Thin Solid Films 520 (2012) 6608–6613
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Thin Solid Films
journal homepage: www.elsevier.com/locate/tsf
Electrochemical deposition and characterization of cupric oxide thin films
V. Dhanasekaran a, T. Mahalingam a,⁎, R. Chandramohan b, Jin-Koo Rhee c, J.P. Chu d
a
Department of Physics, Alagappa University, Karaikudi 630 003, India
Department of Physics, Sree Sevugan Annamalai College, Devakottai 630 303, India
c
Millimeter-wave INnovation Technology Research Center (MINT), Dongguk University, Seoul 100‐715, Republic of Korea
d
Department of Polymer Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan
b
a r t i c l e
i n f o
Article history:
Received 5 August 2011
Received in revised form 3 July 2012
Accepted 5 July 2012
Available online 14 July 2012
Keywords:
Electrodeposition
Thin film
Cupric oxide
Surface morphology
Structural properties
Indirect band gap
a b s t r a c t
Polycrystalline cupric oxide (CuO) thin films are deposited using an alkaline solution bath employing cathodic
electrodeposition method. Thin films are electroplated at various bath temperatures onto conducting indium
tin oxide coated glass substrates. The bath temperature effects on the structural, optical and morphological properties of copper oxide films are studied and reported. X-ray diffraction studies revealed mixed phases of monoclinic and cubic for films grown at lower bath temperatures and that the deposited films at temperatures
optimized as 75 °C exhibited cubic structure with preferential orientation along a (111) plane. Texture coefficient (Tc) values are calculated for all diffraction lines and the films were highly textured (Tc > 1). The surface
morphology and surface roughness are estimated using scanning electron microscopy and atomic force microscopy, respectively and a morphology made up of pyramid shaped grains is presented. Energy dispersive analysis
by X-rays revealed that the near stoichiometric CuO thin films are obtained at optimized preparative parameters.
The refractive index is calculated using the envelop method. Also, the optical constants of CuO thin films such as
complex dielectric constant (ε) and extinction coefficient (k) are also evaluated and reported.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
In the past couple of decades, a large amount of research is carried
out on semiconducting thin films for various device applications [1]. A
good amount of literature is available on the preparation and characterization of semiconductor chalcogenide materials. The development
of semiconductor thin films is one of the key technologies for
pn-junction based devices such as diode, transistors, and light emitting diodes. Cupric oxide (CuO) has unique features such as low
cost, non-toxicity, the abundant availability of copper, a theoretical
solar cell efficiency of 18% and relatively simple formation of the
oxide layer, etc. [2]. In order to utilize CuO for electrochemical and
photoelectrochemical applications, it needs to be prepared as thin
film type electrodes. Thin films of cupric oxide have been prepared
by a number of techniques including spray pyrolysis [3], sol–gel synthesis [4] and electrodeposition [5]. Electrodeposition is an attractive
method for preparation of semiconducting thin layer materials on
conducting substrates, the main advantage being the easy control of
the growth rate through control of various deposition parameters
[6]. This method has been actively investigated for the growth of elemental, binary, and ternary thin films for variety of applications. Electrodeposition offers a facile route to control the morphologies of
interfacial structure of films by including additives in the plating
media [7], and altering pH conditions [8]. The morphological details
⁎ Corresponding author. Tel.: +91 4565 230251.
E-mail address: [email protected] (T. Mahalingam).
0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2012.07.021
of films can significantly affect the physical and chemical properties
of the resulting films. Previously, CuO films were anodically deposited
from highly alkaline solutions (pH > 13) containing Cu (II) ions [9,10].
The electrodeposition of CuO is very difficult to deposit and very few
groups have achieved this [1,5]. The effect of bath temperatures on
the preparation and characterization of CuO has not been reported in
earlier reports [11,12]. Usually, CuO exhibits mixed phases of monoclinic and cubic at low temperatures and occurrence of single cubic phase is
very difficult. In this study, the single cubic phase CuO was obtained by
altering bath temperatures without any other parameter change or agitation. The salient aim of this report is on the low cost synthesis of cubic
phase CuO thin films by electrodeposition. In the present study, we report the preparation of cupric oxide thin films by electrodeposition in
potentiostatic mode on indium tin oxide (ITO) coated glass substrates.
The structural, morphological and optical properties of thin films in
as-deposited condition were studied using X-ray diffraction, scanning
electron microscopy (SEM), atomic force microscopy (AFM) and optical
absorption techniques. The refractive index and extinction coefficient of
the CuO thin films were also estimated for deposited film and the results are discussed.
2. Experimental details
CuO thin films were grown by electrodeposition technique using
potentiostatic method. The deposition process depends on various
parameters such as deposition potential, bath temperature, solution
pH and electrolyte concentration. A standard three electrode cell
V. Dhanasekaran et al. / Thin Solid Films 520 (2012) 6608–6613
was used for the electrodeposition CuO. Indium doped tin oxide (ITO)
was used as working electrode, graphite rod as counter electrode, and
a saturated calomel electrode (SCE) as the reference electrode. ITO
coated glass substrates were first cleaned in acetone, and thoroughly
rinsed with distilled water. The electrodeposition of CuO was carried
out cathodically from an aqueous bath composed of CuSO4 and L(+)
tartaric acid. The deposition of CuO thin films was carried out from an
aqueous electrolyte containing equimolar (0.03 M) concentrations of
CuSO4 and L(+) tartaric acid at a deposition potential of − 650 mV vs
SCE. The solution pH was adjusted to 11 by the addition of NaOH solution. The deposition bath temperature was varied from 30 °C to
90 °C during the deposition of CuO thin films.
Electrodeposition was carried out using an electrochemical system
consisting of PAR (EG&G Princeton Applied Research, USA Model
362A) potentiostat/galvanostat unit. Thickness of the deposited films
was measured using stylus profilometer (Mitutoyo SJ 301). X-ray diffraction [X'PERT PRO PANalytical, Netherlands] study was carried out
on CuO thin films in θ–θ geometry using CuKα (λ = 0.1540) line. Surface
morphological study was carried out using a scanning electron microscopy (Philips Model XL 30, USA). The SEM operating voltage was
employed at 20 kV. Model DSR-XE-100™ non-contacting mode atomic
force microscopy has been used for the surface analysis of CuO thin
films. Optical properties of the samples were analyzed using a UV–
Vis‐NIR double beam spectrophotometer (HR‐2000, M/S Ocean Optics,
USA).
3. Results and discussion
The variation of film thickness with deposition time for CuO thin
films prepared at various bath temperatures ranging from 45 °C to
90 °C is shown in Fig. 1. Fig. 1 represented that the film thickness increases linearly with deposition time and tends to attain saturation
after 40 min of deposition. The film thickness rapidly increases in the
first 10 min of the deposition which may be due to the presence of
more ions in electrolyte bath. The temperature of the electrolytic bath
plays an important role to control the rate of deposition and film thickness by: (i) increasing the precursor solubility and (ii) increasing the
diffusion coefficient of the species and decreasing the viscosity [13].
As soon as the deposition is started, the film thickness increases with
time due to effective mass transfer. The plating rate is almost reaching
an optimal value and if at all there is a change it is minimal and decreases due to reduction in ionic species which in turn reduces the
rate of mass transfer to the cathode. Film thickness is found to influence
the microstructural [14] and optical properties [15] of CuO thin films. In
Fig. 1. Variation of film thickness with deposition time for CuO thin films obtained at
various bath temperatures.
6609
microelectronic applications, the film thickness is to be scaled down
proportional to the device size. In this context, it is valuable to examine
the effect of film thickness on the physical properties of CuO thin films.
The structural properties of electrodeposited CuO thin films were
investigated by X-ray diffraction using CuKα radiation with λ =
0.154 nm. Fig. 2 shows the typical X-ray diffractogram of CuO films
deposited at various bath temperatures such as 45 °C, 60 °C, 75 °C
and 90 °C grown on ITO substrates. X-ray diffraction studies revealed
that deposited films are polycrystalline in nature and belong to the
cubic phase with a preferential orientation along (111) direction.
The observed peaks in the diffraction patterns were indexed and the
corresponding values of lattice spacing d were calculated and compared with standard values (JCPDS-ICDD data card no. #78-0428).
The (111) peak position is located at 2θ = 36.43 corresponding to
the lattice parameter value 0.2464 nm, for films grown at various
bath temperatures. The copper content increased considerably for
films deposited at various bath temperatures. It is found that when
the bath temperature is increased, the intensity of cubic peak is also
increased. At higher bath temperature (90 °C), high intensity (111)
peak with cubic phase of CuO thin film is observed in the X-ray diffraction patterns. CuO thin films prepared at lower bath temperatures
(45 °C and 60 °C) yielded X-ray diffraction patterns with monoclinic
peaks (− 111) and (311) (JCPDS-ICDD data card no. #89-5899) with
more intensities compared with films deposited at higher bath temperature of 90 °C. From these observations, it is found that cupric
oxide thin films possess mixed phase formation prepared at bath
temperatures 45 °C and 60 °C and tends to change into single phase
formation for film deposited at 75 °C and 90 °C as shown in Fig. 2.
The quantitative variation of crystallite size versus bath temperature of CuO thin film is shown in Fig. 3. The crystallite size D of the
films is calculated from the Debye Scherer's formula from the
full-width at half-maximum intensity (FWHM) expressed in radians,
using the formula D = [0.9λ/βcosθ], where D is the crystallite size and
β is the FWHM. The crystallite sizes are estimated as 32, 41, 43
and 38 nm for films deposited at 45 °C, 60 °C, 75 °C and 90 °C, respectively. It is observed from Fig. 3 that the deposition temperature
increases the peak intensities and crystallite size along the preferential orientation direction (111) which indicates an improvement in
the crystallinity of the films. This improvement may be due to the effective mass transfer and increased rate of deposition at higher
Fig. 2. X-ray diffraction patterns of electrodeposited CuO thin films deposited at various bath temperatures (a) 45 °C, (b) 60 °C, (c) 75 °C and (d) 90 °C.
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Fig. 3. Crystallite size and number of crystallites per unit area versus various bath
temperatures.
temperatures. The bath temperature mainly affects the crystallite size of
the film as reported by Li et al. [16]. When the bath temperature increases above 75 °C, crystallite size decreases which may be due to
the arrangement of atoms in the monophase formation of CuO thin
films.
The crystallite size increases as the peak intensities decrease and
maximum crystallite size was observed for pure cubic phase CuO
thin films deposited at 75 °C. Also, stoichiometric composition is observed for films deposited at these bath temperatures. For single
phase CuO thin films, the crystallinity and peak width are found to increase as shown in Fig. 2. The number of crystallites per unit area (N)
of the films was estimated using the formula:
N¼
t
D3
ð1Þ
where t is the thickness of the film and D is the crystallite size. It is observed that the number of crystallites per unit area is found to decrease with bath temperature. As the bath temperature is increased
up to 75 °C, it is evident that the crystallite size increases and in
turn the number of crystallites per unit area is decreased. The crystallite size decreases and the number of crystallites per unit area increases beyond 75 °C of bath temperature.
The texture coefficient (Tc) values are calculated using relations [17],
T C ðhi ki li Þ ¼
Iðhi ki li Þ 1 Iðhi ki li Þ −1
Σ
I0 ðhi ki li Þ n I 0 ðhi ki li Þ
ð2Þ
where I0 represents the standard intensity, I is the observed intensity
of (hikili) plane and n is the reflection number. The texture coefficients
for various peaks such as (111), (− 111), (220) and (222) with bath
temperature are shown in Fig. 4. Fig. 4 represents that the maximum
values of texture coefficient are observed for CuO thin film prepared
at 90 °C and also texture coefficient increases with increase of bath
temperature. The texture coefficient value of predominant peak
(111) is found to be 4.0 at bath temperature 90 °C. However, the texture coefficients of films along (− 111), (220) and (222) are found to
be less than 1. It has been reported for copper oxide film [13] earlier
that texture coefficient higher than 1 indicates preferential orientation and also indicates the abundance of grains in a given (hikili) direction. Poor texture coefficient (less than 1) in (− 111), (220) and
(222) planes indicates lack of grains oriented in the above planes.
Fig. 4. Variation of texture coefficients with bath temperatures at various lattice planes
for CuO thin films.
The high textured films are grown at maximum bath temperature
and it is revealed that the lower Tc has to be greater than unity
which indicates the abundance of grains in a given (hikili) direction.
The predominant plane orientation of the film has a high texture coefficient value.
Scanning electron microscopy is proved to be a unique, convenient and versatile method to analyze surface morphology of thin
film and to determine the grain size. Fig. 5(a–e) shows the scanning
electron micrographs of CuO thin films deposited at room temperature 30 °C, 45 °C, 60 °C, 75 °C and 90 °C, respectively. The powdery
deposit, slight amorphous nature and smooth surface morphology
are observed in CuO thin films prepared at 30 °C as shown in Fig. 5(a).
The pyramid shaped background with amorphous nature grains covered the entire surface of the film and no well defined grain boundaries
are observed but uneven shaped grains are observed in this micrograph.
In fact, bath temperature was varied from 30 °C to 90 °C. The films
obtained at 30 °C were not uniform and also surface discontinuity was
observed. The scanning electron micrograph of the film is shown in
Fig 5(a) for comparison purposes. Since the films deposited at 30 °C
are not good, the films are not subjected to any other characterization.
However, the film deposited at temperature 45 °C revealed grains
with different sizes and grain boundaries are not observed. The grain
size of the film is found to be in the range of 100 to 150 nm. The well
defined pyramid shaped grains with good crystalline nature are
observed as shown in Fig. 5(c) but various sizes of pyramid shaped
grains with coalesce of grain boundaries are observed and it may
be due to small stress in the film. However, in Fig. 5(d) the SEM
micrograph obtained at bath temperature 75 °C reveals a uniformly
constituted surface. It is observed from Fig. 5(d) that the surface
homogeneity of the films is improved. It is also observed that small
grains agglomerate to form larger grains. The surface is covered well
with more number of pyramid shaped grains. The grain sizes of CuO
thin film covering the entire surface of the film are estimated to be in
the range between 200 and 250 nm when the deposition bath
temperature is increased to 75 °C. More crystallites agglomerate to
form grains thereby the average grain size is increased considerably.
The grains tend to agglomerate as the tension may be comparatively
more over the surface of the films. It is evident that by altering the
bath temperature the surface features are modified. When the bath
temperature is increased, the surface mobility is enhanced. This in
turn allows the films to lower its total energy by grain growth and
decrease in the grain boundary areas. However, the films deposited at
temperature 90 °C revealed grains with different sizes and grain
boundaries are not observed. It is also observed that small grains
agglomerate to further bunches of larger grains but high bath
V. Dhanasekaran et al. / Thin Solid Films 520 (2012) 6608–6613
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Fig. 5. SEM micrographs of electrodeposited CuO thin films deposited at bath temperatures (a) 30 °C, (b) 45 °C, (c) 60 °C, (d) 75 °C, and (e) 90 °C.
temperature induces large amount of stress in cupric oxide thin film
and it diffuses the shape of the grains and its regular arrangement of
atoms. The grain size of CuO thin films deposited at 90 °C is estimated
to be in the range between 300 and 400 nm. The energy dispersive
analysis by X-rays shows the near stoichiometric composition for
films deposited at various bath temperatures.
Atomic force microscope studies reveal smaller grains for CuO film
grown at 45 °C and 75 °C as shown in Fig. 6a and b, respectively.
However, larger grains were observed on the surface of the films deposited at higher bath temperatures. Atomic force microscopic studies exhibit the formation of uniform CuO thin films with average
size of 150 nm for film deposited at 75 °C. AFM reveals that the granular nature of particles and agglomeration of particles are seen from
the 2D micrographs. The micrograph of film deposited at low bath
temperature shows some valleys and hillocks in different shapes.
When the bath temperature increases, valleys are found to decrease
due to the formation of regular shaped grains. The surface roughness
of the films was also measured by atomic force microscopy. A strong
dependence of the roughness on the bath temperature is observed as
shown in Fig. 7. It is found that the surface roughness changes with
bath temperature. Grain size of the films increases with bath temperature and in turn the surface roughness also increases with bath temperature. Similar behavior was reported earlier for chemical bath
deposited bismuth selenide thin films [18].
The optical parameters such as absorption coefficient and band
gap are determined from optical absorption measurements. Optical
absorption study of CuO thin films was carried out in the wavelength
range between 400 and 1100 nm at room temperature. The optical
absorption data is used to plot a graph of hν versus (αhν) 1/2, where
α is the optical absorption coefficient of the material and hν is the
photon energy. Extrapolation of the plots to the x-axis gives the
band gap energy of the CuO film deposited at various bath temperatures (Fig. 8). When the bath temperature increases, the amplitude of
atomic vibrations also increases leading to larger interatomic spacing.
The interaction between the lattice phonons and the free electrons
and holes will also affect the band gap to a smaller extent. One of the
mechanisms for electrons to be excited to the conduction band is due
to thermal energy and the conductivity of semiconductors is strongly
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Fig. 8. Band gap of CuO thin film deposited at various bath temperatures and inset figure shows the optical transmittance spectra.
s2 þ 1
T M −T m
N1 ¼ 2s
þ
2
TMTm
Fig. 6. AFM topography of CuO thin films deposited at bath temperatures (a) 45 °C and
(b) 75 °C.
dependent on the temperature of the material. The band gap energy of
CuO thin film deposited at optimized deposition conditions is 1.45 eV
and this value is in good agreement with the value reported earlier
[19,20].
Optical transmission spectra were recorded at room temperature to
obtain information on the optical properties of cupric oxide thin films.
Transmission spectrum is used to calculate the refractive index using
the envelop method proposed by Swanepoel [21,22].
1=2
2
2 1=2
n ¼ N1 þ N1 þ s
ð3Þ
ð4Þ
TM and Tm are the values of maximum and minimum transmission
values at a particular wavelength, s is the refractive index of the substrate. Refractive index can be estimated by extrapolating envelops
corresponding to TM and Tm and can be used to calculate the refractive
index (n) of the thin films applying Eqs. (3) and (4). The refractive
index was estimated using the expressions [23],
k¼
αλ
4π
ð5Þ
where α is absorption coefficient, λ is wavelength of the CuO thin film.
The optical constants are estimated in the wavelength range between
500 and 1100 nm and very good interference patterns are observed in
transmittance spectra in this range. The variation of extinction coefficient with wavelength for CuO thin films is shown in Fig. 9. The refractive index of the CuO thin films calculated using the envelop method
and the values plotted against a function of wavelength is also shown
in the inset of Fig. 9. The value of the refractive index lies between 2
and 3.5 for electrodeposited CuO thin films.
The complex dielectric constant is known to be a fundamental intrinsic material property. The real part of dielectric constant is associated with the property of slowing down the speed of light in the
material. The complex dielectric constant was determined using the
relation [23]
2
ε ¼ ε r þ εi ¼ ðn þ ikÞ
ð6Þ
where εr and εi are the real and imaginary parts of the dielectric constant respectively and are given by
2
εr ¼ n −k
2
ð7Þ
and
ε i ¼ 2nk:
Fig. 7. Surface roughness of electrodeposited CuO thin films at various bath temperatures.
ð8Þ
The imaginary part of the dielectric constant also showed the
same behavior as that of the real part, the only difference is that
V. Dhanasekaran et al. / Thin Solid Films 520 (2012) 6608–6613
Fig. 9. Variation of extinction coefficient (k) as a function of wavelength and inset figure
shows the refractive index (n) as a function of wavelength.
their values seem to be very less as compared to that of real dielectric
constant values. Fig. 10 shows the plot of the imaginary dielectric
constant as a function of wavelength for films deposited at various
bath temperatures. Also, the inset figure shows the real part of dielectric constant. The imaginary part of the dielectric constant resembles
the variation similar to that of extinction coefficient values and the
variation is also plotted. The trend observed is typical to that of a
semiconductor. In this case, the complex dielectric constant values
are increased with an increase of electrolytic bath temperature of cupric oxide thin films. The dielectric constants of both imaginary and
real values are more different beyond the visible region at various
bath temperatures prepared by CuO thin film. Hereafter all the films
have nearly equal values of dielectric constant. The real and imaginary parts of the dielectric constant provide information about the
electronic band structure [24].
4. Conclusions
Thin films of CuO were deposited onto indium doped tin oxide
coated conducting glass substrates using potentiostatic electrodeposition technique employing a potential of − 650 mV versus SCE, at a
pH of 11 ± 0.1 and with solute concentration of 30 mM. X-ray diffraction studies reveal the formation of polycrystalline CuO thin films
with cubic structure with preferential orientation along (111) plane.
However the films exhibited mixed phases of monoclinic and cubic
which become cubic at higher bath temperatures. The structural parameters such as crystallite size are evaluated and their dependence
with bath temperature is studied and reported. The optimum bath
temperature has been identified as 75 °C on the basis of structural homogeneity. The surface is covered well with more number of pyramid
shaped grains. The grain sizes of CuO thin film covered the entire surface of the film and are estimated to be in the range between 200 and
250 nm. The band gap of CuO thin films obtained at optimized condition in the present work is found to be around 1.45 eV which is in
confirmation to the value reported earlier. The value of refractive
index and extinction coefficient is calculated and reported. Also,
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Fig. 10. Imaginary (εi) part of dielectric constant as a function of wavelength and inset
figure shows the real (εr) part of dielectric constant as a function of wavelength for CuO
thin films.
highly textured films have higher crystallinity and low band gap
which are suitable for opto-electronic applications.
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