Available online at www.sciencedirect.com ScienceDirect Energ Procedia 36 (2013 ) 42 49 TerraGreen 13 International Conference 2013 - Advancements in Renewable Energ and Clean Environment Some optical properties of CdO thin films Ghazi Y. Naser a*, Waleed N. Raja b, Ali S. Faris c, Zenhe J. Rahem d, Mohammed A. Salih e, Auda H. Ahmed f a*,b,c,d,e,f Al-Iraqia Universit, College of Education, Phsics department ; Baghadad, Iraq Abstract In this research, thin films of CdO of thicknesses 500nm;450nm;400nm;350nm;300nm±30 have been prepared using chemical spra method. The films have been deposited on Crown glass substrates. Optical studies show that in these films the electronic transition is of the direct transition tpe. The optical energ gap for the films of as deposited are within the range 2-2.08eV and for those heat treated for an hour at different temperatures: (as deposited 300K; 523K; 548K; 573K) is estimated to be in the range 2.08-2.85eV. Results analsis ehibits the dependence of optical energ gap on the temperature of heat treatment and the dependence on films thickness was not significant. 2013 20 The Published Authors. Published b Elsevier b Elsevier Ltd. Selection Ltd. Open and/or access peer-review under CC BY-NC-ND under responsibilit license. of [Terragreen Selection Academ] and/or peer-review under responsibilit of the TerraGreen Academ Thin films, optical energ gap, chemical spra, * Corresponding author. Tel.: +9647816772940; fa: +0-000-000-0000. E-mail address: gaz2005@ahoo.com 1876-62 2013 The Authors. Published b Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibilit of the TerraGreen Academ doi:.16/j.egpro.2013.07.006
Ghazi Y. Naser et al. / Energ Procedia 36 ( 2013 ) 42 49 43 1. Introduction Cadmium oide CdO is one of transparent conducting oide materials whose thin films are regarded as a material with man attractive properties such as large energ band gap, high optical transparenc in the visible spectral region, remarkable luminescence characteristics etc.. Due to these properties CdO is a promising material for electronic or optoelectronic application, such as solar cells application, photodiodes and gas sensors. Thin films of CdO have been prepared b using several deposition techniques which include chemical vapor deposition, oidation of evaporated metallic film, thermal evaporation, spra prolsis etc. [1-5]. Among these methods, the spra prolsis technique has several advantages such as simplicit, safet, and low cost of the equipments and raw materials. With the spra process, the solution is spraed directl onto the substrate b means of a nozzle assisted b a carrier gas. When the fine droplets arrive at the substrate, the solid compounds react to become a new chemical compound. The qualit and phsical properties of the films depend on the various process parameters, such as substrate temperature, molar concentration of the starting solution, spra rate, pressure of the carrier gas and the geometric characteristics of the spra sstem. In this research work, we report a stud on optical properties of, CdO thin films prepared b spra prolsis technique. 2. Eperimental details CdO films were prepared on crown glass substrates b spra prolsis technique. The spra solution used was of 0.1M of high purit cadmium acetate (6.66gm) dissolved in a miture of 250mL methanol and distilled water (Volume ratio 1 to 1). The atomization of the solution into a spra of fine droplets was carried out b the nozzle, with the help of compresses air as carrier gas. The flow rate of solution was 5ml/min and the substrate temperature was held constant at 473K using a chromel alumel thermocouple with the help of a digital multimeter supplied b Pasco. The nozzle to substrate distance was 25cm and the diameter of nozzle was 0.3mm. The substrates were cleaned b acetone, alcohol and finall with distilled water before coating. Each coated substrate was dried at 473K for min. to evaporate the solvent and remove the organic residuals. The process repeated man times to obtain the desired thicknesses. The heat-treatment of the films done b placing the films into evacuated tube furnace and annealed under law pressure of -1 Torr using a mechanical rotar pump for 1h. Film thickness determined b the weight difference method (d=m/a), where A, is the area of the film, M its mass, d its thickness and its densit (8.15g/cm 3 ), using an electronic precision balance. The optical transmission and absorbance of the films were obtained in the Ultraviolet/Visible/near infrared region up to 00nm using 800 Philips double beam spectrophotometer. 3. Results and discussion Figure 1 shows the optical transmission spectra of as deposited CdO thin films of thicknesses 500nm,450nm.400nm.350nm.300nm ±30.These spectra show that the average transmission decreases from 90% to 70% in the wavelength range 650-800nm with increasing film thickness from 300nm up to 500nm.
44 Ghazi Y. Naser et al. / Energ Procedia 36 ( 2013 ) 42 49, % Transmission T 90 80 70 60 50 40 Series 3 Series 4 Series 5 t 500nm t 450nm t 400nm t 350nm t 300nm 30 20 150 250 300 350 400 450 500 550 600 650 700 750 800 850 900 Wavelength, nm Fig. 1 Transmission spectra of the spraed CdO thin films of different sample thicknesses. Absorbance, A % 90 80 70 60 50 Series 3 Series 4 Series 5 t 500nm t 450nm t 400nm t 350nm t 300nm 40 30 S4 S1 20 150 250 300 350 400 450 500 550 600 650 700 750 800 850 900 Wavelength, nm S2 S3 S5 Fig. 2 Absorbance spectra of the spraed CdO thin films of different sample thicknesses.
Ghazi Y. Naser et al. / Energ Procedia 36 ( 2013 ) 42 49 45 It is clear from figure 2, for the absorption spectra of these samples that the films have high absorbance in the visible region of the spectra. The absorption coefficient () was calculated using Lambert law as follow [6]. = (1/d) 2.303A (1) Where, d is the thickness of the films, A is the optical absorbance. These absorption coefficients values were used to determine optical energ gap. 2 2 9 h ev/cm 16 14 12 8 Series 3 Series 4 Series 5 f()=16.012658*-33.237975; R²=1 f()=14.815789*-30.018421; R²=1 f()=16.307453*-32.90441; R²=0.99 f()=16.1266*-32.921519; R²=1 f()=18.20437*-36.391131; R²=0.99 S1 500nm Eg = 2.08 ev S2 450nm Eg = 2.02 ev S3 400nm Eg = 2.02 ev S4 350nm Eg = 2.04 ev S5 300nm Eg = 2 ev 6 4 2 0.5 1 1.5 2 2.5 3 3.5 h (ev) Fig. 3 Plot of (h) 2 vs. h for spraed CdO thin films of different samples thicknesses.
46 Ghazi Y. Naser et al. / Energ Procedia 36 ( 2013 ) 42 49 90 80, % Transmission T 70 60 50 40 CdO film of thickness 500nm 30 20 Series3 Series4 As deposited (RT ) Heat treated (523 K ) for 1 h Heat treated (548 K ) for 1h Heat treated (573 K ) for 1h 150 250 300 350 400 450 500 550 600 650 700 750 800 850 900 Wavelength, nm Fig. 4 Transmission spectra of the spraed CdO thin films of as deposited and heat treated samples of 500nm±30 thickness Figure 3 shows the plot of (h) 2 vs. h, where h is the energ of the incident photon. The energ gap was estimated b assuming a direct transition between valence and conduction bands from the epression: h =K(h -E g ) 1/2 (2) Where K is a constant, E g is determined b etrapolating the straight line portion of the spectrum to h = 0. From this graph, the optical energ gap of the different film thicknesses were in the rang of 2-2.08 ev and show no dependenc on film thicknesses these values in good agreement with previousl reported data of CdO thin films [7-]. Figure 4 shows the optical transmission spectra of as deposited CdO thin films of thicknesses 500nm±30.These spectra show that the average transmission increases from 70% to 90% within the wavelength range 550-800nm with increasing heat treatment from 523K up to 573K. It is clear from figure 5, for the absorption spectra of these samples, that the films have high absorbance in the visible region of the spectra and the shift toward the UV region with heat treatment increase from 523K to 573K. The films following a rapid linear increase in absorbance starting from about 58% to abut 90% within the wavelength range 455nm to 400nm. Figure 6 shows the plot of (h) 2 vs. h, where h is the energ of the incident photon. From this graph, the optical energ gap value show a distinct increase with increasing heat treatment temperature, it was 2.08eV for as deposited film and increased to 2.2eV for the film heat treated at 523K, then increases to 2.56eV when the film heat treated at 548K, finall the optical energ gap increased to 2.85eV for the film that heat treated at 573K. Figure 7 summarizes these results.
Ghazi Y. Naser et al. / Energ Procedia 36 ( 2013 ) 42 49 47 90 Absorbance, A % 80 70 60 50 40 CdO film of thickness Series3 Series4 500nm As deposited (RT ) Heat treated (523 K ) for 1 h Heat treated (548 K ) for 1h Heat treated (573 K ) for 1h 30 20 150 250 300 350 400 450 500 550 600 650 700 750 800 850 900 Wavelength, nm Fig.5 Absorbance spectra of the spraed CdO thin films of as deposited and heat treated samples of 500nm thickness 16 14 Series 3 Series 4 S1 as deposited S2 Heat treated at 523 K for 1h S3 Heat treated at 548 K for 1h S4 Heat treated at 573K for 1h Eg= 2.08 ev Eg= 2.2 ev Eg= 2.56 ev Eg= 2.85 ev 2 2 9 h ev/cm 12 8 6 4 2 0.5 1 1.5 2 2.5 3 3.5 h (ev) Fig. 6 Plot of (h) 2 vs. h for spraed CdO thin films of as deposited and heat treated samples of 500nm thickness
48 Ghazi Y. Naser et al. / Energ Procedia 36 ( 2013 ) 42 49 (ev) 2.5 E Energ gap g 2 1.5 250 300 350 400 450 500 550 600 Temperature, K Fig.7 Optical energ gap vs. heat treatment of the CdO thin films of as deposited and heat-treated samples of 500nm±30 thickness. The optical absorption edge has been observed at a wavelength of about (590nm) for the band gap of about (2.08eV) for as deposited sample S1, at about 570nm for the band gap of about (2.2eV) for S2 sample which heat treated at 523K, at about 470nm for the band gap of about (2.56eV) for S3 sample that heat treated at 548K and finall at about 425nm for S4 sample that heat treated at 573K. We think that most annealing treatments cause evaporation of ogen and the sample become more Cd-rich. Since the O-rich samples usuall have lower band gap energ [11], relativel more Cd-rich samples after annealing due to the evaporation of ecess ogen from the surface will have higher band gap energies in comparison to the as deposited samples 4. Conclusion CdO thin films of as deposited and heat treated at different temperatures prepared b spra prolsis technique have been studied for their optical properties. 1- Analsis of UV/VIS spectra of the films reveals that the materials to be of direct electronic transition with an energ gap in the range 2.08eV to 2.85eV. 2- The optical characterization on both as deposited and heat-treated samples have shown that the heat treatment carried out at 548 K has a great influence on the optical energ gap. 3- There was no significant change in optical energ gap as a function of films thickness. 4- It has been observed that the direct band gap was increased from 2.08eV, 2.2eV, 2.56eV and to 2.85eV for as deposited, heat treated samples at 523K, 548K and 573K for 1h respectivel. We think that this increase in band gap energ ma be due to the low ogen content of the sample surface after heat treatment.
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