O A

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Journal of Applied Sciences Research, 9(9): 5460-5467, 2013
ISSN 1819-544X
This is a refereed journal and all articles are professionally screened and reviewed
ORIGINAL ARTICLES
Structural and optical characterization of ZnO thin films annealed at different
temperatures
1,2
R. Mahendran, 3M. Kashif, 4M. Saroja, 5M. Venkatachalam,
Ayeshamariam, 8C. Sanjeeviraja and 3U. Hashim
6
T. Siva Kumar,
7
A.
1
Research and development center, Bharathiyar University, Coimbatore, India
Department of Electronics, Government Arts College, Kulithalai, India
3
Nano Biochip Research Group, Institute of Nano Electronic Engineering (INEE), Universiti Malaysia Perlis
(UniMAP), Kangar 01000, Perlis, Malaysia
4,5
Thin Film Center, Department of Electronics, Erode Arts and Science College Erode, India.
6
Department of Electronics, RVS College of Arts and Science, Sulur, Coimbatore, India
7
Department of Physics, Khadir Mohideen College, Adirampattinam 614 701, India
8
Department of Physics, Alagappa Chettiar College of Engineering and Technology, Karaikudi, 630 004, India
2
ABSTRACT
ZnO thin films were deposited on glass substrate using a sol-gel method. The structural and optical
properties at different annealing temperatures were studied using X- ray diffraction (XRD), ultra-violet-visible
spectroscopy and Raman spectroscopy. X- ray diffraction results show that the c-axis orientation became
stronger as the annealing temperature increased from 300 to 500 º C. the optical band gap energy was calculated
from the optical absorption using UV-Vis spectrophotometer. The optical band gap of ZnO thin films decreases
from 3.378 eV to 3.338 eV as the annealing temperature increases from 300 to 500 ºC, the experimental data are
in agreement with the calculated results by specific models of refractive index.
Key words: sol-gel, annealing, bandgap, structural, optical
Introduction
Zinc oxide (ZnO), a wide-gap compound II–VI semiconductor that has the direct band gap of about 3.37 eV
at room temperature, is a well known material suitable for generating ultraviolet (UV) light. Furthermore, a
large exciton binding energy of about 60 meV in ZnO, which is significantly larger than the thermal energy at
room temperature (26 meV), can ensure an efficient exciton emission at room temperature under low excitation
energy (Bagnall DM, et al, 1997). Thus, considerable efforts have been made on the synthesis and study of
nano-scale ZnO materials.
Different methods of growing zinc oxide thin film with consistent morphology and reproducible optical and
electrical properties with long-term stability have been tried in the past by the researchers. These techniques for
growing ZnO films on a variety of substrates include pulsed laser deposition (PLD) (Drmosh QA, et al, 2013),
sol–gel technique (Kashif M, et al, 2012, 2013 - Kumar SA et al., 2008, Hariharan C et al., 2006 ), metal
organic chemical vapor deposition (MOCVD) (Kim DC et al., 2007), RF sputtering (Wang J et al., 2007) and
thermal evaporation technique (Abdulgafour HI et al, 2010). Among the above techniques, the vacuum
evaporation-deposition technique has several distinct advantages, e.g. process simplicity, cost effectiveness and
easy thickness monitoring over a very large area.
In this article, we study the influence of post-thermal annealing on the structural and optical properties of
ZnO thin films on glass substrates using the sol–gel spin coating method by analyzing the observed X-ray
diffraction (XRD) patterns, UV absorption spectra, and Raman spectra.
Materials and Methods
ZnO thin films were coated on glass substrates by sol gel spin coating technique. The seed solution
preparation was performed following the procedure described in literatures (Kashif et al. 2013). Briefly,
10.975gm zincacetate dihydrate was dissolved in 50mL 2-methoxyethanol and 3.055gm monoethanolamine.
Monoethanolamine acts as a stabilizer and was added drop wise under stirring and constant temperature. Zinc
acetate dehydrate, 2- methoxyethanol and monoethanolamine were used as the precursor, solvent and stabilizer
respectively. The molar ratio of MEA to zinc acetate was maintained as 1M and the concentration of zinc
Corresponding Author: A.Ayeshamariam, Department of Physics, Khadir Mohideen College, Adirampattinam 614 701,
India
E-mail: [email protected]
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J. Appl. Sci. Res., 9(9): 5460-5467, 2013
acetate was maintained at 1 M. The solution was stirred at 60oC for 2 h to yield a clear and homogeneous
solution. The prepared solution was aged at room temperature for 1 day before spin coating.
The glass substrates were cleaned by soap solution, chromic acid, distilled water, acetone, and finally with
deionized water. Then the films were deposited on the glass substrates using spin coater rotated at 3000 rpm for
30 sec. The films were heated at 300oCfor 10 min after each coating. The coating-to-drying process was
repeated for several times in order to get the desired thickness. The films were annealed at 300, 450, and 500 oC
for 1 hour, respectively.
The structural properties of the ZnO thin films were analyzed using X-ray diffractometer with Cu-Kα
radiation. The thickness of the films was determined by surftest SJ-301 stylus profilometer. For optical
characterization, the optical transmittance spectra of ZnO thin films were recorded using spectrophotometer
(Hitachi-3400 UV-Vis-NIR).
Results and Discussion
Figure 1 shows the XRD spectra of as deposited ZnO films and the ZnO films annealed at different
temperatures (300, 450, 500 C
̊ ) with a fixed annealing time of 1h.The XRD patterns indicate a ZnO phase with a
hexagonal wurzite structure. We can see from Fig. 1 that the as-deposited film have amorphous phase and
annealed films have a polycrystalline structure with orientations at the (100),(002), and (101) planes. The peak
intensity at the (002) plane increased with an increase in the annealing temperature. The intensification of the
peak appearing at (002) plane indicates that the crystal quality improves with the increase in annealing
temperature. The lattice constants ―a‖ and ―c‖ for the ZnO thin films were calculated using the Bragg‘s law
(Kashif M, et al, 2013) as:
Fig. 1: XRD pattern of ZnO thin films at different annealing temperatures
𝑛λ = 2𝑑𝑠𝑖𝑛θ
(1)
where𝑛 is the order of diffraction, 𝜆 is the X-ray wavelength and 𝑑 is the spacing between planes of given
Miller indices ℎ,k, and l. The plane spacing is related to the lattice constants ―a‖ and ―c‖ and the Miller indices
by the following relation (Kashif M, et al, 2012);
1
2
𝑑(ℎ𝑘𝑙
)
=
(ℎ2 + ℎ𝑘 + 𝑘 2 ) 𝑙 2
+ 2
𝑎2
𝑐
(2)
The average crystalline size in ZnO films was estimated using Scherrer‘s equation,
𝐷=
𝑘𝜆
𝛽𝑐𝑜𝑠𝜃
where D is grain size, β the full width at half maxima and λ the wavelength of X-ray used (1⋅5409 Å).
(3)
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J. Appl. Sci. Res., 9(9): 5460-5467, 2013
The average crystallite size is 27.92, 29.39, 32.86, and 39.2439 nm were calculated for the as-deposited and
annealed ZnO thin films, respectively. All the diffraction peaks are in good agreement with the JCPDS (S6-314)
data showing that the main structure of the sample is hexagonal wurzite. The Full Width Half Maximum
(FWHM) of thin film shows a decreasing trend with increasing annealing temperature. From the crystallite size
the dislocation density value was calculated by the formula (Lahewil ASZ et al 2012)
𝛿=
1 𝑙𝑖𝑛𝑒
𝑚2
𝐷2
(4)
The measured line-widths and the other physical parameters are calculated and are given in Table 1. From
the Table 1, it can be concluded that the crystallite size increases due to the heat treatment.
For our films we obtained lower values of ‗a‘ and ‗c‘ than the JCPDS values, and the values decrease as the
annealing temperature and the annealing time increase. Similar results are also reported by other research groups
(Lahewil ASZ et al., 2012, Lupan O et al, 2010, K. Laurent, et al., 2008). This may be due to the lattice
contraction resulting from the presence of dangling bonds on the surface of the ZnO films.
Fig 2 (a-d) is the presentation of the AFM surface topography (3D images) of ZnO films deposited on glass
substrates. The topography of the films shows significant variation with temperature when the films. The grain
size values are in the range of about 35-65 nm. The roughness values were obtained from an area of 1 µm x 1
µm (2D pictures) wherein a closure picture at the surface with ups and downs between the grains or clusters of
grains is visible as seen from Figure. 2(a-d). The Root-Mean-Square value of roughness (Rrms), the difference in
height between the highest and lowest points on the surface height deviations measured from the mean plane
derived as peak-to-valley roughness (hp-v, nm) and the average differences of heights (hav, nm) have been
calculated from the surface profile given in Figure a, b, c, d and e. These values (R rms, hp-v, hav) for the ZnO
films deposited with above mentioned values respectively are: (as deposited) 9.6, 19.2, and 10.4 nm, (300 ºC)
8.2, 18.3 and 9.2 nm, (450 ºC) 7.1, 29.4 and 16.6 nm, and (500 ºC) 3.5, 4.5 3.2 nm. The ZnO films deposited
with a thickness of nearly 40 nm shows larger grains with less crystallites for which minimum surface roughness
value has observed. It shows more uniform surface topography with bigger of nano grains. These observations
show that the ZnO films deposited with thickness 30-60 nm in the present study show nano-scale topography
with uniform sized grains and completely covered surfaces.
Fig. 2(a-d): Morphological studies of ZnO as-deposited to different annealing temperature
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J. Appl. Sci. Res., 9(9): 5460-5467, 2013
Figure 3 shows the absorbance spectra of the films in the wavelength region ranging from 325 to 800 nm.
The absorbance edge shifts towards a lower wavelength as the annealing temperature increases; this shift
indicates, shrinking in the optical band-gap.
Fig. 3: Absorbance spectra of ZnO thin films at different annealing temperatures
The optical bandgap of the transition semiconductor ZnO films were calculated using the Tauc model;
(𝛼ℎ𝑣)2 =
ℎ𝑣 − 𝐸𝑔
1 2
(5)
where α is the absorbance coefficient, h is the photon energy, A is an energy independent constant, and Eg
is energy band gap. The optical bandgap was estimated by extrapolating the straight line of (αh)2 versus the
photo energy (h) plot as shown in figure 6. The intercept of the tangent to the plot provides a good
approximation of the band gap energy of ZnO of films. The band gap of the as deposited ZnO thin film was
3.416 eV. This value gradually decreased to 3.378, 3.358, and 3.338eV, as the annealing temperature increased
to 300, 450 and 500 C
̊ , respectively. The decrease in the band gap may be due to the improved crystalline
structure of the ZnO thin films with the increase in annealing temperature. Due to the disorder arrangements of
the thin films grown at low temperature, the lowest states in the conduction bands are localized, called traps
(Shan FK et al.,2007).
At low temperatures, the amorphous nature of the ZnO phase increases. As a result, the extended
localization in the conduction and valance band increases. Thus, the absorption edge will be mainly attributed to
the amorphous ZnO and the films at lower temperature will have larger band gaps than the films grown at higher
temperatures, and the band gap decreases as the temperature increases due to the decrease in the amorphous
nature of ZnO (J. P. Mathew et al., 2012).
Figure 5 shows the transmittance spectra of ZnO thin films annealed at various temperatures. The optical
transmittance measurements are strongly dependent on annealing temperature as shown in figure 4. The
transmission spectra of as deposited and ZnO films annealed at 300, 450 and 500 ºC exhibit more than 80%
transmittance in the visible region.
A sharp decline in the transmittance of the UV region corresponds to the bandgap. The transmittance
decreases as the annealing temperature increases. This reduction is due to the increase in surface roughness and
surface scattering of the light as it becomes translucent.
Many attempts have been made to relate the refractive index and the energy gap Eg through simple
relationships (Moss TS 1950, Gupta VP et al 1980, Hervé P, 1995 et al). However, these relations of n are
independent of temperature and incident photon energy. In this paper, the various relations between n and Eg
will be reviewed. Ravindra et al. (Ravindra NM et al, 1979) presented a linear form of n as a function of Eg;
𝑛 = 𝛼 + 𝛽𝐸𝑔
(6)
where α = 4.048 eV-1 and β = -0.62 eV-1. Light refraction and dispersion will be inspired. Herve and
Vandamme (Herve PJL et al, 1995) proposed an empirical relation as follows:
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J. Appl. Sci. Res., 9(9): 5460-5467, 2013
𝑛=
1+
𝐴
𝐸𝑔 + 𝐵
2
7
where A = 13.6 eV and B = 3.4 eV. For group IV semiconductors, Ghosh et al. (Ghosh DK et al., 1984)
published an empirical relationship based on the band structure and quantum dielectric considerations of Penn
(Penn DR et al., 1962) and Van Vechten (Van Vechten JA et al, 1976),
𝑛2 − 1 =
𝐴
𝐸𝑔 + 𝐵
2
(8)
where A = 25Eg + 212, B = 0.21Eg + 4.25, and (Eg + B) refers to an appropriate average energy gap of the
material. The calculated refractive indices of the end-point compounds and energy gaps are listed in Table 2;
they are in good agreement with the experimental values (Vandamme H et al, 1981).
This result is verified by the calculation of the optical dielectric constant ε∞, which depends on the
refractive index. Note that the optical dielectric constant ε∞= n2(Samara GA et al.,1983). Our calculated
refractive index values are in accordance to the experimental value shown in Table 2, which provide an
appropriate model of Ravindra et al., for solar cell applications.
Raman spectroscopy is generally is used to study the vibrational, rotational, and other low-frequency modes
in a system. It is an important non-destructive tool that can be used to investigate amorphous to crystalline phase
transitions, oxygen defects, stress state, quantum size effects in transition metal oxides. Raman scattering can be
employed as a simple, rapid and effective technique to evaluate nanocrystalline particle size (Willander, M., et
al, (2009)).
Raman modes were analysed with He-Ne laser (633 nm) as a source. The shift in energy gives information
on the phonon modes in the systems. Apart from that the crystalline and amorphous nature of the materials can
be identified from Raman. For example, crystalline silicon will have a Raman mode at 522 cm-1 (and line width
3.5 cm-1) but amorphous silicon will have the mode at 480 cm-1 that too as a very broad hump. But if the
microcrystalline material is in between the above two extremes (crystalline and amorphous), then, only a red
shift and a very large line broadening will be observed. ZnO crystallizes usually in the hexagonal wurtzite
structure, which lattice belongs to the space group
. The group theory predicts that the observed phonon
belongs to the 2E2, 2E, 2A and 2B symmetries. Out of these, the two B, symmetry modes are not Raman active
(Abdulgafour HI et al., 2010).
Fig. 4: A plot of (αhv)2 versus photon energy for ZnO thin films at different annealing temperatures
Figure 5 shows the Raman spectra measured for the as deposited ZnO film and the films annealed at 300,
450, and 500oC. In low frequency region, the peaks are observed at about at about 420-440cm-1 for all the films.
These are assigned to the two non-polar optical phonons (E2) modes of the sol-gel prepared ZnO films. It was
observed that the peak position of the E2H mode shifts from 438.67 to 421.9 cm-1 when the annealing
temperature was raised. The following peaks, 557.51, 554, 550, and 543.16 cm-1were observed for the as
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J. Appl. Sci. Res., 9(9): 5460-5467, 2013
deposited film and the ZnO films annealed at 300, 450 and 500oC, respectively. This is attributed to the E1 (LO)
mode of ZnO. The intensity of this Raman peak is found varying with the oxidation of excess zinc in the ZnO
film. This mode is created by the defects of O-vacancy and Zn interstitial.
Fig. 5: Transmittance spectra of ZnO thin films at different annealing temperatures
Fig. 6: Raman spectra of ZnO thin films at different annealing temperatures
Table 1: Measured structural properties of ZnO nanostructures using XRD at different annealing temperatures
Annealing temp
a
c
Grain
Dislocation density
(A0)
(A0)
size(nm)
x 10-15 lines/m2\
As deposited
3.2407
5.290
25.22
1.57 x 1015
300 ̊C
3.241
5.289
49.19
4.13 x 1014
450ºC
3.231
5.287
52.26
3.66 x 1014
500 ºC
3.206
5.276
69.04
2.097 x 1014
Strain
( βcosθ/4) x 10-3
1.6137
1.8751
1.9142
2.113
Table 2: Thickness, energy gap (Eg), refractive index (n) and optical dielectric constant (ε ∞) of ZnO nanostructures using Ravindra et al.
(1979), HerveandVandamme (1995) and Ghosh et al. (1984) models.
Annealing Temperature
Energy Bandgap
Refractive Index
Optical dielectric constant
(ε∞)
1.93a
3.725a
b
As deposited
3.416
2.232
4.98b
c
2.287
5.23c
1.953a
3.81a
300 ̊C
3.378
2.242b
5.026b
c
2.294
5.2464c
1.96a
3.865a
450 ̊C
3.358
2.247b
5.049b
c
2.298
5.282c
1.978a
3.914a
500 ̊C
3.338
2.252b
5.073b
c
2.293
5.256c
a
Ravindra et al. (1979).
b
Herve and Vandamme (1995).
c
Ghosh et al. (1984).
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Conclusion:
ZnO thin films were successfully synthesised on glass substrates by sol-gel method. The effect of different
annealing temperatures on the structural and optical properties of ZnO thin films were investigated using X-ray
diffraction and UV-Vis spectroscopy, respectively. The c-axis orientation of the ZnO thin films increased as
annealing temperatures increased. The c-axis orientation of the ZnO thin films increase as annealing
temperatures increased. The optical band gap energy of the ZnO thin films decreases from 3.378 eV to 3.338 eV
as the annealing temperature increases from 300 to 500 ºC. The decrease in the band gap may be due to the
improved crystalline structure of the ZnO thin films. It is proven that Ravindra model is the appropriate for
optoelectronic devices. In addition, the characterizations, analysis and simulation studies recommended ZnO
nanostructures for optoelectronic applications.
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