International Journal of Modern Research in Engineering & Management (IJMREM) ||Volume|| 2 ||Issue|| 6 ||Pages|| 17-24 || June 2019 || ISSN: 2581-4540
Influence of Doping on the Structural, Optical Properties and Dispersion Parameters of CdO Thin Films 1, 1,2,3,
Amal A. Khalaf, 2,Salah Q. Hazaa,3,Sabria A. Dhaba
Department of Physics, College of Education, Mustansiriyah University Iraq Baghdad
------------------------------------------------------ABSTRACT---------------------------------------------------Un-doped and bismuth (Bi) doped cadmium oxide (CdO) films were deposited on glass substrates by chemical spray pyrolysis method at (400℃) temperature. The effect of Bi concentration (1,3 and 5 wt %) on the structural, optical properties and dispersion parameters of CdO thin films were investigated. X-ray diffraction patterns indicated that the all films had a polycrystalline with cubic structure and crystallite size increases with the increase of doping percentage. The optical properties and dispersion parameters of films have been studied over a wavelength range (400-900) nm. The optical band gap, refractive index, single-oscillator energy, dispersion energy, moments of the optical spectra (M−1) and (M−3), average oscillator strength, average oscillator wavelength were calculated and analyzed as a function of Bi concentration. It was shown that the doping percentage has significant effect on the properties of CdO thin films.
KEYWORDS: CdO, Bi-CdO, Thin films, Structure, Optical properties, Dispersion parameters. -------------------------------------------------------------------------------------------------------------------------------- ------Date of Submission: Date, 14 July 2019 Date of Publication: 27. July 2019 --------------------------------------------------------------------------------------------------------------------- ------------------
I.
INTRODUCTION
In recent years, there have been many works on the production and investigation of the physical properties of transparent conducting oxide (TCO) materials due to their electrical and optical properties such as low resistivity and high optical transmittance TCOs have great importance in the semiconductor, electronic and optoelectronic devices [1]. Cadmium oxide (CdO) is one of the most important TCO materials; it is an n-type semiconductor with a narrow direct band gap of 2.2 eV to 2.5 eV [2,3]. CdO has several attractive properties, such as its high optical transmittance in the visible region of the solar spectrum [4], low resistivity, high density (8150 Kg/m3 ), high melting point (1500 °C), and has a cubic crystal structure [NaCl, face center cubic (FCC) type, and lattice constant (a = 0.4695 nm) [5,6]. Different methods have been adopted to synthesize CdO thin films, such as spray pyrolysis [7], sputtering [8], chemical bath deposition [9], sol-gel method [10], vapor-liquid-solid (VLS) [11], and solid-vapor deposition method [12,13]. The aim of this study is to study the effects of Bi with different concentration on the structural, optical properties and dispersion parameters of CdO thin films. Experimental Procedure: Chemical spray pyrolysis technique was used to deposit un-doped and Bi-doped (CdO) thin films on glass substrates at temperature of (400 ℃) with volumetric concentration (1,3,5) % the thickness of prepared films was about (420±10) nm. The spray solution was prepared by mixing of (0.1M) aqueous solution of cadmium acetate (Cd(CH3COO)2.2H2O) and (0.1M) aqueous solution (Bi(NO3)3.5H2O).The X-ray diffraction patterns for the prepared films were obtained in a (Shimadzu XRD-6000) using copper target (CuKa,1.5416 Å). The optical transmittance and absorbance spectra were recorded in the wavelength range (400900)nm, using double beam (SCHIMADZU UV/VIS-160 Å), all measurements were carried out at room temperature.
II.
RESULTS AND DISCUSSION
Structural analysis: The X-ray diffraction patterns of undoped and Bi doped (CdO) deposited at (400 ℃) are shown in figure (1). The XRD patterns revealed that all the films are polycrystalline in nature. The position of the diffraction peaks fit well with the cubic structure of pure CdO (ASTM: American standard for testing materials) corresponding to (111), (200), and (220), planes. It is seen that the preferential orientation is along the (111) plane for all the films, this result is good agreement with others [14, 15]. Also, the result showed that the intensity of (111) peak is increased and peak becomes sharper with increasing Bi concentration, this can be explained by improvement of crystallinity. This suggests that the grain size becomes larger [16]. The crystallite size D was calculated using the Scherrer’s equation (1) [16,17] and the estimated values are presented in table (1).
D=
0.9 λ
1
β cos(θ)
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Influence of Doping on the Structural, Optical Properties…
Where, λ is the wavelength of X-rays(1.5406 Å), β is the full width at half maximum (rad) and θ is the Bragg diffraction angle of the XRD peak (degree). The crystallite size varies from (25 – 38) nm can be noticed from table (1), which is well supported in literature [16], This implies that doping enhances crystallinity of thin film. Improvement in crystal structure could be attributed to the increase in crystallite size as the small crystallites join each other in the planes by increase doping percentage. The values of lattice constant a were calculated using equation (2) and the calculated values are given in table (1), it is seen that the calculated values are in good agreement with the standard values for CdO cubic structure 1 d2
=
h2 +k2 +l2
2
a2
Where, d is the interplanar spacing and h, k, and l are the Miller indices
Figure (1): XRD pattern of undoped and Bi-doped CdO thin films Table (1) structure parameters of CdS
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Influence of Doping on the Structural, Optical Properties‌
Sample
2 θ(degree)
dhkl(Ă…)
a(nm)
CdO ASTM
33.001
2.7120
0.4695
Pure
33.407
2.6774
0.4637
25
1%
33.4029
2.6786
0.4639
29.5
3%
33.389
2.6805
0.4642
34.62
5%
33.395
2.6736
0.4630
38
D(nm)
Optical properties: Transmittance spectra observed of undoped and Bi doped of CdO thin films are presented in fig. (2), using the measured values of transmission (T) the absorption coefficient (ι) can be written in the form [16,17]. 1 1 � = � ln(� ) 3 Where d is the film thickness
T %
60
40
20
0 300
400
500
600
700
800
900
Wavelength nm
Figure (2) transmittance versus wavelength for undoped and Bi doped CdO thin films. The absorption coefficient can be used to determine the optical band gap Eg following equation [16,17]: đ?›źâ„Žđ?‘Ł= A(â„Žđ?‘Łâˆ’đ??¸đ?‘”)r
4
Where A is a constant, hν is the photon energy and r is index depends on the type of the electronic transitions, where r is equal to 1/2, 3/2, 2 and 3 for allowed direct, forbidden direct, indirect allowed and indirect forbidden transitions, respectively. The optical band gap can be obtained by extrapolating the linear portion of the plots of (ιhʋ)r versus hʋ to (ιhʋ)r = 0 . Using the value r = 1/2, the relation found to be straight line as shown in fig. (3) indicating a direct optical transition. The values of allowed direct band gaps calculated from the graphs and www.ijmrem.com
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they are listed in the table (2). the result shows a decrease in the band gap with increasing doping percentage due to the crystallite growth and decreases in defect states near the bands. 4.E+10
pure 1%
(ÎąhĘ‹)2 (cm−1 eV)
2
3 % 5%
3.E+10
2.E+10
1.E+10
0.E+00 1.7
2.2
2.7
3.2
hʋ eV
Figure (3) Allowed direct energy gap for undoped and Bi doped CdO thin films. Optical parameters constant and dispersion parameters: The optical
parameters refractive index (n) and extinction coefficient (k), have been determined according to Taus model [16,17]: 1
ďƒŠďƒŚ 1 + R ďƒś 2 ďƒš 2 1+ R 2 n = ďƒŞďƒ§ ďƒˇ − k +1 ďƒş + ďƒŤďƒŞďƒ¨ 1 − R ďƒ¸ ďƒťďƒş 1 − R
(
)
5
đ?›źđ?œ†
đ?‘˜ = 4đ?œ‹
6
Refractive index
The refractive index and extinction coefficient were plotted in fig. (4) and fig. (5) respectively as a function of the wavelength. It is seen that both the refractive index as well as the extinction coefficient of the CdO films are increased as a result of doping. This can be attributed to the decrease in optical energy gap with doping.
8 6 4 2 0 300
400
500
600
700
800
900
Wavelength nm
Figure (4) Reactive index versus wavelength for undoped and Bi doped CdO thin films. www.ijmrem.com
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Excitation coefficient
Influence of Doping on the Structural, Optical Properties‌ pure
0.02
1%
0.016
3% 5%
0.012 0.008 0.004 0 300
400
500
600
700
800
900
Wavelength nm
Figure (5) Extinction coefficient versus wavelength for undoped and Bi doped CdO thin films. According to the single effective-oscillator model proposed by Wemple–DiDomenico is used to analyze experimental data of refractive index, which usually provides physically significant quantities such as the oscillator energy in the interband transition region. The dispersion behavior, as we all know, plays an important role in the research for optical materials, because it is a significant factor in optical communication and designing of the devices for spectral dispersion. The optical data can be described to an excellent approximation by the relation [18]: E E n 2 −1 = 2d o 7 Eo − hď ľ Where Eo is the energy of the effective dispersion oscillator, Ed is the dispersion energy or single oscillator constant, which is a measure of the intensity of the interband optical transitions. The curves of (n2 -1)-1 versus (â„Žđ?‘Ł)2 for the undoped and Bi doped CdO films are plotted in Fig. (6) and the data are fitted into straight lines, indicating the W–D dispersion model is applicable to all films in the present work. The values of E0 and Ed can be determined from the slope (1/E0Ed) and intercept (E0/Ed) on the vertical axis. It is clear that E0 decrease with increase doping percentage, and has the same behavior of energy gap. While the values of Ed increased. We found that Eo was related to the direct band gap by Eo ≈ (1.4-1.5) Eg, this is good agreement with the result of other workers [19,20].
(đ?‘›2 -1)−1
0.2 0.15 0.1 0.05 0 5.5
6.5
7.5
(hʋ)
2
8.5
9.5
2
(��)
Figure (6) plot between (n2-1)-1 and (he)2 www.ijmrem.com
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Influence of Doping on the Structural, Optical Properties‌
According to the single-oscillator model, the moments of the optical spectrum M−1and M−3can be calculated from the following relations and their values are listed in table (2) for all samples[21]: M Eo2 = M−1 8 −3
Ed2
M3−1
=M
9
−3
The values of the static refractive index (ns) can be calculated by extrapolating the WempleDiDomenico dispersion equation (7) to hʋ → 0[22] n2s = 1+Ed/Eo
10
Then the values of static dielectric constant are equal to the square of the values of static refractive index and they listed in the table (2) đ?œ€đ?‘ =ns2
11
The refractive index can also be analyzed using Sellmeir dispersion formula that is given by[19,23]: n2 − 1 =
Îť2o So
12
2
Îť 1−( o ) Îť
Where So is the oscillator strength, and Îťo is the average oscillator wavelength. Fig. (7) shows the relation between (n2 -1)-1 versus 1/ Îť2 for the CdO films. Values of So and Îťo can be estimated from the slope (1/So) and the infinite wavelength intercept (1/SoÎťo2). The value of So and Îťo are given in table (2). 0.3
pure 1%
(đ?‘›2 -1)−1
3%
0.2 5%
0.1
0 4.E-06
5.E-06
6.E-06
7.E-06
Îťâˆ’2 (đ?‘›đ?‘š)−2
Figure (7) plot between (đ?‘›2 -1)−1 and Îťâˆ’2
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Influence of Doping on the Structural, Optical Properties‌
Table (2) optical parameters of undoped and Bi-doped CdO thin films Sample
ns
đ?‘şđ?’? x1012 đ?’Žâˆ’đ?&#x;?
đ??€đ?’? đ?’™đ?&#x;?đ?&#x;Ž -7 m
9.06
3.011
4.9
3.4
0.63
9.54
3.08
8.4
3.26
9.2
0.73
10.4
3.2
22.4
2.95
9.7
0.8
10.79
3.28
31.51
2.44
Eg eV
Eo eV
Ed eV
M-1
M-3 (eV)-2
Pure
2.61
3.78
30.5
8
0.56
1%
2.52
3.65
31.2
8.5
3%
2.44
3.53
32.5
5%
2.41
3.48
34.1
III.
đ?œşđ??Ź
CONCLUSIONS
The effects of doping on the structural and the optical properties of CdO thin films prepared by chemical spray pyrolysis technique on glass were investigated. The size of the crystallites was found to be in the range of (25 – 38) nm. The optical energy gap was found to decrease with increase the doping percentage. The single oscillator parameters were determined. It was shown that the dispersion parameters of the films obeyed the single oscillator model. From structural and optical measurements, it was observed that CdO thin films were mostly suitable for opto-electronic devices fabrication as the window layer in solar cells applications. REFERENCE
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