Thin films of cadmium sulphide have been prepared using home built spray pyrolysis unit on glass substrate at 400 C. Aqueous solutions of cadmium chloride and thiourea were used for the cadmium sulphide (CdS) films and different proportions by weight/volume of mercury II chloride was used for doping Hg onto cadmium sulphide (Cd1-xHgxS) films. The films obtained are having continuous, smooth surface with good transmittance. The thickness of all films is of the order of 320 nm. Determination of the crystalline nature has been done using XRD pattern. The effect of Hg on the surface morphology of CdS film has been studied by Scanning Electron Microscopy. The optical band gap has been calculated using the data from transmission spectra. Resistance before and after doping with Hg is also presented in the paper.

1. International Journal of Recent advances in Physics (IJRAP) Vol.3, No.1, February 2014
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STRUCTURAL, OPTICAL AND ELECTRICAL STUDIES
ON SPRAY DEPOSITED MERCURY DOPED CADMIUM
SULPHIDE THIN FILMS
Vijaya M G
Department of Physics, MGM College, Udupi 576 102, Karnataka.
ABSTRACT :
Thin films of cadmium sulphide have been prepared using home built spray pyrolysis unit on glass
substrate at 400 C. Aqueous solutions of cadmium chloride and thiourea were used for the cadmium
sulphide (CdS) films and different proportions by weight/volume of mercury II chloride was used for
doping Hg onto cadmium sulphide (Cd1-xHgxS) films. The films obtained are having continuous, smooth
surface with good transmittance. The thickness of all films is of the order of 320 nm. Determination of the
crystalline nature has been done using XRD pattern. The effect of Hg on the surface morphology of CdS
film has been studied by Scanning Electron Microscopy. The optical band gap has been calculated using
the data from transmission spectra. Resistance before and after doping with Hg is also presented in the
paper.
KEY WORDS: spray pyrolysis, CdS thin film, (Cd1-xHgxS)
1. INTRODUCTION
A layer of material having thickness of the order of few nanometres is generally called ‘thin film’.
The surface effect and interface effects are mainly responsible for the thin films to exhibit
completely different properties from the bulk. The use of thin film has been growing at rapidly
increasing rate due to their potential applications in various fields of science & technology. There
are different physical and chemical methods for preparing thin films namely - pulsed laser
deposition, ion beam sputtering, thermal evaporation, vacuum deposition, chemical vapour
deposition, co-precipitation, sol-gel, chemical bath deposition etc. Chemical spray pyrolysis
technique, because of its simplicity and inexpensiveness has found to be better chemical method
for the preparation of thin films with larger area. It also provides an easy method of doping an
element in the required proportion through solution method. It is found that in the chemical spray
pyrolysis technique, various parameters like air pressure, rate of deposition, substrate
temperature, distance between nozzle to substrate, cooling rate after deposition affect the
physical, electrical and optical properties of the thin films [1-4]. Cadmium Sulphide (CdS) is a
semiconductor belonging to II-VI group of compounds which has been used in making
heterojunction thin solar cells. CdS is preferred because it is a wide band gap semiconductor
having good thermal stability & can be used as light dependent resistors sensitive to visible &
near infrared light. The addition of different dopants to different extents has also found to alter the
properties of pure CdS thin films [4-9]. The effect of Hg on structural, optical and electrical
properties of CdS on glass substrate obtained by newly fabricated spray pyrolysis unit has been
reported in the paper.

2. International Journal of Recent advances in Physics (IJRAP) Vol.3, No.1, February 2014
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2. FABRICATION OF THE INSTRUMENT
Figure 1 shows the photograph of the home built spray pyrolysis unit fabricated with the help of
Neha Scientific Internationals, Bengaluru, India. It contains a spray nozzle, a heater for heating
the substrate, thermocouple included temperature controller and an air compressor. The spraying
system and heater are kept inside a metallic chamber of size 90 cm × 60 cm × 80 cm and the
outlet of the box is connected to an exhaust fan to remove the toxic gases produced during the
experiment. The inner surface of the box is coated with epoxy liquid to reduce the heat loss
through the surface.
Figure 1. Photograph of spray pyrolysis unit
3. EXPERIMENTAL DETAILS
In the preparation of Sample 1, pure CdS thin film, an equal amount of solutions of 0.1 M
cadmium chloride (pure CdCl2 H2O procured from Sisco Research Laboratories, India) and 0.2
M thiourea (pure CH4N2S procured from Sisco Research Laboratories, India) was mixed with
distilled water in the ratio of 3:2. Different volumes of Mercury II chloride (procured from
MERCK, India) of 0.02 M were used, for Hg doped CdS films. Solutions of cadmium sulphide
and Mercury II chloride were taken in the volume ratio of 5:1 respectively for Sample 2, 4:1 for
Sample 3 and 2:1 for Sample 4. In the preparation of all samples, cleaned glass substrate was
maintained at a temperature of 4000
C, the distance between nozzle & substrate was kept at 25 cm
and the solution was sprayed at a rate of 15 cm3
per minute. The thickness of the films was
calculated using gravimetric method and the average thickness of the film deposited was found to
be 320 nm.
4. RESULTS AND DISCUSSION
4.1. Structural study and Surface morphology
XRD patterns obtained using XRD, Miniflex 600, Rigaku corporation - Japan, for pure CdS,
Sample 1 and Hg doped CdS, Sample 2 is shown in Figure 2. XRD patterns for the other doped
samples are also similar to that of Sample 2. This clearly indicates that, the crystallinity is lost
with the addition of Mercury as dopant.

3. International Journal of Recent advances in Physics (IJRAP) Vol.3, No.1, February 2014
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Figure 2. XRD spectra
Figure 3 shows the SEM micrographs obtained using SEM EVO MA18 with Oxford EDS (X-
act) for pure CdS, Sample 1 and Hg doped CdS, Sample 2. There are some spheroid shape growth
appears as the creation of nucleation centre on the film surface. These are probably aggregates
due to colloidal particles formed in solution & then absorbed on the film.
Figure 3. SEM micrographs
4.2. Compositional studies
During deposition, Hg was incorporated with different concentrations. Hence Cd was substituted
by Hg in the films. Figure 4.1 and 4.2 shows EDX spectra of Samples 1 and 2.
Figure 4.1 EDX spectra

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Figure 4. EDX spectra
Table 1 shows the quantitative results of pure CdS and Hg doped CdS from EDX analysis.
Sulphur deficiency has been observed in all the films. This may be due to the fact that Sulphur
has great affinity towards Oxygen, so it might have converted to SO2 and then evaporated. A
strong peak is observed which corresponds to Si due to substrate. At high operating voltage, the
electron beam penetrates the film and reaches the glass surface, which results in the Si peak. For
different concentrations of Hg in the solution, there is also Hg peak in the spectra. This confirms
the presence of Mercury atoms in the doped sample.
Table 1. Quantitative results from EDX analysis

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4.3. Optical studies
The optical band gap of all the films was calculated using the transmission spectra obtained in the
region 300 nm to 800 nm using SHIMADZU UV-3600, UV-VIS-NIR spectrophotometer.
Figure 5. Energy gap of pure CdS Figure 6. Energy gap of CdS doped with Hg
All films have good transmittance greater than 75%. The optical band gap was estimated [10]
using the relation,
α h ν = A ( h ν - Eg )n
(1)
where, α is the absorption coefficient, Eg is the band gap corresponding to a particular transition,
A is a constant, ν is the transition frequency and n characterizes the nature of band transition. α
the absorbance, is calculated [10], using the relation
α = [ln(1/T)] / d (2)
where T is the raw transmission data obtained from the experiment and d is the thickness of the
film expressed in cm.
The graph of ( hν) verses ( α h ν )2
is shown in Figure 5 & 6. The extrapolation of the linear
region of the curve with X-axis gives the value of energy band gap of thin films. Energy gap for
pure cadmium sulphide is found to be 2.42 eV which is in good agreement with the earlier reports
[2,4]. Addition of mercury as dopant has found to decrease the energy gap considerably to 1.9 eV.
4.4. Electrical resistance
Resistance of CdS films doped with Hg by different compositions is shown in Figure 7. The
decrease is resistance may be explained on the basis of formation of Cd ions or Cd vacancies
being formed for charge compensation.
Figure 7. Variation of resistance with concentration of Mercury
During the film deposition, Mercury replaces divalent cadmium & there is every possibility of
formation of Cd ions rather than Cd vacancies as Hg and Cd are deposited simultaneously.

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5. CONCLUSIONS
Cadmium sulphide thin films were deposited by homemade spray pyrolysis unit using solutions
of cadmium chloride and thiourea. Mercury doped films were prepared by adding solution of
mercury II chloride in different proportions of volume. The films were deposited onto glass
substrate at temperature of 400° C. The films obtained were found to be uniform, smooth and
compact. The XRD studies suggest that addition of mercury results in the loss of crystallinity.
SEM micrographs show that addition of mercury has no considerable effect on the surface
morphology. The optical studies have shown that, with the addition of mercury, the energy gap
has been decreased from 2.42 eV to 1.9 eV. Study on electrical resistance has shown an
enhancement in the conductivity with the addition of mercury atoms as dopants.
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Author :
M G VIJAYA
Obtained M.Sc. from Mysore university in 1983, M.Phil. from Mangalore
university in 1987 and Ph.D. from Mangalore university in 2005. Joined the
teaching profession in 1985 and currently working in Mahatma Gandhi
Memorial College, Udupi, Karnataka, India. Attended many conferences and
given number of invited lectures on various topics in Physics for students of
neighbouring schools & colleges.