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SnO2 thin films prepared by the sol-gel process
thin
ELSEVIER
o
Thin Solid Films 292 (1997) 299-302
SnOe thin films prepared by the sol-gel process T.M. Racheva a, G.W. Critchlow b,. "Higher Institute of Chemical Technology, Department of Semiconductors, 8 Kl. Ohridski Blvd., 1156 Sofia, Bulgaria b Institute of Surface Science and Technology, Loughborough University, Loughborough, Leics. LE12 3TU, UK
Received 21 March 1996; accepted22 May 1996
Abstract SnO2 thin films were prepared by the sol-gel process using an alcoholic solution of Sn (OC2H5)42C2HsOHas the precursor. After subsequent annealing, the temperature of which was varied according to the substrate (oxidised silicon, glass or ceramic glass ), the films were characterised by scanning electron microscopy, electron diffraction and Auger electron spectroscopy. The sensitivity of these films to variations in humidity was established for values in the range 0% to 93% relative humidity. Keywords: Augerelectron spectroscopy;Secondary electron microscopy;Sensors; Tin oxide
1. Introduction Non-stoichiometric and impurity doped S n Q thin films are of widespread interest because of their unique properties. These include: high electrical conductivity; optical transparency over the visible wavelengths; and high chemical and thermal stability. These properties make them useful for many applications, including: transparent electrodes in various display devices and solar cells [ 1,2] ; Schottky barrier-like heterojunction n-Si/SiO2 solar cells [3]; and highly sensitive and selective semiconductor gas sensors [4-6]. The properties of the films are strongly dependent upon the method of preparation. As a result, a wide range of different deposition techniques have been investigated. These include: chemical vapour deposition [7]; spray pyrolysis [8]; electron beam evaporation [9]; and sputtering [10]. The sol-gel technique offers an alternative to the aforementioned methods for the production of both pure and doped SnO2 thin films [ 11-13]. The sol-gel technique offers many advantages over other methods, such as: low temperature processing; precise control of the doping level; and simplicity, not requiring expensive deposition facilities. The films obtained by the sol-gel technique have a porous structure consisting of ultra-fine particles [ 14], thereby offering alarge specific surface area. Such a structure is desirable for gas sensor application. In the present paper, we report the results from SnO2 films produced by the sol-gel technique from an alcoholic solution * Corresponding author.
of tin alkoxide (Sn(OC2Hs)42C2HsOH). In addition, the chemical composition, structure and humidity sensitive properties of the films were studied.
2. Experimental Tin (IV) alkoxide (Sn(OC2Hs)42C2HsOH) was prepared by the reaction of tin (IV) chloride with sodium ethoxide in ethanol [ 15]. The mixture of the reagents with the precipitated NaC1 was refluxed for 6 h and, after filtration, a clear solution was obtained. The solution was concentrated by vacuum distillation and cooled. As a result, crystals of Sn(OC2Hs)42C2HsOH were produced. The crystals were removed and dissolved in absolute ethanol. This was used as the starting material for the preparation of the SnO2 films by the sol-gel process. The tin content of the solution was estimated by inductively coupled plasma atomic emission spectrometry (ICP-AES). The composition of a number of films was established using Auger electron spectroscopy (AES). The spectrometer used was a Varian 10 kV instrument operating with aprimary electron beam energy of 3×103 eV and a current of 0.4 × 10 .6 A. Depth profiling was carried out by combining AES with sequential argon-ion bombardment. The ion source had a primary beam energy of 3 × 103 eV with a current density of 5 0 × 10 .6 A cm -a. Compositions were determined using experimentally derived relative sensitivity factors with Sn:O ratios based upon a SnO2 powder reference material. Depth scale calibration was achieved using a theo-
0040-6090/97/$17.00 Copyright© 1997ElsevierScience S.A. All fights reserved PIIS0040-6090(96)08956-0
300
T.M. Racheva, G, W. Critchlow / Thin Solid Films 292 (1997) 299-302
Table 1 A summary of experimental parameters used in the productionof thin SnQ films Initial solution Sn c o n t e n t (gl -t)
Experimental solution Sn(OC2Hs)+ content (moll -t )
Mole ratio H20:Sn(OC2Hs),
88.9 88.9 88.9 88.9 70.02 64
0.374 0.374 0.347 0.347 0.294 0.269
0.15 0.37 1.03 1.48 0.47 0.31
OH \ I
o
Sn I
/ \
OH OH --
si
\ I 0
--
OH
I
o
Sn ]
I \
OH OH --
l
I si t
[ \ I
0
--
Time for gelation
5 min-10 h 3 min-18 rain 5 rain-20 rnin 3 min-24 rain
OH
1
Spinnability period
Sn
I \
-r~I20 _).
\ I
OH OH --
I si
11 h 20 min 2 rain 0 22 rain 25 min
OH I Sn
/
\
I
\
/
o
0
0
I
I si I
1
\
/ 0
--
Gel
60-70 nm 70-90 nm 60-80 nm 60-80 nm
Transparentjelly-like Transparentjelly-like White jelly-like White precipitate Transparentjelly-like Transparentjelly-like
I si I
I
/
\
0
I --
Film
[O'] - electrical defect I o Sn 1 \ Sn
0
I --
Resultant product
I --
--
I si I
Si 0 2 based substrate
Fig. 1. To show the mechanismby which metalloxanebonds form at the film-substrate interface. retically derived etch rate for S n Q of 40 nm m i n - 1 using the ion-beam parameters previously mentioned. The coating solutions were prepared by mixing 40 ml of the previously mentioned solution with 40 ml ofethyt acetate. Amsmall amount of water was added to cause partial hydrolysis and subsequent dehydration of the alcoholate ensuring the formation o f chain-like polymers. The viscous state was attained after a critical period of time. After this time, the sol converted to a gel. The concentration of tin in the initial solution and in the experimental solutions, the amount of water (expressed in terms o f th e molar ratio of water to tin alcoholate), and the behaviour of the solutions as a function of water content are given in Table 1. The films were formed on oxidised silicon (wafers, 76 mm diameter), plain soda lime-silca glass ( 2 0 m m × 20 mm) and polished ( to a mirror finish) ceramic-glass plate (40 mm × 40 m m ) substrates. Spinning and heating stages were employed. In order to+ prepare the films, a few cm 3 of the coating solution was, dropped onto the substrate which was then spun at a constant rate (2500 r.p.m. ) for several seconds. The films were then dried in air at 250 °C for 30 min. In addition to the drying process the films were annealed at a higher temperature, in the range 500-700 °C, depending upo n the type of substrate. W i t h the oxidised silicon substrates the films were annealed at 700 ?C for 30 min, whilst the glass and ceramicglass substrates were heated to 500 °C and 600 °C respectively for. 1, h in both cases. The heating was carried out in air. During the heat treatment the films firmly adhered to the substrates due to the presence of metaltoxane bonds at the film-substrate interface (Fig. 1). In addition, there are randomly distributed electrical defects with electrons shared with tin atoms thereby affecting the electrical conductivity of the films [ 16].
3.
Results
The resultant films were transparent, uniform in thickness across the substrates, and free from cracks. The thickness o f the films depended upon the viscosity of the solution and was in the range 6 0 - 9 0 nm. The film thickness was established from both the interference colour and from the AES data, measuring the time taken to sputter through the films. Fig. 2 ( a ) presents a scanning electron microscopy ( S E M ) image (plan view) of a SnO2 film on oxidised silicon, whilst
Fig. 2. (a) Scanning electron micrograph, and (b) electron diffraction pattern from a 60 nm SnO2 thin film on oxidised silicon, annealed at 700 °C for 30 rain.
T.M. Racheva, G. W. Critchlow / Thin Solid Films 292 (1997) 299-302
301
Table 2 A summary of sensor response data as a function of relative humidity Reladve humidity(RH) (%)
Resistance R (Mg2)
Responsetime (s) 0%toRH%
Recovery time (s) RH%to 0%
0 32.3 43 55 66 76 86 93
10000 5000 2000 500 310 220 190 180
-
_ 1 1 1 I 1 1 2
80
8 i0 12 13 I4 16 17
Composition (atomg) 80
ComposiU.on (atomY**)
70 60
•
60
5O 4O
a.
_¢
30
40
+
~
~
20
2o!
+
iO 20
40
60 Depth (rim)
BO
100
120
0 --+--Sn ~ Na -e--Si Fig. 3. An AES composition depth profile through a 75 nm S n Q film on oxidised silicon, annealed at 700 °C for 30 rain.
Fig. 2(b) illustrates the corresponding electron diffraction pattern. These techniques indicate a uniform, granular surface morphology with a very small grain size. The polycrystalline structure corresponds to the cassiterite form of S n Q . The aforementioned surface morphology and structure was typical of all the SnOa films irrespective of substrate type. AES depth profiling was used in order to determine the composition of the films, in particular, to determine the degree of sodium and carbon contamination and the Sn:O ratios. The results of AES analyses through the films on oxidised silicon and glass substrates are given in Figs. 3 and 4 respectively. Sodium levels up to ~ 2.5% were observed throughout the film on the oxidised silicon substrate. However, no carbon could be observed at depth in this instance. With the glass substrate, sodium levels reached a maximum of ~ 4 % with carbon levels of 1-2% observed throughout the film. The detection limits for carbon and sodium were estimated to be ~ 0.3 and ~ 0.4 at.% respectively. Furthermore, the AlES results indicate that these films are slightly sub-stoichiometric with Sn:O ratios in the range 1:1.8 to 1:1.7. Some calcium contamination could be observed in the films produced on both glass and ceramic-glass substrates. This was attributed to diffusion of calcium from the substrates into the films during the heat treatment. The films prepared on glass and ceramic-glass substrates with thicknesses of 60-70 nm were used for the preparation of sensor elements. For this investigation silver electrodes were attached onto the surface of these films. The distance
20
40
60
80
100
120
Depth 0~m) +
0
--S- Sn
+
C
--8--Na
-->4--Si
--¢--Ca
Fig. 4. An AES composition depth profile through a 90 nm SnO2 film on a glass substrate, annealed at 500 °C for 1 h.
between the electrodes was 1 ram. Silver lead wires were attached to the contact pads to measure the resistance (R) between the pads. These sensor devices were placed into flasks and exposed to relative humidities in the range 0-93 % RH. The flasks contained saturated water solutions of salts which maintain a known RH in the surrounding ambient [ 17]. The glass flasks were placed in a chamber which was thermostatically controlled to be at a constant temperature of 20 °C. The humidity sensitivity characteristics were measured by placing the sensor element in the flask with a dryer at 0% RH. After that, the sensors were placed in a flask at a known RH and then again in the original flask at 0% RH. The results of these experiments are presented in Table 2. The films responded to humidities in the range 32-93% RH. The response times were very short, as was the recovery time. A disadvantage, however, was the very high electrical resistivity due mainly to the limited thickness of the films and the fine granular structure. This problem could be solved by repeating the spinning and heating procedure a number of times with the same substrate. In addition, the structure could be modified by additives and subsequent annealing.
4. Conclusions Sn02 films were prepared by the sol-gel process using Sn(OCzHs)42C2HsOH as the starting material. After subsequent heating and annealing (in the range 500 °C to 700 °C
302
T.M. Raeheva, G.W. CritchIow / Thin Solid Films 292 (1997) 299-302
d e p e n d e n t upon the substrate) films were successfully obtained on oxidised silicon, glass and ceramic-glass substrates. The deposited films were converted from being amorphous to being polycrystalline with a very small grain size. The A E S analyses show the films to be sub-stoichiometric with Sn:O ratios in the range 1:1.8 to 1:1.7. Carbon could only be detected within the film on the glass substrate whilst s o d i u m was detected at levels of between ~ 2 , 5 - 4 % in films on both oxidised silicon and plain glass. The films were sensitive to humidities in the range 3 2 - 9 3 % RH.
References [ 1] W.A. Badaway, J. Electroanal. Chem., 28t (1990) 85. [2] C. Geoffroy, G. Campet et al., Active Passive Electron. Comp., 14 (t991) 111.
[3] W.A. Badaway, H.H. Afify and E.M. Elgiar, J. Elecrochem. Soc., 137 (5) (1990) 1592. [4] G. Heiland, Sensors Actuators, 2 (1982) 343. [5] D. Kohl, Se~tsorsActuators, 18 (1989) 71. [6] N. Yamazoe, Sensors Actuators B, 5 ( 1991) 7. [7] A. Mani, N. Karuppian and R. Mahalingam, Mater. Res. Bull., 25 (1990) 799. [8] W.M. Sears and M.A, Gee, Thin Solid Films, 165 (1988) 265. [9] D. Das and R. Banerjee, Thin Solid Films, 147 (1987) 321. [ 10] E. Leja, J. Korecki and K. Toll, Thin Solid Fihns, 59 (1979) 147. [ 11] S. Sakka and K. Kamiya, J. Non-Crystalline Solids, 42 (1980) 403. [12] H. Dislich, Angew. Chem., 83 (1971) 428. [ 13l S, Sakka, J. Non-Crystalline Solids, 73 (1985) 651. [ 14] S.-S. Park, H. Zheng and J.D. Mackenzie, Mater. Lett., 17 ( 1993) 376. [15] Z. Thiessen, Inorg. Chem., B195 (1931) 383. [ 16] J.C. Giuntini, W. Garanier, J.W. Zanchetta and A. Taha, J. Mater. Sci. Lett., 9 (1990) 1383. [ I7] V.V. Kliuev (ed.), Ispitatelnaia Technika, Mashinostroenie, Moscow, 1982, Chapt. 2, p. 488.
ELSEVIER
o
Thin Solid Films 292 (1997) 299-302
SnOe thin films prepared by the sol-gel process T.M. Racheva a, G.W. Critchlow b,. "Higher Institute of Chemical Technology, Department of Semiconductors, 8 Kl. Ohridski Blvd., 1156 Sofia, Bulgaria b Institute of Surface Science and Technology, Loughborough University, Loughborough, Leics. LE12 3TU, UK
Received 21 March 1996; accepted22 May 1996
Abstract SnO2 thin films were prepared by the sol-gel process using an alcoholic solution of Sn (OC2H5)42C2HsOHas the precursor. After subsequent annealing, the temperature of which was varied according to the substrate (oxidised silicon, glass or ceramic glass ), the films were characterised by scanning electron microscopy, electron diffraction and Auger electron spectroscopy. The sensitivity of these films to variations in humidity was established for values in the range 0% to 93% relative humidity. Keywords: Augerelectron spectroscopy;Secondary electron microscopy;Sensors; Tin oxide
1. Introduction Non-stoichiometric and impurity doped S n Q thin films are of widespread interest because of their unique properties. These include: high electrical conductivity; optical transparency over the visible wavelengths; and high chemical and thermal stability. These properties make them useful for many applications, including: transparent electrodes in various display devices and solar cells [ 1,2] ; Schottky barrier-like heterojunction n-Si/SiO2 solar cells [3]; and highly sensitive and selective semiconductor gas sensors [4-6]. The properties of the films are strongly dependent upon the method of preparation. As a result, a wide range of different deposition techniques have been investigated. These include: chemical vapour deposition [7]; spray pyrolysis [8]; electron beam evaporation [9]; and sputtering [10]. The sol-gel technique offers an alternative to the aforementioned methods for the production of both pure and doped SnO2 thin films [ 11-13]. The sol-gel technique offers many advantages over other methods, such as: low temperature processing; precise control of the doping level; and simplicity, not requiring expensive deposition facilities. The films obtained by the sol-gel technique have a porous structure consisting of ultra-fine particles [ 14], thereby offering alarge specific surface area. Such a structure is desirable for gas sensor application. In the present paper, we report the results from SnO2 films produced by the sol-gel technique from an alcoholic solution * Corresponding author.
of tin alkoxide (Sn(OC2Hs)42C2HsOH). In addition, the chemical composition, structure and humidity sensitive properties of the films were studied.
2. Experimental Tin (IV) alkoxide (Sn(OC2Hs)42C2HsOH) was prepared by the reaction of tin (IV) chloride with sodium ethoxide in ethanol [ 15]. The mixture of the reagents with the precipitated NaC1 was refluxed for 6 h and, after filtration, a clear solution was obtained. The solution was concentrated by vacuum distillation and cooled. As a result, crystals of Sn(OC2Hs)42C2HsOH were produced. The crystals were removed and dissolved in absolute ethanol. This was used as the starting material for the preparation of the SnO2 films by the sol-gel process. The tin content of the solution was estimated by inductively coupled plasma atomic emission spectrometry (ICP-AES). The composition of a number of films was established using Auger electron spectroscopy (AES). The spectrometer used was a Varian 10 kV instrument operating with aprimary electron beam energy of 3×103 eV and a current of 0.4 × 10 .6 A. Depth profiling was carried out by combining AES with sequential argon-ion bombardment. The ion source had a primary beam energy of 3 × 103 eV with a current density of 5 0 × 10 .6 A cm -a. Compositions were determined using experimentally derived relative sensitivity factors with Sn:O ratios based upon a SnO2 powder reference material. Depth scale calibration was achieved using a theo-
0040-6090/97/$17.00 Copyright© 1997ElsevierScience S.A. All fights reserved PIIS0040-6090(96)08956-0
300
T.M. Racheva, G, W. Critchlow / Thin Solid Films 292 (1997) 299-302
Table 1 A summary of experimental parameters used in the productionof thin SnQ films Initial solution Sn c o n t e n t (gl -t)
Experimental solution Sn(OC2Hs)+ content (moll -t )
Mole ratio H20:Sn(OC2Hs),
88.9 88.9 88.9 88.9 70.02 64
0.374 0.374 0.347 0.347 0.294 0.269
0.15 0.37 1.03 1.48 0.47 0.31
OH \ I
o
Sn I
/ \
OH OH --
si
\ I 0
--
OH
I
o
Sn ]
I \
OH OH --
l
I si t
[ \ I
0
--
Time for gelation
5 min-10 h 3 min-18 rain 5 rain-20 rnin 3 min-24 rain
OH
1
Spinnability period
Sn
I \
-r~I20 _).
\ I
OH OH --
I si
11 h 20 min 2 rain 0 22 rain 25 min
OH I Sn
/
\
I
\
/
o
0
0
I
I si I
1
\
/ 0
--
Gel
60-70 nm 70-90 nm 60-80 nm 60-80 nm
Transparentjelly-like Transparentjelly-like White jelly-like White precipitate Transparentjelly-like Transparentjelly-like
I si I
I
/
\
0
I --
Film
[O'] - electrical defect I o Sn 1 \ Sn
0
I --
Resultant product
I --
--
I si I
Si 0 2 based substrate
Fig. 1. To show the mechanismby which metalloxanebonds form at the film-substrate interface. retically derived etch rate for S n Q of 40 nm m i n - 1 using the ion-beam parameters previously mentioned. The coating solutions were prepared by mixing 40 ml of the previously mentioned solution with 40 ml ofethyt acetate. Amsmall amount of water was added to cause partial hydrolysis and subsequent dehydration of the alcoholate ensuring the formation o f chain-like polymers. The viscous state was attained after a critical period of time. After this time, the sol converted to a gel. The concentration of tin in the initial solution and in the experimental solutions, the amount of water (expressed in terms o f th e molar ratio of water to tin alcoholate), and the behaviour of the solutions as a function of water content are given in Table 1. The films were formed on oxidised silicon (wafers, 76 mm diameter), plain soda lime-silca glass ( 2 0 m m × 20 mm) and polished ( to a mirror finish) ceramic-glass plate (40 mm × 40 m m ) substrates. Spinning and heating stages were employed. In order to+ prepare the films, a few cm 3 of the coating solution was, dropped onto the substrate which was then spun at a constant rate (2500 r.p.m. ) for several seconds. The films were then dried in air at 250 °C for 30 min. In addition to the drying process the films were annealed at a higher temperature, in the range 500-700 °C, depending upo n the type of substrate. W i t h the oxidised silicon substrates the films were annealed at 700 ?C for 30 min, whilst the glass and ceramicglass substrates were heated to 500 °C and 600 °C respectively for. 1, h in both cases. The heating was carried out in air. During the heat treatment the films firmly adhered to the substrates due to the presence of metaltoxane bonds at the film-substrate interface (Fig. 1). In addition, there are randomly distributed electrical defects with electrons shared with tin atoms thereby affecting the electrical conductivity of the films [ 16].
3.
Results
The resultant films were transparent, uniform in thickness across the substrates, and free from cracks. The thickness o f the films depended upon the viscosity of the solution and was in the range 6 0 - 9 0 nm. The film thickness was established from both the interference colour and from the AES data, measuring the time taken to sputter through the films. Fig. 2 ( a ) presents a scanning electron microscopy ( S E M ) image (plan view) of a SnO2 film on oxidised silicon, whilst
Fig. 2. (a) Scanning electron micrograph, and (b) electron diffraction pattern from a 60 nm SnO2 thin film on oxidised silicon, annealed at 700 °C for 30 rain.
T.M. Racheva, G. W. Critchlow / Thin Solid Films 292 (1997) 299-302
301
Table 2 A summary of sensor response data as a function of relative humidity Reladve humidity(RH) (%)
Resistance R (Mg2)
Responsetime (s) 0%toRH%
Recovery time (s) RH%to 0%
0 32.3 43 55 66 76 86 93
10000 5000 2000 500 310 220 190 180
-
_ 1 1 1 I 1 1 2
80
8 i0 12 13 I4 16 17
Composition (atomg) 80
ComposiU.on (atomY**)
70 60
•
60
5O 4O
a.
_¢
30
40
+
~
~
20
2o!
+
iO 20
40
60 Depth (rim)
BO
100
120
0 --+--Sn ~ Na -e--Si Fig. 3. An AES composition depth profile through a 75 nm S n Q film on oxidised silicon, annealed at 700 °C for 30 rain.
Fig. 2(b) illustrates the corresponding electron diffraction pattern. These techniques indicate a uniform, granular surface morphology with a very small grain size. The polycrystalline structure corresponds to the cassiterite form of S n Q . The aforementioned surface morphology and structure was typical of all the SnOa films irrespective of substrate type. AES depth profiling was used in order to determine the composition of the films, in particular, to determine the degree of sodium and carbon contamination and the Sn:O ratios. The results of AES analyses through the films on oxidised silicon and glass substrates are given in Figs. 3 and 4 respectively. Sodium levels up to ~ 2.5% were observed throughout the film on the oxidised silicon substrate. However, no carbon could be observed at depth in this instance. With the glass substrate, sodium levels reached a maximum of ~ 4 % with carbon levels of 1-2% observed throughout the film. The detection limits for carbon and sodium were estimated to be ~ 0.3 and ~ 0.4 at.% respectively. Furthermore, the AlES results indicate that these films are slightly sub-stoichiometric with Sn:O ratios in the range 1:1.8 to 1:1.7. Some calcium contamination could be observed in the films produced on both glass and ceramic-glass substrates. This was attributed to diffusion of calcium from the substrates into the films during the heat treatment. The films prepared on glass and ceramic-glass substrates with thicknesses of 60-70 nm were used for the preparation of sensor elements. For this investigation silver electrodes were attached onto the surface of these films. The distance
20
40
60
80
100
120
Depth 0~m) +
0
--S- Sn
+
C
--8--Na
-->4--Si
--¢--Ca
Fig. 4. An AES composition depth profile through a 90 nm SnO2 film on a glass substrate, annealed at 500 °C for 1 h.
between the electrodes was 1 ram. Silver lead wires were attached to the contact pads to measure the resistance (R) between the pads. These sensor devices were placed into flasks and exposed to relative humidities in the range 0-93 % RH. The flasks contained saturated water solutions of salts which maintain a known RH in the surrounding ambient [ 17]. The glass flasks were placed in a chamber which was thermostatically controlled to be at a constant temperature of 20 °C. The humidity sensitivity characteristics were measured by placing the sensor element in the flask with a dryer at 0% RH. After that, the sensors were placed in a flask at a known RH and then again in the original flask at 0% RH. The results of these experiments are presented in Table 2. The films responded to humidities in the range 32-93% RH. The response times were very short, as was the recovery time. A disadvantage, however, was the very high electrical resistivity due mainly to the limited thickness of the films and the fine granular structure. This problem could be solved by repeating the spinning and heating procedure a number of times with the same substrate. In addition, the structure could be modified by additives and subsequent annealing.
4. Conclusions Sn02 films were prepared by the sol-gel process using Sn(OCzHs)42C2HsOH as the starting material. After subsequent heating and annealing (in the range 500 °C to 700 °C
302
T.M. Raeheva, G.W. CritchIow / Thin Solid Films 292 (1997) 299-302
d e p e n d e n t upon the substrate) films were successfully obtained on oxidised silicon, glass and ceramic-glass substrates. The deposited films were converted from being amorphous to being polycrystalline with a very small grain size. The A E S analyses show the films to be sub-stoichiometric with Sn:O ratios in the range 1:1.8 to 1:1.7. Carbon could only be detected within the film on the glass substrate whilst s o d i u m was detected at levels of between ~ 2 , 5 - 4 % in films on both oxidised silicon and plain glass. The films were sensitive to humidities in the range 3 2 - 9 3 % RH.
References [ 1] W.A. Badaway, J. Electroanal. Chem., 28t (1990) 85. [2] C. Geoffroy, G. Campet et al., Active Passive Electron. Comp., 14 (t991) 111.
[3] W.A. Badaway, H.H. Afify and E.M. Elgiar, J. Elecrochem. Soc., 137 (5) (1990) 1592. [4] G. Heiland, Sensors Actuators, 2 (1982) 343. [5] D. Kohl, Se~tsorsActuators, 18 (1989) 71. [6] N. Yamazoe, Sensors Actuators B, 5 ( 1991) 7. [7] A. Mani, N. Karuppian and R. Mahalingam, Mater. Res. Bull., 25 (1990) 799. [8] W.M. Sears and M.A, Gee, Thin Solid Films, 165 (1988) 265. [9] D. Das and R. Banerjee, Thin Solid Films, 147 (1987) 321. [ 10] E. Leja, J. Korecki and K. Toll, Thin Solid Fihns, 59 (1979) 147. [ 11] S. Sakka and K. Kamiya, J. Non-Crystalline Solids, 42 (1980) 403. [12] H. Dislich, Angew. Chem., 83 (1971) 428. [ 13l S, Sakka, J. Non-Crystalline Solids, 73 (1985) 651. [ 14] S.-S. Park, H. Zheng and J.D. Mackenzie, Mater. Lett., 17 ( 1993) 376. [15] Z. Thiessen, Inorg. Chem., B195 (1931) 383. [ 16] J.C. Giuntini, W. Garanier, J.W. Zanchetta and A. Taha, J. Mater. Sci. Lett., 9 (1990) 1383. [ I7] V.V. Kliuev (ed.), Ispitatelnaia Technika, Mashinostroenie, Moscow, 1982, Chapt. 2, p. 488.