PSS-BST-2010

solidi status physica pss Phys. Status Solidi A 207, No. 4, 824–830 (2010) / DOI 10.1002/pssa.200983310 a www.pss-a.com applications and materials science pH-sensitive properties of barium strontium titanate (BST) thin films prepared by pulsed laser deposition technique 1,2 2,3 1 3 Vahe V. Buniatyan , Maryam H. Abouzar , Norayr W. Martirosyan , Jürgen Schubert , 4 2,3 ,2,3 Spartak Gevorgian , Michael J. Schöning , and Arshak Poghossian* 1 Department of Microelectronics and Biomedical Devices, State Engineering University of Armenia, 0009 Yerevan, Armenia Institute of Nano- and Biotechnologies, Aachen University of Applied Sciences, 52428 Jülich, Germany 3 Institute of Bio- and Nanosystems, Research Centre Jülich GmbH, 52425 Jülich, Germany 4 Department of Microtechnology and Nanoscience, Chalmers University of Technology, 41296 Gothenburg, Sweden 2 Received 20 October 2009, revised 17 December 2009, accepted 19 December 2009 Published online 19 March 2010 Keywords barium strontium titanate, PLD, biosensor, field effect sensor * Corresponding author: e-mail a.poghossian@fz-juelich.de, Phone: þ49 2461 612605, Fax: þ49 241 6009 53235 pH-sensitive properties of barium strontium titanate (BST) high-k thin films as alternative gate material for field-effect capacitive (bio-)chemical sensors based on an electrolyteinsulator-semiconductor system have been investigated. The BST films of different compositions (Ba0.31Sr0.69TiO3, Ba0.25Sr0.75TiO3 and Mg-doped Ba0.8Sr0.2Mg0.1Ti0.9O3) were deposited by pulsed laser deposition technique from targets fabricated by self-propagating high-temperature synthesis. The realised sensors have been electrochemically characterised by means of impedance-spectroscopy, capacitance–voltage and constant-capacitance method. The sensors possess a Nernstianlike pH sensitivity in the concentration range between pH 3 and 11 with a response time of 5–10 s. An equivalent circuit model for the BST-based capacitive field-effect sensor is discussed. ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction Because of the unique multifunctional material properties (ferroelectric, pyroelectric, piezoelectric, mirowave and electro-optic), the high permittivity, low loss and electric tuneable features, perovskite-type composite oxides of the ABO3 system have received intensive research activities in many applications [1–5]. Perovskite oxides are used in high-density dynamic random access memories (DRAM) [1], voltage-tuneable capacitors (varactors) [1–4], ferroelectric field-effect transistors [6, 7], optical memories and electro-optic modulators [1], solidoxide fuel cells [8], etc. In addition, the mixed conductive properties and high catalytic activity of perovskite oxides have been used to develop a large variety of gas and vapour sensors. Examples of realised devices include sensors sensitive to carbon monoxide and hydrocarbon [9], hydrogen [10], ethanol [11, 12] and acetone [12] vapour, humidity [13], etc. The sensing principle of these sensors is mainly related to the catalytic oxidation of the compounds to be detected in the presence of the oxygen vacancies in perovskite oxides. Due to the multifunctional material properties and large variety in the oxide composition by choosing different A and B elements, perovskite oxides are also very attractive for the creation of chemical sensors and biosensors working in liquids. For instance, pH-sensitive characteristics of perovskite oxides of different compositions (Li2xCa0.5
xTaO3 (0.05  x  0.25), lead titanate, Mg2þ-doped lead titanate, PrTiO3 and Li0.30La0.56TiO3) have been investigated in [14–19]. The suitability of La0.5Sr0.5CoO3 thin films and BaTiO3 nanotubes for the detection of hydrogen peroxide was shown in Ref. [20, 21]. The possibility of application of perovskite oxides for immuno-electrodes was demonstrated in Ref. [22]. Perovskite oxides of the barium strontium titanate (BST) composition belong to the most popular ferroelectric materials. While their ferroelectric, pyroelectric, piezoelectric, microwave and electro-optic properties have been well studied [1–4], to our knowledge, very little is known so far about the behaviour of high-k BST thin films in electrolyte solutions and their application for (bio-)chemical sensors [22–24]. In the present work, pH-sensitive properties of BST thin films of different compositions prepared by means of pulsed ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Original Paper Phys. Status Solidi A 207, No. 4 (2010) laser deposition (PLD) technique from targets fabricated by the self-propagating high-temperature synthesis (SHS) have been investigated using field-effect capacitive electrolyteBST-insulator-semiconductor (EBSTIS) structures. In contrast to the conventional high-temperature ceramic technology, the SHS method is simple, ecologically clean, wasteless, energy-saving, high productive and cost-effective. By variation of the initial mixture components and the combustion conditions, it is possible to produce multiphase materials with a given chemical and phase composition as well as to control their microstructure and properties. In addition, the high velocity of the process provides a short synthesis time. These important features of the SHS technology often result in remarkably improved material properties in comparison with the same composites prepared by conventional ceramic techniques. The possibility of application of SHS-prepared BST thin films as pH-sensitive gate material for capacitive field-effect sensors has been recently demonstrated for the first time by the authors in Ref. [23]. 2 Experimental 2 . 1 H i g h - t e m p er a t u re se l f - p r o p a g at i n g synthesis of BST materials Perovskite oxides of different compositions (Ba0.31Sr0.69TiO3, Ba0.25Sr0.75TiO3 and Mg-doped Ba0.8Sr0.2Mg0.1Ti0.9O3) have been synthesised using the SHS technological equipment schematically shown in Fig. 1 [23, 25]. In the SHS process, a highly exothermic combustion of a powder mixture is locally initiated from the sample surface by means of a heat flux. After initiation, the reaction proceeds in the mode of selfpropagation, resulting in a formation of a high-temperature front that propagates and converts the initial materials into the final high-quality product with a minimal amount of impurities due to the so-called self-purification process [26, 27]. For the SHS experiments, the initial materials (Ti, TiO2, SrCO3 and BaO2) have been milled to powder with grain sizes of about 5–10 mm, dried and thoroughly mixed. The grinding has been done by planetary milling method for 3 h under wet conditions with acetone as a milling medium. The mixture of reactant powders is then placed in a quartz-tube reactor and ignited by the heated wire. The use of oxygen as 825 an oxidant and Ti as a fuel provides combustion temperatures of 1 400–1 800 8C and wave-front propagation velocities of 1–4 mm/s. To prepare targets for the PLD process, the synthesized SHS product has been milled to powders of 0.5–5 mm, pressed into pellets at a pressure of 4.5 ton/cm2, and then heated at 1 350 8C for 4–5 h. The resulting target had a cylindrical shape with a height of 15 mm and a diameter of 15 mm. 2.2 PLD deposition of BST thin films The amorphous BST thin films (100 nm thick) of Ba0.25Sr0.75TiO3, Ba0.31Sr0.69TiO3 and Mg-doped Ba0.8Sr0.2Mg0.1Ti0.9O3 composition were prepared onto Si-SiO2 substrates (p-Si, r ¼ 5–10 Vcm; 50 nm SiO2, chip size: 10  10 mm2) by PLD technique using the targets fabricated by the SHS method. The main advantages of the PLD technique are the compatibility with silicon technology, the controlled deposition of multicomponent materials as perovskite oxides in a defined stoichiometry as well as the short deposition time due to the high growth rates [28, 29]. The BST films were deposited at 400 8C in an oxygen ambient (2  10
3 mbar) using a KrF excimer laser with a wavelength of 248 nm. The laser pulse length, frequency and energy were 20 ns, 10 Hz and 2.5 J/cm2, respectively. Before the PLD growth, a 300 nm thick Al film was deposited on the rear side of the chip as contact layer. The prepared BST layers have been physically characterised (thickness, surface morphology, composition) by means of ellipsometry, scanning-electron microscopy (SEM), Rutherford backscattering spectrometry (RBS) and X-ray diffraction analysis (XRD) methods. As an example, Fig. 2a shows the cross-sectional SEM picture of a Si-SiO2BST layer structure with a 47 nm SiO2 and a 92 nm BST film. RBS measurements (Fig. 2b) could verify the stoichiometric transfer from the original target material to the thin-film state. 2.3 Measurement setup The realised p-Si-SiO2BST capacitive field-effect structures were characterised in different pH buffer solutions by means of capacitance– voltage (C–V), constant-capacitance (ConCap) and impedance-spectroscopy (in a frequency range from 1 Hz to 1 MHz) methods using an impedance analyser (Zahner Figure 1 (online colour at: www.pss-a.com) Schematic of the technological equipment used for the SHS process. 1: quartz-tube reactor; 2: tungsten/rhenium (W/Re) thermocouple; 3: green mixture; 4: combustion front; 5: igniter; 6: signal amplifier; 7: analogue-digital convertor; 8: personal computer. www.pss-a.com ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim solidi physica a 826 status pss V. V. Buniatyan et al.: pH-sensitive properties of barium strontium titanate (BST) thin films O-ring and contacted on its front side by the electrolyte and a reference electrode (conventional liquid-junction Ag/AgCl electrode, Metrohm), and on its rear side by a gold-plated pin. For operating the sensor, a dc polarisation voltage is applied via the reference electrode to set the working point in the depletion range of the capacitance–voltage curve, and a small superimposed ac voltage with an amplitude of 20 mV is applied to measure the capacitance of the sensor. All potential values are referred to the Ag/AgCl reference electrode. The contact area of the sensor with the solution was about 0.5 cm2. The measurements have been performed in a dark Faraday cage at room temperature. For the details of the experimental setup, see e.g., Ref. [30]. 3 Results and discussion Figure 4 depicts a set of C–V curves for the p-Si-SiO2-Ba0.31Sr0.69TiO3 (a) and p-SiSiO2-Ba0.8Sr0.2Mg0.1Ti0.9O3 (b) structure, respectively, as a function of the frequency (so-called capacitance–voltage spectroscopy or frequency-dependent C–V curves) measured Figure 2 (online colour at: www.pss-a.com) Cross-sectional SEM picture of the Si-SiO2-BST layer structure (a) and results of RBS measurements (b). Elektrik). The measurement setup for the electrochemical characterisation of the sensors and schematic of the layer structure is presented in Fig. 3. For the measurements, the Si-SiO2-BST chip was mounted into a home-made measuring cell, sealed by an Figure 3 (online colour at: www.pss-a.com) Measurement setup for the electrochemical characterisation of BST-based capacitive field-effect sensors (a), and schematic of the layer structure (b). ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Figure 4 (online colour at: www.pss-a.com) Frequency-dependent C–V curves for the p-Si-SiO2-Ba0.31Sr0.69TiO3 (a) and p-Si-SiO2Ba0.8Sr0.2Mg0.1Ti0.9O3 (b) structure measured in buffer solution of pH 7 at different frequencies from 30 Hz to 1 kHz. For comparison, the C–V curve for a bare p-Si-SiO2 structure, measured at a frequency of 100 Hz, is presented, too. www.pss-a.com Original Paper Phys. Status Solidi A 207, No. 4 (2010) in Titrisol buffer solution of pH 7 at different frequencies from 30 Hz to 1 kHz. For comparison, the C–V curve of a bare p-Si-SiO2 EIS (electrolyte-insulator-semiconductor) structure, measured at a frequency of 100 Hz, is presented, too (Fig. 4a). As can be seen, similar to the metal-insulator-semiconductor or EIS capacitor, dependent on the magnitude and polarity of the applied gate voltage, Vg, the C–V curves of the BST-based sensors show three distinct regions: accumulation (Vg <
1.5 V), depletion (approximately between
1 and 0 V) and inversion (Vg > 0.5 V). However, in comparison with the bare p-Si-SiO2 structure, the C–V curves and flat-band voltage of the p-Si-SiO2-BST structure are shifted (DVfb  0.6 V) to more negative gate voltages. This is a first indication of the presence of additional positive charges in the EBSTIS structure, in particular, in the BST layer and/or at the BST/SiO2 and electrolyte/BST interfaces. In the accumulation range, the capacitance of the whole EBSTIS structure is determined by the geometrical capacitance of the gate insulator, i.e. the capacitances of the SiO2 and BST in series. For a capacitive field-effect sensor application, the more useful range represents, however, the depletion region of the C–V curve. Because the BST films usually exhibit a certain ionic or mixed conductivity, they can be described as a parallel network of the BST geometric resistance, RBST, and the capacitance, CBST, which is in series with the capacitance of the bare p-Si-SiO2 structure. Due to the high dielectric constant (e.g., a dielectric constant value of 80 has been reported for 150 nm thick amorphous Ba0.3Sr0.7TiO3 films at room temperature [5]), and therefore, higher capacitance of the BST films, the maximum capacitance in the accumulation region is not significantly influenced by the BST layer, compared to the bare p-Si-SiO2 structure (36 nF for a 50 nm SiO2 layer, see Fig. 4a). On the other hand, with increase in the frequency, the C–V curves slightly shift along the capacitance axis towards smaller capacitance values that is due to the presence of the RBST resistance. Similar behaviour has also been observed for EIS structures covered with an additional ion-sensitive membrane [31–35]. This suggestion is supported by impedance spectroscopy measurements. As an example, Fig. 5a shows impedance spectra for a pSi-SiO2-Ba0.31Sr0.69TiO3 structure recorded in a pH 7 buffer solution in the accumulation, depletion and inversion range at polarisation voltages of
3.5,
1 and 2 V, respectively. A linear behaviour has been observed at low frequencies, which bends over to a plateau of a constant impedance at frequencies>1 kHz. In addition, at frequencies of less than 1 kHz, the impedance is increased by passing from the accumulation to the depletion region and even further raised in case of the inversion region. Such behaviour of the impedance curves can be explained due to the decrease in total capacitance of the sensor structure. However, at very low frequencies (<20–30 Hz), the impedance of the sensor in the inversion range recovers to values that correspond to the impedance in the accumulation range, which can be www.pss-a.com 827 Figure 5 Impedance spectra for a p-Si-SiO2-Ba0.31Sr0.69TiO3 structure recorded in pH 7 buffer solution in the accumulation, depletion and inversion range (a), and simplified equivalent circuit of an EBSTIS structure (b). BST is described as parallel network of the BST geometric resistance, RBST, and the capacitance, CBST. attributed to the low frequency C–V behaviour of the sensor, where the total capacitance of the sensor is again increased passing from the depletion to the inversion range (see Fig. 4). The complete ac equivalent circuit of an EBSTIS system is complex and combines components, like the bulk resistance and space-charge capacitance of the Si, capacitance of the SiO2, resistance and capacitance of the BST film, double-layer capacitance at the electrolyte/BST interface, resistance of the electrolyte solution, and impedance of the reference electrode. However, for usual values of an insulator thickness of 30–100 nm and in high ionic-strength solutions (>10
4 M), the interferences from several components can be ignored, and the equivalent circuit of an EBSTIS structure can be simplified as a series connection of the space-charge capacitance of the semiconductor, insulator capacitance and impedance of the BST film. Figure 5b shows a simplified equivalent circuit of an EBSTIS structure, where the BST layer is described as a parallel network of the BST geometric resistance, RBST, and the capacitance, CBST. Then, the experimentally measured capacitance of the EBSTIS structure, Cmeas, can be expressed as: Cmeas ¼ C 2 1 þ R2BST CBST v2
; 2 1 þ R2BST CCBST þ CBST v2 (1) where C is the capacitance of the original EIS structure without BST layer, v ¼ 2pf and f is the measuring frequency. Thus, the measured capacitance is affected by the BST ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim solidi physica a 828 status pss V. V. Buniatyan et al.: pH-sensitive properties of barium strontium titanate (BST) thin films Figure 6 Typical set of C–V curves for BST-based field-effect sensors with Ba0.31Sr0.69TiO3 (a) and Ba0.8Sr0.2Mg0.1Ti0.9O3 (b) film as pH-sensitive material measured in buffer solutions with different pH values from pH 3 to 11. resistance. As it has been demonstrated in Ref. [31–35], generally, any series resistance (e.g., bulk resistance of lowionic strength electrolyte, resistance of Si substrate, backside contact resistance as well as resistance of the reference electrode) can affect the measured capacitance and deform the C–V curves. Figure 6 depicts a typical set of C–V curves for sensors with a Ba0.31Sr0.69TiO3 and a Ba0.8Sr0.2Mg0.1Ti0.9O3 film measured in buffer solutions with different pH values from pH 3 to 11. As expected, the C–V curves of the EBSTIS structure are pH-dependent due to the contribution of the potential at the BST/electrolyte interface. With decrease in pH, the C–V curves are shifted along the voltage axis in the direction of a more negative flat-band voltage due to the change of the additional potential drop at the electrolyte/BST interface. A similar behaviour has been observed for an EBSTIS sensor with a Ba0.25Sr0.75TiO3 film as pH-sensitive gate material. The EBSTIS sensors with both Ba0.25Sr0.75TiO3 and Ba0.31Sr0.69TiO3 films show an average pH sensitivity of 46–50 mV/pH in the range of pH 3–11 (the pH sensitivity of the sensors was evaluated from the linear region of the ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim C–V curves at 60% of the maximum capacitance). For some sensors, a pH sensitivity of 54–56 mV/pH has been observed that is comparable with pH-sensitivity values reported for Si3N4 (46–56 mV/pH), Al2O3 (49–57 mV/pH) and Ta2O5 films (55–59 mV/pH), which have often been utilised as pH-sensitive transducer material in ionsensitive field-effect transistors [35–40]. The pH sensitivity of Ba0.8Sr0.2Mg0.1Ti0.9O3 films was 35–40 mV/pH in the range from pH 3 to 11 that is comparable with the average pH sensitivity reported for SiO2 (37 mV/pH in the range of pH 3–9 [35]). The smaller pH sensitivity of the Ba0.8Sr0.2Mg0.1Ti0.9O3 layer in comparison with Ba0.25Sr0.75TiO3 and Ba0.31Sr0.69TiO3 films might be explained due to the partial replacement of both the lattice and surface Ti4þ ions by Mg2þ ions [41]. The mechanism of the pH sensitivity of BST films can be explained by the protonation/deprotonation of the hydroxyl groups at the BST surface according to the site-binding model (see e.g., Ref. [35] and references there). The resulting pH-dependent electrical surface charge of BST leads to a modulation of the space-charge capacitance in the Si, thus generating a pH-dependent sensor signal that in fact, was observed during the C–V measurements. Figure 7 exemplarily demonstrates the dynamic pH response of an EBSTIS sensor with a Ba0.31Sr0.69TiO3 (a) and a Ba0.8Sr0.2Mg0.1Ti0.9O3 (b) layer recorded in different pH buffer solutions at a frequency of 100 Hz. In this experiment, the capacitance of the EBSTIS sensor has been kept at a fixed value within the depletion region of the C–V curve (a working point was chosen at 60% of the maximum capacitance) using a feedback-control circuit, and the pHdependent signal changes were directly recorded. As can be seen, the sensor signal shows a clear dependence on the pH value of the solution within the loop cycle of pH 7 ! 13 ! 11 ! 13 ! 3 ! 13 (a) and pH 7 ! 3 ! 11 ! 3 ! 11 (b). The response time (t90%) was about 5–10 s. In contrast to pH sensors based on sputtered Ba0.7Sr0.3TiO3 films reported in Ref. [24], which were almost insensitive at pH > 10, our structures are pH-sensitive up to a pH value of 13 (see Fig. 7a). On the other hand, in comparison with often used pH-sensitive materials such as Si3N4 or Ta2O5, the hysteresis of the BST sensors was relatively large. In addition, surprisingly, the pH sensitivity values evaluated from the ConCap curves were smaller than that from the C–V curves. The performed experiments do not allow to explain the origin for this phenomenon. Since such an effect has not been observed for most of the pH-sensitive materials, like SiO2, Ta2O5, Al2O3, it can be attributed to possible polarisation effects in the bulk BST, BST/ electrolyte or BST/SiO2 interfaces and/or relatively large hysteresis within the loop cycle. In further works, this effect will be studied in detail. 4 Conclusions The obtained results demonstrate the potential of PLD-prepared BST films as alternative gate material for capacitive field-effect pH sensors based on an electrolyte-high-k dielectric-insulator-semiconductor www.pss-a.com Original Paper Phys. Status Solidi A 207, No. 4 (2010) 829 References Figure 7 ConCap response of an EBSTIS sensor with Ba0.31Sr0.69TiO3 (a) and Ba0.8Sr0.2Mg0.1Ti0.9O3 (b) film recorded in buffer solutions of different pH values. system. The main advantages of the realised sensors are their simple structure, and the ecologically clean and costeffective synthesis of the BST targets of desired composition and doping elements by means of the SHS technology. Future experiments will focus on a detailed investigation of the effects of composition, crystalline structure (amorphous, polycrystalline), post-treatment and oxygen vacancies in the BST films on the device performances. Although the obtained values of pH sensitivity of BSTbased sensors were comparable or smaller than in case of Ta2O5 films [35–40], nevertheless, BST films are very attractive in the (bio-)chemical sensor field as catalytically active multifunctional materials, for instance, for the development of so-called high-order multisensor systems [36, 37, 39] that is based on the same material but different transducer concepts (e.g., field-effect, amperometric electrodes, changes in work function and/or dielectric constant). Acknowledgements The authors gratefully thank H. P. Bochem for technical support. V. B. gratefully thanks the German Academic Exchange Service (DAAD) for financial support. www.pss-a.com [1] N. Setter, D. Damjanovic, L. Eng, G. Fox, S. Gevorgian, S. Hong, A. Kingon, H. Kohlstedt, N. Y. Park, G. B. Stephenson, I. Stolitchnov, A. K. Tagantsev, D. V. Taylor, T. Yamada, and S. Streiffer, J. Appl. Phys. 100, 051606 (2006). [2] A. K. Tagantsev, V. O. Sherman, K. F. Astafiev, J. Venkatesh, and N. Setter, J. Electroceram. 11, 5 (2003). [3] H. V. Alexandra, C. Berbecaru, A. Ioachim, M. I. Toacsen, M. G. Banciu, L. Nedelcu, and D. Ghetu, Mater. Sci. Eng. B 109, 152 (2004). [4] S. Gevorgian and A. Vorobiev, J. Appl. Phys. 99, 124112 (2006). [5] T. Yamada, V. O. Sherman, A. Nöth, P. Muralt, A. K. Tagantsev, and N. Setter, Appl. Phys. Lett. 89, 032905 (2006). [6] T. Sato, K. Shibuya, T. Ohnishi, K. Nishio, and M. Lippmaa, Jpn. J. Appl. Phys. 46, L515 (2007). [7] M. Brandt, H. Frenzel, H. Hochmuth, M. Lorenz, and M. Grundmann, J. Vac. Sci. Technol. B 27, 1789 (2009). [8] S. J. Skinner, Fuel Cells Bull. 4, 6 (2001). [9] F. L. Brosha, R. Mukundan, D. R. Brown, F. H. Garzon, J. H. Visser, M. Zanini, Z. Zhou, and E. M. Logothetis, Sens. Actuators B 69, 171 (2000). [10] X. Chen, W. Zhu, and O. Tan, Mater. Sci. Eng. B 77, 177 (2000). [11] S. Zhao, J. K. O. Sin, B. Xu, M. Zhao, Z. Peng, and H. Cai, Sens. Actuators B 44, 83 (2000). [12] A. S. Poghossian, H. V. Abovian, P. B. Avakian, S. H. Mkrtchian, and V. M. Haroutunian, Sens. Actuators B 4, 545 (1991). [13] G. Sharma and S. Agarwal, Sens. Actuators B 85, 205 (2002). [14] Q. N. Phan, C. Bohnke, J. Emery, O. Bohnke, F. Le Berre, M.-P. Crosnier-Lopez, J.-L. Fourquet, and P. Florian, Solid State Ion. 176, 495 (2005). [15] S.-S. Jan, Y.-C. Chen, J.-C. Chou, C.-C. Cheng, and C.-T. Lu, Jpn. J. Appl. Phys. 41, 942 (2002). [16] S.-S. Jhan, J.-L. Chiang, Y.-C. Chen, J.-C. Chou, and J.-F. Su, Ferroelectrics 383, 111 (2009). [17] S.-S. Jan, Y.-C. Chen, and J.-C. Chou, Sens. Actuators B 108, 883 (2005). [18] T.-M. Pan and K.-M. Liao, Sens. Actuators B 133, 97 (2008). [19] M. Roffat, O. Noel, O. Soppera, and O. Bohnke, Sens. Actuators B 138, 193 (2009). [20] D. T. V. Anh, W. Olthuis, and P. Bergveld, Sens. Actuators B 103, 165 (2004). [21] X. He, C. Hu, Y. Xi, B. Wan, and C. Xia, Sens. Actuators B 137, 62 (2009). [22] X. Y. Fang, O. K. Tan, Q. Wei, M. W. Yao, and S. C. Tjin, Sens. Actuators B 119, 78 (2006). [23] V. Buniatyan, N. Martirosyan, M. Abouzar, J. Schubert, W. Zander, S. Gevorgian, M. J. Schöning, and A. Poghossian, Proceedings Sensors-2009 Conference, Vol. 2, 26–28 May 2009, Nürnberg, Germany, pp. 317–322 [24] C.-Y. Chen, J.-C. Chou, and H.-T. Chou, Jpn. J. Appl. Phys. 48, 045501 (2009). [25] N. W. Martirosyan, V. V. Buniatyan, P. B. Avakyan, V. R. Khachatryan, and T. V. Vandunts, Proceedings IX Int. Symp. on SHS, Dijon, France, 1–5 July 2007, T3-P05. [26] A. Varma, A. S. Rogachev, A. S. Mukasyan, and S. Hwang, Adv. Chem. Eng. 24, 79 (1998). ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim solidi physica a 830 status pss V. V. Buniatyan et al.: pH-sensitive properties of barium strontium titanate (BST) thin films [27] R. Licheri, S. Fadda, R. Orru, G. Caoa, and V. Buscaglia, J. Eur. Ceram. Soc. 27, 2245 (2007). [28] M. J. Schöning, Yu. G. Mourzina, J. Schubert, W. Zander, A. Legin, Yu. G. Vlasov, and H. Lüth, Sens. Actuators B 78, 273 (2001). [29] Ch. Buchal, L. Beckers, A. Eckau, J. Schubert, and W. Zander, Mater. Sci. Eng. B 56, 234 (1998). [30] M. J. Schöning, D. Brinkmann, D. Rolka, C. Demuth, and A. Poghossian, Sens. Actuators B 111/112, 423 (2005). [31] A. Demoz, E. M. J. Verporte, and D. J. Harrison, J. Electroanal. Chem. 389, 71 (1995). [32] A. Poghossian, D.-T. Mai, Yu. Mourzina, and M. J. Schöning, Sens. Actuators B 103, 423 (2004). [33] Yu. Mourzina, D.-T. Mai, A. Poghossian, Yu. Ermolenko, T. Yoshinobu, Yu. Vlasov, H. Iwasaki, and M. J. Schöning, Electrochim. Acta 48, 3333 (2003). ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [34] P. Fabry and L. Laurent-Yvonnou, J. Electroanal. Chem. 286, 23 (1990). [35] A. Poghossian and M. J. Schöning, Silicon-Based Chemical and Biological Field-Effect Sensors, in: Encyclopedia of Sensors, edited by C. A. Grimes, E. C. Dickey, and M. V. Pishko, Vol. 9 (American Scientific Publishers, Stevenson Ranch, USA, 2006), pp. 463–733. [36] A. Poghossian and M. J. Schöning, Electroanalysis 16, 1863 (2004). [37] A. Poghossian, H. Lüth, J. W. Schultze, and M. J. Schöning, Electrochim. Acta 47, 243 (2001). [38] A. Poghossian, A. Baade, H. Emons, and M. J. Schöning, Sens. Actuators B 76, 634 (2001). [39] A. Poghossian, J. W. Schultze, and M. J. Schöning, Sens. Actuators B 91, 83 (2003). [40] A. Poghossian, Sens. Actuators B 7, 367 (1992). [41] S. Xu, Y. Qu, and C. Zhang, J. Appl. Phys. 106, 014107 (2009). www.pss-a.com