African Physical Review (2008) 2:0007
59
Comparitive Studies of Improved Chemical Bath Deposited Copper Sulphide (CuS)
and Zinc Sulphide (ZnS) Thin Films at 320K and Possible Applications
P. A. Ilenikhena*
Department of Physics, University of Benin, Benin City, Nigeria
Semi-conducting thin films of copper sulphide (CuS) and zinc sulphide (ZnS) were deposited on glass microscope
slides at 320K and pH values of 7, 9, 10, 11 and 12 using a simple, convenient and cost effective improved chemical bath
deposition (CBD) method. Ethylene diamine-tetra acetate (EDTA), a complexing agent with pH opposite to that of bath
constituents, was used to vary the deposition pH values. X-ray diffractiometry method was used to obtain the structural
characterization. Absorbance spectra data of the films were obtained by a single beam spectrophotometer (Pharmacia LKB
Biochrom 4060) at wavelength range 200 to 900nm. Other optical and solid state properties were calculated from the data
and compared with other deposited thin films. The average optical and solid state properties of copper sulphide (CuS) thin
films include absorbance ranging from 0.069 to 0.329, transmittance 0.467 to 0.853, refractive index 1.78 to 2.63, electrical
conductivity 0.43 to 0.69 (ohm-cm)-1, film thickness 0.022 to 0.408µm and band gap 2.10 to 2.55 +0.05 eV. For zinc
sulphide (ZnS) thin films, the absorbance ranges from 0.089 to 0.112, transmittance 0.773 to 0.820, refractive index 1.89 to
2.03, electrical conductivity 0.49 to 0.53 (ohm-cm)-1, film thickness 0.005 to 0.051µm and band gap 2.55 to 3.05 +0.05 eV.
Possible applications of the deposited copper sulphide (CuS) and zinc sulphide (ZnS) thin films are also discussed.
1. Introduction
Chemical bath deposition (CBD) is a simple,
reproducible and cost effective technique of
fabricating high quality compound semiconductor
metal halide and chalcogenide thin films on both
metallic and non-metallic substrates [6, 4]. The
method is well studied and produces films that
have comparable structural and optoelectric
properties to those produced using other
sophisticated thin film deposition techniques [4, 3,
24, 26]. The technique has been applied in
producing emerging materials for solar cells,
protective coating, solar thermal controls in
buildings and is being adopted by some industries
[14, 9, 26]. The technology is based on slow
controlled precipitation of the desired compound
from its ions in a reaction bath solution. A ligand or
complexing agent acting as a catalyst is usually
employed to control the reaction in a suitable
medium as indicated by the pH to obtain crystal
growth. Otherwise, spontaneous reaction and
sedimentation of materials will be obtained. The
condition for compound to be deposited from a
solution bearing its ions is that its ionic product
(I.P) should be greater than the solubility product
(Ksp) [6, 17]. The complexing agent of a metal in
solution forms a fairly stable complex ions of the
_____________
*
[email protected]
metal and provides a controlled release of free ions
according to an equilibrium reaction of the form:
M(A)2+ ⇄ M2+ + A, where M2+ is the metal ions
and A is the complexing agent. The concentration
of an ion at any temperature is given by [M2+] [A] /
M(A)2+ = Kd , where Kd is the dissociation or
instability constant of the complex ion. The
negative ions required for the precipitation of the
compound are also generated slowly by suitable
complex compounds bearing them [6].
The
deposition technique can be improved by
controlled addition of another complexing agent
with pH oppose to that of bath constituents to vary
the deposition conditions at different suitable pH
values [15, 16].
This work reports the successful deposition of
copper sulphide (CuS) and zinc sulphide (ZnS) thin
films at 320K and pH of 7, 9, 10, 11 and 12 using
improved chemical bath deposition (CBD) method.
Controlled addition of ethylenediamine-tetra
acetate (EDTA), a complexing agent with pH
opposite to that of deposition bath constituents, was
used to vary the initial deposition pH values. X-ray
diffractometry method was used to obtain structural
characterization. Optical and solid state properties
of the deposited films were determined and
compared with those obtained using some other
sophisticated thin film deposition techniques.
Possible applications of the films were also
discussed.
African Physical Review (2008) 2:0007
60
2. Experimental details
2.1 Film Preparation
Glass microscope slides (7.6 x 26 x 1mm3) were
cleaned by degreasing them in concentrated nitric
acid (HNO3) for 2 days, washed in detergent
solution, rinsed in distilled water and dried in air.
Reaction baths were 50ml glass beakers containing
different molar solutions and volumes of deposition
reagents. The bath constituents for the deposition
of copper sulphide (CuS) thin films were copper
chloride – 2 – water (CuCl2. 2H2O) as source of
(Cu2+), thiourea
[(NH2)2CS] as a source of
sulphide ions (S2-) in the presence of sodium
hydroxide and ammonia (NH3) as complexing
agent. Distilled water was added to raise the
volume of the bath solutions to a certain level. A
controlled addition of ethylenediamine-tetra acetate
(EDTA), a complexing agent with pH opposite to
that of bath constituents was used to enhance
variation of the deposition pH values from 7 to 12.
For the deposition of zinc sulphide (ZnS) thin
films, zinc chloride (ZnCl2) was used to replace the
copper chloride – 2 – water (CuCl2. 2H2O) in a
similar reaction bath. Details of bath constituents
for the preparation of the metallic sulphide (YS)
thin films are shown in Table 1. The symbol Y
represents Cu and Zn in the deposition of CuS and
ZnS, respectively. The solution baths were stirred
with a glass rod and their initial pH values noted.
The baths were placed in a hot water bath that was
maintained at a steady temperature of 320K by a
Stuart magnetic stirrer hot plot. A cleaned glass
microscope slide was suspended in each reaction
bath for 3 hours. After deposition time, the coated
glass slides were rinsed with distilled water and
dried in air. Pretest runs were carried out to
determine the optimum deposition parameters such
as deposition time, pH and volumes of bath
constituents. The complexing agent ammonia
(NH3) formed complex ions with Y2+. It slowly
released Y2+, ensured ion by ion condensation of
Y2+ and S2-, controlled the growth rate of the
deposited thin films and eliminated spontaneous
precipitation of the chemical reagents in the bath.
The most probable reaction equation for the
deposition of (CuS) thin films is
(CuCl2.2H2O) + NH3 + (NH2) 2CS + 2NaOH
→ CuS ↓ + NH3 + CH2 N2 + 4H2O + 2NaCl
The basic reaction equation for deposition of ZnS
thin films is
ZnCl2 + NH3 + (NH2) 2CS + 2NaOH
→ ZnS ↓ NH3 + CH2 N2 + 2H2O + 2NaCl
Table 1: Bath constituents for preparation of copper sulphide (CuS) and zinc sulphide (ZnS) thin films
Initial
Bath
PH
0.3M
YCl2(2H2O)r
Vol. (ml)
5.0M
NH3
Vol. (ml)
100m
TEA
Vol. (ml)
2.5M
NaOH
Vol. (ml)
0.8m
(NH2)2CS
Vol. (ml)
7
8
3
2
2
6
9
7
4
3
3
7
10
6
4
3
3
8
11
6
5
4
4
8
12
5
5
4
4
9
The symbols Y = Cu and r = 1 for CuCl2* 2H2O , Y = Zn and r = 0 for ZnCl2
2.2 Film characterization
The absorbance (A) spectra data of the deposited
films were obtained by a computerized single beam
spectrophotometer (Pharmacia LKB Biochrom
4060) at wavelength range of 200nm to 900nm.
The reference and coated glass microscope slides
were mounted on a rotating holder at the reference
and sample compartments, respectively, and
scanned to obtain the absorbance spectra data.
Other optical and solid-state properties were
H2 O
Vol. (ml)
19
16
16
13
13
0.2M
EDTA
Vol. (ml)
26
21
20
18
15
obtained from the spectra data by calculations
based on the theory. Structural characterization of
the films was obtained by x- ray diffractometry
method
using
Diano
cooperation
x-ray
diffractometer (model XRD 2100 E*) and copper
target (CuKα) with wavelength 1.540502 Å,
current 30mA and voltage 45 kV. The surface
microstructure of the films was viewed using
electron microscope at magnification 100x.
African Physical Review (2008) 2:0007
3. Theory and calculations
The measured absorbance (A) of the semiconductor films is related to the transmittance (T)
by A= log (1/T) = log Io/I or T = 10-A, where T =
I/Io, I is the transmitted light and Io is the incident
light [7, 20, 27, 13]. The absorbance (A),
transmittance (T), and reflectance (R) satisfy the
law of conservation of energy by the equation: A +
T + R = 1 [11]. According to [12, 27, 31] and
[22], the normal reflectance (R) and refractive
index (n) are related by the equation: n = (1 + R1/2)
/(1 - R1/2). Following [7, 27] and [13], the
transmittance (T), coefficient of absorption (α) and
distance (dµm) transversed in the film are related
by T = exp (-αd), and for a unit distance, α = In
(1/T) x 106 m-1. The coefficient of absorption (α)
is also related to coefficient of extinction (k) by α
= 4πk/λ, where λ is the wavelength of radiation
[34, 13, 32]. Near the absorption edge, absorption
coefficient (α) is related to band gap (Eg) by α =
(hν - Eg)b, where hν is photon energy and b is
constant for a given transition. For allowed direct
transition, b = ½ . The band gap was obtained from
allowed direct transition by plotting α2 against hv
and extrapolating the graph to the point where α2 =
0 [6]. Again, following [27, 1] and [8], the
transmittance of light through a weak absorbing
film of thickness (t) based on optical method is
given by t = In [(1 - R)2 /T] / α. The complex
dielectric constant (εc) is given by εc = (n + ik)2 = εr
+ εi, where the real dielectric constant εr = n2 - k2
and imaginary dielectric constant εI = 2nk [27, 5].
The optical conductivity (σo) is given by σo = αnc /
4π, where c is the speed of light in vacuum.
Following [10], the electrical conductivity (σe) is
given by the expression σe = 2π σo / α.
4. Results and analysis
Absorbance spectra data of the deposited copper
sulphide (CuS) and zinc sulphide (ZnS) thin films
are displayed in Figs. 1 and 2, respectively. The
absorbance depends on the deposition pH and on
the wavelength of radiation. The films have high
absorbance for wavelengths lower than 300nm and
low absorbance for wavelength range 350 –
900nm. Fig. 1 shows that copper sulphide (CuS)
thin film produced at pH of 7 has lowest
absorbance at wavelength range of 350nm to
900nm while the films produced at pH of 12 has
the highest absorbance. Fig. 2 also shows that zinc
sulphide (ZnS) thin films have similar absorbance
spectra. Film produced at pH of 10 has relatively
high absorbance spectra compared to the ZnS films
61
produced at pH of 9 and 12, respectively. Figs. 3
and 4 show the transmittance (T) and reflectance
(R) spectra of copper sulphide and zinc sulphide
thin films, respectively. Both films have low
transmittance for wavelengths lower than 300nm
and high transmittance for wavelength range
300nm to 900nm. For copper sulphide thin films in
Fig. 3, the transmittance varies from 0.313 to 0.926
for wavelength lower than 300nm and from 0.515
to 0.995 for wavelength range 350 to 900nm. Film
produced at pH of 7 has the highest transmittance,
while film produced at pH of 12 has relatively low
transmittance. For zinc sulphide thin films in Fig.
4, the transmittance varies from 0.289 to 0.964 for
wavelength lower than 300nm and from 0.847 to
0.995 for wavelength range 350nm to 900nm. The
films produced at pH of 9 and 12 have the highest
transmittance of 1.033 at 350nm. Figs. 3 and 4 also
show the reflectance of the deposited copper
sulphide (CuS) and zinc sulphide (ZnS) thin films,
respectively.
Both thin films exhibited high
reflectance (R) for wavelengths lower than 300nm.
The reflectance of CuS thin films varies from 0.003
to 0.192 for wavelength range 350 to 900nm. For
ZnS thin films, the reflectance varies from 0.003 to
0.081 in the same wavelength range. The thin
films with high transmittance and low reflectance
characteristics produced at pH of 7 and 9 for CuS
and ZnS, respectively, could be employed in
antireflection coatings for solar thermal devices
and eyeglass coatings to reduce solar reflectance
and increase the transmittance of glass. The high
transmittance and low reflectance properties of
CuS thin films produced at pH of 11 and ZnS thin
films produced at pH of 10 and 12 in the visible
region could be employed in solar thermal control
coatings [21]. The thin films of CuS produced at
pH of 12 with high absorbance, relatively low
transmittance and high reflectance could be useful
in construction of poultry houses to allow enough
infrared radiation to the warm the very young
chicks during the day. This could also reduce the
cost of energy consumption through the use of
stoves, heaters, electric bulbs, etc. and the hazards
associated with them while at the same time
protecting the chicks from ultraviolet radiation. The
application of solar energy as a source of heat in
chick breeding is environmentally acceptable and
promotes sustainable development [25]. Solar
energy technologies are also applicable to egg
incubation and the drying of chicken manure [23].
These films could also be used for anti dazzling
coatings for car windscreens and driving mirrors to
reduce the dazzling effects of light at night. The
high absorbance of CuS thin film at pH of 12 could
African Physical Review (2008) 2:0007
62
band gap varies from 2.55 to 3.05 + 0.05 eV. These
results compare well with 2.40 eV for CdS films
reported by [18] and could be employed in thin
films solar cells. The average optical and solidstate properties at wavelength of 550nm for ZnS
and CdS are shown in Tables 2 and 3, respectively.
The high magnitude of optical conductivity (σo) of
1013 s-1 of both films shows they have good photo
response. The magnitude of average electrical
conductivity 10-1 (ohm-cm) –1 for both thin films is
within electrical conductivity range 10-12 to 102
(ohm – cm)-1 for semiconductors [28, 30] and [33].
The thickness of CuS thin films varies from 0.022
to 0.408µm. For ZnS thin films, the thickness
varies from 0.005 to 0.051µm. Other optical and
solid-state properties determined include extinction
coefficient (k), real dielectric constant (∈r) and
imaginary dielectric constant (∈i). The x-ray
diffraction patterns of the uncoated glass and
deposited copper sulphide (CuS) films on glass
slides obtained by diffractometry method using
Diano corporation x-ray diffractometer (XRD
model 2100* E) and copper targer (CuKα) with
radiation of wavelength 1.540502 Å are shown in
Fig. 9. The corresponding x-ray diffraction patterns
of uncoated glass and deposited ZnS film on glass
slides are shown in Fig. 11. The diffraction patterns
reveal diffraction peaks at some 2θ values.
Electronmicrographs of the copper sulphide (CuS)
films at magnification of 100x is shown in Fig. 10.
Fig. 12 shows the corresponding electron
micrograph of zinc sulphide (ZnS).
be employed in p-n junction solar cells [26]. The
variation of refractive index (n) with photon energy
for CuS and ZnS thin films are shown in Figs. 5
and 6, respectively. Both films exhibited high
refractive index (n) for photon energies higher than
4.14 eV and low refractive index (n) values for
photon energy range 1.46 eV to 4.14 eV. For
copper sulphide thin films in Fig. 5, values of
refractive index vary from 1.56 to 2.63 for photon
energy higher than 4.14eV and from 1.14 to 2.59
for photon energy range 1.46eV to 4.14 eV. In the
case of zinc sulphide (ZnS) thin films in Fig. 6,
values of refractive index vary from 1.33 to 2.63
for photon energy higher than 4.14 eV and from
1.11 to 2.57 for photon energy range 1.46eV to
4.14 eV. The films of both CuS and ZnS with very
low refractive index values could find useful
applications in antireflection coatings. Such films
with refractive index lower than 1.9 could be
employed to reduce reflectance of photovoltaic
from 0.36 to 0.04 and increase the transmittance of
glass from 0.91 to 0.96 [2, 29]. The variations of
coefficient of absorption (α) with photon energy
for copper sulphide (CuS) and zinc sulphide (ZnS)
thin films are shown in Figs. 7 and 8, respectively.
The magnitude of the coefficient of absorption α =
106 m-1 is within α range 106 to 107 m-1 for semiconductor thin films suitable for polycrystalline
thin film solar cell [19]. The coefficient of
absorption method was used to determine the band
gap. Values of band gap for the deposited copper
sulphide (CuS) films varies from 2.10 to 2.55 +
0.05 eV. For the zinc sulphide (ZnS) films, the
Table 2: Average optical and solid-state properties for copper sulphide thin films
PH
T
n
k
x 10-2
7
11
12
0.853
0.764
0.467
1.78
2.05
2.63
0.696
1.18
3.33
∈r
α
x 106
(m-1)
0.159
0.269
0.761
3.17
4.22
6.94
σo
x 1013
s-1
0.68
1.32
4.78
σe
(ohm-cm)-1
t
(µm)
0.43
0.54
0.69
0.022
0.059
0.408
Eg
± 0.05
(eV)
2.45
2.10
2.55
Table 3: Average optical and solid-state properties for zinc sulphide thin films
pH
T
n
k
x 10-2
9
10
12
0.817
0.773
0.820
1.90
2.03
1.89
0.895
1.127
0.867
α
x 106
(m-1)
0.205
0.257
0.198
∈r
3.61
4.11
3.57
σo
x 1013
(s-1)
0.92
1.25
0.89
σe
(ohm-cm)-1
t
(µm)
0.49
0.53
0.49
0.013
0.051
0.005
Eg
±0.05
(eV)
3.05
2.55
2.65
African Physical Review (2008) 2:0007
63
5. Conclusion
pH 9 A
pH 10 A
0.6
pH 12 A
0.5
0.4
0.3
0.2
0.1
0
0
200
400
600
800
1000
wavelengt h ( nm)
Fig. 2: Absorbance (A) spectra for ZnS t hin films produced
at 320K and pH of 9, 10 and 12.
Transmittance (T) - Reflectance (R)
Semi conductor thin films of copper sulphide (CuS)
and zinc sulphide (ZnS) were successfully
produced on glass microscope slides at 320K and
pH values of 7, 9, 10, 11 and 12 using improved
chemical bath deposition method. Controlled
addition of ethylenediamine-tetra acetate (EDTA),
a complexing agent with pH opposite to that of
bath reagents, was used to vary the initial
deposition pH values. X-ray diffractometry method
was used to obtain the structural characterizations.
A single beam spectrophotometer (Pharmacia LKB
Biochrom 4060) was used to obtain the spectra
absorbance data. Other optical and solid-state
properties of the films were obtained by
calculations based on theory. These properties
include transmittance, refractive index, extinction
coefficient,
optical
conductivity,
electrical
conductivity, film thickness, coefficient of
absorption, band gap, etc. Different values of
optical and solid state properties were obtained at
the various bath deposition pH values. The films
with refractive index lower than 1.9 could be used
in antireflection coatings, eyeglass coatings and
solar thermal control coatings. Those with
refractive index greater than 1.9 could be useful in
poultry production, antidazzling coatings and solar
cells.
1.2
pH 7 T
1
pH 7 R
pH 11 T
0.8
pH 11 R
0.6
pH 12 T
pH 12 R
0.4
0.2
0
0
500
1000
wave length (nm)
Fig. 3: Transmittance (T) - Reflectance (R) spectra for CuS thin
films produced at 320k and pH of 7, 11 and 12
pH 7
pH 12
Absorbance (A)
0.5
0.4
0.3
0.2
0.1
pH 9 T
1.2
Transmittance (T) - Reflectance (R)
pH 11
0.6
pH 9 R
1
pH 10 T
pH 10 R
0.8
pH 12 T
pH 12 R
0.6
0.4
0.2
0
0
0
200
400
600
800
1000
Wave length (nm)
fig. 1 Absorbance(A) spectra for CuS thin films produced at
320K and pH of 7, 11 and 12.
0
200
400
600
800
1000
wavelength (nm)
Fig.4: Transmittance (T) - Reflectance (R) spectra for ZnS thin
films produced at 320K and pH of 9, 10 and 12.
African Physical Review (2008) 2:0007
64
3
1.4
Refractive index
6
Coeficient of absorption x 10 m
-1
2.5
2
pH 7 n
pH 11 n
1.5
pH 12 n
1
0.5
1.2
1
0.8
pH 7 ?
0.6
pH 11 ?
0.4
pH 12 ?
0.2
0
0
0
1
2
3
4
5
6
0
7
1
2
3
4
5
6
7
Photon energy hv (eV)
Photon energy hv (eV)
Fig. 7:Variation of coeficient of absorption (?) against photon
energy for CuS thin films produced at 320K and pH of 7, 11
and 12.
fig 5: Variation of refrractive index (n) against photon energy for
CuS thin films produced at 320K and pH of 7, 11 and 12.
pH 9
pH 10
3
pH 12
Refractive index (n)
2.5
2
1.5
1
pH 9 n
pH 10 n
0.5
pH 12 n
0
Coeficient of Absorption x 106m-1
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
1
2
3
4
5
6
Photon energy hu (eV)
Fig.6:Variation of refractive index (n) against photon energy for
ZnS thin films produced at 320K and pH of 9, 10 and 12.
7
0
1
2
3
4
Photon energy hu (eV)
5
6
Fig 8: Variation of coeficient of absorption against photon energy
for ZnS thin films produced at 320K and pH of 9, 10 and 12.
7
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Received: 30 July, 2007
Accepted: 21 February, 2008