Duration
1 Formation of CdS/ZnO structure CdCl2:CH4N2S:NH3 =0.1 M:0.1 M:5 M 5 h
2 Rinsing 1, removal of excess reactionproducts Distilled H2O 2 min
3 Formation of CdxTeyOz/CdS/ZnOheterostructure
Aqueous solution
0.01 M Na2TeO3
10 min
4 Rinsing 3, removal of excess reactionproducts H2O2 2 min
5 Formation of CdxTeyOz/CdS/ZnOheterostructure
Alcohol solution
0.01 M Cd(NO3)2
10 min
6 Rinsing 3, removal of excessreaction products H2O2 2 min
2.3. Characterization of Synthesised Structures
The morphology of the obtained structures was studied using an SEO-SEM Inspect S50-
B (Novations LLC, Kyiv, Ukraine) scanning electron microscope. The chemical composition
of the surface layers was studied using the EDX method on an AZtecOne spectrometer
with an X-MaxN20 (Oxford Instruments Group, Oxford, UK) detector. X-ray diffraction
was performed on a Dron-3 M diffractometer with unfiltered Cu Ka-radiation in the range
of angles 2ϑ 10–80◦ with a step of 0.01◦. Raman measurements were performed at room
temperature in a RENISHAW inVia Reflex system (Renishaw Technology Group, Wotton-
under-Edge, UK) with an excitation wavelength of 532 nm at an intensity of 5%.
To describe the morphological and structural characteristics, ImageJ (version 1.53q,
2022, Wayne Rasband and contributors, Bethesda, MD, USA), Vesta (version 3.5.7, 2021,
Koichi Momma and Fujio Izumi, Tsukuba-shi, Japan), and databases of crystallographic
structures Crystallography Open Database (COD) software packages and The Cambridge
Crystallographic Data Centre (CCDC) software packages were used.
3. Results
3.1. SEM Analysis
Figure 2 shows an image of the surface morphology of the CdxTeyOz/CdS/ZnO het-
erostructure, taken at the maximum allowable approximation (magnification 100,000 times).
As a result of the excessive magnification and translucency of the material, the quality of
the image is not very high, but it allows us to assess the main morphological characteristics
of the formed structure. The surface is covered with small spherical crystallites which are
packed very tightly. The diameter of the crystallites is in the range of 70–90 nm (on the
cutouts of the morphology image in Figure 2b). At this size of nanoobjects, the presence
of quantum-dimensional effects is observed. Additionally, on the cuts of Figure 2c, one
can see the presence of another phase—in these areas, the structure is denser but has
pores with a cross-sectional size of 50–70 nm. Interestingly, the phases do not have clearly
defined boundaries; they smoothly transition to each other, forming a loose structure. This
may indicate the presence of several substances related to composition on the surface.
Figure 2d shows a transverse cleavage of the resulting heterostructure. The CdS film
is quite wide (2 µm), which is a consequence of the long exposure of the sample to the
CdCl2:CH4N2S:NH3 solution, the concentration of reagents in the electrolyte [57], and the
temperature of the electrolyte [58]. The CdxTeyOz surface film has a much smaller width
(500 nm) and exhibits a loose morphology with islands on the surface.
In addition, the interface of the heterostructure demonstrates the presence of pinholes
(Figure 3). It is believed that such holes can be a consequence of the poor wettability of the
substrate [59]. Such morphological defects can lead to increased surface recombination [60].
Coatings 2023, 13, 639 5 of 13
However, it was shown in [61] that the presence of narrow holes does not actually harm
the performance of the device and may even improve its performance to some extent.
Coatings 2023, 13, 639 5 of 14 
 
 
3. Results 
3.1. SEM Analysis 
Figure 2 shows an image of the surface morphology of the CdxTeyOz/CdS/ZnO heter-
ostructure, taken at the maximum allowable approximation (magnification 100,000 times). 
As a result of the excessive magnification and translucency of the material, the quality of 
the image is not very high, but it allows us to assess the main morphological characteristics 
of the formed structure. The surface is covered with small spherical crystallites which are 
packed very tightly. The diameter of the crystallites is in the range of 70–90 nm (on the 
cutouts of the morphology image in Figure 2b). At this size of nanoobjects, the presence 
of quantum-dimensional effects is observed. Additionally, on the cuts of Figure 2c, one 
can see the presence of another phase—in these areas, the structure is denser but has pores 
with a cross-sectional size of 50–70 nm. Interestingly, the phases do not have clearly de-
fined boundaries; they smoothly transition to each other, forming a loose structure. This 
may indicate the presence of several substances related to composition on the surface. 
Figure 2d shows a transverse cleavage of the resulting heterostructure. The CdS film is 
quite wide (2 µm), which is a consequence of the long exposure of the sample to the 
CdCl2:CH4N2S:NH3 solution, the concentration of reagents in the electrolyte [57], and the 
temperature of the electrolyte [58]. The CdxTeyOz surface film has a much smaller width 
(500 nm) and exhibits a loose morphology with islands on the surface. 
In addition, the interface of the heterostructure demonstrates the presence of pin-
holes (Figure 3). It is believed that such holes can be a consequence of the poor wettability 
of the substrate [59]. Such morphological defects can lead to increased surface recombina-
tion [60]. However, it was shown in [61] that the presence of narrow holes does not actu-
ally harm the performance of the device and may even improve its performance to some 
extent. 
 
Figure 2. SEM images of the surface (a–c) and cross-sectional view (d) of the CdxTeyOz/CdS/ZnO 
heterostructure. 
Figure 2. SEM images of the surface (a–c) and cross-sectional view (d) of the CdxTeyOz/CdS/ZnO
heterostructure.
Coatings 2023, 13, 639 6 of 14 
 
 
 
Figure 3. SEM images of the surface CdxTeyOz/CdS/ZnO heterostructure showing pinholes on the 
surface: (a) 1300× magnification shows the surface interface, (b) magnification of 108,514× shows the 
micromorphology of the surface. 
3.2. EDAX Analysis 
Figure 4 shows the EDX spectrum and the ratio of components on the surface of the 
formed structure. It can be seen that there are spectra from Zn, S, Cd, Te, and O. Zn and S 
are present at low concentrations (3.23% and 9.69%, respectively); this indicates that the 
surface of the samples is quite densely overgrown with cadmium-tellurium oxides. The 
highest concentration is oxygen, which indicates its presence in various compounds of 
cadmium and tellurium. Reflexes of other elements were not detected. 
 
Figure 4. EDX spectrum and component composition of the CdxTeyOz/CdS/ZnO surface. 
3.3. XRD Analysis 
Figure 5 shows the diffractometric spectra of the formed structure. The main peak 
corresponds to the (310) plane of CdTe3O8 (Monoclinic, Space group P2/c, Space group 
Figure 3. SEM images of the surface CdxTeyOz/CdS/ZnO heterostructure showing pinholes on the
surface: (a) 1300×magnification shows the surface interface, (b) magnification of 108,514× shows
the micromorphology of the surface.
3.2. EDAX Analysis
Figure 4 shows the EDX spectrum and the ratio of components on the surface of the
formed structure. It can be seen that there are spectra from Zn, S, Cd, Te, and O. Zn and S
are present at low concentrations (3.23% and 9.69%, respectively); this indicates that the
surface of the samples is quite densely overgrown with cadmium-tellurium oxides. The
Coatings 2023, 13, 639 6 of 13
highest concentration is oxygen, which indicates its presence in various compounds of
cadmium and tellurium. Reflexes of other elements were not detected.
Coatings 2023, 13, 639 6 of 14 
 
 
 
Figure 3. SEM images of the surface CdxTeyOz/CdS/ZnO heterostructure showing pinholes on the 
surface: (a) 1300× magnification shows the surface interface, (b) magnification of 108,514× shows the 
micromorphology of the surface. 
3.2. EDAX Analysis 
Figure 4 shows the EDX spectrum and the ratio of components on the surface of the 
formed structure. It can be seen that there are spectra from Zn, S, Cd, Te, and O. Zn and S 
are present at low concentrations (3.23% and 9.69%, respectively); this indicates that the 
surface of the samples is quite densely overgrown with cadmium-tellurium oxides. The 
highest concentration is oxygen, which indicates its presence in various compounds of 
cadmium and tellurium. Reflexes of other elements were not detected. 
 
Figure 4. EDX spectrum and component composition of the CdxTeyOz/CdS/ZnO surface. 
3.3. XRD Analysis 
Figure 5 shows the diffractometric spectra of the formed structure. The main peak 
corresponds to the (310) plane of CdTe3O8 (Monoclinic, Space group P2/c, Space group 
Figure 4. EDX spectrum and component composition of the CdxTeyOz/CdS/ZnO surface.
3.3. XRD Analysis
Figure 5 sh ws the diffractometric spectra of the formed structure. The main peak
corresponds to the (310) plane of CdTe3O8 (Monoclinic, Space group P2/c, Space group
number 13; a = 14.0660 A; b = 5.8720 A; and c = 10.5210 A). The crystallite size calculated
by Scherrer’s formula from this spectrum is 24.9 nm. Additionally, the diffraction peaks at
2θ = 20.82, 27.805, 28.513, and 33.74 correspond to the (111), (112), (440), and (020) planes of
the monoclinic structure CdTe3O8, respectively. Peaks associated with CdTeO3 are observed
at 2θ = 31.4, 54.40, 63.85, and 75.75, which correspond to the (040), (220), (004), and (110)
planes of the orthomorphic structure, respectively. The intensity of these peaks is very
weak compared to the peaks associated with CdTe3O8. This indicates the predominance of
CdTe3O8 oxide over CdTeO3 on the surface of the synthesized structure.
Coatings 2023, 13, 639 7 of 14 
 
 
number 13; a = 14.0660 A; b = 5.8720 A; and c = 10.5210 А). The crystallite size c lculated 
by Scherrer’s formula from this spectrum is 24.9 nm. Additionally, the diffraction peaks 
at 2θ = 20.82, 27.805, 28.513, and 33.74 correspond to the (111), (112), (440), and (020) planes 
of the monoclinic structure CdTe3O8, respectively. Peaks associated with CdTeO3 are ob-
served at 2θ = 31.4, 54.40, 63.85, and 75.75, which correspond to the (040), (220), (004), and 
(110) planes of the orthomorphic structure, respectively. The intensity of these peaks is 
very weak compared to the peaks associated with CdTe3O8. This indicates the predomi-
nance of CdTe3O8 oxide over CdTeO3 on the surface of the synthesized structure. 
No CdS peaks were detected. This may indicate a fairly dense CdxTeyOz nanocompo-
site film embedded in the matrix of the substrate. It may also indicate that the surface of 
the CdS/ZnO heterostructure was characterized by the presence of an oxide amorphized 
layer. 
 
Figure 5. XRD spectrum of the CdxTeyOz/CdS/ZnO heterostructure. 
In addition, it is possible to determine the presence of peaks from tellurium oxides 
TeO2, TeO3, and TeO4. Their weak intensity and proximity to the peaks of CdTe3O8 and 
CdTeO3 indicate that tellurium oxides are components of the structures of cadmium-tel-
lurium oxides. When CdTe3O8 is oversaturated with oxygen over CdTeO3, it can replace it 
with cadmium and form the compound TeO3 [62]. The transition to TeO4 also occurs due 
to the addition of oxygen: 
TeO3 + O2 → TeO4 
Noise is present in the right region of the spectrum, a halo is present in the (50–70) 
2θ region, and a very weak intensity indicates surface amorphization due to the transition 
from CdTe3O8 and CdTeO3 oxides to TeO4 and TeO2. Furthermore, in [63], the haloes are 
associated with the presence of TeO2. Evidence of the presence of tellurium oxides is also 
seen in the yellow color of the sample surface, which is characteristic of the amorphous α-
TeO3 phase. These results agree well with the previously made assumption of the presence 
of several oxides on the surface of the synthesized heterostructure. 
  
Figure 5. XRD spectrum of the CdxTeyOz/CdS/ZnO heterostructure.
Coatings 2023, 13, 639 7 of 13
No CdS peaks were detected. This may indicate a fairly dense CdxTeyOz nanocompos-
ite film embedded in the matrix of the substrate. It may also indicate that the surface of the
CdS/ZnO heterostructure was characterized by the presence of an oxide amorphized layer.
In addition, it is possible to determine the presence of peaks from tellurium oxides
TeO2, TeO3, and TeO4. Their weak intensity and proximity to the peaks of CdTe3O8 and
CdTeO3 indicate that tellurium oxides are components of the structures of cadmium-
tellurium oxides. When CdTe3O8 is oversaturated with oxygen over CdTeO3, it can replace
it with cadmium and form the compound TeO3 [62]. The transition to TeO4 also occurs due
to the addition of oxygen:
TeO3 + O2 → TeO4
Noise is present in the right region of the spectrum, a halo is present in the (50–70) 2θ
region, and a very weak intensity indicates surface amorphization due to the transition
from CdTe3O8 and CdTeO3 oxides to TeO4 and TeO2. Furthermore, in [63], the haloes are
associated with the presence of TeO2. Evidence of the presence of tellurium oxides is also
seen in the yellow color of the sample surface, which is characteristic of the amorphous
α-TeO3 phase. These results agree well with the previously made assumption of the
presence of several oxides on the surface of the synthesized heterostructure.
3.4. Raman Analysis
The Raman spectrum demonstrates the presence of spectra of high (300 cm−1, 603 cm−1)
and medium (108 cm−1, 211 cm−1, 253 cm−1) intensity (Figure 6). Some peaks extend
from the right edge and have minor low-intensity peaks (e.g., 145 cm−1, 319 cm−1, and
369 cm−1). It can also be seen that starting from 600 cm−1 and above, there is a sharp rise
in the spectrum line (“tail”).
Coatings 2023, 13, 639 8 of 14 
 
 
3.4. Raman Analysis 
The Raman spectrum demonstrates the presence of spectra of high (300 cm−1, 603 
cm−1) and medium (108 cm−1, 211 cm−1, 253 cm−1) intensity (Figure 6). Some peaks extend 
from the right edge and have minor low-intensity peaks (e.g., 145 cm−1, 319 cm−1, and 369 
cm−1). It can also be seen that starting from 600 cm−1 and above, there is a sharp rise in the 
spectrum line (“tail”). 
 
Figure 6. Raman scattering spectra of the CdxTeyOz/CdS/ZnO heterostructure. 
Spectra from the ZnO substrate were not recorded. This indicates the complete over-
growth of the surface with cadmium-containing films of sufficient thickness. The intense 
peak at 300 cm−1 is due to the main LO phonon mode of CdS. This peak is asymmetric 
toward a higher frequency compared to the typical spectrum of bulk CdS (305 cm−1). It 
also contains minor peaks at 248 and 369 cm−1. which may indicate the presence of particles 
with a large spread in size. 
The low-frequency peaks at 108 cm−1 and 145 cm−1 are attributed to zone-centered 
phonons of crystalline Te (A1 and E-symmetry, respectively). The intensity of the peak at 
108 cm−1 is medium, while that of the peak at 145 cm−1 is quite weak. This indicates a low 
concentration of free crystalline tellurium on the surface of the formed structure. 
The peak at 253 cm−1 is typical for the cubic phase of cadmium oxide (CdO). A peak 
in the region of 211 cm−1 is also attributed to СdO. Its shift to the low-frequency region of 
the spectrum compared to the typical one (216 cm−1) and a rather large width indicate the 
presence of nanometer-sized particles. 
It is interesting to note that typical CdTe peaks (165 cm−1 and 325 cm−1) [64] are not 
observed. However, there is strong noise in the 310–370 cm−1 region, because of which 
there is a rightward broadening of the 300 cm−1 peak. The appearance of such noise indi-
cates that the surface of the structure was completely oxidized with the formation of var-
ious oxide compounds of Cd and Te. 
Figure 6. Raman scattering spectra of the CdxTeyOz/CdS/ZnO heterostructure.
Spectra from the ZnO substrate were not recorded. This indicates the complete
overgrowth of the surface with cadmium-containing films of sufficient thickness. The
intense peak at 300 cm−1 is due to the main LO phonon mode of CdS. This peak is
asymmetric toward a higher frequency compared to the typical spectrum of bulk CdS
Coatings 2023, 13, 639 8 of 13
(305 cm−1). It also contains minor peaks at 248 and 369 cm−1. which may indicate the
presence of particles with a large spread in size.
The low-frequency peaks at 108 cm−1 and 145 cm−1 are attributed to zone-centered
phonons of crystalline Te (A1 and E-symmetry, respectively). The intensity of the peak at
108 cm−1 is medium, while that of the peak at 145 cm−1 is quite weak. This indicates a low
concentration of free crystalline tellurium on the surface of the formed structure.
The peak at 253 cm−1 is typical for the cubic phase of cadmium oxide (CdO). A peak
in the region of 211 cm−1 is also attributed to CdO. Its shift to the low-frequency region of
the spectrum compared to the typical one (216 cm−1) and a rather large width indicate the
presence of nanometer-sized particles.
It is interesting to note that typical CdTe peaks (165 cm−1 and 325 cm−1) [64] are not
observed. However, there is strong noise in the 310–370 cm−1 region, because of which
there is a rightward broadening of the 300 cm−1 peak. The appearance of such noise
indicates that the surface of the structure was completely oxidized with the formation of
various oxide compounds of Cd and Te.
That is, during the ion deposition and annealing of samples in an oxygen stream, we
formed the compound CdxTeyOz. Extreme cases are those when one of the coefficients
becomes zero (TeyOz, CdxTey, and CdxOz at x = 0, y = 0, and z = 0, respectively).
The most intense peak at 603 cm−1 can be associated with the first overtone mode of
2LO CdS. However, there are reports that indicate that the intense peaks in the region of
550–600 cm−1 are due to the vibrational modes of TeO4 [65]. At the same time, the intensity
of the spectrum depends on the concentration of oxygen in the compounds. That is, the
TeO4 compound is present in various types of cadmium-tellurium oxide. Note that in our
case, the intensity of the peak may be caused by the manifestation of quantum-size effects
owing to the presence of nanometer-sized crystallites. This size of the crystallites allows
them to be classified as quantum dots.
In the 600 to 800 cm−1 region, low-intensity vibrational modes are observed which are
related to the Te-O-Te vibrational bonds. It can be argued that TeO3, when saturated with
oxygen, changes to the form TeO4, which is observed in oxides CdTeO3 and CdTe3O8. In the
Raman light scattering spectrum, peaks from these structural units are observed in the region
of 850–950 cm−1. The intensity of the spectra in the region after 603 cm−1 is very low and has
a rather fluctuating characteristic. This indicates a significant amorphization of the structure
and the formation of glassy oxides, which are tellurium oxides (TeO2→ TeO3→ TeO4). This
is also evidenced by the formation of a “tail” of the spectrum in the high-frequency region.
These results agree well with the XRD and SEM analysis results.
In other words, at low oxygen concentrations, TeO4 compounds are formed which are
compatible with orthorhombic oxide CdTeO3 [66]. When the structures are oversaturated
with oxygen, oxygen atoms displace cadmium atoms with the formation of the TeO3
compound. Then, it becomes possible to transition to TeO2 oxide, which can be a component
of CdTe3O8, CdTe2O5, and CdTeO compounds [67].
4. Discussion
The SILAR method is based on the adsorption of ionic particles of solutions on the
substrate surface. Adsorption triggers a chemical reaction at the interface which leads
to the formation of an insoluble product. This is why the primary precursor from which
the thin film is formed is of crucial importance (Table 1). The atoms of this film will be
adsorption centers for the further deposition of material. The removal of excess and weakly
bonded atoms is a crucial factor for the good adhesion of films to the substrate. Therefore,
washing the sample is an important step (Figure 1).
The method of heterostructure growth chosen by us is characterized by a gradual
transition from one phase to another which allows for the removal of elastic deformations
at the interface of two materials. Thus, the porous layer formed on the ZnO surface acts as a
buffer layer for the CdS film which crystallizes in a hexagonal lattice similar to that of ZnO
but differs in the crystal lattice parameters. This buffer layer helps minimize the effects
Coatings 2023, 13, 639 9 of 13
of lattice mismatch. Then, by oxidation of the CdS surface, cubic CdO is formed. When
processing this structure in electrolytes containing tellurium, a complex nanocomposite
containing TeO2, TeO3, TeO4, CdTeO3, and CdTe3O8 is formed. The composition and
thickness of the film depend significantly on the exposure time in the precursors.
This method is a modification of the chemical precipitation method, and its advantages
include simplicity, cost effectiveness, the ability to quickly generate large-area films, and the
capability of controlling the composition of the film over a wide range without requiring a
vacuum or special temperatures [67,68].
However, the adhesion of the material to the substrate may be insufficient because
of the weak interaction and incoherence of the crystal lattices. Therefore, it is advisable to
use a buffer layer. The CdS buffer layer we used allowed us to effectively form a film of
cadmium tellurium oxide. In [69], a layer of cadmium sulfide was also used to grow CuS
films. The authors showed that such a buffer significantly improves the adhesion of the
film to the substrate. Therefore, compared to ZnO, CdS shows a better affinity for crystal
lattices with the synthesized materials CdO, CdTeO3, and CdTe3O8 (Table 2, Figure 7).
Thus, CdS serves as a reliable buffer layer and, thanks to oxidation, allows for the formation
of a transition layer of CdO for the further formation of CdxTeyOz.
Table 2. Crystal lattice parameters of ZnO, CdS, CdTe, CdO, TeO2, TeO3, TeO4, CdTeO3, and CdTe3O8.
Characteristic ZnO CdS CdTe CdO TeO2 TeO3 TeO4 CdTeO3 CdTe3O8
COD
number 1,011,258 1,011,054 - 1,011,003 9,008,125 7,035,629 - 7,041,644 -
Crystal
system Hexagonal Hexagonal Hexagonal Cubic Orthorhombic Trigonal Monoclinic Orthorhombic Monoclinic
Space group P63-mc P63mc P63mc Fm-3 m Pbca R-3c P2/c Pnma P2/c
Space group
number 186 186 186 225 61 167 14 62 13
Volume
of cell, Å−3 46.692 104.86 145.82 103.76 395.35 97.26 136.77 1196.38 771.50
a, Å 3.220 4.207 4.684 4.699 5.6 5.285 4.96 7.458 14.066
b, Å 3.220 4.207 4.684 4.699 5.75 5.285 5.23 14.522 5.872
c, Å 5.220 6.843 7.674 4.699 12.3 5.285 5.77 11.046 10.521
α, o 90 90 90 90 90 57.051 65.83 90 90
β, o 90 90 90 90 90 57.051 90 90 117.4
γ, o 120 120 120 90 90 57.051 90 90 90
Coatings 2023, 13, 639 10 of 14 
 
 
Thus, CdS serves as a reliable buffer layer and, thanks to oxidation, allows for the for-
mation of a transition layer of CdO for the further formation of CdxTeyOz. 
Table 2. Crystal lattice parameters  , dS, CdTe, CdO, TeO2, TeO3, TeO4, CdTeO3, and CdTe3O8. 
Characteristic ZnO CdS CdTe CdO TeO2 TeO3 TeO4 CdTeO3 CdTe3O8 
COD number 1,011,258 1,011,054 - 1,011,003 9,008,125 7,035,629 - 7,041,644 - 
Crystal system Hexagonal Hexagonal Hexagonal Cubic Orthorhombic Trigonal Monoclinic Orthorhombic Monoclinic 
Space group P63-mc P63mc P63mc Fm-3 m Pbca R-3c P2/c Pnma P2/c 
Space group 
number 
186 186 186 225 61 167 14 62 13 
Volume  
of cell, Å−3 46.692 104.86 145.82 103.76 395.35 97.26 136.77 1196.38 771.50 
a, Å 3.220 4.207 4.684 . 99 5.6 5.285 4.96 7.458 14.066 
b, Å 3.220 4.207 4.684 . 99 5.75 5.285 5.23 14.522 5.872 
c, Å 5.220 6.843 7.674 4.699 12.3 5.285 5.77 11.046 10.521 
α, o 90 90 90 90 90 57.051 65.83 90 90 
β, o 90 90 90 0 90 57.051 90 90 117.4 
γ, o 120 120 120 90 90 57.051 90 90 90 
 
Figure 7. Crystal lattices of the materials that make up the CdxTeyOz/CdS/ZnO heterostructure (a) 
and a schematic representation of probable CdxTeyOz oxides at different concentrations of elements 
(b). 
Solid CdTe crystallizes in a hexagonal crystal lattice of the sphalerite type [70], and 
the cadmium and tellurium atoms each have four paired atoms of a different type which 
are located in the vertices of the tetrahedron. Instead, CdO crystallizes in a cubic rock salt-
like crystal lattice. The Cd and O atoms each have six bonds with atoms of different types 
(Table 2). For these reasons, CdTe and CdS exhibit a crystal lattice match compared to the 
CdS and CdO pair. The absence of Raman peaks from CdTe indicates that Cd–O and Te–
O bonds are formed by oxygen absorption, which may block the formation of Cd–Te 
bonds [71]. In addition, the formation of CdTe on the surface of CdS is energetically dis-
advantageous compared to CdO because of the higher chemical activity of oxygen ions in 
the electrolyte, which may be represented by the hydroxyl group. 
Figure 7. Crystal lattices of the materials that make up the CdxTeyOz/CdS/ZnO heterostructure (a) and
a schematic representation of probable CdxTeyOz oxides at different concentrations of elements (b).
Coatings 2023, 13, 639 10 of 13
Solid CdTe crystallizes in a hexagonal crystal lattice of the sphalerite type [70], and
the cadmium and tellurium atoms each have four paired atoms of a different type which
are located in the vertices of the tetrahedron. Instead, CdO crystallizes in a cubic rock
salt-like crystal lattice. The Cd and O atoms each have six bonds with atoms of different
types (Table 2). For these reasons, CdTe and CdS exhibit a crystal lattice match compared
to the CdS and CdO pair. The absence of Raman peaks from CdTe indicates that Cd–O
and Te–O bonds are formed by oxygen absorption, which may block the formation of
Cd–Te bonds [71]. In addition, the formation of CdTe on the surface of CdS is energetically
disadvantageous compared to CdO because of the higher chemical activity of oxygen ions
in the electrolyte, which may be represented by the hydroxyl group.
The results of the experimental data indicate that the formed heterostructure contains
a dense layer of CdxTeyOz/CdS/ZnO oxide. Therefore, the binary oxides TeO2 and TeO4
are detected on the surface.
Table 2 also shows the affinity of the binary oxides TeO2 and TeO4 with the ternary
compounds CdTeO3 and CdTe3O8, respectively. Figure 6 shows that TeO2 is bonded to four
oxygen atoms, while TeO4 and TeO3 have six bonds each, with the difference that TeO4
has one non-bridging oxygen, that is, it is bonded to only one oxygen atom. The ternary
crystals of CdTeO3 and CdTe3O8 have only Cd-O and Te-O bonds, but no Cd-Te bonds [72].
On this basis, it can be assumed that the oxides CdO and TeO4 initially form on the surface
and later evolve into TeO2 and TeO3 when saturated with oxygen. These oxides, in turn,
are components of the ternary oxides CdTeO3 and CdTe3O8.
The resulting nanocomposite may be interesting for applications in optical waveg-
uides, photonic devices, and solar cells because of the ability to adjust the optical energy
gap by controlling the oxygen content in the oxide lamonas. Further research should
aim to establish and optimize all stages of the technological process for the synthesis of
CdxTeyOz/CdS/ZnO heterostructures with a predetermined composition of components.
5. Conclusions
We have demonstrated the possibility of forming a CdxTeyOz/CdS/ZnO heterostruc-
ture using a simple and inexpensive SILAR method. For this purpose, a six-step procedure
was applied, three of which involved keeping the sample in ionic electrolytes, and another
three which involved washing the samples in distilled water to remove weakly bound
atoms from the surface. This, in turn, ensured good adhesion of the films.
Morphological analysis of the obtained heterostructure showed the presence of
nanometer-sized spherical islands. According to the results of X-ray structural analy-
sis and Raman light scattering, binary and ternary oxides CdxTeyOz are formed on the
surface of the heterostructure, which are in both crystalline and amorphous phases. The
presence of CdTe on the surface of the structure was not recorded.
Author Contributions: Conceptualization, Y.S. and A.P.; Methodology, Y.S. and S.K.; Software, S.K.;
Validation, S.K.; Formal analysis, A.M.; Investigation, Y.S. and E.P.; Resources, Y.S., I.B. and A.P.;
Writing—original draft, Y.S. and I.B.; Writing—review and editing, A.P.; Funding acquisition, A.P. All
authors have read and agreed to the published version of the manuscript.
Funding: The study was supported by the Ministry of Education and Science of Ukraine via Project
No. 0122U000129 “The search for optimal conditions for nanostructure synthesis on the surface of
A3B5, A2B6 semiconductors and silicon for photonics and solar energy” and Project No. 0121U10942
“Theoretical and methodological bases of system fundamentalization of the future nanomaterials
experts training for productive professional activity”. In addition, the research of A.P. and Y.S. was
partly supported by COST Action CA20129 “Multiscale Irradiation and Chemistry Driven Processes
and Related Technologies” (MultIChem). A.P. thanks to the Institute of Solid-State Physics, University
of Latvia. ISSP UL as the Center of Excellence is supported through the Framework Program for
European universities, Union Horizon 2020, H2020-WIDESPREAD-01–2016–2017-TeamingPhase2,
under Grant Agreement No. 739508, CAMART2 project.
Coatings 2023, 13, 639 11 of 13
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data sharing is not applicable to this article.
Conflicts of Interest: The authors declare no conflict of interest.
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