Electrical and optical properties of pyrolytically sprayed SnO2 film—Dependence on substrate temperature and substrate-nozzle distance

Thin Solid Films, 189 ELECTRONICS

(1990)21 l-225

AND OPTICS

217

ELECTRICAL AND OPTICAL PROPERTIES OF PYROLYTICALLY SPRAYED SnO, FILMS-DEPENDENCE ON SUBSTRATE TEMPERATURE AND SUBSTRATE-NOZZLE DISTANCE V. VASU AND A. SUBRAHMANYAM Department of Physics, Indian Institute of Technology, Madras-600036 (Received June 24, 1989; revised October

31, 1989; accepted

February

(India) 15, 1990)

The dependence of electrical and optical properties of SnO, films, prepared using the spray pyrolysis technique, on the substrate temperature and substratenozzle distance was studied. A 0.05 cm diameter spray nozzle was employed (l/4 JAU, Spraying Systems Inc., U.S.A.). Films of about 10m3Szcm resistivity and high visible transparency of about 90% were obtained at a substrate temperature of 400°C and a substrate-nozzle distance of 30cm. It has been observed that substrate-nozzle distance plays an important role in the reaction kinetics of SnO, films. At a substrate-nozzle distance greater than 30 cm, the homogeneous reaction enhances and hence impedes the growth rate, resulting in inferior electrical and optical properties of the films.

1. INTRODUCTION

Investigations of the physical properties of thin films of undoped and doped tin oxide (SnO,) have been carried out by several workersip in view of their variety of applications. Of the various techniques available for preparing SnO, films, spray pyrolysis is widely used because of its simplicity and commercial viability. The method is based on the pyrolytic decomposition of a tin compound (usually chlorides) dissolved in methyl or ethyl alcohol and sprayed onto a preheated substrate. The pyrolytic reaction takes place above 350°C and gives reasonably good films. The physical properties of these films are observed to depend on the nature of the substrate4, substrate temperature, liquid flow rate and the droplet size. It may be noted that the droplet size depends upon carrier gas pressure, liquid flow rate and nozzle diameter. This paper reports the dependence of electrical and optical properties of SnO, films on (i) the substrate temperature T, and (ii) the substrate-nozzle distance D,,. The authors believe that the variation of electrical and optical properties of SnOl films as a function of substrate-nozzle distance has not previously been reported. 004~6090/90/%3.50

0 Elsevier Sequoia/Printed

in The Netherlands

V. VASU.

218 -. 7

EXPERIMENTAL

A. SUBRAHMANYAM

METHOD

The spray pyrolysis set-up consisted of a reaction chamber (furnace) and a spray atomizer (nozzle). A furnace 15 cm in diameter and 35 cm in height was used as the reaction chamber. A maximum temperature of 900°C could be attained in a uniform zone of 15 cm in the furnace. A stainless steel plate (diameter 12 cm, thickness 2.5 cm) was placed in the uniform zone of the furnace to hold the substrates. The substrate temperature was measured using a chromel-alumel thermocouple (placed below the substrates) to an accuracy of i2 C. As the thickness of the soda lime glass substrate was 0.1 cm, the actual temperature at which the film was formed may be slightly inaccurate (this cannot be avoided in the spray pyrolysis technique). The spray atomizer (Model l/4 JAU, M/s Spraying Systems Co., U.S.A.) had a nozzle of diameter 0.05 cm. The flow of liquid through the nozzle was controlled by a needle valve which could be operated automatically by pressurized gas. Oxygen and nitrogen (of 99.9% purity) were used as carrier gases. It was observed that the minimum pressure of the carrier gas should be 2.5 atmospheres or greater to operate the needle valve and to obtain a perfect cone-shaped spray. (The needle valve may be operated below these pressures by suitably adjusting the spring tension.) The tip of the atomizer was kept at a distance of 5 cm above the furnace edge such that the substrate-nozzle distance D,, was 25 cm. This distance was changed by moving the nozzle tip above the furnace edge and was measured to an accuracy of + 0.2 cm. The spraying liquid consisted of tin chloride (SnCl,.SH,O, 2 g) of 99.9:/i purity dissolved in ethyl alcohol (10cm3). The flow rate of the spraying liquid was controlled (through the carrier gas) to 9-10 cm3 mini ‘. All the films were sprayed for 1 min. Optical transmission data in the wavelength range 300-900 nm were recorded on a ratio recording spectrophotometer (Hitachi 220 A) and Hall effect measurements were carried out with an electromagnet (7.5 kG) using a precision digital microvoltmeter. An indigenously fabricated Van der Pauw setup was employed for measuring the resistivity of the samples. The multiple beam interferometry technique, along with the interference pattern observed in optical transmission spectrum, were used to measure the thickness of the film and its refractive index3-5. The thickness values reported are accurate to + 1%. Structural evaluation of the films was carried out by X-ray diffraction. Reproducibility was ascertained by comparing the data on several samples prepared under nearly identical conditions. 3.

RESULTS

Figure 1 represents the variation in Hall mobility ,LL~,carrier density n and resistivity p of SnO, films as a function of substrate-nozzle distance D,, at a substrate temperature of 400 “C. It may be observed that the values of pn and IZare maximum and p is minimum at D,, = 30 cm. The physical properties of these films are presented in Table I. Figure 2 presents the variation in pn, n and p for SnO, films with substrate temperature K at D,, = 30 cm. It may be seen that pn and n increase and p decreases

PYROLYTICALLY

SPRAYED

25

&lo,

FILMS

30

35

Substrate

Fig. 1. Variation substrate-nozzle TABLE I PnoPE~rmso~SnO~ DS”

nozzle

LO distance

FILMS WITHSUBSTRATE--NOZZLEDISTANCE,AT

;x 1V3)

PH

(cm)

(nm)

(Qcm)

(cm’

25 30 35 40 45

515 761 620 250 250

13.7 4.8 7.8 22.3 30.3

5.3 8.5 6.8 7.0 6.5

n,, refractive

VmlsK1)

L5

(cm)

in Hall mobility pa, carrier density n and electrical distance D,,, at a substrate temperature of 400°C.

d

T, transmittance;

219

resistivity

p of SnO, films with the

T, = 400 “C

4

;x KY9,

ft

;t

(cmm3)

500 nm)

500 nm)

teV)

8.6 15.3 11.7 4.0 3.2

0.64 0.82 0.70 0.66 0.67

2.09 2.02 2.27 2.08 2.20

3.52 3.83 3.82 3.76 3.77

index of the film; d, thickness

of the films.

with increase in substrate temperature. Hall effect measurements have shown that these films are n type. The optical transmission spectra for these samples prepared at different substrate temperatures are presented in Fig. 3, and their physical properties, i.e. resistivity, thickness, refractive index, band gap and transmittance (at 500 nm) are presented in Table II. It may be noted that the transmission, in the range 300-900 nm, increases with increasing substrate temperature up to 440 “C. Noticeable powder formation is observed with a reduction in optical transmission at temperatures above 440 “C. Figure 4 represents the X-ray diffraction pattern for SnO, films prepared at different temperatures. All these films show a polycrystalline nature with the

220

V. VASU, A. SURRAHMANYAM

preferential orientation alorig the (211) direction. The lattice parameters of these films are found to be u0 = 0.473 nm and co = 0.3 18 nm, which are in agreement with the reported values4. The increase in sharpness of the diffraction peaks indicates

I”

5

8 6

ii-

L

L-

O

Reslstlvlty

0

Mobollty

24 280

320

360

Substrate

LOO

temperature

(‘C

Fig. 2. Variation in Hall mobdity 11~. carrier density !I and electrical substrate temperature, at a substrate -nozzle distance D,, of 30 cm.

500

LLO

I reslstivity

,’ of SnOz lilms with

700 Wavelength

h

(nm)

Fig. 3. Optical transmittance of SnO, films at substrate 30 cm: curve I, 360°C; curve 2,400 ‘C: curve 3,440 C.

-

temperatures

for a substrate-nozzle

distance

of

PYROLYTICALLY

SPRAYED

&lo,

221

FILMS

‘12111

Ts =LLO*C

Ts =LOo’C

V

1c

.n”

-I”

-20 Fig. 4. X-ray diffraction pattern and 440 “C, for substrate-nozzle

of SnO, films prepared distance D,, of 30 cm.

at substrate

temperatures

T, of 360 “C, 400 “C

222

TABLE

V. VASU. A. SURRAHMANYAM

II

PKOPEKTIES OF

SnO,

WILMA WITH ~UB~~~ATE

T~~~PEKATUKC. A

_~ r,

(I

( Cl

(nm)

P (X10 %I (Rcml

280 320 360 400 440

302 29x 796 761 744

14.3 12.9 8.6 4.x ‘.I

7’. transmittance;

n,, refractive

1D,, = 30cm

;‘x IO’“)

_. I (at

ll:t

(cm’ V ’ 5 ‘)

(cm

SO0 nm)

500 nm

6.h 7.2 7.Y x.5 Il.4

6.6 6.7 9.2 15.3 25.0

0.76 0.x0 0.80 0.82 0 6X

2.06 2.0’) 2.05 2.02 ‘.I I

1’11

index of the film; d, thickness

‘)

k,

)

(CVI 3.76 3.76 3.77 3.83 3.X6

of the films.

how the crystalline nature of SnOz films improves qualitatively with increase in temperature. The observed data do not show any noticeable variation when the carrier gas is nitrogen. 4. DISCUSSION The salient features of the present investigation can be summarized as follows. (i) With an increase in substrate temperature (for D,, = 30 cm), (a) there is an increase in thickness, optical transmission, carrier density, Hail mobility and band gap;(b) there is a decrease in resistivity. (ii) With an increase in substrate-nozzle distance D,, at a temperature of 400 ‘C, all the above-stated parameters, except resistivity, attain a maximum value at D,, = 30cm, and then decrease. The resistivity attains a minimum value at D,, = 30 cm, and then increases. 4. I. Thickness dependence Figure 5 represents the dependence of the thickness of SnO, films on the substrate temperature and the substrate-nozzle distance. An important mechanism which controls the thickness of the film is the reaction kinetics of the droplet. 4. I 1. Suhstrrrte temptwturc The growth rate of the films has three distinct temperature-dependent regionsh.7. Primarily, the thickness of the films depends upon the size of the droplet undergoing pyrolytic reaction. With the details discussed by Siefert7 on the dependence of growth rate on temperature and size of the droplet, the present data indicate that (i) at substrate temperatures below, 35O’C the growth rate (or thickness, since the spraying time is kept constant) is controlled by activated processes leading to a homogeneous reaction; (ii) as the substrate temperature is increased above 35O”C, the growth rate is diffusion limited and the reaction is mostly heterogeneous due to the optimized size of the droplet; (iii) at substrate temperatures above 440°C the size of the droplet decreases

PYROLYTICALLY SPRAYED

SnO, FILMS

Substrate

223

Nozzle

distance

temperature

(‘C

km1

‘i

Substrate

1

Fig. 5. Thickness of SnO, films as a function of(i) substrate temperature T, at D,, = 30 cm, (ii) substratenozzle distance D,, at T, = 400°C.

appreciably due to the evaporation of the water molecule, resulting in a homogeneous reaction; the reaction may be complete well above the substrate, leading to powder formation; this is a common observation6*7. 4.1.2. Substrate-nozzle distance Increasing the substrate-nozzle distance introduces considerable change in the forces operating on the droplet. These forces are entirely responsible for determining the reaction kinetics*. The important force acting on the droplet before pyrolytic reaction takes place is thermophoretic force Fth, which is given by8 F

=

-

th

3vrgraW’d

r$l

Y,T,

32, grad T, gradT,=

21 +A a

d

where r is the radius, Td is the temperature and 1, is the thermal’conductivity of the SnOz droplet; q is the viscosity, ya is the density, 1, is the thermal conductivity, T, is the temperature and 1is the mean free path of air molecules.

224

V. VASU.

A. SUBRAHMANYAM

The thermal energy gained by the droplet depends on the factor grad Td. In the present investigation. it is assumed that the distribution of the droplet sizes is constant in all experiments as the carrier gas pressure and flow rate are kept constant. The thermal energy gained by the droplets will be greater with increasing D,,, resulting in preheating ofthe droplets by carrier gas through heat radiation. It is known that preheating enharces the pyrolytic reaction3. At D,, = 30 cm, therefore. it seems likely that a heterogeneous reaction takes place due to the preheating of the optimized droplet size. But beyond D,, = 30 cm, the thermal energy gained by the droplet may evaporate more water molecules well above the substrate, reducing the size of the droplet, and the homogeneous (growth-limiting) reaction may take over (impeding the growth rate) as stated earlier. This explanation of changing reaction kinetics with variation in D,, seems plausible. 4.2. Elt~I,tric,ul proprrtic~.v The increase in carrier density with increase in substrate temperature may be attributed to the following. (i) Enhanced crystallinity of the films, as indicated by the X-ray diffraction data (Fig.4) which helps to reduce the loss of carriers at the grain boundaries”. This mechanism may dominate the possible reduction in carrier density due to oxidation at higher substrate temperatures’O. (ii) Tin may be present in a tetravalent state at higher temperatures’ ‘. The increase in Hall mobility with increase in substrate temperature is due to the improved crystalline nature of the SnO, films. As a consequence of an increase in carrier density and Hall mobility, the resistivity of these films decreases with increase in temperature. At a given substrate temperature, though the crystallinity remains the same, the nature of the reaction from heterogeneous to homogeneous, and hence oxidation of the films, seems to change with variation of the substrate-nozzle distance. Our present data indicate that 30cm is the optimum substrateenozzle distance, at a substrate temperature of 400 C, to obtain SnO, films of good quality. 4.3. Optic,ulpropprtir.\ The increase in transmission of these films with an increase in substrate temperature may be due to reduced grain boundary scattering (enhanced crystallinity). The refractive index values reported here are practically constant with temperature. At very high substrate temperatures (around 440 -C). the small powdery formation (due to homogeneous reaction) affects the transmission appreciably. The optical band gap E, has been evaluated from the absorption coefficient Y of the films”. Consequently the changes in E, values are due to the changes in carrier density with the substrate temperature and substrate-nozzle distance. 5. (‘ON(‘I.USIONS

It is seen that the electrical spray pyrolysis exhibit improved

and optical properties of SnOz films prepared by performance at higher substrate temperatures. The

PYROLYTICALLY

SPRAYED

ho,

FILMS

225

present data indicate that the substrate-nozzle distance is significant in the pyrolytic reaction, whether it is homogeneous or heterogeneous. It is also seen that the homogeneous reaction (which affects, the conductivity and the visible transparency of the SnO, films) can be minimized by suitable adjustment of the substrate-nozzle distance and suitable substrate temperature. ACKNOWLEDGMENT

V. Vasu acknowledges with thanks the financial grant from the Council of Scientific and Industrial Research to carry out this work. REFERENCES

I 2 3 4 5 6 7 8 9 10 1I 12

K. L. Chopra, R. C. Kainthala, D. K. Pandya and A. P. Thakoor, Phys. Thin E. Shanthi, A. Banerjee, V. Dutta and K. L. Chopra, J. Appl. Phys., 53 (1982) J. C. Manifacier, M. De Murcia, J. P. Fillard and E. Vicario, Thin Solid Films, J. C. Manifacier and J. P. Fillard, Thin SolidFilms, 77(1981) 67. R. Swanepoe1.J. Phys. E., 16(1983) 1214. W. A. Byrant, J. kfuf. Sci., 12 (1977) 1285. W. Siefert, Thin Solid Films, I20 (1984) 275. W. Siefert, Thin Solid Films, 120 (1984) 267. N. Balasubramanian and A. Subrahmanyam, J. Phys. D, 22 (1989) 206. A. Noguchi and H. Sakata, J. Phys. D, 13 (1980) 1129. J. C. C. Fan and G. B. Goodenough, J. Appl. Phys., 48 (1977) 3524. S. Ray, R. Banerjee, N. Basu, A. K. Batabyal and A. K. Barua, J. Appl. Phys.,

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