Optimization of sintering temperature in CdZnS films using reflection spectroscopy.
Keywords Sintering, Semiconductors, Thin films, X-ray diffraction, Reflection spec, Band gap
[Cd.sub.x][Zn.sub.1-x]S films are of considerable interest for photo electrochemical and heterojunction solar cells. (1-4) Padam et al. (5) studied the physical and electrical properties of [Cd.sub.x][Zn.sub.1-x]S films deposited on alumina substrates sintered at 800[degrees]C in a nitrogen atmosphere. They found an excess of ZnS in their films. They also reported X-ray diffraction analysis and photosensitivity with these films. Seol and Im (6) reported sintering behavior and electrical properties of [Cd.sub.x][Zn.sub.1-x]S films deposited on borosilicate glass substrates sintered at 600[degrees]C in a nitrogen atmosphere. They observed that X-ray diffraction analysis did not give any significant amount of ZnS in these sintered films. The mole percentage of zinc in the solid solution was 60% of the added ZnS, indicating that 40% of added zinc evaporated during sintering. Sharma et al. (7), (8) and Rincon et al. (9) have demonstrated that reflection spectroscopy can be used to check the formation of solid solutions of CdS-ZnS sintered films.
The reflection spectra of the slurry consisting of CdS, ZnS, [CdCl.sub.2], and ethylene glycol dried at 120[degrees]C on glass substrates show two sharp falls that correspond to the energy band gaps of CdS and ZnS. When these dried films were sintered, their reflection spectra exhibited only one sharp fall, which corresponds to the band gap of solid solution CdS-ZnS. We carried out reflection spectroscopic studies of [Cd.sub.x][Zn.sub.1-x]S films deposited on ordinary glass substrates sintered over a range of temperatures in an air atmosphere to optimize the sintering temperature.
Commercially available CdS, ZnS, anhydrous [CdCl.sub.2], and ethylene glycol of high purity were used to prepare [Cd.sub.0.4][Zn.sub.0.6]S sintered films. A paste of 40% mole of CdS, 60% mole of ZnS, and [CdCl.sub.2] as 10% of (CdS + ZnS) is prepared in ethylene glycol by mixing and dispensing with a mortar and a pestle. The paste thus formed was coated on clean glass substrates using screen printing. These [Cd.sub.0.4][Zn.sub.0.6]S films were dried at 120[degrees]C for 3 h and then at 400[degrees]C for 15 min to remove the remaining organic materials. We sintered several films of the same composition at different temperatures ranging from 440 to 600[degrees]C in a quartz tube furnace closed at one end. For temperatures above 500[degrees]C, ordinary glass substrates start melting, therefore we used mica substrates. Reflection spectra of all these films were recorded using a Hitachi U-3400 spectrophotometer. The reflection attachment has a 5[degrees] specular reflection arrangement. We used a spectrophotometer in repetitive wavelength scanning mode with a scanning rate of 120 nm/min and a band pass of 2 nm. The energy band gap of [Cd.sub.0.4][Zn.sub.0.6]S film, sintered at 500[degrees]C for 10 min, was determined by reflection spectra. The films of other compositions may also be prepared and characterized in a similar manner. (8)
Results and discussion
Figure 1 contains four curves I-IV showing the reflection spectra of [Cd.sub.0.4][Zn.sub.0.6]S films sintered at 440, 460, 480, and 500[degrees]C, respectively. It was observed that as the temperature increases, a sharp downfall approaches towards the high wavelength side, indicating the formation of lower band gap material--i.e., CdS-dominated films. (10) This is possible due to the evaporation of [ZnCl.sub.2], which is formed during the reaction
ZnS + Cd[Cl.sub.2] [right arrow] CdS + Zn[Cl.sub.2]
[FIGURE 1 OMITTED]
Hence, as the temperature increases, the concentration of ZnS in the films decreases while that of CdS increases. Therefore, the mole percentage of ZnS in the prepared films is smaller than that of ZnS added in the slurry. This behavior agrees with the results quoted by Seol and Im. (6)
Above 500[degrees]C, evaporation of CdS also starts. Since the vapor pressure of CdS is higher than that of ZnS, the rate of evaporation of CdS is more than that of ZnS. Hence, as the temperature exceeds 500[degrees]C, the formation of ZnS-dominated films begins.
Figure 2 presents reflection curves I-V for the films sintered at 520, 540, 560, 580, and 600[degrees]C, respectively. In curve I, a second sharp fall between 420 and 380 nm in reflection spectra takes place in addition to the first fall at 525-500 nm. The second sharp fall at 420-380 nm represents the formation of a solid solution of CdS-ZnS, but they do not yield desired results due to the dominance of ZnS. As the sintering temperature increases, the second sharp fall increases, while the first sharp fall decreases. At 600[degrees]C, the first fall at 525-500 nm disappears and we get only a second sharp fall at 420-380 nm. All these films show results of ZnS dominance. Besides, above 550[degrees]C, the oxidation of ZnS starts to produce ZnO.
[FIGURE 2 OMITTED]
Thus we arrive at the conclusion that 500[degrees]C is the proper temperature at which [Cd.sub.x][Zn.sub.1-x]S films should be sintered. We also recorded that final composition is the same as the initial composition. (8), (11) The [Cd.sub.1-x][Zn.sub.x]S films obtained using this method are smooth, uniform, and adherent to the substrate surface. We sintered the above set of films for 5, 10, and 15 min and found that 10 min is the proper sintering time to get the desired results.
The reflection spectra of [Cd.sub.0.4][Zn.sub.0.6]S film sintered at 500[degrees]C for 10 min in air atmosphere is shown in Fig. 3. The energy band gap of this film is determined by plotting a graph between ([[alpha]hv).sup.2]) or [hv ln([R.sub.max] - [R.sub.min])/(R - [R.sub.min])].sup.2] vs (hv), (8), (11-15) as shown in Fig. 4, where hv is the photon energy, the reflection falls from [R.sub.max] to [R.sub.min] due to absorption by the material, and R is the reflectance for any intermediate energy photons. Therefore, [alpha] is used in terms of reflectance as [hv ln([R.sub.max] - [R.sub.min])/(R - [R.sub.min])] and the extrapolation of a straight line to the ([alpha]hv.[sup.2]) = 0 axis give the value of the energy band gap of film material. The energy band gap of [Cd.sub.0.4][Zn.sub.0.6]S film comes out to be 3.01 eV. The X-ray diffraction pattern of [Cd.sub.0.4][Zn.sub.0.6]S film is shown in Fig. 5. The calculated d-values from the XRD pattern are in good agreement with that of ASTM data confirming the formation of [Cd.sub.0.4][Zn.sub.0.6]S film, except two peaks at 26[degrees] and 47[degrees] that may be attributed to the presence of Cd[Cl.sub.2] because Cd[Cl.sub.2] is used as adhesive in the preparation of [Cd.sub.0.4][Zn.sub.0.6]S sintered films. The X-ray analysis confirmed the hexagonal wurtzite structure of [Cd.sub.0.4][Zn.sub.0.6]S material as reported in the literature. (16) The presence of sharp structural peaks in this XRD confirmed the polycrystalline nature of [Cd.sub.0.4][Zn.sub.0.6]S films.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
We conclude that sintering temperature and time to prepare polycrystalline [Cd.sub0.4][Zn.0.6]S film should be 500[degrees]C and 10 min, respectively. If we increase the sintering temperature, then cadmium-deficient films will be produced, as reported by Padam et al. (5) In our method, the films do not show any change in composition. (8) We have not found any deficiency of zinc in the films as reported by Seol and Im. (6) Therefore, we suggest that if [Cd.sub.0.4][Zn.0.6]S films are produced by the sintering technique, then 500[degrees]C is an appropriate sintering temperature. The films of other compositions can also be made and characterized in a similar manner. We also conclude here that the sintering technique is a simple, viable, and attractive means of obtaining films of II-VI semiconductors.
Acknowledgments The authors are thankful to the Department of Science & Technology, Govt of India, New Delhi, for the financial support to carry out the present work. Corresponding author is also thankful to Dr. Ajay Sharma (Director General, KIET, GZB), Professor O.P. Jain (Director, KIET, GZB), and Professor C.M. Batra (Head, Dept. of Basic Science, KIET, GZB) for their constant support and encouragement to carry out this work on time.
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V. Kumar (*), S. Juneja
Department of Physics, Krishna Institute of Engineering & Technology, Ghaziabad, India
S. K. Sharma
Department of Physics, D.A.V. College, Muzaffarnagar, India
Department of Physics, S.G. (P.G.) College, Saroorpur Khurd, Meerut, India
T. P. Sharma
Department of Physics, Rajasthan University, Jaipur, India
J. Coat. Technol. Res., 7 (3) 399-402, 2010
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|Title Annotation:||BRIEF COMMUNICATION|
|Author:||Kumar, Vipin; Juneja, Soniya; Sharma, Sachin K.; Singh, V.; Sharma, T.P.|
|Date:||May 1, 2010|
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