Photo-physical Study of Al doped ZnO thin films grown by sol-gel method
Kishor M. More, Sudam D. Chavhan, R. R. Ahire, Anup J.
More*
Department of Physics, V.V.M’s S.G. Patil College,
Sakri, Dist.-Dhule, Pin-424304
Corresponding Author:dranupmore@gmail.com
Abstract
In
the present study ZnO:Al Thin films were prepared by sol-gel method. We have
fabricated the aluminium doped ZnO thin films by varying the deposition cycle
and annealing temperature. The spin-coating speed of the samples were 3000 rpm
for 30 sec and subjected to 300 oC and 500 oC annealing
temperatures to obtain uniform films. The obtained films were characterized by
using X-ray diffraction to determine the crystalline structure of the prepared
films. The surface morphology study was carried out by high resolution field
emission scanning electron microscopy (FE-SEM). Optical properties of AZO. The
XRD pattern of AZO thin films revealed the hexagonal crystal structure.The
surface morphology of AZO thin films was uniform and nanocrystalline particle
growth was observed. AZO thin films were highly transparent (>90 %) in the
visible region
Keywords:
Thin Films, AZO, solar cell, Sensor XRD, FE-SEM.
1.
Introduction
ZnO is well-known functional n-type
metal oxide semiconductors of the II-VI group. These materials have drawn
significant attention because of their favorable physical and chemical
properties such as wide band gap, good transparency, low processing cost,
excellent gas sensing ability, good stability in ambient atmosphere and non-toxicity
[1-2].The functionalities of these compound semiconductors can be improved by
doping the appropriate metals into the host materials and it plays a vital role
in solar cell technology. The gas sensing property is the surface phenomenon,
in which interaction of analyte gas molecules with the adsorbed oxygen alters
the sheet resistance of the material [3-4]. Therefore, it is very important to
select an appropriate dopant for a particular gas sensing application to
improve its sensing performance and selectivity to a single gas [5].
Undoped and doped ZnO and SnO2
thin films have been widely used as a gas sensor for the different gases [6].
Generally for the high performance gas applications, ZnO thin films preferably
doped with Al, Ga, or In are used.
There are different ways to prepare
the thin films of ZnO:Al, metal organic
chemical vapor deposition (MOCVD), Chemical vapor deposition (CVD), DC or RF
Sputtering, hydrothermal method, facial spray pyrolysis technique, pulsed laser
deposition and sol-gel [7-14]. Among these techniques, sol-gel method offers
some particular advantages because it has the ability to produce a solid state
material from a chemically homogenous precursor. In sol-gel method, atomic
level mixing of reagents is able to produce the complex inorganic materials
such as ternary and quaternary oxides at lower processing temperature and
shorter synthesis time. Furthermore, by using sol-gel technique, it is possible
to achieve greater control over the growth of particle size and surface
morphology of the thin films.
Therefore,
in the present investigation, for all three different kinds of thin films
sol-gel method was employed. The Al doped ZnO thin films were prepared with
different Al doping concentration for humidity sensor. The effects of doping
concentration and annealing temperatures on the structures, morphology of thins
films were studied via X-ray diffraction, filed effect scanning electron
microscopy techniques, respectively. Finally, sensing properties of all three thin
films were measured.
2. Experimental Details
The chemicals used in all three
different experiments were purchased from commercial sources and used without
further purification. For the
preparation of ZnO:Al thin films only alumina substrates were used. The glass
and alumina substrates were cleaned with soap solution, acetone and ethanol via
ultra-sonication process for 15 min at each stage. Finally, the substrates were dried at 110 oC
for 10 min prior to spin coating.
2.1. Al doped ZnO (AZO) thin film preparation
Al doped ZnO thin films were
deposited on soda-lime glass and alumina substrates by sol-gel method. For the preparation of AZO thin films, zinc
acetate dihydrate and aluminum nitrate nonahydrate were used as source
materials for zinc and aluminum. 0.55 M zinc acetate solution was prepared in
absolute ethanol and desire amount of aluminum nitrate was added to zinc
precursor solution. The complexing agent was added to obtain the clear
solution. The AZO precursor solution was spin coated on the glass or alumina
substrates at 3000 rpm for 30 sec and samples were dried at 300 oC
for 10 min. This process was repeated to number of times in order to obtain the
required thickness of the AZO thin films. Finally the films were sintered at
different annealing temperatures such as 550 oC.
2.4. Characterization Techniques
The formed phases in the prepared thin films were
identified via X-ray diffraction. Field-effect scanning electron microscope is used to study
surface morphology of the samples. The UV-VIS spectrophotometer was used to
study the optical properties of the AZO.
3. Results and Discussion
3.1. Al doped ZnO (AZO) Thin Films
3.1.1 Structural Study
Figure 1 shows the X-ray diffraction
pattern of glass/ZnO:Al thin films annealed at 500 oC for 30 min.
The Al doping concentration was 0.6 at% with respect to zinc. The XRD pattern shows the (101) peak
orientation at 36.207 degree, confirming the hexagonal structure of ZnO thin
film (JCPDS: 80-0074). The intensity of (101) peak increased with increasing in
thickness of Al:ZnO films. The obtained results are in consistence with the
previously reported data [1,8].
The poor crystallinity of AZO thin films can be
attributed to the excess amount of aluminum present in the AZO film. Therefore, to improve the crystallinity of the
Al doped ZnO thin film; the AZO thin films were prepared on alumina instead of
soda-lime glass substrates so that annealing temperature can be increased
beyond the 500 oC.
Figure 1:
X-ray diffraction pattern of Al:ZnO thin films annealed at 500oC
3.1.2 Surface morphology study
Figure
2: FE-SEM image of Al:ZnO thin films deposited on glass substrates
The
small crystalline size of Al:ZnO thin films confirms the nanocrystalline growth
of Al:ZnO thin films.
Fig.3: Plot
of optical transmittance versus wavelength for AZO films deposited on soda lime
glass substrates.
Conclusions
Al doped ZnO (AZO) thin films
were successfully deposited via sol-gel method. The XRD study of AZO films
annealed at 500 oC revealed the hexagonal structure of ZnO thin
film. The broad orientation of (100) peak confirms nanocrystalline nature of
the grown films. Uniform and smooth surface morphology was noticed from SEM
analysis.
Acknowledgement
Authors are thankful to
the Principal Dr. R. R. Ahire of Vidya
Vikas Mandal’s Sitaram Govind Patil Sakri, for providing the laboratory
facility.
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