Thin Solid Films, 108 (1983) 277-283

ELECTRONICS AND OPTICS 277

CHARACTERIZATION OF A N T I M O N Y - D O P E D TIN OXIDE FILMS FOR SOLAR CELL APPLICATIONS*

N. SRINIVASA MURTY AND S. R. JAWALEKAR

Department of Electrical Engineering, Indian Institute of Technology, Powai, Bombay 400076 (India)

(Received March 21, 1983; accepted April 5, 1983)

Transparent conducting undoped tin oxide (SnO2) and antimony-doped tin oxide (ATO) films were deposited onto Pyrex glass and single-crystal silicon substrates using an inexpensive chemical vapour deposition system. SnC12 and SbC13 were used as the source reagents with oxygen and nitrogen respectively as the carrier gases. The deposition conditions were as follows: temperature, 350-500 °C; oxygen flow rate, 0.8-3.25 I min-1; nitrogen flow rate, 0-0.1 1 rain-~; deposition time, 5-20 min. The antimony concentration in the film and its physical properties were the same on both substrates. A figure of merit (Trl°/Rsh where Tr is the transmission at a particular wavelength and Rsh is the sheet resistance) was used to compare the performance of these films. The maximum figure of merit for SnO2 films (1.43 x 10 -3 ~-~-1 (Tr = 95% and Rsh ---- 420 fl/I--q)) was obtained when they were deposited at 500 °C with oxygen at a flow rate of 1 1 min- ~. The sheet resistance of antimony-doped films is a minimum at 3 mol.% Sb and the transmission decreases as the antimony concentration increases. The maximum figure of merit obtained for ATO films was 6.78 x 10- 3 I'~- 1 (Tr = 90.6% and Rsh = 55 f~/I-1) for an antimony content of 3 mol.% and a nitrogen flow rate of 0.07 1 min-1. These results are explained theoretically and are compared with those reported by other workers.

1. INTRODUCTION

Research on transparent conducting tin oxide films has been intensified recently owing to their promising applications in solar cells L2. SnO2 films and SnO2/Si solar cells fabricated by the oxidation of SnCI 2 have not been as extensively studied as those prepared by other chemical vapour deposition (CVD) and vacuum methods 3'4. In this paper we report the electrical and optical properties of undoped tin oxide (SnO2) and antimony-doped tin oxide (ATO) films deposited by the oxidation of SnC12 and SbCl 3. The optimization of the deposition parameters for achieving the maximum figure of merit for solar cell applications is described and the results are compared with those reported by other workers.

* Paper presented at the International Conference on Metallurgical Coatings, San Diego, CA, U.S.A., April 18-22, 1983.

0040-6090/83/$3.00 © Elsevier Sequoia/Printed in The Netherlands

278 N. SRINIVASA MURTY, S. R. JAWALEKAR

2. EXPERIMENTAL DETAILS

The method and the CVD system used to deposit the SnO z and ATO films have been described earlier 5'6. The films were deposited onto Pyrex glass and silicon substrates under the following conditions: deposition temperature T~, 350-500 °C; oxygen flow rate F(O2), 0.8-3.25 1 min 1; nitrogen flow rate F(N/), 0-0 . l 1 min- 1; deposition time t, 5-20 min. The sheet resistance R~h of the films was measured using a four-probe method and the optical transmission Tr in the wavelength range 400-1100nm was measured using an Aminco-DW 7M UV-visible spectro- photometer.

3. RESULTS AND DISCUSSION

3.1. General features All the films deposited at temperatures above 350 °C were polycrystalline 7 and

contained SnO/ and Sn30 4. The mean grain size of the films was in the range 80-300 nm 6. The physical properties of the S n O 2 and ATO films were the same on both Pyrex glass and silicon substrates. Even though the crystal structures of Pyrex (amorphous) and silicon (cubic diamond) are different, since both these structures are also different from that of SnO2 (tetragonal rutile) they may not influence the properties of SnO2. A similar result has been reported previously 8.

3.2. Sheet resistance, transmission and figure of merit of Sn02 films The sheet resistance Rsh, the average transmission Tr(av) in the 400-1100 nm

wavelength range and the transmission Tr(600) at a wavelength of 600 nm of SnO2 films 100 nm thick deposited at 400 and 500 °C are shown as functions of F(O2) in Fig. 1. Rsh initially decreases, reaches a minimum and then increases again as F(O2) increases from 0.8 to 3.25 1 min- 1. At low values of F(Oz) some SnO and/or Sn304 (which are both highly resistive) may be present in the film resulting in a relatively high Rsh, As F(02) increases the SnO and Sn30 4 concentrations may decrease because of their conversion into SnOz, causing a reduction in Rsh. The SnO2 films obtained at F(O2) values where Rsh is a minimum may contain the optimum number of oxygen vacancies to give the lowest value of Rsh. A further increase in F(02) produces a decrease in the oxygen vacancy concentration and hence Rsh increases 5. Rsh decreases as T~ increases owing to an improvement in the crystalline structure and an increase in the free-carrier concentration. Figure ! shows that the variation in Tr(av) is similar to that of Rsh. The carrier concentration increases as R~h decreases and hence the photon absorption by the carriers also increases causing the transmission to decrease.

Both the conductivity and the transmission of the films should be as high as possible for solar cell applications. However, they are inversely proportional to each other. Hence the optimum values of these two parameters should be established using a figure of merit.

The most commonly used definition of the figure of merit qSTC of a transparent conducting film 9-11 was first given by Haacke 12 as

~TC = TrJ°/Rsh (1)

CHARACTERIZATION OF Sb-DOPED S n O 2 FILMS 2 7 9

where Tr is the transmission at a particular wavelength. The transmission is a function of the wavelength, and therefore the use of Tr(av) for the calculation of qSxc is inappropriate because the solar flux is concentrated in a small wavelength range near the green ~3. Moreover the spectral response of the SnO2/Si cells has been reported 1'2'~4 to be a maximum near a wavelength of 600nm. Therefore the transmission Tr(600) at a wavelength of 600 nm was used to calculate ~TC in this study.

I / ~ ~,,, ~ - \~ . ..o'~ / ~

E

70

0 50 o.8 ,'-2 ,!6 ;.o ~-~ ~.8 3'.,

Oxygen f)ow rate (~. rnin 1)

Fig. 1. Dependence of R,h (- - -), Tr(600) ( ) and Tr(av) (--. --) for SnO 2 films 100 nm thick on the oxygen flow rate: O,A, [7, T~ = 500 °C; O,A, I , T~ = 400 °C.

In Fig. 2 ~bxc for S n O 2 films deposited at 400 and 500 °C, calculated from the Rsh

and Tr(600) values given in Fig. l, is shown as a function of F(O2) . The variation in q~xc for films deposited at 400 °C is small since the changes in Rsh and Tr(600) are small. For films deposited at 500 °C ~TC is high at both high and low values of F(O2). However, the uniformity of the films deposited at high F(O2) is poor 6. Therefore films deposited at low flow rates were chosen for further investigations.

The above results show that the opt imum deposition parameters for S n O 2 films are T, = 500 °C and F ( O 2 ) = 1.0 lmin -1 . The corresponding film properties are Rsh = 420 f~/[Z, Tr(av) = 90.5~o, Tr(600) = 95~o and ~brc = 1.43 x 10- 3 f l - 1.

3.3. Sheet resistance, transmission and figure of merit of A TO films Once the opt imum deposition parameters for SnO2 films were established,

ATO films were deposited with T, and F(O2) kept at the optimized values. The

2 8 0 N. SRINIVASA MURTY, S. R. JAWALEKAR

1-6

I-4

(

I I-2

J= 1.0 o

? 0

O-a ® E 'S

a,O.E

i,2 0.4

0-2

o . ~ ' ~ - . . . . . .

44

40

\,

\ \

/ "

/ /

/

\ /

1.6 2!o - - - - ~ - ~ ~ 3-4 2-4 2.6 Oxygen f low rate (,~ min -1 )

Fig. 2. Influence of the oxygen flow rate on ~bvc for SnO: films deposited at 400°C ( - - - ) and 500 °C ( ).

36(

o \ ~ 24 g g

~ 2c

m 16

12

8

4

O - -

\ \

\ °\

\ .

-....

" ~ " - o ~ I t ~ ' "

100

5O ~ - - ~ 4

Mole % Antimony

Fig. 3. Variation in R,h ( - - . - - - ) and the percentage transmission at various wavelengths 2 ( - - ) for ATO films 100nm thick with the ant imony content of the films: x , 2 = 600nm; Z~,2= 900nm; 0 , 2 = l l 0 0 n m ; I , 2 = 400-1100 nm.

C H A R A C T E R I Z A T I O N OF S b - D O P E D S n O 2 FILMS 2 8 1

antimony content in the films was varied from zero to 4 mol.~ by varying F(N2) from zero to 0.1 1 min- 1. ATO films doped with more than 4 mol.~ Sb were not studied because their transmission is too low to be useful in solar cell applications.

In Fig. 3 Rsh, Tr(600), Tr(900), Tr(1100) and Tr(av) for ATO films 100 nm thick are shown as functions of the antimony content. Rsh initially decreases, goes through a minimum and then increases again as the antimony content increases from zero to 4 mol.~o. The increase in the carrier concentration as a result of doping is responsible for the initial decrease in Rsh. However, since the ionic radius of antimony is about 1.2 times that of tin, the crystal defects which create the trap levels in the forbidden gap also increase with doping 15. Thus above a critical antimony content the trap concentration dominates the concentration of donated electrons, and on further doping the antimony atoms act as traps rather than donors. Hence Rsh increases with increasing antimony content above 3 mol.~o Sb. The optical transmission decreases as the antimony content increases owing to the increased scattering of photons by the crystal defects created by doping. The free-carrier absorption of photons may also contribute to this reduction in transmission.

The ~bxc of ATO films at 600, 900 and 1100 nm is shown as a function of the antimony content in Fig. 4. At all wavelengths ~bxc increases rapidly up to about 2 mol.Yo Sb, then increases slowly up to 3 mol.~ Sb and finally falls rapidly if the antimony content is increased further. R~h initially decreases very rapidly and the transmission decreases slowly; therefore ~xc increases rapidly as the antimony content increases. The decrease in R~h and the transmission is slow between 2 and 3 mol.~o Sb and the increase in q~xc is also slow. However, above 3 mol.~o Sb R~h increases and the transmission decreases causing ~bxc to fall rapidly. It can also be seen from Fig. 4 that q~rc is a maximum at 3 mol.~o Sb for all wavelengths.

6

i 0,4

,o x

E

~n iE

o ~ ~ Mole Olo Antimony

Fig. 4. Dependence of thrc for A T O films on the a n t i m o n y con ten t : curve a, 2 = 6 0 0 n m ; curve b, 3. = 900 n m ; curve c, 2 = l l 0 0 n m .

2 8 2 N. SRINIVASA MURTY, S. R. JAWALEKAR

In Fig. 5 ~TC for the ATO films is shown as a function of the wavelength. The profile of each of the curves is a magnified version of the transmission curve since q~a-c is the ratio of Tr 1° to Rsh. ~bxc is a maximum at a wavelength of 600 nm. This indicates that the film satisfies the antireflection condition at a wavelength near that of the peak in the solar flux density.

'o x

.63

L~

4oo ~ o 66o 76o 86o 9~o ' ' 1000 1100 Wavelength (nm )

Fig. 5. Variation in ~bTc for ATO films with the wavelength: curve a, 0 mol.~o Sb; curve b, 3 mol.~o Sb; curve c, 4 mol.% Sb.

The above results show that the opt imum deposition parameters for ATO films are T~ = 500°C, F(Oz) = 1 l min 1 and F(N2) = 0.07 lmin-1 . The correspond- ing film properties are Rsh = 55ti')/[2], Tr(av) = 89.2~, Tr(600) = 90.6~, ~bTC = 6.78 x 10- 3 D - 1 and an ant imony content of 3 mol.~o Sb.

The maximum values of ~bTc obtained for SnO 2 and ATO films in this study are 1.48 x 10 - 3 f2 -1 and 6.78 x 10 - 3 ~ - l respectively. Kane e t a l f l 6 have deposited SnO2 films by the CVD of dibutyltin diacetate, and the maximum ~bxc for their films calculated using eqn. (1) was 0.48 x I0 -3 D-1. q~vc for the ATO films deposited by

CHARACTERIZATION OF Sb-DOPED SnO 2 FILMS 283

K a n e et al. 17 was 5.7 x 10- 3 f~- 1. Shanth i et al.~ o ob ta ined ~bxc = 8.2 x 10- 3 ~ 1 at

a wavelength of 600 nm for A T O films depos i t ed by spray ing SnC14 and SbC13. Therefore ~xc for the films ob t a ined in this s tudy is of the same o rde r of magn i tude as tha t for films ob t a ined by the C V D of d ibu ty l t in d iace ta te and by a spray ing technique.

4. CONCLUSION

Highly t r anspa ren t conduc t ing SnO 2 and A T O films required for solar cell app l i ca t ions can be depos i t ed by the ox ida t ion of SnC12 and SbCl 3. The m a x i m u m figure of meri t ob ta ined for the SnO 2 films is 1.48× 10 -3 ~-~-1 (T r (600)= 95~o; R~h = 420 f~/[3), which increases to 6.78 x 10 -3 ~ - 1 (Tr(600) = 90.6~o; Rsh = 55 f~/[Z) when the films are d o p e d with 3 mol.~o Sb. The o p t i m u m depos i t ion pa r ame te r s are T~ = 500°C, F(O2) = 1 1 min 1 and F(N2) = 0.07 1 r a i n - 1. The wavelength of the m a x i m u m figure of mer i t ma tches tha t of the m a x i m u m solar flux density. The figure of mer i t of the films ob t a ined in this s tudy is of the same o rde r of magn i tude as those for films depos i t ed by the C V D of d ibu ty l t in d iace ta te and the spray ing of SnC14.

REFERENCES

1 T. Feng, A. K. Ghosh and C. Fishman, J. Appl. Phys., 50 (1979) 8070. 2 T. Nagatome, M. Endo and O. Omote, Jpn. J. Appl. Phys., 18 (1979) 1103. 3 J.L. Vossen, Phys. ThinFilms, 9(1977) l. 4 Z.M. Jarzebski and J. P. Marton, J. Electrochem. Soc., 123 (1976) 199c, 299c, 333c. 5 N.S. Murty, G. K. Bhagavat and S. R. Jawalekar, Thin Solid Films, 92 (1982) 347. 6 N.S. Murty and S. R. Jawalekar, Thin Solid Films, 102(1983) 283. 7 N.S. Murty and S. R. Jawalekar, Thin Solid Films, 100 (1983) 219. 8 J.S. Maudes and T. Rodriguez, Thin Solid Films, 69 (1980) 183. 9 J.C. Manifacier, L. Szepessy, J. E. Bresse, M. Perotin and R. Stuck, Mater. Res. Bull., 14 (1979) 163.

10 E. Shanthi, A. Banerjee and K. L. Chopra, Thin Solid Films, 88 (1982) 93. 11 A. Bhardwaj, B. K. Gupta, A. Raza, A. K. Sharma and O. P. Agnihotri, Sol. Ceils, 5 (1981-1982) 39. 12 G. Haacke, J. Appl. Phys., 47 (1976) 4086. 13 H.J. Hovel, in R. K. Willardson and A. C. Beer (eds.), Semiconductors and Semimetals, Vol. 11,

Academic Press, New York, 1975, p. 86. 14 V.B. Baranyuk, A. I. Malik and V. A. Mancisson, Appl. Sol. Energy (U.S.S.R.), 15 (1979) 85. 15 T. Inagaki, J. Nakajima and Y. Nakamura, Fujitsu Sci. Tech. J., 5 (1969) 235. 16 J. Kane, H. P. Schweizer and W. Kern, J. Electrochem. Soc., 122 (1975) 1144. 17 J. Kane, H. P. Schweizer and W. Kern, J. Electrochem. Soc., 123 (1976) 270.