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, Taiwanb 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

Introduction Experimental details Results and discussion Conclusions

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Introduction

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

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 range using non-polarized light.

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Results and discussion

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

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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