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Gas sensing properties of hydrogen-terminated diamond
Available online at www.sciencedirect.com
Sensors and Actuators B 133 (2008) 156–165
Gas sensing properties of hydrogen-terminated diamond A. Helwig a,∗ , G. M¨uller a , J.A. Garrido b , M. Eickhoff b a
b
Innovation Works Germany, EADS Deutschland GmbH, D-81663 M¨unchen, Germany Walter Schottky Institut, Technische Universit¨at M¨unchen, Am Coulombwall 3, D-85748 Garching, Germany Received 16 July 2007; received in revised form 4 February 2008; accepted 5 February 2008 Available online 12 February 2008
Abstract Hydrogen-terminated diamond (HD) samples possess a p-type surface conductivity (SC) which is caused by transfer doping to an adsorbed liquid electrolyte layer. We report on gas sensing experiments on such samples and show that these selectively respond to analyte gases that can undergo electrolytic dissociation in the surface electrolyte layer. These gas sensing interactions occur at room temperature and are far more selective than sensing interactions at heated metal oxide layers. Successive substitution of surface hydrogen atoms by oxygen atoms causes the sensor baseline resistance and the gas-induced resistance changes to increase. This latter observation suggests that a small number of O-termination sites may have a catalytic effect on the gas sensing interactions. Increased temperature, O3 and UV light exposure all reduce the sensor recovery time constants. Heating beyond the water evaporation threshold (∼200 ◦ C) causes the surface electrolyte layer to disappear and the gas sensing effect to vanish. Re-adsorption of the surface electrolyte layer re-establishes both the sensor baseline resistance and the gas sensing effect. A model for the dissociative gas response is proposed that accounts for the observed experimental facts. © 2008 Elsevier B.V. All rights reserved. Keywords: Diamond surfaces; Electrolytic dissociation; Gas sensing mechanism; Surface transfer doping
1. Introduction In recent years semiconductor gas sensors have found widespread commercial application in gas monitoring and alarm applications. To date most of these sensors employ metal oxide (MOx ) semiconductors as gas sensitive materials [1–7]. A common feature of all kinds of MOx materials is that they are ionic and that they exhibit (almost) completely oxidised surfaces. As oxides, such sensor materials are stable against oxidation when operated at elevated temperatures in ambient air. In addition, as ionic materials, metal oxides do not tend to irreversibly form covalent bonds to molecular species that may abound in the surrounding gas atmosphere. It is exactly for these two reasons that MOx materials have become very popular subjects of investigations in the field of gas sensors. The common view of the metal oxide gas sensitivity [8–11] is that MOx materials naturally tend to adsorb O2 molecules in the ∗ Corresponding author at: EADS, Innovation Works Germany, Sensors, Electronics & Systems Integration, IW - SI, D-81663 M¨unchen, Germany. Tel.: +49 89 607 28197; fax: +49 89 607 24001. E-mail address: [email protected] (A. Helwig).
0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.02.007
form of O− ions on their surfaces. On the sensor surface the O− ions in turn may undergo reactions with adsorbed combustible gases forming H2 O and CO2 molecules that may desorb into the gas phase again [12–15]. As these reaction products can only be emitted in a neutral state, the electrons initially trapped on the surface O− ions are re-injected into the MOx conduction band and thus generate an enhanced n-type conductivity which serves as a sensor signal. With combustibility being a major criterion for detectability on MOx surfaces, it is also clear that MOx gas sensors have become ill-famed for their cross-sensitivity to various reactive gases. In order to enable the above surface reactions, MOx gas sensors are usually heated to temperatures in the range between 300 and 500 ◦ C. More recent work has shown that MOx gas sensors are also able to respond to gases at room temperature and slightly above [16,17]. In this latter case photoexcitation is often used to accelerate the surface reactions [18–20]. In recent papers we have shown that semiconductor sensors in general and metal oxides in particular,are likely to carry a very thin and firmly adsorbed water film on their surfaces [21,22,29]. There we have further shown that these liquid electrolyte layers do not prevent the semiconductors to exhibit a gas response at all
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but rather restrict their response to molecular species which are able to undergo electrolytic dissociation in the adsorbed liquid electrolyte layer. Among those semiconductors which exhibit a lowtemperature dissociative gas response, hydrogenated diamond (HD) plays a particular role in that it represents an ideal model system for studying this kind of low-temperature gas response. In comparison to metal oxides, which represent an extremely diverse class of materials, and hydrogenated amorphous silicon, which is a non-crystalline material with a variable composition and microstructure, hydrogenated diamond is a well-defined monocrystalline material, which has received a great deal of attention because of its peculiar p-type surface conductivity. Although details are still under debate, there is a general consensus that this p-type surface conductivity arises from surface transfer doping to an adsorbed liquid electrolyte layer [23–25]. On the application side HD has already found a number of device applications such as field effect transistors [26], single-hole transistors [27] or pH-sensors [28]. In the following we go beyond our initial work on the HD gas sensitivity [29] and report on a wider range of gas sensing experiments involving more species than previously. In addition we report on the accelerating effect of temperature and UV radiation. Last not least we derive from these experiments a comprehensive model of the HD gas sensing effect that builds on the surface transfer-doping model of diamond [25]. 2. Experimental The samples investigated in this work were nominally undoped (1 0 0)-oriented type Ib diamond substrates from Sumitomo. For hydrogen termination the samples were first cleaned in CrO3 /H2 SO4 for 1 h at 180 ◦ C to remove non-diamond components and then in a 30% H2 O2 solution for 15 min. This procedure results in an oxygen-terminated diamond surface. The functional hydrogen termination was accomplished by exposing the samples 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 were used to define insulating surface areas. Finally, Ti/Au (20 nm/200 nm) metal pads were deposited on the remaining patches of Hterminated diamond to form ohmic contacts. The contact pad area was 1000 m × 230 m and the spacing between the contacts was 1000 m (Fig. 1). Before performing gas sensing tests, these samples were stored in ambient air and were exposed to normal daylight during that period. After a set of initial measurements on these samples, the level of H-termination was reduced in several steps by ozone (O3 ) treatments in an arc-discharge ionization system (Sander LabOzonizer) [22]. This allowed the reactive O3 species to directly interact with the surface C H bonds. In order to monitor the ozonisation process the surface conductivity was monitored in situ at a fixed voltage of 1.0 V and the process was interrupted after a significant increase in the sensor baseline resistance had been achieved. In this way, the density of surface C-atoms with a H-termination and the hole
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Fig. 1. Top: cross section through a hydrogen-terminated diamond sensor. Bottom: top view onto a processed diamond sensor.
density in the sub-surface hole gas was reduced. It can clearly be seen that after each treatment the surface conductivity (SC) was decreased [30]. Fig. 2 shows the different I/V characteristics of one and the same diamond sample after repeated ozone plasma treatments. After each ozonisation step the same range of gas sensing tests was repeated as on the fully hydrogenated samples to assess the impact of the oxygen-terminated surface sites on the gas sensing performance. The gas sensing experiments were performed at the gas test rig of EADS Innovation Works. In this laboratory up to six gases or vapors can be mixed and injected into a test chamber. In our experiments all analyte gases investigated (H2 , various hydrocarbons, CO, NH3 , O3 , and various nitrous oxides) were diluted in a background of synthetic air (80% N2 /20% O2 ). Whereas
Fig. 2. I–V characteristic of the diamond after increasing ozone treatment times.
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the high-purity analyte gases were supplied from gas cylinders, small admixtures of O3 (<500 ppb) were produced using an Ansyco ozone generator. Controlled relative humidity levels ranging from 10 to 90% were generated by vaporizing de-ionized H2 O into the synthetic background air. 3. Results 3.1. Fully hydrogenated diamonds In this first paragraph we consider gas sensing experiments that had been performed on a diamond sample with a fully Hterminated surface. In the experiment reported in Fig. 3 we have exposed the HD sample towards high concentrations of C2 H4 , NO2 , C2 H5 OH, CO, H2 and NH3 with the HD sample being kept at room temperature. Going from left to right, the first exposure to C2 H4 clearly does not produce any sensor response. A significant response, however, is observed upon exposing the sensor to NO2 . As expected for a room-temperature-operated gas sensor, the response time is long and the recovery time even longer. The recovery from the 50 ppm NO2 pulse in fact overlaps in duration with the subsequent H2 and CO exposure pulses. The insensitivity to these latter two gases, which has already been described in our previous publication [29], reveals here from the absence of any disturbance of the NO2 recovery tail. The final NH3 exposure pulse again produces a strong response, which is opposite in sign to the NO2 response. Again, we observe a very long recovery from the NH3 exposure. Comparing to gas detection at heated MOx surfaces, we note that the HD gas response is much more restricted, exhibiting no response to H2 , hydrocarbons and CO, i.e. to a range of gases that can easily undergo combustion reactions at heated metal oxide surfaces. NO2 and NH3 , on the other hand, can be detected on both kinds of surfaces. The room-temperature response of HD samples, however, is associated with longer response and recovery time constants than the response of heated MOx gas sensors. The concentration-dependent response towards those two gases that are detectable with HD gas sensors is shown in
Fig. 3. Response of a fully hydrogenated HD sample towards a range of analyte gases that were diluted into a background of synthetic air. During measurements the HD sample was kept at room temperature.
Fig. 4. Concentration dependence of the NH3 and NO2 response of a fully hydrogenated diamond sample. During these tests the HD sample was kept at room temperature.
Fig. 4. In this plot the NO2 gas sensitivity was calculated from S = R0 /Rgas − 1, where R0 stands for the sensor resistance under clean-air conditions (baseline resistance) and Rgas for the sensor resistance under the influence of NO2 . In the case of NH3 we used S = Rgas /R0 − 1 to account for the fact that opposite resistance changes are encountered in the case of NO2 and NH3 exposures and to arrive at consistently positive values of S in both cases. Interestingly, this plot reveals a roughly linear concentration dependence of the gas response S. Combustion reactions on MOx surfaces, on the other hand, give rise to sub-linear concentration dependencies. Among the range of oxidizing gases that can be detected with MOx sensors, O3 stands out as it can easily be detected in ppb (10−9 ) concentrations. For this reason we have also performed O3 sensing tests on HD surfaces. In such RT exposures we could not find any sensitivity to the O3 itself. Rather, we could observe that the recovery from preceding NH3 exposures was accelerated. Fig. 5 demonstrates this effect and shows that a short ozone exposure following a NH3 exposure pulse dramatically reduces the time that is need to reach the sensor baseline resistance again. In comparison, thermal agitation alone would have needed much longer times to reset the sensor. This latter effect is demonstrated by the initial slow recovery that immediately follows each NH3 exposure pulse and that precedes the respective O3 recovery pulse. An ever-present background gas that abounds in high and highly variable concentrations is H2 O vapor. Fig. 6 shows that the H2 O response is negligibly small in comparison to the NH3 and NO2 sensitivities. The complete absence of a humidity effect is consistent with the observation that HD surfaces are covered by thin layers of adsorbed water in which the other analyte gases may or may not dissolve [24,25]. This vanishing humidity dependence is at variance with the case of high-temperature operated MOx gas sensors, which do not accommodate such multi-layer adsorption layers and where direct single-
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Fig. 5. Response of a HD sample towards increasing NH3 concentration steps. The ozone gas pulses demonstrate the O3 -induced resetting of the sensor surface to baseline by NH3 oxidation. During these tests the HD sample was kept at room temperature.
molecule interactions with the MOx surface are likely to prevail. The results reported so far have all been obtained with the HD samples being kept at room temperature during the gas exposure tests. In the case of MOx gas sensors, the sensor operation temperature is a very important parameter that determines the sensor behaviour. We, therefore, have also carried out tests at higher sensor operation temperatures. In this context it should be noted that the H-termination on top of diamond surfaces is stable up to annealing temperatures in the order of 800 ◦ C in vacuum [31,32]. The adsorbed surface electrolyte, however, is much less stable. Electrolyte desorption was observed at temperatures above 150 ◦ C [29]. Respecting these much more rigid temperature constraints we have performed gas sensing tests up to surface temperatures of 140 ◦ C. With regard to the crosssensitivity behaviour we observed results that were similar to the ones reported above. The main difference was that those gases that can be detected exhibited a faster response and recovery behaviour than at room temperature. Fig. 7 shows such results
Fig. 6. Response of a fully hydrogenated HD sample towards water vapor. During these tests the HD sample was kept at room temperature.
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Fig. 7. Response of a fully hydrogenated HD sample towards NH3 gas pulses. In this experiment a fully hydrogenated HD sample was operated at successively higher temperature.
for the special case of NH3 . From these data it is seen that both the response and recovery time constants decrease with temperature with the recovery times being much longer than the response times. As a second effect we observe a reduction in the absolute magnitude of the steady-state gas response. Compared to MOx materials, however, this temperature effect is small [33]. 3.2. Partially oxygen-terminated diamond surfaces The above described results all relate to diamonds with a high degree of H-termination. In order to assess the effects of a reduced level of H-termination, ozonisation treatments were performed and the same range of measurements as above was repeated after each ozonisation step. From these measurements it was revealed that ozonisation causes a significant increase in the baseline resistance R0 and an even larger increase in the gas response S as the ozonisation time increases. The obtained results are visualized in Fig. 8.
Fig. 8. Response of a HD sample towards NO2 and NH3 gases as a function of the sensor baseline resistance.
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Fig. 9. Response of a HD sample with different baseline resistance towards various analyte gases. Dilution of the hole gas was performed by repeated exposure of the original HD sample towards an intense oxygen plasma.
In order to assess possible changes in the cross sensitivity profile, the gas sensing sequence of Fig. 3 was repeated after each ozonisation step. In this way the data of Fig. 9 was obtained. These results again emphasize the effect of ozonisation on the sensor baseline resistance R0 and on the gas response S, but they do not indicate any change in the cross sensitivity behaviour. The effect of the O-termination sites therefore seems to be restricted towards facilitating gas sensing interactions that are in principle possible at H-terminated diamond surfaces, rather than enabling qualitatively new gas sensing interactions. Fig. 9 also shows that the gas response S rises as the baseline resistance of the diamond sample is increased. The O-termination sites therefore may be regarded as performing a catalytic role on predominantly Hterminated diamond surfaces. Another important observation is that – in case a non-zero gas sensing effect is observed – the room-temperature desorption of the detectable species is quite slow and not influenced by the presence of O-terminated sites. Fig. 10 displays the concentration-dependent response of a partially oxygen-terminated diamond sample towards different nitrogen-containing gases. The response to NOx species (x = 1, 2) is quite strong, enhancing the p-type surface conductivity of the HD samples. A strong response is also obtained in the case of NH3 exposure. This time, however, a conductivity decrease is observed. A gas response into the same direction as for NH3 is also observed upon N2 O exposure. This latter effect, however, is comparatively small – even at high concentration levels.
Fig. 10. Concentration-dependent response characteristic of a partially hydrogenated diamond sample towards different nitrogen containing gases. NO and NO2 exhibit an acidic (resistance decrease) and NH3 and N2 O a basic (resistance increase) response.
sensor by roughly one order of magnitude. Upon NH3 exposure a significantly shorter response time is observed as compared to the second exposure pulse that was carried out in the dark. When the UV illumination is continued beyond the NH3 exposure pulse, the UV light also leads to a decrease of the desorption time. 3.4. Effect of electrolyte desorption In this set of experiments we have performed measurements aiming at elucidating the HD gas sensing mechanism. Within the framework of the transfer doping model the occurrence of a 2D hole gas at a diamond surface is due to transfer doping, i.e. a transfer of valence electrons from the top of the diamond valence band to an adsorbed surface electrolyte layer. In comparison the origin of the HD gas sensing effect is less clear. In order to find more clues to the gas sensing effect, we tried to reduce
3.3. Effect of UV light exposure The above-reported experiments have already revealed the accelerating effect of temperature on the speed of the adsorption and desorption processes. Similar effects were observed upon illumination with UV light (Fig. 11). There a HD sample with a partially oxidised surface was exposed to two successive NH3 pulses. During the first pulse light from a UV LED (λ ∼ 255 nm) irradiated the HD surface. Before the NH3 exposure pulse the UV light decreased the baseline resistance of the
Fig. 11. Response of a HD sample with a partially oxidised surface towards two NH3 exposure pulses. During the first pulse the HD sample was illuminated with UV light.
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Fig. 12. Change of the HD baseline resistance and of the NH3 gas sensing effect in response to a desorption step induced by an intense flow of hot air (∼90 ◦ C).
the amount of adsorbed electrolyte up to the limit of complete desorption. Desorption was performed by exposing the HD sample to an intense flow of hot air (∼90 ◦ C). After each desorption step the resulting baseline resistance was measured and a NH3 gas sensing test was performed. From the results displayed in Fig. 12, two effects stand out: firstly, after each desorption step the baseline resistance increases in agreement with the transfer doping model. Secondly, we observed a reduction in the NH3 response after each desorption step. This latter finding indicates that the transfer doping and the gas sensing effects are connected to the same cause, i.e. the presence of an adsorbed electrolyte layer. The break on the time axis after the second NH3 exposure indicates the very long time needed to reach the initial sensor baseline resistance again. During this time interval the HD surface was sequentially exposed to high levels of humidity and UV light irradiation. After reaching the original baseline again, the HD sample was exposed to an additional NH3 gas pulse. Fig. 12 demonstrates that after removing and re-adsorbing the surface electrolyte layer, the magnitude of the induced gas response is restored.
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Fig. 13. Transfer of valence electrons from the diamond surface to an adsorbed electrolyte layer. The adsorbed water layer is assumed to be slightly acid due to the dissolution of atmospheric CO2 .
uncertain stays the question, which of the two Redox-couples (H3 O+ /H2 O or H2 O/OH− ) is actually responsible for the electron transfer between diamond bulk and surface electrolyte. A much higher surface conductivity arises, in case acid-forming molecules become dissolved in the electrolyte layer. In the literature [34] it has been suggested that acid surface electrolyte layers may arise from the dissolution of atmospheric CO2 : CO2 + 2H2 O ↔ H2 CO3 + H2 O ↔ H3 O+ + HCO3 −
(2)
In this latter case a much higher sub-surface hole density is expected and the HCO3 − ions in the electrolyte now take over the role of the counter-ions. The latter case is illustrated in Fig. 13. For completeness, the real-space picture of Fig. 13 needs to be complemented by a band-structure diagram, which summarises the energetic side of the surface transfer doping model. This latter aspect is illustrated in Fig. 14. In this figure we visual-
4. Model of the HD gas sensing effect In this section we suggest a model of the HD gas sensing effect that builds on the established model of surface transfer doping [25]. To this end we first consider Fig. 13. This figure visualizes the basic idea of the surface transfer doping effect: exposing a HD surface to normal ambient air, a thin layer of adsorbed water – about 1 nm thick – forms at the HD surface. Assuming that this adsorbed water layer is neutral, i.e. in case it does not contain any acid- or base-forming impurities, small and equal densities of H3 O+ and OH− ions are formed: 2H2 O ↔ H3 O+ + OH−
(1)
Transferring valence electrons from the top of the diamond valence band to the dissolved H3 O+ ions, a thin sheet of holes below the diamond surface and a layer of compensating negative OH− ions in the adsorbed surface electrolyte are formed. Still
Fig. 14. Position of the valence and conduction bands of H-terminated diamond (left) and O-terminated diamond (right) relative to the H3 O+ /H2 O and H2 O/OH− redox levels in water (middle). The energetic match of the H3 O+ /H2 O redox level in water with the top of the valence band in H-terminated diamond facilitates the transfer of diamond valence electrons into the surface electrolyte and thus the formation of a p-type surface layer in undoped diamond.
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ize the energetic situation that arises when an adsorbed water layer is brought into contact with a diamond surface. In considering such contacts, two principally different situations need to be considered: hydrogen- and oxygen-terminated diamond. Due to the fact that hydrogen is more electropositive and oxygen more electronegative than carbon, the C(δ− ) − H(δ+ ) and the C(δ+ ) − O(δ− ) dipoles point into opposite directions. In the case of hydrogenated surfaces this leads to a negative electron affinity (NEA) of the diamond surface and to a positive electron affinity in the case of oxygen termination. Quantitatively, the EA difference between both kinds of diamond may amount up to 3 eV [35]. These different energetic positions of the conduction and valence bands in H- and O-terminated diamonds have drastic consequences for the exchange of electronic charge between diamond and adsorbed surface electrolyte. In the H-terminated case the maximum of the diamond valence band happens to coincide with one of the redox levels in the electrolyte layer. H3 O+ ions therefore can become neutralised by the transfer of valence electrons to the liquid electrolyte. As a consequence, a high hole density is able to build up easily at the diamond surface. This is the essence of the surface transfer doping model of the p-type surface conductivity (SC) in diamond. In the other case, namely that the diamond surface is oxygen-terminated, a huge thermal activation energy is required for the transfer doping. A p-type SC therefore does not arise on oxygen-terminated diamond. This latter effect relates to our above-described results in so far as it explains the rapid increase in the sensor baseline resistance as a HD surface becomes partially converted from Hto O-termination. A straight-forward generalisation of these ideas leads to an explanation of the HD gas sensing effect: molecules in the neighbouring gas phase with the ability of electrolytic dissociation can dissolve in the surface electrolyte either forming H3 O+ or OH− ions. These ions in turn determine the overall H3 O+ concentration and thereby the level of p-type surface conductivity in the diamond. With regard to those species that we could detect on HD surfaces, the following detection reactions are proposed: NH3 + H2 O ↔ NH4 + + OH−
(3)
NO2 + 2H2 O ↔ NO3 − + H3 O+ + (1/2)H2
(4)
NO + 3H2 O ↔ NO3 − + H3 O+ + (3/2)H2
(5)
N2 O + 5H2 O ↔ 2NH4 + + 2OH− + 2O2
(6)
With OH− decreasing and H3 O+ enhancing the p-type surface conductivity, we note that these dissolution reactions correctly predict the observed sign of the gas response. Concerning the gas response we note that the above reactions are overall results of two partial reactions in which the analyte molecules first become absorbed in the surface electrolyte and then electrolytically dissociated forming H3 O+ or OH− ions. Considering the absorption reactions first, we expect that the ease of absorption decreases in the order of reactions (7)–(10), as an increasing number of H2 O molecules needs to be broken up and covalent bonds to be rearranged to form HNO3 and NH3
Table 1 Comparison of measured gas sensitivities to chemical data of the different analyte gases Analyte gas
Sgas [c = 100 ppm]
Solubility
pKa
NO2 NO CO2 NH3 N2 O
30 28 n.a. 5 0.003
Hydrolyses (<2550 g/l) 0.067 g/l 0.0005 g/l 541 g/l 1.2 g/l
−1.32 −1.32 3.3
pKb
4.79 4.79
molecules: NH3 + nH2 O ↔ (NH3 )naq
(7)
NO2 + H2 O ↔ HNO3 + (1/2)H2
(8)
NO + 2H2 O ↔ HNO3 + (3/2)H2
(9)
N2 O + 3H2 O ↔ 2NH3 + 2O2
(10)
Considering the second step, namely electrolytic dissociation, we expect gas sensing reactions involving the dissociation of HNO3 to be favoured over those involving NH3 : NH3 + H2 O ↔ NH4 + + OH− ; HNO3 + H2 O ↔ NO3 − + H3 O+ ;
pKb = 4.79 pKa = −1.32
(11) (12)
This latter ordering is revealed from the fact that the dissociation constant of HNO3 is larger than that of NH3 . Finally, considering the facts that CO2 does not dissolve readily in water and that the corresponding acid H2 CO3 is only a relatively weak one, it is suggested – and also confirmed by our experiments – that the sensor baseline resistance is more likely to be determined by the trace levels of NO2 in the ambient air (<1 ppm), rather than by the more abundant CO2 (∼400 ppm). These latter considerations are further supported by the data collected in Table 1. 5. Discussion The above model successfully accounts for the insensitivity of HD to water vapor and for its varying degrees of response to acid- and base-forming gases. Things that do not directly unfold from this model are the lengths of the response and recovery time constants, the accelerating effect of ozone and UV light on the speed of the sensor response and the catalytic activity of surface OH groups that increases the magnitude of the sensor response. In this final chapter we attempt to provide tentative explanations for these effects, based on the current state of our knowledge. Finally, and most importantly, we should like to discuss how far the mechanisms, that are operative at HD surfaces, might be generic in the sense that they determine the room-temperature response of other semiconductor materials as well. Turning to the response and recovery time constants first, we note that the above model explains the gas response as a result of the electrolytic dissociation of analyte gas molecules in the surface electrolyte. We have already argued above that in the response case covalent bonds need to be broken and reformed to arrive at ionic species that can undergo electrolytic dissociation.
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Further considering the fact that such rearrangements need to proceed at room temperature, long response times are no surprise. Once formed, the dissolved ionic species are stabilized by a coordination sphere of associated H2 O dipoles. In this way, the dissolved ions attain a largely increased effective molecular mass. Upon termination of the gas exposure pulse, the dissolved ionic species need to reform according to the reversible detection reactions above and the neutralised species need to evaporate into gas phase again. On the way towards re-evaporation, the breaking up of the sphere of associated H2 O dipoles is likely to represent a major kinetic hindrance that elongates the recovery process of the sensor surface. Turning to the accelerated recovery from NH3 exposures under the action of O3 , an oxidation reaction between the O3 and the dissolved NH3 is likely to occur:
H3 O+ + O− s ↔ OHs + H2 O
2NH3 + 4O3 ↔ NH4 NO3 + 4O2 + H2 O
OH− + OH2 + s ↔ H2 O + OHs .
(13a)
The ammonium nitrate, on the other hand, is likely to dissociate again forming NH4 + and NO3 − counter ions: NH4 NO3 ↔ NH4 + + NO3 −
(13b)
with opposing effects on the HD baseline resistance. Considering the accelerating effect of UV light exposure, two explanations seem plausible: considering the short UV wavelength (λ ∼ 255 nm), O3 could be generated in the ambient air that drives the above two reactions; secondly the response and recovery time constants could be accelerated due to electronic charge carriers photo-excited in the HD bulk and transported to the diamond-electrolyte interface. So far, we could not find compelling evidence for any of these explanations. Here further research is required. The third effect, namely the enhancement of the steady-state gas response Sgas upon partial conversion of H-termination into O-termination sites could tentatively be explained by the fact that the more dilute hole gases in partially oxygenated HD sensors are more easily disturbed by the ionic charges in the electrolyte layer. According to the theoretical investigations of Ahlers et al. [36], however, the relative magnitude of the gas response Sgas should be largely independent on the magnitude of the baseline resistance. Fig. 8, on the contrary, shows that Sgas increases more than proportionally as the magnitude of the baseline resistance is increased. In this context it is relevant to note that it has been proposed that the surface OH− groups (OHs ) might have a catalytic effect in that they enable a more efficient transfer of electrical charge across the liquid electrolyte/semiconductor interface [32]. The difficulty in enabling such charge exchange reactions is that electronic charge needs to be interconverted into ionic charge and vice versa as charge is transferred across the interface. Such exchange interactions are enabled by the fact that OH surface groups can interconvert between negatively and positively charged states, i.e.: O− s , OHs ·OH2 + s Within this picture, the transfer of an electron from the diamond valence band into the surface electrolyte is proposed to proceed via two partial reactions: e− + OHs ↔ O− s + H•
(14a)
163
(14b)
where H• stands for a H radical. In the overall reaction a valence band electron in the diamond disappears, i.e. a hole is generated and a H3 O+ ion in the electrolyte is annihilated, which corresponds to the generation of a negative ionic charge in the electrolyte: e − + H 3 O+ ↔ H 2 O + H • .
(14c)
The H atom in turn may be trapped on a neighbouring OH surface group in case a valence band hole is trapped at that site: H• + OHs + h+ ↔ OH2 + s .
(15a)
This latter site in turn may trap an OH− ion from the electrolyte and thus reform the original OH surface group: (15b)
In these latter two interactions a hole in the diamond is annihilated and a negative ion in the electrolyte is neutralised, which corresponds to a gain of net positive charge in the electrolyte (Fig. 15). On the whole these considerations show that an electrical charge originally residing in the diamond valence band can be injected into the surface electrolyte and transferred back again without consuming OH surface groups. This, obviously, demonstrates the catalytic activity of the surface OH groups and their impact on the HD gas sensitivity. A final point to be noted is that, in equilibrium, there are equal numbers of charges crossing the interface in either direction. In effect this means that there will be equal numbers of positively charged OH2 + s and negatively charged O− s centres, which means that statistically the entirety of the OHs surface groups will be neutral, i.e. these catalytic centres will not have an overall effect on the band bending at the semiconductor/electrolyte interface. Turning to the question how far the observed HD gas sensing effects are generic to a wider class of sensing materials, we note that we have observed a very similar type of low-temperature gas response on thin films of SnO2 [21] and on hydrogenated amorphous silicon [22]. In both cases the response is highly selective to NH3 and NO2 , i.e. to gases capable of electrolytic dissociation. At the same time there is no response to water vapor, which is consistent with a thin layer of adsorbed water. A
Fig. 15. Catalytic effect of OH-terminated surface sites. Valence alternation reactions involving such sites mediate the transfer of electronic charge between semiconductor and electrolyte.
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response to water vapor, however, is recovered once the surface temperature of the semiconductor materials is raised beyond 200 ◦ C, i.e. into a range of temperatures at which no adsorbed water layer can exist. Considering the specific case of metal oxide materials, the normal combustive gas response pattern takes over as the surface temperature is raised beyond 200 ◦ C, i.e. the well-known broad-range response to all kinds of combustible gases. Conditions that might be responsible for such common behaviour patterns are an approximate matching of redox levels in the liquid electrolyte and semiconductor band edges. As on HD surfaces, surface OH groups also might play a role in enabling charge transfer processes across other semiconductor electrolyte interfaces. The existence of OH surface groups has been firmly established on metal oxide surfaces. On hydrogenated surfaces such as on a-Si:H and on porous silicon, surface OH groups emerge as these surfaces become partially oxidised. Compared to these latter materials, HD presently seems to be the best and most intensively investigated case of a semiconductor material that exhibits a surface conductivity that arises from a thin layer of adsorbed water and that is sensitive to gases in the immediate neighbourhood of the adsorbed electrolyte layer. HD, therefore, seems to be an ideal model substance for elucidating the mechanism underlying the room temperature gas response of a wider range of semiconductor materials. 6. Conclusions Results of gas sensing experiments on hydrogenated diamond surfaces have been presented. The conclusions that we derive from these results are listed below. Concerning the steady-state gas response Sgas the following features stand: - gas sensing interactions at HD surfaces occur at room temperature under conditions at which the sensor surface is covered by an extremely thin adsorbed water layer (d ∼ 1 nm); - Sgas of HD sensors is limited to analyte gases that can undergo electrolytic dissociation in the surface liquid electrolyte layer; - Sgas of HD surfaces is enhanced by a small number of Otermination sites; - the Sgas features of HD sensors can be satisfactorily explained in terms of the surface transfer doping model of diamond. With regard to the kinetics of the gas response, we note that: - the above room-temperature dissociative gas response is associated with long response and particularly long recovery time constants; - both response and recovery time constants decrease with increasing surface temperature. The highest maximum operation temperatures are determined by the evaporation limit of the surface electrolyte layer, which approaches 200 ◦ C under no-flow conditions; - in addition to temperature, O3 and UV light exposure effectively reduce the room-temperature sensor recovery time
constants – likely by oxidation reactions on the dissolved analyte species. With regard to other gas sensing materials, our main conclusions are: - the dissociative gas sensing effect operative on HD surfaces is far more selective than the well-established combustive gas sensing effect operative on heated metal oxide surfaces; - similar dissociative gas sensing effects as in the HD case can also be observed on other kinds of semiconductor surfaces, when operated under low-temperature conditions in ambient air. A particularly noteworthy example is thin-film SnO2 operated at temperatures below 150–200 ◦ C; - due to the extensive work on surface transfer doping at diamond surfaces [23–25], HD represents a valuable model substance for the further investigation of the low-temperature dissociative gas response. With regard to other types of diamond gas and chemical sensors [37–39] we note that CVD and PECVD-deposited diamond films may prove to be a commercially viable low-cost approach towards diamond gas sensors. In particular this latter type of films is compatible with standard silicon and silicon MEMS processing. Acknowledgement Part of the work at the Walter Schottky Institute was supported by the Deutsche Forschungsgemeinschaft DFG (Ei 182/1-3). References [1] W. G¨opel, K.D. Schierbaum, SnO2 sensors: current status and future trends, Sens. Actuators B 26 (1995) 1–12. [2] D.E. Williams, Conduction and gas response of semiconductor gas sensors, in: Solid State Gas Sensors, Adam Hilger, Bristol, 1987. [3] K. Ihokura, J. Watson, The Stannic Oxide Gas Sensor—Principles and Applications, CRC Press, Boca Raton, 1994. [4] S.R. Morrison, Chemical sensors, in: S.M. Sze (Ed.), Semiconductor Sensors, Wiley, New York, 1994. [5] H.V. Shurmer, J.W. Gardner, H.T. Chan, The application of discrimination technique to alcohols and tobaccos using tin-oxide sensors, Sens. Actuators 18 (1989) 361–371. [6] J.W. Gardner, P.N. Bartlett, Electronic Noses: Principles and Applications, Oxford University Press, Oxford, 1999. [7] R. Menzel, J. Goschnick, Gradient gas sensor microarrays for on-line process control—a new dynamic classification model for fast and reliable air quality assessment, Sens. Actuators B 68 (2000) 115–122. [8] S. Ahlers, G. M¨uller, T. Doll, in: C.A. Grimes, E.C. Dickey, M.V. Pisko (Eds.), Factors Influencing the Gas Sensitivity of Metal Oxide Materials, Encyclopedia of Sensors, The Pennsylvania State University, University Park, USA, 2006, ISBN 1-58883-059-4. [9] N. Barsan, M. Schweizer-Berberich, W. G¨opel, Fundamental and practical aspects in the design of nanoscaled SnO2 gas sensors: a status report, Fresenius J. Anal. Chem. 365 (1999) 287–304. [10] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, J. Electroceram. 7 (2001) 143–167. [11] S.C. Chang, Oxygen chemisorption on tin oxide: Correlation between electrical conductivity and EPR measurements, J. Vac. Sci. Technol. 17 (1980) 366–369.
A. Helwig et al. / Sensors and Actuators B 133 (2008) 156–165 [12] A. Cabot, A. Vil`a, J.R. Morante, Analysis of the catalytic activity and electrical characteristics of different modified SnO2 layers for gas sensors, Sens. Actuators B 84 (2002) 12–20. [13] D. Koh, Surface processes in detection of reducing gases with SnO2 -based devices, Sens. Actuators 18 (1989) 71–113. [14] T. Doll (Ed.), Advanced Gas Sensing, The Electroadsorptive Effect and Related Techniques, Kluwer Academic, Boston, 2003, ISBN 1-4020-74336. [15] T. Becker, S. M¨uhlberger, C. Bosch-vB.raunm¨uhl, G. M¨uller, T. Ziemann, K.V. Hechtenberg, Air pollution monitoring using tin-oxide-based microreactor systems, Sens. Actuators B 69 (2000) 108–119. [16] Z. Jie, H. Li-Hua, G. Shan, Z. Hui, Z. Jing-Gui, Alcohols and acetone sensing properties of SnO2 thin films deposited by dip coating, Sens. Actuators B 115 (2006) 460–464. [17] H.C. Wang, Y. Li, M.J. Yang, Fast response thin film SnO2 gas sensors operating at room temperature, Sens. Actuators B 119 (2006) 380–383. [18] E. Comini, G. Faglia, G. Sberveglieri, UV light activation of tin oxide thin films for NO2 sensing at low temperatures, Sens. Actuators B 78 (2001) 73–77. [19] K. Anothainhart, M. Burgmair, A. Karthigeyan, M. Zimmer, I. Eisele, Light enhanced NO2 gas sensing with tin oxide at room temperature: conductance and work function measurements, Sens. Actuators B 93 (2003) 580–584. [20] T.Y. Yang, H.M. Lin, B.Y. Wie, C.Y. Wu, C.K. Lin, UV enhancement of the gas sensing properties of nano-TiO2 , Rev. Adv. Mater. Sci. 4 (2003) 48–54. [21] A. Helwig, G. M¨uller, M. Eickhoff, G. Sberveglieri, Dissociative gas sensing at metal oxide surfaces, IEEE Sens. J. 7 (2007) 1675–1679. [22] A. Helwig, G. M¨uller, G. Sberveglieri, G. Faglia, Gas sensing properties of hydrogenated amorphous silicon films, IEEE Sens. J. 7 (2007) 1506–1512. [23] M.I. Landstrass, K.V. Ravi, Hydrogen passivation of electrically active defects in diamond, Appl. Phys. Lett. 55 (1989) 1391–1393. [24] T. Maki, S. Shikama, M. Komori, Y. Sakaguchi, K. Sakuta, T. Kobayashi, Hydrogenating effect of single-crystal diamond surface, Jpn. J. Appl. Phys. 31 (1992) L1446–L1449. [25] F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Origin of surface conductivity in diamond, Phys. Rev. Lett. 85 (2000) 3472–3475. [26] A. Hokazono, H. Kawarada, Enhancement/depletion surface channel field effect transistors of diamond and their logic circuits, Jpn. J. Appl. Phys. 36 (1997) 7133–7139. [27] K. Hayashi, S. Yamanaka, H. Okushi, K. Kajimura, Study of the effect of hydrogen on transport properties in chemical vapor deposited diamond films by Hall measurements, Appl. Phys. Lett. 68 (1996) 376–378. [28] J.A. Garrido, A. H¨artl, S. Kuch, M. Stutzmann, pH sensors based on hydrogenated diamond surfaces, Appl. Phys. Lett. 86 (2005) 073504. [29] A. Helwig, G. M¨uller, O. Weidemann, A. H¨artl, J.A. Garrido, M. Eickhoff, Gas sensing interactions at hydrogenated diamond surfaces, IEEE Sens. J. 7 (2007) 1349–1353. [30] J.A. Garrido, A. H¨artl, S. Kuch, M. Stutzmann, O.A. Williams, R.B. Jackman, pH sensors based on hydrogenated diamond surfaces, Appl. Phys. Lett. 86 (2005) 073504. [31] J.B. Cui, J. Ristein, L. Ley, Dehydrogenation and the surface phase transition on diamond (1 1 1): kinetics and electronic structure, Phys. Rev. B 59 (1999) 5847–5856. [32] J. Ristein, M. Riedel, L. Ley, Electrochemical surface transfer doping mechanism behind the surface conductivity of hydrogen-terminated diamond, J. Electrochem. Soc. 151 (2004) E315–E321. [33] A. Helwig, G. M¨uller, G. Sberveglieri, G. Faglia, Gas response times of nano-scale SnO2 gas sensors as determined by the moving gas outlet technique, Sens. Actuators B 126 (2007) 174–180. [34] M. Szameitat, X. Jiang, W. Beyer, Influence of adsorbates on the surface conductivity of chemical vapor deposition diamond, Appl. Phys. Lett. 77 (2000) 1554–1556.
165
[35] F. Maier, J. Ristein, L. Ley, Electron affinity of plasma-hydrogenated and chemically oxidized diamond (1 0 0) surfaces, Phys. Rev. B 64 (2001) 165411. [36] S. Ahlers, G. M¨uller, T. Doll, A rate equation approach to the gas sensitivity of thin film metal oxide materials, Sens. Actuators B 107 (2005) 587– 599. [37] W.P. Kang, Y. Gurbuz, J.L. Davidson, D.V. Kerns, New hydrogen sensor using a polycrystalline diamond-based Schottky diode, J. Electrochem. Soc. 141 (1994) 2231–2234. [38] Y. Gurbuz, W.P. Kang, J.L. Davidson, D.V. Kerns, Diamond microelectronic gas sensor for detection of benzene and toluene, Sens. Actuators B 99 (2004) 207–215. [39] K.L. Soh, W.P. Kang, J.L. Davidson, Y.M. Wong, A. Wisitsora-at, G. Swain, D.E. Cliffel, CVD diamond anisotropic film as electrode for electrochemical sensing, Sens. Actuators B 91 (2003) 39–45.
Biographies Andreas Helwig received a ME degree from the Munich University of Applied Sciences studying in the field of nano-technology in 2003 after graduating in precision- and micro-engineering in 2001. Also in 2003, he started as doctoral candidate at the EADS Corporate Research Centre Germany in the Department Microsystems and Electronics. Currently he is working amongst other things in the field of smart sensors for maintenance, safety and emission control. ¨ Gerhard Muller was graduated in physics from the University of Heidelberg in 1974 and obtained a PhD degree in 1976. Subsequently he was employed at the Max-Planck-Institute for Nuclear Physics in Heidelberg, where he performed work on ion implantation and nuclear solid state physics. In 1979, he changed to the University of Dundee, UK, 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 Chemical Sensors Group of the EADS Corporate Research Centre Germany (∼15 employees). Gerhard M¨uller is author and co-author of about 200 articles in scientific journals and conference proceedings. Since 2000, he is a lecturer in the Munich University of Sciences. Jose Antonio Garrido graduated from the E.T.S.I. Telecomunicaci´on (Universidad Polit´ecnica de Madrid) in 1996 and obtained his PhD degree in 2000. Between 2000 and 2003, he worked as a post-doc at the Walter Schottky Institute (WSI, Department of Physics, T.U. Munich). Since year 2003, he is the team leader of the diamond group at the 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 bio-chemical sensors. Martin Eickhoff studied theoretical physics at the University of Dortmund. From 1995 to 2000, he was part of the DaimlerChrysler AG Central Research Department in Munich where he worked on the development of 3C-SiC technology for high temperature sensor applications. From 2000 to 2001, he was part of the microsensor development department of Infineon Technologies AG in Munich where he worked on surface micromachined microsensors in polysilicon technology. Since 2001, he leads the research group for GaN-based sensors of the Walter Schottky Institute at the Technical University of Munich.
Sensors and Actuators B 133 (2008) 156–165
Gas sensing properties of hydrogen-terminated diamond A. Helwig a,∗ , G. M¨uller a , J.A. Garrido b , M. Eickhoff b a
b
Innovation Works Germany, EADS Deutschland GmbH, D-81663 M¨unchen, Germany Walter Schottky Institut, Technische Universit¨at M¨unchen, Am Coulombwall 3, D-85748 Garching, Germany Received 16 July 2007; received in revised form 4 February 2008; accepted 5 February 2008 Available online 12 February 2008
Abstract Hydrogen-terminated diamond (HD) samples possess a p-type surface conductivity (SC) which is caused by transfer doping to an adsorbed liquid electrolyte layer. We report on gas sensing experiments on such samples and show that these selectively respond to analyte gases that can undergo electrolytic dissociation in the surface electrolyte layer. These gas sensing interactions occur at room temperature and are far more selective than sensing interactions at heated metal oxide layers. Successive substitution of surface hydrogen atoms by oxygen atoms causes the sensor baseline resistance and the gas-induced resistance changes to increase. This latter observation suggests that a small number of O-termination sites may have a catalytic effect on the gas sensing interactions. Increased temperature, O3 and UV light exposure all reduce the sensor recovery time constants. Heating beyond the water evaporation threshold (∼200 ◦ C) causes the surface electrolyte layer to disappear and the gas sensing effect to vanish. Re-adsorption of the surface electrolyte layer re-establishes both the sensor baseline resistance and the gas sensing effect. A model for the dissociative gas response is proposed that accounts for the observed experimental facts. © 2008 Elsevier B.V. All rights reserved. Keywords: Diamond surfaces; Electrolytic dissociation; Gas sensing mechanism; Surface transfer doping
1. Introduction In recent years semiconductor gas sensors have found widespread commercial application in gas monitoring and alarm applications. To date most of these sensors employ metal oxide (MOx ) semiconductors as gas sensitive materials [1–7]. A common feature of all kinds of MOx materials is that they are ionic and that they exhibit (almost) completely oxidised surfaces. As oxides, such sensor materials are stable against oxidation when operated at elevated temperatures in ambient air. In addition, as ionic materials, metal oxides do not tend to irreversibly form covalent bonds to molecular species that may abound in the surrounding gas atmosphere. It is exactly for these two reasons that MOx materials have become very popular subjects of investigations in the field of gas sensors. The common view of the metal oxide gas sensitivity [8–11] is that MOx materials naturally tend to adsorb O2 molecules in the ∗ Corresponding author at: EADS, Innovation Works Germany, Sensors, Electronics & Systems Integration, IW - SI, D-81663 M¨unchen, Germany. Tel.: +49 89 607 28197; fax: +49 89 607 24001. E-mail address: [email protected] (A. Helwig).
0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.02.007
form of O− ions on their surfaces. On the sensor surface the O− ions in turn may undergo reactions with adsorbed combustible gases forming H2 O and CO2 molecules that may desorb into the gas phase again [12–15]. As these reaction products can only be emitted in a neutral state, the electrons initially trapped on the surface O− ions are re-injected into the MOx conduction band and thus generate an enhanced n-type conductivity which serves as a sensor signal. With combustibility being a major criterion for detectability on MOx surfaces, it is also clear that MOx gas sensors have become ill-famed for their cross-sensitivity to various reactive gases. In order to enable the above surface reactions, MOx gas sensors are usually heated to temperatures in the range between 300 and 500 ◦ C. More recent work has shown that MOx gas sensors are also able to respond to gases at room temperature and slightly above [16,17]. In this latter case photoexcitation is often used to accelerate the surface reactions [18–20]. In recent papers we have shown that semiconductor sensors in general and metal oxides in particular,are likely to carry a very thin and firmly adsorbed water film on their surfaces [21,22,29]. There we have further shown that these liquid electrolyte layers do not prevent the semiconductors to exhibit a gas response at all
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but rather restrict their response to molecular species which are able to undergo electrolytic dissociation in the adsorbed liquid electrolyte layer. Among those semiconductors which exhibit a lowtemperature dissociative gas response, hydrogenated diamond (HD) plays a particular role in that it represents an ideal model system for studying this kind of low-temperature gas response. In comparison to metal oxides, which represent an extremely diverse class of materials, and hydrogenated amorphous silicon, which is a non-crystalline material with a variable composition and microstructure, hydrogenated diamond is a well-defined monocrystalline material, which has received a great deal of attention because of its peculiar p-type surface conductivity. Although details are still under debate, there is a general consensus that this p-type surface conductivity arises from surface transfer doping to an adsorbed liquid electrolyte layer [23–25]. On the application side HD has already found a number of device applications such as field effect transistors [26], single-hole transistors [27] or pH-sensors [28]. In the following we go beyond our initial work on the HD gas sensitivity [29] and report on a wider range of gas sensing experiments involving more species than previously. In addition we report on the accelerating effect of temperature and UV radiation. Last not least we derive from these experiments a comprehensive model of the HD gas sensing effect that builds on the surface transfer-doping model of diamond [25]. 2. Experimental The samples investigated in this work were nominally undoped (1 0 0)-oriented type Ib diamond substrates from Sumitomo. For hydrogen termination the samples were first cleaned in CrO3 /H2 SO4 for 1 h at 180 ◦ C to remove non-diamond components and then in a 30% H2 O2 solution for 15 min. This procedure results in an oxygen-terminated diamond surface. The functional hydrogen termination was accomplished by exposing the samples 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 were used to define insulating surface areas. Finally, Ti/Au (20 nm/200 nm) metal pads were deposited on the remaining patches of Hterminated diamond to form ohmic contacts. The contact pad area was 1000 m × 230 m and the spacing between the contacts was 1000 m (Fig. 1). Before performing gas sensing tests, these samples were stored in ambient air and were exposed to normal daylight during that period. After a set of initial measurements on these samples, the level of H-termination was reduced in several steps by ozone (O3 ) treatments in an arc-discharge ionization system (Sander LabOzonizer) [22]. This allowed the reactive O3 species to directly interact with the surface C H bonds. In order to monitor the ozonisation process the surface conductivity was monitored in situ at a fixed voltage of 1.0 V and the process was interrupted after a significant increase in the sensor baseline resistance had been achieved. In this way, the density of surface C-atoms with a H-termination and the hole
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Fig. 1. Top: cross section through a hydrogen-terminated diamond sensor. Bottom: top view onto a processed diamond sensor.
density in the sub-surface hole gas was reduced. It can clearly be seen that after each treatment the surface conductivity (SC) was decreased [30]. Fig. 2 shows the different I/V characteristics of one and the same diamond sample after repeated ozone plasma treatments. After each ozonisation step the same range of gas sensing tests was repeated as on the fully hydrogenated samples to assess the impact of the oxygen-terminated surface sites on the gas sensing performance. The gas sensing experiments were performed at the gas test rig of EADS Innovation Works. In this laboratory up to six gases or vapors can be mixed and injected into a test chamber. In our experiments all analyte gases investigated (H2 , various hydrocarbons, CO, NH3 , O3 , and various nitrous oxides) were diluted in a background of synthetic air (80% N2 /20% O2 ). Whereas
Fig. 2. I–V characteristic of the diamond after increasing ozone treatment times.
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the high-purity analyte gases were supplied from gas cylinders, small admixtures of O3 (<500 ppb) were produced using an Ansyco ozone generator. Controlled relative humidity levels ranging from 10 to 90% were generated by vaporizing de-ionized H2 O into the synthetic background air. 3. Results 3.1. Fully hydrogenated diamonds In this first paragraph we consider gas sensing experiments that had been performed on a diamond sample with a fully Hterminated surface. In the experiment reported in Fig. 3 we have exposed the HD sample towards high concentrations of C2 H4 , NO2 , C2 H5 OH, CO, H2 and NH3 with the HD sample being kept at room temperature. Going from left to right, the first exposure to C2 H4 clearly does not produce any sensor response. A significant response, however, is observed upon exposing the sensor to NO2 . As expected for a room-temperature-operated gas sensor, the response time is long and the recovery time even longer. The recovery from the 50 ppm NO2 pulse in fact overlaps in duration with the subsequent H2 and CO exposure pulses. The insensitivity to these latter two gases, which has already been described in our previous publication [29], reveals here from the absence of any disturbance of the NO2 recovery tail. The final NH3 exposure pulse again produces a strong response, which is opposite in sign to the NO2 response. Again, we observe a very long recovery from the NH3 exposure. Comparing to gas detection at heated MOx surfaces, we note that the HD gas response is much more restricted, exhibiting no response to H2 , hydrocarbons and CO, i.e. to a range of gases that can easily undergo combustion reactions at heated metal oxide surfaces. NO2 and NH3 , on the other hand, can be detected on both kinds of surfaces. The room-temperature response of HD samples, however, is associated with longer response and recovery time constants than the response of heated MOx gas sensors. The concentration-dependent response towards those two gases that are detectable with HD gas sensors is shown in
Fig. 3. Response of a fully hydrogenated HD sample towards a range of analyte gases that were diluted into a background of synthetic air. During measurements the HD sample was kept at room temperature.
Fig. 4. Concentration dependence of the NH3 and NO2 response of a fully hydrogenated diamond sample. During these tests the HD sample was kept at room temperature.
Fig. 4. In this plot the NO2 gas sensitivity was calculated from S = R0 /Rgas − 1, where R0 stands for the sensor resistance under clean-air conditions (baseline resistance) and Rgas for the sensor resistance under the influence of NO2 . In the case of NH3 we used S = Rgas /R0 − 1 to account for the fact that opposite resistance changes are encountered in the case of NO2 and NH3 exposures and to arrive at consistently positive values of S in both cases. Interestingly, this plot reveals a roughly linear concentration dependence of the gas response S. Combustion reactions on MOx surfaces, on the other hand, give rise to sub-linear concentration dependencies. Among the range of oxidizing gases that can be detected with MOx sensors, O3 stands out as it can easily be detected in ppb (10−9 ) concentrations. For this reason we have also performed O3 sensing tests on HD surfaces. In such RT exposures we could not find any sensitivity to the O3 itself. Rather, we could observe that the recovery from preceding NH3 exposures was accelerated. Fig. 5 demonstrates this effect and shows that a short ozone exposure following a NH3 exposure pulse dramatically reduces the time that is need to reach the sensor baseline resistance again. In comparison, thermal agitation alone would have needed much longer times to reset the sensor. This latter effect is demonstrated by the initial slow recovery that immediately follows each NH3 exposure pulse and that precedes the respective O3 recovery pulse. An ever-present background gas that abounds in high and highly variable concentrations is H2 O vapor. Fig. 6 shows that the H2 O response is negligibly small in comparison to the NH3 and NO2 sensitivities. The complete absence of a humidity effect is consistent with the observation that HD surfaces are covered by thin layers of adsorbed water in which the other analyte gases may or may not dissolve [24,25]. This vanishing humidity dependence is at variance with the case of high-temperature operated MOx gas sensors, which do not accommodate such multi-layer adsorption layers and where direct single-
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Fig. 5. Response of a HD sample towards increasing NH3 concentration steps. The ozone gas pulses demonstrate the O3 -induced resetting of the sensor surface to baseline by NH3 oxidation. During these tests the HD sample was kept at room temperature.
molecule interactions with the MOx surface are likely to prevail. The results reported so far have all been obtained with the HD samples being kept at room temperature during the gas exposure tests. In the case of MOx gas sensors, the sensor operation temperature is a very important parameter that determines the sensor behaviour. We, therefore, have also carried out tests at higher sensor operation temperatures. In this context it should be noted that the H-termination on top of diamond surfaces is stable up to annealing temperatures in the order of 800 ◦ C in vacuum [31,32]. The adsorbed surface electrolyte, however, is much less stable. Electrolyte desorption was observed at temperatures above 150 ◦ C [29]. Respecting these much more rigid temperature constraints we have performed gas sensing tests up to surface temperatures of 140 ◦ C. With regard to the crosssensitivity behaviour we observed results that were similar to the ones reported above. The main difference was that those gases that can be detected exhibited a faster response and recovery behaviour than at room temperature. Fig. 7 shows such results
Fig. 6. Response of a fully hydrogenated HD sample towards water vapor. During these tests the HD sample was kept at room temperature.
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Fig. 7. Response of a fully hydrogenated HD sample towards NH3 gas pulses. In this experiment a fully hydrogenated HD sample was operated at successively higher temperature.
for the special case of NH3 . From these data it is seen that both the response and recovery time constants decrease with temperature with the recovery times being much longer than the response times. As a second effect we observe a reduction in the absolute magnitude of the steady-state gas response. Compared to MOx materials, however, this temperature effect is small [33]. 3.2. Partially oxygen-terminated diamond surfaces The above described results all relate to diamonds with a high degree of H-termination. In order to assess the effects of a reduced level of H-termination, ozonisation treatments were performed and the same range of measurements as above was repeated after each ozonisation step. From these measurements it was revealed that ozonisation causes a significant increase in the baseline resistance R0 and an even larger increase in the gas response S as the ozonisation time increases. The obtained results are visualized in Fig. 8.
Fig. 8. Response of a HD sample towards NO2 and NH3 gases as a function of the sensor baseline resistance.
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Fig. 9. Response of a HD sample with different baseline resistance towards various analyte gases. Dilution of the hole gas was performed by repeated exposure of the original HD sample towards an intense oxygen plasma.
In order to assess possible changes in the cross sensitivity profile, the gas sensing sequence of Fig. 3 was repeated after each ozonisation step. In this way the data of Fig. 9 was obtained. These results again emphasize the effect of ozonisation on the sensor baseline resistance R0 and on the gas response S, but they do not indicate any change in the cross sensitivity behaviour. The effect of the O-termination sites therefore seems to be restricted towards facilitating gas sensing interactions that are in principle possible at H-terminated diamond surfaces, rather than enabling qualitatively new gas sensing interactions. Fig. 9 also shows that the gas response S rises as the baseline resistance of the diamond sample is increased. The O-termination sites therefore may be regarded as performing a catalytic role on predominantly Hterminated diamond surfaces. Another important observation is that – in case a non-zero gas sensing effect is observed – the room-temperature desorption of the detectable species is quite slow and not influenced by the presence of O-terminated sites. Fig. 10 displays the concentration-dependent response of a partially oxygen-terminated diamond sample towards different nitrogen-containing gases. The response to NOx species (x = 1, 2) is quite strong, enhancing the p-type surface conductivity of the HD samples. A strong response is also obtained in the case of NH3 exposure. This time, however, a conductivity decrease is observed. A gas response into the same direction as for NH3 is also observed upon N2 O exposure. This latter effect, however, is comparatively small – even at high concentration levels.
Fig. 10. Concentration-dependent response characteristic of a partially hydrogenated diamond sample towards different nitrogen containing gases. NO and NO2 exhibit an acidic (resistance decrease) and NH3 and N2 O a basic (resistance increase) response.
sensor by roughly one order of magnitude. Upon NH3 exposure a significantly shorter response time is observed as compared to the second exposure pulse that was carried out in the dark. When the UV illumination is continued beyond the NH3 exposure pulse, the UV light also leads to a decrease of the desorption time. 3.4. Effect of electrolyte desorption In this set of experiments we have performed measurements aiming at elucidating the HD gas sensing mechanism. Within the framework of the transfer doping model the occurrence of a 2D hole gas at a diamond surface is due to transfer doping, i.e. a transfer of valence electrons from the top of the diamond valence band to an adsorbed surface electrolyte layer. In comparison the origin of the HD gas sensing effect is less clear. In order to find more clues to the gas sensing effect, we tried to reduce
3.3. Effect of UV light exposure The above-reported experiments have already revealed the accelerating effect of temperature on the speed of the adsorption and desorption processes. Similar effects were observed upon illumination with UV light (Fig. 11). There a HD sample with a partially oxidised surface was exposed to two successive NH3 pulses. During the first pulse light from a UV LED (λ ∼ 255 nm) irradiated the HD surface. Before the NH3 exposure pulse the UV light decreased the baseline resistance of the
Fig. 11. Response of a HD sample with a partially oxidised surface towards two NH3 exposure pulses. During the first pulse the HD sample was illuminated with UV light.
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Fig. 12. Change of the HD baseline resistance and of the NH3 gas sensing effect in response to a desorption step induced by an intense flow of hot air (∼90 ◦ C).
the amount of adsorbed electrolyte up to the limit of complete desorption. Desorption was performed by exposing the HD sample to an intense flow of hot air (∼90 ◦ C). After each desorption step the resulting baseline resistance was measured and a NH3 gas sensing test was performed. From the results displayed in Fig. 12, two effects stand out: firstly, after each desorption step the baseline resistance increases in agreement with the transfer doping model. Secondly, we observed a reduction in the NH3 response after each desorption step. This latter finding indicates that the transfer doping and the gas sensing effects are connected to the same cause, i.e. the presence of an adsorbed electrolyte layer. The break on the time axis after the second NH3 exposure indicates the very long time needed to reach the initial sensor baseline resistance again. During this time interval the HD surface was sequentially exposed to high levels of humidity and UV light irradiation. After reaching the original baseline again, the HD sample was exposed to an additional NH3 gas pulse. Fig. 12 demonstrates that after removing and re-adsorbing the surface electrolyte layer, the magnitude of the induced gas response is restored.
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Fig. 13. Transfer of valence electrons from the diamond surface to an adsorbed electrolyte layer. The adsorbed water layer is assumed to be slightly acid due to the dissolution of atmospheric CO2 .
uncertain stays the question, which of the two Redox-couples (H3 O+ /H2 O or H2 O/OH− ) is actually responsible for the electron transfer between diamond bulk and surface electrolyte. A much higher surface conductivity arises, in case acid-forming molecules become dissolved in the electrolyte layer. In the literature [34] it has been suggested that acid surface electrolyte layers may arise from the dissolution of atmospheric CO2 : CO2 + 2H2 O ↔ H2 CO3 + H2 O ↔ H3 O+ + HCO3 −
(2)
In this latter case a much higher sub-surface hole density is expected and the HCO3 − ions in the electrolyte now take over the role of the counter-ions. The latter case is illustrated in Fig. 13. For completeness, the real-space picture of Fig. 13 needs to be complemented by a band-structure diagram, which summarises the energetic side of the surface transfer doping model. This latter aspect is illustrated in Fig. 14. In this figure we visual-
4. Model of the HD gas sensing effect In this section we suggest a model of the HD gas sensing effect that builds on the established model of surface transfer doping [25]. To this end we first consider Fig. 13. This figure visualizes the basic idea of the surface transfer doping effect: exposing a HD surface to normal ambient air, a thin layer of adsorbed water – about 1 nm thick – forms at the HD surface. Assuming that this adsorbed water layer is neutral, i.e. in case it does not contain any acid- or base-forming impurities, small and equal densities of H3 O+ and OH− ions are formed: 2H2 O ↔ H3 O+ + OH−
(1)
Transferring valence electrons from the top of the diamond valence band to the dissolved H3 O+ ions, a thin sheet of holes below the diamond surface and a layer of compensating negative OH− ions in the adsorbed surface electrolyte are formed. Still
Fig. 14. Position of the valence and conduction bands of H-terminated diamond (left) and O-terminated diamond (right) relative to the H3 O+ /H2 O and H2 O/OH− redox levels in water (middle). The energetic match of the H3 O+ /H2 O redox level in water with the top of the valence band in H-terminated diamond facilitates the transfer of diamond valence electrons into the surface electrolyte and thus the formation of a p-type surface layer in undoped diamond.
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ize the energetic situation that arises when an adsorbed water layer is brought into contact with a diamond surface. In considering such contacts, two principally different situations need to be considered: hydrogen- and oxygen-terminated diamond. Due to the fact that hydrogen is more electropositive and oxygen more electronegative than carbon, the C(δ− ) − H(δ+ ) and the C(δ+ ) − O(δ− ) dipoles point into opposite directions. In the case of hydrogenated surfaces this leads to a negative electron affinity (NEA) of the diamond surface and to a positive electron affinity in the case of oxygen termination. Quantitatively, the EA difference between both kinds of diamond may amount up to 3 eV [35]. These different energetic positions of the conduction and valence bands in H- and O-terminated diamonds have drastic consequences for the exchange of electronic charge between diamond and adsorbed surface electrolyte. In the H-terminated case the maximum of the diamond valence band happens to coincide with one of the redox levels in the electrolyte layer. H3 O+ ions therefore can become neutralised by the transfer of valence electrons to the liquid electrolyte. As a consequence, a high hole density is able to build up easily at the diamond surface. This is the essence of the surface transfer doping model of the p-type surface conductivity (SC) in diamond. In the other case, namely that the diamond surface is oxygen-terminated, a huge thermal activation energy is required for the transfer doping. A p-type SC therefore does not arise on oxygen-terminated diamond. This latter effect relates to our above-described results in so far as it explains the rapid increase in the sensor baseline resistance as a HD surface becomes partially converted from Hto O-termination. A straight-forward generalisation of these ideas leads to an explanation of the HD gas sensing effect: molecules in the neighbouring gas phase with the ability of electrolytic dissociation can dissolve in the surface electrolyte either forming H3 O+ or OH− ions. These ions in turn determine the overall H3 O+ concentration and thereby the level of p-type surface conductivity in the diamond. With regard to those species that we could detect on HD surfaces, the following detection reactions are proposed: NH3 + H2 O ↔ NH4 + + OH−
(3)
NO2 + 2H2 O ↔ NO3 − + H3 O+ + (1/2)H2
(4)
NO + 3H2 O ↔ NO3 − + H3 O+ + (3/2)H2
(5)
N2 O + 5H2 O ↔ 2NH4 + + 2OH− + 2O2
(6)
With OH− decreasing and H3 O+ enhancing the p-type surface conductivity, we note that these dissolution reactions correctly predict the observed sign of the gas response. Concerning the gas response we note that the above reactions are overall results of two partial reactions in which the analyte molecules first become absorbed in the surface electrolyte and then electrolytically dissociated forming H3 O+ or OH− ions. Considering the absorption reactions first, we expect that the ease of absorption decreases in the order of reactions (7)–(10), as an increasing number of H2 O molecules needs to be broken up and covalent bonds to be rearranged to form HNO3 and NH3
Table 1 Comparison of measured gas sensitivities to chemical data of the different analyte gases Analyte gas
Sgas [c = 100 ppm]
Solubility
pKa
NO2 NO CO2 NH3 N2 O
30 28 n.a. 5 0.003
Hydrolyses (<2550 g/l) 0.067 g/l 0.0005 g/l 541 g/l 1.2 g/l
−1.32 −1.32 3.3
pKb
4.79 4.79
molecules: NH3 + nH2 O ↔ (NH3 )naq
(7)
NO2 + H2 O ↔ HNO3 + (1/2)H2
(8)
NO + 2H2 O ↔ HNO3 + (3/2)H2
(9)
N2 O + 3H2 O ↔ 2NH3 + 2O2
(10)
Considering the second step, namely electrolytic dissociation, we expect gas sensing reactions involving the dissociation of HNO3 to be favoured over those involving NH3 : NH3 + H2 O ↔ NH4 + + OH− ; HNO3 + H2 O ↔ NO3 − + H3 O+ ;
pKb = 4.79 pKa = −1.32
(11) (12)
This latter ordering is revealed from the fact that the dissociation constant of HNO3 is larger than that of NH3 . Finally, considering the facts that CO2 does not dissolve readily in water and that the corresponding acid H2 CO3 is only a relatively weak one, it is suggested – and also confirmed by our experiments – that the sensor baseline resistance is more likely to be determined by the trace levels of NO2 in the ambient air (<1 ppm), rather than by the more abundant CO2 (∼400 ppm). These latter considerations are further supported by the data collected in Table 1. 5. Discussion The above model successfully accounts for the insensitivity of HD to water vapor and for its varying degrees of response to acid- and base-forming gases. Things that do not directly unfold from this model are the lengths of the response and recovery time constants, the accelerating effect of ozone and UV light on the speed of the sensor response and the catalytic activity of surface OH groups that increases the magnitude of the sensor response. In this final chapter we attempt to provide tentative explanations for these effects, based on the current state of our knowledge. Finally, and most importantly, we should like to discuss how far the mechanisms, that are operative at HD surfaces, might be generic in the sense that they determine the room-temperature response of other semiconductor materials as well. Turning to the response and recovery time constants first, we note that the above model explains the gas response as a result of the electrolytic dissociation of analyte gas molecules in the surface electrolyte. We have already argued above that in the response case covalent bonds need to be broken and reformed to arrive at ionic species that can undergo electrolytic dissociation.
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Further considering the fact that such rearrangements need to proceed at room temperature, long response times are no surprise. Once formed, the dissolved ionic species are stabilized by a coordination sphere of associated H2 O dipoles. In this way, the dissolved ions attain a largely increased effective molecular mass. Upon termination of the gas exposure pulse, the dissolved ionic species need to reform according to the reversible detection reactions above and the neutralised species need to evaporate into gas phase again. On the way towards re-evaporation, the breaking up of the sphere of associated H2 O dipoles is likely to represent a major kinetic hindrance that elongates the recovery process of the sensor surface. Turning to the accelerated recovery from NH3 exposures under the action of O3 , an oxidation reaction between the O3 and the dissolved NH3 is likely to occur:
H3 O+ + O− s ↔ OHs + H2 O
2NH3 + 4O3 ↔ NH4 NO3 + 4O2 + H2 O
OH− + OH2 + s ↔ H2 O + OHs .
(13a)
The ammonium nitrate, on the other hand, is likely to dissociate again forming NH4 + and NO3 − counter ions: NH4 NO3 ↔ NH4 + + NO3 −
(13b)
with opposing effects on the HD baseline resistance. Considering the accelerating effect of UV light exposure, two explanations seem plausible: considering the short UV wavelength (λ ∼ 255 nm), O3 could be generated in the ambient air that drives the above two reactions; secondly the response and recovery time constants could be accelerated due to electronic charge carriers photo-excited in the HD bulk and transported to the diamond-electrolyte interface. So far, we could not find compelling evidence for any of these explanations. Here further research is required. The third effect, namely the enhancement of the steady-state gas response Sgas upon partial conversion of H-termination into O-termination sites could tentatively be explained by the fact that the more dilute hole gases in partially oxygenated HD sensors are more easily disturbed by the ionic charges in the electrolyte layer. According to the theoretical investigations of Ahlers et al. [36], however, the relative magnitude of the gas response Sgas should be largely independent on the magnitude of the baseline resistance. Fig. 8, on the contrary, shows that Sgas increases more than proportionally as the magnitude of the baseline resistance is increased. In this context it is relevant to note that it has been proposed that the surface OH− groups (OHs ) might have a catalytic effect in that they enable a more efficient transfer of electrical charge across the liquid electrolyte/semiconductor interface [32]. The difficulty in enabling such charge exchange reactions is that electronic charge needs to be interconverted into ionic charge and vice versa as charge is transferred across the interface. Such exchange interactions are enabled by the fact that OH surface groups can interconvert between negatively and positively charged states, i.e.: O− s , OHs ·OH2 + s Within this picture, the transfer of an electron from the diamond valence band into the surface electrolyte is proposed to proceed via two partial reactions: e− + OHs ↔ O− s + H•
(14a)
163
(14b)
where H• stands for a H radical. In the overall reaction a valence band electron in the diamond disappears, i.e. a hole is generated and a H3 O+ ion in the electrolyte is annihilated, which corresponds to the generation of a negative ionic charge in the electrolyte: e − + H 3 O+ ↔ H 2 O + H • .
(14c)
The H atom in turn may be trapped on a neighbouring OH surface group in case a valence band hole is trapped at that site: H• + OHs + h+ ↔ OH2 + s .
(15a)
This latter site in turn may trap an OH− ion from the electrolyte and thus reform the original OH surface group: (15b)
In these latter two interactions a hole in the diamond is annihilated and a negative ion in the electrolyte is neutralised, which corresponds to a gain of net positive charge in the electrolyte (Fig. 15). On the whole these considerations show that an electrical charge originally residing in the diamond valence band can be injected into the surface electrolyte and transferred back again without consuming OH surface groups. This, obviously, demonstrates the catalytic activity of the surface OH groups and their impact on the HD gas sensitivity. A final point to be noted is that, in equilibrium, there are equal numbers of charges crossing the interface in either direction. In effect this means that there will be equal numbers of positively charged OH2 + s and negatively charged O− s centres, which means that statistically the entirety of the OHs surface groups will be neutral, i.e. these catalytic centres will not have an overall effect on the band bending at the semiconductor/electrolyte interface. Turning to the question how far the observed HD gas sensing effects are generic to a wider class of sensing materials, we note that we have observed a very similar type of low-temperature gas response on thin films of SnO2 [21] and on hydrogenated amorphous silicon [22]. In both cases the response is highly selective to NH3 and NO2 , i.e. to gases capable of electrolytic dissociation. At the same time there is no response to water vapor, which is consistent with a thin layer of adsorbed water. A
Fig. 15. Catalytic effect of OH-terminated surface sites. Valence alternation reactions involving such sites mediate the transfer of electronic charge between semiconductor and electrolyte.
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response to water vapor, however, is recovered once the surface temperature of the semiconductor materials is raised beyond 200 ◦ C, i.e. into a range of temperatures at which no adsorbed water layer can exist. Considering the specific case of metal oxide materials, the normal combustive gas response pattern takes over as the surface temperature is raised beyond 200 ◦ C, i.e. the well-known broad-range response to all kinds of combustible gases. Conditions that might be responsible for such common behaviour patterns are an approximate matching of redox levels in the liquid electrolyte and semiconductor band edges. As on HD surfaces, surface OH groups also might play a role in enabling charge transfer processes across other semiconductor electrolyte interfaces. The existence of OH surface groups has been firmly established on metal oxide surfaces. On hydrogenated surfaces such as on a-Si:H and on porous silicon, surface OH groups emerge as these surfaces become partially oxidised. Compared to these latter materials, HD presently seems to be the best and most intensively investigated case of a semiconductor material that exhibits a surface conductivity that arises from a thin layer of adsorbed water and that is sensitive to gases in the immediate neighbourhood of the adsorbed electrolyte layer. HD, therefore, seems to be an ideal model substance for elucidating the mechanism underlying the room temperature gas response of a wider range of semiconductor materials. 6. Conclusions Results of gas sensing experiments on hydrogenated diamond surfaces have been presented. The conclusions that we derive from these results are listed below. Concerning the steady-state gas response Sgas the following features stand: - gas sensing interactions at HD surfaces occur at room temperature under conditions at which the sensor surface is covered by an extremely thin adsorbed water layer (d ∼ 1 nm); - Sgas of HD sensors is limited to analyte gases that can undergo electrolytic dissociation in the surface liquid electrolyte layer; - Sgas of HD surfaces is enhanced by a small number of Otermination sites; - the Sgas features of HD sensors can be satisfactorily explained in terms of the surface transfer doping model of diamond. With regard to the kinetics of the gas response, we note that: - the above room-temperature dissociative gas response is associated with long response and particularly long recovery time constants; - both response and recovery time constants decrease with increasing surface temperature. The highest maximum operation temperatures are determined by the evaporation limit of the surface electrolyte layer, which approaches 200 ◦ C under no-flow conditions; - in addition to temperature, O3 and UV light exposure effectively reduce the room-temperature sensor recovery time
constants – likely by oxidation reactions on the dissolved analyte species. With regard to other gas sensing materials, our main conclusions are: - the dissociative gas sensing effect operative on HD surfaces is far more selective than the well-established combustive gas sensing effect operative on heated metal oxide surfaces; - similar dissociative gas sensing effects as in the HD case can also be observed on other kinds of semiconductor surfaces, when operated under low-temperature conditions in ambient air. A particularly noteworthy example is thin-film SnO2 operated at temperatures below 150–200 ◦ C; - due to the extensive work on surface transfer doping at diamond surfaces [23–25], HD represents a valuable model substance for the further investigation of the low-temperature dissociative gas response. With regard to other types of diamond gas and chemical sensors [37–39] we note that CVD and PECVD-deposited diamond films may prove to be a commercially viable low-cost approach towards diamond gas sensors. In particular this latter type of films is compatible with standard silicon and silicon MEMS processing. Acknowledgement Part of the work at the Walter Schottky Institute was supported by the Deutsche Forschungsgemeinschaft DFG (Ei 182/1-3). References [1] W. G¨opel, K.D. Schierbaum, SnO2 sensors: current status and future trends, Sens. Actuators B 26 (1995) 1–12. [2] D.E. Williams, Conduction and gas response of semiconductor gas sensors, in: Solid State Gas Sensors, Adam Hilger, Bristol, 1987. [3] K. Ihokura, J. Watson, The Stannic Oxide Gas Sensor—Principles and Applications, CRC Press, Boca Raton, 1994. [4] S.R. Morrison, Chemical sensors, in: S.M. Sze (Ed.), Semiconductor Sensors, Wiley, New York, 1994. [5] H.V. Shurmer, J.W. Gardner, H.T. Chan, The application of discrimination technique to alcohols and tobaccos using tin-oxide sensors, Sens. Actuators 18 (1989) 361–371. [6] J.W. Gardner, P.N. Bartlett, Electronic Noses: Principles and Applications, Oxford University Press, Oxford, 1999. [7] R. Menzel, J. Goschnick, Gradient gas sensor microarrays for on-line process control—a new dynamic classification model for fast and reliable air quality assessment, Sens. Actuators B 68 (2000) 115–122. [8] S. Ahlers, G. M¨uller, T. Doll, in: C.A. Grimes, E.C. Dickey, M.V. Pisko (Eds.), Factors Influencing the Gas Sensitivity of Metal Oxide Materials, Encyclopedia of Sensors, The Pennsylvania State University, University Park, USA, 2006, ISBN 1-58883-059-4. [9] N. Barsan, M. Schweizer-Berberich, W. G¨opel, Fundamental and practical aspects in the design of nanoscaled SnO2 gas sensors: a status report, Fresenius J. Anal. Chem. 365 (1999) 287–304. [10] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, J. Electroceram. 7 (2001) 143–167. [11] S.C. Chang, Oxygen chemisorption on tin oxide: Correlation between electrical conductivity and EPR measurements, J. Vac. Sci. Technol. 17 (1980) 366–369.
A. Helwig et al. / Sensors and Actuators B 133 (2008) 156–165 [12] A. Cabot, A. Vil`a, J.R. Morante, Analysis of the catalytic activity and electrical characteristics of different modified SnO2 layers for gas sensors, Sens. Actuators B 84 (2002) 12–20. [13] D. Koh, Surface processes in detection of reducing gases with SnO2 -based devices, Sens. Actuators 18 (1989) 71–113. [14] T. Doll (Ed.), Advanced Gas Sensing, The Electroadsorptive Effect and Related Techniques, Kluwer Academic, Boston, 2003, ISBN 1-4020-74336. [15] T. Becker, S. M¨uhlberger, C. Bosch-vB.raunm¨uhl, G. M¨uller, T. Ziemann, K.V. Hechtenberg, Air pollution monitoring using tin-oxide-based microreactor systems, Sens. Actuators B 69 (2000) 108–119. [16] Z. Jie, H. Li-Hua, G. Shan, Z. Hui, Z. Jing-Gui, Alcohols and acetone sensing properties of SnO2 thin films deposited by dip coating, Sens. Actuators B 115 (2006) 460–464. [17] H.C. Wang, Y. Li, M.J. Yang, Fast response thin film SnO2 gas sensors operating at room temperature, Sens. Actuators B 119 (2006) 380–383. [18] E. Comini, G. Faglia, G. Sberveglieri, UV light activation of tin oxide thin films for NO2 sensing at low temperatures, Sens. Actuators B 78 (2001) 73–77. [19] K. Anothainhart, M. Burgmair, A. Karthigeyan, M. Zimmer, I. Eisele, Light enhanced NO2 gas sensing with tin oxide at room temperature: conductance and work function measurements, Sens. Actuators B 93 (2003) 580–584. [20] T.Y. Yang, H.M. Lin, B.Y. Wie, C.Y. Wu, C.K. Lin, UV enhancement of the gas sensing properties of nano-TiO2 , Rev. Adv. Mater. Sci. 4 (2003) 48–54. [21] A. Helwig, G. M¨uller, M. Eickhoff, G. Sberveglieri, Dissociative gas sensing at metal oxide surfaces, IEEE Sens. J. 7 (2007) 1675–1679. [22] A. Helwig, G. M¨uller, G. Sberveglieri, G. Faglia, Gas sensing properties of hydrogenated amorphous silicon films, IEEE Sens. J. 7 (2007) 1506–1512. [23] M.I. Landstrass, K.V. Ravi, Hydrogen passivation of electrically active defects in diamond, Appl. Phys. Lett. 55 (1989) 1391–1393. [24] T. Maki, S. Shikama, M. Komori, Y. Sakaguchi, K. Sakuta, T. Kobayashi, Hydrogenating effect of single-crystal diamond surface, Jpn. J. Appl. Phys. 31 (1992) L1446–L1449. [25] F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Origin of surface conductivity in diamond, Phys. Rev. Lett. 85 (2000) 3472–3475. [26] A. Hokazono, H. Kawarada, Enhancement/depletion surface channel field effect transistors of diamond and their logic circuits, Jpn. J. Appl. Phys. 36 (1997) 7133–7139. [27] K. Hayashi, S. Yamanaka, H. Okushi, K. Kajimura, Study of the effect of hydrogen on transport properties in chemical vapor deposited diamond films by Hall measurements, Appl. Phys. Lett. 68 (1996) 376–378. [28] J.A. Garrido, A. H¨artl, S. Kuch, M. Stutzmann, pH sensors based on hydrogenated diamond surfaces, Appl. Phys. Lett. 86 (2005) 073504. [29] A. Helwig, G. M¨uller, O. Weidemann, A. H¨artl, J.A. Garrido, M. Eickhoff, Gas sensing interactions at hydrogenated diamond surfaces, IEEE Sens. J. 7 (2007) 1349–1353. [30] J.A. Garrido, A. H¨artl, S. Kuch, M. Stutzmann, O.A. Williams, R.B. Jackman, pH sensors based on hydrogenated diamond surfaces, Appl. Phys. Lett. 86 (2005) 073504. [31] J.B. Cui, J. Ristein, L. Ley, Dehydrogenation and the surface phase transition on diamond (1 1 1): kinetics and electronic structure, Phys. Rev. B 59 (1999) 5847–5856. [32] J. Ristein, M. Riedel, L. Ley, Electrochemical surface transfer doping mechanism behind the surface conductivity of hydrogen-terminated diamond, J. Electrochem. Soc. 151 (2004) E315–E321. [33] A. Helwig, G. M¨uller, G. Sberveglieri, G. Faglia, Gas response times of nano-scale SnO2 gas sensors as determined by the moving gas outlet technique, Sens. Actuators B 126 (2007) 174–180. [34] M. Szameitat, X. Jiang, W. Beyer, Influence of adsorbates on the surface conductivity of chemical vapor deposition diamond, Appl. Phys. Lett. 77 (2000) 1554–1556.
165
[35] F. Maier, J. Ristein, L. Ley, Electron affinity of plasma-hydrogenated and chemically oxidized diamond (1 0 0) surfaces, Phys. Rev. B 64 (2001) 165411. [36] S. Ahlers, G. M¨uller, T. Doll, A rate equation approach to the gas sensitivity of thin film metal oxide materials, Sens. Actuators B 107 (2005) 587– 599. [37] W.P. Kang, Y. Gurbuz, J.L. Davidson, D.V. Kerns, New hydrogen sensor using a polycrystalline diamond-based Schottky diode, J. Electrochem. Soc. 141 (1994) 2231–2234. [38] Y. Gurbuz, W.P. Kang, J.L. Davidson, D.V. Kerns, Diamond microelectronic gas sensor for detection of benzene and toluene, Sens. Actuators B 99 (2004) 207–215. [39] K.L. Soh, W.P. Kang, J.L. Davidson, Y.M. Wong, A. Wisitsora-at, G. Swain, D.E. Cliffel, CVD diamond anisotropic film as electrode for electrochemical sensing, Sens. Actuators B 91 (2003) 39–45.
Biographies Andreas Helwig received a ME degree from the Munich University of Applied Sciences studying in the field of nano-technology in 2003 after graduating in precision- and micro-engineering in 2001. Also in 2003, he started as doctoral candidate at the EADS Corporate Research Centre Germany in the Department Microsystems and Electronics. Currently he is working amongst other things in the field of smart sensors for maintenance, safety and emission control. ¨ Gerhard Muller was graduated in physics from the University of Heidelberg in 1974 and obtained a PhD degree in 1976. Subsequently he was employed at the Max-Planck-Institute for Nuclear Physics in Heidelberg, where he performed work on ion implantation and nuclear solid state physics. In 1979, he changed to the University of Dundee, UK, 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 Chemical Sensors Group of the EADS Corporate Research Centre Germany (∼15 employees). Gerhard M¨uller is author and co-author of about 200 articles in scientific journals and conference proceedings. Since 2000, he is a lecturer in the Munich University of Sciences. Jose Antonio Garrido graduated from the E.T.S.I. Telecomunicaci´on (Universidad Polit´ecnica de Madrid) in 1996 and obtained his PhD degree in 2000. Between 2000 and 2003, he worked as a post-doc at the Walter Schottky Institute (WSI, Department of Physics, T.U. Munich). Since year 2003, he is the team leader of the diamond group at the 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 bio-chemical sensors. Martin Eickhoff studied theoretical physics at the University of Dortmund. From 1995 to 2000, he was part of the DaimlerChrysler AG Central Research Department in Munich where he worked on the development of 3C-SiC technology for high temperature sensor applications. From 2000 to 2001, he was part of the microsensor development department of Infineon Technologies AG in Munich where he worked on surface micromachined microsensors in polysilicon technology. Since 2001, he leads the research group for GaN-based sensors of the Walter Schottky Institute at the Technical University of Munich.