Characterization of Nanocrystalline ZnO:Al Films by SolGel Spin Coating Method
P.L. Gareso1*, N. Rauf1, E. Juarlin1, Sugianto2 and A. Maddu2
1
Department of Physics, Faculty of Mathematics and Natural Sciences, Hasanuddin University, Makassar 90245
2
Department of Physics, Faculty of Mathematics and Natural Sciences, Bogor Institute of Culture, IPB Bogor
* Email: pgareso@gmail.com
Abstract. Nanocrystalline ZnO films doped with aluminium by sol-gel spin coating method have been investigated
using optical transmitance UV-Vis and X-ray diffraction (X-RD) measurements. ZnO films were prepared using zinc
acetate dehydrate (Zn(CH3COO)2.2H2O), ethanol, and diethanolamine (DEA) as a starting material, solvent, and
stabilizer, respectively. For doped films, AlCl3 was added to the mixture. The ZnO:Al films were deposited on a
transparent conductive oxide (TCO) substrate using spin coating tecnique at room temperature with a rate of 3000 rpm
in 30 sec. The deposited films were annealed at various temperatures from 400 oC to 600oC during 60 minutes. The
transmitance UV-Vis measurement results showed that after annealing at 400oC, the energy band gap profile of
nanocrystalline ZnO:Al film was a blue shift. This indicated that the band gap of ZnO:Al increassed after annealing due
to the increase of crystalinity size. As the annealing temperature increased the bandgap energy was a constant. In
addition to this, there was a small oscillation occuring after annealing compared to the as–grown samples. In the case of
X-RD measurements, the crystalinity of the films were amorphous before annealing, and after annealing the crystalinity
became enhance. Also, X-RD results showed that structure of nanocrystalline ZnO:Al films were hexagonal
polycrystalline with lattice parameters are a = 3.290 Å and c = 5.2531 Å.
Keywords: Nanocrystalline, ZnO, spin-coating, transmitance
PACS: 78.66 Hf
INTRODUCTION
EXPERIMENTAL DETAILS
ZnO film is one of II-IV compound semiconductor
that has been widely used for many devices
application such as thin film sensors [1], light emitting
diodes [2], spintronic devices [3] and nanolasers [4,5].
Also, ZnO thin film is used as solar cell window since
it has high optical transmittance in the visible region.
Furthermore, ZnO has a large band gap of 3.3 eV,
large exitonic binding energy of 60 meV and high
carrier mobility at room temperature.
Various techniques have been applied for
fabricating ZnO thin film such as sputtering [6],
molecular beam epitaxy (MBE) [7], spray pyrolysis
[8], pulse laser deposition (PLD) [9], metal-organic
chemical vapour deposition (MOCVD) [10] and sol
gel method [11-12]. Compare to other tecniques, the
sol-gel method is widely used to deposite the thin
films ZnO due to its simplicity, low cost to investigate
the structures and optical properties, and the easy to
control of chemical compound.
In this study, nanocrystalline ZnO:Al films have
been deposited by sol-gel spin coating tecniques on
transparent conducctive oxide (TCO) substrate. The
films were characterized by X-ray diffraction and
optical transmitance UV-Vis.
Nanocrystalline ZnO:Al films were prepared using
Zinc acetate dehydrate (Zn(CH3COO)2.2H2O), ethanol,
and diethanolamine (DEA) as a starting material,
solvent and stabilizer, respectively. For doped films,
AlCl3 was added to the mixture with an atomic
percentage fixed at 1 or 2 at % Al. The precursor
solution was deposited on TCO substrate by spin
coating method with a speed of 3000 rpm for 30
second. After depositing by spin coating, the films
were dried at 300oC for 15 min in a furnace to
evaporate the solvent and remove an organic solvent.
Then, the films were inserted to the tube furnace and
annealed at various temperatures from 400oC to 600oC
for 60 minutes.
X-ray diffraction (X-RD) was used to investigate
the crystalline structure of ZnO:Al films using single
scan diffractometer with Cu Kα (λ = 1.5406 Å)
radiation and scanning range of 2θ between 20 o and
70o. During the measurement, the current and the
voltage of X-RD were maintained at 30 mA and 40 kV,
respectively and the scan speed was 2o/min. The
Transmission spectrum of crystalline ZnO:Al films
were performed to characterize the optical properties
using a single beam UV-Vis spectrophotometer with a
wavelength range of 250 nm – 800 nm. The band gap
energy of ZnO:Al films were obtained from the
transmission specrum.
It also shows in Figure 1 that the energy band gap of
ZnO:Al after annealing is blue shift in comparison to
the as-grown samples. This is probably due to the
increase of the grain size of ZnO:Al.
RESULTS AND DISCUSSION
Figure.1 shows the optical transmitance spectrum
of nanocrystalline ZnO:Al films before and after
annealing at various temperatures from 400oC to
600oC. It can be seen in this graph that the optical
transmitance spectrum is a broadening in comparison
to the samples after annealing. The transmitance is
below 80% in the range of wavelength 300 – 500 nm.
After annealing the transmitance spectrum become
narrow and there is a sharp absorption edge located at
370 – 380 nm. This is indicated that in the as-grown
sample, the absorption edge is smaller than after
annealing due to the crystalinity in the as-grown
sample is lower in comparison to the sample after
annealing. In addition to this, after annealing there is a
oscillation spectrum occuring in the range of
wavelength 400 nm-800 nm. The corresponding
optical band gap of ZnO:Al films is estimated by
extrapolation of the linier relationship between (αhυ)2
and hυ according to the equation
αhυ = A (hυ – Eg)1/2
(1)
where α is the absorption coefficient, hυ is the photon
energy, Eg is the optical band gap and A is a constant.
Based on this equation the band gap value of ZnO:Al
will be determined.
Fig.2. X-RD spectra of nanocrystalline ZnO:Al films
before and after annealing at various temperature from
400oC to 600oC for 60 minutes
Figure 2 displays the X-ray diffraction spectrum of
nanocrystalline ZnO:Al films after annealing at with
different annealing temperature from 400 oC to 600oC.
The X-RD patterns of as-grown sample are also
included for comparison. In the case of as-grown
sample ZnO:Al films, the spectra of X-RD merely
show for aluminium holder of the sample, while the XRD spectrum of ZnO:Al were not observed. This
indicates that the ZnO:Al is still amorphous due to the
grain size of the crystallinity is small. After annealing,
more additional peaks were observed due to the
increase of the crystalinity of ZnO:Al films. As
temperature annealing increase, the spectrum width
become narrowing that indicates that the grain size
increases.
The unit cell ‘a” and “c” of the nanocrystalline
ZnO:Al films (002) orientation are calculated using the
relation (1) and (2)
𝑎 = √1⁄3 𝜆⁄𝑠𝑖𝑛𝜃
c = 𝜆/𝑠𝑖𝑛𝜃
Fig.1. The optical transmitance spectrum of
nanocrystalline ZnO:Al films before and after
annealing at various temperature from 400oC to 600oC
for 60 minutes.
(2)
(3)
From the XRD spectrum, grain size (D) of the film is
calculated using the Debye Scherrer formula,
D = kλ/β cosθ
(4)
Where k is a constant, λ, β, and θ are the X-ray
wavelength, full width at half maximum (FWHM) and
Bragg angle respectively. Table.1 shows the structural
parameters of nanocrystalline ZnO:Al film
Table.1. Structural parameters of nanocrystalline
ZnO:Al films
Sample
plane FWHM 2 θ(deg)
d(Å)
As-grown
0
0
0
002
400oC
0.1917
35.4683 2.5606
500oC
0.2350
35.4683 2.5335
600oC
0.2433
35.4683 2.5288
The dislocation density (δ), which represents the
amount of defects in the crystal, is estimated from the
following equation:
δ = 1/D2
(5)
While the strain (ε) of the film is determined from the
following equation:
ε = β cosθ/4
(6)
SUMMARY
We have investigated the structural and optical
characterization of nanocrystalline ZnO:Al films
grown by sol-gel spin coating using x-ray diffraction
and optical transmitance UV-Vis diffractophotometer.
X-RD measurement shows that there is no diffraction
pattern was observed in ZnO:Al before annealing.
After annealing there is more additional X-RD peaks
were observed. This indicates that the cyrstallinity the
sample of nanocrystalline ZnO:Al became enhance.
The results are also confirmed in the optical
transmitance that the sample of ZnO:Al become
amorphous before annealing.
ACKNOWLEDGMENTS
The authors acknowledge the financial support
from the higher education of Indonesia (DIKTI)
through out LP2M-UNHAS under contract number of
110/UN4-42/LK.26/SP-UH/2013.
REFERENCES
1. S. T. Shishiyanu, T. S, Shishiyanu and O. I. Lupen,
Sensors and Actuators B: Chemical, 107, 379-386
(2005).
2. N. Saito, H. Haneda, T. Sekiguchi, N. Ohashi, I.
Sakaguchi and K. Koumoto, Advanced Materials,
14, 418-421 (2002).
3. T. Meron and G. Morkovich, Journal of Physical
Chemistry B, 109, 20232-20236 (2005)
4. M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H.
Kind, E. Weber, R. Russo and P. Yang, Science,
292, 1897-1899 (2001)
5. C. Jagadish, and S. Pearton (Editors), Zinc Oxide
Bulk, Thin Film and Nanostructures, Elsevier,
2006.
6. P. Nanes, D.Costa, E. Furtunato and R. Martins,
Vacuum, 64, 293-297 (2002)
7. D.C. Look, D. C. Reynolds, C. W. Litton, R. L.
Jones, D. B. Eason and G. Gantwell, Appl. Phys.
Lett, 81, 1830-1832 (2002).
8. M. Krunks and E. Mellikov, Thin Solid Films, 270,
33-36 (1995).
9. X. L. Wu, G. G. Siu, C. L. Fu, and H. C. Ong,
Appl. Phys. Lett, 78, 2285-2287 (2001)
10. K. Tominaga, T. Takao, A. Fukushima, T. Moriga
and L. Nakabayashi, Vacuum, 66, 505-509 (2002)
11.E. J. Luna-Arredondo, A. Mmaldonado, R.
Asomoza, D. R. Acosta, M. A. Melendez-lina and
M. de la L. Olvera, Thin solid Film, 490, 132-136
(2005).
12.L. J. Mandalapu, F. X. Xiu, Z. Yang, D. T. Zhao
and J. L. Liu, Apply, Phys. Lett, 88, 112108112110 (2006).