International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 3, March 2014
ISSN 2319 - 4847
Effects of Cd, F and Fe dopants on the physical
properties of ZnO thin films synthesized by
spray pyrolysis
Salam Amir Yousif
Department of Physics, College of Education, the University of Mustansiriyah, Baghdad, Iraq
ABSTRACT
Effect of Cd, Fe and F dopants on the structural, morphological and optical properties of ZnO thin film deposited on glass
substrate at a substrate temperature equal to 450 oC by spray pyrolysis technique have been investigated. X-ray diffraction spectra
indicate that the films have polycrystalline nature with a hexagonal wurtzite structure, all the doped ZnO thin films have (002)
reflection as the preferred orientation along with the other (100), (101), (102), (110) and (103) reflections, except ZnO: Cd film
that appears additional reflection (112) in the XRD pattern. The values of crystallite size are found to be (31, 21.8, 34.1nm) for
Cd, Fe and F doped ZnO thin films, respectively. The properties of the thin films changes with the dopants and this cause light
loss on the films. Thus, the changes in the transmission take place. The inclusion of dopant into film expands width of localized
states as: E U(Cd) >E U(Fe) >E U(F) . AFM study reveals that the dopants in the ZnO film had a significant influence on the morphology.
1. INTRODUCTION
Zinc oxide (ZnO) is a compound semiconductor of the II-VI family, which has a direct bandgap of (3.37 eV) and large
exiton binding energy (60 meV) at room temperature in the ultraviolet (UV) range [1], [2]. Transparent conducting oxide
films, such as ZnO thin films, have found extensive applications in optoelectronic devices [3] for example, solar cells [4],
liquid crystal displays, heat mirrors and multiplayer photothermal conversion system [5]. ZnO thin films have been
deposited by various techniques, such as thermal evaporation [6], [7], sputtering [8], [9], chemical vapor deposition [10],
[11], sol-gel [12], [13], pulsed laser deposition [14], [15] and electrochemical deposition [16]-[18]. In addition to these
techniques, spray pyrolysis has received a little bit of extra attention because of its simplicity and cost-effectiveness as it
does not require sophisticated vacuum apparatus. In this technique, ZnO thin films can be prepared from spray solution of
zinc nitrate [19] or zinc chloride [20] or zinc acetate [21]-[24]. Zinc acetate was used in the present study.
In this paper, the structural, morphological and optical properties of cadmium, fluorine and iron doped zinc oxide thin
films deposited onto glass substrate by spray pyrolysis technique have been investigated. Such doped ZnO thin films are
useful in technology of different optoelectronic devices.
2. EXPERIMENTAL
Doped ZnO thin films have been deposited on glass substrate at a substrate temperature equal to 450 0C by a home-made
chemical spray pyrolysis technique. The experimental setup and other details have been reported elsewhere [25]. A
homogeneous solution with 0.1M was prepared by dissolving zinc acetate Zn(CH3COO)2.2H2O in distilled water at room
temperature. One drop of HCl was added to (50 ml) solution to prevent the formation of zinc hydroxide Zn(OH)2 and to
increase the solubility of the compounds. Similarly aqueous solutions of cadmium acetate dehydrate Cd(CH3 COO)2 .2H2O,
ammonium fluoride NH4F and iron chloride anhydrate FeCl3 were used to obtained Cd, F and Fe doped films. The glass
substrates were cleaned with ethanol and acetone for (15 min) by using ultrasonic cleaner and then rinsed in distilled
water, and subsequently dried before deposition. The flow of the solution was 6 – 7 ml/min and the oxygen was used as a
carrier gas. The substrate to nozzle distance was fixed at (40 cm), and the process involves spraying cycles of (8 s),
followed by periods with no spray of (90 s).
The structure of the samples was analyzed by X-ray diffraction (XRD) using SHIMADZU X-Ray diffractometer system
(XRD - 6000). The optical measurements of the doped ZnO thin films are calculated from the transmittance and
absorbance spectra at normal incidence in the wavelength range (300 – 900), using UV-VIS spectrophotometer type
(SHIMADZU) (UV-1600/1700 series). The surface morphology of the doped ZnO thin films has been examined using
atomic force microscopy (AFM, scanning probe microscope).
3. RESULTS AND DISCUSSION
3.1 STRUCTURAL PROPERTIES
The crystal structure and orientation of the doped ZnO thin films were investigated by X-ray diffraction (XRD) patterns.
X-ray diffraction spectra of the doped ZnO thin films taken at room temperature are shown in figure 1. These spectra
indicate that the films have polycrystalline nature with a hexagonal wurtzite structure, and the peaks with Miller indices
given belong to the ZnO (JCPDS card file no. 36-1451) [26]. As shown in figure 1 all the doped ZnO thin films have
(002) reflection as the preferred orientation along with the other (100), (101), (102), (110) and (103) reflections, except
ZnO: Cd film that appears additional reflection (112) in the XRD pattern. This (002) preferred orientation is due to the
Volume 3, Issue 3, March 2014
Page 1
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 3, March 2014
ISSN 2319 - 4847
minimal surface energy which the hexagonal structure, c-plane to the ZnO crystallites, corresponds to the densest packed
plane. This result is in a good agreement with literature data [27]-[31].The observed d-values are presented in Table 1,
and compared with the standard ones from the (JCPDS) [26]. The matching of the observed and standard d-values
confirms that the deposited films are zinc oxide with hexagonal structure.
Figure 1 XRD pattern of doped ZnO thin films
(hkl)
Table 1: XRD parameters of doped ZnO thin films.
Sample
ZnO: Cd
ZnO: Fe
ZnO: F
2θ(deg.)
31.6813
31.6713
31.6713
(100)
d(Ǻ)
2.82200
2.82286
2.82286
Int.
30
42
45
2θ(deg.)
34.3793
34.5141
34.3268
(002)
d(Ǻ)
2.60646
2.59658
2.61032
Int.
100
100
100
2θ(deg.)
36.1433
36.2668
36.1299
(101)
d(Ǻ)
2.48319
2.47501
2.48408
Int.
87
52
86
JCPDS
31.77
2.816
57
34.422
2.602
44
36.253
2.476
100
The unit cell parameter (a, c) and the ratio of (c/a) belong to the (002) plane as a preferred orientation of doped ZnO thin
films deposited on glass substrate have been calculated using equation (1) [32]. The calculated values of lattice parameter
(a, c) and (c/a) are listed in Table 2, found to be in a good agreement with the standard (JCPDS) values [26].
(1)
Sample
ZnO: Cd
ZnO: Fe
ZnO: F
(JCPDS)
Table 2: Lattice constants of doped ZnO thin films
a (Å)
c (Å)
3.25856
5.21292
3.25955
5.19316
3.25955
5.22064
3.24982
5.20661
c/a
1.59976
1.59321
1.60164
1.60212
The crystallite size was calculated using a well-known Scherre's formula [33].
(2)
Where D is the crystallite size, λ is the wavelength of X-ray used (=1.5406Å), β the broadening of diffraction line
measured at half its maximum intensity in radians and θ is the angle of diffraction. The values of crystallite size are found
to be (31, 21.8, 34.1nm) for Cd, Fe and F doped ZnO thin films, respectively and listed in Table 3. The crystallite size of
the thin films is changed dependence on the different dopant elements, which may be due to the stress arisen from the
difference
in
the
ion
radii
of
zinc,
cadmium,
iron
and
fluorine
.
Volume 3, Issue 3, March 2014
Page 2
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 3, March 2014
The preferred growth orientation was determined using a texture coefficient
following relation [34]:
(3)
ISSN 2319 - 4847
. This factor is calculated using the
Where I(hkl) is the measured relative intensity of a plane (hkl), I0(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. A sample with randomly
oriented crystallite presents TC(hkl)=1, while the larger this value, the larger abundance of crystallites oriented at the
(hkl) direction. The texture coefficient calculated for the three main diffraction peaks (100), (002) and (101) are listed in
Table 3. It is seen that the highest TC was in (002) plane for all the doped ZnO films. The texture coefficient of (002)
plane of the doped ZnO films were between 1.738 and 1.931. The decreasing texture coefficient causes the decreasing in
the crystallite size of the films. Therefore the crystallite size of the films decreases with decreasing the texture coefficient,
which shows the deterioration in the crystallinity of the films.
The micro strain depends directly on the lattice constant (c) and its value related to the shift from the ASTM standard
value. The values of micro strain of doped ZnO thin films are listed in Table 3. This strain can be calculated from the
following formula [35]:
(4)
The dislocation density ( ) , defined as the length of dislocation lines per unit volume, has been estimated using the
following equation [36], [37].
(5)
To derive the film stress (σfilm) parallel to the film surface, the following formula is used, which is valid for a hexagonal
lattice [38]:
(6)
For the elastic constants Cij, the following values for single crystal ZnO are used: C11 = 208.8 GPa, C33 = 213.8 GPa, C12
= 119.7 GPa, C13 = 104.2 GPa
Where: Cfilm and Cbulk are the lattice constants (C) of ZnO film and bulk (or powder).
Sample
ZnO: Cd
ZnO: Fe
ZnO: F
Table 3: Structural properties of doped ZnO thin films
Texture
Micro
Crystallite
Dislocation Density
Coefficient
Strain
Size (nm)
(δ)
ε%
(101)
(100) (002)
31
0.711
0.430 1.858
0.121
1.0405
21.8
0.441
0.626 1.931
0.258
2.1041
34.1
0.657
0.603 1.738
0.269
0.8599
Flim
stress
(GPa)
-0.282
0.601
-0.627
3.2 OPTICAL PROPERTIES
For most applications, such as solar cells, high transmission in the visible range is important. The transmission of the
films changes due to Cd, Fe and F dopants. The properties of the thin films changes with the dopants and this cause light
loss on the films. Thus, the changes in the transmission take place.
Figure 2 Transmittance spectra of the doped ZnO thin films
Volume 3, Issue 3, March 2014
Page 3
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 3, March 2014
ISSN 2319 - 4847
Figure 3 Absorbance spectra of the doped ZnO thin films
The fundamental absorption edge of the films corresponds to electron transition from valence band to conduction band
and this edge can be used to calculate the optical band gap of the films. In the direct transition, the absorption coefficient
can be expressed by [39]
(7)
Where: A is a constant, hυ is the photon energy and Eg is the optical bandgap. Figure 4 shows plots of (αhυ)2 vs. (hυ).
The optical band gaps of the films were calculated from these plots and are given in Table 4.
Figure 4 plots of (αhυ)2 vs. (hυ) of the doped ZnO thin films
In the exponential edge region, where
, Urbach rule is expressed as [40]
(8)
Where:
is a constant,
is the Urbach energy, which characterizes the slope of the exponential edge. Equation (8)
describes the optical transition between occupied states in the valence band tail to unoccupied states of the conduction
band edge. Figure 5 shows Urbach plots of the doped films. The value of
was obtained from the inverse of the slope of
versus
and is given in Table 4. The dopants change the width of localized states in the optical band.
values
change inversely with optical band gap. The inclusion of dopant into film expands width of localized states
as:
.
Volume 3, Issue 3, March 2014
Page 4
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 3, March 2014
ISSN 2319 - 4847
Figure 5 The Urbach plots of the doped ZnO thin films
Dopant
Cd
Fe
F
Table 4: Optical properties of the doped ZnO thin films
Transmittance at 900 nm
Eg eV
70
3.12
74
3.16
77
3.24
EU meV
240
224
162
3.3 MORPHOLOGY
The surface morphology of Cd, Fe and F doped ZnO thin films was analyzed using atomic force microscope (AFM),
which indicated that the dopants in the ZnO film had a significant influence on the morphology. Figure 6 shows the
typical three-dimensional AFM image of Cd, Fe and F doped ZnO films deposited at 450 0C. The pictures show spherical
grains that were uniformly distribution on the substrate. The grain size of the films evaluated from AFM pictures are
listed in Table 5.The value of grain size was less than (100 nm) which confirms the presence of nanostructures. The
difference in the surface roughness of the doped ZnO film belongs to the difference in the ionic radii of the dopants that
acts as interstitial or substitution defects in the ZnO lattice. AFM study reveals that the roughness of the film is
dependent on the type of the dopant.
Table 5: RMS roughness, Grain size and peak-peak height of film
Sample
RMS roughness (nm) Grain size (nm) Peak-Peak height (nm)
ZnO: Cd
1.77
98
34.6
ZnO: Fe
0.80
89
4.3
ZnO: F
1.70
77
7.13
Volume 3, Issue 3, March 2014
Page 5
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 3, March 2014
ISSN 2319 - 4847
Figure 6 AFM images of doped ZnO thin film (1) ZnO: Cd, (2) ZnO: Fe, (3) ZnO: F
4. CONCLUSION
The effect of Cd, Fe and F dopants on the structural, morphological and optical properties of ZnO thin films have been
investigated by X-ray diffraction technique and optical characterization method. X-ray diffraction spectra indicate that the
films have polycrystalline nature with a hexagonal wurtzite structure. The values of crystallite size are found to be 31,
21.8, 34.1nm for Cd, Fe and F doped ZnO thin films, respectively. The optical properties of the ZnO thin films changes
with the dopants and this cause light loss on the films. Thus, the changes in the transmission take place. The inclusion of
dopant into film expands width of localized states as: EU(Cd)˃EU(Fe)˃EU(F). AFM study reveals that the roughness of the
film is dependent on the type of the dopant.
References
[1] Y. F. Chen, D. M. Bagnall, H. J. Koh, K. T. Park, K. J. Hiraga, Z.Q. Zhu, T. F. Yao, J. Appl. Phys. 84 (7) (1998)
3912.
[2] U. Ozgur, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, S. J. Cho, V. Avrutin, H. Morkoc, J. Appl.
Phys. 98 (2005) 041301.
[3] A. L. Dower, J. C. Joshi, J. Mater. Sci. 19 (1984) 1.
[4] J. A. Ronovich, D. Golmoya, R. H. Bube, J. Appl. Phys. 51 (1980) 4260.
[5] K. L. Chopra, S. Major, D. K. Panday, Thin Solid Films 102 (1983) 1.
[6] J. Ma, F. Ji, D. Zhang, H. Ma, S. Li, Thin Solid Films 357 (1999) 98.
[7] S. A. Aly, N. Z. El Sayed, M. A. Kaid, Vacuum 61 (2001) 1.
[8] T. Minami, Y. Ohtani, T. Miyata, T. Kuboi, J. Vac. Sci. Technol. A 25 (2007) 1172.
[9] B. Y. Oh, M. C. Jeong, J. M. Myoung, Appl. Surf. Sci. 253 (2007) 7157.
[10] J. G. Lu, T. Kawaharamura, H. Nishinaka, Y. Kamada, T. Ohsahima, S. Fujita, J. Cryst. Growth 299 (2007) 1.
[11] A. Martin, J. P. Espinos, A. Justo, J. P. Holgado, F. Yubero, A. R. G. Elipe, Surf. Coat.Technol. 151 (2002) 289.
[12] V. Musat, B. Teixeira, E. Fortunato, R. C. C. Monteiro, Thin Solid Films 502 (2006) 219.
[13] H. M. Zhou, D. Q. Yi, Z. M. Yu, I. R. Xiao, J. Li, Thin Solid Films 515 (2007) 6909.
[14] Y. Liu, J. Lian, Appl. Surf. Sci. 253 (2007) 3727.
Volume 3, Issue 3, March 2014
Page 6
International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org
Volume 3, Issue 3, March 2014
ISSN 2319 - 4847
[15] Y. Liu, I. Zhao, J. Lian, Vacuum 81 (2006) 18.
[16] N. Fathy, M. Ichimura, J. Cryst. Growth 294 (2006) 191.
[17] Y. F. Gao, M. Nagai, Y. Masuda, F. Sato, K. Koumoto, J. Cryst. Growth 286 (2006) 445.
[18] Y. Takashige, T. Nebiki, T. Narusawa, Solid State Phenom. 124-126 (2007) 135.
[19] P. Singh, A. Kumar, Deepak,D. Kaur, J. Cryst. Growth 306 (2007) 303.
[20] C. Messaoudi, D. Sayan, M. A. Lefdil, Phys. Stat. Sol. (a) 151 (1995) 93.
[21] M. Krunks, E. Mellikov, Thin Solid Films 270 (1995) 33.
[22] D. F. Paraguay, J. Morales, L. W. Estrada, E. Andrade, M. M. Yoshida, Thin Solid Films 366 (2000) 16.
[23] B. J. Lokhande, M. D. Uplane, Appl. Surf. Sci. 167 (2000) 243.
[24] J. L. Zhao, X. M. Li, J. M. Bian, W. D. Yu, C. Y. Zhang, Thin Solid Films 515 (2006) 1763.
[25] P.S. Patil, Mater. Chem. Phys. 59 (1999) 185-198.
[26] Joint Committee on Powder Diffraction Standards, Powder DiffractionFile, International Center for Diffraction Data,
Swarthmore, PA(1997) card 36-1451
[27] Y. Caglar, S. Ilican, M. Caglar, F. Yakuphanoglu, Spec. Act. Part A 67 (2007) 1113-1119.
[28] A. Bougrine, A. El Hichou, M. Addou, J. Ebothe, A. Kachouane, M. Troyon, Mater Chem. Phys. 80 (2003) 45.
[29] J. Lee, B. Park, Thin Solid Films 426 (2003) 94.
[30] E. Luna-Arredondo, A. Maldonado, R. Asomoza, D. Acosta, M. Melendez-Lira, M. Dela, L. Olvera, Thin Solid
Films 490 (2005) 132.
[31] X. Zi-qiang, D. Hong, L. Yan, C. Hang, Mater.Sci. Semiconductor. Process. 9 (2006) 132.
[32] A. Goswami, Thin Film Fundamentals, New Age International (P) Ltd. Publishers, New Delhi, (2005) 69.
[33] B. Cullity, S. Stock, Elements of X-ray Diffraction, 3rd ed. Prentice Hall, (2001).
[34] C. Barret, T. Massalski, Structure of Metals, Pergamon Press, Oxford, 1980.
[35] T. Obata , K. Komeda , T. Nakao , H. Ueba , and C. Tasygama , J. Appl. Phys. , 81 , (1997) , 199.
[36] V. Bilgin, S. Kose, F. Atay, I.Akyuz, Mater. Chem. Phys. 94 (2005) 103.
[37] S. Prabahar, M. Dhanam, J. Cryst. Growth 285 (2005) 41.
[38] R. Hong, J. Huang, H. He, Z. Fan, J. Shao, Appl. Surf. Sci. 242 (2005) 346-352.
[39] N. F. Mott, R.W. Gurney, Electronic Processes in Ionic Crystals, Oxford Univ. Press, London, (1940).
[40] F. Urbach, Phys. Rev. 92 (1953) 1324.
Volume 3, Issue 3, March 2014
Page 7