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Characterization of tellurium-based films for NO2 detection
Thin Solid Films 485 (2005) 252 – 256 www.elsevier.com/locate/tsf
Characterization of tellurium-based films for NO2 detection D. Tsiulyanua,*, A. Tsiulyanua, H.-D. Liessb, I. Eiseleb a
b
Technical University, Department of Physics, bul. Dacia 41, MD-2060 Kishinau, Moldova University of the Bundeswehr Munich, Faculty of Electrical Engineering and Information Technology, Institute of Physics, D-85577 Neubiberg, Germany Received 29 November 2003; accepted in revised form 29 March 2005 Available online 17 May 2005
Abstract Sensing characteristics of tellurium-based thin films for NO2 monitoring was studied systematically. The influence of contact materials, thermal treatment, temperature and thickness of the samples on the electrical conductivity and sensitivity to NO2 with respect to scanning electron microscopy analyses is given. The possibility is shown to optimize the properties of the films for the development of a simple and stable NO2 sensor device with rapid response/recovery time and low operating temperature. The sensing mechanism is discussed for the direct interaction of gaseous species with lone-pair electrons of chalcogen atoms. D 2005 Elsevier B.V. All rights reserved. Keywords: Gas sensors; Tellurium; Nitrogen dioxide
1. Introduction The detection and emission control of nitrogen dioxide, released by combustion, plants and automobiles is of great importance. Therefore, much effort is made to develop sensors for monitoring the concentration of this pollutant in our environment. Until now, SnO2 based nitrogen dioxide (NO2) sensors were the mainly investigated and also of commercial value [1,2]. They are, however not very selective and usually operate at temperatures above 300 -C. Only recently a novel class of gas sensors, based on chalcogenide materials was proposed for monitoring pollutants in ambient air [3– 5]. Most attractive in this respect were thin films based on pure tellurium or its alloys from the As – Ge – Te system. These films show remarkable sensing properties to NO2 at even room temperature, with the response time of only several minutes and the sensitivity being especially high at concentrations less than 1 ppm. The mechanism of gas detection of these materials is, however, still not completely understood, because of the lack of sufficient experimental data. There are no reports in
* Corresponding author. Tel.: +37 32 2564269. E-mail address: [email protected] (D. Tsiulyanu). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.03.045
particular on the influence of the contact materials, the film thickness, the thermal treatment and the temperature on the electrical and sensing properties of the films. In the present study the dependence of the NO2 sensing properties of tellurium-based films on these process parameters have been systematically investigated. The possible mechanism of gas sensing is also discussed.
2. Experimental details Tellurium (purity 99.999%) based thin films of different thicknesses were deposited onto Pyrex glass substrates with thermal vacuum evaporation. The evaporation was performed from a quartz crucible at the working pressure of å104 Pa. The growing rate of the film was around 10 nm/ s, the area of deposition being around 50 mm2. Rectangular samples of different (20, 60, 110 and 190 nm) thicknesses were prepared by a variation of the distance between the evaporation crucible and the substrate from 10 to 25 cm, while the evaporation time has been kept the same. The thicknesses of the films have been measured using microinterferometer MII-4. The surface morphology of the films was investigated, using a scanning electron microscope (SEM) TESLA BS 340.
D. Tsiulyanu et al. / Thin Solid Films 485 (2005) 252 – 256
In order to investigate the possible effect of film contacting, the electrode materials with greater work function (gold, ¨5.3 eV) or lower work function (indium, 4.12 eV) than tellurium work function (4.95 eV [6]) have been tested. Gold electrodes were deposited onto the film surface through thermal vacuum evaporation. Copper wires were then attached to the electrodes by silver paste. Indium contacts have been made with two ‘‘indium pillows’’, which were pressed on top of the tellurium film. NO2 vapor with a concentration of 0.75 to 18 ppm was obtained by using the experimental set up described in [7]. Gaseous NO2 media was obtained by using a calibrated permeation tube (Vici Metronics, USA), which was introduced into the experimental set-up. Ambient air was used as the carrier, as well as the reference gas. The data was processed, using a PC and a data acquisition board manufactured by National Instruments Inc. The thin film sensing devices were put into a test cell (of 10 ml volume), which was placed inside the furnace. To measure the temperature, a thermocouple was placed on the external wall of this cell. The gases were injected with a flow rate of 100 ml/min, parallel to the film surface. Current/voltage and current transient characteristics have been carried out with different gas concentrations on samples, which were heated and annealed at different temperatures. In all cases the applied voltage varied between 2.5 V and +2.5 V in steps, increasing by 20 mV at each step, while the respective values of the current were measured. The delay time between two measurements was 2 s. The measurements were performed at temperatures between 20 -C and 200 -C. The experiments were carried out as follows: (a) Measurement of the I/U characteristics at room temperature in pure air and in gaseous media with different concentration of NO2. (b) Heating up from room temperature to 50 -C; 100 -C; 150 -C and 200 -C and annealing at these temperatures for 2 h. (c) Measurement of I/U characteristics with and without NO2 vapor at indicated temperatures. (d) Cooling down to room temperature and performing the measurements of I/U characteristics in pure air and in gaseous media with different concentration of NO2 again. During each annealing process an additional sample was placed inside the furnace in order to check the influence of thermal treatment to surface morphology of the film. The transient characteristics have been measured using the computer controlled switching of NO2 vapor and pure air at constant voltage of 4 V. In order to transform the resistance signal into a voltage signal, the sample was connected in series with a load resistance and the D.C. voltage supply. In all measurements the load resistance was
253
chosen approximately by an order of magnitude lower than the sample resistance. The sensor sensitivity was defined as the relative resistance variation expressed in percent: ð1Þ S ¼ 100 jRa Rg j =C I Ra where R a and R g are the electrical resistances of the sensor in the air and in the presence of NO2 respectively. C is the gas concentration.
3. Results and discussion 3.1. Effect of contacts and film thickness Fig. 1 shows the current/voltage characteristic of tellurium-based films with Au electrodes in the air and in the presence of NO2 vapor. The devices with In electrodes exhibit the same characteristics. In all cases the I/U characteristics are linear and follow the Ohm’s law but the electrical conductivity of the sensitive layer depends on the layer thickness. The inset of Fig. 1 shows the conductivity versus layer thickness at room temperature. It is seen that until thickness is below 100 nm the conductivity increases with the increase of the layer thickness, then when thickness reaches 100 nm, the conductivity becomes almost constant. The influence of NO2 vapor leads to an increase of the current independently of the direction of the bias voltage. It is significant to note the high value of the gas-induced current, which is in the range of dozens of microamperes, depending on the film’s thickness and bias voltage. The sensor sensitivity as a function of film thickness is shown in Fig. 2. As can be seen the film sensitivity strongly increases with thickness decrease. Such behavior can be expected in the case of a compact layer [8]. Inset of Fig. 2 shows the SEM view of an as-deposited sensitive film. It can be seen that the morphology of the film
Fig. 1. Current/voltage characteristics of tellurium-based film with Au electrodes in air and in presence of 1.5 ppm of NO2. Inset shows the conductivity of the films versus their thickness at room temperature.
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Fig. 2. Effect of thickness on sensitivity to 1.5 ppm of NO2 at room temperature. Inset shows the SEM micrograph of an as-grown film.
shows a compact layer. In the case of a compact layer [8], the current flows through two parallel channels, one of them being the surface channel, which is affected by the gas reaction, and the other is the gas-unaffected bulk. Because the surface part of the grains can be oxidized under interaction with atmospheric oxygen, the appearance of a grain boundary resistance occurs, which is put in series with a resistance of the surface channel of current flow. Decreasing the layer thickness leads to enhancing the influence of the surface grain boundary resistance and removing the bulk, gas-unaffected parallel resistance. That is why the film conductivity decreases, while its sensitivity strongly increases with the thickness decrease. 3.2. Effect of annealing Fig. 3 shows the resistivity of a sensitive tellurium-based layer with gold contacts versus the annealing temperature. The resistance was simply calculated from the slope of the
Fig. 3. Resistivity of the film with thickness of 190 nm versus temperature of annealing. Inset shows the SEM micrograph of the film annealed at 200 -C.
I/U characteristic, which was carried out at room temperature after annealing of the sample at the indicated temperature. This resistivity increases with the annealing temperature rise up to 100 -C. After this temperature a rather sharp decrease is observed. The little enhancement of the electrical resistivity, observed at annealing temperatures below 100 -C, is not accompanied by some visible structural evolution. We suppose that the increase of film resistance in this case is due to the mentioned above possible oxidation of the surface, which results in an increase of the surface grain boundary resistance. The decrease of electrical resistivity at an annealing temperature of more than 150 -C occurs simultaneously with the appearance and development of large-sized crystallites inside the film. Inset of Fig. 3 shows the surface morphology of a film after annealing at 200 -C. The decrease of the electrical resistance of the annealed films should be attributed to this crystallization, which also affects the gas sensing. Fig. 4 shows the sensitivity of the films to 0.75 and 1.5 ppm of NO2 as a function of annealing temperature. Testing was carried out at room temperature. It is seen that the sensitivity of the films diminishes with increasing annealing temperature, although weak its increasing (like a shoulder) between 100 -C and 150 -C can be observed. Besides, the data points, obtained for the curves for 0.75 ppm and 1.5 ppm of NO2, occurs coincidently at 200 -C. Hence, the sensitivity should not depend much on concentration if the samples are preliminary treated at high temperatures. 3.3. Effect of temperature Taking into account the influence of annealing on electrical properties and gas sensitivity, the effect of temperature was studied on previously annealed samples. Fig. 5 shows the sensitivity of tellurium-based sensors to 0.75 ppm NO2 versus operating temperature. The temperature reduces the sensitivity of the films, especially strong at temperatures higher than 80 -C. The insert in Fig. 5 illustrates the temperature-dependent electrical conductivity
Fig. 4. Sensitivity to NO2 of 110 nm thick films versus temperature of annealing.
D. Tsiulyanu et al. / Thin Solid Films 485 (2005) 252 – 256
Fig. 5. Sensitivity of tellurium film to 0.75 ppm of NO2 versus operating temperature. The film was of 60 nm thick and has been preliminary annealed 2 h at 150 -C. Inset shows the temperature dependence of conductivity.
of the films. This dependence is typical for polycrystalline tellurium thin films and shows the exponential conductivity growth with increasing temperature. The activation energy is about 0.16 eV. Fig. 6 shows the response kinetics at different temperatures, when the squared concentration pulse with 0.75 ppm of NO2 in amplitude was applied. As temperature increases, the film’s conductance increases as well and the sensitivity decreases, while the response time and recovery processes become faster. The response and recovery times (defined as the times taken to reach 90% of steady-state values of R g and R a respectively) versus operating temperature are shown in Fig. 7. Both response and recovery times decrease with increasing operating temperature, while the decrease of the recovery time is more noticeable. Following this, the response and recovery times are almost the same at temperature of about 100 -C having a value of around 2 min to 4 min. These time values are much shorter compared
Fig. 6. Response of tellurium-based sensor to squared pulse of 0.75 ppm NO2 at different temperatures.
255
Fig. 7. The response and the recovery times of 110 nm thick film versus operating temperature.
to those (t = 30 min) reported for SnO2 based sensors [9]. Thus, the best compromise between sensitivity, effect of annealing and velocity of response/recovery times is achieved at around 50 -C. Fig. 8 shows the current flow through a sensor under repeated switching on– off of the NO2 gas mixture at constant bias voltage and 50 -C operation temperature. Squared pulses of NO2 vapor with concentration of 0 ppm, 0.75 ppm, and 1.5 ppm were applied. The dotted line shows the switching schedule. It is seen that the current follows the schedule and the recovery time is longer than the response time. It is significant that there is no baseline drift or noticeable drift of the gas induced current. The results can be interpreted, taking into consideration that tellurium is an elemental chalcogen semiconductor, belonging to so called lone-pair semiconductors, which are materials that contain a large concentration of elements from
Fig. 8. Transient characteristics of gas-induced current by exposure to various concentrations of NO2 according to the profile shown in dotted lines at the bottom. The film thickness and the operating temperature were 110 nm and 50 -C respectively.
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V and VI groups of the periodic table. The main peculiarity of lone-pair semiconductors is that the lone-pair orbitals (in tellurium—5p4 state) form the upper part of the valence band. If the crystalline network contains defects, such as unsaturated chemical bonds (dangling bonds), the interaction between these defects and the lone-pair electrons occurs. The dangling bond interacts with neighboring lone-pair, bonding with it by distorting its environment [10]. Such interaction results in the release of about 1013 – 1015 holes/cm3 and cause the p-type of conductivity. The last affirmation was confirmed experimentally in the present work by measuring the thermoelectric power, which appears to be positive. In such case, the temperature dependent electrical conductivity (inset of Fig. 5) shows that lattice defects create acceptors with activation energy E a = 0.16 eV above the edge of the valence band. These are rather deep acceptors, situated below the center of the gap with the gap of crystalline tellurium being about 0.38 eV [11]. Thus, the dangling bonds act as dopant for lone-pair semiconductors and this is probably the key in understanding the gas sensing when using these materials. In fact, the surface of the semiconductor is the region where the periodicity of the crystal is interrupted and the maximum concentration of dangling bonds occurs. Because of their interaction with lone-pairs electrons, the hole-enriched region is formed near the surface of the grains. Hence, the band edges of p-tellurium bend up at the surface and in the intra-grain regions. When the chalcogenide semiconductor is introduced into gaseous environment, the adsorption of gas molecules occurs, which can produce either donor or acceptor levels. Let us consider the adsorption of nitrogen dioxide. The molecule of nitrogen dioxide has an odd electron [12], i.e. after covalently bonding the nitrogen to oxygen, one of the atoms remains with a single, unpaired electron. Being adsorbed on the surface of the chalcogenide semiconductor, the molecule of NO2 acts as a dangling bond, and can accept a lone-pair electron to form an electron pair. Capture of a lone-pair electron means a transition of an electron from the upper part of the valence band to an NO2 acceptor level, which results in formation of an additional free hole. Thus, the adsorption of nitrogen dioxide causes an increase of concentration of majority carriers and hence increasing the film conductivity.
4. Conclusions The electrical conductivity and the sensitivity to NO2 of tellurium films are strongly dependant on the thickness of
the films when the thickness is less than 100 nm. Thermal treatment of the films at temperatures greater than 100 -C leads to re-crystallization of the films and results in decreasing of their resistivity and sensitivity to NO2. The sensitivity decreases with increasing operating temperature, especially strong at temperatures higher than 80 -C. The response and recovery times strongly improve with increasing operating temperature. At 100 -C the values of these times became nearly the same and were 2 min to 4 min. The best compromise between sensitivity, effect of annealing, response and recovery times can be achieved at operating temperature 50 -C. There is no noticeable drift of baseline or of gas-induced current at this temperature. The sensing mechanism of tellurium films can be explained by interaction of adsorbed species with lone-pair electrons, which form the upper part of the valence band. Tellurium thin films are suitable for the development of simple, reliable, low cost and low power consumption NO2 detection device, which operates at low temperature.
Acknowledgements This work is partially supported by the BMBF Project MDA 02/001.
References [1] W. Gopel, K.-D. Shierbaum, Sens. Actuators, B, Chem. 26 – 27 (1995) 1. [2] G. Sberveglieri, Sens. Actuators, B, Chem. 23 (1995) 103. [3] S. Marian, K. Potje-Kamloth, D. Tsiulyanu, H.-D. Liess, Thin Solid Films 359 (2000) 108. [4] D. Tsiulyanu, S. Marian, V. Miron, H.-D. Liess, Sens. Actuators, B, Chem. 73 (2001) 35. [5] D. Tsiulyanu, S. Marian, H.-D. Liess, Sens. Actuators, B, Chem. 85 (2002) 232. [6] H.B. Mihaelson, J. Appl. Phys. 48 (1977) 4729. [7] R. Cabala, V. Meister, K. Potje-Kamloth, V. Meister, J. Chem. Soc., Faraday Trans. 93 (1) (1997) 131. [8] I. Simon, N. Barsan, M. Bauer, U. Weimer, Sens. Actuators, B, Chem. 73 (2001) 1. [9] I. Sayago, J. Gutierrez, L. Ares, J.I. Robla, M.S. Horrilo, J. Gettino, J. Rino, J.A. Agapito, Sens. Actuators, B, Chem. 26/27 (1995) 19. [10] M. Kastner, H. Fritzsche, Philos. Mag., B 37 (1978) 199. [11] D.M. Chijikov, V.P. Schaslivii, Tellurium and Telluride (In Russian), Nauka, Moscow, 1966. [12] J. Greyson, Carbon, Nitrogen and Sulfur Pollutants and their Determination in Air and Water, Marcel Dekker, Inc., New York, 1990.
Characterization of tellurium-based films for NO2 detection D. Tsiulyanua,*, A. Tsiulyanua, H.-D. Liessb, I. Eiseleb a
b
Technical University, Department of Physics, bul. Dacia 41, MD-2060 Kishinau, Moldova University of the Bundeswehr Munich, Faculty of Electrical Engineering and Information Technology, Institute of Physics, D-85577 Neubiberg, Germany Received 29 November 2003; accepted in revised form 29 March 2005 Available online 17 May 2005
Abstract Sensing characteristics of tellurium-based thin films for NO2 monitoring was studied systematically. The influence of contact materials, thermal treatment, temperature and thickness of the samples on the electrical conductivity and sensitivity to NO2 with respect to scanning electron microscopy analyses is given. The possibility is shown to optimize the properties of the films for the development of a simple and stable NO2 sensor device with rapid response/recovery time and low operating temperature. The sensing mechanism is discussed for the direct interaction of gaseous species with lone-pair electrons of chalcogen atoms. D 2005 Elsevier B.V. All rights reserved. Keywords: Gas sensors; Tellurium; Nitrogen dioxide
1. Introduction The detection and emission control of nitrogen dioxide, released by combustion, plants and automobiles is of great importance. Therefore, much effort is made to develop sensors for monitoring the concentration of this pollutant in our environment. Until now, SnO2 based nitrogen dioxide (NO2) sensors were the mainly investigated and also of commercial value [1,2]. They are, however not very selective and usually operate at temperatures above 300 -C. Only recently a novel class of gas sensors, based on chalcogenide materials was proposed for monitoring pollutants in ambient air [3– 5]. Most attractive in this respect were thin films based on pure tellurium or its alloys from the As – Ge – Te system. These films show remarkable sensing properties to NO2 at even room temperature, with the response time of only several minutes and the sensitivity being especially high at concentrations less than 1 ppm. The mechanism of gas detection of these materials is, however, still not completely understood, because of the lack of sufficient experimental data. There are no reports in
* Corresponding author. Tel.: +37 32 2564269. E-mail address: [email protected] (D. Tsiulyanu). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.03.045
particular on the influence of the contact materials, the film thickness, the thermal treatment and the temperature on the electrical and sensing properties of the films. In the present study the dependence of the NO2 sensing properties of tellurium-based films on these process parameters have been systematically investigated. The possible mechanism of gas sensing is also discussed.
2. Experimental details Tellurium (purity 99.999%) based thin films of different thicknesses were deposited onto Pyrex glass substrates with thermal vacuum evaporation. The evaporation was performed from a quartz crucible at the working pressure of å104 Pa. The growing rate of the film was around 10 nm/ s, the area of deposition being around 50 mm2. Rectangular samples of different (20, 60, 110 and 190 nm) thicknesses were prepared by a variation of the distance between the evaporation crucible and the substrate from 10 to 25 cm, while the evaporation time has been kept the same. The thicknesses of the films have been measured using microinterferometer MII-4. The surface morphology of the films was investigated, using a scanning electron microscope (SEM) TESLA BS 340.
D. Tsiulyanu et al. / Thin Solid Films 485 (2005) 252 – 256
In order to investigate the possible effect of film contacting, the electrode materials with greater work function (gold, ¨5.3 eV) or lower work function (indium, 4.12 eV) than tellurium work function (4.95 eV [6]) have been tested. Gold electrodes were deposited onto the film surface through thermal vacuum evaporation. Copper wires were then attached to the electrodes by silver paste. Indium contacts have been made with two ‘‘indium pillows’’, which were pressed on top of the tellurium film. NO2 vapor with a concentration of 0.75 to 18 ppm was obtained by using the experimental set up described in [7]. Gaseous NO2 media was obtained by using a calibrated permeation tube (Vici Metronics, USA), which was introduced into the experimental set-up. Ambient air was used as the carrier, as well as the reference gas. The data was processed, using a PC and a data acquisition board manufactured by National Instruments Inc. The thin film sensing devices were put into a test cell (of 10 ml volume), which was placed inside the furnace. To measure the temperature, a thermocouple was placed on the external wall of this cell. The gases were injected with a flow rate of 100 ml/min, parallel to the film surface. Current/voltage and current transient characteristics have been carried out with different gas concentrations on samples, which were heated and annealed at different temperatures. In all cases the applied voltage varied between 2.5 V and +2.5 V in steps, increasing by 20 mV at each step, while the respective values of the current were measured. The delay time between two measurements was 2 s. The measurements were performed at temperatures between 20 -C and 200 -C. The experiments were carried out as follows: (a) Measurement of the I/U characteristics at room temperature in pure air and in gaseous media with different concentration of NO2. (b) Heating up from room temperature to 50 -C; 100 -C; 150 -C and 200 -C and annealing at these temperatures for 2 h. (c) Measurement of I/U characteristics with and without NO2 vapor at indicated temperatures. (d) Cooling down to room temperature and performing the measurements of I/U characteristics in pure air and in gaseous media with different concentration of NO2 again. During each annealing process an additional sample was placed inside the furnace in order to check the influence of thermal treatment to surface morphology of the film. The transient characteristics have been measured using the computer controlled switching of NO2 vapor and pure air at constant voltage of 4 V. In order to transform the resistance signal into a voltage signal, the sample was connected in series with a load resistance and the D.C. voltage supply. In all measurements the load resistance was
253
chosen approximately by an order of magnitude lower than the sample resistance. The sensor sensitivity was defined as the relative resistance variation expressed in percent: ð1Þ S ¼ 100 jRa Rg j =C I Ra where R a and R g are the electrical resistances of the sensor in the air and in the presence of NO2 respectively. C is the gas concentration.
3. Results and discussion 3.1. Effect of contacts and film thickness Fig. 1 shows the current/voltage characteristic of tellurium-based films with Au electrodes in the air and in the presence of NO2 vapor. The devices with In electrodes exhibit the same characteristics. In all cases the I/U characteristics are linear and follow the Ohm’s law but the electrical conductivity of the sensitive layer depends on the layer thickness. The inset of Fig. 1 shows the conductivity versus layer thickness at room temperature. It is seen that until thickness is below 100 nm the conductivity increases with the increase of the layer thickness, then when thickness reaches 100 nm, the conductivity becomes almost constant. The influence of NO2 vapor leads to an increase of the current independently of the direction of the bias voltage. It is significant to note the high value of the gas-induced current, which is in the range of dozens of microamperes, depending on the film’s thickness and bias voltage. The sensor sensitivity as a function of film thickness is shown in Fig. 2. As can be seen the film sensitivity strongly increases with thickness decrease. Such behavior can be expected in the case of a compact layer [8]. Inset of Fig. 2 shows the SEM view of an as-deposited sensitive film. It can be seen that the morphology of the film
Fig. 1. Current/voltage characteristics of tellurium-based film with Au electrodes in air and in presence of 1.5 ppm of NO2. Inset shows the conductivity of the films versus their thickness at room temperature.
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Fig. 2. Effect of thickness on sensitivity to 1.5 ppm of NO2 at room temperature. Inset shows the SEM micrograph of an as-grown film.
shows a compact layer. In the case of a compact layer [8], the current flows through two parallel channels, one of them being the surface channel, which is affected by the gas reaction, and the other is the gas-unaffected bulk. Because the surface part of the grains can be oxidized under interaction with atmospheric oxygen, the appearance of a grain boundary resistance occurs, which is put in series with a resistance of the surface channel of current flow. Decreasing the layer thickness leads to enhancing the influence of the surface grain boundary resistance and removing the bulk, gas-unaffected parallel resistance. That is why the film conductivity decreases, while its sensitivity strongly increases with the thickness decrease. 3.2. Effect of annealing Fig. 3 shows the resistivity of a sensitive tellurium-based layer with gold contacts versus the annealing temperature. The resistance was simply calculated from the slope of the
Fig. 3. Resistivity of the film with thickness of 190 nm versus temperature of annealing. Inset shows the SEM micrograph of the film annealed at 200 -C.
I/U characteristic, which was carried out at room temperature after annealing of the sample at the indicated temperature. This resistivity increases with the annealing temperature rise up to 100 -C. After this temperature a rather sharp decrease is observed. The little enhancement of the electrical resistivity, observed at annealing temperatures below 100 -C, is not accompanied by some visible structural evolution. We suppose that the increase of film resistance in this case is due to the mentioned above possible oxidation of the surface, which results in an increase of the surface grain boundary resistance. The decrease of electrical resistivity at an annealing temperature of more than 150 -C occurs simultaneously with the appearance and development of large-sized crystallites inside the film. Inset of Fig. 3 shows the surface morphology of a film after annealing at 200 -C. The decrease of the electrical resistance of the annealed films should be attributed to this crystallization, which also affects the gas sensing. Fig. 4 shows the sensitivity of the films to 0.75 and 1.5 ppm of NO2 as a function of annealing temperature. Testing was carried out at room temperature. It is seen that the sensitivity of the films diminishes with increasing annealing temperature, although weak its increasing (like a shoulder) between 100 -C and 150 -C can be observed. Besides, the data points, obtained for the curves for 0.75 ppm and 1.5 ppm of NO2, occurs coincidently at 200 -C. Hence, the sensitivity should not depend much on concentration if the samples are preliminary treated at high temperatures. 3.3. Effect of temperature Taking into account the influence of annealing on electrical properties and gas sensitivity, the effect of temperature was studied on previously annealed samples. Fig. 5 shows the sensitivity of tellurium-based sensors to 0.75 ppm NO2 versus operating temperature. The temperature reduces the sensitivity of the films, especially strong at temperatures higher than 80 -C. The insert in Fig. 5 illustrates the temperature-dependent electrical conductivity
Fig. 4. Sensitivity to NO2 of 110 nm thick films versus temperature of annealing.
D. Tsiulyanu et al. / Thin Solid Films 485 (2005) 252 – 256
Fig. 5. Sensitivity of tellurium film to 0.75 ppm of NO2 versus operating temperature. The film was of 60 nm thick and has been preliminary annealed 2 h at 150 -C. Inset shows the temperature dependence of conductivity.
of the films. This dependence is typical for polycrystalline tellurium thin films and shows the exponential conductivity growth with increasing temperature. The activation energy is about 0.16 eV. Fig. 6 shows the response kinetics at different temperatures, when the squared concentration pulse with 0.75 ppm of NO2 in amplitude was applied. As temperature increases, the film’s conductance increases as well and the sensitivity decreases, while the response time and recovery processes become faster. The response and recovery times (defined as the times taken to reach 90% of steady-state values of R g and R a respectively) versus operating temperature are shown in Fig. 7. Both response and recovery times decrease with increasing operating temperature, while the decrease of the recovery time is more noticeable. Following this, the response and recovery times are almost the same at temperature of about 100 -C having a value of around 2 min to 4 min. These time values are much shorter compared
Fig. 6. Response of tellurium-based sensor to squared pulse of 0.75 ppm NO2 at different temperatures.
255
Fig. 7. The response and the recovery times of 110 nm thick film versus operating temperature.
to those (t = 30 min) reported for SnO2 based sensors [9]. Thus, the best compromise between sensitivity, effect of annealing and velocity of response/recovery times is achieved at around 50 -C. Fig. 8 shows the current flow through a sensor under repeated switching on– off of the NO2 gas mixture at constant bias voltage and 50 -C operation temperature. Squared pulses of NO2 vapor with concentration of 0 ppm, 0.75 ppm, and 1.5 ppm were applied. The dotted line shows the switching schedule. It is seen that the current follows the schedule and the recovery time is longer than the response time. It is significant that there is no baseline drift or noticeable drift of the gas induced current. The results can be interpreted, taking into consideration that tellurium is an elemental chalcogen semiconductor, belonging to so called lone-pair semiconductors, which are materials that contain a large concentration of elements from
Fig. 8. Transient characteristics of gas-induced current by exposure to various concentrations of NO2 according to the profile shown in dotted lines at the bottom. The film thickness and the operating temperature were 110 nm and 50 -C respectively.
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D. Tsiulyanu et al. / Thin Solid Films 485 (2005) 252 – 256
V and VI groups of the periodic table. The main peculiarity of lone-pair semiconductors is that the lone-pair orbitals (in tellurium—5p4 state) form the upper part of the valence band. If the crystalline network contains defects, such as unsaturated chemical bonds (dangling bonds), the interaction between these defects and the lone-pair electrons occurs. The dangling bond interacts with neighboring lone-pair, bonding with it by distorting its environment [10]. Such interaction results in the release of about 1013 – 1015 holes/cm3 and cause the p-type of conductivity. The last affirmation was confirmed experimentally in the present work by measuring the thermoelectric power, which appears to be positive. In such case, the temperature dependent electrical conductivity (inset of Fig. 5) shows that lattice defects create acceptors with activation energy E a = 0.16 eV above the edge of the valence band. These are rather deep acceptors, situated below the center of the gap with the gap of crystalline tellurium being about 0.38 eV [11]. Thus, the dangling bonds act as dopant for lone-pair semiconductors and this is probably the key in understanding the gas sensing when using these materials. In fact, the surface of the semiconductor is the region where the periodicity of the crystal is interrupted and the maximum concentration of dangling bonds occurs. Because of their interaction with lone-pairs electrons, the hole-enriched region is formed near the surface of the grains. Hence, the band edges of p-tellurium bend up at the surface and in the intra-grain regions. When the chalcogenide semiconductor is introduced into gaseous environment, the adsorption of gas molecules occurs, which can produce either donor or acceptor levels. Let us consider the adsorption of nitrogen dioxide. The molecule of nitrogen dioxide has an odd electron [12], i.e. after covalently bonding the nitrogen to oxygen, one of the atoms remains with a single, unpaired electron. Being adsorbed on the surface of the chalcogenide semiconductor, the molecule of NO2 acts as a dangling bond, and can accept a lone-pair electron to form an electron pair. Capture of a lone-pair electron means a transition of an electron from the upper part of the valence band to an NO2 acceptor level, which results in formation of an additional free hole. Thus, the adsorption of nitrogen dioxide causes an increase of concentration of majority carriers and hence increasing the film conductivity.
4. Conclusions The electrical conductivity and the sensitivity to NO2 of tellurium films are strongly dependant on the thickness of
the films when the thickness is less than 100 nm. Thermal treatment of the films at temperatures greater than 100 -C leads to re-crystallization of the films and results in decreasing of their resistivity and sensitivity to NO2. The sensitivity decreases with increasing operating temperature, especially strong at temperatures higher than 80 -C. The response and recovery times strongly improve with increasing operating temperature. At 100 -C the values of these times became nearly the same and were 2 min to 4 min. The best compromise between sensitivity, effect of annealing, response and recovery times can be achieved at operating temperature 50 -C. There is no noticeable drift of baseline or of gas-induced current at this temperature. The sensing mechanism of tellurium films can be explained by interaction of adsorbed species with lone-pair electrons, which form the upper part of the valence band. Tellurium thin films are suitable for the development of simple, reliable, low cost and low power consumption NO2 detection device, which operates at low temperature.
Acknowledgements This work is partially supported by the BMBF Project MDA 02/001.
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