OPTICAL CHARACTERIZATION OF ZINC SULFIDE (ZnS) THIN FILMS BY AEROSOL ASSISTED CHEMICAL ... ·...

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

mailto:[email protected]

mailto:[email protected]

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


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;


• 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.


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).


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


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…


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


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


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


wavelength

annealed(350ºC)

0,000

0,500

0,00 200,00 400,00 600,00 800,00 1000,001200,00

Abso

rban

ce

Wavelength


wavelength absorbance[unannealed…


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


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


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


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


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.

REFERENCES

Alireza Goudarzi, Ghaffar Motedayen Aval, Reza Sahraei, & Hiva Ahmadpoor, (2008).Thin solid

films., vol. (5)16, 4953-4957

Asel A., Jasib A., & Yousif A. (2008).The Effect of Thickness Nanoparticle ZnS Films on Optical

Properties, International journal of basic and applied science, Insan akademika publications,

vol. 2, 2-23.

Balachander, M. P., Saroja, M. P., Venkatalachalam, M. P., & Shankar, S. P.(2015).Preparation and

Characterization of Zinc Sulfide thin film deposited by Dip Coating Method, International

Journal of Innovative Science, Engineering & Technology,Vol. 2 Issue 10, 1-14. ISSN 2348 –

7968

Ben, T., Nasr, N., Kamoun, M., & Kanzari, R. B. (2006). Effect of pH on the properties of ZnS

thin films grown by chemical bath deposition, Thin Solid Films, vol. 500, 4-8.

Ben, T., Nasr. N., Kamoun. M., & Kanzari, R. B., (2006). Morphological and Structural properties

of ZnS thin films, Thin solid films, vol.5, 4-8.

Echendu, O. K., & Dharmadasa, I. M. (2013). Journal of electronic materials vol. 42( 4), 12-33.

Echendu, O. K., & Dharmadasa, I. M. (2015). Thin films, Energies 8, 4416-4435.

Doi:10.3390/en8054416.

Echendu, O. K., Fauzi, F., Weerasinghe, A. R., & Dharmadasa, I. M. (2014). Thin Solid Films, vol. 5,

529-534


Hasanzadeh, J., Taherkhani A. & Ghorbani, M. (2013). Luminescence and Structural Properties

of ZnS:Cu Nanocrystals Prepared Using a Wet Chemical Technique, Chinese journal of

physics, vol. 51(3), 540-550.

Jasib & Yousif (2015). International Journal of Basic and Applied Science, Vol. 03, No. 03, 38-51.

www.insikapub.com 51

Madugu, Mohammad L., Olusola, Olajide I., Echendu, Kingsley, O., Kadem, Burak & Dharmadasa, I.

(2016). Intrinsic Doping in Electrodeposited ZnS Thin Films for Application in Large-Area

Optoelectronic Devices. Journal of Electronic Materials, 45(6), 2710-2717.

Nabiyouni, G., Sahraei, M., Toghiany M. H., Majles A., & Hedayati, K. (2011). Preparation and

Characterization of Nanostructured Zns Thin Films Grown on Glass and N-Type Si Substrates

Using a New Chemical Bath Deposition Technique, Adv Mater. Sci., vol. 27, 52-57.

Nada, M. S. (2011). Structural and Optical Properties of ZnS Thin Films Prepared by Spray Pyrolysis

Technique. Journal of Al-Nahrain University, vol. 14(2), 86-92.

Nadeem, M. Y., Waqas Ahmed, & Wasiq, M. F. (2005). Journal of Research (science) Vol.16(2),

105-112.

Nadeem, M., & Waqas, Y. Ahmed. (1999). Turk J Phy, Journal of mater. Sci.,vol 24, 651.

Ndukwe, I. C. (1996). solar Energy materials and solar cells, Bull.Mater.Sci., vol. 40,123-131.

Nur Ubaidah Saidin, Kok Kuan Ying & Ng Inn Khuan. (2006). Electrodeposition: Principles,

Applications and Methods, Industrial Technology Division, Malaysian Nuclear Agency, vol. 4,

231-245

Ravi Sharma, Chandra, B. P., Bisen, D. P (2009). Optical properties of ZnS: Mn nanoparticles

prepared by chemical bath routs, Chalcogenide Letters, vol. 6(8) 339 – 342.

Romero, P. G. & Sanchez C., (2003). Functional Hybrid Materials. Wiley & Weinheim

publication, Pp 86.

Tec-Yam, S., Rojas, J., Rejon, V., & Oliva, A. I. (2012). Materials Chemistry and Physics, vol. 136,

386-393.

Ubale, A. U. & Kulkarni D. K. (2005). Preparation and study of thickness dependent electrical

Characteristics of zinc sulfide thin films. Bull. Mater. Sci., vol. 28(1), 43–47.

Valkon, M. P., Lindroos, S., & Leskela, M. (1998). Appl.Sur.Sci, journal of materials research,

vol 3(12), 134- 283.


Wu, Y., Xiang, J., Yang, C., Lu, W., & Liber, M.C. (2004). Nature 430, 61; Kim, B. H., Jung, J. H.,

Hong, S. H., Joo, J., Epstein, A. L., Mizoguchi, K., Kim, J. W., & Choi, H. J. (2002).

Macromolecules 35:1419; Huynh, W. U., Dittmer, J. J., Alivisatos, A. P. (2002). Science 295:

425;

Xiaochun, W., Fachun, L., Yongzhong, L., Zhigao, H., & Rong, C. (2007). Effects of substrate

temperature and annealing on the structure and optical properties of ZnS film," Proc. of

SPIE," vol. 6722, 1-5.

Yang Y., & Zhang, W., (2004). mater.Lett.”International journal of material chemistry, vol. 58,

336.

Yasuda, T., Hara, K., & Kukimoto, H. (1986). Low resistivity Al-doped ZnS grown by MOVPE,

Journal of crystal growth, vol. 77, 485-489.


OPTICAL CHARACTERIZATION OF ZINC SULFIDE (ZnS) THIN FILMS BY AEROSOL ASSISTED CHEMICAL ... ·...

Documents

Transcript of OPTICAL CHARACTERIZATION OF ZINC SULFIDE (ZnS) THIN FILMS BY AEROSOL ASSISTED CHEMICAL ... ·...