Download Journal of science and technolog Effects of Varying Copper (Cu) Ion

International Journal of Science and Technology Volume 5 No. 8, August, 2016
Effects of Varying Copper (Cu) Ion Concentrations of Ternary Compound of
Copper Iron Sulfide (Cufes) Thin Films
B. I. Obasi1, J. C. Osuwa2, and D. A. Odu3
1Physics/Electronics
Department, Federal Polytechnic Nekede, P.M.B. 1036, Owerri Imo State, Nigeria.
Department, Michael Okpara University of Agriculture, Umudike, P.M.B. 7226, Umuahia, Abia State, Nigeria.
3Chemistry/Biochemistry Department, Federal Polytechnic Nekede, P.M.B. 1036, Owerri Imo State, Nigeria.
2Physics
ABSTRACT
Ternary thin films of copper iron sulfide (CuFeS) were prepared by chemical bath deposition (CBD) technique at room temperature
(300 K) with varying molar concentrations of copper (Cu) ions (0.02, 0.04 and 0.06 M). The thin films were characterized using
spectrophotometer, scanning electron microscope (SEM), X-ray diffraction (XRD) and the four-point probe. The results suggest that
the films are of mono-crystal structures, with single XRD peaks broadened by higher copper ion concentrations indicating smaller
crystallite sizes. The absorbance, transmittance, absorption coefficient and some other properties such as refractive index and
dielectric constants of the films show appreciable change for copper ion concentration of 0.04 M or greater. The refractive index for
instance, changed from 1.68 for 0.02-0.04 M of Cu ions to 2.0 for 0.06 M of Cu ion concentration at photon energy of 1.5 eV.
However, the energy band gaps for both direct and indirect transitions decreased with increase in Cu ion concentration ranging from
2.56-2.75 eV and 1.8-2.25 eV respectively. In addition, both the absorption coefficient and imaginary dielectric constant possess
peak values in the visible range of the spectrum for all Cu ion concentrations. Also reported in this paper are the electrical properties.
Keywords: Copper Iron Sulfide, Energy Band Gap, Molar Concentrations, Photon Energy, Ternary
1. INTRODUCTION
Research in ternary and binary compound semiconductor
materials are currently in great focus because they enable
switching, amplification and tuning when used for electronic
devices. Today, they are used in most efficient and high-speed
semiconductor applications because of their direct “band gap” k
= 0 momentum electron states, including for example the many
uses of GaAs and ZnSe [1]. The tuning of semiconductor
properties of thin films includes the energy band gap and
therefore their spectral sensitivity [2]-[3]. Also affected are the
specific electrical, magnetic and resistivity properties of the thin
films [4].
Many materials are functional in the usage of thin films due to
their particular electrical, magnetic and optical properties. The
properties of thin films are particularly sensitive to the
preparation method; a number of methods have been established
for the synthesis of the thin films of metals, alloys, polymer and
superconductors on a range of substrate materials [5]. Binary
iron sulfide belongs to group VI-VIl compound semiconductor
and some of the ternary chalcogenides of sulfur already
researched include copper indium sulfide [6], cadmium zinc
sulfide [7], cadmium indium sulfide [8], [9], etc.
This research is focused on the deposition of ternary compound
semiconductor of copper iron sulfide (CuFeS) doped with
varying concentrations of copper ions using CBD method and
the investigation of their thin film properties for more versatile
applications.
2. DETAILS EXPERIMENTAL
2.1. Materials and Procedures
The apparatus used for the thin film deposition includes reagent
bottles, distilled water, electronic scale, measuring cylinders,
glass rods, 25 ml, 50 ml and 100 ml beakers, laboratory spoon,
synthetic foam/substrate holder, petri dishes and clips.
Commercial quality glass microscope slides of dimensions 26
mm x 76 mm x 1 mm were used as substrates and were
thoroughly cleaned and degreased for 48 hours by dipping in
HCl and HNO3 in the ratio of 3:1 respectively. The slides were
rinsed in distilled water after degreasing and then dried in air
giving the slides the advantage of providing nucleation centers
for the growth of highly adhesive thin films.
2.2. Deposition of CuFeS Thin Films Doped with Varying
Concentration of Cu Ions
Stoichiometric quantities of analytical grade reagents of copper
chloride, iron nitrate and thiourea served as precursors. The
reaction baths consist of a mixture of 5 ml 1 M ferrous nitrate, 5
ml of various molar concentrations of cuprous chloride, l0 ml of
1 M thiourea, 3 ml of ammonia, 3 ml of 7.4 M triethanolamine
(TEA), 5 ml of 0.1 M ethylenediaminetetraacetate (EDTA) and
20 ml of distilled water. The mixtures were stirred with glass
rods and the substrates were then vertically placed inside the
beakers and left for 5 hours. The TEA and EDTA as
complexing agents slowed down the reactions for the formation
of solid thin films on the substrates, while the ammonia (NH 3)
solution served to stabilize the pH of the mixtures. The reaction
mechanism is as follows [10].
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International Journal of Science and Technology (IJST) – Volume 5 No. 8, August, 2016
CuCl ∙ 2H2 O + TEA → [Cu(TEA)]+ + Cl–
[Cu(TEA)]+ → Cu+ + TEA
Fe(NO3 )3 ∙ 9H2 O + EDTA → [Fe(EDTA)]+ + NO–3
[Fe(EDTA)]+ → Fe3+ + EDTA2−
(NH2 )2 CS + OH − → (NH2 )2 CO + HS –
HS − + OH − → H2 O + S −
Cu+ + Fe+ + S 2− → CuFeS
wavelengths portion of the visible light in the electromagnetic
spectrum.
(a)
3. RESULTS AND DISCUSSION
3.1. Equipment Used for Data Acquisition and Analysis
The optical transmittance spectra and other optical properties
were
measured
using
a
UNICO-UV-2102
PC
spectrophotometer in the wavelength range of 200-900 nm. The
samples were, deposited onto transparent substrates so that the
transmittance of the thin films can be determined. SEM images
were, obtained with a FEI Nova Nano SEM field emission SEM
in high vacuum mode with a spot size of 3.5 nm and an
accelerating voltage of 15 kV. Carbon strapping was run from
the surface of the samples to the stub (earth) to minimize any
possible charging effects on the sample surface during image
acquisition.
(b)
XRD studies were carried out on a Bruker 08 wide-angle
diffractometer Bruker D8 Advance wide-angle diffractometer,
using a graphite-monochromatic Cu-Kα source of wavelength λ
= 1.5406 Å. Data were collected within a scan range of 2θ=10100° in step size of 0.020 nm and count rate of 2 s per step.
Finally, the resistance, R was measured using Vander Pauw
four-Point Probe and the thin film thickness, t, was obtained
using height analysis with a Dektak II Profilometer. The
resistivity of a sample is given by;
𝜌 = 𝑅𝑡
(1)
where ρ is the electrical resistivity, R = sheet resistance, t is
thickness and the electrical conductivity 𝜎 = 1/𝜌 was
determined [11].
(c)
3.2. Presentation of Results
Fig. 1(a-b) shows the decreasing absorbance and the increasing
transmittance of the copper iron sulfide (CuFeS) thin films
versus wavelength. The average absorbance was 0.69, 0.84, and
0.83 in the order of decreasing Cu ion concentration.
Observation shows that transmittance increases with increasing
Cu ion concentration. The film suggests transmission in the
long wavelength portion of the visible spectrum. The
absorbance spectra are almost the same for Cu concentrations of
0.02 M and 0.04 M difference really shows for 0.06 M Cu
concentration. The film deposited from 0.02 M Cu
concentration shows more absorbance that may be a result of
availability of more surface area. Fig.1(c) shows the plot of
absorption coefficient against photon energy. Absorption
coefficient is determined by using the relation;
Fig.1. (a) Plot of absorbance versus wavelength (b) plot of
transmittance versus wavelength and (c) plot of absorption
coefficient versus photon energy for varying Cu ion concentration
on FeS thin films
𝑛
𝐴(ℎ𝑣 − 𝐸𝑔 )
𝛼=
ℎ𝑣
(2)
where ℎ𝑣 is the photon energy, Eg is the band gap energy, A and
n are constants. The absorption coefficient peaked in the higher
Fig. 2(a-c) show graphs of the imaginary dielectric constants,
indirect gap energies, and refractive index of the CuFeS thin
films against photon energies. The figures clearly indicate the
positions of the peak values for the various properties.
IJST © 2016– IJST Publications UK. All rights reserved.
370
International Journal of Science and Technology (IJST) – Volume 5 No. 8, August, 2016
(a)
Fig. 3 shows the XRD result depicting peak broadening for
higher concentrations of copper ions and Table 1 shows the
crystallite sizes obtained. Calculation revealed that gain sizes
increased with increasing Cu ion concentration. The diffraction
peak for CuFeS thin films appear at 2θ = 26.23°, 2θ = 26.29°
and 2θ = 23.27° for 0.06, 0.04 and 0.02 M Cu ion concentration.
The XRD patterns also show peak broadening and increase in
Bragg’s angle for higher concentrations of Cu ions, which may
indicate changes in crystallite sizes, more stacking faults as
depicted in the SEM micrographs. Other suggestions are that
peak broadening were due to micro strain, and other defects in
the crystal structure, for instance an inhomogeneous
composition in a solid solution or alloy may result to peak
broadening [12]
(b)
(c)
Fig.3. XRD pattern of CuFeS thin films with varying concentration
of Cu on FeS.
Table 1: The FWHM of XRD and calculated grain
sizes of varying concentrations of Cu ion on FeS
Sample
label
X1
X2
X3
Fig.2. (a) Plot imaginary dielectric constant, (b) plot of (αhf)1/2, and
(c) plot of refractive index against photon energies for varying Cu
ion concentration on FeS thin films.
Diffraction
angle (°)
26.23
26.29
23.17
FWHM
0.059
0.063
0.064
Lattice
strain
0.0011
0.0011
0.0013
Crystallite
sizes (nm)
142.06
142.01
141.23
The SEM micrographs of CuFeS films obtained for different
concentration of copper ions in the solution bath is depicted in
Fig. 4(a-c). The figures show that the distribution of grains is
not uniform throughout all the regions. A crack appears on the
film Fig. 4(a), as it is not homogenous and uniform. The crack
on the film shows that the film is unstable. At lower
concentrations, the grains are very few as seen in Fig 4(b) and
Fig 4(c), as the concentration increases the particles becomes
closely packed. The films appear to be more homogenous with
decreasing concentration.
The extrapolation for indirect energy gaps 𝐸𝑔𝑖 obtained was
1.91 eV, 2.11 eV and 2.13 eV for Cu ion concentration of 0.06,
0.04 and 0.02 M respectively. The refractive index range from
1.68 to 2.0, results to low Fresnel reflection loss. The low
refractive indices also mean that Rayleigh scattering losses are
low.
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371
International Journal of Science and Technology (IJST) – Volume 5 No. 8, August, 2016
(a)
increase in Cu ion concentration. The ion concentration greater
than 0.04 M did not show significant change in sheet resistance.
(a)
(b)
(b)
(c)
Fig. 5 Plots of (a) electrical conductivity with varying molar
concentration of Cu and (b) thickness with varying molar
concentration of Cu on FeS thin films.
4. CONCLUSIONS
Fig. 4. SEM micrograph of CuFeS thin film with (a) 0.06M Cu
concentration (b) 0.04M Cu concentration and (c) 0.02M Cu
concentration.
Fig. 5 shows the plots of conductivity and thickness of CuFeS
thin films versus copper ion concentrations. Observations show
that (Fig. 5(a)) conductivity increases with increase in Cu ion
concentration and that the films (Fig. 5(b)) grown increased in
thickness with increase in concentration of Cu. Results also
revealed that the sheet resistance decreased slightly with
The deposition and characterization of copper iron sulfide
(CuFeS) have been successful. The results show that varying
concentrations of copper ions in CuFeS thin films mostly
increase and in some cases decrease the properties of the thin
films. This provides wide latitude for applications of the thin
film. Anticipation is that the doping of CuFeS thin films with
required specific concentration of copper ions will lead to more
applications of the ternary compound semiconductor.
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