ANNEALING EFFECT OF CUTE THIN FILMS

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1 (An International Peer Reviewed Journal), ISSN Volume XI, Issue I, Jan- October 218 ANNEALING EFFECT OF CUTE THIN FILMS A. Selvaraj & G. Kanchana Assistant Professor, Department Of Physics, NPR Arts & Science College, Natham, Dindigul. ABSTRACT Thin films of cupric telluride (CuTe) of thickness 9nm have been prepared by thermal evaporation technique,deposited at the rate of 15.3 Α/sec on to well-cleaned glass substrate kept at 2 K under vacuum of better than 1-5 Torr.The bulk sample of CuTe also has been taken for investigations. The deposited films were annealed at two different temperature (15 C and 25 C) for one hour under vaccum atmosphere and then used for characterization. X ray diffraction studies confirmed that the composition and the polycrystalline nature of CuTefilms.The SEM studies have been confirmed that the smooth surface of the CuTe thinfilm. The elemental analyzed by EDAX. The optical properties were observed by UV visible and PL spectrum. In the PL spectrum whereas the peak intensity has been varied.. The grain size of CuTe thin films were estimated by around 4.6nm for 15 C and 54.1nm for 25 C sample. Keyword: CuTe, Thin films, Thermal evaporation, XRD, SEM&EDAX, UV Visible and PL. 1. Introduction Copper telluride belongs to the copper chalcogenide (I-VI) compound) group of material. Chalcogenide is a chemical compound consisting of at least one chalcogen anion and at least one more electropositive element. The term chalcogenide is more commonly reserved for sulfide, selenides, tellurides, rather than oxide. Many metal ores exist as chalcogenides. Photoconductive chalcogenide glasses are used in xerography. Some pigments and catalysts are also based on chalcogenides. The Copper telluride have different crystal structure depending upon the value of x (1<x<2) Cu2-xTe and are usually p-type compound semiconductor. It is suitable for application in solar cells, photodetectors, electrodetector, electrode and other electronic application such as for micro wave shielding coating and nonvolatile memories [1].The binary semiconductor CuTe has an energy gap Eg around 1.5 ev at room temperature, very close to the range for optimum solar energy conversion. For this reason this material is of considerable interest for device application. The crystallographic structure of mineral vulcanite CuTe (a=3.16 Å, b= 4.8 Å and c=6.94 Å is orthorhombic, Pmmn(59) space group andit is highly birefringent and pleiochroic.[2].bahl [3] hasstudied the K absorption edge on CuTe and he has estimated that the K absorption edge shift towards the high-energy side is due to the transfer of electron from tellurium to copper. A. Selvaraj & G. Kanchana 1

2 (An International Peer Reviewed Journal), ISSN Volume XI, Issue I, Jan- October 218 Attempts have been made to use CuTe as a photovoltaic cell. Detailed structural studies of CuTe and optical studies have been made by several workers[4-6].however, CopperTelluride has rarely been the subject of study. In recent years, Annealing effect of CuTe thin films are very useful technique for the study of application of solar cells and their interactions with other excitations. The efficiency solar cell CuTe thin film was 14% for the prepared sample. In this paperwe present the preliminary results of the study of the optical characteristics of thin films.the efficiency solar cell CuTe thin film was 14% for the prepared sample. The have attempt has been made previously to investigate the optical characteristics of thin films of CuTe was 22%. 2. Experimental Cooper Telluride (CuTe) alloy purchased from M/S Aldrich (India) company with 99.99% purity was used for preparing thin films by thermal evaporation. A knownamount of CuTe material was taken and evaporated the entire charge from a molybdenum boat under a vacuum better than mbar on well cleaned glass substrates of.1.3 m 2. The glass substrates were cleaned with hot chromic acid and distilled water before mounting them in the vacuum chamber. Copper telluride films of thickness 9 nm were deposited at the rate of 15.5 Å/s. The thickness of films and the deposition rate were monitored using a digital quartz-crystal thickness monitor. The as grown CuTe films were annealed at 15 C and 25 C for 2 hours at a pressure of mbar and afterannealing the films were allowed to cool down to a room temperature in vacuum.fig. 1 shows the X ray diffraction pattern of CuTe thin films. This has confirmed composition of CuTe in films and its polycrystalline nature. The surface morphological studies were analyzed by SEM (Scanning Electron Microscope). The compositional information was also obtained from Energy Dispersive Analysis of X-rays (EDAX)measurements. Optical studies were analyzed by UV Visible and Photoluminescence spectrum. Intensity 14 7 ( 1 ) ( 1 ) ( 2 ) ( 2 ) (1 2 1) Figure 1 XRD pattern of copper telluride thin films of 9nm thickness annealed at 15 C(a) and 25 C(b) (1 2 1) (1 3 1) (1 3 1) ( 4 1) ( 4 1) 15 C(a) 25 C(b) (a) (b) A. Selvaraj & G. Kanchana 2

3 (An International Peer Reviewed Journal), ISSN Volume XI, Issue I, Jan- October 218 cps/ev Te Cu Te Cu kev Figure3(a): EDAX spectra for 15 C CuTe 1 cps/ev 8 Figure 2(a) SEM image for 9nm thickness of copper telluride thin film annealed at 15 C Te Cu Te Cu kev Figure 3(b): EDAX spectra for 25 C CuTe Table 3 (a) Element are present in thesample Figure 2(b) SEM image for 9nm thickness of copper telluride thin film annealed at 25 C Composition 15 C 25 C Atomic percentage (%) Weight percentage (%) Cu Te Tot Cu Te Tot A. Selvaraj & G. Kanchana 3

4 (An International Peer Reviewed Journal), ISSN Volume XI, Issue I, Jan- October C 6 15 C Absorbance (a) h ev photonenergy(ev) wavelength(nm) C 5 25 C Absorbance ( h ) Wavelength (nm) Figure4(a)&(b) The absorption spectra for CuTe thin films of thickness 9nm annealed at 15 C and 25 C (b) 2.4 ev Photonenergy (ev) Figure 4(a)&(b)Tauc-extrapolation graph for CuTe thin films for different annealing temperature (15 C and 25 C) 4(a)&(b) Band gap are presented in table A. Selvaraj & G. Kanchana 4

5 (An International Peer Reviewed Journal), ISSN Volume XI, Issue I, Jan- October 218 Annealing Band gap (ev) temperature ( C) 15 C C 2.4 and the particle size gets increased. The XRD result confirmed that the structure is in orthoromphic and these results are good agreement with JCPDS NO : (7-11) data The crystallite size have been calculated using Debye-Scherrer formula (D), Intensity C(a) 25 C(b) (b) D = Where, Kλ x 18 β cos Ө π K Shape factor (Taken as.94 for spherical particles) β Full Width Half Maximum of the prominent peak λ Wavelength of X-ray (a) wavelength(nm) Figure 4.1 photoluminescence spectra of CuTe thin films for different annealing temperature (15 C and 25 C) 3. Results and discussion Structural analysis Figures 1 shows that the XRD patterns of copper telluride thin films of thickness 9nm annealed at 15 C and 25 C.From the XRD analysis of CuTe thin film the diffraction peaks observed at 2θ=12.87, 25.76, 44.54, and 6.61, which is indexed the corresponding hkl plane values (1), (2), (121), (131) and (41) respectively. When the annealing temperature increases from 15 C to 25 C CuTe thin film, the peak intensity increases θ Diffraction (Bragg) angle The grain size are presented in table Annealing Temperature ( C) 2Ө Deg FWHM hkl Grain Size(D) nm The amorphouse phase is reduced with increasing annealing temperature, since more energy is supplied for crystallite growth, thus resulting in an improvement in crystallinity of CuTe films. Therefore, it is believed that the annealing temperature increases with increase in crystalline size and the reduction in amorphouse phase of CuTe films. Morphological studies A. Selvaraj & G. Kanchana 5

6 (An International Peer Reviewed Journal), ISSN Volume XI, Issue I, Jan- October 218 Figures 2(a) and 2(b) shows that SEM pictures of typical 9nm thickness of copper telluride thin films which are annealed at temperatures 15 C and 25 C. SEM was used for morphology and size distribution investigation of the thin films. The SEM images have a good appearance and useful for judging the surface structure of the coated surface. The SEM image of 15 C annealed film clearly shows the crystallite size of CuTe thin film. The SEM image of 25 C annealed film are uniformly high smooth surface [7]. Elemental analysis Figures 3(a) shows the EDAX spectrum of CuTe thin film annealed at 15 C. The spectrum clearly indicates that the presence of Cu and Te without other impurities. From the EDAX data it was found that the weight percentage of Copper and telluride varied between (%). Figures 3(b) shows the EDAX spectrum of CuTe thin film annealed at 25 C. The spectrum clearly indicates that the presence of Cu and Te without other impurities.from the EDAX data it was found that the weight percentage of Copper and telluride varied between (%). From the table 3(a), we conclude that, when increasing the annealing temperature of copper telluride thin films, the weight percentage of copper increases and telluride decreases. Optical studies The present study is aimed to study the optical properties of various annealing temperature of 9nm thin films of copper telluride. Here we have taken two optical measurement such as UV-Vis and photoluminescence studies to estimate the optical band gap of the sample. Absorption studies Figures 4(a)&(b) shows that the absorption spectrum for the different annealing temperature (15 C and 25 C) of copper telluride thin films with the thickness 9nm. The figure indicates that the films have high absorbance in the visible regions. It is observed that the maximum absorption peak shift slightly towards the smaller wavelength with the increasing annealing temperature. The absorption tends to be very high in the UV region for all the annealing temperature. There is a very high absorption of energy in the near visible region. The deposited films have high absorbance in the UV visible region[8]. Optical band gap (Eg) was determined by analyzing the optical data with the expression for the optical absorption coefficient (α) and photon energy (һν) using the Tauc relation, α = K(hν E g) hν Where K is a constant, the value of n is equal to one for a direct-gap material, and four for an indirect-gap material. Plots of (αһν) 2 versus (һν) were drawn using the above equation. Extrapolation of the linear portion of the plot of energy axis yielded the direct band gap value. The amorphous phase is reduced with increasing annealing temperature, since more energy is supplied for crystallite growth, the resulting in an improvement in crystallinity of CuTe films. Therefore, it is believed that both the increasing in crystallite size and the reduction in amorphous phase, may be the reason for n 2 A. Selvaraj & G. Kanchana 6

7 (An International Peer Reviewed Journal), ISSN Volume XI, Issue I, Jan- October 218 decreasing in band gap of annealed CuTefilms[9]. Photoluminescence studies Figures 4.1 shows that the photoluminescence spectra of CuTe thin films with thickness 9nm at different annealing temperature (15 C and 25 C). The photoluminescence spectra of CuTe thin films of thickness 9nmn recorded at different annealing temperature (15 C and 25 C). A strong emission band in visible range is observed for the CuTe films of different annealing temperature 15 C and 25 C. There is a strong peak observed at about 53nm. The emission peak intensity increased with increasing annealing temperature. As the bandgap of CuTe is about 3.92eV. This value is relatively very close to the value of UV-bandgap. There is no remarkable shift is observed from the PL curve. Only the intensity variation is observed. 4. Conclusion Copper telluride thin film of 9nm were prepared on a glass substrate by thermal evaporation technique under the vaccum pressure of 1-5 Torr. The prepared thin film of CuTe was annealed at two different temperatures (15 C and 25 C) for 1hour under vaccum atmosphere. The films were subjected to XRD, SEM and EDAX, UV- Visible spectroscopic and photoluminescence study. From the XRD patterns of the annealed CuTe thin films of different temperature showed polycrystalline nature and have a orthoromphic structure with a preferred orientation along (1) plane. The crystallite size increases (4.6nm to 54.1nm) with increasing annealing temperature with decrease of microstrain and dislocation density. The surface morphology of the sample were characterized by scanning electron microscopy. The SEM image have a good appearance and the film surface structure is highly smooth. The crystallite quality increased with increased annealing temperature which understood by studying the microstructural properties. From the EDAX analysis, The spectrum clearly indicates that the presence of Cu and Te without other impurities. The weight percentage of Cu and Te ( ) % varies when increasing the annealing temperature of 15 C and The weight percentage of Cu and Te % varies when increasing the annealing temperature of 25 C. From the UV-Visible absorption spectra, while increasing the annealing temperature, the absorption peaks were obtained at 572nm for 15 C and 655nm for 25 C. The deposited films have high absorbance in the UV visible region. It is observed that the maximum absorption peak shift slightly towards the smaller wavelength with the increasing annealing temperature. The optical band gap CuTe decreases ( 3.94eV to 2.4eV ) with increasing annealing temperature (15 C to 25 C). From the PL studies, A strong emission band in visible range is observed for the CuTe films of different annealing temperature 15 C and 25 C. There is a strong peak observed at about 53nm. As the bandgap of CuTe is about 3.92eV. There is no remarkable shift is observed from the PL curve. Only the intensity variation is observed. Acknowledgement I express my deepest gratitude and indebtedness to my Supervisor and guide Dr.K.Neyvasagam, M.Sc., M.Phil.,PGDCA., A. Selvaraj & G. Kanchana 7

8 (An International Peer Reviewed Journal), ISSN Volume XI, Issue I, Jan- October 218 Ph.D. Associate professor, Department of Physics, The Madura College, Madurai for his immeasurable help and valuable suggestion throughout this study and for the successful completion of the report in time. Reference [1] V. J. Fulari, V. P. Malekar, and S. A. Gangawane, Progress In Electromagnetics Research, 12, 21, [2] M. K. Bhal, J. Phys. C: Solid State phys. 8, 417(1975). [3] F. Hanus, M. Wautelet, J. Appl. Phys. 68, 337 (199). [4] F. Hanus, M. Wautelet, Appl. Surf. Sci43, 271 (1989). [5] F. Pertlik, Mineralogy and Petrology, 71, 149 (21). [6] Y. Lefebvre, J. L. Bocquet, J. Phys: B. Atom. mole.phys.8, 1322 (1975). [7] J. Goldstein, D. Newbury, D. Joy, C. Lyman, P. Echlin, E. Lifshin, L. Sawyer and J. Michael, Scanning electron microscopy and X-ray microanalysis, 9 th Edition, Kluwer Academic/Plenum Publishers, New York, 23. [8] Ikhioya I. L, International Journal of Research in Chemistry and Environment, 22, 1996, [9] Suresh Kumar, Virender Singh, A.Vohra1, and S.K.Chakarvarti, Columbia International Publishing American Journal of Materials Science and Technology, 17, 213, A. Selvaraj & G. Kanchana 8