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Electrical, structural and optical properties of
fluorine-doped zinc oxide thin films:
Effect of the solution aging time
S.M. Rozatia, S. Moradia, S. Golshahia, R. Martinsb, E. Fortunatob
a
Department of physics, University of Guilan, Rasht 41335, Iran
b Materials Science Department/CENIMAT, Faculty of Sciences and Technology, New University of Lisbon,
Campus da Caparica, 2829-516 Caparica, Portugal
Adviser：林克默
Advisee：郭俊廷
Date ：99/03/12
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Outline
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Introduction
Experimental details
Results and discussion
Conclusions
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Introduction
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TCO films produced using binary compounds namely In2O3,
SnO2 and ZnO doped with an impurity are in practical use.
ZnO is naturally an n-type semiconductor, due to the
presence of native donors inside the lattice, known as defects.
The most prominent ones are oxygen vacancies, zinc
interstitial atoms, and hydrogen, which are always present in
all growth methods and can easily diffuse into ZnO, in large
amounts due to its large mobility.
In this work we report a study concerning the effect of aging
of the starting solution used to fabricate films by spray
pyrolysis (SP) technique, on the structural, electrical, optical
and morphological properties of ZnO:F thin films.
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Experimental details
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Fluorine-doped zinc oxide thin films were deposited on glass
substrates by the SP technique from a staring 0.4 M solution
containing zinc acetate dehydrated (Merck) dissolved in a
mixture of double distilled water, methanol (3:7 volume
proportion ) and acetic acid.
Doping of the films was achieved by adding ammonium
fluoride to the starting solution, with a fixed [F]/[Zn] ratio of
2 at.%.
Temperature of the substrate and the spray rate were fixed at
450 °C and 22 l/min, respectively.
The SP apparatus used in this work consists of a homemade
spraying unit, substrate holder with heater and enclosure.
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The glass substrate was kept on a stainless steel (ss) plate that
was heated by a 3 kW heater using canthal heating coils.
The temperature of the substrate is controlled by through a
temperature controller connected to a Chromel–Alomel
thermocouple kept at the center of the ss plate.
The air produced by the compressor was first filtered and
then connected to the glass spray-gun (atomizer) through a
sensitive flow-meter to control its flow.
The custom glass spray gun having a nozzle diameter of 0.2
mm was positioned at a distance of 25 cm above the substrate.
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Electrical resistivity was measured using two-probe method.
The optical transmittance was measured with a
spectrophotometer Varian model Cary 100 in the UV–visible
region (200–800 nm).
The optical band gap (Eg) of the films has been calculated
from the dependence of absorption coefficient (α) on the
photon energy (hυ), taking into account that ZnO is a direct
band gap semiconductor.
The structural properties were studied by X-ray diffraction
measurements (Philips PW1840) using the Cu-Kα radiation
with λ=1.5418 Å.
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The surface morphology of the films deposited were analyzed
by a Scan Electron Microscope (SEM), from Philips, model
XL30.
The data obtained from spectral transmittance were used to
calculate the thickness of the films. Film thickness and
refractive index are calculated using unconstrained
optimization method.
Thicknesses value of around 550 nm was kept constant, for
all set of samples analyzed.
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Results and discussion
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This value decreases with the age of the starting solution,
reaching a minimum of about 24 Ω/□, after 15–21 days of
solution ageing.
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These data seem to indicate that during the doping process, a
fraction of F atoms move into the ZnO lattice, increasing the
incorporation efficiency with the age of the solution, up to a
certain aging time, after which the solution may start
degrading.
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Zinc Oxide is a tetrahedral coordinated solid that with a
wurtzite structure.
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For films deposited using 15 and 24 days aging solutions, it
can be seen that the intensity of the XRD peak associated
with (100) and (110) planes predominate.
A(002) preferential orientation is observed from a first-day
solution in all the cases of chemically sprayed ZnO:F thin
films[21,22].
Small signals of the increase of peak intensity, corresponding
to the (110) and (101) planes, start appearing as the age
solution increases. We attribute this to fluorine ions
replacement by oxygen ions in the ZnO lattice.
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The average mean crystallite size was calculated for the (002)
diffraction peak, using Scherrer formula[23].
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For films deposited using 21 days aging solutions, the
average diameter of the crystallites is of the order of 24 nm.
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Fig. 3a shows the porous structure and non-uniform coverage
observed for the undoped ZnO films, while by doping with
fluorine, a change in the surface morphology is observed, as
can be seen in Fig. 3b. Using solutions 9 days aged, the
surface looks to be compact, consisting of small grains as
seen in Fig. 3c.
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Reaching values below55% for films deposited using
solutions aged by 36 days. This behavior is ascribed to the
decrease in the Zn/[O+F] ratio in the films deposited from
aged solutions
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There was a shift in the absorption edge to shorter
wavelength for the optimum film(15 days aged solution),
which was due to Burstein–Moss shift.
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Hence, transparency in the shorter wavelength region is
better for the optimum film than undoped ZnO film that can
be considered as an indication of the incorporation of F
doping.
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Fig. 6. plot of refractive index as a
function of wavelength for (a) undoped and fluorine-doped ZnO thin
films deposited from (b) first day (c)
6 days aged solution.
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Fig. 6 shows the refractive index as a function of wavelength
for undoped and fluorine-doped ZnO thin films deposited
from fresh and 6 days aged solutions. It is clear that the aged
solutions raise the refraction index value (between 1.7 and 2).
The calculated values lead to optical gaps ranging between
3.3 and 3.4 eV, for all the films.
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Conclusions
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從實驗數據得知，溶液的熟化對薄膜的結構與表面形態
有很大的影響。
(110)波峰強度隨著熟化時間的增加而增強，主要是因為
薄膜的結晶性受到了改善。
從SEM之表面觀測得知，在摻雜F原子熟化九天後，所獲
得之晶粒大小約36nm。
從實驗數據得知，FZO溶液最佳的熟化時間約在15-21天，
所沉積出的樣品其膜厚約550nm，擁有高穿透率與低片
電阻(約24Ω/□)。
隨著熟化時間增加到六天，其穿透率最高，但之後穿透
率便開始降低。
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從實驗數據得知，在可見光範圍下，熟化效應對薄膜之
折射率並無明顯的影響；在溶液熟化六天後，所沉積出
FZO薄膜其折射率為1.7-2(400-800nm)。
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Thanks for your attention
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