Characterization and Sensing Properties of ZnO Film In FG-FET Sensor System for NO 2 Detection

Transcription

1 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: Characterization and Sensing Properties of ZnO Film In FG-FET Sensor System for NO 2 Detection W. Widanarto 1, C. Senft 2, O. Senftleben 2, W. Hansch 2,I. Eisele 2 1 Physics Study Program, Jenderal Soedirman University, Jl. dr. Soeparno 61 Purwokerto 53123, Indonesia 2 Institut für Physik, Universität der Bundeswehr München, Werner-Heisenberg-Weg 39, Neubiberg, Germany Abstract- A new type of NO 2 gas sensor has been made using the Floating Gate Field Effect Transistor (FG-FET) sensor system. 200 nm ZnO films were deposited on Si/Ti/Pt electrodes, which are mounted on FG-FET chips. SEM and EDX characterization methods were employed to study the surface of these films. The change in the work function of the film due to their interaction with NO 2 has been measured at various temperatures and relative humidities. The sensor exhibits high sensitivity and selectivity to 1-20 ppm NO 2 at a temperature range between room temperature and 165 C. With t 50 ~10 s, the response time is quite fast. The work function change due to physisorption or chemisorptions of a gas sensitive film atop electrode can generate a reversible potential difference. The potential difference causes a source-drain voltage change of a conventional p-channel MOSFET via a capacitive voltage divider. An equivalent electrical circuit of the FG-FET is shown in Fig. 2. Index Term-- Gas Sensor, Floating Gate Field Effect Transistor (FG-FET), Zinc Oxide, Nitrogen Dioxide. I. INTRODUCTION The existence of Nitrogen dioxide (NO 2 ) is one of the most valid indicators of poor air quality and cause acid rain, photochemical smog and corrosion. Therefore, monitoring NO 2 level is necessary for environmental monitoring as well as protection. Various types of NO x sensing materials have been reported [1,2], mostly based on the conductivity change of materials due to NO 2 exposure. However, this system requires high operating temperature (above 200 C) to obtain interaction between sensitive materials and gas molecules. The aim of this work is to introduce a new type of NO 2 gas sensor with zinc oxide (ZnO) as the gas sensitive film. The sensor uses a FET (field effect transistor) as a transducer to measure the work function change of the film and convert it into an electrical signal. A versatile design in terms of deposition method of chemically sensitive layers is the hybrid suspended gate FET developed by I. Eisele et al. [4]. An advanced fully integrated concept is the FG-FET [5]. It combines the hybrid design with the large amplification of conventional MOSFETs. Fig. 1. Schematic of the FG-FET cross section Fig. 2. Equivalent electrical circuit of the FG-FET This principle is used to detect gases that react with the sensitive film such as hydrogen sulphide [6] or that adheres to the sensitive film by physisorption such as ammonia [7]. II. EXPERIMENTAL Hybrid top electrodes have been made out of p-doped silicon. For the deposition of ZnO on the electrodes an intermediate layer is needed to ensure good adhesion. It consists of 110 nm titanium (Ti) and 250 nm platinum (Pt), which were deposited by DC sputtering. Subsequently, pure zinc (Zn) was thermally evaporated on the top electrode at a pressure of mbar in oxygen environment. The evaporation rate was maintained between nm/sec for a total film thickness of 200 nm. Finally the electrodes were mounted on FG-FET chip to build a complete NO 2 sensor. The sensing properties of the sensor against NO 2 concentrations were measured at various temperatures and relative humidity. 12 test gases were made available for the measurements. They are humidified by flowing dry synthetic air through a water bubbler at room temperature. A. Surface Characterization The surface morphology of ZnO was characterized using a Scanning Electron Microscope (SEM). The result can be seen

2 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: in Fig. 3. The surface is rough but its roughness is smaller than the air gap (~1.6 m) in order to avoid contact with the reference electrode. Fig. 4. EDX spectrum of the ZnO surface Fig. 3. SEM image of the ZnO surface The surface was also characterized also using the Energy- Dispersive X-Ray (EDX) method to determine its qualitative composition. It can be seen in Fig. 4 that the EDX spectrum is dominated by energy peaks of zinc and oxygen. It is evident that the film is ZnO. B. XPS Investigation The gas sensing mechanism on the ZnO surface was directly analysed using X-ray Photoelectron Spectroscopy (XPS). The surface was measured before and after exposing it to NO 2 gas. The corresponding XPS spectra of the surface are depicted in Fig. 5 and 6. Fig. 5. XPS spectra for the O1s region of ZnO before NO 2 gas exposure. Peak A: O - ions-peak adsorbed on the ZnO surface. Peak B: O-peak of ZnO.

3 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: Fig. 5 shows the results before NO 2 exposure. Two peaks of oxygen (O1s line) are observed. Peak A appears at a binding energy of ev with concentration of % and peak B appears at ev with concentration of %. Peak A corresponds to O - ions adsorbed onto the ZnO surface. Peak B is related to the O-peak of ZnO. Fig. 6 depicts the condition of the surface after exposing it to NO 2 at room temperature under dry conditions. The concentration of peak A increases significantly up to %, whereas the concentration of peak B decreases down to %. One possible explanation is that NO 2 adsorption on the surface generates surface acceptor states because NO 2 molecules capture electrons from the bulk conduction band of the ZnO film [8]. With further adsorption the surface becomes more negatively charged, which corresponds to increasing concentration of peak A. This causes broadening of the depletion layer near the surface and thus the change in the work function. The detection of NO 2 is consistent with the following reaction [8] ZnO ads NO 2 e NO O (1) The electrons consumed in this reaction are captured from the bulk conduction band of the ZnO film. Fig. 6. XPS spectra for the O1s region of ZnO after exposing it to 5ppm NO 2 gas at room temperature III. RESULTS AND DISCUSSION A. Temperature effect on the sensor The temperature of a sensor is one of the most important parameters, because adsorption and desorption are temperature activated processes [9]. Increasing the temperature improves the sensor s sensitivity to test gases. On the other hand, temperature has an effect on the physical properties of the film used in the sensor. Therefore, an experiment relating to the effect of temperature on the FG-FET sensor was carried out. The sensor was exposed to 10 ppm NO 2 at a temperature range between room temperature and 165 C. The thermal behavior of the sensor is described in Fig. 7. It can be seen that the sensitivity of the sensor is strongly dependent on temperature. Its sensitivity increases with increasing temperature which evident from the significantly decreasing output signal. The optimum operation is achieved at 165 C. At this temperature negligible base line changes and reversible signals to NO 2 gas exposure are observed. Fig. 7. The effect of temperature on the sensor during measurement of 10 ppm NO 2 under dry conditions. B. Concentration dependence of the sensor The concentration dependence of the sensors on NO 2 concentrations in the range of 1-20 ppm at 165 C under dry condition is shown in Fig. 8. Response time increases and the signal height decreases with increasing concentrations. A stable base line can be observed under this condition. The

4 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: signal height of -250 mv with response time t 50 of ~10 s is observed for 5 ppm NO 2 (maximum allowed concentration). t 50 is the elapsed time which is needed to achieve 50% of maximum output signal. D. Selectivity of the sensor Selectivity of the sensor was characterized by exposing it to H 2, CO 2, CO, O 2, H 2 S, SO 2, NH 3 and H 2 O vapour at 165 C under dry conditions. It can be seen in Fig. 10 that weak signal changes occur with H 2 S, SO 2 and NH 3. A significant negative signal change occurs when the sensor is exposed to NO 2 concentration. Fig. 8. Response time of the sensor to various NO 2 concentrations at 165 C under dry conditions C. Humidity effect on the sensor Humidity is a serious problem for sensors based on FG-FET device because surface current disturbs the potential measurement. In reducing the transducers signal drift due to humidity, a surface passivation process with n-octadecyltrichlorsilan (ODTS) was hence carried out [10]. However, despitethis passivation, the humidity still influences the chemical sensitive layer of the sensors. It can be seen in Fig. 9 that the sensitivity of the sensor decreases with increasing relative humidity. Interaction between zinc oxide and water vapour at 165 C generates chemisorption of OH groups on the surface. Because the chemisorption process is quite strong, high temperature is required to remove OH groups [11]. The signals remain stable under humid condition during NO 2 gas exposure. The following reactions were stipulated for an interaction of H 2 O vapour with the ZnO surface. Fig. 10. Selectivity of the sensor to 8 different gas species at 165 C under dry conditions E. Long-term stability of the sensor The investigation of the sensor s long-term for stability was carried out 1.5 month without testing after a 12-hour initial conditioning. The result gives evidence that the signal pattern of the sensor remains stable. ZnO H 2 O ZnOH OH (2) Fig. 11. The long-term stability of the sensor to 5 ppm NO 2 exposure after 1.5 months at 165 C under dry conditions. Fig. 9. Response time of the sensor to 3 ppm NO 2 exposure at 165 C under various relative humidity. IV. CONCLUSION It has been shown that work function change of the ZnO film due to NO 2 interaction can be measured using the FG-FET sensor. The sensor exhibits high sensitivity and selectivity to 1-20 ppm NO 2 at a temperature range between room temperature and 165 C. With the response time ~10 s, t 50 is quite fast. Thus we conclude that ZnO film is a potential are a promising new material as a gas sensitive film in FG-FETs for NO 2 detection at low concentration and temperature.

5 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: ACKNOWLEDGEMENTS This work was supported by a grant from Directorate General of Higher Education (DGHE) of Indonesia and Universität der Bundeswehr München. REFERENCES [1] Jun Tamaki, Atsushi Hayashi, Yoshifumi Yamamoto, Masao Matsuoka, Detection of dilute nitrogen dioxide and thickness effect of tungsten oxide thin film sensors, Sensors and Actuators, vol. B 95, pp , [2] Z. Ling, C.Leach, The effect of relative humidity on the NO 2 sensitivity of a SnO 2/WO 3 heterojunction gas sensor, Sensors and Actuators, vol. B 102, pp , [3] I. Eisele, T. Doll, M. Burgmair, Low power gas detection with FET sensors, Sensors and Actuators, vol. B 78, pp , [4] T. Doll, B. Flietner, and I. Eisele; German patent DE , [5] M. Burgmair, H.-P. Frerichs, M. Zimmer, M. Lehmann, I. Eisele, Field effect tranduscers for work function gas measurements: device improvements and comparison of performance, Sensors and Actuators, vol. B 95, pp , [6] G. Freitag, T. Knittel, W. Widanarto, I. Eisele, E. Simon, R. Pohle, A Gas FET for Hydrogen Sulfide (H 2S) Detection, Eurosensors XVIII, pp , Roma, 2004 [7] T. Knittel et. al., Combined Ammonia and Hydrogen Gas Sensor, Proc. IEEE Sensors, pp , [8] S. T. Shishiyanu, T. S. Shishiyanu, O. I. Lupan, Sensing characteristics of tin-doped ZnO thin films as NO 2 gas sensor, Sensors and Actuators, vol. B 107, pp , [9] S. Roy Morrison, The chemical physics of surfaces, Plenum Press, New York, [10] T. Knittel, Stabilisierung von Gassensoren auf FET -Basis, Ph.D thesis, UniBw München, [11] A. Karthigeyan, R.P. Gupta, K. Scharnagl, M. Burgmair, M. Zimmer, S.K Sharma, I Eisele, Low temperature NO 2 sensitivity of nanoparticulate SnO 2 film for work function sensors, Sensors and Actuators, vol. B 78, pp , 2001.