Role of Deposition Potential on the Optical Properties of SnSSe Thin Films. Department of Physics, Alagappa University, Karaikudi , India.

Transcription

1 / The Electrochemical Society Role of Deposition Potential on the Optical Properties of SnSSe Thin Films T. Mahalingam a, V. Dhanasekaran a*, G. Ravi a, R.Chandramohan b, A. Kathalingam c, Jin-Koo Rhee c a) Department of Physics, Alagappa University, Karaikudi , India. b) Department of Physics, Sree Sevugan Annamalai College, Devakottai , India. c) Millimeter-wave INnovation Technology Research Center (MINT), Dongguk University, Seoul , Republic of Korea SnSSe solid solution thin films were electrosynthesized from an aqueous solution by potentiostatic method. Films were deposited at different potentials in the range of -700 mv to mv vs SCE. XRD patterns revealed that the deposited films exhibited polycrystalline orthorhombic structure with preferred orientation along (111) crystallographic plane. The optical band gap value was calculated from transmittance and absorption data and the films were found to have direct band gap found to be 1.08 ev-1.25 ev. The refractive index, extinction coefficient, real and imaginary part of dielectric constants and optical conductivity of the SnSSe thin films has been calculated. The value for the refractive index lies between 2.5 and 3.1 and value of extinction coefficient lies between and Surface morphological studies revealed pellet shaped grains occupying the entire surface of the film distributed homogeneity. Introduction The synthesis and characterization of metal chalcogenides via different techniques have attracted considerable attention due to their application prospects. These compounds are reported to be used as sensor and laser materials, thin films polarizers and thermoelectric cooling materials [1]. The IV VI semiconductors have been found to be quite useful in optoelectronic devices working in far IR region as thermoelectric transducers and solar cells [2, 3]. This ternary compound has recently attracted much interest in solar energy conversion because of the band gap tailoring effected by the incorporation of sulfur (S) in tin selenide (SnSe). Structural properties of the material play a dominant role in the performance of the devices and knowledge of the influence of various deposition parameters on the structural properties of thin films is essential before the application of this material in devices [4-6]. Number of attempts have been made by different investigators to measure electrical and optical properties of SnS, and SnSe thin films. Tin sulphide shows narrow band gap type semiconductor behavior and has been used in variety of electronic devices [7, 8]. Semiconductors with a band gap of 1.2 ev have appropriate electrical and optical characteristics suitable for use in diversified applications [9, 10]. Considerable efforts have been made in recent years in the search for low-cost materials for solar energy conversion. These can be synthesized by means of low-temperature chemical precipitation or vapour deposition techniques, as described in previous works [11, 12]. The materials may also be electrosynthesized, either cathodically [13-15] or anodically [16, 17] from an aqueous or non-aqueous medium. Literature shows very few reports on SnSSe in mixed crystals of composition SnS 0.5 Se 0.5 Downloaded on to IP address. Redistribution subject to ECS terms 1 of use (see ecsdl.org/site/terms_use)

2 have the same crystal structure as SnS and SnSe. No superstructure was observed [18]. The unit cell contains eight atoms which are arranged in two adjacent double layers orthogonal to the largest cell dimensions [19]. Within either double layer, each atom has three nearest neighbors and two next nearest neighbors. A sixth nearest neighboring atom lies in the other double layer and provides the bond between double layers. The resulting highly layered structure, typical of all orthorhombic chalcogenide crystals, causes a strong anisotropy of the physical properties of these compounds [20]. Electrochemical methods, especially those performed in an aqueous medium, have recently received enormous attention due to their simplicity and cost effectiveness. The previous reports on this interesting system are depleted of the detailed structural and optical studies on the solid solution and only a near stoichiometry results on the band gap alone was dealt. This study reports on the detailed studies on the structural and optical properties for various film compositions SnS x Se 1-x. Also the reaction kinetics is also discussed. In the present study, we report the preparation of SnSSe thin films by electrodeposition employing potentiostatic mode onto indium tin oxide (ITO) coated conducting glass substrates. The structure, morphology and optical properties of the prepared thin films under as-deposited condition were studied using X-ray diffraction, scanning electron microscopy (SEM) and optical absorption techniques respectively and the results are discussed. Experimental Details Electrochemical experiments were carried out using a Potentiostat/Galvanostat (EG&G Princeton Applied Research, USA Model 362A). A standard three electrode cell was used for the electrodeposition SnSSe thin films. Indium doped tin oxide (ITO) was used as working electrode, while graphite rod is used as counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. ITO conducting glass substrates of 20 Ω/square where used as substrate in this deposition with dimensions 2cm x 1cm. ITO coated glass substrates were first cleaned in acetone, and subsequently thoroughly rinsed with distilled water. SnSSe thin films were prepared using an electrolyte containing tin chloride (SnCl 2 ), sodium sulphate (Na 2 S 2 O 3 ) and selenium dioxide (SeO 2 ). SnCl 2, Na 2 S 2 O 3 and SeO 2 were taken with 0.02 M, 0.1 M and M, respectively and ph of the solution 2.5 ± 0.1 was adjusted using sulphuric acid. Electrodeposition was performed with deposition potential varying from V to V insteps of 100 mv with respect to SCE. The electrolytic bath temperature and deposition time were maintained at 75 0 C and 30 minutes, respectively. An X-ray diffractometer [X PERT PRO PANalytical, Netherlands] with Cuk α radiation (λ = nm) was used to identify the crystal structure of the films. Surface morphological studies were carried out using a scanning electron microscope (Philips Model XL 30, USA). Optical properties of the deposited samples were analyzed using a UV- Vis- NIR double beam spectrophotometer (HR , M/S ocean optics, USA). Results and Discussion X-ray Diffraction (XRD) studies were carried out with am view to determining the structural properties of the deposited films. From XRD data, the crystalline nature and orientation of the deposited films were estimated. Figure 1 shows typical X-ray diffractogram of electrodeposited SnSSe thin films prepared at various potentials from mv to mv vs SCE. SnSSe thin films prepared at -700 mv vs SCE is found to be poorly crystallized, whereas the films deposited at -900 mv vs SCE potential exhibit well defined peaks in the diffractogram. At solution ph value lower than 2.5 the Downloaded on to IP address. Redistribution subject to ECS terms 2 of use (see ecsdl.org/site/terms_use)

3 formation of films may be hindered due to hydrogen evolution reaction. If the solution ph value is greater than 2.5 precipitation of electrolytic bath occurs which in turn yield poor quality films. Therefore, the deposition potential, bath temperature and ph values are fixed as -900 mv vs SCE, 75 0 C and 2.5 respectively to get better deposition. The observed diffraction peaks are found to be corresponding to orthorhombic SnSSe polycrystalline structure at 2 values 26.88, 30.33, 31.29, 35.22, and corresponding to the lattice planes (210),(011), (111), (400), (511) and (502), whereas the diffraction peaks of orthorhombic (SnS) are observed at 2θ value is (marked by *) corresponding to the lattice plane (112) respectively. The observed d values of semiconducting SnSSe thin films are found to be in good agreement with standard JCPDS values for orthorhombic (SnSSe) and orthorhombic (SnS) phases [21, 22]. It is also observed from figure 1 that the intensity of orthorhombic (SnS) peaks increase, with the decrease of deposition potential from -700 mv to -900 mv vs SCE. Below this deposition potential the binary compound peak s intensity eventually decreases. Films with good crystallinity are exhibited by films when deposited at-900 mv vs SCE. Figure 1. X-ray diffraction patterns of typical SnSSe thin films deposited at various potentials (a) -700 mv (b) -800 mv (c) -900 mv and (d) mv vs SCE The optical parameters such as absorption coefficient and band gap are determined from optical absorption measurements. The value of absorption coefficient for strong absorption region of thin film is calculated using the equation [23]. The nature of transition is determined using the equation [23]. The optical band gap of SnSSe thin films deposited at -900 mv vs SCE is estimated using optical absorption data derived from optical measurements. The band gap values are obtained by extrapolating the linear portions of the plots hν versus (αhν) 2 to the energy axis is shown in figure 2. It is observed that electrodeposited SnSSe thin films exhibit direct transition. The band gap energy of SnSSe thin film deposited at optimized deposited conditions is 1.08 ev for a stoichiometric composition and this value is in good agreement with the value reported earlier report of SnS and SnSe thin films. The band gap energy varies between ev for various S, Se compositions. Downloaded on to IP address. Redistribution subject to ECS terms 3 of use (see ecsdl.org/site/terms_use)

4 Figure 2. Direct band gap of the SnSSe thin film deposited at bath temperature 75 0 C and deposition potential -900 mv vs SCE Figure 3. Variation of refractive index (n) and extinction coefficient (k) of SnSSe thin films with deposition potentials Optical transmission spectra were recorded at room temperature in air to obtain information on the optical properties of SnSSe thin films. The transmission spectrum is used to calculate the refractive index using the modified envelope method proposed by Swanepoel [24, 25]. 1/ 2 1/ n = N 1 + N + s 1 T N 1 = 2s M T m T M T m + ( s 2 + 1) 2 [1] [2] T M and T m are the values of maximum and minimum transmission values at a particular wavelength s is the refractive index of the substrate. Refractive index can be estimated by extrapolating envelops corresponding to T M and T m. These fringes can be used to calculate the refractive index (n) of the thin films using equations (1), & (2). The refractive index corresponding to T M and T m for same wavelengths are calculated. Extinction coefficient (k) of SnSSe films are estimate using the expressions [26], αλ = 4π k [3] where α is absorption coefficient, λ is the wave length and R is reflectance of the SnSSe thin films. The refractive index and the extinction coefficient spectra of SnSSe thin films are shown in figure 3. It was observed from this figure 3 that the refractive Downloaded on to IP address. Redistribution subject to ECS terms 4 of use (see ecsdl.org/site/terms_use)

5 index increases with deposition potential, while the extinction coefficient increases for electrodeposited SnSSe thin films. The value for the refractive index lies between 2.5 and 3.1 and value of extinction coefficient lies between and respectively. It is also seen from figure that the refractive index significantly depends on thickness of the films. It is observed that in the refractive index spectra, the lower thickness films are sensitive in the higher energy region, while the higher thickness films in the lower energy region. The complex dielectric constant is known to be a fundamental intrinsic material property. The real part of dielectric constant is associated with the property of slowing down the speed of light in the material. The real and imaginary parts (Figure 4) of the dielectric constant were determined using the relation 2 ε = ε r + ε i = ( n + ik) [4] where ε r and ε i are the real and imaginary parts of the dielectric constant respectively and are given by 2 2 ε r = n k [5] and ε i = 2nk [6] The imaginary part of the dielectric constant also showed the same behavior as that of the real part, only thing is that their values seem to be very less compared to than that of real dielectric constant values. Figure 4 shows the plot of the real and the imaginary dielectric constant versus the deposition potential. The imaginary part of the dielectric constant resembles the variation similar to extinction coefficient values and the variation is plotted simultaneously. The trend showed is typical of a semiconductor. The knowledge of the real and imaginary parts of the dielectric constant provides information about the electronic band structure; details of it are found [25]. The values of ε r and ε i are found to vary with the increase in photon energy. This is based on stoichiometry of the samples and the formation energy of defect levels. This may also be due to the band gap dependence on the deposition potential. Figure 5 shows optical conductivity and optical thickness of SnSSe thin films with various deposition potentials are shown in figure 5. The optical thickness of the thin film can be calculated by knowing the values of the refractive indices n 1 and n 2 at two adjacent maxima or minima corresponding to their wavelengths λ 1 and λ 2. Then the thickness is given by, Downloaded on to IP address. Redistribution subject to ECS terms 5 of use (see ecsdl.org/site/terms_use)

6 λ λ d = λ n λ n [7] The optical conductivity is determined using the relation [27] αnc σ = [8] 4π where c is the velocity of light. The optical thickness is the distance travelled by the light in the nedia and the conductivity is the photonic conductivity through this distance. Since the formula contains refractive index the spectra resembles the n spectra. The films are less absorbing hence the absorption modification of the n spectra is very less. For highly absorbing films we can easily see the difference between n spectra and σ spectra. Figure (5) shows the variation of optical conductivity with the incident photon energy. The thickness of the films is related with refractive index and wavelength of SnSSe thin films using relation (7). Also it is predicted that the deposited film thickness linearly increases with the increase of deposition potential as shown in figure (5). It is also the optical conductivity that directly depends on the absorption coefficient and found to increase sharply for higher energy values due to large absorption coefficient for these films. From the plots of figure 9 we find that the optical conductivity of the films increases with increase of deposition potential indicating the semiconducting behavior of the samples. The spectra do not show any signature related to phonon induced scattering. This once again can be reasoned out based on stoichiometry of samples and the formation energy of defect levels. Figure 4. Variation of real (ε r ) and imaginary (ε i ) part of dielectric constant of SnSSe thin films with deposition potentials Figure 5. Variation of optical conductivity and optical thickness of SnSSe thin films with deposition potentials Downloaded on to IP address. Redistribution subject to ECS terms 6 of use (see ecsdl.org/site/terms_use)

7 The surface morphology of SnSSe thin films prepared at bath temperature of 75 0 C and at a deposition potential of -700 mv and -900 mv versus SCE are shown in figure 6 (a, b). From figure 6(a) it is seen that the grain boundary could not be obviously be observed on the surface, but the micrographs reveal various types of distribution of the particles on the film s surface depending on the deposition potential. It is observed that the film surface constitutes inhomogeneous mixer of rod like and spherical shaped grains. The reason for this state may be attributed to the tense state of the surface of the film. It is observed from figure 6(b), the surface is observed to be smooth and uniform and pellet shaped grains for films obtained at deposition potential -900 mv versus SCE. The sizes of the grains are found to be in the range between 80 and 150 nm. The average size of the grain is found to be 100 nm. Hence the deposition potential is fixed as -900 mv versus SCE for further studies. (a) (b) Figure 6. SEM picture of SnSSe thin film deposited at (a) -700 mv vs SCE and (b) -900 mv vs SCE. Downloaded on to IP address. Redistribution subject to ECS terms 7 of use (see ecsdl.org/site/terms_use)

8 Conclusions Cathodic potentiostatic deposition of SnS x Se 1-x thin films onto tin oxide coated conducting glass substrates have been carried out under various deposition potentials. The deposition potential and bath temperature were optimized as -900 mv vs SCE and 75 0 C, respectively, on the basis of structural studies and film composition. They are found to depend on the deposition potential and film thickness. The band gap energy is estimated to be 1.08 ev for stoichiometric films and the films are found to be suitable for opto-electronic devices. The value of refractive index and extinction coefficient are found to be 2.65 and 0.037, respectively, for optimized stoichiometric condition. Complex dielectric constants and optical conductivity of the SnSSe thin films are estimated. The surface is observed to be smooth, uniform and made of pellet shaped grains for films obtained at deposition potential -900mV versus SCE using SEM studies. Downloaded on to IP address. Redistribution subject to ECS terms 8 of use (see ecsdl.org/site/terms_use)

9 References 1. K. Zweibel, Sol. Energy Mater. Sol. Cells 63, 375 (2000). 2. J.P. Singh, R.K. Bedi, Thin Solid Films 199, 9 (1991). 3. J.J. Loferski, J. Appl. Phys. 27, 77 (1956). 4. D. Bhattacharya, S. Chaudhuri, A.K. Pal, S.K. Battacharya, Vacuum 42, 1113 (1991). 5. R.D. Englken, A.K. Berry, T.P. Van Doren, J. Boone, A. Shahnazary, J. Electrochem. Soc. 133, 851 (1985). 6. M.F. Ladd, R.A. Palmer, Structural Determination By X-Ray Crystallography, Plenum Press, New York, 1964, P T. Chattopadhyay, J. Pannetier, H.G. Von Schnering, J. Phys. Chem. Solids 44, 879 (1986). 8. P. Pramanik, P.K. Basu, S. Biswas, Thin Solid Films 150, 269 (1987). 9. P.K. Nair, M.T.S. Nair, V.M. Garceia, O.L. Arenas, Y. Pena, A. Castillo, I.T. Ayala, O. Gomezdaza, A. Sanchez, J. Campos, H. Hu, R. Suarez, M.E. Rincon, Sol. Energy Mater. Sol. Cells 52, 313 (1998). 10. R. D. Engelken, H. E. Mccloud, C. Lee, M. Slayton and H. Ghoreishi, J. Electrochem. Soc. 134, 2696 (1987). 11. A. Mondal, T. K. Chaudari And P. Pramanik, Solar Energy Mater. 7, 431 (1983). 12. M. P. R. Panicker, M. Knaster And F. A. Kroger, J. Electrochem. Soc. 125, 566 (1978). 13. K. Jackowska and M. Skompska, Polish. J. Chem. 60, 60 (1986). 14. G. Hodes, J. Mannassen and D. Cahan, Nature 261, 403 (1976). 15. B. Miller and A. Heller, Ibid. 262, 680 (1976). 16. L. M. Peter, Electrochim. Acta. 23, 165 (1978). 17. K. Rajeshwar, Adv. Mater. 4, 23 (1992). 18. F. Huiller, Structural Chemistry of Layer Type Phases, In: F. Ley, D. Reifel (Eds.), Dordrecht-Holland, Boston, W. Albers, C. Hass, H. Oar, G.R. Schodder, J.D. Wascher, J. Phys. Chem. Solids 23, 215 (1962). 20. V. Dhanasekaran, T. Mahalingam, Jin Koo Rhee, and J. P. Chu J. Adv. Microsc. Res. 6, 1 (2011) 21. SnSSe JCPDS Card No Downloaded on to IP address. Redistribution subject to ECS terms 9 of use (see ecsdl.org/site/terms_use)

10 22. SnSe JCPDS Card No T. Mahalingam, V. Dhanasekaran, G. Ravi, Soonil Lee, J. P. Chu, Han-Jo Lim J. Optoelectron. Adv. Mater. 12, 1327 (2010). 24. R. Swanepoel, J. Phys. E 16, 1214 (1983). 25. R. Swanepoel, J. Phys. E 17, 896 (1984) C. Vijayan, M. Pandiaraman, N. Soundararajan, R. Chandramohan, V. Dhanasekaran, K. Sundaram, T. Mahalingam and A. John Peter, J Mater Sci: Mater Electron. 22, 545 (2011). 27. J.I. Pankove, Optical Processes in Semiconductors, p Dover Publications, New York (1971). Downloaded on to IP address. Redistribution subject to ECS 10 terms of use (see ecsdl.org/site/terms_use)