R.Bekkari & al. / Mor. J. Chem.

4 N°2 (2016)289- 298

Influence of precursor concentration and annealing treatment on the

structural and optical properties of sol gel ZnO thin films.

R. Bekkari1, L. Laânab1 and B. Jaber2

1LCS, Faculty of Sciences. Mohammed V University, Rabat, Morocco

2CNRST, Angle Allal El Fassi / FAR, B.P. 8027, Hay Riyad 10000 Rabat, Morocco

* Corresponding author: jaber@cnrst.ma

Received 10 Dec 2015, Revised 02 Fev 2016, Accepted 06 May 2016

Abstract:
In this work, Nanocrystalline ZnO thin films are successfully deposited, by sol-gel spin coating
process, on glass and silicon substrates, using Zinc acetate dehydrate as the precursor material. The
structural and optical characteristics of the films are investigated for various precursor concentrations
and annealing temperatures. X-ray diffraction (XRD) results show that the obtained ZnO films have
a preferential (002) orientation, which depends strongly on the precursor concentration and substrate
nature. The Scherer formula reveals that the crystallites have a nanometric character and their average
size varies between 10 and 40 nm. Scanning electron microscopy (SEM) exam shows granular surface
with a relatively dense structure. Using spectrophotometry, the measured transmittance of the ZnO
thin films, in the visible region, is ranged from 75 to 95%. However, no significant variation of the
direct band gap energy with the concentration of precursor or the annealing temperature is observed.
The evaluated band gap energy value of the prepared ZnO thin films is close to 3.31eV.
Keywords: ZnO, sol-gel, spin-coating, thin films, XRD.
I .Introduction:
Zinc oxide is a n-type semiconductor with a broadband gap of 3.3 eV, which crystallizes in the
Hexagonal Wurtzite structure (c = 0,521nm and a = 0,325nm) [1]. It has been extensively applied in
many fields such as electronic or optoelectronic [2, 3], sensors [3], chemical and biomedical sciences
[4]. Recently, ZnO in the form of thin films has attracted more attention as transparent electrode in
the panel displays and solar cells [2, 3, 5]; and also as metal oxide in optoelectronic and piezoelectric
devices [6]. The hardware requirements of these applications are behind the development of several
ZnO growing techniques including sputtering [5, 7, 8], pulsed laser deposition [9], chemical vapor
deposition [1, 10], chemical bath deposition [11, 12], spray pyrolysis [11], electron beam evaporation
[13] and sol-gel method [12, 14]. Among all these, the sol gel process has advantages over the other
techniques due to i) the excellent stoichiometry control that leads to homogeneous products, ii) the
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lower crystallization temperature and iii) the easy fabrication of large-area film with low cost [3, 5,
15].
The optimization of the nanoparticles synthesis process and the best understanding of the relationship
between the physical and microstructural properties are the key issue for the future insertion of ZnO
films in nanotechnology devices. As it was reported, the quality of the ZnO films is particularly
determined by its transparency, conductivity and crystalline orientation [16, 17]. In this framework,
Sol gel ZnO films can crystallize in various orientations which strongly depend on the sol
concentration, annealing temperature, deposition technique and many other factors [7]. The
crystalline orientation is the most important parameter to optimize when seeking to develop ZnO
films with piezoelectric properties [17]. It is around this perspective that turns our work in which we
project to develop ZnO-based transducers. To excite several modes, the preparation of ZnO films
presenting different orientations is strongly preferred. In this context, the influence of the precursor
concentration and annealing treatment on the structural and optical properties of sol gel ZnO thin
films has been investigated.
II. Experimental details:
To prepare the ZnO thin films, zinc acetate dehydrate (Zn (CH3COO) 2, 2H2O), isopropanol (as
solvent) and MonoEthanolAmine (MEA as a stabilizer) were used as starting materials. Firstly, zinc
acetate (at chosen concentration) was dissolved in isopropanol at room temperature, and then the
solution was heated at 60°C for one hour before adding MEA to the solution drop by drop. The molar
ratio MEA and zinc acetate was maintained at 1:1. A clear homogeneous solution was obtained after
one hour of reaction. To ensure a clean surface, all used substrates are first degreasing in a boiling
isopropanol, washing with acetone using ultrasonic for 5 min and then cleaned in a boiling ethanol
using ultrasonic for 5 min.
The ZnO films were deposited on the glass and silicon substrates by spin coating at 3000 rpm for 10
s under normal conditions of temperature and pressure. The as-synthesized films were preheated in
air at 200°C for 15 min. The coating and the preheating treatment processes were repeated several
times to achieve the desired thickness. Finally, the films were subsequently annealed at 500°C for
crystallization and densification. The annealing temperature study was carried out using a horizontal
furnace under normal atmospheric conditions from ambient to 700°C while the precursor
concentration and annealing time are fixed at 1M and 2 hours respectively.
After the elaboration step, the film structure was analyzed by X-ray diffractometer (Panalytical Expert
Pro) working in Bragg–Brentano geometry with Cu Kα radiation. The morphology of the film surface
was examined using an FEI Quanta 200 Scanning Electron Microscopy (SEM). The optical properties
were carried out using a PerkinElmer Lambda 900 Spectrophotometer in the UV/ Vis/ Nir regions.

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III. Results and discussion:

A-Effect of Zn-precursor concentration:
Fig.1 (a) shows the XRD patterns of ZnO thin films deposited on glass substrates at various Zn
concentrations. As observed, the recorded peaks matched well with the hexagonal wurtzite structure
of ZnO. It was clearly found that an increase in the zinc concentration leads ZnO films to adopt (002)
preferred orientation. Comparable results were observed by F. Zahdi et al. [18] who have used the
spray pyrolysis as growth method.
To quantify the preferred orientation of the obtained ZnO films, we compute the relative intensity
ratio (Ir) defined as:
𝐼ℎ𝑘𝑙
𝐼𝑟 = (1)
∑𝐼ℎ𝑘𝑙

As can be deduced from fig.1 (b) the relative intensity ratio of (002) increases up to 0.5 (0.2 in bulk)
when increasing the Zn concentration. This result indicates that (002) is the preferential orientation
of growth at high Zn concentrations. However, the film prepared with low Zn-precursor concentration
behaves like the bulk (polycrystalline) with no preferred orientation.
The structural evolution of the films deposited on silicon substrate was illustrated by the XRD patterns
in the fig.2 (a). All the recorded diffraction peaks are indexed and matches with a pure hexagonal
wurtzite structure (JCPDS card n° 36-1451) without any secondary phase. It was found that, contrary
to films prepared on glass substrate, increasing Zn concentration does not promote the preferred (002)
orientation. This behavior attributed to the difference in the crystalline nature of substrates. In fact
the surface of the glass is amorphous whereas that of the silicon is crystalline.

Fig.1: Evolution of the XRD spectra (a) and Relative intensity ratio (b) of ZnO films deposited on
glass substrates at different precursor concentrations.

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Fig.2: Evolution of the XRD spectra (a) and Relative intensity ratio (b) of ZnO films deposited on
silicon substrate at various Zn concentrations.
As can be noted from Fig.2 (b), the obtained ZnO films using precursor with low Zn concentration
(0.5M) have a strong (002) preferred orientation, while those prepared from charged sol (≥0.7M)
behave as Bulk. Similar results were also reported by Y. H. Hwang et al. [19].
The crystallite size D of the film was evaluated using Scherer’s formula:
0,9λ
𝐷= (2)
βcosѲ

Where λ is the wavelength of the used X-ray radiation  is the Full width at Half Maximum and 2θ is
the highest diffraction angle.
In the same way, the micro-strain in the films was calculated using the following formula:
βcosѲ
𝜀= (3)
4
Fig. 3 (a) & (b) demonstrates the relation between particle size and strain of the films as a function
of Zn precursor concentration. As expected, the crystallite size increases with increasing the Zn
precursor concentration. This behavior, reported also by many authors [7, 20, 21], has been usually
attributed to the increase in the amount of material. Furthermore, the evaluated strain decreases as the
increases. This behavior, observed also by N. Nagayasamy [6], is due to the fact that low Zn precursor
concentration promotes nano-sized crystallites growth.

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Fig.3: Relation between particle size (a) and strains (b) with respect to Zinc concentration for
ZnO/glass films.

Fig.4 depicts the SEM images of ZnO thin films prepared on silicon at different concentrations. As
can be seen, all the ZnO films have a smooth microstructure with uniform spherical shape in the
nanometer order. It was found that increasing Zn precursor concentration results in slight growth of
the grain size.

Fig. 4: SEM Images of ZnO/Si thin films elaborated at 0.3M, 0.5M and 1M.

B-Effect of annealing treatment:

We report in Fig. 5 the X-ray diffraction patterns of ZnO films deposited on silicon at room
temperature, preheated at 200°C and then annealed at various annealing temperatures. This analysis
shows that the films are polycrystalline. The three main peaks are identified as (100), (002) and (101)
of the ZnO wurtzite structure. As can be observed, the initiation of the crystallization occurs already

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at 300°C. Note that films annealed between 400 and 600°C, are preferentially oriented in the (002)
direction, while those annealed above 600°C behave as the bulk.

Fig.5: XRD patterns of ZnO /Si thin films at various annealing temperatures.

The evolution of the calculated average crystallite size, using the Scherer formula, at different
annealing temperatures is illustrated in fig.6. As it was reported [22, 24], the ZnO crystallite size
increases with the increase of the annealing temperature.

Fig.6: Particle size at various annealing temperatures for ZnO/Si films

Fig.7 shows SEM images of the samples obtained at various annealing temperatures where the Zn
concentration was fixed to 1M. As expected, increasing the annealing temperature increases the grain
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size. The coalescence of small grains, due the increment of the energy on the surface when the
annealing temperature increases, is behind the observed behavior [25, 26].

Fig.7: Morphology of ZnO/Si films at different annealing temperatures

B. Optical Properties:
The measured transmittance and reflectance spectra of the ZnO films are presented in fig.8. The
optical absorption corresponds to the electron transition from valence to the conduction band. As
illustrate by fig.8 (a), a sharp absorption edge in the transmittance spectra is observed near 380 nm
which corresponds to the ZnO bulk band gap. The transmittance of the all obtained ZnO thin films is
greater than 80% in the visible light region. We note that when the Zn concentration becomes higher
the optical transmission increases up to 95% (for 1M). This phenomenon is attributed to the fact that
the number of Zn atoms on the surface becomes higher, as the Zn concentration increases, especially
when the deposited thickness is substantially less than the micrometer. In another side, an excessive
thickness leads to an opposite effect: a reduction of the transmittance when the Zn concentration
increases [6, 7].

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Fig.8 (a): Optical transmittance spectra of ZnO/glass thin films for 0.5M, 0.7M and 1M

Fig. 8 (b): the reflectance spectra of ZnO/Si films at different precursor concentrations.

Fig.8 (b) shows the reflectance spectra of spin coated ZnO/Si films at different precursor
concentrations. The observed interference fringes are due to multiple reflections of the light at the
two film interfaces (air/film and film/substrate). Near the band gap absorption, the increase of the
ZnO film reflectance with increasing the precursor concentration is generally related to an
improvement of the crystal and surface morphology of the film.

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Fig.9 presents the reflectance spectra of ZnO/Si films prepared at different annealing temperatures.
It is observed that the reflectance of the elaborated films is improved when increasing the annealing
temperature. An optimal condition with exceptional maximum reflectance is obtained when the
annealing temperature is close to 500° C.

Fig. 9: The reflectance spectra of ZnO/Si films prepared at different annealing temperatures.

In order to estimate the band gap value, the curve (𝛼ℎ𝜈)2 = 𝑓(ℎ𝜈) has been extrapolated to the value
(𝛼ℎ𝜈)2 = 0 using Tauc Equation:
1
(𝛼ℎ𝜈) = 𝐵(ℎ𝜈 − 𝐸𝑔)2 (4)
Where h is the energy of the incident photons, B a constant and α is the absorption coefficient. α can
be calculated from the transmittance T (%), the reflection coefficient R (%) and the thickness d of the
thin layer using the following equation:

T = (1 − 𝑅)2 exp(−𝛼𝑑 ) (5)

From the resulting interference pattern (interference fringes), the thickness of the film can be
determined by the following expression:

m
d= (6)
2Dn √n2 - sin2 θ

Where m is the number of fringes in the used wave number region Dn (1 - 2; cm-1), n is the refractive
index and θ is the angle of incidence. Table 1 presents the variations of the optical band gap and the
thickness of ZnO films as a function of the precursor concentrations.

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Table 1: values of the optical gap and thickness as a function of the precursor concentrations for
ZnO/Si films.

[Zn] (M) m d (nm) Eg (eV)

0,5 2 397 3,29

0,7 3 597 3,31

1 3 597 3,32

It is observed that the energy band gap and the film thickness increase slightly with increasing the
precursor concentration. As it has been reported that the band gap of ZnO films doesn’t depend
significantly on the thickness [27], the observed decrease in the band gap can be therefore attributed
to the generation of strain in ZnO film which leads to a variation in the inter-atomic spacing [18].

Table 2: Values of the thickness d and optical gap for different annealing temperatures

Annealing Temperature (°) m d (nm) Eg (eV)

300 4 794 3,27

400 4 794 3,29

500 3 595 3,31

600 2 397 3,28

700 2 397 3,28

In table 2 are presented the band gap and thickness values as function of the annealing temperature.
It is observed that the energy band gap of ZnO films rises from 3.27 to 3.31eV when the temperature
is varied from 300°C to 500°C. The observed band gap increase is attributed to the fact that annealing
at relatively high temperatures improves the crystal quality of the (002) plane (the polar axe of ZnO)
as it's obvious in the XRD patterns. This statement is valid only up to 500 °C. Above this temperature,
band gap energy decreases again, and that may be related to the increase in the grain sizes.

Conclusion
In this work, the effect of the precursor concentration and annealing treatment on the structural and
optical properties of sol gel derived ZnO thin films has been studied and discussed. XRD results
confirm clearly that the synthesis parameters affect both the film orientation and the crystal quality.
SEM observations show that ZnO film microstructure consists of uniform spherical nanoparticles
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whose size is ranging between 10 and 40 nm. The increase of the precursor concentration and
annealing temperature leads to the increasing in the particle size. The spectrophotometer
measurements show high transparency (>90%) in the visible region. The measured band gap values
of the obtained ZnO films are between 3.27 and 3.32 eV, that is in good agreement with literature
values.
These experimental results suggest that 0.5M and 500°C are respectively the optimal precursor
concentration and annealing temperature for the production of the sol gel ZnO film having good
structural and optical properties.

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