International Paper publication

Vol 2 | Issue 1 | Spring Edition | DOI : February 2014 | Pp 52-57 | ISSN 2279 – 0381
Structural,Optical and Electrical properties of nano structured flower
like ZnO thin films by CBD
R.Radhika *a
a
Department of physics, Vellalar college for women, Erode, India.
* e-mail id: radhikamsc88@gmail.com
www.indiasciencetech.com 52
Keywords: ZnO, Chemical bath deposition, XRD,
UV, SEM, EDAX analysis, Electrical property.
Abstract
Investigations on the effect of annealing
temperature on the structural, optical, electrical
properties and morphology of nano structured
ZnO thin films deposited on glass substrate by
chemical bath deposition have been carried out. X-
ray diffraction studies revealed that deposited
films are in crystalline nature with Hexagonal
structure along the prominent crystallographic
plane. Such as crystalline size, dislocation density,
and micro strain were calculated. The UV- Visible
spectroscopy studies revealed that all the films
have high optical transmittance (>60%) in the
visible range. The optical band gap values are in
the range of 3.23-3ev. The films have increased
transmittance with increase of heat treatment.
Scanning electron microscope images revealed
that the flower like shaped grains that occupy the
entire range of several nm in size. The electrical
conductivity of ZnO thin film is determined by
using Four probe method. The electrical
conductivity of ZnO thin film increases with the
increase of annealing temperature.
Introduction
Zinc oxide is a very important II-VI semiconductor
material with a direct band gap of 3.37ev at room
temperature and a large exciton binding
energy[1]. This semiconductor has several
favorable properties, including good transparency,
non toxicity, good electrical optical
andpiezoelectric property. Since the propertiesof
ZnO strongly depend on its morphology and
microstructure. It has several applications such as
solar cells[2], heat mirrors[3],gas sensors[4], wear
resistant applications[5] , thin film resistors[6]
,dye-sensitized solar cells[7], and in biological
applications[8]. Further more, Zinc is a cheap and
abundant raw material. Different methods such as
Spray pyrolysis[9], pulsed laser deposition[10],
sputtering[11], chemical vapour deposition(CVD)
[12]. However all these techniques sophisticated
instruments and/or a high temperature of
deposition. Among the thin film deposition
methods chemical bath deposition(CBD) from
aqueous solution is the simplest and most
economical one[13]. Wet chemical techniques
such as CBD is extremely attractive due to its
advantageous features over other thin film
deposition techniques, such its simple, low
temperature, low cost, low evaporation
temperature and easy coating of large
surface[14].In this deposition involves two steps,
nucleation and particle growth, and is based on
the formation of a solid phase from a solution. In
the present work, the chemical bath deposition
using a double dip technique has been used for
preparing ZnO thin films deposited on glass
substrate and the characterization results of the
grown films. The structure, surface morphology,
optical and electrical parameters are investigated
and reported.
Experiment
ZnO thin film were deposited on glass substrate
by using a two step chemical bath deposition
technique. A solution comprising 1.2M zinc
sulphate was dissolved in aqueous solution and
3M sodium hydroxide was dissolved in aqueous
solution with pH value of 10±0.2 deposited at bath
temperature 90◦C under optimized condition. This
step sodium zincate was prepared. The glass
substrates by alternately dipping of the substrates
in a sodium zincate bath at room temperature and
hot water maintained nearly at boiling point. The
clean substrate was first immersed in the sodium
zincate bath for five minutes (first dip) and then
dipped in hot water (second dip) for the same
duration. It is repeated for known number of
times. A thin layer of sodium zincate complexes is
formed during the first dip, which decomposed to
ZnO when subsequently dipped in hot water. Part
of the ZnO was formed on the glass substrate as a
strongly adherent film, and the remaining formed
as precipitate in the solution. ZnO films deposited

Journal of NanoScience and NanoTechnology | Vol 2 | Issue 1 | Spring Edition| ISSN 2279 – 0381
53 www.indiasciencetech.com
with a number of dippings were dried in air and
subsequently annealed at a temperature of 100◦C
and 200◦C for 20 minutes in oven. ZnO thin films
were prepared using a double dip techniques by
varying deposition parameter, such as solvent
medium, concentration, pH value, temperature,
number of dippings etc. For air annealing the
samples a furnace is employed.The post annealing
temperatures and time were selected by the
stability of the sample.
Results and discussion
Fig 1 shows the XRD spectra of the pure ZnO
films prepared at different temperature from 30°C
to 200°C . The diffractogram indicates the
presence of prominent peaks corresponding to (1 0
0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2)
planes of the material. The studies revealed that
the ZnO films are polycrystalline in nature and
exhibiting hexagonal (Wurtzite) structure. Films
are annealing at 200°C and 100°C for 20 minutes,
produces a considerable improvement in
crystallinity, and showing more intense XRD
peaks. In room temperature produces sharper
XRD peaks. The highest peak at angle 2θ=
36.0199 corresponding to the (1 0 1) reflection,
and other small peaks are at angle 2θ= 31.5391
and 2θ= 34.1868 corresponding to the (1 0 0) and
(1 0 1) directions respectively. Fig 1 indicates that
the intensity of the peak increase and the
broadening of the peak (1 0 1 ) decrease with
increase in the annealing temperature. It may be
due to increase in the crystallinity and crystallite
size of the films.No new peaks appeared during
increase of annealing temperature. The annealing
of the samples upto 200°C produced no phase
transformation. The observed peaks in the
diffraction pattern were indexed and
corresponding values of d were calculated and
compared with standard values (JCPDS Card
No.89-0511). Fig 2 shows the calculated crystallite
size along with their corresponding peak width
full width half maximum(FWHM) and
2 values.The grain size of ZnO thin films were
evaluated using Debye- Scherer formula(15)
D= m


cos
9.0
Where,  is the wavelength of the X-ray (
1.54A°) and  is the full width at half maximum
of the corresponding peak of the XRD pattern. 
is the Bragg’s angle.The lattice constant also
calculated.
In Fig 2 The large FWHM observed for all the
XRD peaks show that the ZnO films have grains
made of nanocrystallites and calculated crystallite
size is of the order of 47.660 - 50.333nm. As the
annealing temperature increases, the film shows
a slight increase in the grain size . Growth of the
crystallites and the expansion of the matrix are
due to post heat treatment. The micro strain is
calculated using the relation ξ =
4
cos
where,  is Full Width Half Maximum of the
peak, s  is the Bragg’s angle. As the post
heat treatment get increased, the micro strain of
the films are decreased. It may be due to removal
of defects in the lattice with increase in annealing
temperature.The dislocation density ( ) can be
evaluated from the relation
2
1
D
 lines/m2
where, D is a Grain size. The dislocation density
of the film decreased with increase in grain size.
10 20 30 40 50 60 70 80 90 100
0
500
1000
1500
2000
2500
112
103
110102
101
002
100
intensity
2 theta
30
0
C
10 20 30 40 50 60 70 80 90 100
0
500
1000
1500
2000
2500
112
103
110
102
101
002
100
intensity
2 theta
100
0
C
Fig 1(b) X – ray differaction patterns of Zno thin
film at 100°C

10 20 30 40 50 60 70 80 90 100
0
500
1000
1500
2000
2500
112
103
110102
101
002
100
200
0
C
intensity
2 theta
Fig 1(c) X – ray differaction patterns of Zno thin
film at 200°C
Fig 2 Variation of Grain size with Annealing
temperature
The fig 3 shows the Absorption spectra of
annealed and unannealed ZnO films The
unannealed films have 30% of absorption and the
absorption values increases up to 80% for
annealed films. This increase in absorption is due
to decrease in the band gap values. The relation
between the absorption co-efficient (α) and photon
energy (hv) for direct allowed transition is
α hv = (hv-Eg)nwhere , α is Absorption co-efficient
, hv is Photon energy, Eg is the Energy gap, n is
an integer depending on the nature of electronic
transition, for the direct allowed transitions n has
a value of ½ while for indirect allowed transition
n=2. The band gap energy of the grown thin films
can be determined by extrapolation of the linear
part of the plot of (αhv)2versus the incident
radiation energy hv. the band gap of ZnO thin
films was found to be 3.23ev, 3.08ev, 3.00ev
corresponding to the temperatures30ºC, 100ºC,
200°C respectively.However, the band gap can be
changed by the alterations in the oxygen sites.
The increase in band gap may be associated with
decrease in oxygen content in the film, which
might have been caused due to the annealing. As
temperature of annealing increases, more of
oxygen is absorbed in the process that could cause
a decrease in oxygen content of the films(16-21).
200 300 400 500 600 700 800
50
100
150
200
250
300
Absorption
wavelangth(nm)
Fig. 3 (a) Absorption spectra of ZnO thin film at
30°C
200 300 400 500 600 700 800
450
500
550
600
650
700
Absorption
wavelangth(nm)
Fig. 3 (b) Absorption spectra of ZnO thin film at
100°C

200 300 400 500 600 700 800
20
30
40
50
60
70
80
90
100
Absorption
wavelength(nm)
Fig. 3 (c) Absorption spectra of ZnO thin film at
200°C
Fig 4 shows the SEM micrographs of the surface
morphology of pure ZnO films deposited with
different temperature at 30°C, 100°C and 200°C of
the order of magnification 5µm. Increasing the
annealing temperature result, increase in grain
size and homogeneity(22). ZnO film in room
temperature have a particle of the order of 32 nm
in size, the substrate surface being entirely
covered for the film. ZnO films annealed at 100°C
ha flower like particles of the order of 23nm. The
surface of the ZnO film annealed at 200°C have
nano rods about at 14 nm. It is observed fig
4(a),4(b) and 4(c) the grain sizes are small and
distributed uniformly throughout the entire
surface. All micrographs show films with excellent
unchromity and due to hexagonal structure. An
decrease in the size of the crystallites with anneal
temperature was also evident from the SEM
picture.
Fig.4(a) SEM image of ZnO thin film at 30°C
Fig.4(b) SEM image of ZnO thin film at 100°C
Fig.4(c) SEM image of ZnO thin film at 20s0°C
Fig 5 shows the composition of the ZnO nano
crystalline thin film investigated with the help of
Energy Dispersive X-ray Spectroscopy (EDS). Fig
shows the clear peaks of Zinc(Z) and Oxide(O) at
various points on the sample. But some additional
peaks also present which could be due to impurity
in the reagents. The EDAX spectrum of ZnO thin
films deposited under optimised deposition
conditions. The quantitative elemental analysis is
carried out only for Zn and O.
Fig 5 EDAX spectrum of pure ZnO thin films

Fig.5 shows the electrical conductivity of ZnO thin
film is determined by using Four Probe method.
The electrical conductivity of ZnO is directly
related to the number of electrons formed by the
ionization of the interstitial Zinc atom and the
Oxygen vacancies. The electrical conductivity of
the films increase with the increase of the (0 0 2)
orientation of the film.
The electrical conductivity of the films can be
determined using the relation
ρ = V/I×2π×S (ohm/cm)
C=1/ ρ (ohm/cm)-1
where, R is Resistance in Ohms,V is Voltage in
volts,I - Current in ampere, C - Conductivity in
(ohm/cm)-1,ρ – Resistivity in ohm/cm, S – Distance
of the probe (0.2).The electrical resistivity of the
films are decreased gradually with the increase of
annealing temperature of the order of 0.9152×(10-2
(Ω.Cm). The electrical conductivity of the ZnO thin
films increases with the increase of annealing
temperature in the order of 1.10381×102 (ΩCm) -1 .
Fig 5 Plot of Temperature Vs. Conductivity of
annealed and unannealed ZnO thin films.
Conclusion
In present work, The Optical, Structural and
Electrical characterization of both annealed,
annealed ZnO thin films were made to grow over
glass substrates by a low cost Chemical Bath
Deposition, for the annealing temperatures from
100°C to 200°C. The XRD results revealed that
the deposited thin films have a Hexagonal
(Wurtzite) structure. The unannealed films have
the crystallite size lower than those films that are
annealed at temperatures from 100°C and 200°C.
The micro strain and dislocation density of the
films are decreased with the increase in size of the
crystal due to post heat treatment.ZnO thin films
had high absorptions with regard to its Structural
properties. It has been observed that the direct
band gap energy decreased by 3.23ev, 3.08ev and
3.00ev corresponding to the temperatures 30°C,
100°C and 200°C respectively. From the SEM
analysis, it has studied that, the grains are
distributed consistently in this hexagonal
structured ZnO thin films. Also, it has observed
that, the size of the grains get increased with the
annealing temperature up to 200°C. From the
composition analysis EDAX spectra of ZnO thin
films deposited under optimised conditions the
quantitative elemental analysis is carried out only
for Zn and O. The Electrical conductivity of ZnO
thin films are increased with the increase of
annealing temperature of the order of
1.10381×102 (Ω.Cm) -1. The electrical resistivity of
the film are decreased gradually with the increase
of annealing temperature of the order ofsss
0.9152×10-2 (Ω.Cm)(23).ZnO thin films are the
most prominent transparent conducting oxides for
applications such as solar cells, photovoltaic, gas
sensors, piezoelectric devices.
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