phys. stat. sol. (c) 4, No. 6, 2059– 2062 (2007) / DOI 10.1002/pssc.200674371
Room temperature gas sensor based on porous silicon/metal oxide structure V. M. Arakelyan, Kh. S. Martirosyan, V. E. Galstyan, G. E. Shahnazaryan, and V. M. Aroutiounian* Department of Physics of Semiconductors and Microelectronics, Yerevan State University, 375025 Yerevan, Armenia Received 17 March 2006, revised 15 September 2006, accepted 15 November 2006 Published online 9 May 2007 PACS 07.07.Df, 73.40.–c, 84.37.+q N-type TiO2–x and In2O3 · SnO2 thin films were deposited onto p-type porous silicon layer which was formed by common electrochemical anodization. The current-voltage characteristics of obtained structures and sensitivity to different concentrations of hydrogen in air were studied. Measurements were carried out at room temperature. As shown results of measurements, an exponential growth of the current in forward branch of the current-voltage characteristics of the device made of TiO2–x layer was detected. Higher sensitivity to hydrogen of the TiO2–x-porous silicon sensor in comparison to structure made of In2O3 · SnO2 film was detected at room temperature (without preheating of work body of the sensor). © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction The use of hydrogen as a potential energy currier in the transportation, residential and industrial sectors as well as for other applications in different fields made the investigations of hydrogen sensors very promising. Unfortunately, when the hydrogen volumetric concentration in air ranged between 4.65 and 93.9 %, the possibility of the explosion is exist. Therefore, the development of reliable and safe hydrogen-gas sensors becomes of great importance. In recent years, semiconductor hydrogen sensors are the subject of intensive study (see review-paper [1]). Note that in the most of such gas sensors different concentration of hydrogen changes values of the current and voltage, which can be easily measured. Therefore, relatively simple circuit was adjusting for the formation of the gas detector. Today metal oxide based gas detectors (in particularly, hydrogen) have found large application as they have several advantageous features such as a simplicity in device structure, low cost of fabrication, stability, robustness in practical applications, and adaptability to a wide variety of reductive or oxidative gases. For example, in the past, TiO2 thin film hydrogen sensors have been proposed [2–7], where particles of a catalytic metal were in contact with TiO2. But sensors made of metal oxides have shown high sensitivity to gases usually after pre-heating of work body up to high temperatures (200-700 °C). For further integration, it is preferable to make gas sensors on silicon, which is the main material used in microelectronics, and works without preheating of the work body of sensor in order to avoid limitations related to high temperatures. Unfortunately, single- and multicrystalline silicon do not have good sensitivity to different gases. But silicon can be easily prepared in porous form. The latter offers several advantages: porous silicon has a large specific surface area (200-1000 m2/cm3), which provides high sensitivity to external agents (for example, to hydrogen [8, 9]); it can be used to produce a low cost, compact *
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V. M. Arakelyan et al.: Gas sensor based on porous silicon/metal oxide structure
sensor system on a silicon chip, where both the sensing part (the porous silicon) and the transducer part can be integrated. At the same time it is well known that the parameters of porous silicon degrade with time, which means that porous silicon gas sensors cannot fulfil the durability requirement. In this paper we propose a method for protecting protect the porous silicon layer from ambient air, in order to prevent it from further degradation with time. We decide to combine the stability in time of a metal oxide film covered with fine particles of Pt with sensitivity of a large specific area of the porous silicon layer.
2 Experimental Porous silicon films were grown on the surface of p-type Si. Before anodization, samples were boiled in isopropyl alcohol, immersed into the HF aqueous solution for the native oxide removing from the silicon surface, washed in distilled water and ethyl alcohol and then dried in air. The anodization current density and anodization time were 10-50 mA/cm2 and 10–600 sec, correspondingly. The following electrolyte HF (48%):C2H5OH (96%) in ratio 1:1 by volume was used for the porous silicon (PS) formation. The anodization was carried out in the Teflon electrochemical cell with Pt cathode. After the porous silicon layer formation, samples were immersed into ethyl alcohol, dried in air and placed in the electron-beam evaporation chamber. N-type TiO2 and n-type In2O3·SnO2 ceramic pills were used as a target, which were formed from a powder of these materials. Then, the heat treatment was carried out to form final ceramic samples at the temperatures 1100 °C and 1200 °C for TiO2 and In2O3·SnO2, respectively. Thereafter ceramic pill-targets were boiled in acetone, washed in hot distilled water and finally cleaned in ethyl alcohol. TiO2–x or In2O3·SnO2 thin films were evaporated from ceramic target on the surface of porous silicon under the following conditions: electron beam current was equal to 15 mA, target bias was 2 kV and process duration was 1 hour for TiO2–x samples. Electron beam current was 10 mA, bias on the target 1.5 kV and process duration 1 minute for In2O3·SnO2 samples. Then, very thin platinum catalytic film in the form of fine particles as well as gold electrical contacts were deposited for further measurements by ion-beam sputtering over the metal oxide – porous silicon – p-Si structures (Fig. 1). It is well known that Pt layer acts as catalyst for catalytic splitting of regenerative gases. Therefore, when gas molecules pass through Pt layer, they split into atoms or ions, which, in turn, enhance the interaction between the sensitive surface and gas. Hence, the presence of Pt layer leads to improvement of gas sensitivity and response time.
Au electrical contacts Pt catalytic layer Metal oxide layer Porous silicon layer
p-Si Fig. 1 Schematic diagram of gas sensor.
3 Results and discussion Current-voltage characteristic and sensitivity to different concentration of hydrogen of the samples were examined. The gas sensitivity was defined as the ratio ( Rgas / Rair ) of the resistance in hydrogen gas ( Rgas ) to that in air ( Rair ). Measurements of sensitivity to different concentrations of hydrogen gas carried out in the testing chamber at room temperature. All measurements were carried out using a computer controlled system.
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Results of measurements of the current-voltage characteristics and gas sensitivity are shown in the Fig. 2 [a) and b)] and Fig. 3 [a) and b)], respectively. As can be clearly seen from Fig. 2b, current-voltage characteristics of PS/TiO2–x structures are typical for a diode. It is evident from Fig. 3a and 3b that the PS/TiO2–x samples are characterized with higher sensitivity to hydrogen in comparison to the PS/InO3٠SnO2 samples. Sensitivity of structures without Pt particles was significantly less in comparison to the structure with a small amount of a catalytically active metal. The existence of a large specific surface area of porous silicon leads to repetition cavities by a metal oxide film during evaporation, which is envisaged some textured surface of TiO2–x films and an increase in their specific area. TiO2–x forms such a heterojunction with porous silicon, which provides better sensitivity to hydrogen when compared to an In2O3·SnO2 layer. As mentioned above, the properties of porous silicon layer are subject to degradation with time which in turn leads to worsening of device parameters. To ensure that degradation processes in porous silicon layer are negligible, when the porous silicon layer is covered by a metal oxide layer, we have carried out continuous measurements of the current-voltage characteristics and sensitivity of the samples, every week, for six month. Results of measurements indicated that a large changes in both measured characteristics are absent, which implies that the evaporated metal oxide layers on the top of porous silicon layer are effectively protecting it from ambient condition. This implies that it is possible to obtain samples which are sensitive to hydrogen at room temperature. They can be characterized by their durability. 1.2
a)
I, mA
1 0.8 0.6 0.4 0.2 -10
-7.5
-5
-2.5 -0.2
U, V 2.5
5
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0.3 I, mA
b)
0.25 0.2 0.15 0.1 0.05 -4
-3
-2
-1
-0.05
U, V 1
2
3
4
-0.1 Fig. 2 Current-voltage characteristic of the a) porous silicon/In2O3·SnO2 structure, b) porous silicon/TiO2–x structure.
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V. M. Arakelyan et al.: Gas sensor based on porous silicon/metal oxide structure
Sensitivity, Rgas/Rair
1. a)
1. 1 0. 1
2
3
4
5
3
Hydrogen concentration, *10 ppm
Sensitivity, Rgas/Rair
3 b)
2.5 2 1.5 1 1
2
3
4
5
3
Hydrogen concentration, *10 ppm Fig. 3 Sensitivity of the a) porous silicon/In2O3·SnO2 structure, b) porous silicon/TiO2–x structure versus hydrogen gas concentration.
4 Conclusion Hydrogen gas sensors based on porous silicon/TiO2–x and porous silicon/In2O3·SnO2 structures were realized. The sensors work at room temperature. Measurements were carried out of current-voltage characteristics and sensitivity of the two structures. As shown in the results of measurements, the porous silicon/TiO2–x structure is showing better sensitivity to hydrogen in comparison to porous silicon/In2O3·SnO2 structure. Acknowledgements This work has been supported by the ISTC A-1232 and CRDF-IPP ARE2-10838-YE-05 grants as well as by “Semiconductor Nanoelectronics” Armenian National Program. Authors are also thankful to Dr. K.J. Touryan from American University of Armenia for revision of English.
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