Influence of the annealing temperature on Optical properties of (CuInSe2) thin films Influence of the Annealing Temperature on optical Properties of (CuInSe2) Thin Films Sabah A. Salman Muhammad H. Abdalah Dept. of Physics/College of Science/University of Diyala Received in : 28 November 2012 , Accepted in : 25 February 2013 Abstract The Influence of annealing temperature on the optical properties of (CuInSe2) thin films was studied. Thermal evaporation in vacuum technique has been used for films deposited on glass substrates, these films were annealed in vacuum at (100C°, 200C°) for (2 hours). The optical properties were studied in the range (300-900) nm. The obtained results revealed a reduction in energy band gap with annealing temperature . optical parameters such as reflectance, refractive index, extinction coefficient, real and imaginary parts of the dielectric constant, skin depth and optical conductivity are investigated before and after annealing. It was found that all these parameters were affected by annealing temperature. Key words: CuInSe2 thin films, optical properties, vacuum thermal evaporation, annealing temperature. 179 | Physics @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹1a26@@ÖÜ»€a@I3@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (3) 2013 Introduction Semiconducting thin films have been extensively studied for a long time due to their significant role in modern science and technology. CuInSe2 (CIS) semiconductor thin film has recently received much attention as an absorber layer in solar cells owing to its excellent optical and semiconducting properties[1]. It has a direct band gap around (1.0 eV), high optical absorption coefficient (>105 cm-1) [2,3], and reasonable work function, high long-term stability and largest efficiency in solar cell applications[4], photo detectors and light emitting diodes[5]. In order to meet the cost-effective large area requirement for CuInSe2 production, there is continuous research into new processing techniques which are favourable for large scale deployment[6]. The most common methods for preparing of CIS thin films on glass substrates are sputtering, molecular beam epitaxy (MBE) and electrodeposition (ED) [7]. In this work, the optical properties of (CIS) thin films was studied extensively at two different annealing temperatures. Experimental details Copper indium diselenide thin films have been deposited by using thermal evaporation in vacuum technique (10-6 Torr) on to well cleaned glass substrates by using ultrasonic path. The weighed high purity (99.999 %) elemental copper, indium and selenium are taken in a pure cleaned quartz ampoule of diameter (10 mm) and length (65 mm) under vacuum. Using a CuInSe2 ingot as the source material. The elements for the ingot were mixed and sealed in quartz ampoules under vacuum. The ampoules were heated at temperature (1200 C°) over (24 hours), and then cooled to room temperature. The thickness of the films was measured by using the weight methods. The accuracy of thickness measurements was (200 nm). These films were annealed in vacuum at (100C°, 200C°) for (2 hours). Optical transmission spectra of CuInSe2 films have been recorded from (300 nm) to (900 nm) wavelength using a double beam (UV/VIS) (Shimadzu Corporation Japan) at room temperature. Results and Discussions Fig. (1) shows the spectral distribution of transmittance for the as deposited and annealed CuInSe2 films at (100C°, 200C°) for (2 hours), in the wavelength range (300-900) nm. In the spectral region, transmittance of annealed films at (200 C°) is lower than that for as deposited and annealed films at (100 C°). Decreasing the transmittance for CuInSe2 might be due to increase scattering of photons by crystal defects, and the free carrier absorption of photons contributed to the reduction in optical transmittance, or might be due to increase of the crystallite size. The increased roughness of the annealed thin films contributed to the drastic decrease of optical transmittance [8], from this figure it is observed that the transmittance decreases at the low wavelength region, which is the spectral region of fundamental absorption, in this region the incoming photons have sufficient energy to excite electrons from the valence band to the conduction band and thus these photons are absorbed within the material to decrease the transmittance. For this reason, this region carries the information of the band gap of the material [7]. Fig. (2) shows the reflectance spectrum of as prepared CuInSe2 films and the films annealed .The reflectance after annealing became lower than that before annealing. The following relation could be used for calculating the absorption coefficient (α) [9]: (1) t Aοα 303.2 = 180 | Physics @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹1a26@@ÖÜ»€a@I3@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (3) 2013 Where (A°) is the absorbance and (t) is the film thickness. Fig. (3) shows the absorption coefficient (α) of annealed by two temperatures (100 C°, 200 C°) and as deposited CuInSe2 films versus Wavelength, all films show higher absorption on the shorter wavelength side (α >104 cm-1). From this figure α (annealed films at 200 C°) > α (annealed films at 100 C°) > α (films as deposited), this might be attributed to the increase of defect states which leads to increase absorption coefficient. Absorption of photons creates electron-hole pairs. In turn, the field of such pairs may modify the electronic structure and hence of optical properties of CuInSe2 films [10]. The energy band gaps of these films were calculated with the help of the absorption spectra. To determinate of the energy band gap, we plotted (αhf) 2 versus photon energy using the relation [11] : (2) n gEhfBhf )( −=α in which (B) is a constant, (Eg) is the energy band gap and (n) is the (½) and (2) for the allowed transition being direct and indirect, respectively . Fig. (4) shows a plot of (αhf)2 as a function of photon energy. The energy band gap was shifted from (1.35 eV) to (1.33 eV) and (1.312 eV) due to annealing. This may be the cause for the increase in band tail width, and then decreases energy band gap. And the Fig. (5) shows the relation between energy band gap (Eg) and annealing temperature for CuInSe2 films. Refractive index (n°) is one of the fundamental properties for an optical material , because it is closely related to the electronic polarizability of ions and the local field inside materials. The refractive index (n°) is related to the optical reflectance (R) by the following relation [12] : (3)         − + −      − − = °° 1 1 )1( 4 2/1 2 R R k R R n Where (k°) is the extinction coefficient. The behavior of refractive index is nearly similar to the reflectance, we can observe from Fig. (6) the refractive index (n°) decreases with annealing temperature. The extinction coefficient (k°) is directly proportional to the absorption coefficient (α) by using the relation [13]: (4) π λα 4 =°k Where (λ) is the wavelength of the incident photon. Fig. (7) Shows the variation in extinction coefficient (k°) as a function of the wavelength, It can be noticed that the extinction coefficient increases with annealing temperature. The variation of the real (εr) and imaginary (εi) parts of the dielectric constant values versus wavelength for CuInSe2 films before and after annealing are shown in Fig. (8) and Fig. (9). The behavior of real (εr) part of the dielectric constant is similar to that of refractive index (n°) because the smaller value of ( k°2) compared with (n°2) [14] : 181 | Physics @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹1a26@@ÖÜ»€a@I3@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (3) 2013 (5) εr = n°2-k°2 while the imaginary (εi) part of the dielectric constant mainly depends on the extinction coefficient (k°) values, which are related to the variation of absorption coefficient [14]: (6) εi =2n° k° The real and imaginary parts of the dielectric constant indicate the same pattern and the values of real part are higher than imaginary part [15]. It is useful to define a characteristic ‘‘skin’’ thickness that is subjected to an appreciable density of optical energy. The skin depth (χ) can be determined by using the relation [16]: (7) °Κ = π λ χ 2 Fig. (10) shows the variation of skin depth (χ) as a function of wavelength for CuInSe2 films. At wavelength shorter than (700 nm), the skin depth for three films has approximately the same values, this might be due to equal probability of absorption at this spectral region. At wavelengths greater than (700 nm), the reduction in amplitude occurs after passing a larger distance, then the skin depth will be large. The skin depth decreases with annealing temperature ,this might be due to increase the probability of absorption as the annealing temperature increase . The optical conductivity (σ) of the films depends directly on the wavelength (λ) and absorption coefficient (α2 ). With the aid of the Drude formula [17]: (8) σ = [ α 2 c εo λ / 4π ] where (c) is the velocity of light, (εo) is the permittivity of free space (εo = 8.85 x 10-12 C/N.m2) Fig. (11) shows the variation of optical conductivity (σ) as a function of wavelength for CuInSe2 films. From the curves, the effect of annealing temperature is increasing of optical conductivity. Under wavelength (700 nm), high absorption of thin film behavior is due to the photon-atom interaction leading to higher carrier concentration. This effect increases the optical conductivity [18]. Conclusions CuInSe2 thin films have been prepared successfully by thermal evaporation in vacuum technique. It was found that the annealing temperature is an important role in the evolution of CuInSe2 properties. The heat treatment changes the optical properties under investigation in this study. The results show that the (Eg) is (1.35 eV) before annealing and (1.33 eV) after annealing in (100 C°) and (1.312 eV) after annealing in (200 C°). The film annealed at (200 C°) is a good optical properties (lower transmittance, reflectance, energy band gap, refractive index, real part of the dielectric constant and skin depth, and a larger absorption coefficient, extinction coefficient, imaginary part of the 182 | Physics @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹1a26@@ÖÜ»€a@I3@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (3) 2013 dielectric constant and optical conductivity), it can be used as an absorber layer in the fabrication of thin film solar cells. References 1. Prabahar, S.;Balasubramanin, V.;Suryanarayanan, N. and Muthukumarasamy, N. (2010) Optical Properties of Copper Indium Diselenide Thin Films" , Chalcogenide Letters , 7(1) : 49–58. 2. Ruthe, D. ;Zimmer, K. and Hoche, T. (2005) Etching of CuInSe2 thin films comparison of femtosecond and picosecond laser ablation , Applied Surface Science , 247 , 447– 452. 3. Rabeh, M. B. ,Kanzari, M. and Rezig, B. (2009) Effect of Zinc Incorporation in CuInSe2 Thin Films Grown by Vacuum Evaporation Method , ACTA PHYSICA POLONICA A , 115 (3) :699-703. 4. Adurodija, F. O.; Song, J. and Kim, S. D. (1999) Growth of CuInSe2 thin films by high vapour Se treatment of co-sputtered Cu-In alloy in a graphite container , Thin Solid Films , 338 , 13-19. 5. Diaz, R. (1988) Preparation and some semiconducting properties of CuIn(SexTe1-x)2 thin films grown by thermal evaporation , photovoltaic solar energy conference , 2 , 1075-1082. 6. Bouraiou, A. ;Aida, M.S. ;Mosbah, A. and Attaf, N. (2009) Annealing time effect on the properties of CuInSe2 grown by electrodeposition using two electrodes system , Brazilian Journal of Physics , 39(3) :543-546. 7. Huang, C.J. ;Meen, T.H. ;Lai, M.Y. and Chen, W.R. (2004) Formation of CuInSe2 thin films on flexible substrates by electrodeposition (ED) technique , Solar Energy Materials & Solar Cells , 82 , 553–565. 8. Tigau, N.;,Ciupin, V.; Prodan, G.;Rusu ,G. I.;Gheorghies, C. and Vasile, E. (2003) Structural, optical and electrical properties of Sb2O3 thin films with different thickness , Optoelectronics Advance Materials , 8 (1) :37-42. 9. Luis, M. A. S. (2006) Thesis , Study of structural, electrical, optical and magnetic properties of ZnO based films produced by magnetron sputtering , University of Puerto Rico. 10. Afaf A.-A. , Bahiga A.-H.M. and Hoda M. , EISSA3 (2005) "Some Physical Properties of Irradiated Gex(As2Te3)100 - x Chalcogenide System" , Turk J. Phys , 29 , 223-232. 11. Pattar J. , Sawant S. N. , Nagaraja M. and Shashank N. (2009) "Structural Optical and Electrical Properties of Vacuum Evaporated Indium Doped Zinc Telluride Thin Films" , Int. J. Electrochem. Sci. 4 , 369-376. 12. Vgwu E. I. and Onah D. U. (2007) Optical Characteristics of chemical bath deposited CdS thin films characteristics Within UV, Visible and NIR Radiation , The Pacific Journal of Science and Technology , 8(1) :155-161. 13. Alnaimi S.M. and AL-Dileamy M.N. (2007) "Determination of the Optical Constants of Cadmium Stannate (Cd2SnO4) Films" , International Journal of Pure and Applied Physics , 3 (1) :30–39. 14. Shokr, E. K. and Wakkad, M. M. (1992) Effect of laser irradiation on the optical properties of amorphous Se96-xTe4Gax thin films , Mater J. Sci. 27 , 1197-1204. 15. Pal U. , Gonzalez R. S. , Montes G. M. , Jimenez M. G. , Vidal M. A. and Torres S. (1997) Optical Characterization of Vacuum Evaporated Cadmium Sulfide Films" , Thin Solid Films , 305 , (1-2) , 345-350. 16. Eloy, J. F. (1984) Power Lasers , National School of Physics , Grenoble , France , John Wiley & Sons, 59 , (3). 17. Allen, M. V. and Blatter A. (1998) Physical Properties and Applications , Laser Beam Interaction With Materials, 2edition , Springer. 183 | Physics @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹1a26@@ÖÜ»€a@I3@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (3) 2013 184 | Physics @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹1a26@@ÖÜ»€a@I3@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (3) 2013 18. Ali, J. A.-J. (2007) Thesis , Studying the Effect of Molarity on the Physical and Sensing Properties of Zinc Oxide Thin Films Prepared by Spray Pyrolysis Technique , University of Technology , Baghdad. Figure No. (1) : Optical transmittance (T%) versus wavelength for CuInSe2 films 185 | Physics @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹1a26@@ÖÜ»€a@I3@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (3) 2013 Figure No. (2) : Reflectance (R) versus wavelength for CuInSe2 films Figure No. (3) : Absorption coefficient (α) versus wavelength for CuInSe2 films Figure No. (4) : Energy band gap (Eg) versus photon energy for CuInSe2 films 186 | Physics @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹1a26@@ÖÜ»€a@I3@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (3) 2013 Figure No. (5) : Energy band gap (Eg) versus annealing temperature for CuInSe2 films. Figure No. (6) : Refractive Index (n°) versus wavelength for CuInSe2 films 187 | Physics @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹1a26@@ÖÜ»€a@I3@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (3) 2013 Figure No. (7) : Extinction Coefficient (k°) versus wavelength for CuInSe2 films. Figure No. (8) : real (εr) part of the dielectric constant versus wavelength for CuInSe2 films. 188 | Physics @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹1a26@@ÖÜ»€a@I3@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (3) 2013 Figure No. (9) : Imaginary (εi) part of the dielectric constant versus wavelength for CuInSe2 films Figure No. (10) : skin depth (X) versus wavelength for CuInSe2 films. 189 | Physics @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹1a26@@ÖÜ»€a@I3@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (3) 2013 Figure No. (11) : Optical conductivity (σ) versus wavelength for CuInSe2 films . الرقیقة (CuInSe2)الخصائص البصریة ألغشیة فيتأثیر درجة حرارة التلدین صباح أنور سلمان محمد حمید عبد هللا جامعة دیالى /كلیة العلوم /قسم الفیزیاء 2013شباط 25، قبل البحث في 2012تشرین الثاني 28استلم البحث في : الخالصة طریق��ة التبخی��ر .ت��م دراس��تھالرقیق��ة (CuInSe2)البص��ریة ألغش��یة ص��ائصالخ ف��يت��أثیر درج��ة ح��رارة التل��دین إن الفراغ ـفي دنت ــــــة لـــــــــــــعلى قواعد زجاجیة. ھذه األغشی الحراري في الفراغ استخدمت في تحضیر األغشیة الرقیقة nm (900-300) . البصریة بمدى صائصمدة ساعتین . درست الخ (°C°,100 C 200)ي حرارة ــــــــبدرجت المعلمات البصریة مث�ل االنعكاس�یة، معام�ل االنكس�ار، .بدرجة حرارة التلدین تتأثرفجوة الطاقة إن المستحصلةبینت النتائج . وج�د وبع�ده حسبت قب�ل التل�دین ، عمق االختراق والتوصیلیة الضوئیةالحقیقي والخیالي بجزئیھ معامل الخمود، ثابت العزل تأثرت بدرجة حرارة التلدین.قد كافة أن ھذه المعلمات اص البصریة ، التبخیر الحراري في الفراغ ، درجة حرارة، الخو أغشیة الكلمات المفتاحیة : (CuInSe2) الرقیقة التلدین. 190 | Physics @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹1a26@@ÖÜ»€a@I3@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (3) 2013