Room temperature electrosynthesis of ZnSe thin films

Article abstract of DOI:10.1016/j.jallcom.2008.11.036

In the present study, we report the room temperature electrosynthesis of Zinc selenide (ZnSe) thin films on stainless steel (SS) and fluorine doped tin oxide (FTO) coated glass substrates. In addition, the influence of the substrate on the microstructural

Full text of DOI:10.1016/j.jallcom.2008.11.036

Journal of Alloys and Compounds 488 (2009) 157–162
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Room temperature electrosynthesis of ZnSe thin films
Y.G. Gudage, N.G. Deshpande, Abhay A. Sagade, Ramphal Sharma∗
Thin film and Nanotechnology Laboratory, Department of Physics, Dr. B.A.M. University, Aurangabad 431004 (M.S.), India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 July 2008
Received in revised form 1 November 2008
Accepted 5 November 2008
Available online 17 November 2008
In the present study, we report the room temperature electrosynthesis of Zinc selenide (ZnSe) thin films on
stainless steel (SS) and fluorine doped tin oxide (FTO) coated glass substrates. In addition, the influence of
the substrate on the microstructural properties of ZnSe is plausibly explained. Voltammetric curves were
recorded in order to characterize the electrochemical behaviour of Zn2+/SeO2 system. The as-deposited
ZnSe thin films have been characterized for structural (X-ray diffraction (XRD)), surface morphological
(scanning electron microscopy (SEM)), compositional (energy dispersive analysis by X-rays (EDAX)), sur-
face topographical (atomic force microscopy (AFM)) and optical absorption analysis. Formation of cubic
structure with preferential orientation along the (1 1 1) plane was confirmed from structural analysis.
A significant observation seen from the SEM micrograph was the formation of porous layer on the FTO
coated glass substrate. However, this is not seen in case of stainless steel substrate. Similar observation
was predicted in case of AFM analysis for the films deposited on FTO. The optical band gap for ZnSe thin
films was found to be 2.7 eV.
Keywords:
Electrosynthesis
ZnSe
X-ray diffraction
Cubic phase
Atomic force microscopy
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
niques for the economical and efficient growth of the films from
aqueous solution, and numerous reports are available on the depo-
Synthesis and characterization of wide gap II–VI semiconduc-
tors are of great importance, since these materials can be utilized
in a variety of optoelectronic applications, viz. solar cells, hetero-
junction LED, photoconductors, etc. Though Zinc selenide (ZnSe)
has a direct wide band gap (2.7 eV) it is exceptionally interesting
material for the applications like optoelectronics, especially for fab-
rication of the blue light diodes, also it acts as a buffer/window
layer in chalcogenide-based thin film solar cells (by constructing a
heterojunction under lattice matching conditions) [1–3].
There are several reports available on the growth of ZnSe thin
films by various techniques such as chemical vapour deposition
(CVD) [1], screen printing [4], vacuum evaporation [5,6], pulsed
laser deposition (PLD) [7], chemical bath deposition (CBD) [8,9],
etc. It has been identified that the deposition from aqueous solution
makes the process simple, economical and easily scalable. CBD and
electrochemical deposition (ECD) are the commonly known pop-
ular techniques for the deposition of films from aqueous solution.
Several authors have reported on the chemical bath deposition of
ZnSe thin films from aqueous solution [9–11]. But, in CBD, the mate-
rial wastage is high and the control over rate of deposition is rather
difficult. Electrodeposition is one of the most widely accepted tech-
sition of various thin films by this technique. But it is very surprising
to note that the electrodeposition of ZnSe thin films has not been
studied in comparison with cadmium chalcogenides. Also it is
worth to note that electrodeposition of the ZnSe is difficult because
of the wide difference in the reduction potential of Zn and Se ions
[12], and only few reports are available on the electrodeposition of
ZnSe thin films [12–17].
Abundant literature is available on preparation and character-
ization of ZnSe by using ZnSO4 and ZnCl2 as source materials foꢀrꢀ
ꢀꢀ
“Zn2+ source [12–18]. Selenious acid has been reported as a “Se2−
source by many workers [15,19]. However, no attempt is made on
electrosynthesis of the ZnSe by using zinc acetate as a source of
Zn2+ from aqueous acidic bath. In view of this, an attempt has been
made to electrosynthesize ZnSe thin films at different pH of the
bath ranging from 2 to 3 at the interval of 0.5. The as-deposited
films were studied for structural, surface morphological and opti-
cal properties. In addition, the influence of the substrate on the
microstructural properties have been studied and compared with
the reported literature wherever necessary.
2. Experimental
2.1. Preparation of ZnSe thin films
ZnSethinfilmswereelectrodepositedpotentiostaticallybyaconventionalthree-
electrode set-up. Cyclic voltammetric measurements and the ZnSe electrodeposition
were made by using a potentiostat (Princeton PerkinElmer, Applied Research Versa
stat II; Model 250/270) in the three-electrode configuration. The cell consists of
stainless steel (SS) and/or fluorine doped tin oxide (FTO) coated glass as the working
∗
Corresponding author at: Inorganic-Nanomaterials Laboratory, Department of
Chemistry, Hanyang University, Sungdong-Ku, Haengdang-dong 17, Seoul 133-791,
South Korea. Tel.: +82 2 22925212; fax: +82 2 22200762.
E-mail address: rps.phy@gmail.com (R. Sharma).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.11.036

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Y.G. Gudage et al. / Journal of Alloys and Compounds 488 (2009) 157–162
electrode, graphite as the counter electrode and saturated calomel electrode (SCE,
E0 = 244 mV vs SHE, the standard hydrogen electrode) as the reference electrode.
The working and counter electrodes were placed parallel to each other separated
by a distance of 0.3 cm. The electrolyte solutions were prepared by dissolving
Zn(CH3COO)2·2H2O and SeO2 in double distilled water. The pH of the solution was
adjusted by using 1 M H2SO4. The pH of the bath was varied from 2 to 3 at the
interval of 0.5. Digital ␮-pH meter (Systronics-361) was used to measure the pH of
electrolytic bath. Electrodepositon was carried out on the unit area (1 cm2) of the
substrates in potentiostatic mode at room temperature (27◦C) from an unstirred
bath. The substrate cleaning process is discussed elsewhere [20]. The as-deposited
films of ZnSe were further studied for various properties.
2.2. Characterization of ZnSe thin film
Thin films of ZnSe were characterized for structural, surface morphological, com-
positional and optical properties. Thickness of the film was measured by Surface
profiler (AMBIOS XP-1). The structural analysis of the films were made by an X-ray
diffractometer (Bruker AXS, D8 Advance, Germany, CuK␣ radiation at ꢀ = 1.5406 Å) in
the 2ꢁ range 20–60◦. The surface morphology and composition was studied by scan-
ning electron microscopy (SEM) and energy dispersive analysis by X-rays (EDAX)
using JEOL-JSM 6360, respectively. An atomic force microscopy (AFM) study was
done using Nanoscope IIIa (silicon nitride cantilever; tip radius of 20 nm; operated in
tapping mode) provided by Vecco digital instruments. To study the optical character-
istics of the film, absorbance and transmittance spectra were recorded in the range
300–1100 nm by means of UV–vis–NIR spectrophotometer (PerkinElmer, Lambda
25).
3. Results and discussion
3.1. Voltammogram study
Cyclic voltammetry is the best tool to monitor the electro-
chemical reactions in precursor solutions and to find the suitable
deposition potential range. Fig. 1(a) shows a cyclic voltammogram
(scan rate 50 mV/s) of 0.1 M Zn(CH3COO)2·2H2O solution on SS elec-
trode at pH 2.5 of the electrolyte bath. From this figure the reduction
potential was found to be −0.8 V vs SCE. Fig. 1(b) shows a cyclic
voltammogram of 0.01 M SeO2 solution on SS electrode at pH 2.5.
The curve shows the gradual increase in current with applied poten-
tial. So it is not possible to locate the exact deposition potential. At
the deposition potential of −0.45 V vs SCE, there is formation of
reddish elemental ‘Se’ on the working electrode.
Fig. 2(a) shows the linear sweep voltammogram (LSV) of 0.1 M
Zn(CH3COO)2·2H2O + 0.01 M SeO2 solution on SS (at pH 2.0, 2.5 and
3.0) and FTO coated glass (at pH 2.5) electrode. It can be seen, that
the overall polarization curves contain two distinct regions: first
corresponding to reduction of Se2+ to Se and second correspond-
ing to reduction of Zn2+ to Zn. These two regions are separated
by a very small plateau region (potential independent region),
where the electrolysis current is limited by mass transport, in par-
ticular by diffusion of chalcogen ions towards the cathode. The
as-controlled Se(VI) and Zn(II) co-reduction results in the forma-
tion of semiconductive ZnSe-containing layers within a potential
range of approximately −0.6 to −0.9 V vs SCE. To get the optimum
conditions for co-deposition, the deposition potential was suitably
choosen so that both the Zn and Se species get deposited uniformly.
ForstoichiometricdepositionoftheZnSeonboththeelectrodes, the
optimum deposition potential was identified to be −0.75 V vs SCE
and pH of the bath equal to 2.5.
Fig. 1. Cyclic voltammogram of (a) 0.1 M Zn(CH3COO)2·2H2O and (b) 0.01 M SeO2
solution on stainless steel (SS) electrode at pH 2.5.
The dependence of the current density on the pH of the elec-
trolyticsolutioncontaining0.1 MZn(CH3COO)2·2H2O + 0.01 MSeO2
solution is shown in Fig. 2(b). When pH of the plating solution is 2.0,
the current density decreased continuously with time. The films
deposited at this pH value were found to have very poor adher-
ence and the deposit dissolves out due to hydrogen evolution at
the electrode.
At pH 2.5 of the electrolytic bath, the current density decreases
rapidly at first and then gradually stabilizes at a constant value (this
behaviourissimilartobothworkingelectrodes;Fig. 2(b)). Whenthe
electrode potential is applied, there is a sharp rise in the current
density at the electrode surface this may be due to formation of the
ZnSe nuclei on electrode surface. After an initial surge the current
density decreases for longer time revealing the coverage of the ZnSe
ions on to the electrode surface. This phenomenon occurs due to
formation of the ZnSe nucleus on surface of the substrate, which
results in decreasing of the current densities, rapidly. When ZnSe
film was covered on the substrate, the current densities gradually
became stable. Furthermore, the pH of the plating solution affected
the composition of electrodeposited thin films also. Results of EDAX
analysis shows nearly stoichiometric Zn/Se composition ratio for
films deposited at pH 2.5.
The electrochemical reaction on the cathode takes place as fol-
lows:
Zn(CH3COO)2·2H2O → Zn2+ + (CH3COO)22− + 2H2O
Zn2+(aq) + 2e− → Zn(s)
(1)
(2)
(3)
SeO2 + H2O → H2SeO3(aq)
On reduction of H2SeO3,
H2SeO3 + 4H+ + 4e− → Se(s) + 3H2O
Zn(s) + Se(s) → ZnSe
(4)
(5)
At pH 3.0, the current density was found to be lowest. Fur-
thermore, the as-grown films were seen to be reddish in colour
(due to excess of selenium present) when observed with necked

Y.G. Gudage et al. / Journal of Alloys and Compounds 488 (2009) 157–162
159
Fig. 3. Thickness profile plot for ZnSe thin film deposited on (a) SS and (b) FTO coated
glass substrate.
Fig. 2. (a) Linear sweep voltammogram of 0.1 M Zn(CH3COO)2·2H2O + 0.01 M SeO2
solution on SS (at pH 2.0, 2.5 and 3.0) and FTO (at pH 2.5) coated glass substrate
(colour online). (b) Plot of variation of current density as a function of deposition
time (colour online). (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of the article.)
(1 1 1) plane. The (1 1 1) direction is the close-packing direction
of the zincblend structure and the deposited films are polycrys-
talline having a cubic zincblend structure. The diffraction peaks at
2ꢁ = 27.2◦, 45.1◦ and 53.5◦ are attributed to (1 1 1), (2 2 0) and (3 1 1)
planes, respectively of cubic ZnSe phase, which is confirmed using
the standard JCPDS data card [21]. Some peaks of substrates (SS
peak in Fig. 4(a) and FTO peak in Fig. 4(b)) were also observed in
the XRD patterns. The various structural parameters for ZnSe thin
films deposited on the SS and the FTO substrates at pH 2.5 are calcu-
lated using standard formulae and are systematically represented
in Table 1.
eye. In fact, the Zn/Se stoichiometric ratio was found to be
10.2/89.8.
3.2. Thickness measurement
Thickness of the film was measured with the help of surface pro-
filer. Fig. 3(a and b) shows the thickness profile plot for ZnSe thin
film deposited on SS and FTO coated glass substrate, respectively.
The edge of the film is scanned in horizontal direction (1 ␮m). At
substrate–film interface (edge of the film) there is sharp rise in ver-
tical direction of the plot. This sharp rise at the edge of the film is
attributed to the thickness of the film. From this plot the thickness
of the film deposited on SS and FTO coated glass substrate is found
out to be 0.761 and 1.062 ␮m, respectively for 60 min deposition
time.
The lattice constant (a) for plane (1 1 1) is obtained from the
X-ray analysis using the following relationship for cubic crystal [22],
ꢀ
a = d h2 + k2 + l2
(6)
Table 1
Various structural parameters of electrodeposited ZnSe thin films.
Substrate
2ꢁobs (◦)
dobs (Å)
dstd (Å)
(h k l)
a (Å)
3.3. X-ray diffraction analysis
Stainless steel (SS)
27.210
45.184
53.558
3.2747
2.0051
1.7096
3.2729
2.0046
1.7093
(1 1 1)
(2 2 0)
(3 1 1)
5.6719
5.6713
5.6703
The crystallinity and orientation of the ZnSe films are deter-
mined by X-ray diffraction (XRD) using CuK␣ radiation. Fig. 4(a and
b) shows the X-ray diffractogram of the ZnSe films grown on SS
and FTO coated glass substrate, respectively. It is observed that the
XRD pattern of the film show a most preferred orientation along
FTO coated glass
27.215
45.190
53.560
3.2741
2.0048
1.7096
3.2729
2.0046
1.7093
(1 1 1)
(2 2 0)
(3 1 1)
5.6709
5.6706
5.6702

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Y.G. Gudage et al. / Journal of Alloys and Compounds 488 (2009) 157–162
film deposited on the FTO gives rise to extremely different mor-
phology (porous nature) as compared to film deposited on the
SS substrate (compact granular nature). Generally, semiconduc-
tor films with porous structures showed improved performance of
solar cells [25]. Not only this, but for various applications like opti-
cal band pass filters [26] and high sensitive chemical sensors also
require the surface to be porous in nature [27]. Therefore consid-
ering our future goal of the photovoltaic device application (as a
window layer), the films deposited on the FTO substrate are better.
The porous nature of the film will improve the photovoltaic perfor-
mance by enhancing light trapping ability (anti-reflection coating
property) [28]. Fig. 6(a and b) shows a typical two-dimensional
(2D) and three-dimensional (3D) AFM image (2 ␮m × 2 ␮m) of the
ZnSe thin film deposited on the FTO substrate. The 2D image of
the film resembles very well with the morphology of SEM micro-
graph (Fig. 5(b)). The surface evolution of the films showed “hill”
like structures (Fig. 6(b)) which is compact and uniform covering
completesubstratesurface. Thesurfaceroughnesswas0.48 nm. The
surface roughness is unavoidable due to three-dimensional growth
of the film [29].
To study stoichiometry of the film quantitative analysis was car-
ried out using the EDAX technique. Fig. 7 shows a typical EDAX
pattern for the ZnSe film deposited on the SS substrate. The ele-
mental analysis was carried out only for Zn and Se; the average
atomic percentage of Zn:Se was 42.18:57.82 showing that the sam-
ple was slightly selenium rich, which is in good agreement with the
reports of Riverous et al. and Bouroushian et al. [16,19].
Fig. 4. XRD pattern of ZnSe thin film deposited on (a) SS and (b) FTO coated glass
substrate at pH 2.5.
where h, k, l are the miller indices and ‘dhkl’ is the distance
between the crystallographic planes. The lattice parameter cal-
culated (a = 5.67 Å) matches very well with the standard value
reported by other workers [23,24].
3.4. Morphological and compositional analysis
The surface morphology of electrosynthesized ZnSe thin film
was investigated using SEM and AFM technique. Scanning elec-
tron microscopy has been proved to be a unique, convenient and
versatile method to analyze surface morphology of a film and to
determine the grain size. Fig. 5(a and b) shows the SEM images
of ZnSe thin film deposited at optimized preparative parameters
on the SS and the FTO substrate, respectively. From SEM images,
it is observed that the as-deposited ZnSe film is uniform, without
cracks with dense surface morphology covering entire substrate
surface area. Films deposited on the SS substrate revealed dense
morphology of irregular grains (average size 220 20 nm). How-
ever, the films deposited on the FTO substrate revealed that grains
were very small in size with no well-defined grain boundaries.
The substrate morphological features, besides any epitaxial effect,
vigorously define the nucleation and subsequent formation of the
semiconductive layers. In addition, the substrate plays an important
role in defining the surface properties like the grain distribution,
porosity, etc. Hence, from the SEM micrograph of the ZnSe thin
Fig. 5. SEM micrograph ZnSe thin film deposited on (a) SS and (b) FTO coated glass
substrate.

Y.G. Gudage et al. / Journal of Alloys and Compounds 488 (2009) 157–162
161
Fig. 8. Plot of variation of (a) absorbance and transmittance vs wavelength (colour
online) and (b) (˛hꢂ)2 vs (hꢂ) for ZnSe thin film. (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of the article.)
Fig. 6. (a) Two-dimensional (2D) and (b) three-dimensional (3D) AFM image of ZnSe
thin film deposited on FTO coated glass substrate.
3.5. Optical analysis
Fig. 8(a) shows transmittance and absorbance spectrum for the
ZnSe thin film. The transmission is high in the red–infrared region
(>80% for ꢀ > 600 nm) and decrease to a value of 55% in the blue
region (the transmission is 55.8% at 405 nm). The transmittance
data was used to calculate the absorption coefficient (˛) using
Tauc’s relation
˛hꢂ = A(hꢂ − Eg)1/2
(7)
where hꢂ is the photon energy, Eg is the band gap energy and A is
some constant. Further, absorption coefficient (˛) can be simplified
as,
− ln(T)
˛ =
(8)
t
where T is the transmittance and t is the thickness of the film. The
transmittance data was used to determine the thickness and extinc-
tion coefficient [30] of the film using envelop method [31]. The
thickness of the film comes out to be 1.023 ␮m, which is very close
tothevaluedeterminedwiththehelpofsurfaceprofiler(1.062 ␮m).
The extinction coefficient and refractive index was found out to be
0.313 and 1.65, respectively.
Fig. 8(b) shows the variation of (˛hꢂ)2 against (hꢂ) for the ZnSe
thin film. Extrapolating the straight-line portion of the plot of
(˛hꢂ)2 vs (hꢂ) for zero absorption coefficient value gives the band
Fig. 7. Representative EDAX spectra of ZnSe thin film deposited on SS substrate.

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Y.G. Gudage et al. / Journal of Alloys and Compounds 488 (2009) 157–162
gap, which is found to be 2.7 eV and is in good agreement with the
value reported earlier [15].
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The authors are thankful to The Head, Department of Physics,
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Sharma wishes to thank the Korean Science and Engineering Foun-
dation for the award of Brain pool fellowship and author Mr. Y.G.
Gudage acknowledges to The Principal, Mulshi Institute of Tech-
nology and Research, Sambhave, Pune for their kind support and
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