- Email: [email protected]
Effect of heat treatment on physical properties of CdO films deposited by sol–gel method
international journal of hydrogen energy 34 (2009) 5191–5195
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Effect of heat treatment on physical properties of CdO films deposited by sol–gel method Seval Aksoy, Yasemin Caglar*, Saliha Ilican, Mujdat Caglar Anadolu University, Faculty of Science, Department of Physics, 26470 Eskisehir, Turkey
article info
abstract
Article history:
CdO film has been deposited by sol–gel spin coating method on the glass substrate and
Received 1 September 2008
then the film has been annealed at 400, 500, 600 C for 1 h. Effect of annealing temperature
Accepted 17 September 2008
on the structural and optical properties of the film has been investigated. The crystal
Available online 14 November 2008
structure and orientation of the as-grown and annealed CdO films have been investigated by X-ray diffraction method. Annealed CdO films are polycrystalline with (111) preferential
Keywords:
orientation. The information on strain and grain size is obtained from the full width-at-
CdO
half-maximum (FWHM) of the diffraction peaks. Texture coefficient and lattice constant
Sol–gel method
have been calculated. The surface morphology of the films has been analyzed. The optical
Crystal structure
band gap value decreased with increasing the annealing temperatures.
Optical band gap
ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The films of transparent conductive oxides (TCO) such as zinc oxide, indium–tin oxide, tin oxide and cadmium oxide (CdO) have been extensively studied because of their use in semiconductor optoelectronic device technology [1]. Among these TCO, CdO films have been successfully used for many applications, including use in gas sensor devices, photo diodes, transparent electrodes, photo transistors, and photovoltaic solar cells [2]. CdO is an n-type semiconductor with a rock-salt crystal structure, possesses a direct band gap of 2.2 eV [3]. Beside, CdO shows very high electrical conductivity even without doping due to the existence of shallow donors caused by intrinsic interstitial cadmium atoms and oxygen vacancies [4]. CdO will be attractive in the field of optoelectronic devices by making heterostructures with ZnO which has a band gap energy of 3.3 eV and the largest exciton binding energy of 60 meV in the II–IV materials family [5]. Different physical and chemical deposition techniques such as spray pyrolysis [6], sol–gel [7], DC magnetron sputtering [8], radio frequency (RF) sputtering [9], chemical bath deposition [10], etc. have been employed to prepare CdO films.
In this work, CdO film has been deposited by sol–gel method, which is a very simple and economical method. The crystalline structure and optical properties of the films were studied as a function of the annealing temperature (Ta) from 400 to 600 C.
2.
Experimental details
CdO film has been deposited by sol–gel method onto glass substrate. The CdO precursor solutions were prepared starting from cadmium acetate dehydrated (1 mol), methanol (46 mol), glycerol (0.2 mol), triethylamine (0.5 mol). The undoped CdO precursor solution was prepared by the following procedure: a) the cadmium acetate was dissolved in half of the methanol (23 mol for each mol of cadmium acetate) at constant magnetic stirring, until a transparent solution was obtained; b) the glycerol was added to the solution; c) the triethylamine previously dissolved in the other half of the methanol was also incorporated. The solution was stirred constantly during its preparation. The procedure was done entirely at room
* Corresponding author. Tel.: þ90 222 3350580. E-mail address: [email protected] (Y. Caglar). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.09.057
5192
international journal of hydrogen energy 34 (2009) 5191–5195
1400
Nomenclature FWSHM (b) full width-at-half-maximum of the diffraction peaks q Bragg angle d lattice spacing Dd differences between observed and standard d values a lattice constant TC texture coefficient D grain size strain 3s NRF Nelson–Riley function I(hkl ) measured relative intensity of a plane, hkl Io(hkl ) standard intensity of the plane, hkl optical band gap Eg
600oC 1200
(111) 1000 (200)
Results and discussion
3.1.
Crystal structure of the CdO films
(311)
Intensity
800
600
temperature. The solution was stirred at 60 C for 2 h to yield a clear and homogeneous solution, which served as the coating solution after cooling to room temperature. The glass substrate was precleaned with a detergent, and then cleaned in methanol and acetone for 10 min each by using an ultrasonic cleaner and then cleaned with deionized water and dried. The coating solution was dropped onto glass substrate, which was rotated at 2000 rpm for 30 s by using spin coater. After the deposition, the film was dried at 100 C for 10 min into a furnace to evaporate the solvent and remove organic residuals. The procedures from coating to drying were repeated five times. The film was then inserted into a tube furnace and annealed in air at 300 C for 45 min. After the CdO film deposited, the film was annealed at 400, 500 and 600 C for 1 h. The thickness of the films were determined with Mettler Toledo MX5 microbalance by using weighing method and found to be 300 nm. X-ray diffraction patterns were obtained with a Rigaku Rint 2200 Series X-Ray Automatic Diffractom˚ ) in the range of 2q eter using the Cu Ka radiations (l ¼ 1.54059 A between 20 and 60 . Surface morphology was studied using Zeiss Supra 50VP model scanning electron microscope (SEM). Optical transmittance measurements were recorded with a double beam Shimadzu UV 2450 spectrophotometer with an integrating sphere in the wavelength range 190–900 nm. The transmission was measured using a glass as reference.
3.
500oC
(220)
400oC
400
200
as-grown
0 20
30
40
50
60
70
2θ (degree) Fig. 1 – XRD pattern of the as-grown and annealed CdO film.
indicate that the CdO films have a polycrystalline structure. For the CdO films, the main characteristic peaks are assigned to the (111), (200), (220) and (311) planes. A light preferential orientation in the (111) plane is observed for the CdO films and similar behavior has also been reported by other researchers [11]. The full width half maximum (b), angle of diffraction (2q), lattice spacing (d), the differences (Dd), between observed and standard d values, lattice constant (a), texture coefficient (TC), grain size (D) and strain of the as-grown and annealed CdO films are given in Table 1. Observed d values for standard CdO film are smaller than the standard d values due to the oxygen vacancies in the lattice. As seen in Fig. 1, the best crystallinity belongs to the film annealed at 400 C. The structure of the film becomes bad when the annealing temperature increases, and the film has amorphous structure at 600 C annealed temperature.
X-ray diffraction (XRD) spectrum of the as-grown and annealed CdO films is shown in Fig. 1. The X-ray results
Table 1 – The structural parameters of the as-grown and annealed CdO film. Film As-grown 400 C 500 C
FWHM
2q
d
Dd
˚) a (A
TC (111)
˚) D (A
3 103
0.420 0.323 0.337
33.119 33.080 33.117
2.7027 2.7058 2.7029
0.0093 0.0062 0.0091
4.7083 4.6961 4.695
1.77 1.80 1.64
168.0 448.4 315.4
6.5 7.1 3.3
international journal of hydrogen energy 34 (2009) 5191–5195
5193
4.71
lattice constant, a (A°)
4.70 4.70 4.69 4.69 as-grown CdO film 400 °C annealed CdO film 500 °C annealed CdO film
4.68 4.68 1.0
1.5
2.0
2.5
3.0
3.5
NRF Fig. 2 – Nelson–Riley plot for the as-grown and annealed CdO film.
In order to obtain the true value of the lattice constant, we use Nelson–Riley function (NRF) defined by the following relation [12,13] 1 cos2 q cos2 q þ (1) NRF ¼ 2 sin q q where q is the Bragg angle. Fig. 2 shows the plot of a vs. NRF for the CdO films. Extrapolation to NRF ¼ 0 yields the true lattice parameter, ao. The true lattice parameters of the as-grown and annealed CdO films are shown in Table 1. It is evaluated that the true lattice parameter may be taken as the average value for all the reflecting planes in the X-ray spectrum. It is observed that the lattice constant value of CdO film which was annealed 400 C is almost same to the JCPDS data (Card no: ˚. 005-0640) value of 4.6953 A The average grain size and strain for the CdO films can be determined using the equation [12,13], b¼
l 3s tan q D cos q
(2)
where b ¼ FWHM and q the Bragg angle, D grain size and 3s is the strain. Fig. 3 shows a plot of (b cos l)/l vs. (sin q)/l for the various reflecting planes. The D and 3s values were determined from the intercept and slope of Fig. 3 and given in Table 1. It
0.06
(βcosθ) / λ
0.05 0.04
Fig. 4 – SEM images of the (400, 500 and 600 8C) annealed CdO film (30,000 magnifications).
0.03 0.02 as-grown CdO film 400 ºC annealed CdO film
0.01
500 ºC annealed CdO film 0.00 1.5
2.0
2.5
3.0
3.5
(sinθ)/λ Fig. 3 – (b cos q)/l vs. (sin q)/l plot of the as-grown and annealed CdO film.
4.0
was observed that the CdO film annealed at 400 C has the highest grain size. The texture coefficient (TC) represents the texture of the particular plane, deviation of which from unity implies the preferred growth. The different texture coefficients TC(hkl ) have been calculated from the X-ray data using the wellknown formula [14]
5194
international journal of hydrogen energy 34 (2009) 5191–5195
show that as-grown film has higher transmittance than the others and the transmittance values reduced by amount of 10% when the films were annealed. In order to show the effects of annealing temperature, the absorption edge was investigated for the film annealed at different temperatures. The optical absorption edge was determined by the optical absorption, a simple method that provides explanation for the features concerning the band structure of the film. The optical absorption edge was analyzed by the following relationship [15], 1=2 (4) ahn ¼ A hn Eg
100
80
T%
60
40 as-grown CdO film 20
400oC annealed CdO film 500oC annealed CdO film
0 400
500
600
700
800
wavelength (nm) Fig. 5 – The transmittance spectra of the as-grown and annealed CdO film.
TCðhklÞ ¼
IðhklÞ=Io ðhklÞ P N1 IðhklÞ=Io ðhklÞ
(3)
where A is a constant, hn is the photon energy and Eg is the optical band gap. Fig. 6 shows the plots of (ahn)2 vs. hn. The optical band gap values were calculated from this plot and were found to be about 2.47 eV, 2.29 eV and 2.22 eV for asgrown and annealed at 400 and 500 C, respectively. The optical band gaps of the film decreased with the increasing annealing temperature. This result can be attributed to the thermally induced defects which increase dramatically. The values of the band gaps are in agreement with the values reported in the literature for CdO [16,17].
n
where I(hkl ) is the measured relative intensity of a plane (hkl ), Io(hkl ) is the standard intensity of the plane (hkl ) taken from the JCPDS data, N is the reflection number and n is the number of diffraction peaks. The variations of texture coefficient calculated for the diffraction peaks of as-grown and annealed CdO films, i.e. (1 1 1), (2 0 0), (2 2 0) and (3 1 1) are shown in Table 1. The CdO film annealed at 400 C has the highest texture coefficient. Fig. 4 shows scanning electron micrographs (SEM) of annealed CdO films at 30,000 magnifications. It is seen that well-crystallized grains in the first image belongs to annealed 400 C. As the annealing temperature was increased the crystallite structure of the film deteriorated and the structure was amorphous in the last image. This is in agreement with the XRD results. The transmission spectra of as-grown and annealed films are shown in Fig. 5. The optical transmittance measurements 1.0
4.
Conclusion
The CdO film was deposited by sol–gel spin-coating method. The structural and optical properties of the CdO film were influenced by annealing process. XRD results showed that asgrown and annealed CdO films have (111) preferred orienta˚ and tion. The lattice parameter was estimated to be 4.6961 A ˚ for the CdO films annealed at 400 and 500 C using the 4.695 A Nelson–Riley plot. The optical band gap values were found to decrease from 2.47 to 2.22 eV with increasing of the annealing temperature. We can say from both the structural and optical results that 400 C annealing temperature is suitable for production of good CdO film. In addition, it is important to note down that a specific annealing temperature (in our case 400 C) is sufficient to get the improved properties of CdO film for solar cell applications.
Acknowledgement
as-grown CdO film 400oC annealing CdO film
(αhν)2x1014 (eV/m2)
This work was supported by Anadolu University Commission of Scientific Research Projects under Grant No. 061039.
500oC annealing CdO film
0.8
0.6
references 0.4
0.2
0.0 1.5
2.0
2.5
hν (eV) Fig. 6 – The plots of (ahn)2 vs. photon energy of the asgrown and annealed CdO film.
3.0
[1] Champness CH, Chan CH. Optimization of CdO layer in a Se– CdO photovoltaic cell. Sol. Energy Mater. Sol. Cells 1995;37(1): 75–92. [2] Liu X, Xu Z, Shen Y. Proceedings of the international conference on solid-state sensors and actuators. Lisbon; 1997. p. 585. [3] Ortega M, Santana G, Morales-Acevedo A. Optoelectronic properties of CdO/Si photodetectors. Solid State Electron 2000;44(10):1765–9. [4] Haul R, Just D. Disorder and oxygen transport in cadmium oxide. J Appl Phys 1962;33:487.
international journal of hydrogen energy 34 (2009) 5191–5195
[5] Ashrafi ABMA, Kumano H, Suemune I, Ok Y-W, Seong T-Y. Single-crystalline rocksalt CdO layers grown on GaAs(001) substrates by metalorganic molecular-beam epitaxy. Appl Phys Lett 2001;79(4):470–2. [6] Gurumurugan K, Mangalaraj D, Narayandass SaK, Sekar K, Girija Vallabhan CP. Characterization of transparent conducting CdO films deposited by spray-pyrolysis. Semicond Sci Technol 1994;9(10):1827–32. [7] Ghosh PK, Das S, Kundoo S, Chattopadhyay KK. Effect of fluorine doping on semiconductor to metal-like transition and optical properties of cadmium oxide thin films deposited by sol-gel process. J Sol–Gel Sci Technol 2005;34(2): 173–9. [8] Subramanyam TK, Srinivasulu Naidu B, Uthanna S. Studies on dc magnetron sputtered cadmium oxide films. Appl Surf Sci 2001;169:529–34. [9] Ueda N, Meada H, Hosono H, Kawazoe H. Band-gap widening of CdO thin films. J Appl Phys 1998;84:6174. [10] Varkey AJ, Fort AF. Transparent conducting cadmium-oxide thin-films prepared by a solution growth technique. Thin Solid Films 1994;239(2):211–3.
5195
[11] Eze FC. Oxygen partial pressure dependence of the structural properties of CdO thin films deposited by a modified reactive vacuum evaporation process. Mater Chem Phys 2005;89(2–3): 205–10. [12] Gurumurugan K, Mangalaraj D, Narayandass SaK. Magnetron sputtered transparent conducting CdO thin films. J Electron Mater 1996;25(4):765–70. [13] King HP, Alexander LE. X-ray diffraction procedures for polycrystalline and amorphous materials. New York: Wiley; 1994. [14] Barret CS, Massalski TB. Structure of metals. Oxford: Pergamon Press; 1980. [15] Mott NF, Gurney RW. Electronic processes in ionic crystals. London: Oxford Univ. Press; 1940. [16] Ran F, Miao L, Tanemura S, Tanemura M, Cao Y, Tanaka S, Shibata N. Effect of annealing temperature on optical properties of Er-doped ZnO films prepared by sol–gel method. Mat Sci Eng B 2008;148:35–9. [17] Gurumurugan K, Mangalaraj D, Narayandass SaK, Nakanishi Y. DC reactive magnetron sputtered CdO thin films. Mater Lett 1996;28(4–6):307–12.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Effect of heat treatment on physical properties of CdO films deposited by sol–gel method Seval Aksoy, Yasemin Caglar*, Saliha Ilican, Mujdat Caglar Anadolu University, Faculty of Science, Department of Physics, 26470 Eskisehir, Turkey
article info
abstract
Article history:
CdO film has been deposited by sol–gel spin coating method on the glass substrate and
Received 1 September 2008
then the film has been annealed at 400, 500, 600 C for 1 h. Effect of annealing temperature
Accepted 17 September 2008
on the structural and optical properties of the film has been investigated. The crystal
Available online 14 November 2008
structure and orientation of the as-grown and annealed CdO films have been investigated by X-ray diffraction method. Annealed CdO films are polycrystalline with (111) preferential
Keywords:
orientation. The information on strain and grain size is obtained from the full width-at-
CdO
half-maximum (FWHM) of the diffraction peaks. Texture coefficient and lattice constant
Sol–gel method
have been calculated. The surface morphology of the films has been analyzed. The optical
Crystal structure
band gap value decreased with increasing the annealing temperatures.
Optical band gap
ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The films of transparent conductive oxides (TCO) such as zinc oxide, indium–tin oxide, tin oxide and cadmium oxide (CdO) have been extensively studied because of their use in semiconductor optoelectronic device technology [1]. Among these TCO, CdO films have been successfully used for many applications, including use in gas sensor devices, photo diodes, transparent electrodes, photo transistors, and photovoltaic solar cells [2]. CdO is an n-type semiconductor with a rock-salt crystal structure, possesses a direct band gap of 2.2 eV [3]. Beside, CdO shows very high electrical conductivity even without doping due to the existence of shallow donors caused by intrinsic interstitial cadmium atoms and oxygen vacancies [4]. CdO will be attractive in the field of optoelectronic devices by making heterostructures with ZnO which has a band gap energy of 3.3 eV and the largest exciton binding energy of 60 meV in the II–IV materials family [5]. Different physical and chemical deposition techniques such as spray pyrolysis [6], sol–gel [7], DC magnetron sputtering [8], radio frequency (RF) sputtering [9], chemical bath deposition [10], etc. have been employed to prepare CdO films.
In this work, CdO film has been deposited by sol–gel method, which is a very simple and economical method. The crystalline structure and optical properties of the films were studied as a function of the annealing temperature (Ta) from 400 to 600 C.
2.
Experimental details
CdO film has been deposited by sol–gel method onto glass substrate. The CdO precursor solutions were prepared starting from cadmium acetate dehydrated (1 mol), methanol (46 mol), glycerol (0.2 mol), triethylamine (0.5 mol). The undoped CdO precursor solution was prepared by the following procedure: a) the cadmium acetate was dissolved in half of the methanol (23 mol for each mol of cadmium acetate) at constant magnetic stirring, until a transparent solution was obtained; b) the glycerol was added to the solution; c) the triethylamine previously dissolved in the other half of the methanol was also incorporated. The solution was stirred constantly during its preparation. The procedure was done entirely at room
* Corresponding author. Tel.: þ90 222 3350580. E-mail address: [email protected] (Y. Caglar). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.09.057
5192
international journal of hydrogen energy 34 (2009) 5191–5195
1400
Nomenclature FWSHM (b) full width-at-half-maximum of the diffraction peaks q Bragg angle d lattice spacing Dd differences between observed and standard d values a lattice constant TC texture coefficient D grain size strain 3s NRF Nelson–Riley function I(hkl ) measured relative intensity of a plane, hkl Io(hkl ) standard intensity of the plane, hkl optical band gap Eg
600oC 1200
(111) 1000 (200)
Results and discussion
3.1.
Crystal structure of the CdO films
(311)
Intensity
800
600
temperature. The solution was stirred at 60 C for 2 h to yield a clear and homogeneous solution, which served as the coating solution after cooling to room temperature. The glass substrate was precleaned with a detergent, and then cleaned in methanol and acetone for 10 min each by using an ultrasonic cleaner and then cleaned with deionized water and dried. The coating solution was dropped onto glass substrate, which was rotated at 2000 rpm for 30 s by using spin coater. After the deposition, the film was dried at 100 C for 10 min into a furnace to evaporate the solvent and remove organic residuals. The procedures from coating to drying were repeated five times. The film was then inserted into a tube furnace and annealed in air at 300 C for 45 min. After the CdO film deposited, the film was annealed at 400, 500 and 600 C for 1 h. The thickness of the films were determined with Mettler Toledo MX5 microbalance by using weighing method and found to be 300 nm. X-ray diffraction patterns were obtained with a Rigaku Rint 2200 Series X-Ray Automatic Diffractom˚ ) in the range of 2q eter using the Cu Ka radiations (l ¼ 1.54059 A between 20 and 60 . Surface morphology was studied using Zeiss Supra 50VP model scanning electron microscope (SEM). Optical transmittance measurements were recorded with a double beam Shimadzu UV 2450 spectrophotometer with an integrating sphere in the wavelength range 190–900 nm. The transmission was measured using a glass as reference.
3.
500oC
(220)
400oC
400
200
as-grown
0 20
30
40
50
60
70
2θ (degree) Fig. 1 – XRD pattern of the as-grown and annealed CdO film.
indicate that the CdO films have a polycrystalline structure. For the CdO films, the main characteristic peaks are assigned to the (111), (200), (220) and (311) planes. A light preferential orientation in the (111) plane is observed for the CdO films and similar behavior has also been reported by other researchers [11]. The full width half maximum (b), angle of diffraction (2q), lattice spacing (d), the differences (Dd), between observed and standard d values, lattice constant (a), texture coefficient (TC), grain size (D) and strain of the as-grown and annealed CdO films are given in Table 1. Observed d values for standard CdO film are smaller than the standard d values due to the oxygen vacancies in the lattice. As seen in Fig. 1, the best crystallinity belongs to the film annealed at 400 C. The structure of the film becomes bad when the annealing temperature increases, and the film has amorphous structure at 600 C annealed temperature.
X-ray diffraction (XRD) spectrum of the as-grown and annealed CdO films is shown in Fig. 1. The X-ray results
Table 1 – The structural parameters of the as-grown and annealed CdO film. Film As-grown 400 C 500 C
FWHM
2q
d
Dd
˚) a (A
TC (111)
˚) D (A
3 103
0.420 0.323 0.337
33.119 33.080 33.117
2.7027 2.7058 2.7029
0.0093 0.0062 0.0091
4.7083 4.6961 4.695
1.77 1.80 1.64
168.0 448.4 315.4
6.5 7.1 3.3
international journal of hydrogen energy 34 (2009) 5191–5195
5193
4.71
lattice constant, a (A°)
4.70 4.70 4.69 4.69 as-grown CdO film 400 °C annealed CdO film 500 °C annealed CdO film
4.68 4.68 1.0
1.5
2.0
2.5
3.0
3.5
NRF Fig. 2 – Nelson–Riley plot for the as-grown and annealed CdO film.
In order to obtain the true value of the lattice constant, we use Nelson–Riley function (NRF) defined by the following relation [12,13] 1 cos2 q cos2 q þ (1) NRF ¼ 2 sin q q where q is the Bragg angle. Fig. 2 shows the plot of a vs. NRF for the CdO films. Extrapolation to NRF ¼ 0 yields the true lattice parameter, ao. The true lattice parameters of the as-grown and annealed CdO films are shown in Table 1. It is evaluated that the true lattice parameter may be taken as the average value for all the reflecting planes in the X-ray spectrum. It is observed that the lattice constant value of CdO film which was annealed 400 C is almost same to the JCPDS data (Card no: ˚. 005-0640) value of 4.6953 A The average grain size and strain for the CdO films can be determined using the equation [12,13], b¼
l 3s tan q D cos q
(2)
where b ¼ FWHM and q the Bragg angle, D grain size and 3s is the strain. Fig. 3 shows a plot of (b cos l)/l vs. (sin q)/l for the various reflecting planes. The D and 3s values were determined from the intercept and slope of Fig. 3 and given in Table 1. It
0.06
(βcosθ) / λ
0.05 0.04
Fig. 4 – SEM images of the (400, 500 and 600 8C) annealed CdO film (30,000 magnifications).
0.03 0.02 as-grown CdO film 400 ºC annealed CdO film
0.01
500 ºC annealed CdO film 0.00 1.5
2.0
2.5
3.0
3.5
(sinθ)/λ Fig. 3 – (b cos q)/l vs. (sin q)/l plot of the as-grown and annealed CdO film.
4.0
was observed that the CdO film annealed at 400 C has the highest grain size. The texture coefficient (TC) represents the texture of the particular plane, deviation of which from unity implies the preferred growth. The different texture coefficients TC(hkl ) have been calculated from the X-ray data using the wellknown formula [14]
5194
international journal of hydrogen energy 34 (2009) 5191–5195
show that as-grown film has higher transmittance than the others and the transmittance values reduced by amount of 10% when the films were annealed. In order to show the effects of annealing temperature, the absorption edge was investigated for the film annealed at different temperatures. The optical absorption edge was determined by the optical absorption, a simple method that provides explanation for the features concerning the band structure of the film. The optical absorption edge was analyzed by the following relationship [15], 1=2 (4) ahn ¼ A hn Eg
100
80
T%
60
40 as-grown CdO film 20
400oC annealed CdO film 500oC annealed CdO film
0 400
500
600
700
800
wavelength (nm) Fig. 5 – The transmittance spectra of the as-grown and annealed CdO film.
TCðhklÞ ¼
IðhklÞ=Io ðhklÞ P N1 IðhklÞ=Io ðhklÞ
(3)
where A is a constant, hn is the photon energy and Eg is the optical band gap. Fig. 6 shows the plots of (ahn)2 vs. hn. The optical band gap values were calculated from this plot and were found to be about 2.47 eV, 2.29 eV and 2.22 eV for asgrown and annealed at 400 and 500 C, respectively. The optical band gaps of the film decreased with the increasing annealing temperature. This result can be attributed to the thermally induced defects which increase dramatically. The values of the band gaps are in agreement with the values reported in the literature for CdO [16,17].
n
where I(hkl ) is the measured relative intensity of a plane (hkl ), Io(hkl ) is the standard intensity of the plane (hkl ) taken from the JCPDS data, N is the reflection number and n is the number of diffraction peaks. The variations of texture coefficient calculated for the diffraction peaks of as-grown and annealed CdO films, i.e. (1 1 1), (2 0 0), (2 2 0) and (3 1 1) are shown in Table 1. The CdO film annealed at 400 C has the highest texture coefficient. Fig. 4 shows scanning electron micrographs (SEM) of annealed CdO films at 30,000 magnifications. It is seen that well-crystallized grains in the first image belongs to annealed 400 C. As the annealing temperature was increased the crystallite structure of the film deteriorated and the structure was amorphous in the last image. This is in agreement with the XRD results. The transmission spectra of as-grown and annealed films are shown in Fig. 5. The optical transmittance measurements 1.0
4.
Conclusion
The CdO film was deposited by sol–gel spin-coating method. The structural and optical properties of the CdO film were influenced by annealing process. XRD results showed that asgrown and annealed CdO films have (111) preferred orienta˚ and tion. The lattice parameter was estimated to be 4.6961 A ˚ for the CdO films annealed at 400 and 500 C using the 4.695 A Nelson–Riley plot. The optical band gap values were found to decrease from 2.47 to 2.22 eV with increasing of the annealing temperature. We can say from both the structural and optical results that 400 C annealing temperature is suitable for production of good CdO film. In addition, it is important to note down that a specific annealing temperature (in our case 400 C) is sufficient to get the improved properties of CdO film for solar cell applications.
Acknowledgement
as-grown CdO film 400oC annealing CdO film
(αhν)2x1014 (eV/m2)
This work was supported by Anadolu University Commission of Scientific Research Projects under Grant No. 061039.
500oC annealing CdO film
0.8
0.6
references 0.4
0.2
0.0 1.5
2.0
2.5
hν (eV) Fig. 6 – The plots of (ahn)2 vs. photon energy of the asgrown and annealed CdO film.
3.0
[1] Champness CH, Chan CH. Optimization of CdO layer in a Se– CdO photovoltaic cell. Sol. Energy Mater. Sol. Cells 1995;37(1): 75–92. [2] Liu X, Xu Z, Shen Y. Proceedings of the international conference on solid-state sensors and actuators. Lisbon; 1997. p. 585. [3] Ortega M, Santana G, Morales-Acevedo A. Optoelectronic properties of CdO/Si photodetectors. Solid State Electron 2000;44(10):1765–9. [4] Haul R, Just D. Disorder and oxygen transport in cadmium oxide. J Appl Phys 1962;33:487.
international journal of hydrogen energy 34 (2009) 5191–5195
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