Physics - 179 مجلة إبن الهيثم للعلوم الصرفة و التطبيقية 2012 السنة 25 المجلد 3 العدد Ibn Al-Haitham Journal for Pure and Applied Science No. 3 Vol. 25 Year 2012 Thickness Influence on Structural and Optical Properties of ZnO Thin Films Prepared by Thermal Evaporation E. y. Abed Department of Physical , College of Education Ibn Al- Haithem ,University of Baghdad Receired in:29 February 2012 : Accepted in: 22 April 2012 Abstract Zinc Oxide transparent thin films (ZnO) with different thickness from (220 to 420)nm ±15nm were prepared by thermal evaporation technique onto glass substrates at 200 with the deposition rate of (10 2) nm sec-1, X-ray diffraction patterns confirm the proper phase formation of the material. The investigation of (XRD) indicates that the (ZnO) film is polycrystalline type of Hexagonal and the preferred orientation along (002) plane. The Optical properties of ZnO were determined through the optical transmission method using ultraviolet- visible spectrophotometer with wavelength (300 – 1100) nm. The optical band gap values of ZnO thin films were slightly increased from (2.9 - 3.1) eV as the film thickness increased. Key words: thin films; structural properties; optical band gap. Introduction Zinc Oxide (ZnO) has a direct band gap semiconductor with 3.37 eV at room temperature.[1] polycrystalline ZnO has found numerous applications, such as transparent electrode in Solar Cells and flat panel displays as well as for the fabrication of grating in Optoelectronic devices, Window in antireflection coatings and Optical filters[2]. There are different methods available to prepare Zinc Oxide, such as pulsed laser deposition, chemical vapor deposition[3], etc. ZnO- based devices have attracted significant interest since they present sensitivity to various gases, high chemical stability for doping, non- toxicity and low cost[1]. Many of searchers works on ZnO thin films as [1-4]. The aim of The present work is to study the structural and optical properties of ZnO films which prepared by the thermal evaporation technique at different thickness. Experiment The metallic Zn films were deposited by thermal evaporation under vacuum onto clean glass substrates at different thickness (220, 320, 420)nm ± 15nm. A cylinder chamber of height 8 cm at top substrate vacuum system used was about Torr. After preparation, the as – deposited Zn films were heated under air at flow rate 400 scuum at temperature 400 for two hours. Physics - 180 مجلة إبن الهيثم للعلوم الصرفة و التطبيقية 2012 السنة 25 المجلد 3 العدد Ibn Al-Haitham Journal for Pure and Applied Science No. 3 Vol. 25 Year 2012 The film thickness was determined by ultrasound method. The crystalline structure of studied films was investigated by X-ray diffraction (XRD) analysis using Cu - radiation (λ = 1.541 A°) in the range (2θ=20° – 80°).The films transmittance was measured using a UV – VIS spectrophotometer in the wavelength range(300 – 1100) nm. Results and Discussion Fig. (1) shows that (ZnO) films of different thicknesses are polycrystalline in nature with orientation along (002) plane. We see an increase in the intensity with the increase thickness material crystallization order will increase. Fig. (2) shows the increase of peak intensity and decrease of the full width at half maxima (FWHM) as the thickness of ZnO film increase[3],where thickness increase make atoms packet as good and improve in structure this make (FWHM) decrease. The crystallite size has been estimated from the FWHM of (002) diffraction peak by the scherrer relation [4]: Where is grain size, λ is the wavelength of X – ray radiation, θ is the Bragg angle of the (002) peak, is the angular width of the (002) peak at a half of its maximum intensity (FWHM). The mean crystallite dimensions calculated from Eq.(1), It is observed that the crystallite size reduces slightly for increased thickness films (19.8, 18.9, 17.7 nm). The lattice parameters have been calculated using the relation [5]: ……………..(2) The parameter was deduced from the XRD pattern. The values are found to be very much close to the standard values of hexagonal ZnO. In a real ZnO crystal the wurtzite structure deviates from the ideal arrangement by changing the c/a ratio or u parameter [6]. The c/a ratio and u parameter for the films deposited are computed and given in table (1). The lattice constants range from 3.253 to 3.258 for "a" parameter and from 5.215 to 5.220 for "c" parameter. The c/a ratio and u parameter vary from 1.603 to 1.602 and from 0.379 to 0.378 respectively. All these films shows good optical transmittance (> 80 %)with wavelength (950 – 1100)nm in the visible and near – infrared range as shown in Fig.(3). Their high transmission in the visible range makes these films excellent candidates for transparent window materials in solar cells. The transmission of ZnO thin films decreased as the thickness increased because the atoms packed to gather as good. This seems to came from the change in reflectance due to light interference of the zinc oxide thin film, similar behavior involving a decrease in the transmission of the film thickness increases has been reported in the literature[7, 8]. The optical band gap was determined by analyzing the optical data with the expression for optical absorption coefficient α and the photon energy where [9]: Where h is the planck constant, is the frequency of the incident light. The optical band gap value ( can be obtained by plotting ( ; is the photon Physics - 181 مجلة إبن الهيثم للعلوم الصرفة و التطبيقية 2012 السنة 25 المجلد 3 العدد Ibn Al-Haitham Journal for Pure and Applied Science No. 3 Vol. 25 Year 2012 energy as shown in Fig. (4) As the film thickness increased from 220 nm to 420nm, the value of optical band gap gradually increased from (2.9, 3.0, 3.1) eV because of crystallization material increase , So position levels reduced near valance and conduction bands, that increased value of optical band gap , similar to other repots [3,4]. The low energy band gap could be attributed to defects and impurities[3]. Conclusion ZnO thin films with various thicknesses were prepared on glass substrates by the thermal evaporation technique. The XRD patterns of the films showed good crystallinity. The optical bad gap values of ZnO thin films decreased as the film thickness increased. The optical properties that were observed for transmittance films of ZnO indicate that they may be used as window layers in solar cells. References 1- Christoulakis, S.; Suchea, M.; Koudoumas, E.; Katherekis, M. and Kiriakidis, G., (2006), Thickness influence on surface morphology and ozone sensing properties of nanostructure ZnO transparent thin films grown by PLD, J. Applied surface science 252: 5351- 5354. 2- Suchea, M.; Christoulakis, S. ; Katharakis, M. and Kiriakidis, G. (2005), Surface characterization of ZnO transparent thin films, J. Physics ,10: 147- 150. 3- Jain, A. ; Sagar, P. and Mehra, R. , (2007), Changes of structural, optical and electrical properties of sol- gel derived ZnO films with their thickness" J. Materials Science – Poland, 25, 1, :233 – 242. 4- Vaezi, M. and Sadruezhead, S. ( 2008), Two – stage chemical deposition of oxide films, J. Transactions, 21, 1,:65 – 70. 5- Lupan, O. ; Shishyanu, S. ; Ursaki, V. ; Khallaf, H. and Sontea, V. , (2009), "Synthesis of nanostructured Al – doped Zinc oxide films on Si for Solar cells applicctions" J. Solar Energy materials and solar cells, 93, : 1417 – 1422. 6- Rakhesh, V. and Vaidyan, K. , (2009), "Effect of substrate temperature and post deposition annealing on the electrical deposited by spray pyrolysis" J. Optoelectronics and Biomedical Materials, 1, 3, : 281 – 290. 7- Faraj, M. and Ibraim, K. , (2008) " Optical and structural properties of thermally evaporated zinc oxide thin films on polyethylene terephthalate substrates" J.Nano – Optoelectronics Resarch and Technology Laboratory (N. O. R. ),3,: 1 – 11. 8- Imanishi, Y. and Nishimura, E. , 2009 "Zinc oxide thin film transparent, conductive film and display device using the sams" J. Patent Application, 5, : 33- 39. 9- Shakti, N. and Gupta, P. , (2010) ,Structural and optical properties of Sol – gel prepared ZnO thin film, J. Applied Physics Research 2, 1, : 19 – 28. Physics - 182 مجلة إبن الهيثم للعلوم الصرفة و التطبيقية 2012 السنة 25 المجلد 3 العدد Ibn Al-Haitham Journal for Pure and Applied Science No. 3 Vol. 25 Year 2012 Physics - 183 مجلة إبن الهيثم للعلوم الصرفة و التطبيقية 2012 السنة 25 المجلد 3 العدد Ibn Al-Haitham Journal for Pure and Applied Science No. 3 Vol. 25 Year 2012 Fig. (1): XRD spectrum of ZnO films with different thickness (A) 222nm, (B) 317nm and (C) 413nm Fig.(2): Dependence of FWHM on the thickness of ZnO films. Physics - 184 مجلة إبن الهيثم للعلوم الصرفة و التطبيقية 2012 السنة 25 المجلد 3 العدد Ibn Al-Haitham Journal for Pure and Applied Science No. 3 Vol. 25 Year 2012 Fig.(3): Transmission as function of wavelength for ZnO films with different thicknesses Fig. (4): A plot of (αhv)² as function of photon energy for ZnO films with different thicknesses Physics - 185 مجلة إبن الهيثم للعلوم الصرفة و التطبيقية 2012 السنة 25 المجلد 3 العدد Ibn Al-Haitham Journal for Pure and Applied Science No. 3 Vol. 25 Year 2012 المحضرة ZnOتأثير السمك على الخواص التركيبية والبصرية لألغشية بطريقة التبخير الحراري الفراغي ايناس ياسين عبد جامعة بغداد ،كلية التربية ابن الهيثم ،قسم الفيزياء 2012نيسان 22،قبل البحث في : 2012شباط 29استلم البحث في الخالصة بتقنية التبخير الحراري الفراغي على قواعد من الزجاج بدرجة ZnOحضرت أغشية رقيقة ألوكسيد الخارصين ، ان nm 15nm)420الى 220ن (لسمك مختلف م nm sec -1)2 10وبمعدل ترسيب( 200حرارة أساس ). حددت الخواص 002حيود األشعة السينة بينت أن األغشية ذو تركيب متعدد التبلور واالتجاه السائد هو للمستوي ( ضمن االطوال Ultra Violet – Visibleالبصرية ألوكسيد الخارصين عن طريق النفاذية البصرية باستعمال مطياف بزيادة 3.1) ( eVالى eV( 2.9 )وكانت قيمة فجوة الطاقة البصرية لألغشية تتراوح بين 300- 1100)(nmالموجية سمك الغشاء. كلمات مفتاحية: األغشية الرقيقة، الخصائص التركيبية، فجوة الطاقة البصرية.