Gas Sensing Interactions at Hydrogenated Diamond ... - IEEE Xplore

IEEE SENSORS JOURNAL, VOL. 7, NO. 9, SEPTEMBER 2007

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Gas Sensing Interactions at Hydrogenated Diamond Surfaces Andreas Helwig, Gerhard Müller, Olaf Weidemann, Andreas Härtl, Jose Antonio Garrido, and Martin Eickhoff

Abstract—Hydrogenated diamond (HD) samples exhibit a p-type surface conductivity (SC) which is caused by transfer doping to an adsorbed liquid electrolyte layer. We report on gas sensing experiments showing that such samples selectively respond to 2 and 3 gases at room temperature. Successive substitution of H-terminated surface sites by O-termination ones causes an increase in both the sensor baseline resistance and the gas-induced resistance changes. Thermal desorption of the surface electrolyte layer, on the other hand, causes the sensor baseline resistance to increase and the gas sensing effect to disappear. Readsorption of the surface electrolyte reestablishes both the sensor baseline resistance and the gas sensing effect. Our results indicate that the gas sensing effect is caused by local pH-changes due to acid/base reactions of the adsorbed gas molecules in the surface electrolyte layer. It is argued that this dissociative gas sensing mechanism represents a valuable complement with regard to the established surface combustion mechanism that is operative on heated metal oxide surfaces.

NO

NH

Index Terms—Diamond surfaces, electrolytic dissociation, gas sensing mechanism, pH sensor, surface transfer doping.

I. INTRODUCTION

D

IAMOND with a bandgap of 5.5 eV is a bonafide insulator in its undoped state. However, it has been shown to exhibit a pronounced p-type surface conductivity (SC) when the surface is terminated by hydrogen [1], [2]. The SC reaches values per square, independent of whether diamond is up to in the form of single crystals or in the form of polycrystalline films prepared by chemical vapor deposition. The best established model to explain this phenomenon is the transfer doping model suggested by Maier et al. [3]. According to this model, a near-surface hole accumulation with densities in the range of cm occurs via a transfer of diamond valence electrons to an adsorbed surface electrolyte layer. This effect, whose exact microscopic origin is still under debate, has been employed for device applications such as field effect transistors [4], single hole transistors [5], or pH-sensors [6]. In the present paper, we report on the gas sensitivity of hydrogen-terminated diamond (HD) samples. In particular, we Manuscript received January 23, 2007; revised March 16, 2007; accepted March 16, 2007. This work was supported in part by the Deutsche Forschungsgemeinschaft DFG (Ei 182/1-3). The associate editor coordinating the review of this paper and approving it for publication was Dr. Hans-Joachim Krause. A. Helwig and G. Müller are with the EADS Innovation Works Germany, EADS Deutschland GmbH, D-81663 München, Germany (e-mail: [email protected]; [email protected]). O. Weidemann, A. Härt, J. A. Garrido, and M. Eickhoff are with the Walter Schottky Institut, Technische Universität München, D-85748 Garching, Germany (e-mail: [email protected]; [email protected]; [email protected]; [email protected]). Digital Object Identifier 10.1109/JSEN.2007.905019

show that HD supports gas sensing interactions that are distinctly different from those encountered in the widely employed metal oxide (MOx) materials [7], [8]. These latter exhibit reversible conductance changes when exposed to combustible gases such as CO, , and a wide range of hydrocarbon species (HC). This sensing effect is explained in terms of surface combustion, in which the adsorbed analyte gas molecules are and O via interactions with surface converted into oxygen ions. The surface oxygen ions, in turn, form naturally on heated MOx surfaces upon exposure to ambient air and, when consumed by reducing gas species, they serve as a source of additional conduction electrons. Considering the fact that a wide variety of molecular species can be oxidized upon thermal activation, the lack of selectivity of MOx gas sensors is evident. In the following, we show that the selectivity of the gas response is significantly enhanced on HD surfaces. II. EXPERIMENTAL The samples investigated were (100)-oriented type Ib diamond substrates (Sumitomo). For hydrogen termination, for 1 h at the samples were first cleaned in 180 C to remove nondiamond components, and then in a -solution for 15 min. This procedure results in 30% an oxygen-terminated diamond surface. Then, the diamond samples were exposed to a hot-wire generated flow of atomic hydrogen in a high vacuum chamber for 30 min at a substrate temperature of 540 C. For the fabrication of resistors as sensor devices, standard photolithography and oxygen-plasma treatment was used to define insulating surface areas. Finally, Ti/Au (20 nm/200 nm) metal pads were deposited on the remaining patches of H-terminated diamond to form Ohmic contacts. The contact pad area was 1000 m 230 m and the spacing between the contacts was 1000 m. After a set of initial measurements, the level of H-termination was reduced in several treatments in an arc-discharge steps by aggressive ozone ozonisation system (Sander Lab-Ozonizer) [6]. During each of these treatments, the SC was monitored in situ at a fixed voltage of 1.0 V and the process was interrupted after a significant reduction of the sensor baseline resistance had been achieved. After each ozonization step, the same gas sensing tests were repeated to assess the impact of the oxygen-terminated surface sites on the gas sensing properties. Measurements of the gas response were carried out in a custom-built gas mixing facility at EADS. This facility allows the preparation of controlled mixtures of up to six gases or vapors. In our experiments, all analyte gases investigated , and various nitrous ( , various hydrocarbons, CO, oxides) were diluted in synthetic air (80% % ). In addition, controlled humidity levels ranging from 10% to

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Fig. 1. Room temperature response of a HD sample towards a range of combustible gases diluted into a background of synthetic air (SA). A selective response to NH is observed.

Fig. 2. Room temperature response of a HD sample towards different concentrations of NO . The response towards a high-concentration NH pulse is shown for comparison.

90% can be generated by vaporizing deionized O into the synthetic background air. Small ozone concentrations up to 500 ppb were generated by guiding the background synthetic air through an Ansyco ozone generator. III. RESULTS AND DISCUSSION Fig. 1 displays the change in the sensor resistance of a freshly H-terminated diamond sample upon exposure to different gases at room temperature. No response to high concentrations of , CO, , and easily combustible gases such as OH was observed. The only combustible gas that could be de, which caused the HD sensor resistance to be tected was reversibly increased, i.e., a decrease in the p-type SC. In comparison, control experiments on a range of n-type MOx materials showed that all the above analyte gases caused large and reversible reductions in their sensor resistances, i.e., increases in the respective n-type conductivity (not shown). In contrast to the gas-surface interactions at the HD samples, those occurring on the MOx samples require surface temperatures in the order of 300 C–500 C for activation [7], [8]. The detection at MOx surfaces reaches down to concentrations limit for of 0.5–5 ppm . is a In contrast to the combustible gases above, strongly oxidizing gas that can be very well detected on . Being MOx surfaces—detection limit about 50 ppb more electronegative than adsorbs in the form of -species by attracting electrons from abounding surface oxygen ions, leading to a reduced conductivity of n-type MOx results materials [7], [8]. As shown in Fig. 2, exposure to in a decrease in the HD sensor resistance, i.e., in a conductance -induced one. change opposite to the as an oxidizing gas can be detected with MOx Ozone sensors down to concentrations of ppb [9]. To compare the sensing mechanisms of MOx and HD surfaces, we have investigated the response of the latter ones to strongly diluted ozone -sensitivity. Rather, we (300 ppb) and did not observe any species are removed from could observe that preadsorbed the HD surface—most likely by oxidation. Fig. 3 demonstrates exposure returns the this effect and shows that repeated sensor signal back to its initial baseline. In comparison, thermally induced desorption at room temperature would lead to

Fig. 3. Response of a HD sample towards NH (left). The three gas exposure pulses on the right demonstrate the O -induced resetting of the sensor surface to baseline by NH oxidation.

much longer recovery times. We observed that this related cleaning effect does not influence the sensitivity of the HD to . Even after multiple exposures to the magnitude of the gas-induced resistance change does not change (not shown). To investigate the underlying sensing mechanism and, in particular, the role of surface termination, the H-termination was successively replaced by oxygen-termination applying the aggressive ozone treatment described above [6]. In contrast to the very low ozone concentrations applied during the gas response measurements, this first treatment had been shown to replace surface H-atoms by more electronegative O-atoms. These latter ones invert the surface dipole layer and, thus, cause the efficiency of the transfer-doping effect to decrease [3]. We have studied the effect of successive O-termination on the gas response of a freshly prepared HD sample by measuring the gas sensitivity after repeated ozone treatments. As shown in Fig. 4, an increasing oxygen surface coverage due to the ozone treatment leads to an increased sample resistance and to an increase of the absolute and relative gas-induced resistance changes, whereas the selectivity pattern remains unchanged. It should be noted that Garrido et al. have observed a similar influence of the oxygen termination on the pH-sensitivity of polycrystalline diamond surfaces in electrolyte solutions [6].

HELWIG et al.: GAS SENSING INTERACTIONS AT HYDROGENATED DIAMOND SURFACES

Fig. 4. Response of HD samples with different baseline resistance towards NO and NH concentration stairs. Dilution of the hole gas was performed by repeated exposure of the original HD sample towards an intense oxygen plasma.

Fig. 5. Change of the HD baseline resistance and of the NH gas sensing effect in response to repeated desorption steps at 350 C. Arrows indicate the NH -induced resistance changes after the different desorption steps.

They have attributed this effect to an increasing number of surface hydroxyl groups as proton adsorption sites and found that a completely H-terminated surface does not show any pH-sensitivity. In order to assess the importance of the adsorbed surface electrolyte layer for the HD gas sensing effect, we have repeatedly exposed a HD sample with a partially oxygen terminated surC) to reduce the electrolyte face to hot air streams ( surface coverage. After each desorption step, an increase in the response sensor baseline resistance and a decrease in the was observed (Fig. 5). Readsorption of the surface electrolyte layer causes the SC and the gas response to be reestablished, however, at a very slow rate. In agreement with Snidero et al. [10], we found that the recovery rate can be significantly increased by simultaneous irradiation with ultraviolet light. These latter observations indicate that the gas response is caused by the adsorbed surface electrolyte on the HD rather than by the H-termination itself. Fig. 6 displays an analogue measurement as described above. Performing this measurement the temperature of the hot air stream for reducing the electrolyte surface coverage was reduced from 350 C to about 90 C. Again, it can be seen

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Fig. 6. Change of the HD baseline resistance and of the NH gas sensing effect in response to repeated desorption steps at a temperature of about 90 C.

that after each desorption step the baseline resistance increases, gas response decreases. The axis while the respective exposure indicates break on the time axis after the second the long time required to reach the initial sensor baseline again. During this time interval, the HD surface was sequentially exposed to high concentrations of humidity and UV light irradiation. After reaching its original baseline an additional gas pulse was introduced. Fig. 6 demonstrates that after removing and readsorbing the surface electrolyte layer, the magnitude of the induced gas response is restored. and On the basis of our results, we interpret the gas sensing effects on HD surfaces as acid/base reactions in the adsorbed surface electrolyte. It should be noted that acidic dissociation leads to an increase in SC, as also reported in [11]. In contrast, a decrease of SC upon decrease of the pH was reported for measurements in electrolyte solutions in [6]. One possible explanation for this apparent contradiction is the difference in experimental conditions. Whereas measurements in electrolyte are performed with a well defined potential difference applied between bulk electrolyte and diamond surface (i.e., not in thermodynamic equilibrium), the measurements in the gas phase are carried out under floating gate conditions. In addition, the structure of the adsorbed surface electrolyte layer in a gaseous environment might differ significantly from that reported for contact with a bulk-electrolyte [12]. Hence, the mechanism which causes electronic detection of the dissociation-induced changes in proton concentration (pH) still needs to be clarified. However, our results demonstrate that this effect is enhanced due to the presence of oxygen-terminated surface sites. In that context, the gas sensitivity of the freshly prepared HD can be attributed to a residual oxygen-termination due to an incomplete H-termination. Consistent with this interpretation, we do not observe sensitivity to gases that do not give rise to acid/base reac). This interpretation tions in water ( , HC, alcohols, and also explains results by Ri et al., who have observed changes in the hole density and hole mobility upon exposure to water vapor and [11]. Other observations which enriched with are in line with this interpretation are that strongly dissociating HCl vapors produce large changes in the SC that are opposite

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to and that can compensate - induced ones [13]. Also, consistent with the much smaller dissociation constant of compared with HCl and , we found an almost zero response [14]. IV. CONCLUSION In conclusion, we have shown that the SC of H-terminated diamond exhibits selective sensitivity towards gases that dissociate in aqueous electrolytes. A decrease in the SC was observed and an increase for acidic dissocifor basic dissociation suggesting detection via changes in pH due to the ation dissociation of the gas species in the surface electrolyte. The reversible disappearance of the gas sensitivity after desorption of the surface electrolyte layer confirms this explanation. Our results further demonstrate that HD samples support gas sensing interactions distinctly different from those observed on more conventional MOx sensor surfaces. This difference does not only arise on the level of the above-reported cross sensitivity behavior but also in the form of reduced heating requirements. Whereas MOx gas sensors need to be operated at temperatures in the range from 300 C–500 C [7], [8], all the above-reported gas sensing tests on HD samples were performed at room temperature. Further tests at sensor operation temperatures up to 140 C did not reveal qualitatively different gas sensing phenomena except for reduced response and recovery time constants. On the whole, our results show that HD sensors support a novel dissociative gas sensing mechanism that is likely to form a valuable complement in gas sensor arrays containing MOx gas sensors [15]. With such arrays combustible gases could easily be separated into groups that can or cannot undergo electrolytic dissociation. REFERENCES [1] M. I. Landstrass and K. V. Ravi, “Hydrogen passivation of electrically active defects in diamond,” Appl. Phys. Lett., vol. 55, p. 1391, 1989. [2] T. Maki, S. Shikama, M. Komori, Y. Sakaguchi, K. Sakuta, and T. Kobayashi, “Hydrogenating effect of single-crystal diamond surface,” Jpn. J. Appl. Phys., vol. 31, p. L1446, 1992. [3] F. Maier, M. Riedel, B. Mantel, J. Ristein, and L. Ley, “Origin of surface conductivity in diamond,” Phys. Rev. Lett., vol. 85, p. 3472, 2000. [4] A. Hokazono and H. Kawarada, “Enhancement/Depletion surface channel field effect transistors of diamond and their logic circuits,” Jpn. J. Appl. Phys., vol. 36, pp. 7133–7139, 1997. [5] K. Hayashi, S. Yamanaka, H. Okushi, and K. Kajimura, “Study of the effect of hydrogen on transport properties in chemical vapor deposited diamond films by Hall measurements,” Appl. Phys. Lett., vol. 68, p. 376, 1996. [6] J. A. Garrido, A. Härtl, S. Kuch, and M. Stutzmann, “pH sensors based on hydrogenated diamond surfaces,” Appl. Phys. Lett., vol. 86, p. 073504, 2005. [7] D. E. Williams, “Conduction and gas response of semiconductor gas sensors,” in Solid State Gas Sensors, Bristol, 1987, pp. 71–123. [8] S. Ahlers, G. Müller, and T. Doll, C. A. Grimes, E. C. Dickey, and M. V. Pisko, Eds., “Factors influencing the gas sensitivity of metal oxide materials,” Encyclopedia of Sensors, vol. 3, pp. 413–447, 2006, ISBN: 1-58883-059-4. [9] T. Becker, L. Tomasi, C. Bosch v. Braunmühl, G. Müller, G. Sberveglieri, G. Faglia, and E. Comini, “Ozone detection using low-powerconsumption metal-oxide gas sensors,” Sens. Actuators A, vol. 74, pp. 229–, 1999.

[10] E. Snidero, D. Tromson, C. Mer, P. Bergonzo, J. S. Food, C. Nebel, O. A. Williams, and R. B. Jackman, “Influence of the post-plasma process conditions on the surface conductivity of hydrogenated diamond surfaces,” J. Appl. Phys., vol. 93, p. 2700, 2003. [11] G. S. Ri, K. Tashiro, S. Tanaka, T. Fujisawa, H. Kimura, T. Kurosu, and M. Iida, “Hall effect measurements of surface conductive layer on undoped diamond films in NO and NH atmospheres,” Jpn. J. Appl. Phys., vol. 38, p. 3492, 1999. [12] A. Härtl, J. A. Garrido, S. Nowy, R. Zimmermann, C. Werner, D. Horinek, R. Netz, and M. Stutzmann, “The ion sensitivity of surface conductive single crystalline diamond,” J. Amer. Chem. Soc., vol. 129, p. 1287, 2007. [13] G. S. Ri, T. Mizumasa, Y. Akiba, Y. Hirose, T. Kurosu, and M. Iida, “Formation mechanism of p-type surface conductive layer on deposited diamond films,” Jpn. J. Appl. Phys., vol. 34, p. 5550, 1995. [14] B. Ostrick, J. Mühlsteff, M. Fleischer, H. Meixner, T. Doll, and C.-D. Kohl, “Adsorbed water as key to room temperature gas-sensitive reactions in work function type sensors: The carbonate—Carbon dioxide system,” Sens. Actuators B, vol. 57, p. 115, 1999. [15] J. W. Gardner and P. N. Bartlett, “Electronic noses. principles and applications,” in 2000 Meas. Sci. Technol.. Oxford, U.K.: Oxford Univ. Press, ISBN: 0-19-85595-5-0, 11, 1087.

Andreas Helwig was born in 1975. After graduating in precision engineering and microengineering in 2001, he received the M.E. degree in the field of nanotechnology from the Munich University of Applied Sciences, Munich, Germany, in 2003. He is currently working towards the Ph.D. degree at the Department of Sensors, Electronics and Systems Integration, EADS Innovation Works, Germany. Currently, he is working amongst other things in the field of smart sensors for maintenance, safety, and emission control.

Gerhard Müller graduated in physics from the University of Heidelberg, Heidelberg, Germany, in 1974, and received the Ph.D. degree from the University of Heidelberg in 1976. Subsequently, he was employed at the MaxPlanck-Institute for Nuclear Physics, Heidelberg, where he performed work on ion implantation and nuclear solid-state physics. In 1979, he changed to the University of Dundee, U.K., where he started research on hydrogenated amorphous silicon. In 1981, he moved to Messerschmitt-Boelkow-Blohm GmbH (MBB), where he performed development work on thin-film solar- cell modules. Since 1986, he has been active in the field of silicon micromachining and sensors working in leading positions for a number of employers: MBB/DASA (1986–1993): building up clean room laboratories for silicon micromachining and thin-film technologies; DaimlerChrysler Central Research (1994–2000): sensors for automotive safety and exhaust gas monitoring; EADS Corporate Research Centre, Germany (2000 onwards): sensors for aircraft safety, security, maintenance, and diagnosis. He is currently managing the Biological and Chemical Sensors Group, EADS Innovation Works ( 15 employees). He is an author and coauthor of about 250 articles in scientific journals and conference proceedings. Since 2000, he has been a Lecturer with the Munich University of Applied Sciences.

Olaf Weidemann received the Diploma for work on gas sensitivite AlGaN Schottky diodes in 2003. Currently, he is working towards the Ph.D. degree studying interfaces in AlGaN structures and GaN/AlN quantum dot systems at the Walter Schottky Institut, TU München. He has been working with the Sensors and Materials Group, Walter Schottky Institut, Technische Universität München, Garching, since 2002.

HELWIG et al.: GAS SENSING INTERACTIONS AT HYDROGENATED DIAMOND SURFACES

Andreas Härtl received the Diploma in physics from the Technische Universität München, Garching, Germany, in 2002. Currently, he is working towards the Ph.D. degree in biosensors based on diamonds at the Walter Schottky Insitut, TU München. His main research interests include (bio-)sensors, new materials and devices.

Jose Antonio Garrido graduated from the E.T.S.I. Telecomunicación, Universidad Politécnica de Madrid, in 1996 and received the Ph.D. degree from the Universidad Politécnica de Madrid, in 2000. From 2000 and 2003, he worked as a Postdoctoral at the Walter Schottky Institute (WSI), Department of Physics, T.U. Munich. Since 2003, he has been the Team Leader of the Diamond Group, WSI. His current research activity is oriented to fundamental aspects of the diamond/water, diamond/biomolecules, and diamond/organic thin films interfaces, as well as the development of diamond-based biochemical sensors.

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Martin Eickhoff received the Ph.D. degree in experimental physics from the Technische Universität München, Garching, Germany, in 2000. He has worked on SiC material and device development with DaimlerChrysler Research and Technology, Munich, Germany, from 1995 to 2000. After that he worked for Infineon Technologies AG, Munich, on the development of surface micromachined integrated sensors. Since 2001, he has been a Work Group Leader of the Sensors and Materials Group at the Walter Schottky Institut, TU München, focusing on growth and characterization of wide bandgap semiconductors, surface functionalization, and on the application in biochemical sensors.