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Temperature dependence of Fluorine-doped tin
oxide films produced by ultrasonic spray pyrolysis
Chin-Ching Lina, Mei-Ching Chianga, Yu-Wei Chenb
a
Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan
b Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan
Adviser：林克默
Advisee：郭俊廷
Data：99/01/29
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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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Among these TCO's the fluorine-doped tin oxide(FTO),
being an n-type, wide band gap semiconductor (≥3 eV) with
special properties, high transmittance in the visible range and
high reflectance in the infrared, excellent electrical
conductivity, greater carrier mobility and good mechanical
stability.
The spray pyrolysis technique is well suited for the
preparation of doped tin oxide thin films because of it is ease
to adding various doping materials, controlling the texture via
various deposition temperatures and mass production
capability for uniform large area coatings.
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In this study, FTO films were deposited with different
working temperatures in a controlled way and the study on
the effect of deposition temperatures on structural and
electrical properties.
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Experimental details
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Thin films of FTO on glass were prepared using a homemade
ultrasonic vertical spray pyrolysis system with a hot plate
heater.
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The initial solution is prepared from 0.5 mol hydrated
stannous chloride (SnCl2×2H2O) in 1.0 L of deionized water
and Corning glass (EAGLE 2000) was used as substrates.
The fluorine doping was achieved by adding ammonium
fluoride (NH4F) to the starting solution. The percentage of
fluorine doping was varied from 0 to 75 mol%.
The substrate temperature (working temperature) was varied
from 360°C to 500°C and the deposition time was 5–10 min
for all the depositions. The thickness of deposited films is
around 550 nm.
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Crystal structure of the films was studied by X-ray diffraction
(XRD) system using Cu-Ka radiation.
The surface morphology of the films, crystallites size and
distribution were examined by JEOL 6500 scanning electron
microscopy (SEM).
The fluorine concentration of the films was examined by
secondary ion mass spectrometry (SIMS).
The electrical studies were carried out by Hall measurements
in van der Pauw configuration.
The visible transmission spectra of FTO films were measured
using UV–Vis spectrometer with 190–1100 nm wavelength
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range using non-polarized light.
Results and discussion
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It is found that the resistivity of the pure tin oxide thin films
(3.0×10−2 Ω-cm) decreases with increasing fluorine
concentration initially and then reaches a saturate value
(6.2×10−4 Ω-cm at 50 mol% F).
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All the patterns correspond to the SnO2 in the rutile structure
and contain the characteristic SnO2 peaks only.
This shows that the preferred orientation of the films depends
on the working temperature. For films deposited on more
than 400°C, this slight preference changes from (211) to
(200).
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The FTO films are characterized by uniform-sized grains
with cubical shape at the deposition temperature below
400°C, which are on the average smaller than the grains in
the high temperature region.
The grain size of FTO films are rapidly increasing as working
temperature increasing, and then the crystallite size would
become saturate as the temperature reaches to 420°C.
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The resistivity was found to decrease from 1.3×10−3 to
6.2×10−4 Ω-cm for the increase the FTO film deposition
temperature from 360°C to 400°C.
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The Hall mobility of FTO films presents the similar trend in
the result of grain size data
In general, the large grain of FTO films presents the high
mobility property due to their less grain boundaries in the
films to improve the electron mobility and resistivity.
However, we found the FTO film deposited on high
temperature shows the worst resistivity than others in this
work.
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The figure clearly reveals that carrier concentration of FTO
films is increasing with increase in working temperature
firstly and reaches a peak value and then decreases with
further increase the deposition temperature.
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The fluorine concentration of FTO films shows the similar
trend to carrier concentration of the films.
When deposition temperature was beyond 400 °C, the
fluorine concentration would also increase again due to
fluorine escaped from FTO films by high temperature
deposition process.
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It was observed that visible transmittance would decrease
with increasing the deposition temperature of FTO films.
Highest transmittance (~77%) was obtained in FTO films on
working temperature at 400°C, whereas a lower
transmittance (~60%) was observed in FTO films on working
temperature at 500 °C.
This important variation could be due to the optical scattering
by surface morphology and grain boundaries of FTO films in
this work.
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Conclusions
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多晶FTO薄膜在高溫沉積時，其優選取向從(211)轉變為
(200)。
隨著FTO薄膜的沉積溫度增加，其表面晶粒也增大。
隨著不同的沉積溫度，FTO薄膜的載子濃度有明顯的影
響。
FTO薄膜在沉積溫度400°C時，獲得最低之電阻率約
6.2×10−4 Ω-cm，在可見光範圍之最高穿透率約77%。
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Thanks for your attention
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