@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 Study the Effect of Thickness on the Electrical Conductivity and Optical Constant of Co3O4 Thin Films Bushra K. H. Al-Maiyaly Dept. of Physics/College of Education For Pure Science(Ibn Al-Haitham) / University of Baghdad Received in: 1April 2012 , Accepted in: 21 May 2012 Abstract In this research the Cobalt Oxide (Co3O4) films are prepared by the method of chemical spray pyrolysis deposition at different thicknesses such that (250, 350, 450, and 550) ± 20 nm. The optical measurement shows that the Co3O4 films have a direct energy gap, and they in general increase with the increase of the thickness. The optical constants are investigated and calculated such as absorption coefficient, refractive index, extinction coefficient and the dielectric constants for the wavelengths in the range (300-900) nm. The electrical conductivity (σ) and the activation energies (Ea1, Ea2) have been investigated on (Co3O4) thin films as a function of thickness. The films contain two types of transport mechanisms, and the electrical conductivity (σ) increases whereas the activation energy (Ea) would decrease as the films thickness increases. Key words :Cobalt Oxide, Optical Constant, Electrical Conductivity, chemical spray pyrolysis 159 | 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 A glass with a Co or Fe oxide film on its surface is generally called a semi-reflective glass, because it partially reflects the solar spectrum. The glass reflects solar energy which reduces the cost of air-conditioning. In recent years the glass has become of interest with the rise in the cost of energy. The oxide films, mainly composed of Co, are widely employed for semi- reflective glasses, not only because of their energy saving effect but also because of their good durability as a glazing of building. [1] Cobalt Oxide films that are highly protective against localized corrosion and depicting a wide variety of bright and uniform colors due to light interference, have been successfully electro generated on polycrystalline cobalt disk electrodes under potentiostatic. [2] Several methods have been developed for (Co3O4) films preparation, such as sputtering pyrolysis [3, 4], chemical vapor deposition [5], electro deposition [6, 7], pulsed laser deposition [8], electro chemical techniques [9], etc. In this research thermal chemical spray technique was used. Experiment a-Preparation of the samples Cobalt Oxide (Co3O4) thin films in molarity (0.1) at different thicknesses were prepared by the method of chemical spray pyrolysis. They are prepared by spraying a solution of Co(NO3)2.6H2O on preheated glass substrates at (663) ºK. A diluted (2.9104) gm of Co (NO3)2.6H2O in (100) ml water as molarity (0.1) was used in accordance with the following equation: [10] M = (Wt / M wt). (1000 / V) ……………………… (1) Where M = molarity, Wt = weight of sample, Mwt = molecular weight, V = water volume. The obtained solution is used for the preparation of (Co3O4) based on the reaction:- ↑+↑+↓→ 243 heat 23 O4NO6OCo)NO(Co3 ……… (2) The thin films of Co3O4 are prepared by spraying the solution on glass substrates which are placed on the hot plate for 25 min before the spraying process; each spraying period lasts for about 15 sec followed by 2 min waiting period to avoid excessive cooling of the hot substrate due to the spraying process. It has been found that the following deposition parameters give good stoichiometric films:-(i) substrate temperature at (663) ºK, (ii)distance between sprayer nozzle and substrate of( 30) cm, (iii)spray rate of (15 ml / min).The films were clear, dark colored and having good adhesive properties . b-Measurement The thickness of the sprayed samples (250, 350, 450, and 550) nm was measured by using the weighing method according to the following relation: [11] t= (m / à .ρ) ……………………. (3) Where t= thickness of film, m= mass of film, ρ= density of films, à= area of film. Using a sensitive balance whose sensitivity is of the order (10-4) gm. To determine the optical parameters of the prepared thin film, we measured the transmission and the absorption spectrum in the range (200-900) nm using a double beam spectrophotometer (UV). The optical absorption spectrum used to determine the optical energy gap (Egopt) from the calculation of the variable absorption coefficient (α) for each wavelength from equation: [12] α = 2.303 (A / t)………………. (4) Where A = absorbance. Also incident photon energy (E=hν) was calculated as a function of wavelength (λ) according to equation: E (eV) = (1240 / λ)………………… (5) The energy dependence of absorption coefficient near the band edge for band to band and exciton transition could be described by Tauc formulas: [13] (α hν) = B` (hν – Egopt) r………………. (6) 160 | 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 Where B` is a constant inversely proportional to amorphousity, r is constant and may take values 2,3,1/2,3/2 depending on the material and the type of the optical transition. The optical constants including, the refractive index (n), extinction coefficient (K), and dielectric constant (ε), they are calculated from transmittance (T) and absorbance (A) spectrum in the range (200-900) nm. The refractive index value can be calculated from the formula: [14] n = {[4R / (R-1)]-K2}1/2 – [(R+1) / (R-1)]……………….. (7) Where R is the reflectance which calculated by using equation: R = 1-T-A ………………………….. (8) The absorption coefficient (α) is related to extinction coefficient (K) by: [14] α = 4π K / λ ………………………. (9) The dielectric constant can be introduced by: [14] ε = ε1-i ε2 ………………………… (10) Where ε1= n2-K2 ………………………… (11) ε2 = 2nK ………………………… (12) Where ε1= real part of dielectric constant, ε2 = imaginary part of dielectric constant. For D.C. measurement films deposited on the glass substrate with Al electrode Keithly models 616 have been used to measure the variation of electric resistance (R) with temperature range (298- 473) ºK, then calculated the resistivity (ρ) by the formula: - [14] L tbR ⋅⋅ =ρ ……………………………………… (13) Where t is film thickness, b is electrodes width; L is distance between two Al electrodes. Then the conductivity (σ) of the films was determined by using the relation:- ρ =σ 1 c.d ………………………………………… (14) Results and Discussion a -Absorption Coefficient Fig (1) shows the variation of the absorption coefficient (α) of (Co3O4) films as a function of photon energy at various thicknesses (250, 350, 450, and 550) nm. It is observed that the absorption coefficient decreases the increase of thickness (t) within the whole range of the spectrum; it is noticed that the absorption is not attributed to the free carriers only, but to impurities or localized electronic states. It is clear from this figure that all the films have high values of absorption coefficient (α > 105 cm-1) this means that the direct transition possible occurs. This result is in agreement with refs. [3, 15]. b- Optical Energy Gap The optical energy gap (Egopt) values were calculated from Tauc equation (6) which is used to find the type of the optical transition for (Co3O4) films by plotting the relation (α hν)2 versus photon energy (hν) and select the optimum linear part, which describes the allowed direct transition, then we determined Egopt by the extrapolation of the portion at (α =0) as shown in Fig (2).Fig (3) and table (1) show the optical energy gap as a function of thickness (t), the noticeable remark is that the Egopt increased from (1.78 eV) to (2.1 eV) when the thickness increased. This behavior can be attributed to the increase of the density of localize states in the Eg which caused the energy gap seems large. The value of the optical energy gap agrees with ref. [4] which found it equals to (2.1 eV). c- Refractive Index Fig (4) shows the variation of the refractive index values (n) versus wavelengths in the range (300-900) nm, at different thicknesses (250, 350, 450, and 550) nm. We can notice from 161 | 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 this figure that the refractive index values increase with the increase of thickness, this behavior may be due to increase in packing density.[2] The values of the refractive index of these films range from (2.69 to 3.38) at λ =625 nm, are in agood agreement with values reported in ref. [2] d- Extinction Coefficient The variation of the extinction coefficient (K) with photon energy for various thickness is shown in Fig (5), from this figure we can notice that the extinction coefficient values decrease with the increase of thickness, this behavior of the extinction coefficient values is similar for all the range of the wavelength spectrum to that of the absorption coefficients for the same reasons as we mentioned before. e- The Dielectric Constants Fig 6( a, b) shows the variation of the real (ε1) and imaginary (ε2) parts of the dielectric constant values versus photon energy at different thicknesses. From this figure we can deduce that the imaginary part of the dielectric constant (ε2) decrease with the increase of thickness in all of the range of the spectrum while the real part of the dielectric constant (ε1) increases with the increase of thickness because the variation of (ε1) mainly depend on the value of the refractive index while the (ε2) value mainly depend on the extinction coefficient values which are related to the variation of absorption coefficient. f- D.C Conductivity The plots of lnσ versus 103/T at different thicknesses for (Co3O4) films are shown in Fig (7), the electrical conductivity (σ) shows an increase behavior with the increase of (t). It is increased from (0.527) ohm-1.cm-1 to (0.82) ohm-1.cm-1. This behavior is in an agreement with ref. [3]. The increase of trend in σ upon increase thickness can be attributed to the improvement in the films structure with the increase of (t), due to reducing dangling bonds, and defects like vacancy sites. We believe that the increases of film thickness (t) decreases the trapping centers of charge carriers, this is, perhaps, because of the decreased grain boundary scattering, moreover it yields more packing density. This figure also shows two mechanisms for electrical conductivity at lower and higher temperature with two values of activation energy (Ea1, Ea2) for all films, which means that there are two mechanisms of transport. Fig (8) shows the activation energies (Ea1, Ea2) of (Co3O4) films as a function of thickness, it is clear from this figure that both Ea1 and Ea2 decrease with the increase of thickness (t) because of the decreased number of carriers available for transport, this behavior can be attributed to the decrease of the density of states in the gap, reducing of dangling bonds, and defects like vacancy sites in the films structure. Table (2) shows the values of the electrical conductivity and the activation energies of deposited films. Conclusion In conclusion, we studied in detail the influence of film thickness on the optical constant of (Co3O4) films. Throughout our research we showed that:- 1. The absorption coefficient decreases with the increase of the thickness of prepared thin films. 2. All films prepared have high values of absorption coefficient (α > 105 cm-1) We can use (Co3O4) films as a window for wavelength (λ > 625 nm) and as 625 nm).< Filter for wavelength (λ 3. The optical energy gap values increases when films thickness increases. 4. The variation in films thickness resulted increase values of refractive index and decrease the extinction coefficient values. 5. The values of real part of the dielectric constant increase while the imaginary part of the dielectric constant decrease with the increase of films thickness. 162 | 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 6. The electrical conductivity shows as the increase of behavior with the increase of thickness, whereas the activation energies decrease as the thickness increases. References 1. Sakata, N.; Hyodo, M. and Kawahara, H. (1982), optical properties and chemical resistance 438.-429:49Cr oxide films, “Journal of Non crystalline Solids, -of Fe 2. Danick Gallant; Michel Pezolet and Stephan Simard (2006), optical and physical properties , 110of cobalt oxide films electro generated in bicarbonate aqueous media,”J.phys.chem.B”, (13),6871-6880. 3. Kadam, L.D and Patil, P.S. (2001), Thickness dependent properties of sprayed cobalt oxide 232.-3, 225-, Issue168thin films,” Materials chemistry and physics”, 4. Shinde, V.R.; Mahadik, S.B.; Gujar, T.P. and Lokhande, C.D.(2006), super capacitive -,Issue 20, 7487252ray pyrolysis,”Applied Surface Science”,cobalt oxide thin films by sp 7492. 5. David Barreca and Cristian Massignan, (2001), composition and micro structure of cobalt 593.-, (2), 58813oxide thin films,” 6. Sunil Kandalkar G.; Hae-Min Lee, Heeyeopchae and Chang-Koo Kim(2011),Structural, Morphological and Electrical characteristics of the electrodeposited cobalt oxide,” Materials 51.-, Issue 1, 4846Research B”, 7. Wang chong; Wang Dian; Wang qiu-Ming and Huan-Jun (2010), preparation and electrochemical performance of three-dimensional structure foam, “Chemical journal of 2062.-, Issue 10, 205831Chinese universities”, 8. Kennedy,Rj (1995),The growth of iron oxide, nickel oxide and cobalt oxide thin films by 3831.-, Issue 6, 382931laser ab targets,”Magnetics”, 9. Nicolae Spataru; Chiaki Terashima; Kenichi Tokuhiro and Akira Fujishima (2003),”Journal 341.-, (7), 337150of the Electrochemical Society”, 10. AL-Ghabsha;Th.S and AL-Abachi.M.Q (1986)”Fundamentals of Analytical chemistry" Baghdad University, p39. 11. AL-Mizban, E.S. (1997),”A study of optical and electrical properties of Cr2O3 and Co3O4 thin films and their mixture”M.Sc thesis University of Baghdad,p.47-49. 12. William,Callister, D. J (2003),”Materials Science& Engineering. An Introduction 6th edition, John Wiley Sons Inc, p.96. 13. Taus, J. (1974),”Amorphous and Liquid Semiconductor”, Plenums Press.NewYork and London. 14. Kasap, S.O. (2002),” Principles of Electronic Materials and Devices”, 2nd edition, Mc Graw Hill. 15. Sunil, Kandalkar, G.; Lokhande C.D.; Mane R.S. and Sung, Hwan Han (2007), Anon- thermal chemical synthesis of hydrophilic and amorphous cobalt oxide films for super 3956.-, Issue 8, 3952253capacitor application,” Applied Surface Science “, Table (1): The optical constant of (Co3O4) films at different thicknesses. Films thickness (nm) Egopt (eV) Optical constant at λ =625 nm α x 105 cm-1 n k ε1 ε2 250 1.78 1.316 2.69 0.655 6.819 3.528 350 1.84 0.9808 2.82 0.488 7.72 2.75 450 1.96 0.838 3.09 0.417 9.41 2.58 550 2.1 0.666 3.38 0.331 11.32 2.24 163 | 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 Table (2): The electrical conductivity and activation energies of (Co3O4) films at different thicknesses Fig. (1): Absorption coefficient behavior as a function of photon energy for (Co 3O4) thin films deposited at different thicknesses. Fig. (2): Variation (α hν)2 & photon energy as a function of thickness for (Co3O4) films. Fig. (3) : Variation optical energy gap as a function of thickness for (Co3O4) films Films thickness (nm) σ at R.T (Ω.cm)-1 Ea1 Tem. rang Ea2 Tem. rang ( eV ) ( ºK ) ( eV ) ( ºK ) 250 0.526 0.0985 303-363 0.254 373-473 350 0.625 0.0934 303-363 0.232 373-473 450 0.724 0.0897 303-363 0.204 373-473 550 0.82 0.0845 303-363 0.175 373-473 Co3O4 1.7 1.8 1.9 2 2.1 2.2 250 350 450 550 Thickness (nm) E g op t (e V) Co3O4 0 0.5 1 1.5 2 1 2 3 4 5 Photon Energy (eV) ( α *1 05 ) cm -1 t=250 nm t=350 nm t=450 nm t=550 nm Co3O4 0 10 20 30 40 50 60 70 0 1 2 3 4 5 Photon Energy (eV) (ah u)2 e1 0 ( cm -1 .e V ) 2 t=250 nm t=350 nm t=450 nm t=550 nm 164 | 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 (a) (b) Fig. (6): Dielectric constant of (Co3O4) thin films & photon energy:- (a) Real part at different thicknesses. (b) Imaginary part at different thicknesses. Fig. (7): Variation lnσ versus 103/T as a function of thickness for (Co3O4) films Fig. (8): Variation activation energies as a function of thickness for (Co3O4) films. -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.5 2 2.5 3 3.5 4 1000/T (K)-1 ln s (oh m.c m)- 1 t=250 nm t= 350 nm t= 450 nm t= 550 nm 0.05 0.1 0.15 0.2 0.25 0.3 200 300 400 500 600 Thickness (nm) Ac tiv ati on En erg y ( eV ) Ea1 Ea2 Co3O4 0 0.2 0.4 0.6 0.8 1 1 2 3 4 5 Photon Energy (eV) Ex tin cti on C oe ffi cie nt (k ) t=250 nm t=350 nm t=450 nm t=550 nm Fig. (4): Variation refractive index & photon Energy as a function of thickness for (Co3O4) films Fig. (5): Variation extinction coefficient & photon Energy as a function of thickness for (Co3O4) films Co3O4 1.5 2.5 3.5 1 2 3 4 5 Photon Energy (eV) Re fra cti ve In de x ( n o) t=250 nm t=350 nm t=450 nm t=550 nm Co3O4 2 4 6 8 10 12 1 2 3 4 5 Photon Energy (eV) Re al Pa rt of D iel ec tri c C on st an t ( e 1 ) t=250 nm t=350 nm t=450 nm t=550 nm Co3O4 0.5 1.5 2.5 3.5 4.5 1 2 3 4 5 Photon Energy (eV) Im ag en ry P ar t o f D ie le ct ric Co ns ta nt ( ε 2) t=250 nm t=350 nm t=450 nm t=550 nm 165 | 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 تأثیر السمك في التوصیلیة الكھربائیة والثوابت البصریةالغشیة Co3O4 الرقیقة بشرى كاظم حسون المیالي الفیزیاء / كلیة التربیة للعلوم الصرفة (أبن الھیثم) / جامعة بغدادقسم علوم 2012أیار 21، قبل البحث في 2012نیسان 1أستلم البحث في : الخالصة بطریق�ة ال�رش الكیمی�ائي الح�راري وعن�د أس�ماك مختلف�ة (Co3O4)في ھذا البح�ث حض�رت أغش�یة أوكس�ید الكوبل�ت (250, 350, 450, and 550) ± 20 nm . فجوة طاقة مباشرة وتزداد قیمتھا بصورة عامة بزیادة السمك .ولقد (Co3O4)من خالل القیاسات البصریة تبین ان الغشیة حسبت الثوابت البصریة، مثل: معامل االمتصاص، ومعامل الخمود، وثابت العزل الكھربائي ضمن مدى االطوال الموجی�ة nm )300-900 . ( كدال�ة لتغی�ر الس�مك ، وق�د أظھ�رت (Co3O4)الغش�یة (Ea1, Ea2)وطاقات التنش�یط (σ)وحسبت التوصیلیة الكھربائیة االغشیة آلیتین لالنتق�ال االلكترون�ي ول�وحظ زی�ادة التوص�یلیة الكھربائی�ة م�ع نقص�ان طاق�ات التنش�یط بزی�ادة س�مك االغش�یة المحضرة. : أوكسید الكوبلت ، الثوابت البصریة، التوصیلیة الكھربائیة ، الرش الكیمیائي الحراري . الكلمات المفتاحیة 166 | Physics