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Group III-nitride-based gas sensors for combustion monitoring
Materials Science and Engineering B93 (2002) 207 /214 www.elsevier.com/locate/mseb
Group III-nitride-based gas sensors for combustion monitoring J. Schalwig a,*, G. Mu¨ller a, M. Eickhoff b, O. Ambacher b, M. Stutzmann b a
b
EADS Deutschland GmbH, Corporate Research Centre, D-81663 Munich, Germany Walter Schottky Institut (E25), Technische Universita¨t Mu¨nchen, Am Coulombwall, D-85748 Garching, Germany
Abstract The paper reports on novel gas-sensing devices based on group III-nitride materials. Both platinum (Pt) /GaN Schottky diodes as well as high-electron-mobility transistors formed from GaN/AlGaN heterostructures with catalytically active platinum gates were investigated. The performance of these devices towards a number of relevant exhaust gas components such as H2, HC, CO, NOx was tested. Test gas concentrations were chosen to simulate exhaust gas emissions from lean-burn 4-stroke petrol engines. We found that GaN-based devices with platinum electrodes are mainly sensitive to hydrogen and unsaturated hydrocarbons with a sizeable cross-sensitivity to CO and NO2. These performance characteristics are similar to those of comparable SiC devices. With GaN devices this performance, however, can be obtained at a reduced complexity of the device processing and a greater freedom in the choice of sensor architectures. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Gas sensors; GaN; GaN/AlGaN-heterostructures; Combustion monitoring
1. Introduction By varying the aluminium content x in the continuous alloy system Alx Ga1x N, a range of optical bandgaps extending from 3.4 up to 6.2 eV is accessible. As for all values of x the bandgap is direct, device-oriented work in the past has focused primarily on optical applications of the group III-nitrides [1]. These materials have allowed the region of semiconductor light emission to be pushed into the blue and UV wavelength range thus making all the three primary colours of the visible spectrum accessible to semiconductor light sources. Besides their outstanding optical properties, group IIInitrides also exhibit high electron saturation velocities, high breakdown voltages and superior thermal and chemical stability. These latter properties also make III-nitride materials interesting candidates for highpower and -frequency devices capable of operating at high temperatures. An application hardly investigated yet are GaN-based high-temperature gas-sensing devices. Recently, however, it has been shown that GaN Schottky diodes with catalytic platinum (Pt) electrodes can be used for the
* Corresponding author. Tel.: 49-89-607-27648; fax: 49-89-60724001. E-mail address: [email protected] (J. Schalwig).
detection of hydrogen and hydrocarbon species [2]. An interesting application of such sensing devices could be the detection of unwanted emission components from internal combustion engines. Tightening environmental legislation forces the car industry to reduce further the harmful emissions such as CO, HC and NOx significantly below the presently allowed limits. Reaching these goals is aggravated by the fact that, at the same time, significant reductions in the overall CO2 emissions also need to be realised. Energy efficiencies can be increased easily by some 20% by introducing lean-burn engines. Exhaust gas emissions from such engines, however, cannot be treated using the conventional three-way catalytic converters and switching l probes. Entirely new concepts of engine and catalytic converter operation therefore need to be developed supported by innovative kinds of gas-sensing devices. Pioneering work on semiconductor gas sensors for combustion gas monitoring purposes was earlier performed by Lloyd Spetz and co-workers [3,4]. This group has performed extensive investigations into siliconcarbide (SiC)-based devices such as metal oxide semiconductor (MOSiC) capacitors, Schottky diodes (MISiC) and field effect transistors. Gas sensitivity was induced using catalytically active materials such as Pt, Pd and Ir as contact or gate metallisation materials. Such sensors have been shown to have millisecond
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response times [5] and are capable of working at operation temperatures up to 800 8C. Sensitivity towards hydrogen-containing molecules like molecular hydrogen and various hydrocarbon species was explained along the same lines of argument as those developed for the corresponding silicon devices [6,7]. SiC devices exhibit a number of properties which make them less desirable for technical applications. Technical drawbacks are that the processing, particularly of SiC field effect transistors, is inherently complicated, requiring high-temperature implantation (Ti 400 8C) and very high-temperature post-implantation annealing steps (Ta 1300 8C). In addition, sensor chips fabricated from small pieces of bulk SiC substrates are hard to assemble and interconnect on ceramic heater substrates that form the backbone of commercial exhaust gas monitoring devices. In view of this situation, in the following we like to address three main topics: firstly, we show that GaNbased gas-sensing devices, with performance characteristics similar to the SiC ones, can be realised, assembled and interconnected to sensor arrays with much less technological effort than the SiC ones. Secondly, we present a wide body of gas-sensing data for GaN devices covering the entire range of relevant exhaust gas components. Finally, we compare the gas-sensing characteristics of GaN devices to those of the state-of-the-art SiC exhaust gas monitors.
2. Device processing GaN was grown on c-plane sapphire substrates by means of plasma-induced molecular beam epitaxy (PIMBE). The deposition was carried out under Garich conditions at a substrate temperature of about 800/ 820 8C. In order to investigate the influence of different crystal polarities on the sensor response, both Ga- and N-face materials were grown. Material with N-face polarity was obtained by depositing GaN directly onto the sapphire substrates, whereas material with Ga-face polarity was obtained by depositing a thin AlN nucleation layer prior to the GaN growth. For more information on the growth process the reader is referred to Ref. [8]. Schottky barrier devices of both Ga- and N-face polarities were formed on epitaxial GaN layers (n-type, Si-doped, ND :1 1018 cm 3, d 2 mm). After removing the native oxide by an HCl dip a 75 nm thick Pt layer was evaporated on the top surface, structured and annealed at 650 8C in nitrogen. Finally, patches of Ti/ Al/Ti/Au multi-layers were formed on the neighbouring parts of the top surfaces for Ohmic contacts. As a second kind of device, high-electron-mobility transistors (HEMTs) based on GaN/AlGaN heterostructures were grown. A cross section through a Ga-
Fig. 1. Device cross-sections of: (a) a Pt /GaN Schottky diode; and (b) a GaN/AlGaN heterostructure, both with Ga-face polarity. The dashed line in (b) indicates the 2-DEG at the GaN/AlGaN interface.
face HEMT structure is shown in Fig. 1(b). No intensional doping was used in the GaN and AlGaN layers. Owing to the band-offsets and the discontinuous change in the spontaneous and piezoelectric polarisation at the GaN/AlGaN interfaces, a two-dimensional electron gas (2-DEG) forms underneath the Al0.3Ga0.7N barrier layer [9]. Hall measurements on these samples yielded an electron mobility of 510 cm2 V s 1 and a free electron concentration of 1.19 1013 cm 2 at room temperature. A gas sensitive 75 nm Pt layer was deposited after an HCl dip onto the top surface, forming a Schottky barrier within the underlying GaN cap layer. Once again, Ohmic contacts for source and drain were formed by depositing patches of Ti/Al/Ti/Au multilayers on both sides of the gate contact as shown in Fig. 1(b). It should be noted that this particular device processing is inherently simple, requiring neither a specific doping profile within the layer sequence to form a 2DEG, nor any implantation or high-temperature annealing steps to form the source and drain contact regions. Last but not least, these devices are planar thinfilm devices deposited on a rugged and electrically insulating sapphire substrate which makes the processing of multi-sensor arrays and their interconnection metallisations simple, mechanically stable and straightforward. For the sake of comparison SiC-based MOS sensors were also made. 4H-SiC bulk material (0.02 V cm) with a 10 mm thick epitaxial layer on top (ND 4.2 1015 cm3) was oxidised to a thickness of 100 nm. A 75 nm Pt/ 30 nm TaSix double layer was then deposited onto the SiO2 and annealed at 800 8C. An AES analysis revealed an intermixing of the layer system and an almost complete oxidation of the additional TaSix buffer layer after annealing. Ohmic back contacts were realised by evaporating Al/Ti/Pt/Au multi-layer stacks onto the back surfaces of the SiC chips.
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3. Gas measurement results For gas measurements the GaN and SiC chips were mounted onto ceramic substrates carrying a Pt heater on the back side and Pt contacts on the front side. The planar GaN devices were fixed on these substrates using a ceramic adhesive. In the case of SiC chips mechanical and electrical contact to the Pt contacts of the ceramic substrates was made by gold die bonding. Electrical contact to the front side metallisations was made by lowering the Pt needles onto the contact areas. Gas measurements were performed in atmospheres containing H2, saturated and unsaturated hydrocarbons as well as CO, NO and NO2 in a background of oxygen in N2. The synthetic test gas mixtures were supplied from a gas-mixing manifold and the sensors were enclosed into a heated stainless steel chamber. Some experiments concerning the hydrogen response of GaN-based sensors were also performed in vacuum. Hence the sensors were fixed onto a heated copper block and mounted inside a vacuum vessel allowing different partial pressures of deuterium. 3.1. Ga-face GaN Schottky diodes Pt/GaN Schottky diodes made from Ga-face material exhibited a good rectifying behaviour in the entire temperature range extending up to 600 8C. Ideality factors n and saturation current densities J0 of n 1.37 and J0 3.5 1012 A cm 2 were measured in the best samples. Plots of measured values of barrier heights over n yielded an effective barrier height of FB eff 1.15 eV, in good agreement with the results published recently [10]. Upon exposure to 1000 ppm of H2 at 400 8C the reverse saturation current of the Schottky diodes increased by more than one order of magnitude (Fig. 2). By increasing the H2 concentration further the IV
Fig. 2. Logarithmic IV-curves for different concentrations of H2 in synthetic air as obtained for Ga-face Pt /GaN Schottky diodes operated at 400 8C.
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characteristics deteriorated until finally Ohmic behaviour was observed. Applying rectangular pulses of H2 it was further shown that H2 response occurred even at room temperature, however, with a very long recovery time after the termination of the H2 pulse. A reasonably rapid recovery to the sensor baseline was observed only after increasing the device temperature to 300 8C. Operating Schottky barrier devices in forward bias under constant-current conditions, a change in the background hydrogen concentration is directly converted into an output voltage. Fig. 3 gives an example where the diode has been operated at 300 8C in vacuum and subsequently exposed to different partial pressures of deuterium. Under these conditions a logarithmic dependence on the D2 partial pressure over more than two orders of magnitude was observed. In order to examine the possible application of such GaN Schottky diodes for combustion monitoring, the sensitivity in response to different kinds of oxidising and reducing gases for the device operation temperature ranging between 200 and 600 8C was tested. Both the concentrations of the individual test gases as well as the oxygen concentration in the background air were chosen to simulate conditions expected for exhaust gas emissions from 4-stroke petrol engines operating in the lean air-to-fuel mixture range. As revealed from Fig. 4, the sensor exhibited a considerable sensitivity towards H2 and at higher temperatures also towards unsaturated hydrocarbons (ethylene). Surprisingly, our GaN Schottky diodes hardly exhibited any response towards saturated hydrocarbon species at the concentrations applied. In addition to the expected cross-sensitivity to CO a sizeable response with opposite sign was also detected in the case of NO2. NO exposure, on the other hand, produced only very small signals with a sign identical to the much stronger hydrogen response. In general, reducing gases led to a less rectifying behaviour of the Schottky diodes and
Fig. 3. Response of a Ga-face Pt /GaN Schottky diode operating in vacuum upon exposure to different partial pressures of deuterium. Operation temperature: T 300 8C.
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Fig. 4. Response of a Ga-face Pt /GaN Schottky diode to various gases diluted in a carrier gas of 4% O2 in N2. Operation temperatures ranged from 200 to 600 8C.
therefore to a positive output voltage signal. Exposure to oxidising gases, such as NO2, produced negative output voltages. In order to obtain further clues to the hydrocarbon detection mechanism, concentration-dependent measurements were performed at a constant operation temperature of 500 8C. The data published in Ref. [11] revealed that Pt/GaN Schottky diodes did not exhibit a sensitivity towards saturated hydrocarbons until these reducing gas species were in excess, i.e. until the ratio of reducing to oxidising gas species a became greater than 1. A transition from a low to a high sensitivity occurred at around 6200 ppm in the case of butane (C4H10) and around 8000 ppm in the case of propane (C3H8), i.e. with both concentrations being close to the stoichiometry point a 1. For mixtures of hydrocarbons and oxygen, the ratio a is defined as: X y × [Cx Hy ]i x 4 a i [O2 ]
3.2. N-face GaN Schottky diodes In the case of N-face Schottky diodes, worse rectifying characteristics were observed. Recently, a barrier height of about 0.2 eV smaller than that of the Pt contacts on the Ga-face GAN was determined [10]. Initial measurements with hydrogen, CO and hydrocarbons on N-face diodes exhibited qualitatively similar response patterns than for Ga-face sensors (Fig. 5). Once again, reducing gases like H2 were found to induce a less rectifying behaviour and a shift of the forward current characteristics towards higher current densities for a given bias voltage. Comparing the results in Figs. 4 and 5, similar H2 response patterns were observed whereas differences appeared in the case of CO and
Fig. 5. Response of a N-face Pt /GaN Schottky diode to various reducing gases diluted in a carrier gas of 4% O2 in N2.
hydrocarbon test gases. For hydrocarbons, smaller responses with similar temperature dependence were observed. Interestingly, no CO response was seen at temperatures below 300 8C, although the response at higher temperatures was comparable to the CO response of Ga-face devices.
3.3. Comparison to MOSiC gas sensors It is interesting to compare the sensor behaviour of Pt /GaN Schottky diodes with that of the MOSiC devices under identical experimental conditions. In order to obtain an output voltage signal from the MOSiC capacitors, we employed an electronic circuit, which kept the capacitance of the device constant, irrespective of the gas concentrations applied. The control voltage necessary to maintain such conditions was then used as a sensor signal. With this circuit positive and negative voltage shifts are obtained in the case of reducing and oxidising gas interactions, respectively. For more details on the underlying sensor mechanism please refer to Ref. [12]. A summary of these measurements is shown in Fig. 6. On the whole we found a very similar gas-sensing behaviour as in GaN-based Schottky diodes, with the response to unsaturated hydrocarbons being stronger than those towards saturated ones. Again a remarkable cross-sensitivity towards CO was observed within the entire temperature range extending from 200 to 600 8C. Further, strong signals with opposite sign were detected on exposure to NO2. NO, on the other hand, was not detected with a significant signal by the MOSiCs at any temperature. Some quantitative differences, however, occurred for the response of the two different sensor devices with respect to hydrocarbon species. First, MOSiCs were able to detect ethylene beforehand at temperatures lower than 300 8C. Secondly, MOSiCs exhibited a much better response to saturated hydrocarbons (propane, butane) already at low concentra-
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Fig. 6. Response of a MOSiC gas sensor to various reducing and oxidising gas species diluted in a carrier gas of 4% O2 in N2 as measured between 200 and 600 8C.
tions around 2000 ppm. Thirdly concentration-dependent measurements at a constant temperature of 500 8C MOSiCs revealed a linearly increasing response towards saturated hydrocarbons (butane, propane) which could not be reproduced with Ga-face GaN Schottky diodes. 3.4. GaN/AlGaN-HEMT Experimental I/V characteristics of gas sensitive GaN/AlGaN-HEMTs with Pt gate metallisation are shown in Fig. 7 both before and during exposure to 1% H2 diluted in a carrier gas of 4% O2 in N2. The device operation temperature in this experiment was again 300 8C. Comparison of the full and dashed lines in Fig. 7 shows that the sign of the H2-induced change in source-drain currents depends on the gate voltage. Increased currents were observed for gate bias voltages larger than 3 V, whereas reduced currents were observed for smaller voltages.
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Fig. 8. Response of a HEMT device to various concentrations (500 ppm to 1.5%) of reducing and oxidising gases diluted in a background of 4% O2 in N2. Operation temperature: 400 8C.
Fig. 8 shows the results of a concentration-dependent response measurement on a HEMT gas sensor exposed to a range of reducing and oxidising gas species. During these measurements the HEMT was operated with a bias of 0 V on the gate. Adding reducing gases to a background of synthetic air increased the source-drain currents, whereas a reduced current was observed upon exposure to NO2. It should be noted that the gasinduced changes in the source-drain current are quite sizeable, being of the order of tenths of milliamperes and thus similar to the current levels observed in comparable SiC devices. On the whole the sensing characteristics of HEMT devices are qualitatively similar to the ones observed on Pt/GaN Schottky devices. As obvious from Fig. 8, however, the stability of the baseline current is still a major issue. Current drifts amounting to almost one order of magnitude during a single gas measurement run have been observed. Performing similar tests on HEMT devices without Pt gates, no such baseline current drifts were observed.
4. Discussion 4.1. Sensing mechanism in GaN Schottky diodes
Fig. 7. Experimental I /V characteristics of a Ga-face GaN/AlGaNHEMT in a gas ambient of 4% O2 and 1% H2 diluted in N2. Operation temperature: 300 8C.
The above-described results have clearly shown that gas adsorption at the surface of GaN Schottky diodes leads to measurable changes in the diode currents. These changes can either enhance or reduce the diode currents. H2, CO and hydrocarbons were found to increase the diode currents in forward and reverse directions. As a consequence, the corresponding gas interactions are identified as reducing. In contrast, adsorbing oxidising molecules such as O2 and NO2 cause the diode currents to be reduced. From the fact that measurements were normally carried out in an ambient of 4% oxygen in N2 and that nevertheless much smaller concentrations of
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NO2 could be detected, it is inferred that NO2 molecules act as more strongly oxidising molecules than O2 ones. With respect to the different molecular gas species considered here, another important property is the ability of penetrating sub-surface layers or the bulk of the sensor devices. In this latter respect, hydrogen plays an outstanding role because it is the only species able to diffuse through a dense noble metal layer into deeper lying layers of the semiconductor device */at least within the range of operating temperatures investigated. All other species can only adsorb at the outer surface of the device, i.e. onto the Pt contact layer. Deeper penetration into the device and interaction with underlying oxide or bare semiconductor layers is only possible in case of thin, porous noble metal layers at the top surface. In view of the experimental results presented above, the most fundamental question concerns the influence of hydrogen on the Schottky diode characteristics of the Pt /GaN interface. In this context the data presented in Figs. 2 and 3 can give some clues. Turning to Fig. 2 first, it is seen that upon exposure to H2 the diode reverse current levels are shifted in a more or less parallel manner towards higher current levels. Such behaviour rules out the generation of sizeable concentrations of hydrogen-induced generation-recombination centres within the depletion layer. A more probable mechanism is that molecular adsorption produces molecular dipoles at the surface of an Schottky barrier device thereby altering the apparent metal work function. Such a work function change in turn can give rise to changes in the device characteristics as observed and displayed in Fig. 2. Referring Fig. 3, we note that a logarithmic dependence of the sensor signal on D2 partial pressure is consistent with Temkin isotherm behaviour, pointing to an interfacial effect with adsorbate coverage of less than a monolayer [13]. Such behaviour has also previously been observed in the case of Si-based MOS capacitors with palladium electrodes [6]. For these devices it is generally agreed that H2 molecules dissociate at the Pd surface and react with the adsorbed oxygen, forming water and atomic hydrogen adsorbed at the platinum surface. The atomic hydrogen, in turn, is supposed to penetrate into the Pd bulk and to adsorb at the Pd/SiO2 interface, leading to an interfacial OH-dipole layer. The very similar H2 detection properties of Pd-Si and Pt-SiC MOS devices, on the one hand, and Pt/GaN diodes, on the other hand, suggest closely similar mechanisms for all three kinds of devices (Fig. 9). Whereas OH molecules are the likely barrier-reducing dipoles in Si and SiC MOS sensors, the molecular nature of the dipoles in the GaN devices, at present, remains speculative. Following previous interpretations we attribute the sensitivity to non-hydrogen containing molecules to the effects of porosity of the Pt electrode. Here, patches of GaN are directly exposed to the gas ambient. An
Fig. 9. Proposed gas-sensing mechanisms underlying the function of Pt /GaN Schottky barrier devices (Ha, Hi: adsorbed and interfacial hydrogen; Xa, Xi: adsorbed and interfacial molecules).
interaction of the gas molecules can occur either directly from the gas phase or via spill-over processes involving species adsorbed at the platinum surface [14]. Such interactions ultimately can lead to polarised adsorbates at the device surface and to changes in the current due to a modulation of the Schottky barrier height at the edge of the pores. Support for porosity-related effects can indeed be obtained from the SEM micrograph in Fig. 10 which displays a Pt film on the top surface of a Pt /GaN Schottky barrier device. The similar behaviour of Ga-face and N-face Schottky diodes, in particular for the response to hydrogen, indicates that the different polarisation directions of the semiconductor base materials does not play a major role in the gas-sensing interactions. On the contrary, our results rather point into the direction of a similar interface structure for both polarities. Indeed, it was recently claimed that, although the crystal struc-
Fig. 10. SEM micrograph of the surface of 75 nm Pt on GaN after annealing at 650 8C.
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tures of Ga- and N-face materials are inverted relatively to each other, both kinds of crystals are Ga-terminated at the outer surfaces [15,16]. Consistent with such a termination, a surface gallium oxide layer might be present [17] which could explain the closely similar kinds of surface interactions. For this explanation to hold, however, we have to suppose that even after carrying out a HCl dip prior to the Pt evaporation oxygen in similar bonding environments is still present at the surface of both N-face and Ga-face GaN. The quantitative differences in the sensor response patterns between GaN Schottky diodes and MOSiCs (Figs. 4 and 6), on the one hand, or between GaN Schottky diodes with Ga- and N-face polarity (Figs. 4 and 5), on the other hand, are likely to arise from different sorption properties of the individual device surfaces; i.e. oxidised and non-oxidised GaN or TaSi in its different oxidation states in the case of MOSiC devices. Turning to the response of Pt /GaN Schottky diodes towards saturated hydrocarbons, we note that a closely similar behaviour had previously been observed on MOSiC capacitors and been explained in terms of a kinetic phase transition [18,19]. This model assumes that the sensor functioning depends both on the reactivity of the gases at the Pt electrodes as well as on the diffusion of reaction educts to and of reaction products away from the catalytic surfaces. In case of a mass-transportlimited reaction, a transition from small to large signal amplitude should occur close to the stoichiometric gas ratio as observed for the Ga-face Schottky diode. Butane and Propane, on the other hand, could also be detected in the region a 1 by the MOSiC. The absence of sensitivity a step at the critical gas composition suggests that in this case the detection reactions are reaction-rate limited. Such a limitation could potentially arise from the intermixing of the Pt/TaSix multilayer gates and a concomitant lowering of the catalytic activity of the Pt surfaces on MOSiC devices. 4.2. Sensing mechanism of GaN/AlGaN-Hemt Analogous to the GaN Schottky diodes, adsorption of gas species on the Pt gate of GaN-HEMTs leads to changes in the effective barrier height and thus either to enhancement or depletion effects in the 2DEG. However, as the rectifying behaviour is decreased by reducing gases, an additional current across the gate contact is observed for high negative gate bias voltages which overcompensates the influence of the gas adsorption, leading to reduced currents for gate voltages smaller than 3 V. In order to avoid such complications, measurements at zero gate bias voltage or with a floating gate contacts are recommended. The strong drift in sensor signal seen in Fig. 1 will be attributed to a continuous deterioration of the 2DEG due to platinum diffusion into the columnar structured GaN bulk
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material and/or to a rearrangement of the platinum within the electrode. The relatively small signal to CO and NO2 with respect to H2 is attributed to a poorer porosity of the platinum gate as no annealing of the device was carried out.
5. Conclusions Group III-nitride-based gas sensors exhibit a sizeable sensitivity towards relevant exhaust gas components such as unsaturated hydrocarbons, CO and also NO2. In the case of GaN Schottky diodes, sufficient device stability was observed to justify more detailed longterm stability tests and investments into sensor packaging efforts. Furthermore, tests in real exhaust gases need to be performed in the next step. For GaN/AlGaNHEMTs, on the other hand, further improvements in the material quality need to be made to allow for prolonged and stable operation at temperatures in excess of 400 8C. In order to achieve further information on the sensing mechanism, structural analysis of the platinum and GaN surfaces and of the Pt/GaN interface will be necessary.
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[15] T.K. Zywietz, J. Neugebauer, M. Scheffler, Appl. Phys. Lett. 74 (1999) 1696. [16] A.R. Smith, R.M. Feenstra, D.W. Greeve, M.-S. Shin, M. Skowronski, J. Neugebauer, J.E. Northrup, Appl. Phys. Lett. 72 (1998) 2114.
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Group III-nitride-based gas sensors for combustion monitoring J. Schalwig a,*, G. Mu¨ller a, M. Eickhoff b, O. Ambacher b, M. Stutzmann b a
b
EADS Deutschland GmbH, Corporate Research Centre, D-81663 Munich, Germany Walter Schottky Institut (E25), Technische Universita¨t Mu¨nchen, Am Coulombwall, D-85748 Garching, Germany
Abstract The paper reports on novel gas-sensing devices based on group III-nitride materials. Both platinum (Pt) /GaN Schottky diodes as well as high-electron-mobility transistors formed from GaN/AlGaN heterostructures with catalytically active platinum gates were investigated. The performance of these devices towards a number of relevant exhaust gas components such as H2, HC, CO, NOx was tested. Test gas concentrations were chosen to simulate exhaust gas emissions from lean-burn 4-stroke petrol engines. We found that GaN-based devices with platinum electrodes are mainly sensitive to hydrogen and unsaturated hydrocarbons with a sizeable cross-sensitivity to CO and NO2. These performance characteristics are similar to those of comparable SiC devices. With GaN devices this performance, however, can be obtained at a reduced complexity of the device processing and a greater freedom in the choice of sensor architectures. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Gas sensors; GaN; GaN/AlGaN-heterostructures; Combustion monitoring
1. Introduction By varying the aluminium content x in the continuous alloy system Alx Ga1x N, a range of optical bandgaps extending from 3.4 up to 6.2 eV is accessible. As for all values of x the bandgap is direct, device-oriented work in the past has focused primarily on optical applications of the group III-nitrides [1]. These materials have allowed the region of semiconductor light emission to be pushed into the blue and UV wavelength range thus making all the three primary colours of the visible spectrum accessible to semiconductor light sources. Besides their outstanding optical properties, group IIInitrides also exhibit high electron saturation velocities, high breakdown voltages and superior thermal and chemical stability. These latter properties also make III-nitride materials interesting candidates for highpower and -frequency devices capable of operating at high temperatures. An application hardly investigated yet are GaN-based high-temperature gas-sensing devices. Recently, however, it has been shown that GaN Schottky diodes with catalytic platinum (Pt) electrodes can be used for the
* Corresponding author. Tel.: 49-89-607-27648; fax: 49-89-60724001. E-mail address: [email protected] (J. Schalwig).
detection of hydrogen and hydrocarbon species [2]. An interesting application of such sensing devices could be the detection of unwanted emission components from internal combustion engines. Tightening environmental legislation forces the car industry to reduce further the harmful emissions such as CO, HC and NOx significantly below the presently allowed limits. Reaching these goals is aggravated by the fact that, at the same time, significant reductions in the overall CO2 emissions also need to be realised. Energy efficiencies can be increased easily by some 20% by introducing lean-burn engines. Exhaust gas emissions from such engines, however, cannot be treated using the conventional three-way catalytic converters and switching l probes. Entirely new concepts of engine and catalytic converter operation therefore need to be developed supported by innovative kinds of gas-sensing devices. Pioneering work on semiconductor gas sensors for combustion gas monitoring purposes was earlier performed by Lloyd Spetz and co-workers [3,4]. This group has performed extensive investigations into siliconcarbide (SiC)-based devices such as metal oxide semiconductor (MOSiC) capacitors, Schottky diodes (MISiC) and field effect transistors. Gas sensitivity was induced using catalytically active materials such as Pt, Pd and Ir as contact or gate metallisation materials. Such sensors have been shown to have millisecond
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response times [5] and are capable of working at operation temperatures up to 800 8C. Sensitivity towards hydrogen-containing molecules like molecular hydrogen and various hydrocarbon species was explained along the same lines of argument as those developed for the corresponding silicon devices [6,7]. SiC devices exhibit a number of properties which make them less desirable for technical applications. Technical drawbacks are that the processing, particularly of SiC field effect transistors, is inherently complicated, requiring high-temperature implantation (Ti 400 8C) and very high-temperature post-implantation annealing steps (Ta 1300 8C). In addition, sensor chips fabricated from small pieces of bulk SiC substrates are hard to assemble and interconnect on ceramic heater substrates that form the backbone of commercial exhaust gas monitoring devices. In view of this situation, in the following we like to address three main topics: firstly, we show that GaNbased gas-sensing devices, with performance characteristics similar to the SiC ones, can be realised, assembled and interconnected to sensor arrays with much less technological effort than the SiC ones. Secondly, we present a wide body of gas-sensing data for GaN devices covering the entire range of relevant exhaust gas components. Finally, we compare the gas-sensing characteristics of GaN devices to those of the state-of-the-art SiC exhaust gas monitors.
2. Device processing GaN was grown on c-plane sapphire substrates by means of plasma-induced molecular beam epitaxy (PIMBE). The deposition was carried out under Garich conditions at a substrate temperature of about 800/ 820 8C. In order to investigate the influence of different crystal polarities on the sensor response, both Ga- and N-face materials were grown. Material with N-face polarity was obtained by depositing GaN directly onto the sapphire substrates, whereas material with Ga-face polarity was obtained by depositing a thin AlN nucleation layer prior to the GaN growth. For more information on the growth process the reader is referred to Ref. [8]. Schottky barrier devices of both Ga- and N-face polarities were formed on epitaxial GaN layers (n-type, Si-doped, ND :1 1018 cm 3, d 2 mm). After removing the native oxide by an HCl dip a 75 nm thick Pt layer was evaporated on the top surface, structured and annealed at 650 8C in nitrogen. Finally, patches of Ti/ Al/Ti/Au multi-layers were formed on the neighbouring parts of the top surfaces for Ohmic contacts. As a second kind of device, high-electron-mobility transistors (HEMTs) based on GaN/AlGaN heterostructures were grown. A cross section through a Ga-
Fig. 1. Device cross-sections of: (a) a Pt /GaN Schottky diode; and (b) a GaN/AlGaN heterostructure, both with Ga-face polarity. The dashed line in (b) indicates the 2-DEG at the GaN/AlGaN interface.
face HEMT structure is shown in Fig. 1(b). No intensional doping was used in the GaN and AlGaN layers. Owing to the band-offsets and the discontinuous change in the spontaneous and piezoelectric polarisation at the GaN/AlGaN interfaces, a two-dimensional electron gas (2-DEG) forms underneath the Al0.3Ga0.7N barrier layer [9]. Hall measurements on these samples yielded an electron mobility of 510 cm2 V s 1 and a free electron concentration of 1.19 1013 cm 2 at room temperature. A gas sensitive 75 nm Pt layer was deposited after an HCl dip onto the top surface, forming a Schottky barrier within the underlying GaN cap layer. Once again, Ohmic contacts for source and drain were formed by depositing patches of Ti/Al/Ti/Au multilayers on both sides of the gate contact as shown in Fig. 1(b). It should be noted that this particular device processing is inherently simple, requiring neither a specific doping profile within the layer sequence to form a 2DEG, nor any implantation or high-temperature annealing steps to form the source and drain contact regions. Last but not least, these devices are planar thinfilm devices deposited on a rugged and electrically insulating sapphire substrate which makes the processing of multi-sensor arrays and their interconnection metallisations simple, mechanically stable and straightforward. For the sake of comparison SiC-based MOS sensors were also made. 4H-SiC bulk material (0.02 V cm) with a 10 mm thick epitaxial layer on top (ND 4.2 1015 cm3) was oxidised to a thickness of 100 nm. A 75 nm Pt/ 30 nm TaSix double layer was then deposited onto the SiO2 and annealed at 800 8C. An AES analysis revealed an intermixing of the layer system and an almost complete oxidation of the additional TaSix buffer layer after annealing. Ohmic back contacts were realised by evaporating Al/Ti/Pt/Au multi-layer stacks onto the back surfaces of the SiC chips.
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3. Gas measurement results For gas measurements the GaN and SiC chips were mounted onto ceramic substrates carrying a Pt heater on the back side and Pt contacts on the front side. The planar GaN devices were fixed on these substrates using a ceramic adhesive. In the case of SiC chips mechanical and electrical contact to the Pt contacts of the ceramic substrates was made by gold die bonding. Electrical contact to the front side metallisations was made by lowering the Pt needles onto the contact areas. Gas measurements were performed in atmospheres containing H2, saturated and unsaturated hydrocarbons as well as CO, NO and NO2 in a background of oxygen in N2. The synthetic test gas mixtures were supplied from a gas-mixing manifold and the sensors were enclosed into a heated stainless steel chamber. Some experiments concerning the hydrogen response of GaN-based sensors were also performed in vacuum. Hence the sensors were fixed onto a heated copper block and mounted inside a vacuum vessel allowing different partial pressures of deuterium. 3.1. Ga-face GaN Schottky diodes Pt/GaN Schottky diodes made from Ga-face material exhibited a good rectifying behaviour in the entire temperature range extending up to 600 8C. Ideality factors n and saturation current densities J0 of n 1.37 and J0 3.5 1012 A cm 2 were measured in the best samples. Plots of measured values of barrier heights over n yielded an effective barrier height of FB eff 1.15 eV, in good agreement with the results published recently [10]. Upon exposure to 1000 ppm of H2 at 400 8C the reverse saturation current of the Schottky diodes increased by more than one order of magnitude (Fig. 2). By increasing the H2 concentration further the IV
Fig. 2. Logarithmic IV-curves for different concentrations of H2 in synthetic air as obtained for Ga-face Pt /GaN Schottky diodes operated at 400 8C.
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characteristics deteriorated until finally Ohmic behaviour was observed. Applying rectangular pulses of H2 it was further shown that H2 response occurred even at room temperature, however, with a very long recovery time after the termination of the H2 pulse. A reasonably rapid recovery to the sensor baseline was observed only after increasing the device temperature to 300 8C. Operating Schottky barrier devices in forward bias under constant-current conditions, a change in the background hydrogen concentration is directly converted into an output voltage. Fig. 3 gives an example where the diode has been operated at 300 8C in vacuum and subsequently exposed to different partial pressures of deuterium. Under these conditions a logarithmic dependence on the D2 partial pressure over more than two orders of magnitude was observed. In order to examine the possible application of such GaN Schottky diodes for combustion monitoring, the sensitivity in response to different kinds of oxidising and reducing gases for the device operation temperature ranging between 200 and 600 8C was tested. Both the concentrations of the individual test gases as well as the oxygen concentration in the background air were chosen to simulate conditions expected for exhaust gas emissions from 4-stroke petrol engines operating in the lean air-to-fuel mixture range. As revealed from Fig. 4, the sensor exhibited a considerable sensitivity towards H2 and at higher temperatures also towards unsaturated hydrocarbons (ethylene). Surprisingly, our GaN Schottky diodes hardly exhibited any response towards saturated hydrocarbon species at the concentrations applied. In addition to the expected cross-sensitivity to CO a sizeable response with opposite sign was also detected in the case of NO2. NO exposure, on the other hand, produced only very small signals with a sign identical to the much stronger hydrogen response. In general, reducing gases led to a less rectifying behaviour of the Schottky diodes and
Fig. 3. Response of a Ga-face Pt /GaN Schottky diode operating in vacuum upon exposure to different partial pressures of deuterium. Operation temperature: T 300 8C.
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Fig. 4. Response of a Ga-face Pt /GaN Schottky diode to various gases diluted in a carrier gas of 4% O2 in N2. Operation temperatures ranged from 200 to 600 8C.
therefore to a positive output voltage signal. Exposure to oxidising gases, such as NO2, produced negative output voltages. In order to obtain further clues to the hydrocarbon detection mechanism, concentration-dependent measurements were performed at a constant operation temperature of 500 8C. The data published in Ref. [11] revealed that Pt/GaN Schottky diodes did not exhibit a sensitivity towards saturated hydrocarbons until these reducing gas species were in excess, i.e. until the ratio of reducing to oxidising gas species a became greater than 1. A transition from a low to a high sensitivity occurred at around 6200 ppm in the case of butane (C4H10) and around 8000 ppm in the case of propane (C3H8), i.e. with both concentrations being close to the stoichiometry point a 1. For mixtures of hydrocarbons and oxygen, the ratio a is defined as: X y × [Cx Hy ]i x 4 a i [O2 ]
3.2. N-face GaN Schottky diodes In the case of N-face Schottky diodes, worse rectifying characteristics were observed. Recently, a barrier height of about 0.2 eV smaller than that of the Pt contacts on the Ga-face GAN was determined [10]. Initial measurements with hydrogen, CO and hydrocarbons on N-face diodes exhibited qualitatively similar response patterns than for Ga-face sensors (Fig. 5). Once again, reducing gases like H2 were found to induce a less rectifying behaviour and a shift of the forward current characteristics towards higher current densities for a given bias voltage. Comparing the results in Figs. 4 and 5, similar H2 response patterns were observed whereas differences appeared in the case of CO and
Fig. 5. Response of a N-face Pt /GaN Schottky diode to various reducing gases diluted in a carrier gas of 4% O2 in N2.
hydrocarbon test gases. For hydrocarbons, smaller responses with similar temperature dependence were observed. Interestingly, no CO response was seen at temperatures below 300 8C, although the response at higher temperatures was comparable to the CO response of Ga-face devices.
3.3. Comparison to MOSiC gas sensors It is interesting to compare the sensor behaviour of Pt /GaN Schottky diodes with that of the MOSiC devices under identical experimental conditions. In order to obtain an output voltage signal from the MOSiC capacitors, we employed an electronic circuit, which kept the capacitance of the device constant, irrespective of the gas concentrations applied. The control voltage necessary to maintain such conditions was then used as a sensor signal. With this circuit positive and negative voltage shifts are obtained in the case of reducing and oxidising gas interactions, respectively. For more details on the underlying sensor mechanism please refer to Ref. [12]. A summary of these measurements is shown in Fig. 6. On the whole we found a very similar gas-sensing behaviour as in GaN-based Schottky diodes, with the response to unsaturated hydrocarbons being stronger than those towards saturated ones. Again a remarkable cross-sensitivity towards CO was observed within the entire temperature range extending from 200 to 600 8C. Further, strong signals with opposite sign were detected on exposure to NO2. NO, on the other hand, was not detected with a significant signal by the MOSiCs at any temperature. Some quantitative differences, however, occurred for the response of the two different sensor devices with respect to hydrocarbon species. First, MOSiCs were able to detect ethylene beforehand at temperatures lower than 300 8C. Secondly, MOSiCs exhibited a much better response to saturated hydrocarbons (propane, butane) already at low concentra-
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Fig. 6. Response of a MOSiC gas sensor to various reducing and oxidising gas species diluted in a carrier gas of 4% O2 in N2 as measured between 200 and 600 8C.
tions around 2000 ppm. Thirdly concentration-dependent measurements at a constant temperature of 500 8C MOSiCs revealed a linearly increasing response towards saturated hydrocarbons (butane, propane) which could not be reproduced with Ga-face GaN Schottky diodes. 3.4. GaN/AlGaN-HEMT Experimental I/V characteristics of gas sensitive GaN/AlGaN-HEMTs with Pt gate metallisation are shown in Fig. 7 both before and during exposure to 1% H2 diluted in a carrier gas of 4% O2 in N2. The device operation temperature in this experiment was again 300 8C. Comparison of the full and dashed lines in Fig. 7 shows that the sign of the H2-induced change in source-drain currents depends on the gate voltage. Increased currents were observed for gate bias voltages larger than 3 V, whereas reduced currents were observed for smaller voltages.
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Fig. 8. Response of a HEMT device to various concentrations (500 ppm to 1.5%) of reducing and oxidising gases diluted in a background of 4% O2 in N2. Operation temperature: 400 8C.
Fig. 8 shows the results of a concentration-dependent response measurement on a HEMT gas sensor exposed to a range of reducing and oxidising gas species. During these measurements the HEMT was operated with a bias of 0 V on the gate. Adding reducing gases to a background of synthetic air increased the source-drain currents, whereas a reduced current was observed upon exposure to NO2. It should be noted that the gasinduced changes in the source-drain current are quite sizeable, being of the order of tenths of milliamperes and thus similar to the current levels observed in comparable SiC devices. On the whole the sensing characteristics of HEMT devices are qualitatively similar to the ones observed on Pt/GaN Schottky devices. As obvious from Fig. 8, however, the stability of the baseline current is still a major issue. Current drifts amounting to almost one order of magnitude during a single gas measurement run have been observed. Performing similar tests on HEMT devices without Pt gates, no such baseline current drifts were observed.
4. Discussion 4.1. Sensing mechanism in GaN Schottky diodes
Fig. 7. Experimental I /V characteristics of a Ga-face GaN/AlGaNHEMT in a gas ambient of 4% O2 and 1% H2 diluted in N2. Operation temperature: 300 8C.
The above-described results have clearly shown that gas adsorption at the surface of GaN Schottky diodes leads to measurable changes in the diode currents. These changes can either enhance or reduce the diode currents. H2, CO and hydrocarbons were found to increase the diode currents in forward and reverse directions. As a consequence, the corresponding gas interactions are identified as reducing. In contrast, adsorbing oxidising molecules such as O2 and NO2 cause the diode currents to be reduced. From the fact that measurements were normally carried out in an ambient of 4% oxygen in N2 and that nevertheless much smaller concentrations of
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NO2 could be detected, it is inferred that NO2 molecules act as more strongly oxidising molecules than O2 ones. With respect to the different molecular gas species considered here, another important property is the ability of penetrating sub-surface layers or the bulk of the sensor devices. In this latter respect, hydrogen plays an outstanding role because it is the only species able to diffuse through a dense noble metal layer into deeper lying layers of the semiconductor device */at least within the range of operating temperatures investigated. All other species can only adsorb at the outer surface of the device, i.e. onto the Pt contact layer. Deeper penetration into the device and interaction with underlying oxide or bare semiconductor layers is only possible in case of thin, porous noble metal layers at the top surface. In view of the experimental results presented above, the most fundamental question concerns the influence of hydrogen on the Schottky diode characteristics of the Pt /GaN interface. In this context the data presented in Figs. 2 and 3 can give some clues. Turning to Fig. 2 first, it is seen that upon exposure to H2 the diode reverse current levels are shifted in a more or less parallel manner towards higher current levels. Such behaviour rules out the generation of sizeable concentrations of hydrogen-induced generation-recombination centres within the depletion layer. A more probable mechanism is that molecular adsorption produces molecular dipoles at the surface of an Schottky barrier device thereby altering the apparent metal work function. Such a work function change in turn can give rise to changes in the device characteristics as observed and displayed in Fig. 2. Referring Fig. 3, we note that a logarithmic dependence of the sensor signal on D2 partial pressure is consistent with Temkin isotherm behaviour, pointing to an interfacial effect with adsorbate coverage of less than a monolayer [13]. Such behaviour has also previously been observed in the case of Si-based MOS capacitors with palladium electrodes [6]. For these devices it is generally agreed that H2 molecules dissociate at the Pd surface and react with the adsorbed oxygen, forming water and atomic hydrogen adsorbed at the platinum surface. The atomic hydrogen, in turn, is supposed to penetrate into the Pd bulk and to adsorb at the Pd/SiO2 interface, leading to an interfacial OH-dipole layer. The very similar H2 detection properties of Pd-Si and Pt-SiC MOS devices, on the one hand, and Pt/GaN diodes, on the other hand, suggest closely similar mechanisms for all three kinds of devices (Fig. 9). Whereas OH molecules are the likely barrier-reducing dipoles in Si and SiC MOS sensors, the molecular nature of the dipoles in the GaN devices, at present, remains speculative. Following previous interpretations we attribute the sensitivity to non-hydrogen containing molecules to the effects of porosity of the Pt electrode. Here, patches of GaN are directly exposed to the gas ambient. An
Fig. 9. Proposed gas-sensing mechanisms underlying the function of Pt /GaN Schottky barrier devices (Ha, Hi: adsorbed and interfacial hydrogen; Xa, Xi: adsorbed and interfacial molecules).
interaction of the gas molecules can occur either directly from the gas phase or via spill-over processes involving species adsorbed at the platinum surface [14]. Such interactions ultimately can lead to polarised adsorbates at the device surface and to changes in the current due to a modulation of the Schottky barrier height at the edge of the pores. Support for porosity-related effects can indeed be obtained from the SEM micrograph in Fig. 10 which displays a Pt film on the top surface of a Pt /GaN Schottky barrier device. The similar behaviour of Ga-face and N-face Schottky diodes, in particular for the response to hydrogen, indicates that the different polarisation directions of the semiconductor base materials does not play a major role in the gas-sensing interactions. On the contrary, our results rather point into the direction of a similar interface structure for both polarities. Indeed, it was recently claimed that, although the crystal struc-
Fig. 10. SEM micrograph of the surface of 75 nm Pt on GaN after annealing at 650 8C.
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tures of Ga- and N-face materials are inverted relatively to each other, both kinds of crystals are Ga-terminated at the outer surfaces [15,16]. Consistent with such a termination, a surface gallium oxide layer might be present [17] which could explain the closely similar kinds of surface interactions. For this explanation to hold, however, we have to suppose that even after carrying out a HCl dip prior to the Pt evaporation oxygen in similar bonding environments is still present at the surface of both N-face and Ga-face GaN. The quantitative differences in the sensor response patterns between GaN Schottky diodes and MOSiCs (Figs. 4 and 6), on the one hand, or between GaN Schottky diodes with Ga- and N-face polarity (Figs. 4 and 5), on the other hand, are likely to arise from different sorption properties of the individual device surfaces; i.e. oxidised and non-oxidised GaN or TaSi in its different oxidation states in the case of MOSiC devices. Turning to the response of Pt /GaN Schottky diodes towards saturated hydrocarbons, we note that a closely similar behaviour had previously been observed on MOSiC capacitors and been explained in terms of a kinetic phase transition [18,19]. This model assumes that the sensor functioning depends both on the reactivity of the gases at the Pt electrodes as well as on the diffusion of reaction educts to and of reaction products away from the catalytic surfaces. In case of a mass-transportlimited reaction, a transition from small to large signal amplitude should occur close to the stoichiometric gas ratio as observed for the Ga-face Schottky diode. Butane and Propane, on the other hand, could also be detected in the region a 1 by the MOSiC. The absence of sensitivity a step at the critical gas composition suggests that in this case the detection reactions are reaction-rate limited. Such a limitation could potentially arise from the intermixing of the Pt/TaSix multilayer gates and a concomitant lowering of the catalytic activity of the Pt surfaces on MOSiC devices. 4.2. Sensing mechanism of GaN/AlGaN-Hemt Analogous to the GaN Schottky diodes, adsorption of gas species on the Pt gate of GaN-HEMTs leads to changes in the effective barrier height and thus either to enhancement or depletion effects in the 2DEG. However, as the rectifying behaviour is decreased by reducing gases, an additional current across the gate contact is observed for high negative gate bias voltages which overcompensates the influence of the gas adsorption, leading to reduced currents for gate voltages smaller than 3 V. In order to avoid such complications, measurements at zero gate bias voltage or with a floating gate contacts are recommended. The strong drift in sensor signal seen in Fig. 1 will be attributed to a continuous deterioration of the 2DEG due to platinum diffusion into the columnar structured GaN bulk
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material and/or to a rearrangement of the platinum within the electrode. The relatively small signal to CO and NO2 with respect to H2 is attributed to a poorer porosity of the platinum gate as no annealing of the device was carried out.
5. Conclusions Group III-nitride-based gas sensors exhibit a sizeable sensitivity towards relevant exhaust gas components such as unsaturated hydrocarbons, CO and also NO2. In the case of GaN Schottky diodes, sufficient device stability was observed to justify more detailed longterm stability tests and investments into sensor packaging efforts. Furthermore, tests in real exhaust gases need to be performed in the next step. For GaN/AlGaNHEMTs, on the other hand, further improvements in the material quality need to be made to allow for prolonged and stable operation at temperatures in excess of 400 8C. In order to achieve further information on the sensing mechanism, structural analysis of the platinum and GaN surfaces and of the Pt/GaN interface will be necessary.
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