@1a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹26@@ÖÜ»€a@I1@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (1) 2013 A Study of Structure and Optical Properties of ZnO Thin Films Deposited by Using Thermal Evaporation Technique under Different Flow Rate of Oxygen O2 Samir A. Maki Alli A. Shehaband Enase Y. Abed Dept. of Physics/College of Education For Pure Science(Ibn Al-Haitham) / University of Baghdad Received in:28 February 2012 , Accepted in:22 April 2012 Abstract Zinc oxide (ZnO) transparent thin films with different oxygen flow rates (0.5, 1.0, and 1.5)Litter/min. were prepared by thermal evaporation technique on glass substrate at a temperature of 200℃ with rate (10±2)nm sec-1, The crystallinity and structure of these films were analyzed by X-ray diffraction (XRD). It exhibits a polycrystalline hexagonal wurtzite structure and the preferred orientation along (002) plane. The Optical properties of ZnO were determined through the optical transmission method using ulta violet–Visible spectrophotometer with in wave length (300-1100)nm. The optical transmittance of the ZnO films increases from 75% to 85% with increase flow rate of O2, and the optical band gap of ZnO films was found to be increased from (3.0) eV to (3.2) eV with increase flow rate of oxygen.The refractive index of ZnO flims increased from (1.1) to (1.27) with increase flow rate of O2 . Key word : ZnO thin films, X-ray diffraction, Optical properties. 137 | Physics @1a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹26@@ÖÜ»€a@I1@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (1) 2013 Introduction Zinc oxide (ZnO) is n – type semiconductor of wurtzite structure exhibits many interesting properties, such as a wide energy band gap (3.37)eV, high excitonic binding energy (60)meV, high photoconductivity and important piezoelectric and pyroelectric properties. These behaviors make this compound useful for numerous technological applications, such as transparent conducting electrodes, ultraviolet, surface acoustic waves filters, gas sensors and recently in nanoelectronics and photonics. A variety of techniques such as pulse laser deposition, molecular beam epitaxy, vapor phase transport, chemical vapor deposition, r.f. sputtering, spray pyrolysis, ect. [1-3]. However, not much attention has been paid to the preparation of ZnO films by thermal oxidation of metallic Zn thin films, such a method is a relatively simple and low- cost procedure that does not require any calalyst or higher- temperature growth[4]. Many of searchers works of ZnO thin films as [1-4]. The aim of this paper is study structural and optical properties at different flow rates of oxygen. Experimental The metallic Zn films were deposited by thermal evaporation under vacuum onto clean glass substrates by vacuum evaporation technique. A cylinder chamber of height 8 cm, closed at the top by the substrate holder. The residual pressure in the standard vacuum system used was about 4.5 × 10 -5 Torr and heating glass substrates at 200℃ . These films prepared at thickness were 500± 50 nm with rate (10 ± 2)nm sec -1. After preparation Zn films were heated under 400℃ at different flow rates of O2 ( 0.5, 1.0, and 1.5) litter/min. for one hour. The crystalline structure of the studied films was investigated by X-ray diffraction (XRD) analysis using Cu-Kα radiation (λ= 1.5418) A◦ in the rang 2θ = 20◦ -80◦. The films transmittance was measured using a UV-VIS spectrometer in the wavelength range (300 – 1100) nm . Results and Discussion Fig. (1) shows the XRD spectra of ZnO films prepared at different flow of oxygen which are (0.5, 1.0, and 1.5) litter/min. , respectively. For all as – grown films, a strong peak corresponding to (002) plane at around 34◦, which presented the ZnO films are polycrystalline and have hexagonal structure. The diffraction intensiveities of the (002) (100) and (101) direction are increased with theincrease of flow rate oxygen, that means improving crystalline material[5,6]. Fig. (2) displays stress along the c-axis for the ZnO films as a function of the flow rate of oxygen. The variation of the curve presents that with oxygen flow rate increase, stress along c-axis slightly increases and the film stress 𝜎 along the c-axis is given by (C◦ - C)/C◦ where C is the lattice constant obtained from the (002) direction in the XRD spectra and C◦ is 0.5205 nm measured from bulk ZnO[5]. The stress 𝜎 along the c- axis with a hexagonal crystal structure is given by the following equation [5]:- 𝜎 = �2𝐶13 2 − 𝐶33𝐶11−𝐶33𝐶12 𝐶13 � 𝐶°−𝐶 𝐶° … … … (1) Where C ij is elastic stiffness constant of ZnO, C11=209.7 GPa, C12 121.1=GPa, C33= 210.9 GPa and C13= 105.1 GPa [5]. Substituting these values in the above equation gives[5]:- 𝜎 �𝑁 𝑚2� � = −453.6 × 10 9 𝐶°−𝐶 𝐶° ………...(2) 138 | Physics @1a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹26@@ÖÜ»€a@I1@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (1) 2013 According to the above equation, if the stress 𝜎 is positive, the biaxial stress is tensile, if the stress is negative, the biaxial stress is compressive [5]. All the as- grown films exhibit tensile stress, when the flow rate of oxygen increases from 0.5 to 1.5 litter/min., the tensile stress increases from 3.962× 10 6 N/m2 to 4.092× 106 N/m2, and this result breffare from result reference [6]. Fig. (3) shows the transmission for ZnO thin films with wavelength at different oxygen gas flow, we can see from this Fig. the transmission increases with the increase of oxygen flow rate, and was attributed mainly to the improvement of crystalline structures of the films. In near infrared (NIR) spectrum region with (900-1100)nm, the average transmission is more than 85% for flow rate 1.5 litter/min. . The optical absorption edge was analyzed by the following equation [7]:- ∝ ℎ𝑣 = 𝐵�ℎ𝑣 − 𝐸𝑔� 0.5 …………..(3) Where B is a constant and ∝ is absorption coefficient. The variation of (∝ ℎ𝑣) 2 with photon energy (ℎ𝑣) for the ZnO thin film is shown in Fig. (4). It has been observed that plots of (∝ ℎ𝑣 ) 2 versus ℎ𝑣 are linear over a wide range of photon energies indicating the direct type of transitions [7]. The optical band gap of ZnO films increases from (3.0 to 3.2) eV with the increase of oxygen gas flow rate because of increase crystallinity material and decreased crystalline defect, so position levels reduced near valance and conduction bands. The refractive index is an important parameter for optical materials and application, Thus, it is important to determine optical constants of the films. The refractive index of the films was determined from the following relation [8]:- 𝑛 = 1+𝑅 1−𝑅 + � 4𝑅 (1−𝑅)2 − 𝐾2 …………………..(4) Where R is the reflection, and K is the extinction coefficient. The extinction coefficient can be obtained from the experimental expression [7]:- 𝐾 = ∝𝜆 4𝜋 ………………….(5) Where λ is the wave length. from Fig.(5), the films show that the refractive index values decrease with the increase of flow rate oxygen. The extinction coefficient dependence on wavelength is shown in Fig.(6), where it is decreased with the increase of flow rate oxygen. Conclusions We have deposited ZnO thin films on glass substrate using thermal evaporation technique at different oxygen flow rates at 200 ℃ . The XRD measurement showed the intensity of ZnO films increase with the increase of oxygen gas flow. The tensile stress along the c-axis in the as – grown thin film attributed to the common effect of amorphous substrate and extruding between grains. The optical transmittance is 80% at flow 1.5 Litter/min., and a value of about 3.2 eV for the optical band gap at flow rate 1.5 Litter/min. . These behaviors are comparable with those of ZnO films grown by other preparation techniques, and it may be considered that the deposited crystalline ZnO thin film was suitable for many optical devices, such as solar cells and electrodes. References 1-lupan, O.; Shishyanu, S.; Ursaki, V.; Khalf, H.; Chow, L.; Monanic, E.; and Railean, S. ,(2009) ,Synthesis of nanostructure Al-doped zinc oxide films on Si for Solar cells application, J. Solar Energy Materials and Solar cells , 93: 1417-1422. 139 | Physics @1a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹26@@ÖÜ»€a@I1@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (1) 2013 2- Suchristou, M. ; Katharakis, M. and Koudoumas, E.( 2007) ,Substrate temperature influence on the properties of nanostructured ZnO transparent ultrathin films grown by PLD, J. Science Direct , 253 : 8141-8145. 3- Zhao, L.; Liu, J.; and Jiang, Q. ;( 2006) ,Structural and optical properties of ZnO thin films deposited on quartz glass by pulsed laser deposition, J. Science Direct, 252 : 8451- 8455. 4- Rusu, G.; Girtan, M. and Rusu, M., (2007) ,Preparation and characterization of ZnO thin films prepared by thermal oxidation of evaporated Zn thin films, J. Science Direct, 42 : 116- 122. 5- Li, C.; Li, X.; Yan, P.; Liu, Y.; and Fan, X. ( 2007) ,Research on the properties of ZnO thin films deposited by using filtered cathodic arc plasma technique on glass substrate under different flow rate of O2, J. Science Direct ,253 : 4000-4005. 6- Yang, Z.; Lim, J.; Chu, S.; Zuo, Z. and Liu, J., (2008) ,Study of the effect of plasma power on ZnO thin films growth using electron cyclotron resonance plasma assisted molecular-beam epitaxy, J. Applied Surface Science, 255 : 3375-3380. 7- Caglar, M.; Caglar, Y. and Lican, S. , (2006) , The determination of the thickness and optical constants of the ZnO crystalline thin film by using envelope method, J. Optoelectronics and advanced materials, 8(4) : 1410-1413. 8- Akoy, S.; Caglare, Y.; Ilican, S. and Caglar, M. (2010) Effect of Sn dopants on the optical and electrical properties of ZnO films, J. Optical Application , 1 : 7-13. Fig. (1): XRD patterns of ZnO thin films at different flow rates of O2: (a) 0.5, (b) 1.0 and (c) 1.5 litter /min 140 | Physics @1a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹26@@ÖÜ»€a@I1@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (1) 2013 Fig.(2): Stress along the c-axis for ZnO films as a function of the flow rate of O2 Fig.(3): Transmission spectra for ZnO films different flow rat of oxygen Fig.(4): Optical energy for ZnO films at different flow rat of oxygen Fig.(5): Refractive index with wave length for ZnO films at different flow rate of oxygen. 141 | Physics @1a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹26@@ÖÜ»€a@I1@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (1) 2013 Fig. (6): Extinction coefficient with wave length for ZnO films at different flow rate of oxygen. 142 | Physics @1a@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ÚÓ‘Ój�n€a@Î@Úœäñ€a@‚Ï‹»‹€@·rÓ:a@Âig@Ú‹©@Ü‹26@@ÖÜ»€a@I1@‚b«@H2013 Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 26 (1) 2013 دراسة الخصائص التركیبیة والبصریة الغشیة اوكسید الخارصین الرقیقة المرسبة بطریقة التبخیر الحراري الفراغي عند اختالف معدل تدفق االوكسجین )O 2( سمیر عطا مكي علیة عبد المحسن شھاب ایناس یاسین عبد جامعة بغداد/ )ابن الھیثمللعلوم الصرفة ( كلیة التربیةقسم الفیزیاء / 2012نیسان 22، قبل البحث في 2012شباط 28إستلم البحث في الخالصة ) بتقنیة التبخیر الحراري الفراغي على قواعد من الزجاج ZnOحضرت اغشیة رقیقة الوكسید الخارصین ( بمعدل تدفق مختلف لغاز االوكسجین nm sec-1 (10 ±2)وبمعدل ترسیب 200 ℃ساس المرسبة بدرجة حرارة ا 0.5, 1.0,and 1.5) litter/min. تركیب سداسي متعدد التبلور واالتجاه والغشاء ذ ان )، ان حیود االشعة السینیة بینت ) . حددت الخواص البصریة الوكسید الخارصین عن طریق النفاذیة البصریة باستعمال مطیاف 002السائد ھو للمستوي ( ultra violet – visible ZnO 1100-300لمدى االطوال الموجیة)nm 85الى %75) فالنفاذیة البصریة تتغیر من% بزیادة معدل تدفق eV (3.2)الى eV(3.0)بزیادة معدل تدفق االوكسجین وفجوة الطاقة البصریة لالغشیة تتراوح بین بزیادة معدل تدفق االوكسجین. 1.7الى 1.1تزداد من ZnOاالوكسجین. معامل االنكسار ألغشیة الرقیقة، حیود االشعة السینیة، الخصائص البصریة. ZnOاغشیة : مفتاحیةالكلمات ال 143 | Physics