Available online at www.sciencedirect.com Energy Procedia 6 (2011) 241 250 MEDGREEN 2011-LB Effect of Deposition Time on the Optical Characteristics of Chemically Deposited Nanostructure PbS Thin Films M.M.Abbas a, *, A. Ab-M. Shehab b, A-K. Al-Samuraee c, and N-A. Hassan b a Physics department, college of science, Baghdad University, Baghdad, Iraq b Physics department, college of Ibn- Al-Haithem, Baghdad University, Baghdad, Iraq c Chemical department, college of Ibn- Al-Haithem, Baghdad University, Baghdad, Iraq Abstract PbS thin films are good materials for antireflection coatings and for solar thermal applications in flat-plat collectors, house heating for solar chick brooding, etc. Nanocrystalline PbS thin films were deposited on glass substrates with various deposition times using chemical bath deposition technique. The study was carried out for thicknesses in the range (500-660 nm). The crystalline size of these films was studied by atomic force microscope AFM. A dense surface composed of multilayered grains of films was obtained with the crystal size around 37.67 nm. The optical properties of these films have been studied and show that PbS thin films have allowed direct transition and the values of energy gap varied between (1.88-1.55 ev) with increasing film thickness, The large optical band gap in the films is attributed to the quantum coefficient effect. Influences of thermal treatments under various annealing temperatures on the optical properties for some deposition films were studied. 2010 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibility of [name organizer] Keywords: PbS thin films,chemical bath deposition,optical properties. 1. Introduction The rapidly growing development of nanotechnology is due to the unique properties of nanocrysta- * Corresponding author..:e-mail address: muna_moussa@yahoo.com 1876 6102 2011 Published by Elsevier Ltd. doi:10.1016/j.egypro.2011.05.028 Open access under CC BY-NC-ND license.
242 M.M.Abbas et al. / Energy Procedia 6 (2011) 241 250 lline materials in comparison with their large-grained analogy. An important line of research is further miniaturization of electronic circuits by passing from the submicron-sized components widely used in present-day integrated circuits to nanodevices. This issue is particularly important in semiconductor technology [1]. Most of the work on nano semiconductor particles reported so far has been restricted to optical absorption. The absorption edge shifted to blue as the particles size reduces. Most studied crystalline semiconductors belong to the II-VI and IV-VI groups, since they are relatively easy to synthesize and are generally prepared as particles or in thin film form in the recent years [2]. Lead sulfide (PbS) belongs to IV-VI group and is an important semiconductor with a narrow band gap [3]. This material has been used in many technological applications due to its useful optoelectronic properties [4-7]. Among the different PbS thin film fabrication methods, chemical bath deposition CBD techniques requiring relatively mild conditions, being cost effective, scalable and technically straight-forward [8]. In the last decade, there has been a renewed interest in this method, mainly associated with its remarkable success in depositing semiconductor layers in thin film photovoltaic cells. By CBD the dimensions of the crystallites can be varied by controlling deposition parameters. Researchers observed that thermal treating process has effect on the rate of absorptivity of PbS thin films and consequently influence the optical characterizations of chemically deposited PbS [9]. Therefore, the essence of this research is to determine the effect of deposition time on the optical properties of the films and their modification after annealing in vacuum and compare their effect on the films. 2. Experimental PbS films were grown on ordinary glass slide substrates by CBD technique. Lead acetate and thiourea were used as sources of Pb 2+ and S 2- respectively. Triethanolamine used as complexing agent. The deposition was made at 30 o C during deposition time t D (30, 120, and 210 min). Details of the deposition process are reported in a recent past work [10]. The PH was measured by PH meter (HANNA- HI-8314) and kept at 12. Microstructure surface topography was estimated by using AFM (AA3000 Angstrom advanced Inc). The structures of the films thickness (500,600, and 660 nm) were obtained using X-ray diffraction XRD (Shimadzue -2006, with Cu-k α radiation). The optical absorption and transmission studies of the films in the wavelength range (200-1000 nm) were conducted using spectrophotometer (Model UV-160, Shimadzu, Kyoto). The thicknesses were measured using digital coating thickness gauge TT 260. The effect of annealing temperatures for samples with different thicknesses of (100,150, and 200 o C) for 1h under vacuum, have been studied. 3. Results and discussion A dense surface composed of multilayered grains was obtained for film deposited at 30 o C during 210 min and annealed at 100 o C for 1 h. The crystal size from AFM was around 37.67 nm, while from XRD 16 nm as shown from Fig. 1 more detailed analysis put forward in [10]. The size of the nanocrystalline observed from the AFM was greater than the average crystallite size calculated from the XRD peak broadening, indicating that a number of nanocrystallites (oriented in the same plane) coalesced together to form PbS nanocrystals. Since the Gibbs free energy of the surface of small-sized nanoparticles is usually very high due to the large surface to volume ratio, these small particles have tendency to aggregate together to decrease the Gibbs free energy of the surface and make
M.M.Abbas et al. / Energy Procedia 6 (2011) 241 250 243 the surface state stable [11]. Thangaraju and Kaliannam [12] also reported that the average grain size of their PbS thin films observed from SEM was greater than average crystallite size calculated using the Debye-Scherrer relation. Fig.1: AFM images for PbS deposited during 210 min. and annealed at 100 o C XRD pattern of the PbS films deposited for (30,120, and 210 min). in Fig. 2 shows three diffraction peaks at 2θ with values of approximately 26,30,and 43 o which correspond to the diffraction lines produced by the (111), (200), and (220) crystalline planes of the PbS cubic phase, respectively. It is observed with a preferred orientation growth along the (200) direction. In addition it is observed that the intensity of the peaks increases and the crystallization of PbS thin films were improved with increasing thickness of the films and annealing temperature. Whereas samples prepared at 30 o C for 120 min in a heat treatment after annealed at 200 o C become amorphous [10]. c b a Fig.2: X-ray diffraction patterns of PbS films for 30 o C at (a) 30min., (b) 120 min. and (c) 210 min.
244 M.M.Abbas et al. / Energy Procedia 6 (2011) 241 250 The absorption spectra of PbS films with different thicknesses were displayed in Fig. 3. It is observed that the shapes of the curves were similar although differences in absorbency were observed. Higher deposition time resulted in thicker thin films, which led to relatively low absorbency in the absorption spectra. All films show low absorption on the longer wavelength region. The thickness effect is noticed in the long wave length portion of the spectra where interference effects take place. Furthermore the spectra of PbS showed clear absorption peaks. These peaks correspond to exciton transition and obviously formed peaks are indicative of a narrower size distribution. And these results similar to those reported by Babu et al., who argues that variation in the size of nanocrystals leads to a distribution in the exciton peak positions, smearing the peaks and rendering the spectrum featureless [13]. The transmittance spectra for PbS at different thicknesses showed a decrease in transmission wavelength with increasing the thickness as shown in Fig. 4, and this is in a agreement with the results obtained by Ileana et al. [7] and Muna et al. [14].This results explained through the film morphology, as the film thickness increases the grain size increases too, as we mentioned previously. The surface roughness increases with increasing grain size which led to decrease in transmission. Also it has been noticed that below 400 nm there is a sharp fall in the transmittance of the film with thickness 500 nm, which is due to the strong absorbance of the films in this region. All The films have low transmittance in the UV region which increased with increasing wavelength towards the NIR regions. It should be emphasized that as the deposition time increases the transparency window decreases, due to the increasing thickness with time. 660 nm Fig. 3: The absorption spectra for PbS films at different thicknesses. Fig. 4: The transmission spectra for PbS films at different thicknesses. Fig. 5 shows the reflectance spectra of films. It is observed that the films with greater thickness show more reflection in the visible NIR range. As it can be seen, film with 500 nm thickness show high transmittance in the ultraviolet, visible and infrared regions of the electromagnetic spectrum. Conversely, the reflectance of the film is found to be low within the same region. Therefore, these high transmittance and low reflectance properties make the film good materials for antireflection coatings and for solar thermal applications in flat-plat collectors, house heating for solar chick brooding, etc. Fig. 6 shows the optical absorption coefficient (α) which was calculated in the wavelength range (300-1100 nm). The behaviour of α is similar to the absorption spectra. It has been found that films with a higher thickness have lower α value at the forbidden gap region than films with lower thickness. The thinner films have high α value in the band- band absorption region. This behaviour effect as suggested by Hulya et al. [15] may be explained by proposing that thicker films have bigger crystallites (grains) so
M.M.Abbas et al. / Energy Procedia 6 (2011) 241 250 245 they are closer to the crystalline PbS, but bigger grain sizes gives results in larger unfilled inter-granular volume so the absorption per unit thickness is reduced. 660 nm Fig. 5: The reflectance spectra for PbS films at different thicknesses. Fig. 6: The absorption coefficient spectra for PbS films at different thicknesses. The optical energy gap (E g ) of PbS thin films are calculated using the following equation [16]: αhυ = A (αhυ - E g ) n (1) Where υ is the frequency of the incident photon, h is Planck s constant, A is constants, and n is the number which characterizes the optical processes. The plots of the (αhυ) 2 as a function of hυ are shown in Fig. 7 in accordance with equation (1). The value of E g values decreases with increasing thicknesses and this is attributed to the increases of density of localized state in the energy gab. These values of E g are agreed with those ones published [17and 18]. Fig. 7: Plot of (αhυ) 2 vs. hυ for PbS films with different thicknesses.
246 M.M.Abbas et al. / Energy Procedia 6 (2011) 241 250 All these thickness dependent properties are given in Table (1). The large optical band gap in the films is attributed to the quantum coefficient effect [19]. Table 1 Values of the optical constants of PbS films. Thickness (nm) Thermal Treatment R (nm) E g (ev) E u (ev) 500 As grown 10.62 1.88 0.73 As grown 18.14 1.65 1.77 600 150 o C 18.4 1.8 0.45 200 o C Amorphous 1.79 0.45 660 As grown 21.24 1.55 2.41 100 o C 16.39 1.85 0.92 Extinction coefficient (k o ) versus wave length spectra is shown in Fig. 8. In the visible range, k o is approximately constant and decreases with increasing thicknesses. It has been noticed that the increasing k o at the wavelength below 400 nm is due to the high absorbance of PbS films in that region. The value of refractive index (n) is shown in Fig. 9 and results show higher values of refractive index in the UV-visible regions, but lower values in the NIR region. This is an indication that the rate at which light is slowed down in the film is very high in the UV-region and decays sharply to relatively low rate in the NIR with similar behavior have been reported in [20]. It has been found that increasing the thickness results in the overall increase in the refractive index. This increase is due to the overall increase in the reflectance with the film thickness. Fig. 8: The extinction coefficient k o as a function of wavelength for PbS films with different thicknesses. Fig. 9: Refractive index as function of hυ for PbS films with different thicknesses. The real and imaginary part of the dielectric constant (ε 1 and ε 2 ) are shown in Fig. 10 respectively, the behaviour of ε 1 is similar to refractive index because of the smaller value of k 2 comparison of n, while ε 2 mainly depends on the values of k, which are related to the variation of absorption coefficient.
M.M.Abbas et al. / Energy Procedia 6 (2011) 241 250 247 Fig. 10: The real and imaginary part of the dielectric constant (ε 1 and ε 2 ) as a function of wavelength for PbS films with different thicknesses. Optical conductivity (σ o ) is calculated using the following equation [16]: σ o = αnc/4π (2) The plots of σ o against hν are displayed in Fig. 11 optical conductivity ranged between (3.03 x 10 12 and 1.62 x 10 11 S -1 ). The tails width (E u ) in the energy gap were obtained by plotting Ln α as a function of hν, and the value of E u extract from the reciprocal slope of the linear part. As shown in Fig. 12. Fig. 11: The optical conductivity as function of hυ for PbS films with different thicknesses. Fig. 12: The variation of Ln α as a function of hυ of PbS films with different thicknesses. Increasing of E u values with increasing thicknesses was attributed to the increases of the defects and localized state in the energy gap. The spectral absorbance of the films before and after thermal annealing at (100,150 and 200 o C ) is shown in Figs. 13 and 14. Note that although absorbance spectra decreased with increasing wavelength a highest absorbance values are obtained from the annealed samples. Thus, the critical annealed
248 M.M.Abbas et al. / Energy Procedia 6 (2011) 241 250 temperature at which optimum absorbance occurs for samples with 600 and 660 nm thickness annealed at 150 and 100 o C respectively. As prepare x 150 o C 200 o C As prepare 100 o C Fig. 13: The absorption spectra for PbS thin films with 600 nm thickness annealing at different temperatures. Fig. 14: The absorption spectra for PbS thin films with 660 nm thickness annealing at annealing at different temperatures. Similar behaviour was indicated by Fajinmi [21] who assumed that high absorptivity obtained in annealed samples may be attributed to minimize the defects in PbS crystalline structure through annealing and that it is grain boundaries were modified and consequently increase the grain size. The spectral transmittance of the films before and after annealing treatments is shown Figs. 15 and 16. The thermal annealing of the samples produced a notable shift in the transmittance curve toward the longer wavelength side of the curve, indicating a decrease in the optical band gap. Fig. 17 shows (αhυ) 2 versus hυ curves for annealed films. As it can be seen, there was no normal Urbach tail on the lower energy side of the absorption threshold is possibly due to excitons in the nanocrystals. Excitons have also been reported in [22]. As prepare x 150 o C 200 o C As prepare 100 o C Fig. 15: The transmission spectra for PbS thin films with 600 nm thickness annealing at different temperatures. Fig. 16: The absorption spectra for PbS films with 660 nm annealed at different temperatures.
M.M.Abbas et al. / Energy Procedia 6 (2011) 241 250 249 600 nm X 150 o C 200 o C 660 nm 100 o C Fig. 17: Plot of (αhυ) 2 vs. hυ for PbS films annealed at different temperatures. 4. Conclusion Nanostructure PbS thin films at low temperatures were prepared by a simple bath deposition method. The results of AFM & XRD tests showed that the deposited PbS thin films consist of nanosize grains and the grain size increases with increasing film thickness. The optical properties of these films have been studied and show that PbS thin films have allowed direct transition and the values of energy gap varied between (1.88-1.55) ev with increasing film thickness. Thus the optical parameters have also been calculated and presented. It is observed that the highest absorbance values are obtained from the annealed samples and optimally from samples with 600 and 660 nm thickness annealed at 150 and 100 o C respectively. References [1] N. S. Belova, A.A. Rempel. PbS Nanoparticles: Synthesis and Size Determination by X-ray Diffraction, Inorganic Materials 40 (2004) 3-10. [2] E. Pentia, L. Pintilie, I. Matei, T. Botila, E. Ozbaya. Chemically prepared nanocrystlline PbS thin films. J. Optoelectronics and Advanced Materials 3, 2 (2001) 525-530. [3] H. Saloniemi. Electro-deposition of PbS, PbSe and PbTe thin films. Academic dissertation university of Helsinki (2000). [4] H. Hirata, K. Higashiyama. Analytical Study of the Lead Ion-selective ceramic membrane electrode, Bull. Chem. Soc. Jpn. 44(1971) 2420-2423. [5] T.K. Chaudhuri, S. Chatterjes. Proceedings of the International Conference on Thermoelectronics 11(1992) 40. [6] P.K. Nair, V.M. Garcia, A.B. Hernandez, M.T.S. Nair. Photoaccelerated chemical deposition of PbS thin films: novel applications in decorative coatings and imaging. J. Appl. Phys. 24 (1991) 1466-1472. [7] I. Pop, C. Nascu, V. Ionescu, E. Indrea, I. Bratu. Structural and optical properties of PbS thin films obtained by chemical deposition. Thin Solid Films 307 (1997) 240-244. [8] A.A. Rempela,b, N.S. Kozhevnikovab, A.J.G. Leenaersa, S. van den Berghe. Towards particle size regulation of chemically deposited lead sulfide (PbS). J. Cryst. Growth 280 (2005) 300-308. [9] J.A. Amusan. Effect of annealed temperature on absorptivity of chemically deposited PbS thin film.res. J. App. Sci. 3, 1 (2008) 1-4.
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