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OPTICAL CHARACTERIZATION OF ZINC SULFIDE (ZnS) THIN FILMS BY
AEROSOL ASSISTED CHEMICAL VAPOUR DEPOSITION (AACVD)
TECHNIQUES
Musa A. Bilya, Sarki M.U, Iro N.A.
[email protected], [email protected],
Nasarawa State University, Keffi, Faculty of Natural and Applied Sciences, Department of Physics, P.M.B 1022,
Keffi, Nasarawa State Nigeria.
Nigeria Meteorological Agency, Abuja Nigeria.
ABSTRACT
This research work gives a report on the optical characterization of Zinc Sulfide (ZnS) thin films
deposited by aerosol assisted chemical vapour deposition (AACVD) technique onto a frosted glass
substrate, using a precursor solution of 1.09g of Zinc Acitate and 1.52g of Thiourea in 50ml solution of
propernol (solvent). The deposition wavelength ranges from 300-1100nm with an average of 10 scans.
Thin films deposited were characterized using UV-VIS 750 Spectrophotometer. The optical density
(absorbance), total transmittance, optical band gap, reflectance and refractive index were obtained and
analysed. ZnS thin films have been found useful in various devices. The applications of ZnS thin films
which cover a wide area of interest are: Anti-reflection coating for the solar cell, Environmental friendly
buffer layer as compared to CdS layer in CIS based thin film solar cell. Wide band gap material for
electroluminescence and opto-electronic Devices; integrated circuits, resistors, capacitors, transistors
and superconductors, Photosynthetic coatings, Blue light emitting laser diodes and as α - particle
detector.
Keywords: Thiourea, Deposition, Characterization, Aerosol, Deposition
Introduction
Nanotechnology has become the most fashionable science since the end of the last century. Since
then, a lot of effort has been made to achieve numbers of multifunctional materials with simple
synthetic procedure. At the same time, investigations of easy processing for subsequent
applications have become more and more popular as well. Chemical and electrochemical
preparation methods are one of the possible and powerful options for the fabrication of a new
class of Nano materials. Many popular methods used for nanostructures fabrication need to
involve very expensive devices like: UHV chambers equipped with MBE or sputtering, etc. It is
useful to develop methods which
M&Ns-19, Paris, 17-19 July 2019 Pag. 54
are less expensive and lead to similar quality final products. From the application point of view,
methods which allow to perform mass production in a relatively easy way were sought. Such a
method is electrochemistry, which allows us to deposit a large variety of materials in many
different forms from various solutions. Chemical methods also provide an opportunity to obtain
nanomaterial’s in big quantities. Recently, hybrid nanomaterial’s and Nano composites have
been studied extensively because of new opportunities of the fabrication of a novel class of
materials which use nanostructures as building blocks. Such subsequent hierarchical ordering of
constituent Nano elements often enhances their needed magnetic, electrical, optical, structural
and mechanical properties and extinct unneeded one (Romero & Sanchez, 2003).
Zinc sulfide (ZnS) was one of the first semiconductors discovered and is also an important
semiconductor material with direct wide band gaps for cubic and hexagonal phases of 3.72 and
3.77eV, respectively (Ben et al., 2006). It has a high absorption coefficient in the visible range
of the optical spectrum and reasonably good electrical properties. This property makes ZnS very
attractive as absorber in hetero-junction thin-film solar cells (Ubale & Kulkarni, 2005).
The optical properties of the prepared film depend strongly on the manufacturing technique. Two
of the most important optical properties; refractive index and the extinction coefficient are
generally called optical constants. The amount of light that transmitted through thin film material
depends on the amount of the reflection and absorption that takes place along the light path
(Nada, 2011). Many growth techniques have been reported to prepare ZnS thin films, such as
sputtering, pulsed-laser deposition, and metal organic chemical vapor deposition, electron beam
evaporation, photochemical deposition, thermal evaporation, sol-gel processing, co-precipitation
and chemical bath deposition (Ravi et al., 2009; Xiaochun et al., 2007).
These nanoparticles ZnS semiconductor materials have a wide range of applications in
electroluminescence devices, phosphors, light emitting displays, and optical sensors.
Zinc sulphide (ZnS) is an important wide band gap II-VI semiconductor material with a direct
optical band gap of 3.70eV (Yasuda et al., 1986). This bandgap is convenient when used as a
window material in solar cells, since it allows for high transparency in the short wavelength
region (350–550) nm when compared to CdS with band gap of 2.42eV. ZnS has a refractive
index (n) of 2.40 which also makes it possible for application as antireflective coatings in thin
M&Ns-19, Paris, 17-19 July 2019 Pag. 55
film solar cells (Tech-Yam et al., 2012). This material is under intense study and has gained
application in the areas
of solar cells (Echendu et al., 2014; Echendu & Dharmadasa, 2015), and various photonic devices
(Echendu & Dharmadasa, 2013).
Materials and Method
This chapter presents the detailed on the materials and methods used in carrying out this research.
A detailed description of the materials and the procedure as well the steps taken of methods used
in this research work.
Materials
*AACVD-Glass 3D machine (ID printer) * UV-VIS Spectrophotometer * Magnetic stirrer hot
plate
*Electronic weight balance * Frosted glass (substrates) *Measuring cylinder * 100ml beaker
*Stirrer band * Zinc acitate (ZAD) (Zn(CH3COO) * Thiourea * Propernol (solvent) * Petri dish
*Filter paper * Aluminium foil *Insulin syringe *Microsoft Excel
Method
AACVD Techniques
Aerosol assisted chemical vapour deposition (AACVD) involves the formation of a thin solid film on a
substrate via chemical reactions of precursors in the vapour phase. It is the chemical reaction which
distinguishes CVD from physical vapour deposition (PVD) processes (Choy, 2003).The chemical
reactions occur homogeneously in the gas phase and/or heterogeneously on the substrate The latter is
desired in order to obtain high quality films (Jone, 2009). CVD requires the delivery of a precursor into
a gas stream,
transport of the precursor to a substrate and the application of energy to cause a reaction. The key steps
of CVD are shown below, which include:
• Evaporation and transport of reagents in the bulk gas flow region into the reactor;
• Mass transport of the reactants to the substrate surface;
• Adsorption of the reactants on the substrate surface;
• Surface diffusion to growth sites;
• Nucleation and surface chemical reactions leading to film growth;
M&Ns-19, Paris, 17-19 July 2019 Pag. 56
• Desorption and mass transport of remaining fragments of the decomposition away from the
reaction zone.
The film growth rate in thermal CVD is determined by the substrate temperature and the pressure of the
reactor as well as the complex gas phase chemistry occurring in the reaction zone. At lower substrate
temperatures the film growth rate is determined by the kinetics of the chemical reactions occurring either
in the gas phase or on the substrate surface and is generally denoted as surface reaction limited film
growth. As the temperature is increased, the film growth rate becomes almost independent of temperature
and the growth is determined by the mass transport of the precursors through the boundary layer to the
growth surface. This is known as mass transport limited film growth.
Fig 1:Experimental Setup of 3D printer
(AACVD deposition) machine
The main advantage of using AACVD, over other CVD techniques, is that this system eliminates the
need for volatile precursors. It utilizes an aerosol vaporization technique where the precursor is dissolved
in a suitable solvent and an aerosol mist is generated. A piezoelectric transducer creates ultrasonic waves
which pass through the solution, this produces a wave pattern on the surface of the liquid. When the
height of the wave is sufficient, the crest of the wave is so unstable that droplets are ejected from the
surface of the precursor solution, as this builds up it creates an aerosol mist. This mist can be transported
into the reaction chamber by diverting an inert carrier gas through the flask containing the precursor
solution, to act as a bubbler. Once in the reaction chamber, the solvent is evaporated and the chain of gas
phase reactions begins in the production of the desired thin film.
M&Ns-19, Paris, 17-19 July 2019 Pag. 57
Fabrication of Zinc Sulfide (ZnS) Thin Films
The fabrication of Zinc Sulfide (ZnS) thin films by aerosol assited chemical vapour deposition (AACVD)
techniques involves various processes as follows;
• Substrates
The substrate cleaning is of key importance, since it affects the adhesion of the film. It has the
function of removing all contaminants from the substrates surface. Substrates used are frosted glasses
which were cleaned using distillate water and cotton wool. Cleaned slides glasses were used as
substrate for the thin film deposition.
• Precursor solution
The chemical compound used involves a precursor solution of 1.09g of Zinc Acetate Zn(CH3COO)
and 1.52g Thiourea were dissolved in 50ml of a solution of propanol (PA) (solvent) which serve as
stabilizer for the mixture, The solution was stirred at normal room temperature until a clear and
homogeneous solution was obtained.
• Deposition using 3D - Printer
The 3D Printer machine is fixed to power supply, which then undergo automatic and manual
calibration (set to work on AACVD mode). A cleaned substrate was fixed in the substrate holder and
the nozzle is set directly at the substrate surface of which the distance between the nozzle and the
substrate is 6mm. Also the target region of the substrate will be set to Y-axis and X-axis respectively.
The mode of deposition onto the substrate is raftering mode. In this research work 0.2ml is deposited
twice on each substrate (0.4ml) of which the time interval for depositing 0.4ml is two (2) minute and
the deposition temperature is 260oc per sample.
UV-VIS Spectrophotometer
UV-VIS Spectroscopy refers to absorption spectroscopy or reflectance spectroscopy in the ultra violet
and visible spectral regions. The measured UV-Vis spectrum can be viewed as an absorption spectrum
or as a transmission spectrum. The transmission spectrum gives the percentage of the incoming light that
actually travelled through the sample. The spectrum can be analyzed to obtain information such as the
energy band gap and thickness of the thin films (Jasif & Yousif, 2015).
M&Ns-19, Paris, 17-19 July 2019 Pag. 58
Fig 3.2: UV-VIS 750 Spectrophotometer
The optical gap is defined as the minimum energy needed to excite an electron from the valence band to
the conduction band. Intrinsic absorption of photons by a semiconductor occurs when the photon energy
is located in the vicinity of the energy band gap of the material. In other words, absorption happens when
the incident photon energy (hν), is greater than the semiconductor’s band gap energy between the bottom
of the conduction band and top of the valence band, Eg. The optical density (absorbance) of the thin
films is given by the expression:
𝐴 = 2 − log 𝑇 (1)
𝑇 = 10(2−𝐴) (2)
Where, A is the absorbance and T is the transmittance of the films. The maximum absorption occurs at
high energy and decreases with optical energy.
The reflectance of each deposited thin films is calculated using the expression
𝑅 + 𝐴 + 𝑇 = 1 (3)
Where, R is reflectance, A is the absorbance and T is the transmittance. The equation below is used in
calculating the refractive index.
𝑅𝑛 =1+√𝑅
1−√𝑅 (4)
The equation below can also be used to determine the band gap energy.
E =ℎ𝑐
(5)
Where, h is the Planck’s constant, c is the speed of light and is the wavelength.
In this research work a UV-VIS 750 Spectrophotometer is used to obtain the optical absorption
parameters of the films in air. It utilizes tungsten halide or deuterium light sources which pass through a
monochromeator, then split into two beams that travel through a sample and a blank glass slide reference,
respectively
the optical properties include: refractive index, reflectance, absorbance, transmittance, and band gap.
Result and Discussion
M&Ns-19, Paris, 17-19 July 2019 Pag. 59
This chapter discussed on the result obtained and were analyzed and calculated for the four consecutive
samples characterized at different temperatures; 0oC, 250oC, 300oC, and 350oC respectively.
Optical properties
The optical properties of Zinc sulfide (ZnS) thin films characterized in this work are; Absorbance (optical
density), Transmittance, Reflectance, Refractive Index, and Band Gap (energy gap).
Absorbance
This is the amount of absorb radiation over the amount of incoming power. The amount of the absorbed
radiation in the ultra violet and visible spectral regions is viewed in the absorption spectrum by the UV-
VIS Spectrophotometer, which give the amount of incoming light that actually travelled through each
sample of temperature 0oC (unannealed), 250oC, 300oC, and 350oC (annealed) respectively at a
wavelength range from 300nm-1100nm.
Fig 2: Sample 1 (unannealed 0oC) graph
of absorbance against wavelength
0,000
0,050
0,100
0,150
0,200
0,250
0,300
0,350
0,400
0,450
0,500
0,00 500,00 1000,00 1500,00
Ab
sorb
ance
Wavelength
Absorbance [a.u] against
wavelength
unannealed…
M&Ns-19, Paris, 17-19 July 2019 Pag. 60
Fig 3: Sample 2 (annealed 250oC) graph of Fig 4: Sample 3 (annealed 300oC) graph of absorbance
absorbance against wavelength against wavelength
Fig 5: Sample 4 (annealed 350oC) graph of Fig 6: Graph of absorbance against wavelength
absorbance against wavelength for all the four (4) samples
Transmittance : The result of the transmittance obtained were calculated using equation (3.2) above,
the result indicated that a decrease in the absorbance lead to an increased in the transmittance, whereas
an increase in temperature of each sample yield an increase in the transmittance. The transmittance
calculated for the four consecutive samples were more than 70%. Thus, the amount of the transmitted
radiation in the ultra violet and visible spectral regions is over the amount of incident radiation.
Table 1: calculated transmittance for each sample
0,000
0,050
0,100
0,150
0,200
0,250
0,300
0,00 500,00 1000,00 1500,00
Abso
rban
ce
Wavelength
Absorbance [a.u] against
wavelength
annealed(250ºC)
0,000
0,050
0,100
0,150
0,200
0,250
0,300
0,350
0,400
0,00 500,00 1000,00 1500,00
Abso
rban
ce
Wavelength
Absorbance [a.u] against
wavelength
anealed(300ºC)
0,000
0,050
0,100
0,150
0,200
0,250
0,300
0,350
0,00 500,00 1000,00 1500,00
Abso
rban
ce
Wavelength
Absorbance [a.u] against
wavelength
annealed(350ºC)
0,000
0,500
0,00 200,00 400,00 600,00 800,00 1000,001200,00
Abso
rban
ce
Wavelength
Absorbance [a.u] against
wavelength absorbance[unannealed…
M&Ns-19, Paris, 17-19 July 2019 Pag. 61
Samples temperature
(oc)
Absorbance [a.u] Transmittance
(%)
0 0.119 76
250 0.078 84
300 0.069 85
350 0.065 86
Fig 7: Graph transmittance for each sample temperature
Reflectance: The result here obtained were calculated using (3.3) above, an increased in the
transmittance yield an increased in the reflectance. Hence, it’s stated that reflectance is the amount of
reflected radiation over the amount of incoming power.
Table 2: Calculated reflectance for each sample
Reflectance
74
76
78
80
82
84
86
88
0 50 100 150 200 250 300 350 400
sam
ple
s
transmittance
transmittance
M&Ns-19, Paris, 17-19 July 2019 Pag. 62
Sample
Temperature
(oc)
Absorbance
[a.u]
Transmittance
(%)
0 0.119 76 74.90
250 0.078 84 82.92
300 0.069 85 83.93
350 0.065 86 84.94
Fig 8: graph of reflectance for each sample temperature
Refractive index
The result here obtained were calculated using (3.4) above, the refractive index of each were calculated
based on their temperature differences.
Table 3: calculated refractive index for each sample
S/N SAMPLE
TEMPERATURE
(Oc)
REFECTANCE REFRACTIVE INDEX
0
50
100
150
200
250
300
350
400
74 76 78 80 82 84 86
sam
ple
s
reflectance
reflectance
M&Ns-19, Paris, 17-19 July 2019 Pag. 63
1. 0 74.90 -1.26
2. 250 82.92 -1.25
3. 300 83.93 -1.25
4. 350 84.94 -1.24
Fig 9: graph of refractive index for each sample temperature
Optical band gap (Energy gap)
This is the difference between the valence band and conduction band. The result of the band gap in this
experiment were obtain using equation (3.5) above. Absorption happens when the incident photon
energy (hν), is greater than the semiconductor’s band gap energy between the bottoms of the conduction
band and top of the valence band. The optical band gap of a material increases due to reduction of
temperature and wavelength.
Table 4: calculated band gap (energy gap) for each sample
Samples (oc) wavelength
(nm)
Band gap (𝐸𝑔)
0 330.08 3.76
250 516.34 2.40
300 1027.86 2.21
350 1030.64 1.20
0
100
200
300
400
-1,265 -1,26 -1,255 -1,25 -1,245 -1,24 -1,235
Sam
ple
Tem
per
ature
Refractive Index
Refractive Index
M&Ns-19, Paris, 17-19 July 2019 Pag. 64
Fig 10: graph of Band gap against wavelength
Summary
Semiconductor particles have attracted much attention because of their novel electric and optical
properties originating from surface and quantum confinement effects. In this work, zinc sulfide (ZnS)
thin films is deposited by aerosol assisted chemical vapour deposition (AACVD) techniques onto a
frosted glass substrate, using precursor solution of 1.09g of Zinc Acitate and 1.52g of thiourea in 50ml
of propernol (solvent). The amount of the absorbed radiation in the ultra violet and visible spectral
regions is viewed in the absorption spectrum by the UV-VIS Spectrophotometer, which give the amount
of incoming light that actually travelled through each sample of temperature 0oC (unannealed), 250oC,
300oC, and 350oC (annealed) respectively at a wavelength range from 300nm-1100nm. The result of the
transmittance obtained were calculated using equation (3.2) above, the result indicated that a decreased
in the absorbance lead to an increased in the transmittance whereas an increased in temperature of each
sample yield an increased in the transmittance. The transmittance calculated for the four consecutive
samples were more than 70%. Thus, the amount of the transmitted radiation in the ultra violet and visible
spectral regions is over the amount of incident radiation, an increased in the transmittance yield an
increased in the reflectance. Hence, the refractive index for each samples were calculated.
Decrease in the particle size gives rise to quantum confinement effect. Wherein an increase in the energy
gap as well as splitting of the conduction and valance band into discrete energy levels become evident.
ZnS is an II-VI semi conducting material with a wide direct band gap. It has potential applications in
optoelectronic devices such as blue light emitting diodes electroluminescence devices and photovoltaic
cells which enable wide applications in the field of displays, sensors and lasers, biological fluorescence
marking and photo detectors. The various effect and phenomena observed during the interaction of light
0
0,5
1
1,5
2
2,5
3
3,5
4
0 200 400 600 800 1000 1200
Ban
d g
ap e
ner
gy
wavelength
M&Ns-19, Paris, 17-19 July 2019 Pag. 65
with matter in an optical experiment are unique to the particular material system. The interaction process
reveals variations in physical properties of this material. This is the basis for use of 206 optical techniques
for precise characterization of the quality of material. Optical techniques have proven to be very sensitive
tools and also non-destructive techniques for studying growth process, structural defects and impurities.
Conclusion
At the end of this research work, the main aim and objectives of this research is achieved. Zinc Sulfide
(ZnS) thin films were deposited on a frosted glass for four (4) samples, and were all characterized at
different temperatures ranging from 0oC (unannealed), 250oC, 300oC, and 350oC (annealed) respectively.
The result obtained are analyzed and calculated such as the optical density (absorbance), transmittance,
reflectance, refractive index and the optical band gap.
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