Journal of Modern Physics, 2012, 3, 1516-1522
http://dx.doi.org/10.4236/jmp.2012.310187 Published Online October 2012 (http://www.SciRP.org/journal/jmp)
Effect of Concentration on the Optical and Solid State
Properties of ZnO Thin Films Deposited by Aqueous
Chemical Growth (ACG) Method
Sylvester Lekoo Mammah1,2, Fidelix Ekeoma Opara2, Friday Barikpe Sigalo2,
Sabastine Chukwuemeka Ezugwu3, Fabian Ifeanyichukwu Ezema3
1Department of Science Laboratory Technology, School of Applied Sciences,
Rivers State Polytechnic, Bori, Nigeria
2Department of Physics, Faculty of Science, Rivers State University of Science and Technology,
Port Harcourt, Nigeria
3Department of Physics and Astronomy, University of Nigeria, Nsukka, Nigeria
Received August 3, 2012; revised September 4, 2012; accepted September 12, 2012
ABSTRACT
Thin films of Zinc Oxide (ZnO) having different concentrations were deposited using the Aqueous Chemical Growth
(ACG) method. The films were characterized using Rutherford Back Scattering (RBS) spectroscopy for chemical com-
position and thickness, X-Ray Diffraction (XRD) for crystallographic structure, a UV-VIS spectrophotometer for the
analysis of the optical and solid state properties which include spectral absorbance, transmittance, reflectance, refractive
index, direct band gap, real and imaginary dielectric constants, absorption and extinction coefficients and a photo-
microscope for photomicrographs. The average deposited film thickness was 100 nm. The results indicate that the val-
ues of all the optical and solid state properties investigated vary directly with concentration except transmittance which
is the reverse. Thus, the optical and solid state properties of ZnO thin film deposited by the Aqueous Chemical Growth
method can be tuned by deliberately controlling the concentration of the precursors for various optoelectronic applica-
tions including its application as absorber layer in solar cells.
Keywords: Component; Formatting; Style; Styling; Insert
1. Introduction
Zinc Oxide is an amphoteric oxide which exist in the
form of white powder (zinc white) at room temperature
[1]. Though zinc oxide exist naturally in the earth crust
(zincite), it is the synthetic ZnO produced in laboratories
that are usually used in industries [2].
Crystalline zinc oxide is thermochromic [3]. Apart
from the ability of ZnO to react with acids, it also react
with bases to produce soluble zincates [4-6]. Zinc oxide
has high stability at room temperature and decomposes
into zinc vapour and oxygen at about 1975˚C [7]. Though
zinc oxide can exist in either of hexagonal wurtzite, cubic
zinc blende and cubic (rock salt) structure, the hexagonal
wurtzite structure is the most common at room tempera-
ture. The different polymorphs of ZnO do not possess the
property of symmetry inversion. This and other proper-
ties such as ionic bonding account for the strong piezo-
electricity of both hexagonal and zinc blende ZnO [8,9].
The hexagonal wurtzite structure has the point group 6
mm (Hermann-Mauguin notation) or C6v (schoenflies
notation) and a space group of P63 me or . Its lattice
constants are a = 3.25 Å and C = 5.2 Å [10].
4
6v
C
The many applications to which ZnO can be put has
made it to be a material for constant scientific study. The
diverse applications of ZnO are derived from its unique
properties which include non-toxicity, good electrical
conductivity, high luminous transmittance, good sub-
strate adherence, good optical behaviour and stability in
plasma atmosphere [11,12].
ZnO has been tested and found to behave as a semi-
conductor having a wide bulk direct band gap of about
3.37 ev at room temperature [13] ZnO has been used for
the fabrication of light emitting diodes, varistors, photo
detectors, piezoelectric cantilever, gas sensors, buffer
layer in solar cells and in photonic crystals [14-21].
As a result of the strategic importance of ZnO to hu-
manity, various deposition methods have been used for
its production in the form of thin films. Among these
methods are the chemical bath deposition (CBD) [22-26].
Successive Ionic Layer Adsorption and Reaction (SI-
LAR) method [27,28].
Spray pyrolysis method [29].
C
opyright © 2012 SciRes. JMP

S. L. MAMMAH ET AL. 1517
Electro deposition method [30,31].
The ACG method is a novel method for the deposition
of ZnO thin films; hence the literature on it is sparse.
Indeed F. I. Ezema (2004) [26] reported that “Thin
films of ZnO have been used but its preparation tech-
niques have been restricted to sputtering, vacuum eva-
poration, chemical vapour deposition, spray prolysis,
molecular beam epitaxy, sol gel and pulse laser”.
Among the advantages which the ACG method have
over the other methods are simplicity, low cost, repro-
ducibility, availability of materials, environmental fri-
endliness (non toxicity of residue), non requirement of
surfactant, templates and complexing agents, low tem-
perature requirements, ability to produce nanostructures,
high purity (absence of surfactants), suitability for large
scale production etc. [32,33].
This paper reports the effect of concentration of pre-
cursors on the optical and solid state properties of ZnO
thin films deposited using the ACG method with
Zn(NO3)2·6H2O as precursor material. The spectral anal-
ysis of the optical and solid state properties was carried
out using a Unico UV-2102 PC spectrophotometer.
2. Experimental Details
ZnO thin films were deposited on clean glass slides using
the Aqueous Chemical Growth method.
Equimolar concentrations of hydrated zinc nitrate
(Zn(NO3)2·6H2O) and hexamine in 80 ml of water were
used as precursors.
Hexamine was used to make the zinc nitrate alkaline.
Three samples having different concentrations of the
precursors were prepared as shown in Table 1.
Masses of samples were measured using an analytical
microbalance. The masses of the components of each
sample was put into a 100 ml pyrex bottle together with
80 ml of water. The contents were properly mixed using
a magnetic stirrer
The pyrex bottles were then tightly corked and care-
fully placed in an oven at a temperature of 90.00˚C.
The average deposition time for the films was twelve
hours
The chemical reactions which resulted in the crystalli-
zation of ZnO are
612 423
CH N6HO6HCHO4NH 
NHH ONHOH

 
32 4
2
2OHZnZnOH O


2 [34].
3. Results and Discussion
The elemental composition of sample C2 was analyzed
using Rutherford Backscattering (RBS).
The result is as shown in Figure 1. Judging from the
film composition shown in the Table 2, we conclude that
the elements contained in sample C2 are Zn (0.050%) and
oxygen (0.950%) while the glass substrate has the com-
position 0 (0.500%), Si (0.120%), Ca (0.100%), Al
(0.100%) and Na (0.180%). This is summarized in Table
2. The thickness of the film is given as 100 nm.
The X-Ray diffractogram of the ACG ZnO thin films
were studied to determine its crystalline nature.
The thin films were scanned continuously between O
and 70 at step size of 0.03 and at time per step of 0.15 s.
Figure 2 shows the intensity of peaks versus diffraction
angle 2θ for sample C2 ZnO thin film using CuKα radia-
tion source having a wavelength of 1.54056 Å. The
X-ray diffractogram reveal several peaks corresponding
to directions of strong reflections. Some of the observed
peaks are 30.00˚, 32.50˚, 34.42˚, 45.56˚ and 54.87˚, and
67.50˚.
The 34.42˚ agree with the preferred orientation along
the (002) plane reported by Ajuba et al. (2010) [35] and
Shinde et al. (2005) [36] who used the CBD and SILAR
methods respectively. Also, the 67.50˚ fairly agree with
the (112) plane reported by Ajuba et al. (2010) [35,36].
C
2
Figure 1. RBS analysis for ACG ZnO Thin Film (sample
C2).
Table 1. Three samples of different concentrations of the precursor.
Sample of mass Zn (NO3)2·6H2O Mass of Hexane Conc. of Zn(NO3)2·6H2O Conc. of Hexane Volume of H2O
C3 0.60 g 0.030 g 0.025 M 0.025 M 80.00 ml
C4 1.20 g 0.060 g 0.050 M 0.050 M 80.00 ml
C2 2.40 g 0.112 g 0.100 M 0.100 M 80.00 ml

S. L. MAMMAH ET AL.
1518
Table 2. Elemental composition of ZnO thin film and substrate from RBS analysis.
Oxygen Zinc Silicon Calcium Aluminium Sodium
ZnO thin film 0.950% 0.050% - - - -
Glass substrate 0.500% - 0.120% 0.100% 0.100% 0.180%
Figure 2. X-Ray diffractogram for ACG ZnO Thin Film
(sample C2).
The diffractogram reveal sharp diffraction peaks
which is indicative of good crystallinity [37-41].
The XRD pattern shows that film is crystallized in the
wurtzite ZnO hexagonal P6 (3) mc structure. (JCPDF
CARD no. 36-1451) [42].
The mean size of the crystallite was estimated to be 12
nm using the Sherrer formula which is given as:
k
cos
D



 (1)
where k = 0.9
λ = 1.541 Å is the diffraction peak angle (34.42˚) and β
is the Full Width at Half Maximum (FWHM) corre-
sponding to the diffraction peak.
The spectral absorbance of films is as shown in Figure
3.
The 0.05 M concentration has an average absorbance
value of 0.28 in the visible region while the 0.025 M
concentration has an average absorbance value of 0.268
in the visible region.
However, the 0.05 M concentration has the lowest ab-
sorbance value of 0.21 in the infrared region.
Absorption peaks clearly occurred in film C2 at 368
nm, 496 nm and 656 nm but were not so clearly defined
in films C3 and C4. Ezema [26] reported absorption
peaks at 3.68 nm, 449 nm and 566 nm using Chemical
Bath Method. The difference in the absorption peaks may
be due to deposition method. The spectral transmittance
of the films is as shown in Figure 4. Transmittance de-
creases with concentration in the visible region. The 0.1
Figure 3. Absorbance vs wavelength for ZnO thin film at
different Conc.
M, 0.05 M and 0.025 M concentrations has average
transmittance of 45.5%, 53.2% and 54.60% respectively
in the visible region.
However, the 0.05 M concentration has the highest
transmittance of about 61.8% in the ultraviolet region.
The transmittance of the films in the visible region is
generally lower than the above 85% transmittance re-
ported using Chemical Bath Deposition method [43].
It is also much lower than the 90% - 95% transmit-
tance in the UV-VIS-NIR regions for ZnO thin films
deposited using spray pyrolysis method [44,45] and Sol-
Gel method [46].
It has been reported that the transmittance of a film in-
creases as its thickness decreases [26].
The average thickness of the films being reported on is
100 nm which is far smaller than the 0.203 μm and 0.069
μm thicknesses which gave average transmittances of
62% and 75% respectively that has been reported [26].
Thus, the transmittance of films deposited by ACG me-
thod are relatively lower compared to the ones deposited
by other methods even though the thicknesses are smaller.
The spectral reflectance of the films being reported on is
as shown in Figure 5. Reflectance increases with con-
centration in the visible region.
The 0.1 M, 0.050 M and 0.025 M concentrations re-
flect an average of 20.25%, 19.38% and 18.88% of visi-
ble electromagnetic waves. These values are higher than
the average of 12% to 17% reflectance of films deposited
using Chemical Bath Method as reported by Ezema [26].
Thus, films deposited by ACG method are better re-

S. L. MAMMAH ET AL. 1519
Figure 4. Transmittance vs wavelength for ZnO thin films
at different Conc.
Figure 5. Reflectance vs wavelength for ZnO thin films at
different Conc.
flectors than those deposited by Chemical Bath Method.
Absorption coefficient increases with concentration in
the visible region as shown in Figure 6.
The average absorption coefficients for the 0.025 M,
0.05 M and 0.1 M concentrations are 0.62, 0.65 and 0.77
respectively. However, in the UV region, 0.05 M has the
lowest absorption coefficient of about 0.49.
Refractive index is fairly constant in the visible region
as shown in Figure 7. The 0.1 M, 0.05 M and 0.025 M
concentrations have average refractive indices of 2.3,
2.225 and 2.2 respectively. However, the 0.05 M con-
centration has the lowest refractive index of about 2.05 in
the UV region.
It has been reported that the refractive index of ZnO
films doped with Li deposited by spray pyrolysis lie be-
tween 1.60 and 2.20 at a wavelength of 500 nm [45],
while Ezema [26] gave the average refractive index of
the ZnO films deposited by Chemical Bath method as
being between 1.64 and 1.98 and also reported observed
Figure 6. Absorption coefficient vs photon energy for ZnO
at different Conc.
Figure 7. Refractive index vs photon energy for ZnO thin
films at different Conc.
peak values of 2.28 at 368 nm and 1.72 at 569 nm. In our
own case, the refractive indices for the different concen-
trations were generally uniform in the visible region.
Extinction coefficient decreases with concentration
and also decreases with increasing photon energy (wave-
length) in the visible region as shown in Figure 8. The
value of extinction coefficient for 0.1 M concentration
ranges from 37.85 × 10−3 for 2 eV to 27.14 × 10−3 for 3
eV in the visible region. Similarly, the values of extinc-
tion coefficient for 0.05 M concentration ranges from 35 ×
10−3 for 2 eV to 19.64 × 10−3 for 3 eV and that of the
0.025 M concentration ranges from 30 × 10−3 to about
21.43 × 10−3 at 3 eV in the visible region However, the
0.05 M concentration has the lowest extinction coefficient
of about 13.93 × 10−3 in the ultraviolet region. In general
all the concentrations have the lowest value of extinction
coefficient in the higher energy region (between 3 eV
and 4 eV) and the maximum value in the low energy re-
gion (between 1 eV and 2 eV). Low and high values of

S. L. MAMMAH ET AL.
1520
Figure 8. Extinction coefficient vs photon energy for ZnO
thin films at different Conc.
extinction coefficient at high and low energy regions
respectively have been reported [26].
The band gap increases with concentration as shown in
Figure 9. The average band gap for the 0.025 M, 0.05 M
and 0.1 M concentrations are 1.16 eV, 1.22 eV and 1.4
eV respectively.
These values are lower than the direct band gaps of
ZnO films deposited by Chemical Bath method [26]
which lie between 1.60 eV and 1.80 eV.
Our values are also far lower than 3.29 eV band gap
obtained for aluminium doped ZnO deposited using radio
frequency (r.f) megnetron sputtering [18], and intrinsic
band gap of 3.20eV for ZnO films deposited by spray
pyrolysis [29]. The low values of the direct band gap
obtained indicate that the ZnO thin film materials pre-
pared by ACG method are suitable for use as absorber
layers in solar cells. The low values of the band gaps
obtained in this work may be due to preparation condi-
tions Ezema et al. [26].
Real dielectric constant increases with concentration in
the visible region as shown in Figure 10. The 0.025 M,
0.05 M and 0.1 M concentrations have average real di-
electric constants of 4.76, 4.85 and 5.15 respectively in
the visible region of the electromagnetic spectrum.
However, the 0.05 M concentration has the lowest real
dielectric constant of about 4.09 in the ultra-violet region.
Figure 11 is the Imaginary dielectric constant that in-
creases with concentration in the visible range.
The 0.1 M, 0.05 M and 0.025 M concentrations have
average values of imaginary dielectric constant which
ranges from 180.3 × 10−3 for 2 eV to 118.18 × 10−3 for 3
eV, 143.93 × 10−3 for 2 eV to 78.785 × 10−3 for 3 eV and
143.93 × 10−3 for 2 eV to 89.39 × 10−3 for 3 eV respec-
tively. This shows that the imaginary dielectric constant
decreases with increasing photon energy (decreasing
wavelength) for all concentrations in the visible region.
Figure 9. Direct band gap plot for ZnO thin films at differ-
ent Conc.
Figure 10. Real dielectric constant vs photon energy for
ZnO thin films at different Conc.
Figure 11. Imaginery dielectric constant vs photon energy
for ZnO thin films at different Conc.

S. L. MAMMAH ET AL. 1521
C
2
200x
C
3
200x
C
4
200x
Figure 12. Photomicrographs of ACG ZnO thin films.
However, the 0.05 M concentration has the lowest
imaginary dielectric constant of about 56.06 in the ultra
violet region. All concentrations have their lowest imagi-
nary dielectric constant values in the ultra-violet region.
The photomicrographs as shown in Figure 12 indicate
uniform deposition of the ZnO thin films on the glass
substrate. While C2 and C3 show stretches of rod C4
shows clusters of crystallites on the substrates.
4. Conclusion
Crystals of ZnO have been successfully grown on glass
slides in the form of thin films from the aqueous solution
of hexahydrated zinc nitrate and hexamine after the fa-
shion of Lionel Vassieres [47]. The concentration of the
precursors was found to vary directly with absorbance,
reflectance, absorption coefficient, extinction coefficient,
refractive index, direct band gap, real dielectric constant
and imaginery dielectric constant. The transmittance of
the thin films was found to vary inversely with the con-
centration of the precursors.
The results indicate the suitability of ZnO thin films
prepared by the Aqueous Chemical Growth method for
various optoelectronic applications such as absorber
layer in solar cells.
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