Microsoft Word - 70-80 Physics | 70 2016) عام 2(العدد 29المجلد مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 The Structural and Optical Properties of Zinc Telluride Thin Films by Vacuum Thermal Evaporation Technique Samir A. Maki Hanan K. Hassun Dept. of Physics/ College of Education for Pure Science (Ibn Al-Haitham) University of Baghdad Received in :1/March/2016,Accepted in :8/May/2016 Abstract Different thicknesseses of polycrystalline ZnTe films have been deposited on to glass substrates by vacuum evaporation technique under vacuum 2.1x10-5 mbar. The structural characteristics studied by X-ray diffraction (XRD) showed that the films are polycrystalline and have a cubic (zinc blende ) structure. The calculated microstructure parameters revealed that the crystallite size increases with increasing film thicknesses. The optical measurements on the deposited films were performed in different thicknesseses [ 400 , 450 and 500]nm, to determine the transmission spectrum and the absorption spectra as a function of incident wavelength. The optical absorption coefficient (α) of the films was determined from transmittance spectra in the range of wavelength (400-1100) nm. The dependence of absorption coefficient, on the photon energy showed the occurrence of a direct transition with band gap energy from 2.24eV to 1.92eV (for ZnTe films of different thicknesseses), where with high film thicknesses there are several energy levels resulting in several overlapping energy bands in the band gap of these films. The overlapping energy bands therefore tend to reduce the energy band gap, resulting in lower band gaps for thicker films. Keywords: ZnTe thin films, optical measurements, thermal evaporation and structural properties Physics | 71 2016) عام 2(العدد 29لمجلد ا مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 Introduction Polycrystalline thin films of II-VI compound semiconductors have reasonable importance in electronic and optoelectronic devices fabrication due to their high absorption coefficients and low fabrication costs [1]. Amongst the wide band gap II-VI semiconductor materials, Zinc Telluride (ZnTe) is the most attractive material and finds several applications in the field of device electronics [2]. In addition, it possesses many interesting properties, such as a wide energy band-gap, large photoconductivity, and high excitonic binding energy high-quality, nanostructured ZnTe films are widely used in various fields such as light emitting diodes, photodetectors, laser diodes , gas sensors, field emission and solar cells[3]. There are several reports on deposition of polycrystalline ZnTe thin films by various techniques ,most of the previous research has focused primarily on the structural, electrical and optical properties of ZnTe both in the form of bulk crystals and sub-micron thin films [4]. In the present work, we focus on the study of the influence of the thicknesses on optical properties of ZnTe. Experimental part Thin films of high purity (99.999) ZnTe was used as a source material for the evaporation. With [400,450 and 500] nm thicknesses were deposited on glass substrate by thermal vacuum evaporation using (Edwards – Unit 306) system with 2.1 x10-5 mbar. The thicknesses of films were determined with ( Precisa -Swiss) microbalance by using weighing method and with deposition rate about (1.2∓0.1) nm/sec. The material was placed into molybdenum boat with a small dimple at the center to act as a point source. The boat was heated indirectly by passing current through the electrodes. Cleaned glass slides were used as a substrate. These glass slides were cleaned with chromic acid, ultrasonic cleaner, soap water, distilled water and then with acetone. The material evaporated in vacuum at room temperature onto cleaned glass substrates with (2.5 x 2 x 1) cm3size. The distance between the substrate and the boat is (18) cm. After reaching high vacuum in the vacuum chamber, slowly current was applied to the electrodes to heat the substance. Optical transmission measurements were performed with (UV/Visible 1800 spectrophotometer). The band gap (Eg) and optical constants of the transparent films were determined from the optical transmission spectra. Results and Discussion Structural properties The structure of ZnTe thin films of different thicknesses were analysed by X-ray diffraction system uses CuKα radiation source, which has wavelength 1.5418 Å. as shown in Fig. (1). In ZnTe films only one prominent peak was observed along (111) plane .The films are crystalline in nature and have cubic structure and the corresponding values of inter planar spacing, dhkl ((hkl) are Milles indices), were calculated from the Bragg equation [5] 2d sin θ = nλ …………………………… (1) Where θ is the angle of incidence and λ is the wavelength of the X-rays, n is an integer and it is the order of reflection, and dhkl is the distance between the lattice planes . And we determined lattice parameter, a, from relationship = …….……………… (2) Physics | 72 2016) عام 2(العدد 29لمجلد ا مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 compared with the standard values (American Standard Testing Materia )(ASTM) [6]. The sizes of the crystallites were also determined from X-ray diffraction data using the Scherer’s relation . . ẞ ………………(3) Where λ is wavelength of radiation, β is full width half maxima and θ the diffraction angle. And the dislocation density (δ) of films was estimated using the equations . ………………………(4) The crystallite size (G.S), dislocation density (δ) and lattice spacing are calculated and presented in Table (1). It is observed from XRD pattern of films that intensity of (111) peak and its grain size have increased with the increase of film thicknesses from 400 to 500nm, the improvement in crystallinity is due to fast growth of crystallite , increased ability of atoms to move towards stable sites in the lattice [7and 9] and due to the decrease in lattice defects among the grain boundaries with the grain size increase [10]. The dislocation is imperfection in the crystal which is created during growth of thin film. The dislocation density decreases with increase of film thicknesses due to fast growth of crystallite [11]. Optical properties Fig.(2) shows the optical transmittance spectra with wavelength from 300nm to 1100 nm of the ZnTe thin films with different thicknesses, It is shown that the transmission of these films increases rapidly within the range 500-900nm reaching the maximum value. After this maximum, the transmission approximately remains constant at near-infrared wavelengths. Also from these curves one can see the transmittance of the thin films varies between 56% - 73% , maximum of 73% being reached for ZnTe thin films with lower thicknesses (400nm). With increasing the thicknesses (from 400nm to 500nm ) the spectra show decrease in the transmission. There is a sharp fall from 73% to 56% in the transmittance as the thicknesses rises from 400 to 500 nm; due to increase in the density of the film. Due to highly transparent in visible and IR region [12,13]. The transmittance spectra of the films decrease as film thicknesses increases, which is identifyies a good crystallinity of obtained thicknesses [14, 15]. From figs.(3) and (4) can see vice versa in the absorption spectra and reflectance spectra of ZnTe films deposited onto a glass substrate, the absorption edge shifts to higher wavelength for higher thicknesses and it changes with film thicknesses .The reflectance spectra of films are shown in Fig.(4). It is observed that the average reflectance film increased rapidly in visible region and then decreases with the increase of wave length from the range of (800 to 1100) nm with the increase in thicknesses. The change in the reflectance of the films suggests that the refractive index films is changed where the shift of transmittance and reflectance indicates changes in the film thicknesses. The ability of a material to absorb light is measured by its absorption coefficient and it is a very strong function of the photon energy and band gap energy [16]. The variation of the optical absorption coefficient with photon energy for various thicknesses is shown in Fig.5. The absorption coefficient (α) of a film of thicknesses (t) can be calculated from the transmittance spectrum using the relation [16]: Physics | 73 2016) عام 2(العدد 29لمجلد ا مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 .   …………………….…….…. (5) Where, A: is the absorbance, which is calculated from the relation {A=log (1/Ʈ)}, (Ʈ) is the transparence. The calculated values of absorption coefficient are in the order of 104 cm−1. The variation of optical absorbance with wavelength reveals a high absorption of energy at shorter wavelength and vice versa. The spectra also confirms that with increasing film thicknesses the absorption effect decreases exponentially due to the effect of index of refraction of films [17]. The fundamental absorption, which corresponds to electron excitation from valance to conduction band, can be used to determine the nature and value of the optical band gap. The optical band gap of the ZnTe thin films is calculated using the expression [18]: (αhν) = B(hν - Eg) r ………………(6) Where, B is constant, α [cm-1] is the absorption coefficient, hν is the photon energy and Eg [eV] is the optical band gap. The parameter (r) is an index related to the nature of the material which is determined by the optical transition involved in the absorption process, it specifies the allowed direct (r = 1/2) and indirect transition (r = 2) in the electronic band structure. The optical band gap energy Eg was obtained from the intercept on the photon energy axis after extrapolating of the straight line section of the curve of (αhν)2 versus (hν) plot as shown in Fig.(6) . Due to increase in film thicknesses the results are decrease of energy band gap from 2.24eV to 1.92eV, the individual levels of free atoms will broaden the energy bands and create overlapping levels. This occurs when atoms become closer to each other. Hence, with high film thicknesses there are several energy levels resulting in several overlapping energy bands in the band gap of these films. The overlapping energy bands therefore tend to reduce the energy band gap, resulting in lower band gaps for increment on films thicknesses [19]. Conclusions Different thicknesses of polycrystalline ZnTe films have been deposited onto glass substrates at room temperature by vacuum evaporation technique. From XRD studies it was found that the film is polycrystalline with only one prominent peak was observed along (111) plane, having the zinc blende structure .The calculated microstructure parameters of the ZnTe thin films such as crystallite size (G.S) showed that the size of crystallites increases with the increase of the film thicknesses, The optical band gap decreases with the increase of the film thicknesses. The increase of ( Eg opt) for direct transition may be attributed to the increase in crystallites size. The increase of grain size with thicknesses can be attributed to the improved crystallinity. The improvement in crystallinity is due to increase ability of adding atoms to move towards stable sites in the lattice. Physics | 74 2016) عام 2(العدد 29لمجلد ا مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 References 1- Yahiya, K.Z.;Salem, E. T. and Muhammad, M. S. (2008), Optical Constants of Zinc Telluride Thin Films in the Visibleand Near-Infrared Regions, Eng.&Tech,26,5, Laser Research Unit, School of Applied Sciences, Univ. of Tech. 2- Pattar,J.; Sawant, S. N.; Nagaraja, M.; Shashank, N.; Balakrishna ,K. M.; Ganesh Sanjeev and Mahesh,H. M. (2009) Structural Optical and Electrical Properties of Vacuum Evaporated Indium Doped Zinc Telluride Thin Films, Int. J. Electrochem. Sci., 4 369 – 376. 3- Sharma,D. C.; Srivastava ,S.; Vijay1, Y.K. and Sharma, Y.K. (2012) Effect of Mn-Doping on Optical Properties of ZnTe Thin Films.. IV, ISSN 2277 –8322 International Journal of Recent Research and Review. 4- Kiran, M.S.R.N.; Kshirsagar, S.; Krishna, M.G. and Surya, P. Tewari (2010), Structural, optical and nanomechanical properties of (1 1 1) oriented nanocrystalline ZnTe thin films, Eur. Phys. J. Appl. Phys. 51, 10502 . 5- Jeetendra, S.; Nagabhushana, H.; Mrudula, K.; Naveen., C. 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XRD analysis of ZnTe thin films Films thicknesses ZnTe (nm) 2 θ d (Å) a(Å) G.S nm δ x 1015 (m-2) 400 25.2530 3.53386 6.10313 26.4103 1.4337 450 25.2068 3.53023 6.11414 31.411 1.0135 500 25.1788 3.53386 6.12082 31.939 0.9802 Physics | 76 2016) عام 2(العدد 29لمجلد ا مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 Ɵ (Degree) Figure(1). XRD of ZnTe thin films 0 500 1000 1500 2000 2500 3000 3500 400 nm 0 500 1000 1500 2000 2500 3000 3500 450 0 500 1000 1500 2000 2500 3000 3500 10 20 30 40 50 60 70 80 500 nm In te n si ty ( co u n t / Physics | 77 2016) عام 2(العدد 29لمجلد ا مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 Figure. (2 ): Transmittance spectra of ZnTe thin films with different thicknesses Figure. (3 ): Absorption spectra of ZnTe thin films with different thicknesses 0 10 20 30 40 50 60 70 80 90 100 400 500 600 700 800 900 1000 1100 tr a n sm itt a n ce % wave length nm 400nm 450nm 500nm 0 10 20 30 40 50 60 70 80 90 100 400 500 600 700 800 900 1000 1100 A b so rb a n ce % wave length nm 400nm 450nm 500nm Physics | 78 2016) عام 2(العدد 29لمجلد ا مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 Figure. (4 ): Reflectance spectra of ZnTe thin films with different thicknesses Figure.(5): Variation of absorption coefficient as a function of photon energy of ZnTe thin films with different thicknesses 0 5 10 15 20 25 30 35 40 45 50 400 500 600 700 800 900 1000 1100 R e fle ct io n % wave length nm 400nm" 450nm 500nm 0.00E+04 5.00E-14 1.00E+04 1.50E+04 2.00E+04 2.50E+04 3.00E+04 3.50E+04 4.00E+04 4.50E+04 5.00E+04 1 1.5 2 2.5 3  (c m -1 ) photon Energy(eV) 400nm 450nm 500nm Physics | 79 2016) عام 2(العدد 29لمجلد ا مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 Figure (6): Variation of (αhv)2 with photon energy of ZnTe thin films with different thicknesses 0.00E+00 1.00E+02 0.5 1 1.5 2 2.5 3 ( h v) 2 (e V . cm -1 )2 photon Energy(eV) 400nm Eg 400nm=2.24 eV 0.00E+00 1.00E+02 0.5 1 1.5 2 2.5 3 ( h v) 2 (e V . cm -1 )2 photon Energy(eV) 450nm Eg 450nm=2 eV 0.00E+00 1.00E+02 0.5 1 1.5 2 2.5 3 ( h v) 2 (e V . cm -1 )2 photon Energy(eV) 500nm Eg 500nm=1.92eV Physics | 80 2016) عام 2(العدد 29لمجلد ا مجلة إبن الهيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham J. for Pure & Appl. Sci. Vol.29 (2) 2016 ZnTeالخواص التركيبية والبصرية الغشية والمحضرة الفراغ في الحراري التبخير بتقنية مكي عطا سمير حنان كاظم حسون ) بغداد جامعة / (الهيثم الصرفةإبن للعلوم التربية كلية /الفيزياء قسم 2016/ايار/8 :،قبل في2016اذار//1 :استلم في الخالصة المتعددة التبلور على ارضية من الزجاج وباسماك مختلفة باستعمال تقنية التبخير الحراري بالفراغ ZnTeرسبت اغشية وبدرجة حرارة الغرفة . تم دراسة الخواص التركيبية من خالل حيود االشعة السينية قد mbar 5-2.1x10تحت ضغط وهوالتركيب) Zinc-blende structure( الزنك ركاز نوع مناظهرت النتائج بأن االغشية متعددة التبلور وهي . وكشفت الحسابات التركيبية للحجم الحبيبي بأنه يزداد مع زيادة سمك االغشية .وحددت القياسات البصرية لالغشية المكعبي طيف النفاذية واالمتصاصية كدالة للطول الموجي ,ومن طيف nm[ and 500 450 ,400 ]المرسبة باسماك مختلفة تم تحديد معامل االمتصاص البصري . وكان االنتقال المباشر لفجوة nm (1100-400)لمدى االطوال الموجية النفاذية ولالسماك المختلفة ) التأثير في اعتماد معامل االمتصاص في فجوة ZnTeالغشية 1.92eV (الى 2.24eVالطاقة من الطاقة واالستدالل على فجوة الطاقة من سمك الغشاء . , القياسات البصرية , التبخير الحراري , الخواص التركيبية ZnTeاالغشية الرقيقة الكلمات المفتاحية :