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Transparent conductive films of tin oxide-preparation and properties
Solid State Communications, Vol. 46, No. 7, pp. 541-544, 1983. Printed in Great Britain.
0038-1098/83/190541-04503.00/0 Pergamon Press Ltd.
TRANSPARENT CONDUCTIVE FILMS OF TIN OXIDE-PREPARATION AND PROPERTIES Joy George and K.S. Joseph Solid State Physics Laboratory, Department of Physics, University of Cochin, Cochin, 682022, India
(Received 17 August 1982 by C.W. McCombie) Transparent conducting films (T = 87%, p = 2 x 10 -2 ohm cm) of tin oxide have been prepared by the oxidation in air of reactively evaporated tin disulphide films. It has been found that the time taken for the onset of oxidation at different temperatures can be represented by a simple rate relationship. The variation of resistivity of the films with oxidation temperature and heating rate is explained on the basis of the rate of crystallization of the as prepared amorphous sulphide films and the rate of oxidation. A direct transition at 4.0 --- 0.1 eV has been detected in agreement with earlier reported values. The fdms obtained show n-type conductivity. The oxide f'llms are homogenous and have strong adhesion to the substrate. 1. INTRODUCTION
2. EXPERIMENTAL
SEVERAL WORKERS have reported the preparation and properties of tin dioxide films because of the industrial applications due to its transparent and conducting nature. Thin f'dms of this material prepared on glass and ceramics are used as resistors, heating elements and other components in electronic industry. Of most interest is the possibility of using it as a transparent conducting window in photocell technology [ 1 - 3 ] . A sufficiently conducting tin oxide layer could be used in photovoltaic applications thus dispensing with the collector grid. The possibility of using this material as a gas sensor [4, 5] is yet another application of recent interest. Transparent conductive tin oxide was first deposited by McMasters [6]. Since then several methods have been reported in the literature which includes the most common spray deposition [7], flash evaporation [8], reactive evaporation [9], and r.f. sputtering [ 10]. Of these r.f. sputtering gives the best fdms with approximately 95% transmission and a sheet resistivity of 2 x 10 -~ ohm cm when sputtered from suitable SnO2-In203 targets. But this method requires complicated apparatus. The main disadvantages of spray deposition are that the substrate is to be heated in air to a temperature of 800-900 K and the resistivity is around 10 -2 ohm cm which is considerably higher than r.f. sputtered fdms. We here report a new method of preparation of tin oxide by the oxidation in air of tin disulphide films deposited by reactive evaporation [ 11 ], and their electrical and optical properties. These oxidized films are highly transparent, conducting and homogeneous and have excellent adherence.
541
Tin disulphide films were deposited on optically flat glass/quartz slides of dimension 2 x 1 × 0.2 cm. It has been found that good stoichiometric films are obtained if the following deposition parameters are satisfied: tin atom flux = 4 x 1014-1.5 x 10 is atomscm -2 sec -t , chalcogen flux = 1.5 x 1016-2.8 x 1016 molcm -2 sec -l, substrate temperature = 295-335 K. The films thus obtained are golden yellow in colour. After deposition the f'llms are taken out of the chamber and oxidized in air at a particular temperature. The setup used for the oxidation of the films in air is shown in Fig. 1. It consists of a heater block made of aluminium on which there is provision for clamping two substrates simultaneously very close together, one with the film on it and the other a dummy substrate. A chromel-alumel thermocouple is placed on the dummy to measure the temperature. The temperature could be controlled only to an accuracy of +-7 K. The heating rate was approximately 0.5 K sec -~ . All the sulphide films used in these
5~
~ 2
4 ~ i
it
i
m ~
Fig. 1. Schematic of the setup for oxidation. (1) Substrates; (2) thermocouple; (3) heater block; (4) clamping arrangement; (5) film.
542
TIN OXIDE-PREPARATION AND PROPERTIES
investigations were of thickness between 250 and 400 rim. The deposition rate of the sulphide films was approximately 1 nm sec -t . The substrate temperature during evaporation o f the sulphide Films was 325 + 4 K. The electrical resistance of the oxidized films were measured using a Hewlett-Packard 3465A digital multimeter after depositing silver electrodes on either ends of the Film. Transmission measurements were made using a Hitachi 2 0 0 - 2 0 u.v.-vis spectrophotometer. A spectral bandwidth of 1 nm was used. Transmission T through a weakly absorbing Film (n 2 ~, k 2) in a medium of refractive index no, thickness t, extinction coefficient k and refractive index n, on a substrate o f refractive index nl is given by 16nlnon2a ' T = C~ + C 22"'2 + 2CIC2~' cos (41rnt/X)
/
A ¢;i
.E I - I02
/ 18
I
19
2O I/kT
where or' = exp [-- (41rkt/X)], Cl = n + no/n1 + n and C2 = n - - no/n1 -- n. From this equation and a knowledge of n, a = 4rrk/X was determined. Refractive index n of the rdrns was obtained from the interference pattern by the method of Manffacier et al. [ 12]. Thickness of the fdms was measured using multiple beam interferometry.
/
/
103 -
io r
(1)
Vol. 46, No. 7
I 21
(eV) l
Fig. 2. Plot of 1/kT vs time taken for the onset of oxidation (t) at different oxidation temperatures (T) for a typical set of samples.
The reaction leading to the formation of tin oxide may be represented as
and the temperature T is shown in Fig. 2 for a typical set of samples. The activation energy required for the process is found to be 2.25 + 0.1 eV. It has been found that activation energy varies with the different set of specimens by about 0.2 eV. Though the monitoring technique used may seem to be crude, the result obtained are quite consistent and reasonable. The characteristic time for the microscopic interaction is found to be of the order of 10 -is sec.
SnS2 + 302 -+ SnO2 + 2SO2t.
(b) Electrical
3. RESULTS AND DISCUSSIONS (a) Oxidation o1" the [film
(2)
Oxidation of the films was monitored by observing the colour of the Film in reflected light. In the initial stages of heating, the reflectivity was found to be poor. Reflectivity increased with temperature after the as prepared film crystallized [13]. At the instant the oxidation began, an abrupt change in the colour of the film was observed. The time taken for the abrupt change in colour to occur after the film had attained the particular temperature was taken as the time required for the onset of oxidation at that temperature. It has been found that the time t taken for the onset of oxidation at different temperatures T can be represented by a simple rate relationship of the form Ea__
t = to exp k T
(3)
where Ea is the activation energy required for the process, k ~he Boltzmann constant and to the characteristic time of the microscopic interaction. A similar relationship is observed in the crystallization of amorphous silicon Films [14]. The plot of time taken for the onset of oxidation (t)
Tin dioxide f'tlms prepared by this method shows n-type conductivity. We say a Film is a low resistivity type if its resistivity is less than 10 ohm cm and high resistivity type if its resistivity is higher than 102 ohm cm. The variation of film resistivity with oxidation temperature is shown in Fig. 3. From the figure it can be seen that the films oxidized at the highest temperature used in this study (625 K) have the lowest resistivity. At this temperature it took only 30 sec for the oxidation to be completed with the rate of heating we used and so heating beyond 625 K was not attempted. It was also observed that high heating rates of about 0.5 K sec -1 had to be used to get low resistivity films. With low heating rates only high resistivity films were obtained even at the highest temperature of 625 K. On heating the as prepared amorphous sulphide films, the amorphous to crystalline transition take place around 410 K and then the grains of the sulphide Fdm begin to grow with time. At low heating rates the small crystallites will have sufficient time to grow into larger crystallites as the rate of crystallization of the amorphous Films far exceed the rate of oxidation at low
Vet. 46, No. 7
TIN OXIDE-PREPARATION AND PROPERTIES
I
543
80
I0 0
E Q.
c
\
i0-1
o
g "E
\ oN m
I
1 500
550
350
I
450
650
T (K)
Fig. 3. Variation of resistivity with oxidation temperature for the same set of samples as shown in Fig. 2. temperatures. On oxidation of such large crystallites it is possible that the bulk of the SOz formed will sublime away resulting in tin oxide crystallites and small amounts of SO2 and free S. This is similar to the oxidation behaviour reported in many II-VI compound crystals [15-17]. The free S atoms may modify the conductivity of the l-flms in two ways depending on whether the S atoms are inside the crystallites or are adsorbed in grain boundaries. It has been reported in the literature that the conductivity in tin oxide arises due to an oxygen vacancy [18, 19]. If the S atoms are inside the crystallites, these atoms can substitutionally occupy oxygen vacancies thereby decreasing the carrier concentration. In the other case the S atoms adsorbed in the grain boundaries can modify the barrier at the grain boundaries thereby decreasing the mobility of the carriers. It is not clear at present which of these mechanisms is responsible for the decreased conductivity. The role of SO2 is also not understood. But when the fllrns are oxidized at high temperatures and at high heating rates, the oxidation rate is almost comparable to the crystallization rate and the oxidation is completed much quicker depending on the temperature of the l-rim. Also there is enhanced probability for the highly volatile S and SO2 to escape from the t'tlm at high temperatures, thereby making it electrically more conductive. Figure 4 shows the transmission spectra of a typical oxidized l-rim. For comparison purposes transmission spectra of tin disulphide l-tim is also included. The variation of refractive index of the l-dm with wavelength is shown in Fig. 5. The value obtained here agrees well with that reported in the literature [8]. This value is slightly less than that reported for tin dioxide crystals (n = 1.9) [201.
550
650
>. ( n m )
Fig. 4. Transmission spectra of the oxidized l-tim (curve 1) and of as prepared sulphide film (curve 2). Oxidation temperature 600 K, film thickness 250 nm. 2.5--
2.0
1.5
1.0
I
I
I
800
I100
X (nrn)
Fig. 5. Variation of refractive index n with wavelength. Absorption data is analyzed in terms of the theory of Bardeen et al. [21 ]. For a direct transition the theory gives
A(hv ¢x =
h~
'
(4)
r = 1/2 for an allowed transition, r = 3/2 for a forbidden transition, where A is a constant, hv photon energy and E~ the direct gap. It has been generally reported that tin dioxide is an indirect gap semiconductor. Because of the thinness of the samples and also due to the low absorption coefficients associated with indirect transitions, the indirect transition could not be observed. In Fig. 6, (ahv) 2 is plotted against hv for a typical f'dm and gives an intercept of 3.93 + 0.01 eV corresponding to a direct
544
TIN OXIDE-PREPARATION AND PROPERTIES REFERENCES
/ 4 X 1012
/ /
%
3. 4.
/
g
1. 2.
/
> o
5. 6.
0000
7. 8.
l
I
3.2
3.6
I
4.0
4.4
hv ( e V )
9. 10. 11.
Fig. 6. Plot of (othv)2 vs hz, for a typical film.
12.
transition. The intercepts obtained for different films range from 3.9 to 4.1 eV. Values reported in the literature for this transition varies from 3.7 to 4.3 eV [2224].
13. 14. 15.
4. CONCLUSIONS
16.
Transparent conducting films of tin oxide (T = 87%, p = 2 x 10 -2 ohm cm) have been prepared by the oxidation in air of reactively evaporated tin disulphide films. It has been found that the time taken for the onset of oxidation at different temperatures can be represented by a simple rate relationship. Low resistivity films are obtained with oxidation temperature around 625 K and heating rate around 0.5 Ksec -1 . The films are n-type and have strong adhesion to the substrate.
17.
Acknowledgements - One of the authors (K.S.J.) is thankful to C.S.I.R., New Delhi for the award of a senior research fellowship.
Vol. 46, No. 7
18. 19. 20. 21. 22. 23. 24.
K. Kajimaya & Y. Furukawa, Japan. J. Appl. Phys. 6,905 (1967). Tom Feng, A.K. Ghosh & Charles Fishman, Appl. Phys. Lett. 35,266 (1979). A. Coche, Appl. Phys. Lett. 35,863 (1979). M. Nita, S. Kanefusa, Y. Taketa & M. Haradome, Appl. Phys. Lett. 32, 590 (1978). H. Windischmann & P. Mark, J. Electrochem. Soc. 126,627 (1979). British Patent 632,256 to H.A. McMasters, LibelyOwens-Ford Glass Co. (1942). K. Ishiguro, T. Sasaki, T. Arai & T. Imai, J. Phys. Soc. Japan 13,296 (1958). J.C. Manifacier, M. De Murcia, J.P. Fillard & E. Vicario, Thin SolidFilms 41,127 (1977). S. Muranaka, Y. Bands & T. Takada, Thin Solid Films 86, 11 (1981). J.L. Vossen, R.C.A. Rev. 32,289 (1971). J. George & K.S. Joseph, J. Phys. D: Appl. Phys. 15, 1109 (1982). J.C. Manifacier, J.Gasiot & J.P. Fillard, J. Phys E: Sci. Instrum. 9, 1002 (1976). J. George & K.S. Joseph, to be published in J. Phys. D: Appl. Phys. N.A. Blum & C. Feldman, Z Non. Cryst. Solids 11,242 (1972). P. Pianetta, I. Lindau, C.M. Garner & W.E. Spear, Phys. Rev. BI8, 2792 (1978). A. Ebina, K. Asano, Y. Suda & T. Takahashi, J. Vac. Sci. Technol. 17, 1074 (1980). A. Ebina, Y. Suda & T. Takahashi, Int. J. Elec. 52, 77 (1982). S. Samson & C.G. Fonstad, J. Appl. Phys. 44, 4618 (1973). J.A. Aboof, V.C. Marcotte & N.J. Chow, J. Electrochem. Soc. 120, 701 (1973). S.F. Redaway & D.A. Wright, British J. Appl. Phys. 16, 195 (1965). J. Bardeen, F.J. Blatt& L.H. Hall, Photoconductivity Conference, Atlantic City p. 146, Wiley, New York (1956). T. Arai, J. Phys. Soc. Japan 15,916 (1960). W. Spence, J. Appl. Phys. 38, 3767 (1967). K.B. Sundaram & G.K. Bhagavat, J. Phys. D: Appl. Phys. 14,921 (1981).
0038-1098/83/190541-04503.00/0 Pergamon Press Ltd.
TRANSPARENT CONDUCTIVE FILMS OF TIN OXIDE-PREPARATION AND PROPERTIES Joy George and K.S. Joseph Solid State Physics Laboratory, Department of Physics, University of Cochin, Cochin, 682022, India
(Received 17 August 1982 by C.W. McCombie) Transparent conducting films (T = 87%, p = 2 x 10 -2 ohm cm) of tin oxide have been prepared by the oxidation in air of reactively evaporated tin disulphide films. It has been found that the time taken for the onset of oxidation at different temperatures can be represented by a simple rate relationship. The variation of resistivity of the films with oxidation temperature and heating rate is explained on the basis of the rate of crystallization of the as prepared amorphous sulphide films and the rate of oxidation. A direct transition at 4.0 --- 0.1 eV has been detected in agreement with earlier reported values. The fdms obtained show n-type conductivity. The oxide f'llms are homogenous and have strong adhesion to the substrate. 1. INTRODUCTION
2. EXPERIMENTAL
SEVERAL WORKERS have reported the preparation and properties of tin dioxide films because of the industrial applications due to its transparent and conducting nature. Thin f'dms of this material prepared on glass and ceramics are used as resistors, heating elements and other components in electronic industry. Of most interest is the possibility of using it as a transparent conducting window in photocell technology [ 1 - 3 ] . A sufficiently conducting tin oxide layer could be used in photovoltaic applications thus dispensing with the collector grid. The possibility of using this material as a gas sensor [4, 5] is yet another application of recent interest. Transparent conductive tin oxide was first deposited by McMasters [6]. Since then several methods have been reported in the literature which includes the most common spray deposition [7], flash evaporation [8], reactive evaporation [9], and r.f. sputtering [ 10]. Of these r.f. sputtering gives the best fdms with approximately 95% transmission and a sheet resistivity of 2 x 10 -~ ohm cm when sputtered from suitable SnO2-In203 targets. But this method requires complicated apparatus. The main disadvantages of spray deposition are that the substrate is to be heated in air to a temperature of 800-900 K and the resistivity is around 10 -2 ohm cm which is considerably higher than r.f. sputtered fdms. We here report a new method of preparation of tin oxide by the oxidation in air of tin disulphide films deposited by reactive evaporation [ 11 ], and their electrical and optical properties. These oxidized films are highly transparent, conducting and homogeneous and have excellent adherence.
541
Tin disulphide films were deposited on optically flat glass/quartz slides of dimension 2 x 1 × 0.2 cm. It has been found that good stoichiometric films are obtained if the following deposition parameters are satisfied: tin atom flux = 4 x 1014-1.5 x 10 is atomscm -2 sec -t , chalcogen flux = 1.5 x 1016-2.8 x 1016 molcm -2 sec -l, substrate temperature = 295-335 K. The films thus obtained are golden yellow in colour. After deposition the f'llms are taken out of the chamber and oxidized in air at a particular temperature. The setup used for the oxidation of the films in air is shown in Fig. 1. It consists of a heater block made of aluminium on which there is provision for clamping two substrates simultaneously very close together, one with the film on it and the other a dummy substrate. A chromel-alumel thermocouple is placed on the dummy to measure the temperature. The temperature could be controlled only to an accuracy of +-7 K. The heating rate was approximately 0.5 K sec -~ . All the sulphide films used in these
5~
~ 2
4 ~ i
it
i
m ~
Fig. 1. Schematic of the setup for oxidation. (1) Substrates; (2) thermocouple; (3) heater block; (4) clamping arrangement; (5) film.
542
TIN OXIDE-PREPARATION AND PROPERTIES
investigations were of thickness between 250 and 400 rim. The deposition rate of the sulphide films was approximately 1 nm sec -t . The substrate temperature during evaporation o f the sulphide Films was 325 + 4 K. The electrical resistance of the oxidized films were measured using a Hewlett-Packard 3465A digital multimeter after depositing silver electrodes on either ends of the Film. Transmission measurements were made using a Hitachi 2 0 0 - 2 0 u.v.-vis spectrophotometer. A spectral bandwidth of 1 nm was used. Transmission T through a weakly absorbing Film (n 2 ~, k 2) in a medium of refractive index no, thickness t, extinction coefficient k and refractive index n, on a substrate o f refractive index nl is given by 16nlnon2a ' T = C~ + C 22"'2 + 2CIC2~' cos (41rnt/X)
/
A ¢;i
.E I - I02
/ 18
I
19
2O I/kT
where or' = exp [-- (41rkt/X)], Cl = n + no/n1 + n and C2 = n - - no/n1 -- n. From this equation and a knowledge of n, a = 4rrk/X was determined. Refractive index n of the rdrns was obtained from the interference pattern by the method of Manffacier et al. [ 12]. Thickness of the fdms was measured using multiple beam interferometry.
/
/
103 -
io r
(1)
Vol. 46, No. 7
I 21
(eV) l
Fig. 2. Plot of 1/kT vs time taken for the onset of oxidation (t) at different oxidation temperatures (T) for a typical set of samples.
The reaction leading to the formation of tin oxide may be represented as
and the temperature T is shown in Fig. 2 for a typical set of samples. The activation energy required for the process is found to be 2.25 + 0.1 eV. It has been found that activation energy varies with the different set of specimens by about 0.2 eV. Though the monitoring technique used may seem to be crude, the result obtained are quite consistent and reasonable. The characteristic time for the microscopic interaction is found to be of the order of 10 -is sec.
SnS2 + 302 -+ SnO2 + 2SO2t.
(b) Electrical
3. RESULTS AND DISCUSSIONS (a) Oxidation o1" the [film
(2)
Oxidation of the films was monitored by observing the colour of the Film in reflected light. In the initial stages of heating, the reflectivity was found to be poor. Reflectivity increased with temperature after the as prepared film crystallized [13]. At the instant the oxidation began, an abrupt change in the colour of the film was observed. The time taken for the abrupt change in colour to occur after the film had attained the particular temperature was taken as the time required for the onset of oxidation at that temperature. It has been found that the time t taken for the onset of oxidation at different temperatures T can be represented by a simple rate relationship of the form Ea__
t = to exp k T
(3)
where Ea is the activation energy required for the process, k ~he Boltzmann constant and to the characteristic time of the microscopic interaction. A similar relationship is observed in the crystallization of amorphous silicon Films [14]. The plot of time taken for the onset of oxidation (t)
Tin dioxide f'tlms prepared by this method shows n-type conductivity. We say a Film is a low resistivity type if its resistivity is less than 10 ohm cm and high resistivity type if its resistivity is higher than 102 ohm cm. The variation of film resistivity with oxidation temperature is shown in Fig. 3. From the figure it can be seen that the films oxidized at the highest temperature used in this study (625 K) have the lowest resistivity. At this temperature it took only 30 sec for the oxidation to be completed with the rate of heating we used and so heating beyond 625 K was not attempted. It was also observed that high heating rates of about 0.5 K sec -1 had to be used to get low resistivity films. With low heating rates only high resistivity films were obtained even at the highest temperature of 625 K. On heating the as prepared amorphous sulphide films, the amorphous to crystalline transition take place around 410 K and then the grains of the sulphide Fdm begin to grow with time. At low heating rates the small crystallites will have sufficient time to grow into larger crystallites as the rate of crystallization of the amorphous Films far exceed the rate of oxidation at low
Vet. 46, No. 7
TIN OXIDE-PREPARATION AND PROPERTIES
I
543
80
I0 0
E Q.
c
\
i0-1
o
g "E
\ oN m
I
1 500
550
350
I
450
650
T (K)
Fig. 3. Variation of resistivity with oxidation temperature for the same set of samples as shown in Fig. 2. temperatures. On oxidation of such large crystallites it is possible that the bulk of the SOz formed will sublime away resulting in tin oxide crystallites and small amounts of SO2 and free S. This is similar to the oxidation behaviour reported in many II-VI compound crystals [15-17]. The free S atoms may modify the conductivity of the l-flms in two ways depending on whether the S atoms are inside the crystallites or are adsorbed in grain boundaries. It has been reported in the literature that the conductivity in tin oxide arises due to an oxygen vacancy [18, 19]. If the S atoms are inside the crystallites, these atoms can substitutionally occupy oxygen vacancies thereby decreasing the carrier concentration. In the other case the S atoms adsorbed in the grain boundaries can modify the barrier at the grain boundaries thereby decreasing the mobility of the carriers. It is not clear at present which of these mechanisms is responsible for the decreased conductivity. The role of SO2 is also not understood. But when the fllrns are oxidized at high temperatures and at high heating rates, the oxidation rate is almost comparable to the crystallization rate and the oxidation is completed much quicker depending on the temperature of the l-rim. Also there is enhanced probability for the highly volatile S and SO2 to escape from the t'tlm at high temperatures, thereby making it electrically more conductive. Figure 4 shows the transmission spectra of a typical oxidized l-rim. For comparison purposes transmission spectra of tin disulphide l-tim is also included. The variation of refractive index of the l-dm with wavelength is shown in Fig. 5. The value obtained here agrees well with that reported in the literature [8]. This value is slightly less than that reported for tin dioxide crystals (n = 1.9) [201.
550
650
>. ( n m )
Fig. 4. Transmission spectra of the oxidized l-tim (curve 1) and of as prepared sulphide film (curve 2). Oxidation temperature 600 K, film thickness 250 nm. 2.5--
2.0
1.5
1.0
I
I
I
800
I100
X (nrn)
Fig. 5. Variation of refractive index n with wavelength. Absorption data is analyzed in terms of the theory of Bardeen et al. [21 ]. For a direct transition the theory gives
A(hv ¢x =
h~
'
(4)
r = 1/2 for an allowed transition, r = 3/2 for a forbidden transition, where A is a constant, hv photon energy and E~ the direct gap. It has been generally reported that tin dioxide is an indirect gap semiconductor. Because of the thinness of the samples and also due to the low absorption coefficients associated with indirect transitions, the indirect transition could not be observed. In Fig. 6, (ahv) 2 is plotted against hv for a typical f'dm and gives an intercept of 3.93 + 0.01 eV corresponding to a direct
544
TIN OXIDE-PREPARATION AND PROPERTIES REFERENCES
/ 4 X 1012
/ /
%
3. 4.
/
g
1. 2.
/
> o
5. 6.
0000
7. 8.
l
I
3.2
3.6
I
4.0
4.4
hv ( e V )
9. 10. 11.
Fig. 6. Plot of (othv)2 vs hz, for a typical film.
12.
transition. The intercepts obtained for different films range from 3.9 to 4.1 eV. Values reported in the literature for this transition varies from 3.7 to 4.3 eV [2224].
13. 14. 15.
4. CONCLUSIONS
16.
Transparent conducting films of tin oxide (T = 87%, p = 2 x 10 -2 ohm cm) have been prepared by the oxidation in air of reactively evaporated tin disulphide films. It has been found that the time taken for the onset of oxidation at different temperatures can be represented by a simple rate relationship. Low resistivity films are obtained with oxidation temperature around 625 K and heating rate around 0.5 Ksec -1 . The films are n-type and have strong adhesion to the substrate.
17.
Acknowledgements - One of the authors (K.S.J.) is thankful to C.S.I.R., New Delhi for the award of a senior research fellowship.
Vol. 46, No. 7
18. 19. 20. 21. 22. 23. 24.
K. Kajimaya & Y. Furukawa, Japan. J. Appl. Phys. 6,905 (1967). Tom Feng, A.K. Ghosh & Charles Fishman, Appl. Phys. Lett. 35,266 (1979). A. Coche, Appl. Phys. Lett. 35,863 (1979). M. Nita, S. Kanefusa, Y. Taketa & M. Haradome, Appl. Phys. Lett. 32, 590 (1978). H. Windischmann & P. Mark, J. Electrochem. Soc. 126,627 (1979). British Patent 632,256 to H.A. McMasters, LibelyOwens-Ford Glass Co. (1942). K. Ishiguro, T. Sasaki, T. Arai & T. Imai, J. Phys. Soc. Japan 13,296 (1958). J.C. Manifacier, M. De Murcia, J.P. Fillard & E. Vicario, Thin SolidFilms 41,127 (1977). S. Muranaka, Y. Bands & T. Takada, Thin Solid Films 86, 11 (1981). J.L. Vossen, R.C.A. Rev. 32,289 (1971). J. George & K.S. Joseph, J. Phys. D: Appl. Phys. 15, 1109 (1982). J.C. Manifacier, J.Gasiot & J.P. Fillard, J. Phys E: Sci. Instrum. 9, 1002 (1976). J. George & K.S. Joseph, to be published in J. Phys. D: Appl. Phys. N.A. Blum & C. Feldman, Z Non. Cryst. Solids 11,242 (1972). P. Pianetta, I. Lindau, C.M. Garner & W.E. Spear, Phys. Rev. BI8, 2792 (1978). A. Ebina, K. Asano, Y. Suda & T. Takahashi, J. Vac. Sci. Technol. 17, 1074 (1980). A. Ebina, Y. Suda & T. Takahashi, Int. J. Elec. 52, 77 (1982). S. Samson & C.G. Fonstad, J. Appl. Phys. 44, 4618 (1973). J.A. Aboof, V.C. Marcotte & N.J. Chow, J. Electrochem. Soc. 120, 701 (1973). S.F. Redaway & D.A. Wright, British J. Appl. Phys. 16, 195 (1965). J. Bardeen, F.J. Blatt& L.H. Hall, Photoconductivity Conference, Atlantic City p. 146, Wiley, New York (1956). T. Arai, J. Phys. Soc. Japan 15,916 (1960). W. Spence, J. Appl. Phys. 38, 3767 (1967). K.B. Sundaram & G.K. Bhagavat, J. Phys. D: Appl. Phys. 14,921 (1981).