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Materials Science in Semiconductor Processing 35 (2015) 144–148
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Materials Science in Semiconductor Processing
journal homepage: www.elsevier.com/locate/mssp
Structural and optical properties of CdTe-nanocrystals
thin films grown by chemical synthesis
E. Campos-González a, F. de Moure-Flores b,n, L.E. Ramírez-Velázquez c,
K. Casallas-Moreno d, A. Guillén-Cervantes a, J. Santoyo-Salazar a,
G. Contreras-Puente e, O. Zelaya-Angel a
a
Departamento de Física, CINVESTAV-IPN, Apdo. Postal 14-740, México D.F. 07360, Mexico
Facultad de Química, Materiales, Universidad Autónoma de Querétaro, Querétaro, Mexico
c
Escuela Superior de Ingeniería Química e Industrias Extractivas, Instituto Politécnico Nacional, México D.F., Mexico
d
Escuela Superior de Ingeniería y Arquitectura del IPN, México D.F., Mexico
e
Escuela Superior de Física y Matemáticas del IPN, México D.F. 07738, Mexico
b
a r t i c l e in f o
Keywords:
CdTe thin films
CdTe nanocrystals
Te excess
Chemical synthesis
abstract
By mans of a chemical synthesis technique stoichiometric CdTe-nanocrystals thin films were
prepared on glass substrates at 70 1C. First, Cd(OH)2 films were deposited on glass substrates,
then these films were immersed in a growing solution prepared by dissolution of Te in
hydroxymethane sulfinic acid to obtain CdTe. The structural analysis indicates that CdTe thin
films have a zinc-blende structure. The average nanocrystal size was 19.4 nm and the
thickness of the films 170 nm. The Raman characterization shows the presence of the
longitudinal optical mode and their second order mode, which indicates a good crystalline
quality. The optical transmittance was less than 5% in the visible region (400–700 nm). The
compositional characterization indicates that CdTe films grew with Te excess.
& 2015 Elsevier Ltd. All rights reserved.
1. Introduction
The recent advances in the efficiency of cadmium telluride
based terrestrial photovoltaic solar cells have achieved efficiencies of 20.4% [1]. However, with this material a predictable efficiency limit of 30% can been reached [2]. Thus, there
is still a lot of work on the CdTe preparation and on solar cells
fabrication in order to reach that limit. CdTe is a II–VI
semiconductor compound with a direct bandgap of 1.5 eV
at room temperature and a high absorption coefficient, which
means that a layer thickness of few micrometers is enough to
absorb 90% of incident photons. CdTe films can exhibit n- or
p-type electrical conductivity; cadmium excess yields n-type
while tellurium excess yields p-type conductivity [3]. In the
case of CdTe nanocrystals, they have been used also in solar
n
Corresponding author. Tel.: þ52 442 192 1200.
http://dx.doi.org/10.1016/j.mssp.2015.03.005
1369-8001/& 2015 Elsevier Ltd. All rights reserved.
cells [4], proton flux sensors [5], electrochemiluminescent
detectors [6] among others. In the solar cells fabrication, the
highest photovoltaic conversion efficiencies have been
obtained by employing the CdS/CdTe heterojunction as basic
element, where CdS is prepared by means of chemical bath
deposition (CBD) and CdTe with the close spaced sublimation
(CSS) technique. A tremendous amount of work exists in the
growth and characterization of CdS layers using chemical
synthesis. On the contrary, very low work has been published
on CdTe films prepared using all-chemical processes. The
material processing by chemical synthesis is very attractive
due to its feasibility to produce large-area thin films at low
cost [7]. As far as we know, five works on polycrystalline CdTe
thin films prepared by chemical bath have been published
until now [8–12]. Up to date, abundant work has been
published on CdTe nanocrystals, but not deposited on a substrate. The application of nanocrystalline layer in thin film
solar cells offer many advantages, the principal: the
E. Campos-González et al. / Materials Science in Semiconductor Processing 35 (2015) 144–148
nanocrystalline absorber film in solar cells can be as thin as
150 nm instead of micrometers [13]. The main aim of this
work is to report the preparation of nanocrystalline CdTe
films with suitable properties to be used in the processing of
ultra-thin CdTe/CdS solar cells by mean of chemical synthesis.
2. Experimental
The CdTe nanocrystals thin films were obtained by
reaction between Cd(OH)2 films and an alkaline stable Te
solution. The Cd(OH)2 films were grown by the chemical
bath technique on glass substrates [14], subsequently
these films were immersed in a solution containing Te at
70 1C and with a pH in the interval 12–14. The preparation
of solution containing Te represents a modification of
procedure described by Sotelo et al. [8]. In 100 ml of
distilled water were mixed: tellurium powder, sodium
hydroxide and hydroxymethane sulfinic. This solution
was stirred until a pink hue was obtained, then the
solution was filtered and then poured (at 70 1C) into a
beaker containing the Cd(OH)2 films. The Cd(OH)2 films
were immersed in this solution for 5 min. The basic
chemical reaction is Cd(OH)2 þTe(sol.)-CdTeþ2OH [8].
After deposition the films were rinsed in distilled water in
ultrasonic bath to remove possible Te excess.
The crystalline structure was determined by X-ray
diffraction (XRD) using a Siemens D5000 X-ray diffractometer, with the CuKα radiation. The nanoparticle size
was determined from the full width at half maximum
(FWHM) of the diffraction peaks using the Scherrer formula and this was supported by Transmission Electron
Microscopy (TEM) with a JEOL JEM microscope operating
at 200 kV and 106 μA. Selected area electron diffraction
(SAED) was done at camera length, L¼20 cm. The interplanar distances (d) were obtained by indexing the electron pattern and the formula d¼ λL/r, where: λ is the
electron wavelength of 0.00273 nm at 200 kV, and r is the
radius from the transmitted beam to diffracted rings.
Raman measurements were achieved by means of a
micro-Raman spectrometer (Jobin Yvon, model Labram)
using the 632.8 nm line from a He–Ne laser. Atomic
concentration measurements of the samples were determined by Energy Dispersive Spectrometry (EDS) with a
Bruker XFlash detector 5010 installed in a JEOL JSM-6300
Scanning Electron Microscope, using an acceleration voltage of 20 kV. The film thickness was measured with a KLA
Tencor D-100 profilometer. The UV–Vis spectra were
obtained with a Perkin-Elmer Lambda-2 spectrophotometer. All the characterization studies were carried out
at room temperature.
145
blende) crystalline structure. The diffraction peak at 28.781
corresponds to (012) diffraction plane of crystalline rhombohedral Te, which indicates that CdTe films grew with Te
aggregates. The peaks were indexed using the powder
diffraction files 15-0770 and 23-1000, respectively. Fig. 1
(b) displays the electron diffraction pattern. The indexation of diffraction rings corresponds to interplanar distances of CdTe cubic structure. The nanocrystal size was
calculated with the Scherrer formula. The CdTe-nanocrystal size value was 19.4 nm, which shows the nanostructured character of CdTe films. The dislocation density
and the micro-strain was calculated using the formulas
δ ¼n/D2 and ε ¼ βcotθ/4, respectively. Where: n is a factor
that when has a value of 1 gives the minimum of dislocation density, D is the nanocrystal size, ε is the micro-strain,
β is the FWHM and θ is the Bragg angle. The dislocation
density was 2.66 1015 lines/m2, while the microstrain
was 8.70 10 3. Shaaban et al. [15] report that the
microstrain of CdTe films increases as decreases the thickness, therefore the high value of microstrain of the CdTe
film grown by chemical synthesis may be due to reduced
thickness 170 nm [15].
A HRTEM image of a CdTe film is illustrated in Fig. 2(a)
and (b), the nanocrystal size observed in this picture is in
agreement with results obtained from the FWHM of the
peak-reflections in the XRD patterns. It is important to
mention that in our experiments, after mixing the sources
materials, the growth temperature increases faster than in
other published work [8] and as a consequence the faster
reaction does not allow to obtain large CdTe crystals. The
zoom in Fig. 2(b) shows the structure of CdTe (111) with
d ¼3.7 Å and Fig. 2(c) Fast Fourier Transformation (FFT)
shows {220} planes.
The Raman vibrational spectrum is exhibited in Fig. 3.
The CdTe longitudinal optical (LO) mode and its first
overtone are observed at 170 cm 1 and 340 cm 1,
respectively. It can be also observed two shoulders at
123 cm 1 and 143 cm 1 (see inset), which correspond to
A1 and E modes of the rhombohedral tellurium [16]. The
presence of Te-Raman modes is usually observed in CdTe
samples: single-crystals, films, nanocrystals, etc. [17–19].
The first CdTe overtone presence at 340 cm 1 is an
indication that the films have good crystalline quality in
spite of the nanocrystalline character of the material. From
Fig. 3 it can be observed that the Raman intensity of the
vibrational modes for the Te is greater than that corresponding to the LO of CdTe, this may be due to the fact that
Te aggregates have a crystalline nature while the CdTe has
a nanocrystalline nature as shown in the XRD analysis.
3.2. Compositional analysis
3. Results and discussion
3.1. Structural characterization
XRD pattern of a CdTe film is shown in Fig. 1(a). It can
be observed that the CdTe film has six diffraction peaks at
23.681, 28.781, 39.301, 46.461, 56.421 and 62.661. The
diffraction peaks at 23.681, 39.301, 46.461, 56.421 and
62.661 correspond to (111), (220), (311), (400) and (331)
diffraction planes, respectively, of the CdTe cubic (zinc-
In order to verify the Te excess detected in XRD, TEM
and Raman characterizations, EDS measurements were
carried out: Te and Cd concentrations are in the ranges
54–58 at% and 46–42 at%, respectively. It is widely
accepted that CdTe films with Te excess have p-type
conductivity; this suggests that these films have p-type
conductivity [3,20]. The most of techniques used to grow
CdTe films require high growth temperature (450–600 1C);
thus the high deposition temperature and the differences
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Fig. 1. (a) X-ray diffraction pattern and (b) SAED-TEM of a CdTe film grown by chemical synthesis. The reflections reveal that CdTe nanoparticles grow with
the cubic-zincblende phase. The electron diffraction pattern reflects the nano-character of the films.
Fig. 2. TEM image of a CdTe film grown by chemical synthesis. (a) High Resolution Transmission Electron Microscopy, (b) zoom of (a) and (c) FFT.
E. Campos-González et al. / Materials Science in Semiconductor Processing 35 (2015) 144–148
Fig. 3. Raman spectrum of a CdTe film grown by chemical synthesis. The
inset illustrates the deconvolution method used to separate the spectrum
in three Lorentzian bands.
147
Fig. 5. The (αhν)2 versus hν plot used to obtain the Eg value. The inset
displays the first derivative of OA with respect to hν, the relative
maximum determines Eg.
3.3. Optical properties
Fig. 4. Optical transmittance of a CdTe thin film grown by chemical
synthesis. Note that the transmittance is very low in the visible region
(400–700 nm).
in vapor pressure of materials make that the CdTe films
present Te excces [21]. It is important to mention that in
this work CdTe thin films with Te excess at atmospheric
pressure and low temperature were obtained. These
results indicate that CdTe films gown by chemical synthesis are suitable as nano-crystalline absorber layer in the
processing of ultra-thin CdTe/CdS solar cells.
Fig. 4 shows the optical transmittance of the CdTe film
grown by chemical synthesis. It can be appreciated that the
transmittance is less than 5% in the visible region of the
electromagnetic spectrum (400–700 nm). Note that the
CdTe film grown by chemical synthesis is very thin
(o200 nm) and the transmittance is less than 5%, indicating
that these CdTe films may be employed in the processing of
ultra-thin CdS/CdTe solar cells [22]. The UV-Vis optical
absorbance (OA) spectrum allows the calculation of the
forbidden energy bandgap (Eg) of the films. From the
(αhν)2 versus hν plot the Eg value was obtained from the
intercept of the linear part of the curve with the energy (hν)
axis, Eg ¼1.5870.04 eV (see Fig. 5). Where, α is the optical
absorption coefficient and hν is the photon energy. The first
derivative of OA with respect to hν is displayed in the inset
of Fig. 5. The relative maximum of the d(OA)/d(hν) versus hν
curve provides the value of Eg since the inflection point of
the OA versus hν plot, determined by means of the maximum of the first derivative of OA with respect to hν,
supplies a good estimation of Eg [23], from which
1.5870.07 eV is obtained. This result confirms the former
calculation. The Bohr radius of CdTe is 7.5 nm, this datum
shows that our CdTe nanoparticles are within both the
strong and the intermediate quantum confinement regimes.
4. Conclusions
CdTe-nanocrystals thin films by chemical synthesis were
deposited on glass substrates at 70 1C. The structural
148
E. Campos-González et al. / Materials Science in Semiconductor Processing 35 (2015) 144–148
characterization (XRD and TEM) showed that CdTe have a
zinc-blende structure. The CdTe crystallite size was 19.4 nm,
as measured from XRD patterns and TEM images. The
compositional characterization showed that CdTe have Te
excess. The film thickness was 170 nm and the transmittance less than 5% in the visible region (400–700 nm). The
band gap value reflect the quantum confinement effect in
the nanocrystals. The results make our CdTe-nanocrystalline
thin films potentially useful for ultra-thin CdS/CdTe solar
cells applications at low cost.
Acknowledgments
The authors are grateful with Paulina González-Arceo,
Marcela Guerrero and A. García-Sotelo for their technical
assistance.
The authors acknowledge financial support for this work
from Fondo Sectorial Conacyt-Sener-Sustentabilidad Energética through CeMIE-sol, within of the strategic project
number 37: “development of new photovoltaic devices and
semi-superconductor materials”.
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