CO-EXISTENCE OF F AND Sb DOPANTS IN TRANSPARENT CONDUCTING SnO2 THIN
FILMS PREPARED BY ULTRASONIC SPRAY PYROLYSIS METHOD
Ngamnit Wongcharoen, Thitinai Gaewdang Department of Applied Physics, Faculty of Science King Mongkut’s Institute of Technology Ladkrabang
3 Moo 2, Chalongkrung Rd., Ladkrabang, Bangkok 10520, Thailand.
kwngamni@kmitl.ac.th
of transparent and conducting oxide (TCO) coatings such as SnO2, CdO, ZnO, Cd2SnO4, CdIn2O4 and In2O3. Among
Thin films of undoped, fluorine(F)- doped, antimony the different transparent conducting oxides, SnO2 films (Sb)-doped, fluorine and antimony co-doped SnO2 thin films doped with fluorine or antimony seem to be the most were prepared by ultrasonic spray pyrolysis technique using appropriate for use in solar cells, owing to its low electrical SnCl2, NH4F and SbCl3 as precursors of Sn, F and Sb elements resistivity and high optical transmittance [1]. SnO2 is respectively. In F and Sb co-doped SnO2 films, the proportions chemically inert, mechanically hard and can resist high of F and Sb to Sn in starting solution were found to be 15 temperature. SnO2 either doped or undoped can be and 2 wt%. XRD patterns showed that the preferred synthesised by numerous techniques such as thermal
evaporation, sputting, chemical vapor deposition, sol-gel orientation of undoped, SnO2:F, SnO2:Sb and SnO2:F,Sb
was dependent on the doping concentration and on dip coating, painting, spray pyrolysis and hydrothermal annealing atmosphere. The variation of doping method. Among the various deposition techniques the spray concentration and preferred orientation of the films was pyrolysis is the well suited for the preparation of doped reflected in their morphology as investigated by SEM. The SnO2 thin films because of its simple and inexpensive
experimental arrangement, ease of adding various doping annealed SnO2:F, Sb films showed the lower transmission
value than the value obtained in the as-prepared SnO2:F, Sb elements, reproducibility, high growth rate and mass films. Photoluminescence spectra of the undoped SnO2 production capability for uniform large area coating, which are films showed a broad emission band peaking around 400, desirable for industrial and solar cell applications. The prime 430, and 520 nm. This emission band may be attributed to aim of this work is to prepare thin films of undoped, F-doped, oxygen vacancy and surface defects. Sb-doped, F and Sb co-doped SnO2 thin films by ultrasonic
spray pyrolysis technique using SnCl2, NH4F and SbCl3 as precursors of Sn, F and Sb elements respectively. The advantages of the SnCl2 are that is cheaper than SnCl4 and can 1. INTRODUCTION be produced easily in a laboratory [2]. Structural and electrical properties of the obtained films were also investigated. An important application of thin film technology from the
point of view of global energy crunch is solar cell, which converts the energy of the solar radiation into useful electrical energy. The main requirement for thin film solar 2. EXPERIMENTAL DETAILS cells is the window materials, which allows the visible
Thin films of undoped, F-doped, Sb-doped, F and Sb region of solar spectrum to pass through but reflect the
co-doped SnO2 were prepared by a home-made ultrasonic infrared region. This can be achieved by the development ABSTRACT
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spray pyrolysis experimental setup. Stannous chloride (SnCl2.2H2O) of 0.2M was used as precursor of tin. The tin precursor dissolved in concentrated HCl and subsequently diluted with methanol served as starting solution. The addition of HCl was required in order to break down the polymer molecules that were formed when diluting within methanol. NH4F and SbCl3 dissolved in deionized water were added into the starting solution in order to dope the films; the variation of proportions of F and Sb to Sn in starting solution were 5 to 20 and 1 to 4 wt % respectively. In the case of F and Sb co-doped SnO2 films, the proportions of F and Sb to Sn were 15 and 2 wt% respectively. The starting solutions were heated at 90°C for 15 min. The amount of spray solution prepared was 200 ml for all depositions. The well-cleaned glass slide was used as substrate. During deposition process, the substrate temperature was kept at 400 °C. The carrier gas flow rate was maintained a pressure of 5 kPa and deposition time is 15 min using ultrasonic wave with frequency 17 MHz to generate mist of solution. The obtained films were annealed in air, vacuum and N2 ambients for 30 min. X-ray diffraction (XRD) measurements using CuKα radiation were performed to study the preferential orientation and crystallinity of the films. Grain size and surface morphology were revealed in accordance to SEM. The optical transmission spectra of films were measured in the wavelength ranging from 300 to 2,500 nm. The electrical properties studies were carried out by Hall effect measurements in van der Pauw configuration.
3. RESULTS AND DISCUSSION The film thickness was estimated from the cross-sectional SEM studies that showed the film-substrate interface clearly and found to vary from 0.3 to 1.5 µm. A typical cross-sectional SEM micrograph of 1.2 µm thick films is shown in Fig. 1. It is found that the films have a sharp interface between the films and substrate. The increase in the film thickness of the undoped SnO2 as a function of spraying time was shown in Fig. 2. Hence, this study indicates that the film thickness can be controlled by spraying time.
Fig. 3 shows the XRD patterns of the undoped- SnO2 films prepared with spraying time during 10 to 40 min. The films are found to be cassiterite with tetragonal rutile structure.
Fig.1: Cross-sectional SEM micrograph of the undoped-SnO2
films obtained from spraying time of 40 min.
Fig. 2: Variation of thickness of the undoped-SnO2 films as
a function of spraying time.
Fig. 3: XRD patterns of undoped-SnO2 films prepared with
different spraying times. The calculated lattice parameter values of the undoped SnO2 films sprayed for 40 min are a = 4.735 Å and c = 3.214 Å. The lattice parameter values are in good agreement with the standard data. The preferred growth of (200) plane slighly decreases with increasing spraying time.
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Effects of annealing atmosphere on the preferred growth of
the undoped- SnO2 films were observed in Fig. 4. The preferred growth of (200) plane increases with increasing F doping concentration. Small difference in preferred growth of the films doped with 15 wt% of F annealed at 500 oC in air, in vacuum and in N2 atmosphere are observed in Fig. 5. Similar behavior is also observed in the films doped with 2 wt% of Sb as shown in Fig. 6. XRD patterns of F and Sb co-doped SnO2 films annealed at 500 oC in different atmospheres are shown in Fig. 7. The absence of preferred growth of the co-doped SnO2 films is observed in vacuum annealing condition. In contrast, the preferred orientation of (200) plane of co-doped SnO2 films is displayed in air annealing condition. However, the (211) diffraction plane is predominant in the co-doped SnO2 films annealed in N2 atmosphere. The SEM micrographs of the undoped films without annealing clearly show that the morphology of the films is strongly depending on thickness (Fig. 8). Grain size increases when the spraying time increases. The films obtained from spraying time of 30 min show the parallelopiped shaped morphology. But the films obtained from spraying time of 40 min show the leaf shaped morphology with random orientation.
Fig. 4: XRD patterns of the undoped-SnO2 films annealed
at 500 oC in different atmospheres. The SEM micrographs of the undoped SnO2 films obtained from spraying time of 15 min and annealed at 500 oC in different atmospheres are shown in Fig. 9. Two types of grain size, large and small size, are appeared in the F and Sb co-doped SnO2 films annealed in vacuum (as shown in Fig. 10). But grain of almost similar size were observed in the films annealed in N2 ambient. Optical transmission
Fig. 5: XRD patterns of SnO2 films doped with 15 wt% of
F and annealed at 500 oC in different atmospheres.
Fig. 6: XRD patterns of SnO2 films doped with 2 wt% of
Sb and annealed at 500 oC in different atmospheres.
Fig. 7: XRD patterns of SnO2 films co-doped with 15 wt%
of F and 2 wt% of Sb and annealed at 500 oC in different atmospheres. spectra of the undoped-SnO2 films with different spraying times and the ones annealed at 500 oC in different atmospheres were shown in Fig. 11 and Fig. 12 respectively. Optical transmission spectra of the F and Sb co-doped SnO2
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Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement
Fig. 8: SEM micrographs of the undoped-SnO2 films
prepared with different spraying times.
Fig. 9: SEM micrographs of undoped-SnO2 films annealed
at 500 oC in different atmospheres.
Fig. 10: SEM micrographs of SnO2 films co-doped with 15
wt% of F and 2 wt% of Sb annealed at 500 oC in different atmospheres.
Fig. 11: Optical transmission spectra of the undoped-SnO2
films prepared with different spraying times.
Fig. 12: Optical transmission spectra of the undoped-SnO2
films annealed at 500 oC in different atmospheres. films annealed at 500 oC were also observed in Fig. 13. From the optical transmission spectra, SnO2 films doped with 15 wt% of F were more transparent than the one doped with 2 wt% of Sb. Among the all annealing atmospheres, the SnO2:F films annealed in air showed the highest transmission value. However, the transmission value of the annealed SnO2:Sb films decreased in all annealing atmospheres. The annealed SnO2:F, Sb films showed the lower transmission value than the value obtained in the as-prepared SnO2:F, Sb films. From electrical measurements, important parameters of undoped, F-doped, Sb-doped, and F and Sb co-doped SnOo2 films without annealing and annealed at 500 C in difference atmospheres were shown in TABLE 1 and TABLE 2. The minimum
Fig. 13: Optical transmission spectra of SnO2 films co-doped
with 15 wt% of F and 2 wt% of Sb annealed at 500 o
C in different atmospheres.
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resistivity values SnO2:F and SnO2:Sb were found in the
films doped with 15 wt% of F and 2 wt% of Sb respectively. Among different annealing conditions, The F and Sb co-doped SnO2 films annealed in N2 ambient show the best results.
Fig. 14 shows the typical excitation and emission spectra of the undoped-SnO2 films without annealing condition. The emission spectrum consists a broad emission band with the main emission band having a maximum intensity around 400 nm and two small bands peaking around 430 and 520 nm. The corresponding excitation spectrum shows a single band with a maximum intensity at 320 nm. Redshift of the main emission band was observed when the temperature increases. It may suggest that the emission at 520 nm to electron transition mediated by defects levels in the band gap, such as oxygen vacancy [3]. In the undoped- SnO2 films, oxygen vacancy acts as luminescence center, can form defect levels located highly in the band gap and trap electrons from the valence band to make a contributuion to the luminescence. Generally, oxygen vacancy is known to be the most common defects and usually act as radiative centers in luminescence processes. The other emission bands peaking around 400 and 430 nm may be corresponding to electron transition from donor to acceptor levels [4]. These levels originate from surface defects. Different annealing atmospheres have not significantly affected the luminescence characteristics of the undoped and co-doped films.
Fig. 14: Photoluminescence spectra of the undoped-SnO2
films without annealing atmospheres.
4. CONCLUSION Transparent conducting undoped and doped-SnO2 thin films
have been prepared by ultrasonic spray pyrolysis technique using SnCl2 precursor solution. Structural and electrical properties of the obtained films have been characterized for
three cases of doped: fluorine, antimony, fluorine and antimony co-doped. Structural investigations using XRD reveal that thin films are composed of SnO2 only. No other phases are detected. Fluorine doped samples are having lower both mobility and resistivity than the antimony doped samples and fluorine and antimony co- doped samples. Among different annealing conditions, The F and Sb co-doped SnO2 films annealed in N2 ambient show the best results. Different annealing atmospheres have not significantly affected the luminescence characteristics of the undoped and co-doped films.
5. ACKNOWLEDGMENTS The authors thank Assoc.Prof. A. Srongprapa for Hall effect measurements, Dr. S. Peramontri and Mr. A. Vitttheeranon for optical transmittance measurements. They are also grateful to Asst.Prof.Dr. C. Poo-Rakkiat and Assco.Prof.Dr. T. Wongcharoen for valuable discussions. This work was partially supported by Faculty of Science, KMITL.
TABLE 1: IMPORTANT PARAMETERS OF UNDOPED, F-DOPED, Sb-DOPED, AND F AND Sb CO-DOPED SnO2 FILMS WITHOUT ANNEALING.
TABLE 2: IMPORTANT PARAMETERS OF F AND Sb CO-DOPED SnO2 FILMS ANNEALED AT 500 ℃ IN DIFFERENT ATMOSPHERES.
6. REFERENCES (1) B. Thangaraju, “Structural and Electrical Studies on
Highly Conducting Spray Deposited Fluorine and
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Antimony Doped SnO2 Thin Films From SnCl2 Precursor” Thin Solid Films, Vol. 402, 2000, pp. 71-78
(2) E. Elangovan, M.P. Singh and K. Ramamurthi, “Studies
on Structural and Electrical Properties of Spray Deposited SnO2:F Thin Films as a function of Films Thickness” Mater. Sci Eng. B, Vol. 113, 2004, pp. 143-148
(3) D. Cai, Y. Su, Y. Chen, J. Jiang, Z. He and L. Chen,
“Synthesis and Photoluminescence Properties of Novel SnO2 Asterisk- Like Nanostructures” Materials Letters, Vol. 59, 2005, pp. 1984-1988
(4) J. Ma, Y. Wang, F. Ji, X. Yu and H. Ma, “UV- Violet
Photoluminescence Emitted From SnO2:Sb Thin Films at Different Temperature” Materials Letters, Vol. 59, 2005, pp. 2142-2145
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