ة مجلة إبن الھیثم للعلوم الصرفة و التطبیقی 2012 السنة 25 المجلد 1 العدد Ibn A l-Haitham Journal f or Pure and Applied Science No. 1 Vol. 25 Year 2012 Effect of Annealing Temperature on the Structural and Optical Properties of The CdO Thin Films Prepared By Vacuum Evaporation Thermal Technique S.A. Mak i and M. Q. Mustafa Departme nt of Physics,College of Education I bn Al-Haitham,Unive rsity of Baghdad Received in : 28 September 2011, Accepted in : 18October 2011 Abstract Cadmium Oxide f ilms have been p repared by vacuum ev aporation technique on a glass subst rate at room temp erature. Structural and optical p rop erties of the films are st udied at different annealing temp eratures (375 and 475) ˚C, for the thickness (450) nm at one hour. The crystal st ructure of the samples was studied by X- ray diffraction. The highest value of the absorbance is equal to (78%) in the wavelength (530) nm, at annealing temp erature (375) ˚C. The value of at a rate of deposition is (10) nm/s. T he value of op tical ener gy gap found is equal to (2.22) eV. Keywords: CdO t hin films, X-ray diffraction, Structural, Op tical p rop er ties, Absorbance, Bandgap Introduction Transp arent conductive oxides (TCOs), an attractive type of semiconducting materials that are both op tically transp arent and electrically conductive, have p otential application in op toelectronic devices such as solar cells, p hotovoltaic, flat p anel disp lay s, transp arent electrodes, ohmic contact t o LEDs, and h eat reflectors [1–4]. In r ecent y ears CdO-based TCOs (binary semiconducting o xides, SnO2, ZnO, In2O3, CdO, Ga2O3 etc, in thin films) r eceived much att ention due to their exceptional carrier mobility , nearly metallic conductivities and simple cryst al st ructure [5]. M any techniques were adopted to grow CdO films such as: thermal evaporation [6], metal v apor organic deposition [7], sp ray py roly sis [5], rap id photothermal o xidation of Cd [8], and p ulsed laser deposition [9]. Cadmium o xide CdO is conducting, transp arent in the visible region with a d irect band gap of (2.5) eV and indir ect band gap of (1.98) eV [10].The p hysics associated with these p rop erties of nanocry st allites of II–VI semiconductors have b een v ery interesting because of thinkin g the p henomena in a new p oint of view with the p rop erties exhibited by them.[11] Various t echniques h ave b een emp loyed to p repare CdO t hin films such as sp ray py roly sis [12], rapid dep osition technique [13], sol – gel sp in coating [14]. In this p aper CdO thin film was p repared by thermal evap oration technique. According to our knowledge, such work on CdO films obtained by thermal evap oration technique has not been reported. In this article we p resent characterization p rop erties CdO films p repared by thermal evap oration method, such as the st ructural and op tical p rop erties. The influence of thickn esses on t he film char acterist ic p rop erties is investigated. Experime nts The st udied CdO thin films p rep ared by thermal vacuum techn ique at room temp erature and annealin g temp eratures (375 and 475) ˚C, for the thickness (450) nm, we obtained the metal bulk Cd ة مجلة إبن الھیثم للعلوم الصرفة و التطبیقی 2012 السنة 25 المجلد 1 العدد Ibn A l-Haitham Journal f or Pure and Applied Science No. 1 Vol. 25 Year 2012 thin films (from Fluka A.G/Germany ), were evap orated in vacuum, at room temp erature, onto p recleaned glass substrates, with (2.6 x 2) area. The distance used between the substrate and the boat is (10) cm, and the thermal p ressure is equal to (4.8  10 -5 )Torr. During Cd film heating, the color varied fro m silver gray at room temp erature to bl ack-brown at annealing temp eratures (375 and 475) ˚C. Aft er deposition, the obtained in the same conditions Cd thin films wer e divid ed in subjected to a gradual heating in air with the rate of (5) K/min. In order to get CdO material is p rep ared from oxidation of samples by the p rocess of cadmium deposition in the thermal oven temp erature (350) ˚C at one hour. The Structural char acterization of the films has been carried out using XPERTPRO X-ray diffractometer. (XRD--6000/7000,SHIM ADZU CORPORATION). The incident radiation was Cu Kα filtered by Ni filter. The grain sizes of the film ( g.s) were d etermined by the measured FWHM values of the st rongest (111) reflection and estimated. The transition sp ectra of the films have been recorded using of the typ e (UV – VIS – Sp ectrop hoto M eter UV - 1800). Results and Discussion Structural studies The X-ray diffraction (XRD) p atterns of the (CdO) films p rep ared at different annealing temp eratures (375 and 475) ˚C, as shown in Figure.1. The p resence of several p eaks in the XRD p atterns reveals t hat all the films are p oly cry stalline. The micrographs and corresp onding diffraction p attern of CdO thin films for different annealin g have be en shown in Fi gur e.1, From the diameter of the rings, whi ch corresp ond to reflection from (111) and (222) p lanes of cubic CdO. For the CdO films, the main characteristic p eaks are assigned to the (111), orientation at (2 ) angles, which are well matched with the st andard data for the CdO cry stal with rock salt cubic st ructure.[JCPDS data card No.050640]. Further it can also be seen that the latt ice parameter (a) for the films works, which also matches very well with the standard value of (0.4695) nm [JCPDS data], as show in Table 1. The relatively st ronger intensity of the p eak (111) indicates p referential or ientation of the f ilm and similar behav ior has also been reported by other researchers [15]. From the XRD p atterns it can be seen that as t he temperatures increases t he p eak intensities increasing and it reaches a maximum at annealin g, indicatin g an imp rovement in cryst allinity up to (475) ˚C. This shows that a p rocess annealin g temp erature of (475) ˚C is op timum for p reparing CdO films by vacuum evap oration method. Such trend that the peak intensity increases up to certain p rocess annealin g temp erature and then falls has been reported by several invest igators who have st udied CdO films grown by various techniques like t hermal o xidation of ev aporation [16]. This trend may be attributed to t he variation in the amount of oxy gen intake and energy of ad atoms. T he mobility of ad atoms and clust ers on the surface of substrate is p rop ortional to their ener gy .[14] Grain size value calculate is by the Scherrer formula.1, that the increasing in annealin g temp erature leads t o an increase in the rate of grain size, as the evident in the (111) p lane, and as in Table:1. This means an imp rovement in the structural prop erties of the film (nanocryst allin), because the disp lay curved at t he mid-intensity inversely p rop ortional to grain size, as in equation 1.[15] where: D = Cry stalline grain size (nm). K = is a const ant (0.94) of the CdO. ة مجلة إبن الھیثم للعلوم الصرفة و التطبیقی 2012 السنة 25 المجلد 1 العدد Ibn A l-Haitham Journal f or Pure and Applied Science No. 1 Vol. 25 Year 2012 = FWHM of the observed p eak (deg). λ = wave len gth of the X-ray diffraction (nm). Thus, the decrease FWHM leads to an increase in the size of p articleboard. Not e from the Table,1, that most of the values that are calcu lated resp onsive to this p rop ortionality. Table: 1, list s the lattice constant (a) and grain size of (111) p eak. From these results we can notice that the fundamental effect of vacuu m ev aporation is related to an increase in cryst allites size and a decrease in the latt ice const ant. The growt h p rocess is affected by the followin g p rocess; namely cold-worked annealed metal with flattened grains, will give essentially the same microstructure of worked and recry st allized grains, and be clear in the article typ e (Fcc), where is the CdO films of this type which is in agreement with p revious results [15,17] The Texture coefficient rep resents the texture of the p articular plane [18], and is used to quantify the preferred orientation. For a p referential orientation the Texture coefficient should be greater than one. The Texture Coefficient (TC (hkl)) for the various p lanes at different annealing temp has been calcu lated using the relation:[19] where: I(hkl) is the observed indensity of the (hkl) p lane, (hkl) is the st andard intensity of the (hkl) p lane, taken from the JCPDS data and N is number of diffraction p eaks. The results obtained for the films grown at (375, and 475) ˚C. It is seen that the TC is maximum for (111) p lane for all the films deposited at different annealing. This shows that t he (111) p lane is the p referred orientation and that t here is neither orientation chan ge nor p hase chan ge with p rocess temp erature [20]. The values of some st ructural p arameters determined for analyzed CdO samples are summar ized in Table.1. Table.1, one can se e that the calculated valu es of TC(hkl) are deviated from unity , esp ecially those corresp onding to (111) and (222) p lanes. This indicates that the resp ective films h ave the lar gest p referred crystallographic orientation alon g the (111) diffraction plane. Such higher values for the texture coefficient have also reported by F.C. Eze for reactive vacuum evaporated CdO films deposited at various partial pressure of oxygen, [14 and 20]. So, one can conclude that in both cases (of CdO films obtained by thermal oxidation of vacuum evaporated Cd films and of those obtained by reactive vacuum evaporation) occur similar grow th mechanism of the CdO crystallites. Optical studies The transmittance and absorbance spectra of spin c oated CdO thin films prepared at annealing temperatures (375 and 475) ˚C. The optical transmission in the visible region of the CdO f ilms increase w ith the annealing and attain a maximum values, as shown in Figure.2. Hence (375) ˚C may be c onsidered at this stage the absorbance is (nearly 78%) in the wavelength (530) nm, from the relationship:[21] ة مجلة إبن الھیثم للعلوم الصرفة و التطبیقی 2012 السنة 25 المجلد 1 العدد Ibn A l-Haitham Journal f or Pure and Applied Science No. 1 Vol. 25 Year 2012 where: A, T are the absorbance, and transmittance, respectively. The increase in visible transmittance of the film with annealing temperature up to (475) ˚C may be attributable to the imp rovement in the crystallinity of the cry stallites and imp rovement in the st ructural and surface ho mogeneity of the films. The band gap which is of the material and exp onent depends on the type of transition. The calculated ener gy gap is for direct t ransition from the relationship :[21] where: A is constant, is the optical band gap ener gy , hv and α is the p hoton energy and absorption coefficient, resp ectively. To determine the p ossible transitions vs. is p lotted and corresp onding band gap was obt ained from extrapolating the straight p ortion of the graph on axis. The direct band gap is calcu lated from vs. p lots by the p oint [ = 0]. T he direct band gap from vs. p lots by the point [ = 0] lies in the ran ge (2.22) eV. as shown in Figur e. 3. The value of the energy gap for CdO films p rep ared at different annealin g temp eratures (375 and 475) ˚C for the films decrease from the valu e of (2.175) eV to the value of (2.1) eV.[22] It is exp lains that so, there may be more than one interband absorp tions for these nanocrystalline CdO samples. These might be due to the fact that for nanop articles (size -few nm) as surface to volume ratio increases, so defect st ates increase due t o increase of surface area and strain than that of bulk CdO. These defects create st ates at different ener gies in the b and gap and there may be some finite transition p robabilities from these st ates. Hence due to the absorp tion of energy by the defect st ates situated at different energies within the forbidden region (for our nanocryst alline CdO thin films), we get different lin ear se gments in Figures.3.[11] Conclusion Thin films of Cadmium O xide have been p repared by vacuum evap oration method. XRD shows the diffraction p eaks characterist ic to the CdO faces centered cubic (FCC), with lattice parameter (a = 0.469±0.004) nm, The average g.s of (111) orientation gra ins of the studied polycrystalline f ilm annealed at (375 and 475) ˚C was found to Direct band gap energies were determined. The annealing temperature increases due to increase of transmittance which is deceasing absorption which of the value (near ly 78%) in the wavelength (530) nm, at annealing temperature (375) ˚C. The optical energy gap value of the range; (2.1 – 2.175) eV have been obtained that is decreasing bandgap where increased of annealin g temp . Re ferences 1.Ramakrishna, K.T. Reddy, Shanthin i G.M., Johnst on D., and Miles R.W. (2003), Highly t ransparent and conduct ing CdO films gro wn by chemical spray pyroly sis. Thin Solid Films. by Elsevier Science B.V., U.S.A, p: 427, 397. ة مجلة إبن الھیثم للعلوم الصرفة و التطبیقی 2012 السنة 25 المجلد 1 العدد Ibn A l-Haitham Journal f or Pure and Applied Science No. 1 Vol. 25 Year 2012 2.Dhage, S.; Ravi, V. and Dat e, S. Bull. (2004). Mat er. Sci.. by Elsevier Science B.V., U.S.A, (2ed) p: 27, 43. 3.Coutts, T.J.; Young, D.L. X. Li. ; Mulligan, W.P . and Wu, X. (2000). J. Vac. Sci. T echnol. by Elsevier Science B.V., U.S.A, A 18, 2646. 4.Shim , T. ;Ma, D. (2002). Thin Solid Films. by Elsevier Science B.V., U.S.A, 8, 410. 5.Deokate, R.J.; P awar, S.M.; Moholk ar, A.V.; Sawant , V.S.an d P awar, C.A. (2008), Spray depo sit ion of highly tran sparent fluorine doped CdO thin films. Appl. Surf. Sci. by John Wiley & Sons Lt d, New York, 254(7): 2187. 6.Dakhel, A.A. and Henari, F. Z. (2003). 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(2ed), 235. ة مجلة إبن الھیثم للعلوم الصرفة و التطبیقی 2012 السنة 25 المجلد 1 العدد Ibn A l-Haitham Journal f or Pure and Applied Science No. 1 Vol. 25 Year 2012 Table (1): T – annealing temperature, D grain size, 2θ (111) - diffraction angle corresponding to (111) plane s, a – lattice constant to (111) planes, TC (hkl) - te xture coefficient for various plane s 2θ 111 (deg ) a 111 (nm ) TC (hkl) (T) Anne alin g (˚C) D 111 (nm ) St andard JCP DS Experiment al XR-D JCP DS XR-D (1 11) (2 00) (2 22) R.T 47.47 33.001 33.150 0.4695 0.4699 0.96 - 1.04 375 49.91 33.001 32.706 0.4695 0.4734 1.59 0.18 1.23 475 51.1 33.001 32.710 0.4695 0.4738 1.25 0.31 1.25 Fig.( 1): X-Ray diffraction of the CdO thin films. before and after anne aling temperatures ة مجلة إبن الھیثم للعلوم الصرفة و التطبیقی 2012 السنة 25 المجلد 1 العدد Ibn A l-Haitham Journal f or Pure and Applied Science No. 1 Vol. 25 Year 2012 Fig.( 2): Transmittance and absorbance spectra at anne aling temp (375and 475) ˚C as a function of wavelength Fig.(3): The band gap is for direct transi tion of the CdO films at annealing temperatures: (375and 475) ˚C, and thickness: (450) nm, as a function of photon ene rgy A T ة مجلة إبن الھیثم للعلوم الصرفة و التطبیقی 2012 السنة 25 المجلد 1 العدد Ibn A l-Haitham Journal f or Pure and Applied Science No. 1 Vol. 25 Year 2012 تأثیر درجة حرارة التلدین في الخواص التركیبیة والبصریة الغشیة CdOالرقیقة المحضرة بوساطة تقنیة التبخیر الحراري الفراغي سمیر عطا مكي ومصطفى قحطان مصطفى قسم الفیزیاء ،كلیة التربیة ابن الهیثم ، جامعة بغداد 2011 تشرین االول 18: قبل البحث في 2011 ایلول 28: استلم البحث في الخالصة بوساطة تقنیة التبخیر الحراري الفراغي على قواعد زجاجیة بدرجة الرقیقة (CdO) ّحضرت اغشیة اوكسید الكادمیوم ص التركیبیة والبصریة لالغشیة عند، الغرفة حرارة داره بزمن ثابت مق C˚ (375,475) التلدین بدرجة حرارة ودرست الخوا امتصاصیة وكانت اعلى، ودرس التركیب البلوري بواسطة حیود االشعة السینیة .nm (450) السمك عند ,ساعة واحدة وكان مقدار معدل .(%78) التي تساوي تقریباC˚ (375) درجة حرارة التلدین عند nm (530) الطول الموجي عند .eV (2.22) قة قیمتهاووجد ان فجوة الطا .nm/s (10) الترسیب هو عند القیمة فجوة الطاقة، االمتصاصیة، الخواص التركیبیة والبصریة، حیود االشعة السینیة، اوكسید الكادمیوم: الكلمات المفتاحیة