Structural and optical properties of PbS thin films deposited by chemical bath deposition

Materials Chemistry and Physics 97 (2006) 71 80 Structural and optical properties of PbS thin films deposited by chemical bath deposition S. Seghaier a,, N. Kamoun a, R. Brini b, A.B. Amara c a Laboratoire de Physique de la Matière Condensée, Faculté des Sciences de Tunis, 2092 El Manar, Tunisia b Laboratoire de Photovoltaïque et des Matériaux Semi-conducteurs, Ecole Nationale, d Ingénieurs de Tunis, El Manar, Tunisia c Laboratoire de Physique des Matériaux, Faculté des Sciences de Bizerte, 7003 Zarzouna, Tunisia Received 10 May 2005; received in revised form 22 July 2005; accepted 23 July 2005 Abstract In the current paper lead sulphide (PbS) thin films have been deposited on glass slide substrates via a simple and easily controlled method, proceeding large area films, using the chemical bath deposition technique. The reactive substances used to obtained the PbS layers were (Pb(NO 3 ) 2 ), (NaOH), (SC(NH 2 ) 2 ) and H 2 O for different concentration and dipping times, at constant room temperature. The structure was determined by X-ray diffraction studies. The films are very adherent to the substrate and well crystallized according to the centered cubic structure with the preferential orientation (2 0 0). The surface morphology and crystallite sizes were determined by scanning electron microscope measurements. The optical properties were carried out from spectroscopy measurements of transmission reflection and ellipsometry. The thickness of these films was controlled by changing the dipping times and the concentrations of the reaction solution. Experiments showed that the growth parameters and thermal treatment influenced the structure, the morphology and the optical properties of PbS films. 2005 Elsevier B.V. All rights reserved. Keywords: PbS thin films; Chemical bath deposition; Structure; Optical properties 1. Introduction Lead sulphide (PbS) is an important direct narrow gap semiconductor material with an approximate energy band gap of 0.4 ev at 300 K and a relatively large excitation Bohr radius of 18 nm [1]. These properties make PbS very suitable for infrared detection application [2]. This material has also been used in many fields such as photography [3],Pb 2+ ionselective sensors [4] and solar absorption [5]. In addition, PbS has been utilized as photoresistance, diode lasers, humidity and temperature sensors, decorative and solar control coatings [6,7]. These properties have been correlated with the growth conditions and the nature of substrates. For these reasons, many research groups have shown a great interest in the development and study of this material by various deposition processes such as electrodeposition [8], spray pyrolysis Corresponding author. Tel.: +216 22536062; fax: +216 71 885073. E-mail address: samia 772003@yahoo.ca (S. Seghaier). [9], photoaccelerated chemical deposition [6], microwaveheating [10,11] and chemical bath deposition (CBD) [12 16]. CBD method is presently attracting considerable attention, as it does not require sophisticated instrumentation. It is relatively inexpensive, easy to handle, convenient for large area deposition and capable of yielding good quality thin films. The characteristics of chemically deposited PbS thin films by CBD strongly depend on the growth conditions. In this paper, we report the structural and optical properties of PbS thin films obtained by CBD method at various concentrations of the precursors and different deposition time. 2. Experimental 2.1. Synthesis of lead sulphide The PbS thin films were grown on ordinary glass slide (2.5 cm 2.5 cm 2 mm) substrates. The deposition was 0254-0584/$ see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.07.061

72 S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71 80 done in a reactive solution prepared in a 50 ml beaker containing lead nitrate, which concentration range [Pb(NO 3 ) 2 ] was set between 0.165 and 0.185 M. This reactive was mixed in alkaline aqueous solution with thiourea at concentration [SC(NH 2 ) 2 ] varying between 0.09 and 0.20 M. The alkalinity is set using sodium hydroxide [NaOH] in the range 0.55 0.60 M. The bidistilled water was added to the solution to achieve a total volume of 50 ml. Cleaned substrates was vertically immersed into the solution. The beaker with the reactive solution was immersed in water heating bath circulator placed on heating magnetic agitator and was maintained at room temperature. The substrates were subsequently taken out of the chemical bath after different dipping time t D (10 t D 70 min), rinsed with bidistilled water, dried and placed into a desiccator. The reaction process for forming lead sulphide films is considered as follows [17]: Pb(NO 3 ) 2 + 2NaOH Pb(OH) 2 + 2NaNO 3 Pb(OH) 2 + 4NaOH Na 4 Pb(OH) 6 Na 4 Pb(OH) 6 4Na + + HPbO 2 + 3OH + H 2 O SC(NH 2 ) 2 + OH CH 2 N 2 + H 2 O + SH HPbO 2 + SH PbS + 2OH The resulting films were homogeneous, well adhered to the substrate with darker surface like mirror. The thickness of each grown film was calculated from the density of 7.596 g cm 3 (of bulk PbS) and the mass of the deposit surface area that is determined by the double weight method. 2.2. Instruments These films were structurally characterized by, X-ray diffraction (XRD) using a bruker D8 Advance X-ray diffractometer with Cu K irradiation (λ = 1.54060 Å) and by scanning electron microscopy (SEM) images carried out with XL 30/EDAX BSE microscope. This study was complemented with ellipsometry and spectroscopy measurements. The ellipsometry analysis were performed at λ = 0.6328 Å using a L 117 Gaertner Manual Ellipsometer. The optical transmittance and reflectance spectrum were examined on UV-3100 Schimadzu UV vis spectrophotometer. Pb 2+ ions sources, 0.10 M thiourea [SC(NH 2 ) 2 ], as sulphurliberating solution and 0.57 M sodium hydroxide [NaOH] that is used as complexing agent for Pb 2+ ions. The deposition is carried out at a constant temperature T D equal to 25 C. The substrates were subsequently taken out of the beaker after dipping time from 10 to 70 min. Fig. 1 shows the XRD patterns of the PbS thin films deposited at several growth time, in the above condition. The crystallography of the film is good and characterized by three principal peaks corresponding to (1 1 1), (2 0 0) and (2 2 0) orientations. The width at half maximum of the (2 0 0) line is in the range 0.130 0.259 for all time deposition, the values are indicative of the good crystallinity. By comparison with the data from JCDF cards No. 1-880, all diffraction peaks can be indexed within experimental error as a face-centered cubic structure of PbS. The narrow peaks show that the materiel has good crystallinity preferentially oriented along the (2 0 0) direction which is perpendicular to the substrate and which intensity depends on t D value. Table 1 displays the ratio values of the relative diffraction peaks intensities (I 111 /I 200 ) and (I 220 /I 200 ) for PbS films produced for various dipping times. From the following table we can note that an increase in the deposition time from 10 to 40 min is accompanied by an increase in the I 111 /I 200 ratio. For t D values varying between 40 and 60 min, we notice that the peak (1 1 1) intensity is practically constant, whereas the intensity of (2 0 0) increases reaching its maximum for a growth time equal to 60 min. Both the intensity ratio I 111 /I 200 and I 220 /I 200 for thin films grown during 60 min, are equal to 0.280 and 0.094, these values are lower than the theoretical one equal to 0.8 and 0.6, respectively, showing that the PbS samples grown by CBD have (2 0 0) preferred orientation with strong intensity and full width at half maximum. That can be related to the larger number of both Pb 2+ and S 2 ions in the reaction solution at t D equal to 60 min. Similar results were obtained by Valenzuela- Jauregui et al. [16] and Larramendi et al. [13]. However, for t D equal to 70 min, the intensity of (2 0 0) decreases. In this case, we notice also that the thin layers obtained are very rough. We tried to make thin layers of PbS for t D more than 70 min and we noticed that the layer is detached from its substrate. Recently, rectangular and rod-like PbS crystals have been successfully prepared in systems containing organic polyamines with N-chelation properties such as triethylenetetramine [18] and ethylenedimine [19], respectively. However, we have determined the average crystallite sizes L by measuring full width at half of the (2 0 0) peaks and by 3. Results and discussion 3.1. Structure and surface morphology 3.1.1. Effect of the deposition time To optimize the deposition time, we produced PbS thin layers on glass from 0.175 M lead nitrate [Pb(NO 3 ) 2 ]as Table 1 Variation of PbS peak orientation degree as a function of the deposition time (t D ) t D (min) 10 20 30 40 50 60 70 I 111 /I 200 0.421 0.423 0.808 0.853 0.557 0.280 1.391 I 220 /I 200 0.185 0.184 0.333 0.322 0.297 0.094 0.383

S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71 80 73 Fig. 1. X-ray diffraction spectra of PbS layer, CBD-prepared with [Pb(NO 3 ) 2 ] = 0.175 M, [NaOH] = 0.57 M, [SC(NH 2 ) 2 ] = 0.1 M, T D =25 C and for various deposition time (t D ). using the Scherrer formula [20]: 0.9λ L = cos θ 0 (2θ) For different deposition time, comparable L values are measured from SEM micrograph as shown in Table 2. Table 2 Effect of the deposition time on PbS grain size deduced from MEB and XRD analysis t D (min) 40 50 60 70 Grain size (Å) SEM 204.2 250.0 320.0 291.7 L (Å) of (2 0 0) XRD 208.4 250.2 312.9 312.9 On the other hand, SEM analysis gives the morphology of the PbS nanocrystals. The prepared thin films have a pyramidal shape with a rather compact surface, which does not show any fissures, faults and disturbances. Fig. 2 shows the thickness variation of PbS thin films as a function of deposition time. The curve shows an initial slow growth involving the creation of nucleation centers; it corresponds to the induction phase. The growth phase (20 60 min) at which the thickness is obtained in a linear dependence. They increase steadily with dipping time up to 60 min to reach the max value of 771.560 nm, so while the film thickness increases the grain size goes up (see Table 2), and the terminal phase in which the growth slows down. We can conclude the uniformity, the good adhesion and the best crystallinity of PbS on glass sub-

74 S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71 80 strate is obtained for an immersion time equal to 60 min from the bath described above. Fig. 2. Thickness of PbS films as a function of deposition time. 3.1.2. Effect of the lead nitrate concentration In this study we set deposition time t D at its optimal value equal to 60 min and we tried to change the [Pb(NO 3 ) 2 ] from 0.165 to 0.185 M per step of 0.005 M. The XRD patterns of as-prepared PbS are shown in Fig. 3 all diffraction peaks, can be indexed to the pure cubic phase of PbS. No peak of any other phase was detected. The strong intensity and narrow peaks show that the material has good crystallinity and (2 0 0) preferred orientation. The crystallography of the film is good and characterized by three peaks corresponding to (1 1 1), (2 0 0) and (2 2 0) orientations. The (2 0 0) reflection peak has very strong intensity and the full width at half maximum of line is the order of 0.130 for a concentration equal to 0.170 M. This indicates a good preferential orientation of the grains with c-axis perpendicular to the plane of the substrate. The values of the intensity ratios I 111 /I 200 and I 220 /I 200 for lead nitrate concentrations equal to 0.170 M are in the order of 0.661 and 0.371, respectively, as it indicated in Table 3. The Fig. 3. XRD pattern of PbS thin layers, CBD-prepared on glass substrate with [NaOH] 0.57 M, [SC(NH 2 ) 2 ] = 0.1 M, t D = 60 min, T D =25 C and for various Pb(NO 3 ) 2 concentration.

S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71 80 75 Table 3 Variation of PbS peak orientation degree as a function of the lead nitrate concentration [Pb(NO 3 ) 2 ](M) 0.165 0.170 0.175 0.180 0.185 I 111 /I 200 0.866 0.661 0.280 0.367 0.886 I 220 /I 200 0.338 0.371 0.094 0.190 0.381 (2 0 0) preferential direction is also observed with a middle intensity [16,17]. As shown in Fig. 4a fat uniform structure can be seen, which in a compact way, carpets the substrate surface with lead sulphide crystallites of various size. So Table 4 shows that the mean size of PbS crystallites, depends on the Pb(NO 3 ) 2 concentration in the bath solution. Besides, dispersed small spherical structures can also be observed, which seem to be formed by cluster of microparticles. These disturbances on surfaces are less marked for a [Pb(NO 3 ) 2 ] equal to 0.170 M. Nevertheless, the pyramidal crystallites Fig. 4. SEM surface micrograph of PbS thin films deposited on glass substrate prepared with [NaOH] = 0.57 M, [SC(NH 2 ) 2 ] = 0.1 M, T D =25 C and for various [Pb(NO 3 ) 2 ]: (a) 0.165 M, (b) 0.170 M, (c) 0.175 M, (d) 0.180 M and (e) 0.185 M.

76 S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71 80 Table 4 Mean PbS grain size as a function of lead nitrate concentration deduced from SEM and XRD analysis [Pb(NO 3 ) 2 ](M) 0.165 0.170 0.175 0.180 0.185 Grain size (Å) SEM 250 280 320 170.750 208.333 L (Å) of (2 0 0) XRD 256.9 316.3 312.9 158.5 208.4 are uniformly distributed on the surface make thin film with strong compactness. The coalescence of crystallites on thin film surface affirms their privileged orientation along (2 0 0) direction. With lead nitrate concentration equal to 0.175 M and dipping time of 40 min, Mahmoud and Hamid [17] show the presence of randomly oriented inter-grown aggregate crystals in a heterogeneous size distributed on the surface film. Elsewhere the study of the dependence of the PbS thickness values on the lead nitrate concentration in same preceding conditions is reported by Fig. 5. While the [Pb(NO 3 ) 2 ] exceeds 0.170 M, the thickness of PbS layer decreases. The result seems to confirm that the nucleation rate is mainly regulated by the concentration of Pb 2+ ions in solution, which results in the development of PbS complex. We can note that for a [Pb(NO 3 ) 2 ] equal to 0.170 M, the quantity of Fig. 5. PbS film thickness as a function of lead nitrate concentration. Pb 2+ is high, so it favourites the growth of lead sulphide thin films with a maximum thickness above 826 nm. As far as lead nitrate concentration increases, PbS is formed and the film is removed from its substrate then, so the solution becomes depleted of ions, resulting in a lower rate of deposition. Fig. 6. XRD pattern of PbS thin films deposited on glass with [Pb(NO 3 ) 2 ] = 0.170 M, [SC(NH 2 ) 2 ] = 0.1 M, T D =25 C, t D = 60 min and for various sodium hydroxide concentration [NaOH]: (a) 0.56 M, (b) 0.57 M, (c) 0.58 M and (d) 0.59 M.

S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71 80 77 3.1.3. Effect of the sodium hydroxide concentration The presence of the sodium hydroxide in this prepared CBD bath is very vital for the deposition of PbS thin films, so it assures the precipitation of Na(OH) 2 and the hydrolysis of thiourea. In this study, we have set the deposition time and the lead nitrate concentration at the previous optimal values according to the structural and morphological analyses. These values are, respectively, 60 min and 0.170 M. We have varied the [NaOH] in the interval [0.55 0.60 M] with a step of 0.01. In addition, we note by variation of the basic nature reagent, that the ph of the solution varies from 13.27 to 13.33. Fig. 6 proves strongly the effect of the [NaOH] on PbS thin films structure. The spectra of the samples deposited with a [NaOH] less than 0.57 M, is practically amorphous. It shows small and broad peaks indicative of a film poor quality. Similarly, Fig. 7 shows that the film thickness is rather low so we concluded that lower concentration of sodium hydroxide gives lower growth rates. For [NaOH] equal to 0.57 M, X-ray diffraction patterns show the appearance of the PbS diffraction lines (Fig. 6b). The resulting film showed good adhesion to the glass substrate with thickness equal to 826 nm (Fig. 7). While the [NaOH] increases from 0.57 M, we note that the intensity of the all PbS peaks gradually decreases. From this analysis we can conclude that the (2 0 0) direction is the preferential orientation of PbS. However, the well crystallinity obtained for a sodium hydroxide concentration equal to 0.57 M, setting the solution ph to 13. 3.1.4. Effect of the thiourea concentration In this study [SC(NH 2 ) 2 ] varies from 0.09 to 0.2 M. The XRD pattern of the films is shown in Fig. 8. From this study, we note that for a concentration value different from 0.1 M the films are amorphous with weak thicknesses as shown in Fig. 9. However, for thiourea concentration equal to 0.1 M the layers essentially consists of PbS compound (Fig. 8), as shown by the three principal orientations (1 1 1), (2 0 0) Fig. 8. XRD pattern of PbS thin layers, CBD-prepared on glass substrate with [NaOH] = 0.57 M, [Pb(NO 3 ) 2 ] = 0.170 M, t D = 60 min, T D =25 C and for various SC(NH 2 ) 2 concentration. and (2 2 0) characterizing a cubic structure. These diffraction peaks are rather narrow, indicating a good crystallinity with a (2 0 0) preferential orientation. By examination of the effect of [SC(NH 2 ) 2 ] on the film thickness we can deduce that if [SC(NH 2 ) 2 ] increases from 0.09 to 0.1 M, the film thickness increases from 305 to 826 nm. For concentration value exceeding 0.1 M, we note a reduction in the layer thickness and the film has bad crystallization. Under these optimal conditions determined in this work the PbS film studied by EDAX analysis (energy dispersive spectrometry) confirmed that the film is only composed by PbS compound with a concentration ratio practically equal to the theoretical value [Pb]/[S] = 1 (see Fig. 10). 3.1.5. Annealing effect We study the effect of the heat treatment under nitrogen during 1 h at 200 C on the structure of lead sulphide carried out from deposition bath with optimal conditions: [Pb(NO 3 ) 2 ] = 0.170 M, [NaOH] = 0.57 M, Fig. 7. Thickness of PbS films as a function of sodium hydroxide concentration. Fig. 9. PbS film thickness as a function of thiourea concentration.

78 S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71 80 Fig. 10. EDS analysis of as-deposited layer on glass with [Pb(NO 3 ) 2 ]= 0.170 M, [SC(NH 2 ) 2 ] = 0.1 M, [NaOH] = 0.57 M, t D = 60 min and T D = 25 C. [SC(NH 2 ) 2 ] = 0.1 M, t D = 60 min at room temperature. Thin layer is well crystallized in the cubic structure before heat treatment (Fig. 11a), but after heating (Fig. 11b) all peaks corresponding to PbS compound have disappeared without trace. This result can be explained by an increase of the disorder in the layer so the film becomes amorphous and/or by a decrease of thin layer thickness after annealing. 3.2. Optical properties In this work, we study the influence of the deposition parameters on the optical properties of chemically deposited PbS films. The first part will be devoted to the study of the effect of lead nitrate concentration and the effect of heat treatment on the transmittance and reflectance spectra. The second part will proceed in determining the refractive index n 1 and the extinction coefficient k 1 in various points of coordinates (x, y) on PbS thin layer surface using an extinction ellipsometer. Fig. 12. Transmittance spectra of PbS thin layers, CBD-prepared on glass substrate with [NaOH] = 0.57 M, [SC(NH 2 ) 2 ] = 0.1 M, t D = 60 min, T D =25 C and for various Pb(NO 3 ) 2 concentration. 3.2.1. Transmittance and reflectance measurements Optical measurements were undertaken in order to compare the optical performances of PbS films prepared with different values of lead nitrate concentration. Transmittance and reflectance measurements at nearnormal incidence were performed over a spectral ranging between 300 and 1800 nm on PbS thin films deposited on glass substrate with optimal deposition parameters, determined according to X-ray diffraction analysis but with different values of lead nitrate concentration varying from 0.165 to 0.185 M. Fig. 12 shows the optical transmission spectra of representative samples of lead sulphide coatings on glass from baths prepared at various lead nitrate concentration. We can explain the decrease of transmission for [Pb(NO 3 ) 2 ] equal to 0.165, 0.180 and 0.185 M by the presence of very thin layer Fig. 11. XRD pattern of PbS thin layers, CBD-prepared on glass substrate with [NaOH] = 0.57 M, [Pb(NO 3 ) 2 ] = 0.170 M, [SC(NH 2 ) 2 ] = 0.10 M, T D =25 C and t D = 60 min, before thermal treatment (a) and after heat treatment under nitrogen for 1 h at 200 C (b). Fig. 13. transmission and reflection spectra of PbS thin layers deposited on glass with [NaOH] = 0.57 M, [Pb(NO 3 ) 2 ] = 0.170 M, [SC(NH 2 ) 2 ] = 0.100 M, T D =25 C and t D = 60 min: (a) before and (b) after annealing under nitrogen at 200 C during 1 h.

S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71 80 79 of Pb(OH 2 ) on the film surface. Such films produce considerably lower specular reflectance and transmittance due to diffuse reflection from the surface. It is known that Pb(OH 2 ) formation can be reduced or avoided by minimizing the quantity of Pb 2+ ions in the bath as well as by optimizing the OH ion concentration in the bath so that the solubility product for the Pb(OH 2 ) is not exceeded [21]. This result can be also explained by the reduction of the thickness for [Pb(NO 3 ) 2 ] equal to 0.165, 0.180 and 0.185 M as can bee seen in Fig. 5. Fig. 13 shows the modifications of the transmittance and reflectance spectra of the PbS with [Pb(NO 3 ) 2 ] = 0.170 M, after heat treatment at 200 C under nitrogen during 1 h, we observe a decrease of T and an increase of R values over the whole spectral range. The result can be interpreted by the PbS layers being amorphous after heat treatment as shown in Fig. 11b. We notice the presence of rather weak interference fringes on both the T and R spectra, which confirm that the PbS films have uniform thickness and good surface homogeneity. 3.2.2. Ellipsometry measurements In this section we determined the complex index N 1 = n 1 ik 1 by measuring the ellipsometric parameter Ψ and at a wavelength of 633 nm and for different point surface, of PbS thin films realized in optimal conditions. Fig. 14 displays the surface topography of the refraction index n 1 (x, y) and the extinction coefficient k 1 (x, y) as a function of point surface coordinates (x, y) of the PbS thin films before and after annealing. As seen in this figure, the topography of n 1 and k 1 along the surface is quite uniform but it get more homogeneous after thermal treatment (Fig. 14b). The ellipsometric analysis shows that the reflective index n 1 and k 1 vary, respectively, between the following ranges n 1 Fig. 14. Surface topography of the refraction index n 1 and the extinction coefficient k 1 of PbS layers CBD-deposited on glass for [NaOH] = 0.57 M, [Pb(NO 3 ) 2 ] = 0.170 M, [SC(NH 2 ) 2 ] = 0.100 M, T D =25 C and t D = 60 min: (a) before and (b) after heat treatment under nitrogen at 200 C during 1 h.

80 S. Seghaier et al. / Materials Chemistry and Physics 97 (2006) 71 80 between them is due to the uncertainties of measurement. There is a good agreement between SEM and ellipsometric analysis. The stoichiometric compound is also confirmed by the EDAX measurements. From these studies, we were able to determine this quality of the surface and, there for a good epitaxy. References Fig. 15. SEM micrograph of the PbS fractured film CBD-deposited on glass for [NaOH] = 0.57 M, [Pb(NO 3 ) 2 ] = 0.170 M, [SC(NH 2 ) 2 ] = 0.100 M, T D =25 C and t D = 60 min. from 2.5 to 3 and k 1 from 1.5 to 2. Elsewhere at a wavelength of 550 nm bulk PbS is a highly absorbing material with a complex refraction index N 1 = 4.3 i1.23 [22]. 4. Conclusions In this study, we have presented a chemical bath deposition method to elaborate the PbS crystallites. From structural analysis we can conclude that the best crystallinity and the great adhesion of PbS on glass substrate is obtained by employing 0.170 M of lead nitrate, 0.57 M of sodium hydroxide and 0.1 M of thiourea as reactants, at room temperature for dipping time equal to 60 min, formed only by lead sulphide compound with face-centered cubic structure preferentially oriented according to the (2 0 0) perpendicular direction to the plane of the substrate. The films gown in this condition were well crystallized, with good surface homogeneity and roughness. Thickness of films elaborated in these conditions have determined by doubles weighting is practically equal to 826 nm and by SEM micrograph we have found 833 nm (Fig. 15). These two values are comparable and the difference [1] J.L. Machol, F.W. Wise, R.C. Patel, D.B. Tanner, Phys. Rev. B 48 (1993) 2819. [2] P. Gadenne, Y. Yagil, G. Deutscher, J. Appl. Phys. 66 (1989) 3019. [3] P.K. Nair, O. Gomezdaza, M.T.S. Nair, Adv. Mater. Opt. Electron. 1 (1992) 139. [4] H. Hirata, K. Higashiyama, Bull. Chem. Soc. Jpn. 44 (1971) 2420. [5] T.K. Chaudhuri, S. Chatterjes, Proceedings of the International Conference on Thermoelectronics, vol. 11, 1992, p. 40. [6] P.K. Nair, V.M. Garcia, A.B. Hernandez, M.T.S. Nair, J. Phys. D: Appl. Phys. 24 (1991) 1466 1472. [7] Ileana Pop, Cristina Nascu, Violeta Ionescu, E. Indrea, I. Bratu, Thin Solid Films 307 (1997) 240 244. [8] Maheshwar Sharon, K.S. Ramaiah, Mukul Kumar, M. Neumann- Spallart, C. Levy-Clement, Electroanal. Chem. 436 (1997) 49 52. [9] B. Thangaraju, P. Kaliannan, Semicond. Sci. Technol. 15 (2000) 849 853. [10] Yonghong Ni, Fei Wang, Hongjiang Liu, Gui Yin, Jianming Hong, Xiang Ma, Zheng Xu, Cryst. Growth 262 (2004) 399 402. [11] Yu Zhao, Xue-Hong Liao, Jian-Min Hong, Jun-Jie Zhu, Mater. Chem. Phys. 87 (2004) 149 153. [12] Cristina Nascu, Valentina Vomir, Ileana Pop, Violeta Ionescu, Rodica Grecu, Mater. Sci. Eng. B 41 (1996) 235 240. [13] E.M. Larramendi, O. Calzadilla, A. Gonzalez-Arias, E. Hernandez, J. Ruiz-Garcia, Thin Solid Films 389 (2001) 301 306. [14] E. Pentia, L. Pintilie, C. Tivarus, I. Pintilie, T. Botila, Mater. Sci. Eng. B 80 (2001) 23 26. [15] Rakesh K. Joshi, Aloke Kanjilal, H.K. Sehgal, Appl. Surf. Sci. 221 (2004) 43 47. [16] J.J. Valenzuela-Jauregui, R. Ramirez-Bon, A. Mendoza-Galvan, M. Sotelo-Lerma, Thin Solid Films 441 (2003) 104 110. [17] S. Mahmoud, O. Hamid, FIZIKA A (Zagreb) 10 (2001) 21 30. [18] T. Sugimoto, S.H. Chen, A. Muramatsu, Colloid Surf. A 135 (1998) 207. [19] M. Chen, Y. Xie, Z. Yao, Y. Qian, G. Zhou, Mater. Res. Bull. 37 (2002) 247. [20] A. Guinier, Théorie et Technique de la radiocristallographie, Ed. Dunod, Paris, 1964. [21] R.A. Zingaro, D.O. Skovlin, J. Electrochem. Soc. 111 (1964) 42. [22] Landolt-Börnstein, Physical Data, Tables and Functions, Series III, 17 (1961) 155.