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LASER CRYSTALLIZATION OF NANOSTRUCTURE CdS THIN FILM AUSAMA I. KHUDIAR * , SIDDHARTHA , M. ZULFEQUAR and ZAHID H. KHAN . Department of Physics, Jamia Millia Islamia (Central University), New Delhi-110025, INDIA 2 Center of Laser and Optoelectronics, Ministry of Science and Technology, Baghdad, IRAQ 12

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*Corresponding Author E-mail: [email protected]

Abstract Cadmium sulfide (CdS) thin films were deposited on a glass substrate using the thermal evaporation method at room temperature. The changes in the optical properties (optical band gap and absorption coefficient) after irradiation by Nd: YAG laser at wavelength 532 nm have been measured in the spectral range 190-650 nm. It is found that optical band gap is decreased after irradiating the thin films. The samples were characterized using XRD and the grain size of the CdS thin film the calculated from the XRD data was found as 25.91 nm as-deposited. The grain size is also found to increase with exposure time of laser irradiation.

1. Introduction

2. Experimental

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2.40

60000 50000 40000

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2.38

Grain Size (nm)

90000

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60

2.36

2.34

40

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2.32 20

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hν (e V )

wavelength (nm)

Figure 4 shows the variation of absorption coefficient (α) with wavelength (λ).

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Figure 5 The estimated band gaps from the plots of (αhν) 2 versus (hν).

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T im e Irradation of Laser (m in.)

Figure 6 shows the variation of optical energy gap and grain size with irradiation time.

Reference 1. P.P. Sahay , R. K. Nath and S. Tewari, Cryst.Res.Technol 42.275 (2007). 2. Shamshad A. Khan, M. Zulfequar and M. Husain Vacuum 72, 291 (2004). 3.A. Sunda-Meya, D. Gracin, J. Dutta, Mat.Res.Soc.Symp.Proc 664 (2001). 4.N J Suthankissinger, M Jayachandran, Bull Mater. Sci 30, 547 (2007). Indian Academy of Sciences. Figure 1 shows the XRD of CdS thin film as-deposited

Figure 2 shows the XRD of CdS thin film after 2 min irradiation by Nd: YAG laser.

Figure 3 shows the XRD of CdS thin film after 5 min irradiation by Nd: YAG laser.

4. Conclusion The XRD data show the crystalline nature increase with irradiation time and the grain size being increase and the structure was change from (ZB) to (W) after laser irradiation. The band gap was decreased and absorption coefficient was increase after laser irradiation due to change in the structure.

5. Joint Comittee on Powder Diffraction Standards, (Newton Square,PA, USA,2000), Diffraction Data Files no. 73-1546 and 80-0006. 6. P.Shindov, R. Kakanakov, L. Kolaklieva, Sv. Kaneva, T. Anastasova. Proc. 26th International Conference on Microelectronics (MIEL 2008), Nis, Serbia. (2008), pp. 11- 14. 7. R B Kale and C D Lokhande, Semicond. Sci. Technol 20, 1 (2005). 8. A.A. Othman, H. H. Amer, M. A. Osman and A. Dahshan, Radiation Effects & Defects in Solids 159, 659-666 (2004). 9. K. Sarmah, R. Sarmaand H. L. Das, Journal of Physics: Conference Series 114, 012041 (2008). 10. F. S. Al-Hazmi, Chalcogenide letters 6, 63-69 (2009).

Optical Energy Gap (eV)

Figure 1 shows the XRD of CdS thin film. The peak small but broad corresponding to the (111) plane at 2θ=26.535º. The type of structure of cubic zincblende (ZB). The crystallite size of the nanocrystalline film was calculated by Scherrer’s formula4 was found as 25.91 nm. Figure 2 shows the XRD of CdS thin film after 2 min laser irradiation. The peak corresponding to the (002) plane at 2θ=26.475º . The type of structure is hexagonal wurtzite (W) 5 . The grain size of the film increases to 69.09 nm after laser irradiation. Figure 3 shows the XRD of thin film CdS after 5 min laser irradiation. The peak corresponding to the (002) plane at 2θ=26.450º The type of structure is hexagonal wurtzite (W) 5 . The grain size increases from 25.91 nm to 103.69 nm after laser irradiation. Figures 2& 3 show the intensity of the reflection peak increases due to lager volumes of crystalline material present. All the parameters were changed after irradiation. The diffraction peaks of laser irradiation samples are very sharp with the high intensity indicating the significant increase in crystallite size with the hexagonal modification. The occurrence of phase transformation is probably due to the increase in crystallite size and the change in atomic configuration of the CdS thin film6,7 .

Figure 4 shows the variation of absorption coefficient (α) with wavelength (λ). The absorption coefficient is found to increase after laser irradiation of the thin film. It is clearly seen from the optical spectra that the absorption edge is red shifted for irradiated films this shift indicates a decrease of the optical band gap. This is possibly due to the increase in grain size and the decrease in the number of defects. Figure 5 shows The estimated band gaps from the plots of (αhν) 2 versus hν are for ‘asdeposited’ and irradiated CdS films by pulsed Nd: YAG laser. It was found to be 2.42 eV1, for asdeposited CdS films and shows ‘red shifts’ by 2.37 eV and 2.31 eV after irradiation by laser for different exposure times of 2 and 5 min, respectively. . Figure 7 shows the variation of optical band gap and grain size as a function of irradiation time in minutes. From this figure it is clear that the optical band gap decrease with increase of irradiation time indicating the photodarkening8 and the grain size increase that means the nanocrystalline nature of thin film increase according to the intensity of XRD increase with irradiation time 9, 10 .

(α hν ) 2 (eV 2 Cm -2 )

3.1. X-ray diffraction studies

3.2: Optical properties

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3. Results and Discussion

Thin films of CdS were deposited on a glass substrate by using the thermal vacuum evaporation technique at room temperature and vacuum of ~2*10-5 torr, using a molybdenum boat. The films were kept inside the deposition chamber for 24 h to achieve the metastable equilibrium. The thin films were irradiated by pulsed Nd: YAG laser (wavelength 532 nm, energy 2mJ, pulse duration 5 nsec) for different durations of time. For measuring the optical absorption and transmittance of thin films, a double beam UV/VIS/NIR Spectrophotometer (Camspee-M550) was used. The XRD measurements were carried out using an X-Ray Diffractometer PW 1830 PANalylical which has tube anode; copper using the wavelength 1.54056Å. The X-ray diffraction and optical absorption measurements were carried at before and after irradiation of the sample by the laser.

absorption coefficients (Cm )

Cadmium sulfide (CdS) is an important metal chalcogenides. Its CdS thin films are regarded as one of the most promising materials for heterojunction thin film solar cells. Wide band gap (Eg =2.4 eV) has been used as the window material together with several semiconductor such as CdTe, Cu2S and InP with 14-16% efficiency1. Laser crystallization of thin films on glass is widely used to improve the electronic transport. In the production of flat panel displays, laser crystallization increases the carrier mobility in thin film transistors. Suitable laser intensity profiles in combination with multiple scanning sequences have been used to reduce the number of grain boundaries2,3 . Among the semiconductors of the group IIVI chalcogenide semiconductors have received much interest as they find applications in the solid-state physics. Such semiconductors have band gaps between 1-3 eV in the visible region. They are also used worldwide in optoelectronic devices.