Temperature dependence of Fluorine-doped tin oxide films produced by ultrasonic spray pyrolysis

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Temperature dependence of Fluorine-doped tin oxide

ﬁlms produced by ultrasonic

spray pyrolysis

Chin-Ching Lin

a,

⁎

, Mei-Ching Chiang

a

, Yu-Wei Chen

b

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}

a b s t r a c t

a r t i c l e i n f o

Available online 11 June 2009 Keywords: Fluorine doping Fluorine concentration Tin oxide Spray pyrolysis SIMS

Fluorine-doped tin oxide (FTO)films were prepared at different substrate temperatures by ultrasonic spray pyrolysis technique on glass substrates. Among F-doped tin oxidefilms, the lowest resistivitiy was found to be 6.2 × 10− 4Ω-cm for a doping percentage of 50 mol% of fluorine in 0.5 M solution, deposited at 400 °C. Hall coefficient analyses and secondary ion mass spectrometry (SIMS) measured the electron carrier concentration that varies from 3.52 × 1020_cm− 3_{to 6.21 × 10}20_cm− 3_{with increasing}_{fluorine content from}

4.6 × 1020_cm− 3_{to 7.2 × 10}20_cm− 3_{in FTO}_{ﬁlms deposited on various temperatures. Deposition temperature}

1. Introduction

An important application of thin_{film technology has been the} development of transparent and conducting oxide (TCO) coatings that serve as window layers transparent to solar radiation and also as electrical contacts. Among these TCO's thefluorine-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 are used in different devices like solar cells as transparent, protective electrodes,[1] flat panel collectors as spectral selective windows, sensors for detection of gases, sodium lamps, gas sensors, and varistors[2_–4]. FTO_{films have} been prepared by various techniques, such as chemical vapour deposition, metalorganic deposition, rf sputtering, sol–gel, and spray pyrolysis[5_–7]. Spray pyrolysis is used to prepare_{films because of its} simplicity and commercial viability [8,9]. Moreover, 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. In this study, FTOfilms were deposited with different working temperatures in a controlled way and the study on the effect of deposition temperatures on structural and electrical properties.

2. Experimental details

Thinﬁlms of FTO on glass were prepared using a homemade ultrasonic vertical spray pyrolysis system with a hot plate heater, as

shown inScheme 1. 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 sample size is around 7.0 cm × 7.0 cm. Theﬂuorine doping was achieved by adding ammonium ﬂuoride (NH4F) to the starting solution. The

percentage offluorine doping was varied from 0 to 75 mol%.(The fluorine concentrations were 0–0.75 M in the starting solution.) 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 depositedfilms is around 550 nm. Air is used as carrier gas at a constantflow rate of 20 L/min. For each concentration, several sets offilms were prepared and found to be reproducible. Crystal structure of thefilms was studied by X-ray diffraction (XRD)

Thin Solid Films 518 (2009) 1241–1244

⁎ Corresponding author. Tel.: +886 3 591 8751; fax: +886 3 582 206.

E-mail address:[email protected](C.-C. Lin). Scheme 1. Illustration of ultrasonic spray pyrolysis deposition (SPD) apparatus.

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system using Cu-Ka radiation. The surface morphology of thefilms, crystallites size and distribution were examined by JEOL 6500 scanning electron microscopy (SEM). The_{fluorine concentration of} thefilms was examined by secondary ion mass spectrometry (SIMS). The electrical studies were carried out by Hall measurements in van der Pauw con_{figuration. The visible transmission spectra of FTO films} were measured using UV–Vis spectrometer with 190–1100 nm wavelength range using non-polarized light.

3. Results and discussion

The variation of sheet electrical resistivity (ρ) and mobility of the FTO films is shown inFig. 1as a function of_{fluorine concentration. It is found} that the resistivity of the pure tin oxide thinfilms (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). However, the} property of Hall mobility of FTOfilms is opposite to resistivity. The Hall mobility of FTOfilms was found to increase from 11 to 18 cm2_{/VS for the}

increase influorine concentration from 0 to 10% and then saturated. Thus, it indicates that optimum doping concentration offluorine could be around 50% in tin oxidefilms.

Fig.2shows the XRD patterns of FTOﬁlms grown with different working temperature. All the patterns correspond to the SnO2in the

rutile structure and contain the characteristic SnO2peaks only. The

films are polycrystalline in nature even though these films were deposited under various temperatures. The film has no preferred orientation as the growth temperature at 360 °C, whereas with progressive increase in deposition temperature,films tend to grow in speci_{fic crystallographic direction. This shows that the preferred} orientation of thefilms depends on the working temperature. For films deposited on more than 400 °C, this slight preference changes from (211) to (200). It may be seen that, while the (211) and (110) texture in thefilms at low deposition temperature noticeable, the (200) texture is stronger in thefilms with higher working tempera-ture. The high intensity of (200) reflection is observed for the films deposited on more than 440 °C. This increase in intensity leads to

Fig. 1. Variation of resistivity and Hall mobility of FTOfilms with different fluorine doping. The deposited temperature is 400 °C and thickness of depositedfilms is around 550 nm.

Fig. 2. XRD patterns of FTOﬁlms (ﬂuorine concentration at 50 mol%) for different deposition temperature.

Fig. 3. SEM micrographs of FTOfilms (fluorine concentration at 50 mol%) obtained for different deposition temperature at (a) 360 °C, (b)400 °C, and (c)500 °C, respectively. (d) Grain size of various working temperature in FTOfilms.

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better crystallinity of the _{films. Further, the XRD data could also} corroborate the trend in microstructure development observed by SEM. The SEM micrographs displaying the surface morphology of FTO films are shown inFig. 3for the different working temperatures in this study. The FTOfilms 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.

The electrical resistivity (ρ) and Hall mobility (µ) of the FTO films with different deposition temperature are shown inFig. 4. The resistivity is found to decrease with increasing working temperature initially but then increased for higher deposition temperature. The resistivity was found to decrease from 1.3× 10− 3to 6.2 × 10−4Ω-cm for the increase the FTOfilm deposition temperature from 360 °C to 400 °C. It is then increased to 1.8 × 10−3Ω-cm for higher working temperature (500 °C). It is apparent from thefigure that the Hall mobility is increasing with increasing deposition temperature till 420 °C and then saturates for the higher temperature. The Hall mobility of FTOfilms presents the similar trend in the result of grain size data (Fig. 3d). In general, the large grain of FTOfilms presents the high mobility property due to their less grain boundaries in thefilms to improve the electron mobility and resistivity. However, we found the FTOfilm deposited on high temperature shows the worst resistivity than others in this work. It is well known that the mobility and carrier concentration can both evidently influence the electrical property. The Hall coef_{ficient measurement and secondary ion} mass spectrometry (SIMS) were carried out on FTOfilms with various deposition temperatures to show the relation between electron carrier

concentration and_{fluorine concentration as shown in}Fig. 5. The_figure clearly reveals that carrier concentration of FTOfilms is increasing with increase in working temperaturefirstly and reaches a peak value and then decreases with further increase the deposition temperature. The fluorine concentration of FTO films shows the similar trend to carrier concentration of thefilms. The increase in the value of fluorine con-centration within a deposition temperature at 400 °C of FTO films probably represents a solubility limit offluorine in tin oxide lattice. When deposition temperature was beyond 400 °C, the fluorine concentration would also increase again due tofluorine escaped from FTOfilms by high temperature deposition process. Hence, it implies that carrier concentration can be obviously controlled by the fluorine concentration in FTOfilms.

Optical transmission spectra of various deposition temperature FTO ﬁlms in the range of 400–900 nm wavelength range are represented in

Fig. 6. It was observed that visible transmittance would decrease with increasing the deposition temperature of FTOfilms. Highest transmit-tance (~77%) was obtained in FTOfilms 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 FTOfilms in this work.

4. Conclusions

Polycrystalline FTOfilms showed the preferential growth along (211) which has been found to changed to (200) on high deposition temperature. The grain size and surface morphology of FTOfilms would be increased by increasing deposition temperature. The carrier concentration of FTOfilms were clearly influenced by concentration of fluorine on variation working temperatures. The FTO film deposited on 400 °C revealed the minimum resistivity of about 6.2 × 10− 4Ω-cm and maximum transmittance of 77% in the visible band. This high conductivity and transparency of FTOfilms suggest that these films are likely to be useful as electrical contacts in various electronic and energy harvest applications.

Acknowledgements

The authors would like to thank Dr. Chia-Hsin Lin for the helpful discussions. The authors also gratefully acknowledge the ﬁnancial support of Industrial Technology Research Institute through Grant Nos.7301XS4H10 and 8301XS4710.

References

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ﬁlms with respect to deposition temperature.

Fig. 5. Variation of carrier concentration andfluorine concentration of FTO (fluorine concentration at 50 mol%)films with different deposition temperature.

Fig. 6. Optical transmittance spectra of FTO (ﬂuorine concentration at 50 mol%) thin ﬁlms prepared at different deposition temperatures.

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