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Sensing characteristics of NiO thin films as NO2 gas sensor
Thin Solid Films 418 (2002) 9–15
Sensing characteristics of NiO thin films as NO2 gas sensor I. Hotovya,*, V. Rehaceka, P. Sicilianob, S. Caponec, L. Spiessd a Department of Microelectronics, Slovak University of Technology, Ilkovicova 3, 812 19 Bratislava, Slovakia Instituto per lo Studio di Nuovi Materiali per l’Elettronica (I.M.E.-C.N.R.), Via Arnesano, 73100 Lecce, Italy c Dipartimento di Ingegneria dell’Innovazione, University of Lecce, Via per Arnesano, 73100 Lecce, Italy d Department of Materials Technology, Technical University of Ilmenau, PF 100565, D-98684 Ilmenau, Germany b
Abstract In this paper we present the results concerning the characterisation of nickel oxide thin films deposited by d.c. reactive magnetron sputtering. Different NiO thin films have been prepared by changing some deposition parameters, as the oxygen content in the reactive plasma and the sputtering mode (metal- or oxide-sputtering mode). The structure and surface morphology of the samples have been analysed by XRD and by atomic force microscopy and scanning electron microscope, respectively. The electrical responses of the NiO films towards NO2 have been also considered. NiO thin films showed good responses to low NO2 concentrations (1–10 ppm) with a maximum at 160 8C operating temperature. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: NiO; Thin film; Gas sensor; NO2
1. Introduction Nitrogen oxide is one of the most harmful gases to the human body and also a cause of air pollution. The major sources of nitrogen oxide are the combustion of fossil fuel and the exhaust of motor engines. Therefore, the development of new sensors for the detection of NO2 in the ambient is an important aspect of the general interest to improve the quality of our environment, to avoid damage to our health and quality of life and to avoid long-term irreversible changes to the composition of the earth’s atmosphere. There have been a lot of efforts to develop many types of nitrogen oxide gas sensors. Among them, the most interesting are the metal oxide-based gas sensors (MOS) that mainly use WO3 w1,2x, V2O5 w3x, TiO2 and SnO2 thin films as sensing layers w12x. A promising approach in this field is both to use novel materials based on semiconducting metal oxides, and to exploit the advantages of microelectronic and micro-mechanical technologies for the fabrication and production of a system compatible with current electron*Corresponding author. Tel.: q421-2-60291594; fax: q421-265423480. E-mail address: [email protected] (I. Hotovy).
ic information systems. Moreover, the surface properties and the deposition conditions of these materials play an important role in the properties of MOS-based gas sensors. Hence, a considerable interest is normally devoted to the preparation and characterisation of the sensing films. Nickel oxide, that is usually taken as model for ptype, is an attractive material well known for its chemical stability as well as for its excellent optical and electrical properties. Indeed, NiO thin films have been studied for applications in electrochromic devices w4,5x and also as functional sensing layers for MOS gas sensors w6x. NiO films can be fabricated by different physical and chemical vapour deposition techniques, such as reactive sputtering and plasma-enhanced chemical vapour deposition. The preparation method and the deposition mode are fundamental in determining the properties of MOS thin films, but the effective dependence of the process parameters on the film properties is not well defined. Nevertheless, it is evident that the improvement of the material properties and performance as gas sensor can be reached by the optimisation of the preparation conditions. Our research has been focused on the preparation and characterisation of NiO thin films deposited by reactive
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 5 7 9 - 5
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I. Hotovy et al. / Thin Solid Films 418 (2002) 9–15
magnetron sputtering. The NiO films have been diversified by changing some deposition parameters. In particular the NiO films were deposited in argon–oxygen mixtures with a relative O2 content varying from 20 to 60% in two operation modes, i.e. metal- and oxidesputtering mode. The samples have been characterised structurally by XRD and morphologically by atomic force microscopy (AFM) and scanning electron microscope (SEM). Tiny gas sensors based on the as-prepared NiO thin films deposited on alumina substrates were thus fabricated. The NO2 sensing properties of all the samples were also investigated in order to evaluate the sensors with the better response towards NO2 and to optimise the experimental conditions of the deposition process. The sensing tests toward CO were also carried out and the related results were presented in details in a previous work w8x. 2. Experiment The NiO films were deposited by d.c. reactive magnetron sputtering from a Ni target in a mixture of oxygen and argon. Two types of substrates were used: Si and alumina substrates. A sputtering power of 600 W was used. Both argon inert and oxygen reactive flow were controlled by mass flow controllers. The relative partial pressure, defined as jsp(O2)yp(O2 qAr), varied from 20 to 60%. The total gas pressure was kept at 0.5 Pa. The preparation of NiO films was realised in two operation modes: metal- and oxide-sputtering mode. Details of these sputtering deposition modes were given elsewhere w7x. The sputtering conditions are listed in Table 1. The thickness of the NiO films under the above mentioned deposition conditions ranged from 100 to 150 nm and was measured by a Talystep. Films for morphological and electrical measurements were deposited on alumina substrates (3=3 mm2) equipped interdigitated Pt electrodes with previously deposited. In order to stabilise the electrical properties of NiO thin films they have been annealed at 500 8C in air for 2 h. The crystal structure was identified by a Theta–Theta ¨ Diffractometer D 5000 with Gobelmirror into BraggBrentano focusing with copper radiation. Surface mor-
phology was observed by a Topometrix Discover TM 2000 AFM. In particular in the AFM observations a 70 mm linearized scanner with a minimal z-resolution of 1.3 nm was used. Moreover, the lateral material distribution was observed by a LEO 1420 SEM with an energy dispersive X-ray (EDX) analyser based on a germanium detector (Oxford Instruments) and operating at 12 kV acceleration voltage. Different lateral parts of the sensor structure were thus analysed in order to check the lateral uniformity of the elemental distribution. The sensor structures were mounted as suspended devices onto standard TO-8 packages and introduced into a test chamber in Teflon for the sensing tests in controlled ambient. A gas flow controller (MKS mod. 647B) connected to mass flow meters (MFCs) and to certified gas bottles allows to dilute the target gas in dry air at different concentrations. The total gas flow rate was kept constant at 100 sccm during the measurements. The test chamber, entirely programmed and realised at the Gas Sensor Laboratory of I.M.E.-C.N.R. in Lecce (Italy), can host up to eight sensors. During operation the sensors were heated by applying a voltage across the Pt heating meander, whose resistivity value is linked to the sensor’s operating temperature. The sensor working temperature was so controlled and varied in the range from 25 to 400 8C. A d.c. voltage of 1 V was applied across the sensing films and the electrical current was measured by using a Keithley (mod. 6517A) electrometer equipped with a multiplexer. A LabVIEW software drives via a PC and a GPIB interface all the operations of the gas-mixing station. First the NO2 sensing tests at different working temperatures and at different NO2 concentrations were carried out by using dry air as reference and carrier gas. Next the same protocol of measurements were repeated in humid air with a relative humidity content of 50% (R.H.s50%). The humidification of the mixtures NO2-dry air was obtained by the saturation method with a glass bubbler; dry air, left to flow through a bubble of water for collecting the saturated water vapours, is be mixed to dry air in fixed ratio just to get the desired level of humidity.
Table 1 Sample preparation conditions Sample
S1
S2
S3
S4
S5
S6
Sputtering mode Pumping speed (lys) Oxygen content in working gas (%) Oxygen partial pressure (Pa) Total pressure (Pa) Average target voltage (V)
Oxide 275 30 0.15 0.5 299
Oxide 275 40 0.2 0.5 308
Metal 275 20 0.1 0.5 336
Metal 136 40 0.18 0.5 374
Oxide 136 45 0.22 0.5 301
Oxide 136 60 0.3 0.5 306
I. Hotovy et al. / Thin Solid Films 418 (2002) 9–15
Fig. 1. XRD diffraction patterns for the NiO thin films prepared in metal-sputtering mode (S3, S4) and oxide-sputtering mode (S1, S2, S5, S6).
3. Results and discussion 3.1. Thin film characterisation Fig. 1a presents the diffraction patterns of all the asprepared NiO samples deposited at different oxygen contents in the ArqO2 plasma (i.e. 20 and 40% of O2) in the metal-sputtering mode (sensors marked S3 and S4) and in the oxide-sputtering mode (sensors marked S1, S2, S5 and S6). The XRD spectra of as-deposited
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NiO films shows that the NiO films have either amorphous or polycrystalline structure. In the diffraction pattern from the as-deposited samples with higher oxygen content in oxide-sputtering mode, only the Si diffraction peaks from the substrate are observed indicating the amorphous nature of these as-deposited NiO layers. The diffraction patterns from other samples prepared at low oxygen content in metal-sputtering mode show the presence of the (fcc) NiO phase. Fig. 1b shows that after annealing at 500 8C all samples contain polycrystalline NiO cubic lattice, although the relative intensities of NiO peaks are different. It should be noticed that the peak positions of NiO (1 1 1), (2 0 0) and (2 2 0) for the samples prepared in metal-sputtering mode (S3, S4) are different from those for the samples prepared in oxide-sputtering mode. The positions of NiO peaks of the samples S3 and S4 are closer to the peak positions of bulk NiO than the positions of the samples prepared in oxide-sputtering mode. An AFM operating in air has been also used to study the surface morphology and the roughness of all the NiO thin films deposited on rough alumina substrates. First, by taking into account the roughness of the alumina substrates, an AFM image of the substrate was acquired before the deposition of the films (Fig. 2a). We found that the substrate surface is covered by a compact granular structure with grains sized 500–700 nm in diameter and 330 nm in height. The average roughness of the substrate defined as the ratio of the root mean square (RMS) value to the average height was approximately 32%. Fig. 2b shows the AFM image of the surface morphology of a NiO thin film. The values of the average roughness are changed in dependence on the sputtering mode (Table 2). The films prepared in the metal-sputtering mode (samples S3 and S4) show a surface roughness smoother in comparison with the films prepared in the oxide-sputtering mode (samples S1 and S5); in particular samples S3 and S4 show similar average roughness values 30.7 and 29.4%, respectively. On the contrary, the average roughness values of samples S1 and S5 are higher, 34.7 and 31.6%,
Fig. 2. AFM images showing the surface (5=5 mm2) of (a) the alumina substrate and (b) a NiO thin film.
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Table 2 RMS and average roughness of NiO films after annealing at 500 8C in air for 2 h as deduced from AFM analysis Sample
S3
S1
S4
S5
Oxygen content in working gas (%) Sputtering mode RMS (nm) Average height (nm) Average roughness (%)
20 Metal 104.71 341.13 30.7
30 Oxide 87.78 252.73 34.7
40 Metal 86.80 295.15 29.4
45 Oxide 86.47 273.38 31.6
respectively. We assume that lower values of the average roughness are due to an increase of the grain sizes in the films prepared in the metal-sputtering mode and by smaller grains filling out the spaces between the larger ones in the case of stoichiometric composition. SEM observations and EDX microanalysis (Fig. 3) revealed a uniform morphology and homogenous dispersion of NiO, Pt and Al2O3 phases. No defects were observed in material distribution in the analysed thin film sensor structures. Fig. 4 shows both the topography and the elements mapping (Ni, O, Pt and Al) within the sample. We can observe not only a clear and equal distribution of Ni and O in the NiO thin film but also sharp boundaries between the sensitive NiO film, interdigitated Pt electrodes and alumina substrate. In particular the oxygen map (Fig. 4b) shows the distribution of oxygen not only in the NiO film but also in the Al2O3 substrate and one can observe that the oxygen density is of course higher in the aluminaqNiO layered structured than in the alumina substrate. 3.2. Gas-sensing properties In order to evaluate the effects of the sputtering parameters on the gas sensing properties of the NiObased sensors, different gas-sensing tests towards NO2
were carried out on all the samples. The sensors, working at constant temperature, were exposed to different low NO2 concentrations (1, 2, 5, 10 ppm); after each exposure to NO2 a recovering exposure period in dry air followed. The above gas sequence protocol was repeated in dry air and in humid air (R.H.s50%) at different working temperatures ranging from room temperature to 400 8C. The NO2 gas response vs. operating temperature is shown in Fig. 5 for all the samples both in dry air (Fig. 5a) and in humid air (Fig. 5b). Here the gas response is defined as Ig yI0, where Ig is the electrical current value at the end of the exposure time to NO2 and I0 is the current value of baseline in air. As it is evident from Fig. 5, the gas response towards NO2 has a maximum value at a temperature of approximately 160 8C for all samples. Moreover we can observe that the NiO-based sensors, prepared at different sputtering conditions and showing different physical microstructure, show also slowly different responses to NO2. The samples S1 and S5 have the lowest gas responses compared to the other sensors prepared in different deposition conditions. Indeed, XRD analysis revealed only the beginning of crystallization for these samples (S1 and S5) after annealing at 500 8C. Moreover, looking at the sensors prepared with the metalsputtering mode in the best temperature range 160–200
Fig. 3. (a) SEM (LEO 1420) backscattered image of sensor structure (sample S5) and (b) calculated phase-distribution of NiO, Pt and Al2O3 by CAMEO programme from EDX analysis (Oxford Instruments).
I. Hotovy et al. / Thin Solid Films 418 (2002) 9–15
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Fig. 4. Elemental distribution of (a) nickel; (b) oxygen; (c) platinum and (d) aluminium within the sensor structure (sample S5).
8C, we can notice that sensor S4, prepared with an higher oxygen content (40% of O2) in the mixture Arq O2, has a bit higher response than sensor S3 prepared with 20% of O2. In the same way among the sensors deposited in the oxide-sputtering mode the higher response was reported for sensor S6 prepared in 60% of O 2. A particularly interesting effect is the enhancement of the response to NO2 (an oxidising gas) in humid air (Fig. 5b). Water vapour is known to act usually as a reducing gases on n-type metal-oxide semiconductors like SnO2, even if interfering effects of humidity on the sensitivity to reducing gases has been reported w9–11x. More intensive studies need to understand the adsorption and the surface interaction of water vapour on p-type metal-oxide semiconductors and its interfering andyor enhancing effect on the response to oxidising and reducing gases. Fig. 6 shows the dynamic response of all the NiObased sensors to the NO2 measurement protocol at the optimum operating temperature (160 8C) in humid air
(50% relative humidity). The sensor signals cannot reach the equilibrium during the exposure time, so that the response time at this temperature is really low. However at higher temperatures the response time increases; as example Fig. 7 shows the electrical current variation vs. time at Ts320 8C and R.H.s50%. In Fig. 8 it is also reported the calibration curve at Ts160 8C and R.H.s50% for the sensors S4 and S6 together with their sensitivity and low detection limit. The last sensor is more sensitive (slopes0.18"0.02y ppm) than S4, while the sensor S4 can detect a lowest critic concentration of NO2 (low limit(0.04 ppm). 4. Conclusions The NiO thin films have been deposited on Si and alumina substrates by d.c. reactive magnetron sputtering using a metallic nickel target at different oxygen contents in the gas mixture and pumping speed. It was observed that by varying these parameters it is possible to prepare NiO films with different structure and surface morphology. A promising sensitivity and response was
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Fig. 7. Dynamic response of all the NiO-based sensors to the NO2 measurement protocol at Ts320 8C) in humid air (R.H.s50%).
working temperature range. Moreover we found that humidity improves the NO2 sensing properties of the NiO thin films, but further analysis are necessary to understand the effective role of water vapour on NiO surface. Further studies are also in progress to improve the sensing properties by means of doping and catalysing with different elements. Fig. 5. Response to 5 ppm NO2 for all the NiO-based sensors as a function of working temperature (a) in dry air and (b) at the R.H.s 50%.
found towards NO2 concentrations in the range 1–10 ppm at low temperatures (160–200 8C). The different NiO-based sensors showed similar gas sensing properties to NO2; however, among the sensors prepared in the metal- and in the oxide-sputtering mode sensors S4 and S6 showed the best response to NO2 in the optimum
Fig. 6. Dynamic response of all the NiO-based sensors to the NO2 measurement protocol at the optimum operating temperature (160 8C) in humid air (R.H.s50%).
Acknowledgments The work was supported by the Scientific Grant Agency of the Ministry of Education of Slovak Republic and the Slovak Academy of Sciences, No. 1y7614y20. We would like to thank Oxford Instruments for EDX microanalysis.
Fig. 8. Calibration curve at Ts160 8C and R.H.s50% for the sensors S4 and S6 together with their sensitivity and low detection limit.
I. Hotovy et al. / Thin Solid Films 418 (2002) 9–15
References w1x D.S. Lee, S.D. Han, J.S. Huh, D.D. Lee, Sensors and Actuators B 60 (1999) 57–63. w2x T.S. Kim, Y.B. Kim, K.S. Yoo, G.S. Sung, H.J. Jung, Sensors and Actuators B 62 (2000) 102–108. w3x S. Capone, R. Rella, P. Siciliano, L. Vasanelli, Thin Solid Films 350 (1999) 264–268. w4x M. Kitao, K. Izawa, K. Urabe, T. Komatsu, S. Kuwano, S. Yamada, Jpn. J. Appl. Phys. 33 (1994) 6656–6662. w5x K. Yoshimura, T. Miki, S. Tanemura, Jpn. J. Appl. Phys. 34 (1995) 2440–2446.
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w6x A. Neubecker, T. Pompl, T. Doll, W. Hansch, I. Eisele, Thin Solid Films 310 (1997) 19–23. w7x I. Hotovy, J. Huran, J. Janik, A.P. Kobzev, Vacuum 51 (1998) 157–160. w8x I. Hotovy, J. Huran, P. Siciliano, S. Capone, L. Spiess, V. Rehacek, Sensors Actuators B 78 (2001) 126–132. w9x N. Barsan, ˆ R. Ionescu, Sensors Actuators B 12 (1993) 71–75. w10x R. Ionescu, A. Vancu, C. Moise, A. Tomescu, Sensors Actuators B 61 (1999) 39–42. w11x M. Caldararu, D. Sprınceana, ˆ V.T. Popa, N.I. Ionescu, Sensors Actuators B 30 (1996) 35–41. w12x A. Dieguez, ´ ´ A. Romano-Rodrıguez, J.L. Alay, J.R. Morante, ˆ N. Barsan, Sensors Actuators B: Chem. 65 (1–3) (2000) 166–168.
Sensing characteristics of NiO thin films as NO2 gas sensor I. Hotovya,*, V. Rehaceka, P. Sicilianob, S. Caponec, L. Spiessd a Department of Microelectronics, Slovak University of Technology, Ilkovicova 3, 812 19 Bratislava, Slovakia Instituto per lo Studio di Nuovi Materiali per l’Elettronica (I.M.E.-C.N.R.), Via Arnesano, 73100 Lecce, Italy c Dipartimento di Ingegneria dell’Innovazione, University of Lecce, Via per Arnesano, 73100 Lecce, Italy d Department of Materials Technology, Technical University of Ilmenau, PF 100565, D-98684 Ilmenau, Germany b
Abstract In this paper we present the results concerning the characterisation of nickel oxide thin films deposited by d.c. reactive magnetron sputtering. Different NiO thin films have been prepared by changing some deposition parameters, as the oxygen content in the reactive plasma and the sputtering mode (metal- or oxide-sputtering mode). The structure and surface morphology of the samples have been analysed by XRD and by atomic force microscopy and scanning electron microscope, respectively. The electrical responses of the NiO films towards NO2 have been also considered. NiO thin films showed good responses to low NO2 concentrations (1–10 ppm) with a maximum at 160 8C operating temperature. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: NiO; Thin film; Gas sensor; NO2
1. Introduction Nitrogen oxide is one of the most harmful gases to the human body and also a cause of air pollution. The major sources of nitrogen oxide are the combustion of fossil fuel and the exhaust of motor engines. Therefore, the development of new sensors for the detection of NO2 in the ambient is an important aspect of the general interest to improve the quality of our environment, to avoid damage to our health and quality of life and to avoid long-term irreversible changes to the composition of the earth’s atmosphere. There have been a lot of efforts to develop many types of nitrogen oxide gas sensors. Among them, the most interesting are the metal oxide-based gas sensors (MOS) that mainly use WO3 w1,2x, V2O5 w3x, TiO2 and SnO2 thin films as sensing layers w12x. A promising approach in this field is both to use novel materials based on semiconducting metal oxides, and to exploit the advantages of microelectronic and micro-mechanical technologies for the fabrication and production of a system compatible with current electron*Corresponding author. Tel.: q421-2-60291594; fax: q421-265423480. E-mail address: [email protected] (I. Hotovy).
ic information systems. Moreover, the surface properties and the deposition conditions of these materials play an important role in the properties of MOS-based gas sensors. Hence, a considerable interest is normally devoted to the preparation and characterisation of the sensing films. Nickel oxide, that is usually taken as model for ptype, is an attractive material well known for its chemical stability as well as for its excellent optical and electrical properties. Indeed, NiO thin films have been studied for applications in electrochromic devices w4,5x and also as functional sensing layers for MOS gas sensors w6x. NiO films can be fabricated by different physical and chemical vapour deposition techniques, such as reactive sputtering and plasma-enhanced chemical vapour deposition. The preparation method and the deposition mode are fundamental in determining the properties of MOS thin films, but the effective dependence of the process parameters on the film properties is not well defined. Nevertheless, it is evident that the improvement of the material properties and performance as gas sensor can be reached by the optimisation of the preparation conditions. Our research has been focused on the preparation and characterisation of NiO thin films deposited by reactive
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I. Hotovy et al. / Thin Solid Films 418 (2002) 9–15
magnetron sputtering. The NiO films have been diversified by changing some deposition parameters. In particular the NiO films were deposited in argon–oxygen mixtures with a relative O2 content varying from 20 to 60% in two operation modes, i.e. metal- and oxidesputtering mode. The samples have been characterised structurally by XRD and morphologically by atomic force microscopy (AFM) and scanning electron microscope (SEM). Tiny gas sensors based on the as-prepared NiO thin films deposited on alumina substrates were thus fabricated. The NO2 sensing properties of all the samples were also investigated in order to evaluate the sensors with the better response towards NO2 and to optimise the experimental conditions of the deposition process. The sensing tests toward CO were also carried out and the related results were presented in details in a previous work w8x. 2. Experiment The NiO films were deposited by d.c. reactive magnetron sputtering from a Ni target in a mixture of oxygen and argon. Two types of substrates were used: Si and alumina substrates. A sputtering power of 600 W was used. Both argon inert and oxygen reactive flow were controlled by mass flow controllers. The relative partial pressure, defined as jsp(O2)yp(O2 qAr), varied from 20 to 60%. The total gas pressure was kept at 0.5 Pa. The preparation of NiO films was realised in two operation modes: metal- and oxide-sputtering mode. Details of these sputtering deposition modes were given elsewhere w7x. The sputtering conditions are listed in Table 1. The thickness of the NiO films under the above mentioned deposition conditions ranged from 100 to 150 nm and was measured by a Talystep. Films for morphological and electrical measurements were deposited on alumina substrates (3=3 mm2) equipped interdigitated Pt electrodes with previously deposited. In order to stabilise the electrical properties of NiO thin films they have been annealed at 500 8C in air for 2 h. The crystal structure was identified by a Theta–Theta ¨ Diffractometer D 5000 with Gobelmirror into BraggBrentano focusing with copper radiation. Surface mor-
phology was observed by a Topometrix Discover TM 2000 AFM. In particular in the AFM observations a 70 mm linearized scanner with a minimal z-resolution of 1.3 nm was used. Moreover, the lateral material distribution was observed by a LEO 1420 SEM with an energy dispersive X-ray (EDX) analyser based on a germanium detector (Oxford Instruments) and operating at 12 kV acceleration voltage. Different lateral parts of the sensor structure were thus analysed in order to check the lateral uniformity of the elemental distribution. The sensor structures were mounted as suspended devices onto standard TO-8 packages and introduced into a test chamber in Teflon for the sensing tests in controlled ambient. A gas flow controller (MKS mod. 647B) connected to mass flow meters (MFCs) and to certified gas bottles allows to dilute the target gas in dry air at different concentrations. The total gas flow rate was kept constant at 100 sccm during the measurements. The test chamber, entirely programmed and realised at the Gas Sensor Laboratory of I.M.E.-C.N.R. in Lecce (Italy), can host up to eight sensors. During operation the sensors were heated by applying a voltage across the Pt heating meander, whose resistivity value is linked to the sensor’s operating temperature. The sensor working temperature was so controlled and varied in the range from 25 to 400 8C. A d.c. voltage of 1 V was applied across the sensing films and the electrical current was measured by using a Keithley (mod. 6517A) electrometer equipped with a multiplexer. A LabVIEW software drives via a PC and a GPIB interface all the operations of the gas-mixing station. First the NO2 sensing tests at different working temperatures and at different NO2 concentrations were carried out by using dry air as reference and carrier gas. Next the same protocol of measurements were repeated in humid air with a relative humidity content of 50% (R.H.s50%). The humidification of the mixtures NO2-dry air was obtained by the saturation method with a glass bubbler; dry air, left to flow through a bubble of water for collecting the saturated water vapours, is be mixed to dry air in fixed ratio just to get the desired level of humidity.
Table 1 Sample preparation conditions Sample
S1
S2
S3
S4
S5
S6
Sputtering mode Pumping speed (lys) Oxygen content in working gas (%) Oxygen partial pressure (Pa) Total pressure (Pa) Average target voltage (V)
Oxide 275 30 0.15 0.5 299
Oxide 275 40 0.2 0.5 308
Metal 275 20 0.1 0.5 336
Metal 136 40 0.18 0.5 374
Oxide 136 45 0.22 0.5 301
Oxide 136 60 0.3 0.5 306
I. Hotovy et al. / Thin Solid Films 418 (2002) 9–15
Fig. 1. XRD diffraction patterns for the NiO thin films prepared in metal-sputtering mode (S3, S4) and oxide-sputtering mode (S1, S2, S5, S6).
3. Results and discussion 3.1. Thin film characterisation Fig. 1a presents the diffraction patterns of all the asprepared NiO samples deposited at different oxygen contents in the ArqO2 plasma (i.e. 20 and 40% of O2) in the metal-sputtering mode (sensors marked S3 and S4) and in the oxide-sputtering mode (sensors marked S1, S2, S5 and S6). The XRD spectra of as-deposited
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NiO films shows that the NiO films have either amorphous or polycrystalline structure. In the diffraction pattern from the as-deposited samples with higher oxygen content in oxide-sputtering mode, only the Si diffraction peaks from the substrate are observed indicating the amorphous nature of these as-deposited NiO layers. The diffraction patterns from other samples prepared at low oxygen content in metal-sputtering mode show the presence of the (fcc) NiO phase. Fig. 1b shows that after annealing at 500 8C all samples contain polycrystalline NiO cubic lattice, although the relative intensities of NiO peaks are different. It should be noticed that the peak positions of NiO (1 1 1), (2 0 0) and (2 2 0) for the samples prepared in metal-sputtering mode (S3, S4) are different from those for the samples prepared in oxide-sputtering mode. The positions of NiO peaks of the samples S3 and S4 are closer to the peak positions of bulk NiO than the positions of the samples prepared in oxide-sputtering mode. An AFM operating in air has been also used to study the surface morphology and the roughness of all the NiO thin films deposited on rough alumina substrates. First, by taking into account the roughness of the alumina substrates, an AFM image of the substrate was acquired before the deposition of the films (Fig. 2a). We found that the substrate surface is covered by a compact granular structure with grains sized 500–700 nm in diameter and 330 nm in height. The average roughness of the substrate defined as the ratio of the root mean square (RMS) value to the average height was approximately 32%. Fig. 2b shows the AFM image of the surface morphology of a NiO thin film. The values of the average roughness are changed in dependence on the sputtering mode (Table 2). The films prepared in the metal-sputtering mode (samples S3 and S4) show a surface roughness smoother in comparison with the films prepared in the oxide-sputtering mode (samples S1 and S5); in particular samples S3 and S4 show similar average roughness values 30.7 and 29.4%, respectively. On the contrary, the average roughness values of samples S1 and S5 are higher, 34.7 and 31.6%,
Fig. 2. AFM images showing the surface (5=5 mm2) of (a) the alumina substrate and (b) a NiO thin film.
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Table 2 RMS and average roughness of NiO films after annealing at 500 8C in air for 2 h as deduced from AFM analysis Sample
S3
S1
S4
S5
Oxygen content in working gas (%) Sputtering mode RMS (nm) Average height (nm) Average roughness (%)
20 Metal 104.71 341.13 30.7
30 Oxide 87.78 252.73 34.7
40 Metal 86.80 295.15 29.4
45 Oxide 86.47 273.38 31.6
respectively. We assume that lower values of the average roughness are due to an increase of the grain sizes in the films prepared in the metal-sputtering mode and by smaller grains filling out the spaces between the larger ones in the case of stoichiometric composition. SEM observations and EDX microanalysis (Fig. 3) revealed a uniform morphology and homogenous dispersion of NiO, Pt and Al2O3 phases. No defects were observed in material distribution in the analysed thin film sensor structures. Fig. 4 shows both the topography and the elements mapping (Ni, O, Pt and Al) within the sample. We can observe not only a clear and equal distribution of Ni and O in the NiO thin film but also sharp boundaries between the sensitive NiO film, interdigitated Pt electrodes and alumina substrate. In particular the oxygen map (Fig. 4b) shows the distribution of oxygen not only in the NiO film but also in the Al2O3 substrate and one can observe that the oxygen density is of course higher in the aluminaqNiO layered structured than in the alumina substrate. 3.2. Gas-sensing properties In order to evaluate the effects of the sputtering parameters on the gas sensing properties of the NiObased sensors, different gas-sensing tests towards NO2
were carried out on all the samples. The sensors, working at constant temperature, were exposed to different low NO2 concentrations (1, 2, 5, 10 ppm); after each exposure to NO2 a recovering exposure period in dry air followed. The above gas sequence protocol was repeated in dry air and in humid air (R.H.s50%) at different working temperatures ranging from room temperature to 400 8C. The NO2 gas response vs. operating temperature is shown in Fig. 5 for all the samples both in dry air (Fig. 5a) and in humid air (Fig. 5b). Here the gas response is defined as Ig yI0, where Ig is the electrical current value at the end of the exposure time to NO2 and I0 is the current value of baseline in air. As it is evident from Fig. 5, the gas response towards NO2 has a maximum value at a temperature of approximately 160 8C for all samples. Moreover we can observe that the NiO-based sensors, prepared at different sputtering conditions and showing different physical microstructure, show also slowly different responses to NO2. The samples S1 and S5 have the lowest gas responses compared to the other sensors prepared in different deposition conditions. Indeed, XRD analysis revealed only the beginning of crystallization for these samples (S1 and S5) after annealing at 500 8C. Moreover, looking at the sensors prepared with the metalsputtering mode in the best temperature range 160–200
Fig. 3. (a) SEM (LEO 1420) backscattered image of sensor structure (sample S5) and (b) calculated phase-distribution of NiO, Pt and Al2O3 by CAMEO programme from EDX analysis (Oxford Instruments).
I. Hotovy et al. / Thin Solid Films 418 (2002) 9–15
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Fig. 4. Elemental distribution of (a) nickel; (b) oxygen; (c) platinum and (d) aluminium within the sensor structure (sample S5).
8C, we can notice that sensor S4, prepared with an higher oxygen content (40% of O2) in the mixture Arq O2, has a bit higher response than sensor S3 prepared with 20% of O2. In the same way among the sensors deposited in the oxide-sputtering mode the higher response was reported for sensor S6 prepared in 60% of O 2. A particularly interesting effect is the enhancement of the response to NO2 (an oxidising gas) in humid air (Fig. 5b). Water vapour is known to act usually as a reducing gases on n-type metal-oxide semiconductors like SnO2, even if interfering effects of humidity on the sensitivity to reducing gases has been reported w9–11x. More intensive studies need to understand the adsorption and the surface interaction of water vapour on p-type metal-oxide semiconductors and its interfering andyor enhancing effect on the response to oxidising and reducing gases. Fig. 6 shows the dynamic response of all the NiObased sensors to the NO2 measurement protocol at the optimum operating temperature (160 8C) in humid air
(50% relative humidity). The sensor signals cannot reach the equilibrium during the exposure time, so that the response time at this temperature is really low. However at higher temperatures the response time increases; as example Fig. 7 shows the electrical current variation vs. time at Ts320 8C and R.H.s50%. In Fig. 8 it is also reported the calibration curve at Ts160 8C and R.H.s50% for the sensors S4 and S6 together with their sensitivity and low detection limit. The last sensor is more sensitive (slopes0.18"0.02y ppm) than S4, while the sensor S4 can detect a lowest critic concentration of NO2 (low limit(0.04 ppm). 4. Conclusions The NiO thin films have been deposited on Si and alumina substrates by d.c. reactive magnetron sputtering using a metallic nickel target at different oxygen contents in the gas mixture and pumping speed. It was observed that by varying these parameters it is possible to prepare NiO films with different structure and surface morphology. A promising sensitivity and response was
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I. Hotovy et al. / Thin Solid Films 418 (2002) 9–15
Fig. 7. Dynamic response of all the NiO-based sensors to the NO2 measurement protocol at Ts320 8C) in humid air (R.H.s50%).
working temperature range. Moreover we found that humidity improves the NO2 sensing properties of the NiO thin films, but further analysis are necessary to understand the effective role of water vapour on NiO surface. Further studies are also in progress to improve the sensing properties by means of doping and catalysing with different elements. Fig. 5. Response to 5 ppm NO2 for all the NiO-based sensors as a function of working temperature (a) in dry air and (b) at the R.H.s 50%.
found towards NO2 concentrations in the range 1–10 ppm at low temperatures (160–200 8C). The different NiO-based sensors showed similar gas sensing properties to NO2; however, among the sensors prepared in the metal- and in the oxide-sputtering mode sensors S4 and S6 showed the best response to NO2 in the optimum
Fig. 6. Dynamic response of all the NiO-based sensors to the NO2 measurement protocol at the optimum operating temperature (160 8C) in humid air (R.H.s50%).
Acknowledgments The work was supported by the Scientific Grant Agency of the Ministry of Education of Slovak Republic and the Slovak Academy of Sciences, No. 1y7614y20. We would like to thank Oxford Instruments for EDX microanalysis.
Fig. 8. Calibration curve at Ts160 8C and R.H.s50% for the sensors S4 and S6 together with their sensitivity and low detection limit.
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