The Effect of Thickness Nanoparticle ZnS Films on Optical Properties

The Effect of Thickness Nanoparticle ZnS Films on Optical Properties Asel A. Jasib 1, Ali A. Yousif 2 1,2 Department of Physics, College of Education, University of Al-Mustansiriyah, Baghdad, Iraq asel.adel56@yahoo.com aliyousif73@gmail.com Abstract In this study, nanoparticle ZnS films have been successfully deposited on glass substrates by chemical spray pyrolysis (CSP) technique at substrate temperature of (400±5 О C) and different thickness (75, 150, 225, 300, 375, 450 and 525 nm). The absorbance and transmittance spectra have been recorded in the wavelength range of (340-1100 nm) in order to study the optical properties. The transmittance for all thin films increases rapidly as the wavelength increases in the range (340-360 nm), and then increases slowly at higher wavelengths, also the transmittance decreases with increased thickness. The absorbance increase with increased thickness and decreases rapidly at short wavelengths (high energies) corresponding to the energy gap of the film, (when the incident photon has an energy equal or more than the energy gap value). The absorption coefficient was estimated for all samples and due to its high values (<104cm-1), it was concluded that the thin films material has a direct band gap. The optical energy gap was calculated for allowed direct electronic transition. It was found that the band gap decreases when the thickness increases and the band gap values ranges between 3.62eV and 3.38eV. The optical constants including (absorption coefficient and optical conductivity) were also calculated as a function of photon energy, (reflectance, Refractive index, extinction coefficient and real and imaginary parts of dielectric constant) for all films were estimated as a function of wavelength. Key Words Nanoparticle, Zinc Sulfide (ZnS), Optical Properties, Chemical Spray Pyrolysis. 1 Introduction 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 3.77 ev, respectively. [1] 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 an absorber in hetero-junction thin-film solar cells. [2] 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 [3]. 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 [4-6]. 38

Jasib and Yousif These nanoparticles ZnS semiconductor materials have a wide range of applications in electroluminescence devices, phosphors, light emitting displays, and optical sensors. Nanoparticles of dimensions below Bohr diameter exhibit interesting optoelectronic properties due to quantum size effect and are potential candidates for variety of applications, the responsible for particular luminescence emission and efficiency of semiconductor nanoparticles.[7-8] In the present work, we report the chemical spray pyrolysis of nanoparticle ZnS thin films and their characterization. The effect of the change of thickness on optical properties of ZnS thin films, by using UV-VIS spectrometry, is investigated with the objective to optimize the conditions of the deposition process. 2 Experiment 2.1 System used for fabricating thin films Chemical spray pyrolysis method was employed in the present work, where in this method, thin films were prepared by spraying the solution on a hot glass substrate to a certain temperature, and the film could be then obtained by the chemical reaction on the hot substrate. However, in some application these thin films could have good properties, for example. It might be used in solar and sensor applications. 2.2 Preparing of the Spray Solution The spraying solution which contains the materials necessary for fabrication of the ZnS film can be prepared by mixing zinc chloride ZnCL 2 and thiourea CS(NH 2 ) 2 as starting materials. The molar concentration of the solution should be equal to 0.1 mole/liter. In order to prepare the solution of 0.1 molar concentrations from these two materials, 0.6814 grams weight of ZnCL 2 and 0.3806 grams weight of CS(NH 2 ) 2 are needed from each of them, melted in 100 ml of distilled water, according to the following equation: Weight of the material (gm) = Volume (ml) Molecular concentration (mol/l) Molecular weight (gm/mol) (1) Finally, the two weights materials melted in (100ml) of distilled water to get the wanted solution (The spray solution).the solution then sprayed and deposited on a cleaned glass (400 ºC) substrate to get the finally ZnS thin films. It is necessary to leave the glass substrate on the electrical heater for one hour at least after finishing the operation of spraying to complete its oxidation and crystalline growth process. 2.3 Thickness Measurements In this work the experimental method of thickness measurement was used: 2.3.1 Optical interferometer method by (He Ne) laser: There are a many optical methods to measure the film thickness, these methods are based on the interference of the light within thin film. The film thickness measurements by optical interferometer method have been obtained. This method is based on interference of the light beam reflection from thin film surface and substrate bottom. He-Ne laser (0.632 µm) is used and the thickness is determined using the following formula: [9] x d ( 2) x 2 www.insikapub.com 39

Jasib and Yousif Where x is the fringe width, x is the distance between two fringes and λ wavelengths of laser light. The film thickness from applying equation (2) is (75, 150, 225, 300, 375, 450 and 525 nm). The interference mode is formed as a result of the phase difference between the rays reflected from the back surface and the rays reflected from the front surface. 2.4 Optical measurements The optical measurements of the ZnS thin film deposited on glass substrate by spray pyrolysis (SP) technique at a substrate temperature 400 0 C for different thickness (x = 75, 150, 225, 300, 375,450 and 525nm) are calculated from the transmittance and absorbance spectrum at normal incidence over the range (340 1100) nm, by using UV-VIS spectrophotometer type ( SHIMADZU ) ( UV- 1600/1700 series ). 3 Results and Discussion 3.1 Optical Properties Measurements Optical properties of great importance in the study of the behavior of the optical semiconductor materials, which it can see the practical application of appropriate, visual behavior is closely associated with the crystal structure of the material, and the installation of energy level. The optical properties of the ZnS films deposited by chemical spray pyrolysis technique on glass substrate at 400 C temperature with different thickness (75,150,225,300,375,450 and 525 nm). Can many of the optical constants account of study change spectral transmittance and absorbance of these membranes, such as the expense of absorption coefficient, and energy gap for direct transitions and calculate the reflectivity and extinction coefficient and refractive index and dielectric constant real and imaginary and optical connectivity. 3.1.1 Transmission (T) Transmission of films depends in general on the thickness of the film, and the nature of the surface and the type of material, and its crystal structure, and the degree of heat-substrate, as well as the algebraic sum of the absorbency and the reflectivity of these films. Transmission were measured for all films in room temperature by UV-VIS spectrophotometer in the range from 340nm to 1100nm. The variation in the transmission of nanoparticles ZnS films depends on the method of preparation of the films. The transmittance spectra of the ZnS films coated with different thicknesses are shown in (Figure 1). The figure shows that films coated with 75 nm thickness have a maximum transmittance of 88% in the visible region. The porosity, crystalline, structural and surface homogeneity influence the film transmission [10]. It has been observed that the over all T% increase with the decrease in the film thickness. When the film thickness is small, the porosity will be high which will tend to increase the transmittance, at the same time the crystallinity, surface and structural homogeneity may be poor tending to reduce the transmittance, but because of the dominance of the effect due to porosity, the transmittance could be high. This is in agreement with the contention of that low film thickness results in enhanced transmission. When the thickness is increased the porosity would decrease tending to reduce the transmission but the crystallinity, surface and structural homogeneity may increase the transmittance. Under these contrasting effects the transmittance may register a fall up to a certain film thickness and then may become more or less a constant. In both cases, the transmittance falls very sharply at the lower wavelength region due to the onset of the fundamental absorption [11]. 40 Insan Akademika Publications

Jasib and Yousif 3.1.2 Absorption (A) Fig. 1: The optical transmission of ZnS thin films with different thickness. Can be found in many of the optical constants of the absorbance spectrum study was conducted as part of a range of wavelengths (340-1100nm) for all films prepared at room temperature. The absorbance spectra of the thin films of ZnS,having different thicknesses, are shown in Figure (2). These spectra reveal that films, grown under the same parametric conditions have low absorbance in the visible and near infrared regions.however, absorbance in the ultraviolet region is high. The enhanced absorption is observed in the neighborhood of λ=360 nm.it has been observed that the maximum absorption peak shifts towards the longer wavelength with increasing film thickness.this suggests the decrease in the band gap with the increasing thickness.the overall absorbance has been increased with the film thickness. This is because of the reason that in case of thicker films more atoms are present in the film so more states will be available for the photons to be absorbed. [12]. Fig. 2: The optical Absorption of ZnS thin films with different thickness. www.insikapub.com 41

Jasib and Yousif 3.1.3 Absorption Coefficient (α) We calculated the absorption coefficient (α) as a function of photon energy of ZnS films with different thickness. Figure (3) shows the absorption coefficient (α) of ZnS films increasing with the increasing of photon energy (or absorption coefficient α of ZnS films increasing with the decreasing of wave length). The optical absorption coefficient α determined from absorbance measurements using equations (3). [13] 2.303A ( 3) t Where (A) is the absorbance, and (t) is the sample thickness The absorption coefficient of nanoparticles ZnS films decreased in the UV/VIS boundary (or at lower photon energy), and then decreased gradually in the visible region (or at high photon energy) because it is inversely proportional to the transmittance. Fig. 3: Absorption coefficient as a function of photo energy ZnS thin films at different thickness. 3.1.4 Optical Energy Gap (Eg) The optical band gap (Eg) of nanoparticle ZnS films was evaluated from the transmission (or absorption ) spectra and optical absorption coefficient (α ) near the absorption edge for allowed direct transitions is given by equation : [14] αhυ = B ( hυ Eg ) r (4) Where Eg: energy gap between direct transition B: constant depended on type of material υ: frequency of incident photon. r: exponential constant, its value depended on type of transition, 42 Insan Akademika Publications

Jasib and Yousif r =1/2 for the allowed direct transition. r =3/2 for the forbidden direct transition. Equation (4) gives the band gap Eg, when the straight portion of α² versus hυ plot is extrapolated to the point α = 0. Figure (4) shows the variation of band gap with the thickness of the films. ZnS thin films grown here have band gap in the range (3.6 ev - 3.38 ev) which shows in table (1). These values are decreasing with the increasing thickness of the films. There is the possibility of structural defects in the films due to their preparation this could give rise to the allowed states near the conduction band in the forbidden region. In case of thick films these allowed states could well merge with the conduction band resulting in the reduction of the band gap. [15] www.insikapub.com 43

Jasib and Yousif 44 Insan Akademika Publications

Jasib and Yousif Fig. 4: Shows the variation of band gap with the thickness of the films Table 1: The values of optical energy gap for ZnS thin film with different Thickness ZnS Thickness (nm) 75 150 225 300 375 450 525 Energy gap (ev) 3.62 3.54 3.5 3.42 3.41 3.4 3.38 3.1.5 Reflectance (R) Defined as the ratio between the reflective intensity of the reflected beam to the amount of incident radiation, it can calculate the reflectivity of the spectral absorbance and transmittance, according to equation (5). [13] R T A 1 ( 5) Figure (5) shows the change in reflectivity as function to the wavelength of the ZnS films. The reflectance of ZnS thin films is small in the near infrared and visible region, the over all reflectance of the film increases with increases film thickness [12]. www.insikapub.com 45

Jasib and Yousif Fig. 5: The optical Reflectance of ZnS thin films with different thickness. 3.1.6 Refractive Index (n) The refractive indices (n) of the ZnS thin films are determined from equation (6). [9] n 4R K 1 2 1 2 0 R 1 1 2 R R (6) Where (R) is the reflectance and (K 0 ) extinction coefficient. As shown in figure (6) shows the variations in the refractive index with the wavelength. The increase in the film thickness results in the over all increase in the refractive index. This increase is due to the over all increase in the reflectance with the film thickness. The peak value of the refractive index for the ZnS thin films of various thickness vary in the range of 2.61 to 2.64, which is in good agreement with the value 2.62 reported by I. C. Ndukwe [16]. 46 Insan Akademika Publications

Jasib and Yousif Fig. 6: Refractive index as a function of wavelength for ZnS thin films at different thickness. 3.1.7 Extinction Coefficient (K ο ) The extinction coefficient (K 0 ) is evaluated using equation (7). [9]. k ( 7) 4 Where (λ) is the wavelength of the incident radiation. Figure (7) shows the extinction coefficient (K 0 ) as a function of wavelength for different thickness of ZnS films. As shown in figure (7). The extinction coefficient decrease as the wavelength increases. And the extinction coefficient increase as the optical thickness increases, the increase of surface roughness with increasing film thickness for crystalline film will increase surface optical scattering and optical loss [17], which induces the increase of extinction coefficient [18]. Fig. 7: Extinction coefficient as a function of wavelength for ZnS thin films at different thickness. www.insikapub.com 47

Dilectric Constant (Real) Jasib and Yousif 3.1.8 Real and Imaginary Part of Dielectric Constant (ε r ), (ε i ) The real and imaginary part of dielectric constant of the ZnS thin films have been investigated using equations (8) and (9) [9] 2 2 r n K ( 8) i 2nK ( 9) As shown in figures (8) and (9), the variation of (ε r ), (ε i ) with wavelength for percentage of different thickness of ZnS films. The obtained results show that the values of real (ε r ) part of dielectric constant are increased with increasing of wavelength for ZnS thin films, especially it increased with increasing of thickness and should be noted that imaginary (ε i ) parts values for ZnS films are increased with increasing of wavelength for ZnS thin films, especially it increased with increasing of thickness. The range of variation of dielectric constant is in agreement with the observations of Ndukwe [16], Nadeem and Waqas [15] Different shapes of the curves for the real part of the dielectric constant have been observed. This is due to the different effective thickness of the insulator. The imaginary part confirms the free carrier's contribution to the absorption [12]. 8 7 6 75 nm 150 nm 225 nm 300 nm 375 nm 450 nm 525 nm 5 4 3 2 1 0 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 Wavelength (nm) Fig. 8: Real part of dielectric constant of the ZnS thin films with different thickness. 48 Insan Akademika Publications

Dilectric Constant ( Imaginary ) Jasib and Yousif 0.3 0.25 0.2 75 nm 150 nm 225 nm 300 nm 375 nm 450 nm 525 nm 0.15 0.1 0.05 0 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 Wavelength (nm) Fig. 9: Imaginary part of dielectric constant of the ZnS thin films with different thickness. 3.1.9 Optical Conductivity (σ) Figure (10) shows the variation of optical conductivity as a function of photon energy for different thickness of ZnS films. The optical conductivity is calculated by using equation (10).[9] nc...(10) 4 From the figure, we can see that the optical conductivity increases with increasing photon energy. This suggests that the increase in optical conductivity is due to electron exited by photon energy, and the optical conductivity of the films increases with increasing thickness in the films. The increased optical conductivity at high photon energies is due to the high absorbance of ZnS thin films in that region [15]. www.insikapub.com 49

Jasib and Yousif Fig. 10: optical conductivity as a function of photon energy for ZnS thin films with different thickness. 4 Conclusions The transmittance for all films increases with the decrease in the films thickness, and increases rapidly as the wavelength increases in the range (300-350nm), and then increases slowly at higher wavelengths. The band gap decreases when the thickness increases and the band gap values range between 3.62eV and 3.38 ev. The optical analysis showed that the influence of thickness on the energy gap of nanoparticle ZnS films is significant and found that the band gap of ZnS films could be wide with decreasing thickness. Because that nanoparticle ZnS films have average transmittance larger than (88%) at thickness 75 nm, we can use these samples as an optical window in solar cell. References [1] T. Ben Nasr, N. Kamoun, M. Kanzari, R. Bennaceur, "Effect of ph on the properties of ZnS thin films grown by chemical bath deposition," Thin Solid Films,vol. 500, pp.4-8,2006. [2] A. U. Ubale and D K Kulkarni, "Preparation and study of thickness dependent electrical Characteristics of zinc sulfide thin films," Bull. Mater. Sci., vol. 28 (1), pp. 43 47,2005. [3] Nada M. Saeed, "Structural and Optical Properties of ZnS Thin Films Prepared by Spray Pyrolysis Technique," Journal of Al-Nahrain University, vol.14 (2), pp.86-92, 2011. [4] Ravi Sharma, B.P.Chandra, D. P. Bisen, " Optical properties of ZnS: Mn nanoparticles prepared by chemical bath routs," Chalcogenide Letters, vol. 6 (8) pp. 339 342, 2009 50 Insan Akademika Publications

Jasib and Yousif [5] J. Hasanzadeh, A. Taherkhani and M. Ghorbani, "Luminescence and Structural Properties of ZnS:Cu Nanocrystals Prepared Using a Wet Chemical Technique, Chinese journal of physics," vol. 51 (3),pp.540-550, 2013. [6] Xiaochun Wu, Fachun Lai, Yongzhong Lin, Zhigao Huang, Rong Chen, Effects of substrate temperature and annealing on the structure and optical properties of ZnS film," Proc. of SPIE," vol. 6722, pp 67222L(1-5), 2007. [7] K. Jayanthi, S. Chawla, H. Chander, and D. Haranath, "Structural, optical and photoluminescence properties of ZnS:Cu nanoparticle thin films as a function of dopant concentration and quantum confinement effect," Cryst. Res. Technol." vol.42 (10), pp. 976 982, 2007. [8] Eman M. Nasir, surface morphological and structural properties of ZnS and ZnS:Al thin films, International Journal of Innovative Research in Science, Engineering and Technology, Vol. 3, Issue 1, January 2014, pp. 8114 8120. [9] Ali A. Yousif, "Preparation and Construction of Efficient Nanostructures Dopant ZnO Thin Films Prepared by Pulsed Laser Deposition for Gas Sensor", Ph.D. Thesis, Dept. of phys., University of Al-Mustansiriya, (2010). [10] Ates A, Y1ld1r1m MA, Kundakc1 M. Annealing and light effect on optical and electrical properties of ZnS thin films grown with the SILAR method. J. Mater. Sci. Semicond. Process., 2007;10(6):281-286. [11] P. Krishnamurthi and E.Murugan, Effect of layer thickness on the structural and optical properties of chemically sprayed ZnS thin films, Journal of current pharmaceutical research, Vol. 11(1), (2013), p.p. 38-42. [12] Raad.M.S.Al-Hadad, Raheem G.K Al-Morshdy and Hussein Kh Al-Lamey, " The effect of thickness on the optical properties of ZnS, Um-Salama Science Journal.,Vol.4(4)2007, pp. 647 652. [13] J. Marien, T. Wagner, G. Duscher, A. Koch, M. Rühle, Ag, Pt, Pd, Nb Doping (110) TiO 2 (Rutile): Growth, Structure, and Chemical Composition of the Interface, Surface Science, Vol. 446, (2000), pp.219. [14] C. Kittel "Introduction to solid stat physics", 5 th ed., Newyork (1976) p.361. [15] M. Y. Nadeem and Waqas Ahmed, "Optical properties of ZnS thin films using resistive heating technique", Turkish Journal of Physics, Vol 24, 2000, p. 651. [16] I. C. Ndukwe, "Solar energy materials and solar cells", vol 40, 1996, p. 123. [17] F. Lai, L. Lin, Z. Huang, R. Gai, Y. Qu, "Optical inhomogeneity of ZnS films deposited by thermal evaporation",appl. Surf. Sci. 253 (2006) 1801. [18] Xiaochun Wu, Fachun Lai, Limei Lin, Jing Lv, Binping Zhuang, Qu Yan and Zhigao Huang, "Optical inhomogeneity of ZnS films deposited by thermal evaporation", Applied Surface Science, 254,2008, p. 6455. www.insikapub.com 51