Abstract. The aim of the present work was to prepare fairly uniform transparent and conductive tin oxide films using an inexpensive and easily controllable ...
JOURNAL
OF MATERIALS
SCIENCE:
MATERIALS
IN ELECTRONICS
1 (1990) 79-83
Preparation of sprayed tin oxide transparent conducting films and their structural and electrical properties O. A. O M A R , H. F. R A G A I E * , W. F. FIKRY
Facu/ty of Engineering, Ain Shams University, Cairo, Egypt The aim of the present work was to prepare fairly uniform transparent and conductive tin oxide films using an inexpensive and easily controllable method. The technique adopted was spray deposition. The design of the spray deposition apparatus takes into consideration its reliability in controlling the different parameters affecting the formation of deposited layers, as well as its adaptability for large-area applications. The growth rate was independent of substrate type but increased with increasing substrate temperature. The crystalline structure of the films was found to be a function of substrate temperature and film thickness. The X-ray diffraction patterns showed that the preferred orientation was (1 1 0) and the grain size was in the range of a few tens of nanometres. The variation of sheet resistance with deposition parameters was studied and an empirical formula relating the sheet resistance to the film thickness was obtained. The prepared tin oxide layers of thicknesses up to 200 nm were found to have a transparency of about 80% to 85%, which makes them suitable for application as a single antireflecting coating of silicon solar cells. 1. Introduction A tin oxide (TO) thin film is an n-type semiconductor with a wide band gap [1]. Its pure form exhibits both high optical transparency (over 80%) in the visible region and high electrical conductivity (about 103~ -l cm -~ or more) [2]. These properties, in addition to its high chemical stability under normal environmental conditions, make it very useful in many fields of application, such as solar cells, liquid crystals, opto-electronic devices, thin film resistors, antireflecting coatings and gas sensors [3]. Several deposition techniques, such as flash evaporation, sputtering, chemical vapour deposition and spray pyrolysis, have been employed to obtain tin oxide films [4, 5]. The structural as well as the electrical properties of the films obtained depend strongly on the deposition method, and can be tailored by controlling the deposition parameters. In the present work the spray deposition technique was implemented, because it is simple and inexpensive compared with other methods. The deposition apparatus can be easily modified for largearea production.
2. Film preparation Tin oxide (SnO2) layers were obtained by pyrohydrolysis of alcoholic solution of stannic chloride (SnCI4) on to silicon or glass substrates, which were kept at constant elevated temperatures.(200 to 500 ° C) according to the following reaction SnCI~ + 2H20 -~ 4HC1 + SnO~
(1)
In this case H20 is the oxidizing agent. Propanol, which possesses high volatility and low surface ten-
sion, was added to help in the dilution of the solution. The addition of a small quantity of HC1 improved the fitrfi quality [6]. Hence all TO films were obtained using a fixed composition of spray solution consisting of 0.8 M SnC14, 10 M propanol and 0.2 M HC1. A schematic diagram of the spray deposition apparatus is shown in Fig. 1. The distance between the nozzle and the substrate, air pressure and the droplet size are the main parameters affecting the layer homogeneity. The design of the system controls the above parameters as well as providing an angular oscillatory motion to the nozzle during the spraying process to improve the film homogeneity. Because the temperature of the substrate drastically affects the characteristics and quality of TO films, the substrate temperature must be maintained within _+5° C throughout the operating temperature range. In order to control such temperature tolerance, an electronic temperature controller was specially designed to satisfy the above requirements. It was found that the optimum homogeneity was achieved at a nozzle distance 19 cm from the substrate and with an air pressure of 0.85 kg cm~-2. The adhesion of all films deposited at a substrate temperature greater than 300 ° C was found to be adequate and they could not be peeled off mechanically. At substrate temperature less than 200 ° C, an opaque and a poorly adhesive film was obtained.
3. Results and discussion 3.1. Deposition rate Under the optimum experimental conditions mentioned above, it was found that the thickness of the
* Present address: Electrical EngineeringDepartment, Collegeof Engineering,King Saud University, Riyad 11421, Saudi Arabia.
0957-4522/90 $03.00 + , 12 © 1990 Chapman and Hall Ltd.
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TO layer increases linearly with the deposition time throughout the range of deposited thicknesses up to 330nm as shown in Fig. 2. Moreover, the rate of growth increased with increasing substrate temperature irrespective of the nature of the substrate. The growth rate was about 375 nm rain-' at 320 ° C, whereas at the higher temperature (420 ° C) it reached 480 nm rain- ~.
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3.2. Structural properties The structural properties of deposited TO film are studied by X-ray diffractometry for samples deposited on glass substrates. A monochromatic X-ray beam of wavelength 0.154 17 nm strikes the film surface at an angle 0. The sample is slowly rotated and the detector simultaneously moves, at twice the rate, along the circumference of a circle with the same centre as the sample. Diffraction maxima appear when 0 coincides with the Bragg angle. The X-ray diffraction pattern, as recorded automatically by the diffractometer, is basically a plot of intensities against 20 given by the sample. Fig. 3 shows the X-ray diffraction patterns for three films deposited at 300, 355 and 4t0° C. The films have the same thickness of 188nm. From Fig. 3 it can be concluded that the film deposited at 300°C has an amorphous nature whereas the films deposited at higher temperatures (355 and 410 ° C) have grown with a certain degree of polycrystallinity. By comparing the X-ray diffraction pattern in Fig. 3 with the standard diffraction data cards of the ASTM [7], it was found that within the range of Bragg angle 20 from 14° to 54°: (i) both films exhibit strong (1 10) orientation referred to the plane parallel to the film surface; (ii) the sample deposited at 410°C has other preferred orientations such as (1 0 1), (200) and (2 1 1) in addition to the dominant (1 1 0) orientation; and (iii) the peak intensity for the sample deposited at 410°C is higher than that ['or the sample deposited at 355 ° C. Such an increase in the peaks indicates that the degree of polycrystallinity increases with increasing deposition temperature. Fig. 4 depicts the X-ray broadening of the (1 1 0) orientation of the two samples deposited at 355 and
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Figure 5 X-ray diffraction peak (1 I 0) of TO films deposited with different thicknesses.
410 ° C. It is obvious from the figure that the broadening of the two samples is the same. This indicates that the grain size of the two structures is independent of the deposition temperature. Calculations, using the Scherrer formula [8] give a grain size of about 30 nm. Fig. 5 shows the broadening of the (1 1 0) orientation for four samples deposited at 410°C with different thicknesses of 40, 85, 110 and 188nm. From Fig. 5, it is clear that the film of thickness 40 nm has an amorphous nature, whereas the other films are polycrystalline. Moreover, the grain size increased from 25 to 30 nm on increasing the film thickness from 85 to 110 nm, but no further increase in grain size f o r the sample with the highest thickness (188nm) was observed. Therefore, it can be concluded that at the higher deposition temperature (410 ° C) the very thin TO film ( < 400 nm) exhibits an amorphous nature but with increasing thickness, the film becomes polycrystalline and the grain size increases towards a value of about 30 nm.
3.3. Resistivity measurements It was found that the main deposition parameters affecting the resistivity are the deposition temperature and the solution flow rate. For constant thickness ( l l 0 n m ) and under optimum homogeneity conditions the relation between the resistivity at room temperature (p) and deposition temperature on both oxidized silicon and glass substrates is shown in Fig. 6. From the figure, the following conclusions can be drawn. (i) On increasing the deposition temperature from 300 to 480°C the resistivity decreases slowly and
reaches a minimum value at about 410 ° C, after which it increases sharply with increasing temperature. Accordingly, the deposition temperature of 410 ° C can be considered as optimum. (ii) Within the range of the optimum deposition temperatures (around the minimum) the resistivity of the film is nearly independent of the nature of the substrate. The decrease in resistivity in the deposition temperature ranges from 300 to 410°C can be explained by the variation in the structure of the film from amorphous to polycrystalline and/or the increase in its non-stoichiometry with increasing temperature [1]. However, the sharp increases in resistivity at higher temperatures may be due to improvement in the film stoichiometry [2], and possibly as a result of the chemisorption of oxygen at higher temperatures, into the grain boundaries. The chemisorption of oxygen results in an enhancement of the barrier height, thereby lowering the mobility and consequently increasing the resistivity [9]. The variation of the resistivity for constant film thickness deposited under optimum condition of homogeneity and deposition temperature with the solution flow rate is shown in Fig. 7. It is obvious that with increasing solution flow rate the resistivity decreases. We believe that the decrease in the resistivity with increasing solution flow rate is due to the rapid formation of the TO film and accordingly less chemisorption of oxygen by the film and oxygendeficient stoichiometry. Hence, the optimum solution 81
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(ii) The rapid decrease in sheet resistance on increasing the film thickness from 50 to 120nm is believed to be due to the change in the film from an amorphous nature to polycrystalline, together with the increase in grain size. This behaviour agrees with the results obtained by other researchers [10, 11]. (iii) The moderate decrease in the sheet resistance may be attributed to the increase in the degree of polycrystallinity. Using the least squares method to carry out a curve fitting over the range 50 to 300nm, the sheet resistance, RD, is found to be related to the thickness according to the following expression
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FiLmre 6 Variation of TO resistivity, p, as a function of the deposition temperature, T, on (o) silicon and (x) glass substrates.
flow rate for our system was found to be in the range 30 ml rain- ~, The variation of sheet resistance with film thickness for films deposited under optimum conditions is shown in Fig. 8. It is clear from the figure that the sheet resistance decreases with increasing film thickness. Moreover, two zones can be observed. In the first zone where the thickness is up to 120rim, the sheet resistance decreases rapidly. In the second zone, where the thickness varies fl'om 120 to 300nm, the sheet resistance decreases moderately. Such behaviour can be explained on the basis of the structure, using the results of X-ray diffraction studies. (i) In case of very thin films, about 50 nm, the film is amorphous or may be partially discontinuous. Therefore, the sheet resistance is considerably high (3000 ~I~).
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3.4. TO films as antireflecting coating of silicon solar cells TO films sprayed in the present work have a refractive index of about 2 in the visible region [12]. This makes them useful as an antireflecting coating on silicon solar cells. In order to give a minimum reflectance at the peak of the solar spectrum, which is near 600 nm for the AM 1 condition, the thickness of this coating
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prepare and characterize transparent conducting films of tin oxide taking into consideration that the preparation technique used should be economical, reliable, and suitable for local technical conditions. The spray pyrolysis apparatus specially designed and constructed for this purpose gave satisfactory results. The electrical and optical properties were studied and it was found that the films can be successfully applied as a single antireflecting coating of silicon solar cells.
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layer was calculated to be 75 nm [13]. Fig. 9 shows the reflection spectra of polished silicon wafer and silicon wafer coated with sprayed TO layers of about 75 nm. From the figure it can be seen that the average reflection coefficient over the range 400 to 1200nm is reduced from 34% for polished silicon to 13% for silicon coated with TO. The good adhesion of TO layers to silicon besides the above obtained results are very promising for their use as an efficient single antireflecting coating of silicon solar cells.
9.
4. Conclusion The main objective of the present research was to
I0. 11. 12. I3.
J. C. M A N I F A C I E R , M. de MURICA, J. P. F1LLARD and E. VICARIO, Thin Solid Fihns 41 (1977) 127. K. L. CHOPRA, S. MAJOR and D. K. PANDYA, ibid. 102 (1983) 1. T. M. UEN, K.F. HUANG, M . S . CHEN and Y. N. GOU, ibid. 158 (I988) 69. W. A. BADAWY and E. A. EL~TArfER, ibid. 158 (1988) 277. K. L. CHOPRA and S. R. DOS, "Thin Film Solar Cell" (Plenum Press, New York, 1983) p. 69. G. HAACHE, H. ANDO and W. E. M E A L M A K E R , J. Electrochem. Soc. 124 (1977) 1923, J. C. M A N I F A C I E R , J.P. FILLARD and J.M. BIND, Thin SolidFilms 77 (I981) 67, B. D. CULLITY, "Elements of X-ray Diffraction" (Addison-Wesley, California, 1967) p. 96. E. SHANTHI, A. BANARJEE, V. DUTTA and K. L. CHOPRA, J. Appt, Phys. 71 (1980) 237. H. DE WAAL and F. SIMONIS, Thin Solid Fi#ns 77 (1981) 253, J. N. AVARITSIOTIS and R. P. HOWSON, ibid. 80 (1981) 63. W. F. F. FIKRY, MSc thesis, Ain Shams University, Cairo (1989). F. A. JENKINS and H. E. WHITE, "Fundamentals of Optics" (McGraw-Hill, Singapore, 198I) p. 295.
Received 24 January and accepted 28 March 1990
