- Email: [email protected]
A study of gas-sensing properties of sputtered α-SnWO4 thin films
-sEmR$
T ELSEVIER
CHEMICAL
Sensors and Actuators B 2425 (1995) 591-595
A study of gas-sensing properties of sputtered films J.L. Solis ‘, V. Lantto Uicrocleehoncs Laboratory, Universiry of Oulu, Limmmaa, FIN-90570
wSnW04 thin
Oulu, Finland
Abstract Thin films of stannous tungstate have been grown by reactive co-sputtering in an 11% OJAr atmosphere where a tungsten target is operated in the r.f. mode and a tin target in the d.c. mode. The deposited amorphous films are annealed for 4 h at 400 or 600 “C in order to crystallize the films. XRD together with RBS, EDS and CEMS measurements are used for the characterization of the films. The films sputtered with 150 W power for both modes are found to have crystalline a-SnWO, structure after annealing at 400 “C. CEMS results show that tin is in a divalent form (St?+) in cr-SnWO, films. The electrical resistivity together with the gas-response properties to different gases of the a-SnWO, films have been measured at different temperatures between room temperature and 400 “C. In the case of response to CO and NO in air, an unusual dual-conductance behaviour is found. For comparison, measurements have also been made with a pure WO, film and with some Sn,WO, films having lower tin concentrations (x< 1). Keywordr: Gas sensors; Stannous
tungstate
1. Introduction Both SnO, and W03 are well-known materials in the semiconductor gas-sensor field and have found applications in commercial sensor devices. A synthesis of these materials in the form of the compound SnWO, was first reported in 1972 [l]. The low-temperature (a) form of stannous tungstate is stable below 670 “C. It is a diamagnetic n-type semiconductor with an orthorhombic crystal structure and a dark-red colour. Both metal atoms have distorted octahedral oxygen coordinations as in SnO, and WO,. However, in contrast to SnO,, tin appears in divalent form, Sn’+, in the aSnWO, structure. A survey of the gas-response behaviour of a relatively large group of semiconducting oxides is given by Moseley et al. [2], including proposed models for sensor operation. They also describe [2,3] an unusual dualconductance behaviour for some semiconducting oxides in which a shift from a reducing to an oxidizing behaviour of conductance is observed at some critical temperature. However, stannous tungstate is missing from the survey and we have not found any data of gas-response prop ’Permanent address: Universidad National de Ciencias, PO Box, 1301-Lima, Peru. 0925-4005/95/$09.50 0 1995 Elsevier SSDI 0925-4005(94)01425-H
Science
de Ingenieria,
Fan&ad
S.A. All rights reserved
erties of &nWO, in the literature. the characterization of our films and properties of the films, especially conductance behaviour in response dry synthetic air.
Here we present some gas-response an unusual dualto CO and NO in
2. Experimental The SnWO, tilms were grown by means of reactive co-sputtering with a Balzers BAS 450 magnetron sputtering system. A tin target was operated in the d.c. mode and a tungsten target in the r.f. mode. Both thermally oxidized Si (111) and glass were used as substrates for the fihns. Oxygen/argon gas mixtures were used to control the oxygen partial pressure. The base pressure before sputtering reached lop6 mbar with a pre-sputtering step lasting as long as 20 min. The sputter deposition was done in an argon atmosphere containing 11% oxygen. Sputtering powers of 75-150 W and of 150 W were applied to the tin and tungsten targets, respectively, in order to obtain films with different Sn/ W ratios. The samples used in the study are identified in Table 1 by the sputtering parameters used in their deposition, the SniW ratios (x in Sn,WO,) and the thickness d of the films. Three different substrate tem-
592
J.L. Solis, K Lanfto
I Senrors and Actuators
B 24-25
(1995) 591-595
10
20
Table 1 Identification of the thin-film samples with sputtering parameters (powers to W and Sn targets and substrate temperatures) used in the deposition and with the Sri/// ratio and thickness d of the films Sample
W/r.f. (W)
Sn/d.c. (W)
T (‘C)
Sri/// ratio
d (nm)
SnWl SnW2 SnW3 SnW4 SnW5 SnW6’ Wl
150 150 150 150 150 1.50 150
150 150 130 100 75 100
29 200 29 100 29 29 29
1.12 0.65 0.48 0.37 0.27 0.03 0.00
135 170 170 150 130 110 135
’Deposition with a mask.
peratures (room temperature, 100 “C and 200 “C) were chosen during the deposition. The thickness and composition of the tilms were obtained from RBS (Rutherford backscattering spectroscopy) and EDS (energydispersive spectroscopy of X-rays) results, respectively. The films were annealed at 400 or 600 “C in air for 4 h in a conventional temperature-controlled electric furnace. The two-point method was used in the electricalconductivity measurements and the wire contacts to the films were made with a low-temperature gold paste. A computer-controlled measuring system employing the flow-through principle (gas flow through the measuring chamber 1 1 min-‘) was used for varying the gas concentration, substrate temperature and for data acquisition, handling and storage [4]. A voltage of 1 V was used in the conductance measurements.
3. Characterization
30
40
50
60
70
DiffractIon angle 2 0 (deg) Fig. 1. XRD pattern measured with Cu Ka radiation from an SnWl (&nWO,) thin film, deposited on an oxidized Si(ll1) substrate. Different diffraction peaks from the orthorhombic a-SnWO, phase are indicated together with a peak from the minor SnO, phase. The inset shows the top of the major peak of two components at 28.4” ((121) of &nWOJ and at 28.6” ((111) of Si).
of films
X-ray diffraction (XRD) patterns were obtained with Cu Kcu radiation in a Siemens D5000 diffractometer. It was found that deposited films (as-grown) were amorphous, but annealing at 400 “C crystallized the films. Films deposited at room temperature with a sputtering power of 150 W to both targets (SnWl in Table 1) were found, after annealing at 400 “C, to contain polycrystalline a-SnWO, in an orthorhombic structure as the major phase and a small amount of SnO,, as is shown in Fig. 1. This was also confirmed from SEM and AFM (atomic force microscopy) images. The sample SnW6 with a very small amount of Sn (the tin target was covered during deposition with a metal mask having only small holes for sputtered tin atoms to reach the substrate) was very similar to pure WO,. A decomposition of a-SnWO, into SnO, and WO, phases was found after annealing the films at 600 “C in air. No Mijssbauer parameter data were found in the literature regarding cu-SnWO, (only tin-tungsten bronzes were studied [5]). Conversion electron Miiss-
-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6
Velocity
(mm/s)
Fig. 2. CEMS spectra of as-grown and annealed (at 4GG and 600 “C) SnWl (rr-SnWO,) samples. In the a-SnWO< structure tin is in the form of Sn2+.
bauer spectroscopy (CEMS) experiments were performed on samples mounted in an acetone-vapour proportional counter attached to a conventional constant-acceleration spectrometer. *lgSnm in CaSnO, was used as the source. The isomer shifts 6 are quoted with respect to this source. Typical CEMS spectra of as-grown and annealed (400 and 600 “C) SnWl (aSnWO,) samples are shown in Fig. 2. All other samples, except SnW6 and pure WOg, showed similar spectra. The Mijssbauer parameters 6 and AE, (the quadrupole splitting) obtained from Fig. 2 are given in Table 2. The spectra of as-grown samples, based on the values of the quadrupole splitting, were in the region char-
J.L. Solis, v. Lmtto,
Table 2 Miissbauer parameters 8 and A& obtained by fitting the CEMS spectra in Fig. 2 with a least-square fitting procedure SnWl films
s (mm s-1)
AEo (mm s-1)
Phases
As-grmvn
- 0.05
3.05
0.50 1.28
SnOz a-snwo,
Annealed at 400 “C
- 0.02 3.32
0.52 1.24
SrlOZ a-SnWO,
Annealed at 600 “C
- 0.04
0.57
SnOZ
0
150
300
450
600
593
I Semors and Ach~otars B 24-25 (1995) 591-595
750
Energy (KeV) Fig. 3. RBS spectra measured with 0.76 MeV ‘He*+ ions from asgrown and annealed (400 “C) SnWl (a-SnWO,) samples.
acteristic of compounds having tin in the divalent Sn2+ form. For as-grown films, 6 was in the range 2.92-3.33 mm s-l with AEo between 1.24 and 1.47 mm s-‘, which corresponds to the divalent Sn2+ form. A very small doublet corresponding to the SnO, phase was also observed. After annealing at 400 “C, the CEMS spectrum revealed the two phases SnO, and c&nWO,, but after annealing at 600 “C only the doublet of the SnO, phase was found by comparing the parameter values with data reported by Soares et al. [6]. XRD and CEMS together made possible a very satisfactory identification of the phases in the films. In addition, Raman spectra up to 1900 cm-’ were also measured. RBS experiments were made with 0.76 MeV 4He2+ ions using a van de Graaff accelerator. Fig. 3 shows measured RBS spectra from both as-grown and annealed (400 “C) SnWl (cx-SnW04) samples. The spectrum from the annealed sample reveals a diffusion of tungsten into the substrate. RBS spectra from SnWl and other samples support the values obtained from EDS measurements for the Sri/W ratio in the films.
4. Gas response of films The band gap of a-SnWO, was found to be in the UV region by examination of the photownductivity. The peak absorption in the blue (origin of the dark-
red wlour) had no effect on conductivity. A localized absorption may involve an electron transfer from Sn2’ to W6+, octahedrally coordinated by oxygen. An increase of Sn content in the films increased their conductivity at room temperature and, simultaneously, decreased the temperature coefficient of conductivity. Only a small increase of conductance with increasing temperature was found for a-SnWO, films in dry air and nitrogen (less than 2 ppm 0,). The increase corresponds to an average activation energy of 0.18 eV up to 400 “C. The conductance response of the films to different gases like CO, NO, S02, H,S, H2 and CH, was measured at different temperatures between room temperature and 400 “C using dry synthetic air as a carrier gas. Only a small response of the flhns was found at different concentrations of S02, H2 and CI%, at different temperatures below 400 “C. Here we restrict the discussion only to CO and NO responses. Moseley et al. [2,3] describe an unusual dual response of conductance for some serniconducting oxides (they call it a transition between n-type and p-type behaviour). The conductance response of the films SnWl, SnW2 and SnW3 (Table 1) showed a dual behaviour in the case of exposure, for instance, to 250 ppm of CO in air at different temperatures. As is shown in Fig. 4, the border temperature T* for the dual behaviour is between 200 and 250 “C!. A conductance decrease is shown in Fig. 4 after exposure to 250 ppm of CO at 150 and 200 “C, while at 250 “C and higher temperatures a conductance increase follows the CO exposure. The amount of tin in the film seems to be an important factor for this unusual dual-conductance response to CO. The response of the films was also measured at 200 and 300 “C to different concentrations of CO between 10 and 350 ppm. The results for an SnWl sample are shown in Fig. 5. At 300 “C, the conductance of all the SnWl, SnW2 and SnW3 samples increased with increasing CO concentration, but at 200 “C, the 1.50
-
coon,------
Time
(min)
Fig. 4. Conductance response G/G, of an annealed (400 “C) SnWl (u-SnWO,) sample to 250 ppm of CO in dry synthetic air at temperatures of 150, 200, 250, 300, 350 and 400 “C. Go is the initial conductance at zero CO concentration.
594
J.L. Solk, K Lantto / Sensors and Actwtom
B 24-25
(1995) 591-595
6.2
SnWl si A
6.7
8
6.2
300%
iif 5 , 4.7 u s ”
8.5
4.2
f”b71 Tz300”C
5;
7.5
A v
6.5
8
_
s
0.35-
‘3 s
030
uz
0.25 0.20
4.6 (b)
0.15 a
5
10
16
20
25
30
36
40
45
Time (min) Fig. 5. Conductance response of an annealed (400 “C) SnWl (aSnWO,) sample to diierent concentrations of CO in dry synthetic air (a) at 200 T and (b) at 300 “C.
conductance of the three films increased on exposure to 10 ppm of CO, while at higher concentrations above 50 ppm, the conductance decreased with increasing CO concentration. It was found that the dual-conductance behaviour depends on temperature, CO concentration in air and the tin content in the film. The conductance response to NO of all the films containing tin also showed a dual behaviour with respect to NO concentration and temperature. Only the pure WO, film did not show the dual behaviour. At 10 ppm of NO in air, the conductance of the films decreased at temperatures below 250 “C and increased above 300 “C. At 250 “C, a conductance increase of the films was observed at low NO concentrations below 5 ppm, while at higher concentrations the conductance decreased. Fig. 6 shows a similar behaviour at 300 “C with only a small shift in the inversion concentration.
5. Discussion The gas-response properties of Sn,WO, thin films are very different from those of SnOz and WO, films. In the &nWO,, structure, tin is in divalent form, which may enable an electron transfer between the Sn2+ lattice ions at the surface and surface adsorbates. Decomposition of cr-SnWO, into SnO, and WO, phases restricts the operation temperature of the films to < 400 “C.
0.10 0.05 0
40
80
120 160 200 240 280 320 360
Time (min) Fig. 6. Conductance response of an annealed (400 “C) SnWl (aSnWO,) sample and of a Wl (pure WO,) sample to different concentrations of NO (I-70 ppm) in synthetic air at 300 “C.
The dual-conductance behaviour in the response to CO was found only with the SnWl, SnW2 and SnW3 films in Table 1. The film SnWl had cr-SnWO, as the major phase and only a small amount of SnO, phase (Fig. 1). The tin-deficient films (SnnV ratio < 1) SnW2 and SnW3 were mixtures of two phases, &nWO, and WO,, but in all three films SnWl, SnW2 and SnW3, c&nWO, was found from SEM and AFM images to be the matrix phase. The dual-conductance behaviour was strongest for the SnWl film and was much weaker for the SnW2 and SnW3 films. In the case of exposure to 250 ppm of CO in synthetic air, for instance, the border temperature T* was about the same, 230 “C, for both SnWl and SnW3 films. The conductivity of the SnWl, SnW2 and SnW3 films was much higher (up to three orders of magnitude at low temperatures) than that of the pure WO, (Wl) film in the whole temperature range from room temperature up to 400 “C. A reducing behaviour of CO was found for WO, films (and also for pure SnO, films) in this whole temperature range. On the basis of these findings, we conclude that the unusual dual-conductance behaviour of the films is a property of only the &nWO_, phase.
J.L. Solis, V. Lantfo, I Sensors and Actuators B 24-25 (1995) 591-595
The role of the adsorptionldesorption mechanism and that of the surface-defect mechanism in creation of the transducing conductance signal in semiconductor gas sensors is, to some extent, controversial. The creation of oxygen surface vacancies in oxide semiconductors is of central importance in the surface-defect mechanism. A dual-conductance behaviour in the response to CO was found also with some CdS thin films in Ref. [7]. In the case of conductance response to CO or to changes in oxygen partial pressure, oxide materials like a-SnWO, may reflect changes in the amount of oxygen vacancies (donors) in addition to changes in the amount of adsorbed oxygen, while the first changes are not possible in the case of CdS films. The appearance of the dual-conductance behaviour in both oxide and sulphide semiconductors may rule out changes in the amount of donors as the origin for the behaviour. Then the origin may be at the film surfaces. A possibility for the oxidizing behaviour of CO, for instance, is the trapping of electrons to its antibonding 2~* orbitals. Changes in polar surface layers of ionic semiconductors and corresponding changes in the electron affinity of the semiconductor may serve as another explanation for the dual-conductance behaviour. Acknowledgements The authors wish to thank Mr M.R. Soares, Dr W.H. Schreiner and Mr J. Frantti for their advice in the
595
preparation of samples and in EDS and SEM experiments. .I. Solis expresses his gratitude to International Science Programs at Uppsala University, Sweden for the financial support.
References 111 W. Jeitschko andA.W. Sleight, aStannousTungstate: 14
131 [41
PI
161
[71
properties, crystal structure and relationship to ferroelectric SbTaO, type compounds, Acar Ctysf., 828 (1972) 317-l-2094. P.T. Moseley, A.M. Stoneham and D.E. Williams, Oxide semiconductors: patterns of gas response behaviour according to material type, in P.T. Moselay, J.O.W. Norris and DE. Wiiiams (eds.), Techniques and A4echanim.s in Gas Sensing Adam Hilger, Bristol, 1991, pp. 108-138. P.T. Moseley, New trends and future prospects of thick- and thin-film gas sensors, Seruors and Actuators B, 3 (1991) 16’7-174. P. Romppainen and V. Lantto, Design and construction of an experimental set-up for semiconductor gas sensor studies.Repotf S 93, Department of Electrical Engineering, University of Oulu, Oulu, Finland, 1987, 22 pp. I J. M&olm, R. Steadman and A. Howe, Preparation, structure, and Mossbauer spectra of tin tungsten bronzes, J. Sorid Sarre Chm., 2 (1970) 555-562. M.R. Soares, P.H. Dionisio, I.J.R. Baumvoland W.H. Schreiner, Influence of sputtering parameters on the composition and crystailinity of tin oxide, Thin Solid Films, 214 (1992) 6-16. V. Golovanov, V. Serdiouk, L. Stys, G. Thchemeresiouk and A. Shmilevitcb, Mechanism of oaygen chemisorption on the CdS polycrystalline films, .I. ukmin Phys., 33 (1988) 157-162 (in Russian).
T ELSEVIER
CHEMICAL
Sensors and Actuators B 2425 (1995) 591-595
A study of gas-sensing properties of sputtered films J.L. Solis ‘, V. Lantto Uicrocleehoncs Laboratory, Universiry of Oulu, Limmmaa, FIN-90570
wSnW04 thin
Oulu, Finland
Abstract Thin films of stannous tungstate have been grown by reactive co-sputtering in an 11% OJAr atmosphere where a tungsten target is operated in the r.f. mode and a tin target in the d.c. mode. The deposited amorphous films are annealed for 4 h at 400 or 600 “C in order to crystallize the films. XRD together with RBS, EDS and CEMS measurements are used for the characterization of the films. The films sputtered with 150 W power for both modes are found to have crystalline a-SnWO, structure after annealing at 400 “C. CEMS results show that tin is in a divalent form (St?+) in cr-SnWO, films. The electrical resistivity together with the gas-response properties to different gases of the a-SnWO, films have been measured at different temperatures between room temperature and 400 “C. In the case of response to CO and NO in air, an unusual dual-conductance behaviour is found. For comparison, measurements have also been made with a pure WO, film and with some Sn,WO, films having lower tin concentrations (x< 1). Keywordr: Gas sensors; Stannous
tungstate
1. Introduction Both SnO, and W03 are well-known materials in the semiconductor gas-sensor field and have found applications in commercial sensor devices. A synthesis of these materials in the form of the compound SnWO, was first reported in 1972 [l]. The low-temperature (a) form of stannous tungstate is stable below 670 “C. It is a diamagnetic n-type semiconductor with an orthorhombic crystal structure and a dark-red colour. Both metal atoms have distorted octahedral oxygen coordinations as in SnO, and WO,. However, in contrast to SnO,, tin appears in divalent form, Sn’+, in the aSnWO, structure. A survey of the gas-response behaviour of a relatively large group of semiconducting oxides is given by Moseley et al. [2], including proposed models for sensor operation. They also describe [2,3] an unusual dualconductance behaviour for some semiconducting oxides in which a shift from a reducing to an oxidizing behaviour of conductance is observed at some critical temperature. However, stannous tungstate is missing from the survey and we have not found any data of gas-response prop ’Permanent address: Universidad National de Ciencias, PO Box, 1301-Lima, Peru. 0925-4005/95/$09.50 0 1995 Elsevier SSDI 0925-4005(94)01425-H
Science
de Ingenieria,
Fan&ad
S.A. All rights reserved
erties of &nWO, in the literature. the characterization of our films and properties of the films, especially conductance behaviour in response dry synthetic air.
Here we present some gas-response an unusual dualto CO and NO in
2. Experimental The SnWO, tilms were grown by means of reactive co-sputtering with a Balzers BAS 450 magnetron sputtering system. A tin target was operated in the d.c. mode and a tungsten target in the r.f. mode. Both thermally oxidized Si (111) and glass were used as substrates for the fihns. Oxygen/argon gas mixtures were used to control the oxygen partial pressure. The base pressure before sputtering reached lop6 mbar with a pre-sputtering step lasting as long as 20 min. The sputter deposition was done in an argon atmosphere containing 11% oxygen. Sputtering powers of 75-150 W and of 150 W were applied to the tin and tungsten targets, respectively, in order to obtain films with different Sn/ W ratios. The samples used in the study are identified in Table 1 by the sputtering parameters used in their deposition, the SniW ratios (x in Sn,WO,) and the thickness d of the films. Three different substrate tem-
592
J.L. Solis, K Lanfto
I Senrors and Actuators
B 24-25
(1995) 591-595
10
20
Table 1 Identification of the thin-film samples with sputtering parameters (powers to W and Sn targets and substrate temperatures) used in the deposition and with the Sri/// ratio and thickness d of the films Sample
W/r.f. (W)
Sn/d.c. (W)
T (‘C)
Sri/// ratio
d (nm)
SnWl SnW2 SnW3 SnW4 SnW5 SnW6’ Wl
150 150 150 150 150 1.50 150
150 150 130 100 75 100
29 200 29 100 29 29 29
1.12 0.65 0.48 0.37 0.27 0.03 0.00
135 170 170 150 130 110 135
’Deposition with a mask.
peratures (room temperature, 100 “C and 200 “C) were chosen during the deposition. The thickness and composition of the tilms were obtained from RBS (Rutherford backscattering spectroscopy) and EDS (energydispersive spectroscopy of X-rays) results, respectively. The films were annealed at 400 or 600 “C in air for 4 h in a conventional temperature-controlled electric furnace. The two-point method was used in the electricalconductivity measurements and the wire contacts to the films were made with a low-temperature gold paste. A computer-controlled measuring system employing the flow-through principle (gas flow through the measuring chamber 1 1 min-‘) was used for varying the gas concentration, substrate temperature and for data acquisition, handling and storage [4]. A voltage of 1 V was used in the conductance measurements.
3. Characterization
30
40
50
60
70
DiffractIon angle 2 0 (deg) Fig. 1. XRD pattern measured with Cu Ka radiation from an SnWl (&nWO,) thin film, deposited on an oxidized Si(ll1) substrate. Different diffraction peaks from the orthorhombic a-SnWO, phase are indicated together with a peak from the minor SnO, phase. The inset shows the top of the major peak of two components at 28.4” ((121) of &nWOJ and at 28.6” ((111) of Si).
of films
X-ray diffraction (XRD) patterns were obtained with Cu Kcu radiation in a Siemens D5000 diffractometer. It was found that deposited films (as-grown) were amorphous, but annealing at 400 “C crystallized the films. Films deposited at room temperature with a sputtering power of 150 W to both targets (SnWl in Table 1) were found, after annealing at 400 “C, to contain polycrystalline a-SnWO, in an orthorhombic structure as the major phase and a small amount of SnO,, as is shown in Fig. 1. This was also confirmed from SEM and AFM (atomic force microscopy) images. The sample SnW6 with a very small amount of Sn (the tin target was covered during deposition with a metal mask having only small holes for sputtered tin atoms to reach the substrate) was very similar to pure WO,. A decomposition of a-SnWO, into SnO, and WO, phases was found after annealing the films at 600 “C in air. No Mijssbauer parameter data were found in the literature regarding cu-SnWO, (only tin-tungsten bronzes were studied [5]). Conversion electron Miiss-
-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6
Velocity
(mm/s)
Fig. 2. CEMS spectra of as-grown and annealed (at 4GG and 600 “C) SnWl (rr-SnWO,) samples. In the a-SnWO< structure tin is in the form of Sn2+.
bauer spectroscopy (CEMS) experiments were performed on samples mounted in an acetone-vapour proportional counter attached to a conventional constant-acceleration spectrometer. *lgSnm in CaSnO, was used as the source. The isomer shifts 6 are quoted with respect to this source. Typical CEMS spectra of as-grown and annealed (400 and 600 “C) SnWl (aSnWO,) samples are shown in Fig. 2. All other samples, except SnW6 and pure WOg, showed similar spectra. The Mijssbauer parameters 6 and AE, (the quadrupole splitting) obtained from Fig. 2 are given in Table 2. The spectra of as-grown samples, based on the values of the quadrupole splitting, were in the region char-
J.L. Solis, v. Lmtto,
Table 2 Miissbauer parameters 8 and A& obtained by fitting the CEMS spectra in Fig. 2 with a least-square fitting procedure SnWl films
s (mm s-1)
AEo (mm s-1)
Phases
As-grmvn
- 0.05
3.05
0.50 1.28
SnOz a-snwo,
Annealed at 400 “C
- 0.02 3.32
0.52 1.24
SrlOZ a-SnWO,
Annealed at 600 “C
- 0.04
0.57
SnOZ
0
150
300
450
600
593
I Semors and Ach~otars B 24-25 (1995) 591-595
750
Energy (KeV) Fig. 3. RBS spectra measured with 0.76 MeV ‘He*+ ions from asgrown and annealed (400 “C) SnWl (a-SnWO,) samples.
acteristic of compounds having tin in the divalent Sn2+ form. For as-grown films, 6 was in the range 2.92-3.33 mm s-l with AEo between 1.24 and 1.47 mm s-‘, which corresponds to the divalent Sn2+ form. A very small doublet corresponding to the SnO, phase was also observed. After annealing at 400 “C, the CEMS spectrum revealed the two phases SnO, and c&nWO,, but after annealing at 600 “C only the doublet of the SnO, phase was found by comparing the parameter values with data reported by Soares et al. [6]. XRD and CEMS together made possible a very satisfactory identification of the phases in the films. In addition, Raman spectra up to 1900 cm-’ were also measured. RBS experiments were made with 0.76 MeV 4He2+ ions using a van de Graaff accelerator. Fig. 3 shows measured RBS spectra from both as-grown and annealed (400 “C) SnWl (cx-SnW04) samples. The spectrum from the annealed sample reveals a diffusion of tungsten into the substrate. RBS spectra from SnWl and other samples support the values obtained from EDS measurements for the Sri/W ratio in the films.
4. Gas response of films The band gap of a-SnWO, was found to be in the UV region by examination of the photownductivity. The peak absorption in the blue (origin of the dark-
red wlour) had no effect on conductivity. A localized absorption may involve an electron transfer from Sn2’ to W6+, octahedrally coordinated by oxygen. An increase of Sn content in the films increased their conductivity at room temperature and, simultaneously, decreased the temperature coefficient of conductivity. Only a small increase of conductance with increasing temperature was found for a-SnWO, films in dry air and nitrogen (less than 2 ppm 0,). The increase corresponds to an average activation energy of 0.18 eV up to 400 “C. The conductance response of the films to different gases like CO, NO, S02, H,S, H2 and CH, was measured at different temperatures between room temperature and 400 “C using dry synthetic air as a carrier gas. Only a small response of the flhns was found at different concentrations of S02, H2 and CI%, at different temperatures below 400 “C. Here we restrict the discussion only to CO and NO responses. Moseley et al. [2,3] describe an unusual dual response of conductance for some serniconducting oxides (they call it a transition between n-type and p-type behaviour). The conductance response of the films SnWl, SnW2 and SnW3 (Table 1) showed a dual behaviour in the case of exposure, for instance, to 250 ppm of CO in air at different temperatures. As is shown in Fig. 4, the border temperature T* for the dual behaviour is between 200 and 250 “C!. A conductance decrease is shown in Fig. 4 after exposure to 250 ppm of CO at 150 and 200 “C, while at 250 “C and higher temperatures a conductance increase follows the CO exposure. The amount of tin in the film seems to be an important factor for this unusual dual-conductance response to CO. The response of the films was also measured at 200 and 300 “C to different concentrations of CO between 10 and 350 ppm. The results for an SnWl sample are shown in Fig. 5. At 300 “C, the conductance of all the SnWl, SnW2 and SnW3 samples increased with increasing CO concentration, but at 200 “C, the 1.50
-
coon,------
Time
(min)
Fig. 4. Conductance response G/G, of an annealed (400 “C) SnWl (u-SnWO,) sample to 250 ppm of CO in dry synthetic air at temperatures of 150, 200, 250, 300, 350 and 400 “C. Go is the initial conductance at zero CO concentration.
594
J.L. Solk, K Lantto / Sensors and Actwtom
B 24-25
(1995) 591-595
6.2
SnWl si A
6.7
8
6.2
300%
iif 5 , 4.7 u s ”
8.5
4.2
f”b71 Tz300”C
5;
7.5
A v
6.5
8
_
s
0.35-
‘3 s
030
uz
0.25 0.20
4.6 (b)
0.15 a
5
10
16
20
25
30
36
40
45
Time (min) Fig. 5. Conductance response of an annealed (400 “C) SnWl (aSnWO,) sample to diierent concentrations of CO in dry synthetic air (a) at 200 T and (b) at 300 “C.
conductance of the three films increased on exposure to 10 ppm of CO, while at higher concentrations above 50 ppm, the conductance decreased with increasing CO concentration. It was found that the dual-conductance behaviour depends on temperature, CO concentration in air and the tin content in the film. The conductance response to NO of all the films containing tin also showed a dual behaviour with respect to NO concentration and temperature. Only the pure WO, film did not show the dual behaviour. At 10 ppm of NO in air, the conductance of the films decreased at temperatures below 250 “C and increased above 300 “C. At 250 “C, a conductance increase of the films was observed at low NO concentrations below 5 ppm, while at higher concentrations the conductance decreased. Fig. 6 shows a similar behaviour at 300 “C with only a small shift in the inversion concentration.
5. Discussion The gas-response properties of Sn,WO, thin films are very different from those of SnOz and WO, films. In the &nWO,, structure, tin is in divalent form, which may enable an electron transfer between the Sn2+ lattice ions at the surface and surface adsorbates. Decomposition of cr-SnWO, into SnO, and WO, phases restricts the operation temperature of the films to < 400 “C.
0.10 0.05 0
40
80
120 160 200 240 280 320 360
Time (min) Fig. 6. Conductance response of an annealed (400 “C) SnWl (aSnWO,) sample and of a Wl (pure WO,) sample to different concentrations of NO (I-70 ppm) in synthetic air at 300 “C.
The dual-conductance behaviour in the response to CO was found only with the SnWl, SnW2 and SnW3 films in Table 1. The film SnWl had cr-SnWO, as the major phase and only a small amount of SnO, phase (Fig. 1). The tin-deficient films (SnnV ratio < 1) SnW2 and SnW3 were mixtures of two phases, &nWO, and WO,, but in all three films SnWl, SnW2 and SnW3, c&nWO, was found from SEM and AFM images to be the matrix phase. The dual-conductance behaviour was strongest for the SnWl film and was much weaker for the SnW2 and SnW3 films. In the case of exposure to 250 ppm of CO in synthetic air, for instance, the border temperature T* was about the same, 230 “C, for both SnWl and SnW3 films. The conductivity of the SnWl, SnW2 and SnW3 films was much higher (up to three orders of magnitude at low temperatures) than that of the pure WO, (Wl) film in the whole temperature range from room temperature up to 400 “C. A reducing behaviour of CO was found for WO, films (and also for pure SnO, films) in this whole temperature range. On the basis of these findings, we conclude that the unusual dual-conductance behaviour of the films is a property of only the &nWO_, phase.
J.L. Solis, V. Lantfo, I Sensors and Actuators B 24-25 (1995) 591-595
The role of the adsorptionldesorption mechanism and that of the surface-defect mechanism in creation of the transducing conductance signal in semiconductor gas sensors is, to some extent, controversial. The creation of oxygen surface vacancies in oxide semiconductors is of central importance in the surface-defect mechanism. A dual-conductance behaviour in the response to CO was found also with some CdS thin films in Ref. [7]. In the case of conductance response to CO or to changes in oxygen partial pressure, oxide materials like a-SnWO, may reflect changes in the amount of oxygen vacancies (donors) in addition to changes in the amount of adsorbed oxygen, while the first changes are not possible in the case of CdS films. The appearance of the dual-conductance behaviour in both oxide and sulphide semiconductors may rule out changes in the amount of donors as the origin for the behaviour. Then the origin may be at the film surfaces. A possibility for the oxidizing behaviour of CO, for instance, is the trapping of electrons to its antibonding 2~* orbitals. Changes in polar surface layers of ionic semiconductors and corresponding changes in the electron affinity of the semiconductor may serve as another explanation for the dual-conductance behaviour. Acknowledgements The authors wish to thank Mr M.R. Soares, Dr W.H. Schreiner and Mr J. Frantti for their advice in the
595
preparation of samples and in EDS and SEM experiments. .I. Solis expresses his gratitude to International Science Programs at Uppsala University, Sweden for the financial support.
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properties, crystal structure and relationship to ferroelectric SbTaO, type compounds, Acar Ctysf., 828 (1972) 317-l-2094. P.T. Moseley, A.M. Stoneham and D.E. Williams, Oxide semiconductors: patterns of gas response behaviour according to material type, in P.T. Moselay, J.O.W. Norris and DE. Wiiiams (eds.), Techniques and A4echanim.s in Gas Sensing Adam Hilger, Bristol, 1991, pp. 108-138. P.T. Moseley, New trends and future prospects of thick- and thin-film gas sensors, Seruors and Actuators B, 3 (1991) 16’7-174. P. Romppainen and V. Lantto, Design and construction of an experimental set-up for semiconductor gas sensor studies.Repotf S 93, Department of Electrical Engineering, University of Oulu, Oulu, Finland, 1987, 22 pp. I J. M&olm, R. Steadman and A. Howe, Preparation, structure, and Mossbauer spectra of tin tungsten bronzes, J. Sorid Sarre Chm., 2 (1970) 555-562. M.R. Soares, P.H. Dionisio, I.J.R. Baumvoland W.H. Schreiner, Influence of sputtering parameters on the composition and crystailinity of tin oxide, Thin Solid Films, 214 (1992) 6-16. V. Golovanov, V. Serdiouk, L. Stys, G. Thchemeresiouk and A. Shmilevitcb, Mechanism of oaygen chemisorption on the CdS polycrystalline films, .I. ukmin Phys., 33 (1988) 157-162 (in Russian).