Materials Sciences and Applicatio ns, 2011, 2, 643-648
doi:10.4236/msa.2011.26088 Published Online June 2011 (http://www.SciRP.org/journal/msa)
643
Growth Mechanism of ZnO Films Deposited by
Spray Pyrolysis Technique
Bhavana Godbole1, Nitu Badera2, Shyambihari Shrivastava2, Deepti Jain3, Vganesan Ganesan3
1Department of Applied Physics, Jabalpur Engineering College, Jabalpur, India; 2School of Studies in Physics, Vikram University,
Ujjain, India; 3UGC-DAE, Consortium of Scientific Researches, Indore, India.
Received November 26th, 2010; revised January 17th, 2011; accepted May 17th, 2011.
ABSTRACT
Transparent Zinc Oxide (ZnO) thin films having different thickness are prepared by using Spray pyrolysis technique.
Structural and morphological behavior has been studied by using atomic force microscopy and x-ray diffraction. The
scaling behavior obtained by using height-height correlation method shows that, films with different thickness are de-
veloped under non-equilibrium condition behaves as self-affine surfaces. With the increase in thickness, the grain size
as well as activation energy have found to be decreased.
Keywords: Nano-Crystalline Materials, Thin Film, Atomic Force Microscope, Structure and Morphology, Thickness
1. Introduction
ZnO is a fascinating material for different applications in
both microelectronic and optoelectronic devices. It is a
wide-band gap oxide semiconductor with a direct energy
gap of about 3.37 eV. It has found numerous applications,
such as surface acoustic wave devices [1], piezoelectric
devices [2], ultra sonic transducers [3], ultraviolet photo
detectors [4], gas sensors [5] etc. The properties of thin
films, which are the basis of their application in techno-
logical devices, depend on their surface properties, par-
ticularly on the surface roughness. Due to the interest
related to the specific properties of transparent metal
oxide thin films, recent studies are focused in the correla-
tion of surface with deposition parameters and physical
properties. Many deposition techniques have been ap-
plied for the production of ZnO films in order to improve
their properties [6-15]. Thus, the knowledge of the
mechanisms that determine the film structure has moti-
vated a large amount of research during the last years.
The physical properties of the thin films depend not only
on the morphology of the sample but also on the prepara-
tion route, thickness of the sample and grain sizes. Re-
producible properties are achieved only when the thick-
ness and deposition parameters are kept constant. In case
of optical devices especially as interference filters, anti-
reflection coatings etc, the success of fabrication depend
on the deposition of specific thickness [16]. In the pre-
sent work, we have reported the dependence of thickness
on surface topography and growth mechanism involved
during the deposition.
2. Sample Preparation
The deposition of ZnO films is carried out by using sim-
ple, economic and home made Spray pyrolysis (SP)
technique. The starting solution consists of 0.2 M of zinc
nitrate dissolved in distilled water. The films have been
deposited on ultrasonically cleaned glass substrates kept
at 400˚C. The atomization of the solution in the fine
droplets was affected by spray nozzle with the help of
compressed air, during the course of the spray. The spray
rate of 1 mL·min–1 was maintained and the distance be-
tween tip of nozzle and the substrate was kept at 28 cm.
In order to obtain different thickness of films different
volumes of precursor solution say 25 mL, 50 mL, 75 mL,
and 100 mL was sprayed on preheated glass substrates
respectively and their thickness is calculated by weight
difference method. The aqueous solution of zinc nitrate
gets thermally decomposes when fallowing over the sur-
face of preheated substrates. This results in the formation
of well adherent and uniform ZnO film. The as deposited
films of ZnO are transparent with faint white tint. The
chemical reaction involved is as fallows:


322
2
22 2
2ZnNO6HOO
2ZnO4NO2O12HO




The structural studies of the prepared ZnO thin films

Growth Mechanism of ZnO Films Deposited by Spray Pyrolysis Technique 644
are done by means of X-ray diffractometer (Rigaku
RINT 2000) with CuKα radiation having the wavelength
of 1.5418˚A and in the range of 2θ = 5˚ - 80˚. Continuous
scanning was done with the slow scanning speed with
small time constant. The surface morphology (grain size
and surface roughness) was measured by using Nano-
scope-E atomic force microscope (AFM). These meas-
urements are carried out by using a silicon nitride tip
with contact mode at room temperature. The electrical
measurements are carried out by using home made
dc-two probe resistivity set up.
3. Results and Discussion
In the present work, we have deposited 25 mL, 50 mL,
75 mL and 100 mL of precursor solution on to the pre-
heated glass substrate. The thicknesses of the films were
calculated by using weight difference method were found
to be increase with the amount of precursor solution used
for spray. The thickness so obtained is shown in Table 1.
The XRD spectra corresponding to the film deposited for
25 mL of precursor solution is shown in Figure 1. It is
found that, the films are polycrystalline with hexagonal
wurtzite structure [17]. In pure ZnO the most intensive
peak has a strong c-axis orientation along (002) direction
and found to be at 2θ = 34.48˚. No extra peak was ob-
served in the XRD pattern. The lattice constant a and c
was found to be 3.2499 nm and 5.20 nm respectively.
Figure 1. XRD spectra for ZnO thin film.
Table 1. The variation in thickness, grain size and activa-
tion energy with amount of spray solution.
Spray Solution
(mL) Thickness (nm) Grain size (nm) Activation band gap Ea (eV)
25 98 167.9 0.55
50 193 127.5, 160.9 0.57
75 284 150.8, 182.5 0.34
100 386 143.0 0.26
The AFM measurements could provide information
regarding the surface morphology of films and this capa-
bility might be used to investigate the nature of the de-
posited thin film like the roughness and size of the grains.
A careful analysis of roughness can yield information
regarding the kind of growth that is taken place while
film is formed.
In order to realize the grain size distribution and
roughness as a function of film thickness, the AFM data
has been analyzed as follows: AFM images along with its
lateral force images for the case of ZnO films with dif-
ferent volumetric concentration have been displayed in
Figure 2(a-d).
The grain sizes were measured for all the films and
their statistical distributions are plotted in Figure 3(a-d).
It is observed that the grain size decreases with the in-
crease in thickness.
The growth mechanism could be understood by using
concept of scaling. It was first introduced by Family et al.
[18] to provide a frame work for fractal like topologies.
The solid films grown under far from equilibrium con-
ditions are predicted to have self-affine surfaces. The
fundamental assumption is that all rough surfaces exhibit
perpendicular fluctuations, which are characterized by
their root mean square (rms) width.

1/2
2
,Zxy   (1)
where




,, ,
Z
xy hxyhxy (2)
and h(x, y) are height fluctuations, h(x, y) is spatial av-
erage over a planner reference structure. The films grown
under non-equilibrium conditions are also expected to
develop the self-affine surfaces [19] whose rms widths
scale with the time t and length L as fallows;



,LtLft L



  (3)
where,


~LL

 for / tL




~tt

 for /0tL


The parameter
 is the static scaling or roughness ex-
ponent and
 is growth exponent. The roughness is also
characterized by height-height correlation function,
 
2
,gRZxyZx y





 (4)
where,
 
12
22
,Rxx yy


 


which scales as,

Growth Mechanism of ZnO Films Deposited by Spray Pyrolysis Technique645
(a)
(b)
(c)
(d)
Figure 2. AFM and LFM images of the film (2 × 2 m) ob-
tained by spraying different amount of solutions. (a) for 25
mL of solution; (b) for 50 mL of solution; (c) for 75 mL of
solution; (d) for 100 mL of solution.
(a)
(b)
(c)
(d)
Figure 3. The log g(R) vs log (R) plots for film for different
thickness. (a) for 25 mL; (b) for 50 mL; (c) for 75 mL; (d)
for 100 mL.

Growth Mechanism of ZnO Films Deposited by Spray Pyrolysis Technique 646
 
2
g
RR

 as (5(a)) 0R
and

2gR

 as (5(b)) R
In order to get the information regarding the roughness
of prepared films with thickness, the quantities g(R) from
the AFM data is derived. The plot of log g(R) vs log (R)
should give the information regarding the roughness ex-
ponent
 and also the growth exponent  , (Equation (5)).
The value of g(R) has been obtained by scanning the film
for smaller areas. The value of g(R) so obtained is plotted
in Figure 4(a-d), by plotting log g(R) vs log(R) for dif-
ferent amount of precursor solution.
The slope of the upper curve will give the value of

and for the films corresponding to the different thickness
is displayed in Figure 5(a). In order to obtain the growth
exponent, one needs to carry out in-situ experiments
while the film grows. We have attempted to correlate the
time factor to the film thickness, as they are proportional
to each other at least upto a first approximation. Figure
5(b) shows the log-log of rms roughness verses film
thickness, where all the data were taking recorded for
0.274  0.274 µm2 pixel area. The slope of Figure 5(b)
should yield β. The value of β was found to be 0.26 ±
0.089 from the plot of log (R) =Atβ.
A comparison of our results with the available litera-
ture [20], suggest the exponent should falls in the range
of 0.2 ≤ α ≤ 1.0 and 0.2 ≤ β ≤ 0.56 for ZnO films. The
results so obtained are in good agreement with the avail-
able literature. Thus, ZnO thin films developed under
non-equilibrium conditions behave as self-affine surfaces.
The obtained results suggest that the roughness of the
film increases with the increases in thickness of the film,
i.e. with the increase in the amount of sprayed solution.
Electrical measurements on ZnO films having differ-
ent thicknesses were made by using conventional dc-two
probe method in the temperature range of 250 K to 400 K.
Contacts made were of copper wires using silver paste on
the surface of films. All the films show semiconducting
behavior. The activation energy were calculated by using
equation as
 = 
o (–Ea/k T), where
 is the resistivity at
the temperature T,

o is a constant, k is the Boltzmann
constant and Ea is the activation energy required for tran-
sition. The activation energy was found to decrease with
increase in thickness of the films. The values so obtained
are given in Table 1.
The variation of particle size distributions and associ-
ated grain boundaries may be responsible for decrease in
activation energy.
4. Conclusions
The ZnO thin films having different thickness prepared
(a)
(b)
(c)
(d)
Figure 4. The log g(R) vs log(R) for different amount of
precursor solutions.

Growth Mechanism of ZnO Films Deposited by Spray Pyrolysis Technique647
(a)
(b)
Figure 5. Variation in  and  exponents of films having
different thickness.
by Spray pyrolysis technique at constant substrate tem-
perature of 400˚C on glass substrates, was investigated
by XRD, AFM and resistivity measurements. The XRD
results suggest that, the film is polycrystalline in nature.
The scaling behavior obtained by using height-height
correlation method shows that films having different
thickness are developed under non-equilibrium condition
behaves as self-affine surfaces. The grain size was found
to decrease while the roughness of the film increases
with increase in thickness of the film. Activation energy
was found to decrease with the increase in thickness of
the film.
5. Acknowledgments
The author from Jabalpur would like to thank the Direc-
tor and Centre Director of UGC-DAE CSR, Indore for
their support under the CRS plan and Mr. L. S. Sharath
Chandra and Ms. Swati Pandya for their assistance.
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