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Zinc Oxide is a direct wide band gap semiconducting
material which is of great interest for numerous technological applications due
to its unparalleled properties and the availability of a variety of synthesis
approaches. In this paper, we have presented the synthesis of ZnO thin films on
micro-glass substrates by sol-gel technique and their structural, morphological
and optical properties were investigated. X-Ray Diffraction (XRD) analysis
inferred the film’s hexagonal wurzite phase with a preferential growth along
(101) plane. The mean crystallite size was obtained as 12 nm along (101) plane
on the basis of the Deby-Scherer model. A small dislocation density of 6.64 × 10-3
(Line/nm2) was obtained along (101) plane, showing the presence of
few lattice imperfections and good crystallinity which make the films suitable
for optoelectronic applications. SEM micrographs inferred the film’s granular
nature composed of spherical nanoparticles. The absorbance spectra show that
the ZnO thin films absorb maximum radiation in the UV region. The refractive index
of the thin films has been evaluated using Tripathi relation and a low
reflectivity 0.15 obtained with the help of Fresnel’s equation which indicates
the good absorbance of the films. The optical band gap (3.36 eV) is estimated
using UV-Visible spectroscopy. A high absorbance of the thin films was found in
ultraviolet region with peaks around 375 nm, indicating that the films have
potential applications in making of solar cells.
Keywords: ZnO, XRD, Thin films, SEM, Nanoparticles, ZAD, Spin coating
INTRODUCTION
Nanotechnology is
one of the areas of knowledge that attract the attention of researchers
worldwide due to innovations created by reducing the size of the materials to
nano range. A material is nanometric when its structural components have at
least one dimension in the nano regime (1-100 nm). Due to their extraordinary
mechanical, electrical, magnetic, optical and chemical properties, ZnO is one
of the most studied materials in nanotechnology.
Zinc oxide (ZnO) is
an interesting direct wide band gap (~3.32 eV) material of II-VI compound
semiconducting family. ZnO has large exciton binding energy (60 meV) and
superconducting properties based on oxygen vacancies and the wurzite structured
[1-4].
ZnO has become one
of the most promising materials and has lots of research interest due to its
unique structure and size-dependent electrical, optical and mechanical
properties. ZnO is suited for various technological applications such as
anti-reflection coatings, transparent electrodes in solar cells [5],
piezoelectric devices [6], gas sensors [7], varistors [8], UV and blue light
emitters and even thin-film transistors [9]. It is also being considered as a
potential candidate in the field of spintronics [10].
Unlike many of its
competitors, ZnO is inexpensive, chemically stable, easy to prepare, non-toxic
and abundant. It has been used in variety of applications such as conductive
films, solar cell windows, photoelectric cells, non-linear optics, bulk and
surface acoustic wave devices [11-13] and micro mechanical devices [14,15]. ZnO
wide band gap opens the possibility of creating ultraviolet emission diodes
[LEDs] and white LEDs with superior color purity. ZnO thin films have good
electrical and optical properties and are of lower material cost in comparison
to the ITO films [16,17].
Due to large exciton
binding energy the exciton remain dominant in optical processes even at room
temperature. Due to its vast industrial applications such as electrophotography,
electroluminescence phosphorous, pigment in paints, flux in ceramic glazes,
fillers for rubber products, coatings for papers, sun screens,
medicines and
ZnO thin films have been
made by a variety of techniques among which there can be mentioned reactive
sputtering [25], spray pyrolysis [26], zinc oxidation [27], electro deposition
[28], pulsed laser deposition (PLD) [29], chemical vapor deposition (CVD) [30],
metal organic chemical vapor deposition (MOCVD) [31], plasma enhanced chemical
vapor deposition (PECVD) [32], low pressure sputtering [33], chemical bath
deposition (CBD) [34] and sol gel route [35]. The synthesis of ZnO
nanostructures can be accomplished by chemical and physical routes. However
chemical methods are more suitable and cost effective for the production of ZnO
on the industrial scale [36].
The sol gel method has emerged as one of the most promising processing
route as it is particularly efficient in processing thin transparent,
homogeneous, multi components oxide films of many compositions on various
substrates at low cost and it allows the tuning of thickness of the film by
varying synthesis parameters.
In this work, we have investigated the structural, morphological and
optical properties of undoped ZnO thin films spin coated on glass substrates.
BACKGROUND
Sol gel process
A sol is a dispersion of the solid particles (~0.1-1 µm) (~0.1-1 μm) in a
liquid where only the Brownian motions suspend the particles. A gel is a state
where both liquid and solid are dispersed in each other, which presents a solid
network containing liquid components. The sol-gel coating process usually
consists of four steps:
Steps:
i.
The desired colloidal particles once dispersed in a liquid to form a sol.
ii.
The deposition of sol-solution produces the coatings on the substrates by
spraying, dipping or spinning.
iii.
The particles in sol are polymerized through the removal of the
stabilizing components and produce a gel in the state of a continuous network.
iv.
The final heat treatments pyrolyze the remaining organic or inorganic
components and form an amorphous or crystalline coating [40].
Advantages of sol-gel
technique:
Ø
Can produce thin bond-coating to provide excellent adhesion between the
substrate and the top coat.
Ø
Can produce thick coating to provide corrosion protection performance.
Ø
Can easily shape material into complex geometries in a gel state.
Ø
Can produce high purity products.
Ø
Can have low temperature sintering capability, usually 200-600°C.
Ø Can provide a simple,
economic and effective method to produce high quality coatings.
MATERIALS AND METHODS
Cleaning of the
substrates
The microscope glass substrates were cleaned ultrasonically. Initially
the ultrasonic bath was filled with distilled water and then quartz substrates
placed in the beaker filled with acetone. The beaker placed in the ultrasonic
bath for cleaning purpose of the substrates at 60°C for 15 min. After cleaning
the substrates are carried out from the beaker and dried at room temperature
for 10 min by keeping the substrates vertically.
Synthesis of thin
films
Zinc oxide thin films were prepared by sol-gel process [24]. As a
precursor material, (ZAD) zinc acetate dehydrate [Zn (CH3COO) 2.2H2O]
was dissolved in isopropyl alcohol with molarities of 0.65 mol L-1and 0.45 mol
L-1 with the help of monoethanolomine (MEA) as a catalyst. The
solution was stirred thoroughly on a magnetic stirrer for 1 h at 60°C
temperature. As prepared solution is filtered using filter paper. The filtered
precursor solution was deposited on quartz substrates by spin coating (3500
rpm, 120 s). As synthesized films were dried for 30 min at nearly 60°C
temperature. The films were subsequently annealed up to 500°C for one hour in
order to obtain crystallized ZnO.
Characterization
The crystal structure and orientation of the ZnO films were studied by
X-ray diffractometery (XRD) using radiation CuKα (λ=1.5418 Å) operating at 30
kV and 25 mA of electric current. The diffractometer reflection was recorded at
room temperature. The morphology of the synthesized ZnO thin films were
analyzed by scanning electron microscopy using a JEOL JSM6380 microscope
operating at 80 kV. The optical parameters of the ZnO thin films analyzed by a
UV-Visible spectrophotometer in the spectral range of 200-1000 nm at normal
incidence.
RESULTS AND
DISCUSSION
Film thickness
measurement
The thickness of the sol gel derived ZnO film is estimated approximately
315 nm by equation (1) given below [41].
t=w2-w1/Aρ × 104 μm (1)
Where w1 and w2 are the weights of the
glass substrate before and after the film deposition in grams; A is the area in
cm2 of the deposition of the film and ρ is the density of ZnO (5.6
g/cm3).
X-ray diffraction
(XRD) analysis
The XRD pattern of synthesized ZnO thin films by sol-gel spin coating
technique on quartz substrates is shown in Figure 1. All the recorded
peaks correspond to the peaks of standard ZnO (JCPDS36-1451). For the
synthesized ZnO films, different diffraction peaks are recorded in XRD pattern
showing in the growth of ZnO along different directions. Strong preferential
growth is observed along the (101) planes [42]. The mean crystallite size (τ)
was estimated using the Deby-Scherer formula shown in equation (2):
τ= kλ/ β2θ cosθ (2)
Where k is 0.94 is a constant and λ=X-ray wavelength (1.5418 Å), β2θ=full
width at half maximum in radian (FWHM) and θ is the Bragg’s angle obtained from
2θ values.
The dislocation density (δ) defined as the length of dislocation lines
per unit volume and strain (ε) have been estimated by the equation (3) and (4)
given below [42,43]:
δ=1/D2 (3)
ε = β cosθ/4 (4)
Lattice constant ‘a’ and ‘c’ of the wurzite structure of ZnO are
calculated using relations [42,44,45]:
a= γ/√3 sinθ100 (5)
c= γ/sinθ002 (6)
The estimated structural parameters of sol gel derived ZnO films are
presented in Table 2 and the calculated lattice parameters are presented
in Table 3. The estimated lattice parameters of ZnO films deposited on
glass substrate from recorded XRD data are in agreement with standards reported
in (JCPDS 36-1451).
The bond length L of nanostructured ZnO films is found 2.01 Å using
equation (7) and (8) [46,52]:
L=√[a2/3+(1/2-μ)2c2] (7)
μ = a2/3c2+1/4 (8)
This bond length is slightly higher than the standard value1.9767 Å for
bulk ZnO which is due to strain produced during synthesis of the thin films
[47]. The ratio c/a is found 1.6 which confirms the wurzite structure of the
prepared thin films.
The refractive index of the ZnO thin films is estimated 2.28 using
Tripathi relation [49] given in equation (9) below:
η=
η0[1+
αe- βEg] (9)
Where α=1.9017, ŋ0=1.73 and β=0.539/eV. The calculated value
is found to be slightly greater than the refractive index of bulk ZnO. The high
refractive index consisted by ZnO thin films make them suitable for use as
anti-reflection coatings. The reflectivity of the ZnO films for air and ZnO
interface is calculated using Fresnel’s equation [49]:
R = [η-1/ η-1]2 (10)
Where ŋ is the refractive index of synthesized Zno film; the reflectivity
is found 0.152 which shows the good absorbance of the ZnO films suitable for
optoelectronic applications. This low value of reflectivity of ZnO thin films
makes them suitable for anti-reflection coating ion solar cells.
Morphological
analysis
The morphology of the synthesized ZnO thin films was investigated using
scanning electron microscopy. Figure 2 shows the surface morphology of
the prepared films in our report. The results recorded by SEM are found to be
are very good with stoichiometric formation of ZnO nano crystal of spherical
shape and demonstrated aggregation of ZnO nano particles. The aggregation of
ZnO nanoparticles occurred probably during the drying process [37,38]. The
surface morphology of ZnO thin films indicates high density of grains implying
nucleation on all sites of the substrates.
Optical
characterization
The absorption spectra of ZnO thin films are recorded in the spectral
range of 200-1200 nm using UV-spectrophotometer. The band gap energy of ZnO
thin films have been calculated using Tauc’s plot.
Tauc’s equation (11) is given by Kashyout et al. [39]:
αhυ = A(hυ-Eg)n (11)
Where α is the absorption coefficient, hυ is the photon energy and A is
the constant, Eg is the band gap energy of the sample. The value of
n is 1/2 or 2 depending upon whether the transition from valence band to
conduction band is direct or indirect. The presence of a single slope in the
plot suggests that the films have direct and allowed transition. Since ZnO has
a direct band gap material, the value of n is taken 1/2 in this case. The
absorbance curves of the sol-gel derived ZnO thin films are recorded in the
range 200-1200 nm shown in Figures 3-5.
The graphs show that ZnO thin
films grown on glass substrates absorb light in ultra violet region of the
electromagnetic spectrum. The results are in accordance with the band gap value
of the bulk ZnO (3.37 eV) according to which ZnO absorbs light in UV.
The maximum absorbance occurs at wavelengths approx 350-375 nm which
indicates that the films have potential application in fabrication of solar
cells.
The energy band gap is evaluated by extrapolating the straight line
portion of the plot to zero absorption coefficient in strong absorption
spectral region when (αhυ)1/2 varies dramatically (Figure 6).
The band gap energy is calculated using Tauc’s plot in Figures 4 and 6
which comes out to be 3.36 eV for M=0.45 of ZAD and 3.32 eV for M=0.65 of ZAD,
which suggest that Eg slightly decreases with increasing of
molarities of the precursor. These values are in good agreement with the values
reported by other researchers [28].
CONCLUSION
ZnO thin films having nanostructure were synthesized via sol-gel
technique. The morphological, structural and optical features of sol-gel
deposited ZnO thin films were characterized. The XRD analysis revealed
hexagonal wurzite structure with preferred orientation along (101) plane. SEM
images of the ZnO thin films showed ZnO nano particles which coalesced and make
clusters on the surface of the film. The surface morphology exhibits that the
quality of the film is well adapted to the considered optoelectronic
applications. The high refractive index of sol-gel derived ZnO thin films make
them suitable for the usage of anti-reflection coatings. The low reflectivity
of the ZnO films for air and ZnO interface make them suitable in the
fabrication of solar cells. Hence, the sol-gel technique has proved to be
effective in the contribution of thin films for optoelectronic applications.
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