Amorphous InGaZnO4 films: Gas sensor response and stability

Sensors and Actuators B 171–172 (2012) 1166–1171

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Amorphous InGaZnO4 films: Gas sensor response and stability Dae Jin Yang a,1 , George C. Whitfield a , Nam Gyu Cho b , Pyeong-Seok Cho a,2 , Il-Doo Kim b , Howard M. Saltsburg a , Harry L. Tuller a,∗ a b

Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 8 May 2012 Received in revised form 18 June 2012 Accepted 21 June 2012 Available online 1 July 2012 Keywords: Semiconducting metal oxides Hydrogen gas sensor NOx gas sensor Amorphous semiconductor

a b s t r a c t The response characteristics of amorphous-InGaZnO4 (a-IGZO4 ) thin films toward reducing/oxidizing gases (H2 /NO2 ), at sensor operating temperatures, are reported for the first time. The lack of grain boundaries eliminates a major source of electrical, microstructural and chemical inhomogeneities associated with polycrystalline semiconducting metal oxides (SMOs), rendering a-IGZO4 a highly promising model sensor system. Gas sensor tests were carried out in the temperature range of 200–400 ◦ C by monitoring changes in DC resistance during cyclic exposure to trace concentrations (between 1.25 and 50 ppm) of H2 or NO2 in dry air. The response (S) to H2 was found to go through a temperature maximum (e.g. S ∼ 0.7 at 350 ◦ C for pH2 = 12.5 ppm) that value being a function of pH2 . The response to NO2 , on the other hand, decreased with increasing temperatures with the highest recorded values at 200 ◦ C (e.g. S ∼ 33 at 200 ◦ C for pNO2 = 5 ppm). The response followed an approximate power law dependence on gas partial pressure (p), S = Apˇ , with ˇ taking on values of ∼0.5–1.0 as temperature increased from 200 to 400 ◦ C. Response times were found to range from 10 s to greater than 1000 s as temperature decreased. The hysteretic behavior exhibited by a-IGZO films between 150 and 400 ◦ C, under temperature sweep conditions, is attributed to kinetically limited adsorption/desorption and reaction rates. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Resistive-type gas sensors based on semiconducting metal oxides (SMOs) have attracted considerable interest for environmental monitoring, air-quality control, and detection of explosive and toxic gases, given their high sensitivity, simple structure, ready modification, and low-cost fabrication [1]. These SMO sensors normally consist of polycrystalline oxides and operate between 200 and 400 ◦ C, where they undergo changes in resistance on exposure to target gases. Although the sensors of this type have long been used in practice, much remains unclear about their operation, given that performance depends simultaneously on a number of sensor parameters including grain size, degree of crystallinity, porosity, dopant type and concentration, surface chemistry stoichiometry, orientation, and catalytic reactivity. [2–4]. Traditional polycrystalline sensor materials such as SnO2 , In2 O3 , WO3 , TiO2 , and ZnO are prepared by post-annealing/sintering processes in excess of 600 ◦ C prior to actual use to insure device stability.

∗ Corresponding author. E-mail address: [email protected] (H.L. Tuller). 1 Present address: Materials Research Laboratory, Samsung Advanced Institute of Technology, Samsung Electronics Corporation, Yongin-Si, Gyeonggi-Do 446-712, Republic of Korea. 2 Present address: Development Group 3, Cheil Industries INC., Uiwang-Si, Gyeonggi-Do 437-711, Republic of Korea. 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.06.057

This serves to minimize subsequent grain growth and bulk stoichiometry changes at the sensor operating temperatures. The majority of SMO gas sensor studies have been conducted based on the existence of back-to-back Schottky barriers between the semiconductor grains whose barrier height is sensitive to the gases that adsorb on the pore walls of the SMOs. The sensitivity is observed to increase rapidly with decreasing grain size as the grain radius approaches the depletion width of the Schottky barriers [5]. Given the complex relationship between gas sensor sensitivity and microstructure in these polycrystalline ceramics, it is clear that an amorphous material with no grain/grain boundary network could serve as a highly useful model system for more in-depth studies of sensing phenomena in SMO based sensors. Recently, amorphous oxide semiconductors (AOSs) such as the In–Ga–Zn–O system (a-IGZO), have been extensively studied as the active, semiconductive channel in transparent/flexible thin film transistors (TFTs) due to their high carrier mobility and lowtemperature compatible fabrication processes [6]. The amorphous nature of a-IGZO films has been reported to remain stable up to relatively high temperatures (∼500 ◦ C). The lack of grain boundaries eliminates a major source of electrical, microstructural and chemical inhomogeneities associated with polycrystalline SMOs. This makes a-IGZO a highly promising model sensor system. At the same time, it has been reported that the properties of a-IGZO TFTs are unstable and highly sensitive to environmental conditions such as temperature, oxygen partial pressure and humidity [7–9].

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2. Experimental The a-IGZO thin films were deposited by RF-magnetron sputtering, using a polycrystalline InGaZnO4 target, at room temperature, onto alumina substrates, fitted with gold interdigitated electrodes (Au-IDEs). The film thickness was ∼100 nm. Au-IDEs were prepared by thermal evaporation through a stainless steel mask, and were comprised of a 100 nm thick gold layer deposited onto a 10 nm thick titanium adhesion layer (16 fingers, 8 mm long and 200 ␮m wide, spaced 200 ␮m apart). As-deposited films were annealed at different temperatures for 1 h in air. The films were examined by X-ray diffractometry (XRD, Rigaku D/MAX-RC using CuK␣ radiation). The morphology and elemental composition of the films were examined by field emission scanning electron microscopy and energy dispersive X-ray analysis (FE-SEM, EDX, Philips XL30). The assembled a-IGZO sensors were placed in a quartz tube chamber and gas sensing tests were carried out in the temperature range of 200–400 ◦ C by monitoring changes in the DC resistance during cyclic exposure to trace concentrations (between 1.25 and 50 ppm) of H2 or NO2 in dry air. To achieve desired gas compositions, certified pre-mixed gas mixtures containing the test gas in dry air were mixed with clean dry air using mass flow controllers (MKS). The total gas flow rate (test gas plus balance gas) was kept constant (200 ml/min) during these tests. To ensure a stable and reproducible baseline at the beginning of each test, the specimens were equilibrated under the baseline conditions, viz. dry air, at the test temperatures for approximately 9 h prior to the exposure to the test gases.

ZnO In2O3 ZnGa2O 4 InGaZn2O5 o

annealed at 850 C Intensity (a.u.)

Therefore, most recent a-IGZO TFTs are covered by passivation layers made of other oxide layers, to insure stability [10]. However, this drawback of environmental instability in TFTs suggests that the active a-IGZO channel material may serve as an excellent candidate for gas sensor applications. To date, there have been no reports in the literature relating to a-IGZO thin films operated specifically as gas sensors. The authors report, for the first time, the response characteristics of a-IGZO thin films toward reducing/oxidizing gases (H2 /NO2 ).

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2 theta (degree) Fig. 1. XRD patterns for IGZO thin films annealed at various temperatures.

atomic ratio of the annealed a-IGZO film, analyzed by EDX, was found to be In:Ga:Zn = 1.25:1.28:1.00 as shown in Fig. 2(b). Fig. 3 shows the DC conductivity () change of an a-IGZO film during ramp up and down in temperature from 25 ◦ C to 400 ◦ C to 25 ◦ C at a ramp rate of 1 ◦ C per minute. Prior to this experiment, the film was annealed at 450 ◦ C for 1 h and then cooled to room temperature. The conductivity of the IGZO film can be, based on its temperature dependence, assigned to three temperature regions (I–III). In region I (60–150 ◦ C), a well defined thermally activated process, independent of the magnitude and direction of the ramp

3. Results and discussion In order to establish the temperature at which IGZO films transform from amorphous to crystalline phase (in this case losing the desired film morphology of the present study), different annealing processes at various temperatures were conducted. Fig. 1 shows the XRD patterns of the as-deposited and annealed IGZO films at temperatures ranging from 450 to 850 ◦ C. The IGZO films annealed below 650 ◦ C remained amorphous, while the film annealed at 850 ◦ C showed mixed diffraction peaks attributable to ZnO (JCPDS No. 36-1451), In2 O3 (JCPDS No. 22-0336), ZnGa2 O4 (JCPDS No. 381240) and InGaZn2 O5 (JCPDS No. 40-0252) polycrystalline phases, consistent with other reports [11]. Considering that the operating temperatures of conventional gas sensors are in the 200–400 ◦ C range, the amorphous nature of IGZO films appears to be sufficiently robust to stand up to standard gas response tests. It is generally recognized that thermal annealing at temperatures higher than the sensor operating temperatures improves the long-term stability and reliability of polycrystalline sensor materials. Likewise, in this study, films were annealed at 450 ◦ C for 1 h in air prior to gas response tests. Fig. 2(a) presents a SEM image of a-IGZO thin film deposited onto an alumina substrate following thermal annealing. The a-IGZO surface is observed to be smooth without grain/grain boundaries, unlike polycrystalline films. Note that the source of the morphology seen in the image derives from the rough alumina substrate. The

Fig. 2. (a) FE-SEM image of a-IGZO thin film on Al2 O3 substrate and Au-IDE pattern (inset) and (b) EDX spectrum.

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Fig. 4. Dynamic sensing transient of an a-IGZO thin film as a function of H2 and NO2 concentrations at 400 ◦ C and 200 ◦ C, respectively.

rate, is evident, likely due to thermal generation of carriers in aIGZO. Above 150 ◦ C, one observes a clear hysteresis in the plot of log() versus inverse temperature (1/T). The area of the hysteresis loop depends on the temperature sweep rate, with a larger area of hysteresis occurring at a higher ramp rate. The source of the hysteresis loop is likely related to the competing reactions of adsorption and desorption of oxygen, and the thermally activated semiconductor charge carrier generation. A maximum in conductance is observed at approximately 250 ◦ C, at which point accelerated chemisorption of oxygen is believed to deplete free electrons in the semiconductor conduction band, resulting in a drop in conductivity consistent with IGZO as an n-type semiconductor. Close to 375 ◦ C, we observe a minimum in conductance, at which point thermally induced desorption of oxygen begins to become important, resulting in a net decrease in electronic charge trapped at the surface and a corresponding increase in free carrier density in the a-IGZO. During the ramp down in temperature in region II, oxygen desorption is apparently sluggish, so the overall conductivity is lower at a given temperature than during heating. By the time region I is reached, one enters back into a region dominated by thermal generation of charge carriers leading to the thermally activated region characterized by Ea = 0.52 eV. In the region I (60–150 ◦ C), one can estimate the position of the Fermi level (EF ) from the film conductance (G = 3.06 × 10−7 S at 128 ◦ C), film dimensions (100 nm thick film and IDE geometry), the effective density of states (NC = 5.21 × 1018 cm−3 ) and the drift mobility ( = 1–10 cm2 V−1 s−1 ) by taking Boltzmann’s approximation for the evaluation of EF ( = ne; n = NC exp{−(EC − EF )/kB T }) [12]. The resultant EC − EF value (0.36–0.43 eV) corresponds roughly to the Ea value (0.52 eV) obtained from the linear curve fitting in Fig. 3, consistent with the expectation that the conductivity change in the low temperature region is caused mainly by the thermal excitation of electrons to the conduction band. The 0.09–0.16 eV difference in the calculated and measured energies are not surprising, given the rough approximations made in the calculations. At higher temperatures (>150 ◦ C), the hysteresis in conductivity is observed. At these temperatures, the surface chemistry should be strongly influenced by adsorption/desorption of water and oxygen species in various forms (H2 O, OH, O2 , O). The anomalous conductivity decrease with increasing

temperature observed in the range of 250–400 ◦ C (region II in Fig. 3) is similar to that observed for other n-type crystalline SMOs. For example, a similar conductivity anomaly appeared for a crystalline Pd-SnO2 sensor, but only in humid atmosphere and disappeared when operated in relatively dry air. The response in humid air was explained by the trapping of additional electrons from the conduction band of SnO2 by dissociation of H2 O on the surface of the device (O− + H2 O + e− → 2OH− ) [13,14]. Another explanation for this phenomenon was ascribed to the transition of pre-adsorbed oxygen molecules to atomic oxygen (O2 − + e− → 2O− ) [15]. In the case of a-IGZO, this anomalous behavior was observed in dry air. According to commonly accepted models relating to the nature of the oxygen species at the surfaces of SMOs, the transition from molecular form O2 − to atomic species O− occurs at around 150 ◦ C, consistent with theoretical and experimental (IR (infrared) and TPD (temperature programmed desorption)) analysis [1,16]. The results observed in this study are therefore more consistent with this molecular oxygen dissociation model. In either case, the desorption of O− and OH− would release electrons back to the bulk, resulting again in an increase in conductivity as observed in region III. Representative results of the electrical response of these sensors are shown in Fig. 4. Here one observes the resistance response resulting from cyclic exposure to increasing H2 or NO2 concentrations ranging between 1.25 and 50 ppm gas in dry air at an operation temperature of 400 ◦ C for H2 and 200 ◦ C for NO2 , respectively. The resistance transients are in agreement with the expected behavior of an n-type semiconducting metal oxide material in the presence of reducing/oxidizing gases [17]. Under exposure to air, oxygen anionic species (typically O− ) induce an electron depleted layer by extracting electrons from the semiconductor surface and thereby increasing the surface band bending and ultimately setting the base resistance of the sensor. Upon exposure to H2 (reducing gas), the pre-adsorbed oxygen is consumed, leading to a recovery of electrons in the semiconductor conduction band, a corresponding decrease in band bending and also in resistance. When the sensor is exposed to NO2 (oxidizing gas), the NO2 competes with the oxygen species for adsorption sites. Given that NO2 adsorbs more strongly than O2 , band bending increases, resulting in an increase in resistance.

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In addition to the cyclic response of the sensor to H2 and NO2 , one also observes a slow drift in the baseline resistance (at constant temperature and in dry air). This drift can be caused by slow rates of adsorption/desorption of oxygen species when exposed to dry air, as noted in Fig. 3. Similar base line drifts are observed in most metal oxide gas sensors [18,19]. Sensor baseline drift could also result from the inherent thermodynamic instability of a-IGZO, whose properties can be sensitive to processing and pre-annealing conditions. Further investigations directed toward elucidating the source(s) of drift are warranted. In order to extract the magnitude of the response (S) as well as response/recovery time constants ( res ,  rec ), the resistance transient curves were fitted by a simple exponential function of the form, R(t) = a + b exp(−t/) with a, b and  serving as fitting parameters. In the case of high NO2 concentration exposure (>12.5 ppm), the resistance change became more complex, as can be observed in Fig. 4. Therefore, the fitting results with one exponential function were unrealistic, indicating that multiple reaction processes were involved at high NO2 concentrations [20]. Fig. 5(a) shows the variation of gas response (S) with operation temperature upon H2 exposure. The sensor response to H2 gas is defined as follows: S = (G − G0 )/G0 , where G is conductance (G0 in dry air). The observed trend in response versus temperature is in agreement with a surface combustion reaction model, as is often seen in the response of semiconducting metal oxides to H2 gas: (H2(g) + O(s) − → H2 O(g) + e− ). With increasing temperature, the surface combustion reaction proceeds more readily with more electrons released from oxygen ion trap states into the conduction band. It is important to note that region II in Fig. 3 corresponds to the temperatures, at which the a-IGZO film becomes sensitive to H2 . The response increases up to a certain temperature (TM ) where a maximum in the responsivity is observed. TM is observed to increase with increasing H2 gas concentration. This bell-shaped variation of gas response with temperature is also observed in most conventional metal oxide gas sensors and can be explained by the strength of adsorption of the analyte gas molecules and the kinetic barrier that needs to be overcome to induce a surface combustion event [21]. It is interesting to note that the maximum H2 sensitivity, at least for concentrations of H2 below 25 ppm, occurs at ∼350 ◦ C, approximately the temperature at which the conductivity goes through a minimum (maximum oxygen adsorption) in region II of Fig. 3. The response to NO2 gas, S = (R − R0 )/R0 , (R0 , in dry air), is shown in Fig. 5(b) as a function of operating temperature. Here one finds a decreasing sensor response with increasing temperature. Similar effects have been seen in SnO2 based sensors and can be correlated with the reduced adsorption of NO2 with increasing temperature [22]. The response of semiconducting oxide gas sensors is often empirically represented as S = Apˇ , where p is the target gas partial pressure and the response is characterized by the prefactor A and exponent ˇ. ˇ variations are considered to be dependent on the charge of the surface species and the stoichiometric reactions [1,23]. For typical gas sensors utilizing n-type semiconducting oxides such as SnO2 , WO3 or In2 O3 , ˇ is approximately 1/2 for reducing gases like H2 and CO and approximately 1 for oxidizing gases like NO2 and O3 [24]. While rational fractions for the response exponent are often observed (usually 1 or 1/2), dependent on the surface species charge and the stoichiometry of the elementary surface reactions, non-rational fractions are also frequently obtained [25]. In the case of a-IGZO, ˇ values increased from 1/2 to 1 with increasing temperature for both H2 and NO2 gases, as shown in Fig. 6. Further studies are required to establish the sources driving these changes. Fig. 7(a) and (b) shows the variation of sensor response and recovery time constants ( res ,  rec ) for the a-IGZO thin film sensor

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Temperature ( C) Fig. 5. Sensor response to (a) H2 and (b) NO2 as a function of operation temperature.

in the presence of H2 and NO2 gases respectively, as a function of temperature assuming an exponential response. However only the hydrogen response was truly exponential, and thus the time constants reflect trends. Both  res and  rec were generally found to decrease with increasing temperature, although for 250 ◦ C and above, these values for NO2 were nearly temperature independent. Faster rates of adsorption, desorption and in the case of H2 , reaction of target gas molecules on the surface of sensing film at higher temperatures are expected. The relative insensitivity at higher temperatures suggested by the convergence of the data possibly reflects the inability of our experiments to distinguish between transients related to the sensor from that of the system. However, response time studies for the system suggest that the time needed to flush out residual gases or the time needed to reach the desired levels of gases in the chamber are not controlling. A more likely scenario is the role of gas diffusion to the surface from the dilute reacting gas, which presents itself as a first order reaction with an exponential response. The temperature dependence would be T3/2 power, as suggested by the hydrogen response data at the higher temperatures. The recovery data for hydrogen is not

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1.2

represented by a simple exponential but by the Elovitch law as commonly observed in semiconductor systems. Further studies will be required to analyze the data in a quantitative manner.

H2 NO2

4. Conclusions

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Acknowledgements

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This work was supported by the Korea Research Foundation Grant funded by the Korean Government (KRF-2009-352-D00135) and a grant from the cooperative R&D Program (B551179-10-0100) funded by the Korea Research Council Industrial Science and Technology, Republic of Korea. I. D. Kim acknowledges the support of Korea Institute of Science and Technology (KIST) research program. References

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The response characteristics of amorphous-InGaZnO4 (a-IGZO4 ) thin films, toward reducing/oxidizing gases (H2 /NO2 ), at sensor operating temperatures, were reported for the first time. The lack of grain boundaries eliminates a major source of electrical, microstructural and chemical inhomogeneities associated with polycrystalline semiconducting metal oxides (SMOs), rendering aIGZO a highly promising model sensor system. The sensitivities (S) of a-IGZO4 to H2 (e.g. S ∼ 0.7 at 350 ◦ C for pH2 = 12.5 ppm) and to NO2 (e.g. S ∼ 33 at 200 ◦ C for pNO2 = 5 ppm) were found to be comparable to those for polycrystalline SMOs as were its response times, rendering this material of interest for future sensor development given its simplified structure. Future studies need be directed toward developing an improved understanding of the role of cation and oxygen stoichiometry on electrical properties and on sensor response.

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Temperature( C) Fig. 7. Temperature dependencies of response/recovery time constants evaluated from the gas response transient characteristics of an a-IGZO film.

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Biographies Dae Jin Yang received his Ph.D. degree (2008) in Materials Science and Engineering at Korea Advanced Institute of Science and Technology (KAIST). From 2008 to 2009, he was a postdoctoral fellow with Dr. Il-Doo Kim at Korea Institute of Science and Technology. He joined at Prof. Harry L. Tuller’s group as a postdoctoral fellow in Department of Materials Science and Engineering at MIT from November 2009. He is now a research staff member at Materials Research Laboratory, Samsung Advanced Institute of Technology, Samsung Electronics Corporation. His research interests are centered mainly in the fields of novel nanomaterial architectures, with particular emphasis on dye-sensitized solar cell, modified photo-electrodes and electrochemical sensors. George C. Whitfield holds the Ph.D. and M.Eng. in Materials Science and Engineering from MIT, and S.B. from MIT in Electrical Engineering and Computer Science, with a minor in materials science in 2004. He has been working on the modeling and characterization of chemical sensors based on thin-film and nanostructured semiconducting metal oxide materials. Nam Gyu Cho received his Ph.D. degree (2011.02) in Materials Science and Engineering from Korea Advanced Institute of Science and Technology (KAIST). His research interests include the synthesis of inorganic hollow nanostructures using polymer templates for application in electrochemical devices such as metal oxide chemical gas sensors, nanostructured catalyst and Li-ion batteries.

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Pyeong-Seok Cho received his Ph.D. (2009) in Materials Science and Engineering at Korea University. From 2010 to 2011, he was a postdoctoral fellow with Prof. Harry L. Tuller at the Department of Materials Science and Engineering, MIT, where he worked on CuO based chemical sensors. Now he is a senior research engineer at Cheil Industries Inc. and concentrates his study on the degradation mechanism of OLEDs. Also, his research interests include IT- and LT-SOFC, improvement of grainboundary conduction in solid electrolytes (GDC), electrochemical sensors, and low temperature sintering. Il-Doo Kim received his Ph.D. degree (2002) in Materials Science and Engineering from KAIST. From 2003 to 2005, he was a postdoctoral fellow with Prof. Harry L. Tuller at the Department of Materials Science and Engineering at MIT, where he worked on low-voltage organic/ZnO based thin film transistors and Si integrated microphotonic circuits. In April of 2005, he returned to Korea Institute of Science and Technology as a senior research scientist. Dr. Kim’s current research emphasizes processing, characterization, modeling, and optimization of solid-state devices for environmental (sensors) and energy applications (batteries and solar cells) as well as ZnO-based electronics including plastic transistors. He joined at KAIST as a faculty member in Department of Materials Science and Engineering from February 2011. He has published over 83 articles and holds 70 patents. Dr. Il-Doo Kim is Deputy Editor of the Journal of Electroceramics, Springer Academic Publisher. Howard M. Saltsburg received his BS in chemistry at City College of New York and a Ph.D. in chemistry at Boston University. He is Professor Emeritus of Chemical Engineering at the University of Rochester, currently Research Professor of Chemical and Biological Engineering at Tufts University and Research Affiliate in the Department of Materials Science and Engineering at MIT. His research activities are focused on the interaction of gases and solid surfaces ranging from scattering of gases from surfaces to a variety of studies in heterogeneous catalysis including the use of solid electrolyte cells as a diagnostic for surface activity of hydrogen or oxygen during catalysis on metals and oxides and conductivity changes resulting from gas interactions with metal oxides. He has published more than 70 articles including several related to educational activities. Harry L. Tuller received his S.B. and S.M. degrees in electrical engineering and his EngScD in solid state science and engineering from Columbia University in New York. He is a member of the faculty of the Department of Materials Science and Engineering at MIT, where he serves as professor of ceramics and electronic materials and director of the Crystal Physics and Electroceramics Laboratory. Dr. Tuller’s current research emphasizes the integration of sensor and actuator materials into microelectromechanical (MEMS) and microphotonic systems and the modeling, processing, characterization and optimization of solid state ionic devices (sensors, batteries, fuel cells). He has published over 370 articles, coedited 15 books and been awarded 26 patents. Dr. Tuller is editor-in-chief of the Journal of Electroceramics and series editor of Electronic Materials: Science and Technology, Springer Academic Publishers. He is a fellow of the American Ceramic Society, recipient of Fulbright and von Humboldt Awards and former holder of the Sumitomo Electric Industries Faculty Chair at MIT. Dr. Tuller was awarded Docteur Honoris Causa (2004), for life-long contributions to the field of electroceramics by the University of Provence, Marseilles, France and Docteur Honoris Causa (2009) from the University of Oulu, Finland. Dr. Tuller is co-founder of Boston MicroSystems Inc., a pioneer in the design and fabrication of harsh environment compatible micromachined Si and SiC-based sensor arrays.