Thin oxynitride grown in NO at low temperature was successfully used as gate insulator for fabricating MISiC Schottky hydrogen sensors. Response properties of the sensors were compared with other MISiC Schottky sensors with thicker or no oxynitride. It was found that the thin oxynitride played an important role in increasing device sensitivity and stability. Even at a low H 2 concentration, e.g., 100-ppm H 2 in N 2, a significant response was observed, indicating a promising application for detecting hydrogen leakage. Moreover, a rapid and stable dynamic response on the introduction and removal of H 2/N 2 mixed gas was realized for the sensor. Improved interface properties and larger barrier height associated with the thin oxynitride are responsible for the excellent response characteristics. As a result, NO oxidation could be a superior process for preparing highly sensitive and highly reliable MISiC Schottky hydrogen sensors.published_or_final_versio

Chan, CL

Lai, PT

Xu, JP

Zhong, DG

IEEE Electron Device Letters

HKU Scholars Hub

Title Improved hydrogen-sensitive properties of MISiC Schottkysensor with thin NO-grown oxynitride as gate insulatorAuthor(s) Xu, JP; Lai, PT; Zhong, DG; Chan, CLCitation Ieee Electron Device Letters, 2003, v. 24 n. 1, p. 13-15Issued Date 2003URL http://hdl.handle.net/10722/42933Rights©2003 IEEE. Personal use of this material is permitted. However,permission to reprint/republish this material for advertising orpromotional purposes or for creating new collective works forresale or redistribution to servers or lists, or to reuse anycopyrighted component of this work in other works must beobtained from the IEEE.IEEE ELECTRON DEVICE LETTERS, VOL. 24, NO. 1, JANUARY 2003 13Improved Hydrogen-Sensitive Properties ofMISiC Schottky Sensor with Thin NO-GrownOxynitride as Gate InsulatorJ. P. Xu, P. T. Lai, Member, IEEE, D. G. Zhong, and C. L. Chan, Member, IEEEAbstract—Thin oxynitride grown in NO at low temperature wassuccessfully used as gate insulator for fabricating MISiC Schottkyhydrogen sensors. Response properties of the sensors were com-pared with other MISiC Schottky sensors with thicker or no oxyni-tride. It was found that the thin oxynitride played an important rolein increasing device sensitivity and stability. Even at a low H2 con-centration, e.g., 100-ppm H2 in N2, a significant response was ob-served, indicating a promising application for detecting hydrogenleakage. Moreover, a rapid and stable dynamic response on the in-troduction and removal of H2/N2 mixed gas was realized for thesensor. Improved interface properties and larger barrier height as-sociated with the thin oxynitride are responsible for the excellentresponse characteristics. As a result, NO oxidation could be a su-perior process for preparing highly sensitive and highly reliableMISiC Schottky hydrogen sensors.Index Terms—Hydrogen sensors, NO, oxynitride, silicon car-bide.I. INTRODUCTIONOVER THE past few years, there has been an increasinginterest in the field of gas sensors that can operate inharsh environments such as hot-engine control for aerospaceand automobile applications, process-gas monitoring, and leakdetection. The requirement of operating in high-temperatureenvironments brought SiC into attention due to its remarkableproperties, e.g., outstanding thermal stability enabling opera-tion up to 1000 C. So far, a large amount of research work oncatalytic metal (e.g., Pt, Pd)-insulator (SiO )-silicon carbide(MISiC) gas sensors, including MISiC Schottky diodes andMISiC capacitors, has been conducted [1]–[4]. It has beenshown that MISiC Schottky diodes are preferably selectedas gas sensors due to their much simpler electronic circuitryrequired for operation [3] and a direct or indirect sensitivity togases like H , hydrocarbons, CO, NO, and O . Unfortunately,a main failure mechanism of the sensors at high temperatures isfailure of the gate insulator due to the mixing of layers and oxideconsumption [5], [6], which lead to long-term high-temperatureManuscript received August 20, 2002; revised November 21, 2002. This workwas supported by the Natural Science Foundation of Hubei Province of Chinaunder Grant 2000J158 and by the RGC Research Grant of Hong Kong. Thereview of this letter was arranged by Editor S. S. Chung.J. P. Xu and D. G. Zhong are with the Department of Electronic Science andTechnology, Huazhong University of Science and Technology, Wuhan 430074,China.P. T. Lai and C. L. Chan are with the Department of Electrical andElectronic Engineering, the University of Hong Kong, Hong Kong (e-mail:laip@eee.hku.hk).Digital Object Identifier 10.1109/LED.2002.807526stability problem of the gas response. Presently, the insulatorused in MISiC Schottky hydrogen sensor is a very thin ( 1 nm)native oxide, or SiO created by various processes such asozone exposure [7]–[9]. Therefore, it is easy to consume theinsulator when intermixing of materials at the metal–oxide andoxide–SiC interfaces occurs due to poor interfacial diffusionbarrier (weak Si–Si and Si–O bonds of the oxides). In addition,high interface-state and fixed-charge densities of the oxidegreatly degrade the device characteristics, e.g., sensitivityand stability. Therefore, development of a high-quality thininsulator for MISiC Schottky hydrogen sensors becomes a keyissue. In this work, a technique of growing thin oxynitridein NO as the thin insulator of MISiC Schottky sensors wasdeveloped for the first time, and greatly improved hydrogenresponse and stability were obtained as compared with MISiCSchottky sensors with thicker or no oxynitride. Relevant resultsare reported, and physical mechanisms involved are analyzed.II. EXPERIMENTSN-type (0001) Si-face 6H–SiC wafers, manufactured by CreeResearch, were used in this study. The SiC wafers had a 5- mepitaxial layer grown on heavily doped substrate. The dopinglevel of the epitaxial layer was 4 10 cm . The waferswere cleaned using the conventional RCA method followed bya 1-min dip in 5% HF to remove the native oxide. The waferswere then loaded into a quartz furnace at required oxidationtemperature in N to have good control on the final thicknessof thin oxynitrides. Oxidation conditions of MISiC Schottkysensors were 800 C/7 min and 900 C/5 min, respectively,both in pure NO ambient (0.5 l/min) to produce thin oxyni-trides with about the same thickness (denoted as NOG10 andNOG15, respectively, with the number indicating the thicknessof the resulting oxynitride in angstroms measured by ellip-sometry). Post-oxidation annealing was done in-situ at the ox-idation temperature in N (1 l/min) for 5 min. For the purposeof comparison, two other samples were prepared: 1) The waferwas loaded into quartz furnace at 900 C in N and oxidized at1100 C for 4 h in pure NO ambient (0.5 l/min) to form a muchthicker oxynitride. Because of the higher oxidation temperatureand longer oxidation time, a longer annealing was carried outin N (1 l/min) for 30 min after cooling down in N with aramping rate of 1 C/min to 950 C (denoted as NOG120,i.e., nm); 2) no oxynitride was grown on the sur-face of wafer (denoted as NOG0, i.e., MSiC Schottky sensor).Electrodes of all samples were formed by dc-magnetron sput-tering at a substrate temperature of 350 C, and 200-nm TaSi0741-3106/03$17.00 © 2003 IEEE14 IEEE ELECTRON DEVICE LETTERS, VOL. 24, NO. 1, JANUARY 2003Fig. 1. Temperature dependence of forward-current change (I =I ) of allsamples when changing environment from air to 800-ppm H in N underV =2:5 V. The inset are the I–V curves of the NOG15 sample in air and 800 ppmH in N at room temperature.and 400-nm Pt formed an ohmic back contact after etching offthe oxide, while 10-nm TaSi and 100-nm Pt were depositedthrough a stainless steel mask, producing a gate electrode witha diameter of 0.5 mm. The samples did not go through any ac-tivation procedure for increasing sensitivity [1].III. RESULTS AND DISCUSSIONSteady-state and transient response measurements wereperformed in a copper reaction chamber located inside a high-temperature thermostat and connected to a gas flow tube witha regulating valve. The forward current–voltage ( – )characteristics of the MISiC Schottky diodes were measuredat different temperatures and hydrogen concentrations byHP4145B semiconductor parameter analyzer to examine thehydrogen-sensitive properties of the devices.Fig. 1 shows the temperature dependency of response signal(current change normalized by corresponding current inair ) of all samples at of 2.5 V when the chamber en-vironment is changed from air (the reference) to 800-ppm Hin N . In fact, is caused by a voltage shift in theH -containing environment, as shown by the – curve shiftin the inset of Fig. 1. Three points can be summarized fromFig. 1 as follows: 1) Sensitivities of all samples increase as tem-perature rises, where sensitivities of NOG10 and NOG15 sam-ples with thin oxynitride are obviously higher than those of theNOG0 and NOG120 devices for the whole measured tempera-ture range; 2) the two sensors with thin oxynitride have goodresponse to hydrogen even at room temperature, and it can befound that the sensitivity increases with thickness for thin oxyni-trides and has a maximum value for an optimal thickness; 3) forthe NOG120 sample, its sensitivity is lower than that of theNOG0 sample at temperatures below 220 C but then exceeds itFig. 2. Forward-current change (I =I ) as a function of hydrogenconcentration in N at T = 25 and 300 C under V = 2:5 V.at higher temperatures. As compared with MSiC Schottky sen-sors, the higher sensitivity of MISiC Schottky sensors with thinoxynitride can be ascribed to two factors: 1) a remarkable reduc-tion of leakage current in air [10] resulting from improved inter-face properties for NO-grown oxynitride [11] and 2) the oxyni-tride-induced larger barrier height associated with thermionicemission providing a wider regime of barrier-height modulationwhen adsorbed hydrogen atoms diffuse to Pt–oxynitride inter-face and form a dipole layer [10]. However, when the oxynitrideis too thick (NOG120), thermionic emission current through theMISiC Schottky diode is very small, especially at low temper-atures, and thus, the current shift under the H -containing en-vironment is small, despite the modulation effect of the barrierheight. As temperature increases, thermionic emission currentbecomes large, and hence, the current shift under the H envi-ronment is significant (e.g., at 300 C is 2.5 times thatat room temperature). This implies that MISiC Schottky sensorswith thicker oxynitride probably have good hydrogen responseand good stability at higher temperatures. From the above dis-cussion, it can be suggested that the oxynitride thickness is a keyfactor affecting the sensitivity and stability of MISiC Schottkysensors and has different optimal values for different tempera-ture ranges. It should be noted that higher oxidation temperaturecan also improve the oxynitride quality and, hence, device sensi-tivity, although the NOG120 sample with the highest oxidationtemperature shows the lowest sensitivity due to the dominantrole of its thick oxynitride.Fig. 2 is the sensor response ( ) on exposure to dif-ferent H concentrations in N at room temperature (25 C)and 300 C under V for all the samples. As canbe seen, response signals of the sensors increase rapidly at lowH concentrations first and then gradually reach saturation athigh H concentrations ( 600 ppm). Again, the sensors withthin oxynitride (NOG10 and NOG15) exhibit high sensitivityto H in the whole measured range of H concentration withthe highest for the NOG15 sample ( nm). Even at thelowest measured H concentration of 200-ppm H in N , a sub-stantial response is obviously observed for the NOG15 sampleat room temperature.XU et al.: IMPROVED HYDROGEN-SENSITIVE PROPERTIES OF MISiC SCHOTTKY SENSOR 15Fig. 3. Forward-voltage (V ) change of the NOG15 sample at 300 C onalternate exposure to air and H /N mixed gas (800 ppm of H ) under I =1 mA.Fig. 4. Response characteristics of the NOG15 sample when continuallychanging H concentrations in N at 300 C under V = 2 V.The forward-voltage response of the NOG15 sample for afixed forward current of 1 mA at 300 C on alternate exposureto air and H /N gas with 800 ppm of H is presented in Fig. 3.It can be observed that a rapid and stable response can be ob-tained for the MISiC Schottky sensor with a thin oxynitride (alarger response is expected if the sample is fully activated [1]).Excellent response to H is further confirmed through contin-ually changing hydrogen concentration, which gives rise to therapid “ladder” response shown in Fig. 4, even for 100-ppm Hconcentration in N .Preliminary testing on the long-term stability of the NOG15sample was performed for 4 days in H /N gas with 800 ppmof H at 300 C under V. No obvious responsedegradation was observed. In fact, the device sensitivity wasincreased by 0.2%, probably due to a slight activation underthe test conditions.IV. SUMMARYMISiC Schottky hydrogen sensors with thin NO-grownoxynitride as gate insulator were successfully prepared, andexcellent H -sensitive properties were obtained at not onlyhigh temperatures but also at room temperature. Experimentalresults showed that the thin oxynitride was the key factor inaffecting sensitivity and stability. Rapid, strong, and stableresponse on a changing environment was realized for thehydrogen sensor with thin oxynitride as gate insulator. Asubstantial response to H even at room temperature andlow H concentrations provides a potential application fordetecting hydrogen leakage. Therefore, it can be suggestedthat low-temperature NO oxidation is a promising processingstep for preparing a high-sensitivity and high-reliability MISiCSchottky hydrogen sensor.REFERENCES[1] A. L. Spetz, P. Tobias, A. Baranzahi, P. Mårtensson, and I. Lundström,“Current status of silicon carbide based high-temperature gas sensors,”IEEE Trans. Electron Devices, vol. 46, pp. 561–566, Mar. 1999.[2] C. K. Kim, J. H. Lee, Y. H. Lee, N. I. Cho, and D. J. Kim, “A study ona platinum–silicon carbide Schottky diodes as a hydrogen gas sensor,”Sensors Actuators B, vol. 66, pp. 116–118, 2000.[3] A. L. Spetz, P. Tobias, L. Unéus, H. Svenningstorp, and L.-G. Ekedahl,“High temperature catalytic metal field effect transistors for industrialapplication,” Sens. Actuators B, vol. 70, pp. 67–76, 2000.[4] H. Svenningstorp, B. Widén, P. Salomonsson, L.-G. Ekedahl, I. Lund-ström, P. Tobias, and A. L. Spetz, “Detection of HC in exhaust gases byan array of MISiC sensors,” Sens. Actuators B, vol. 77, pp. 177–185,2001.[5] A. L. Spetz, D. Schmeisser, A. Baranzahi, B. Walivaara, W. Gopel, andI. Lundström, “X-ray photoemission and auger electron spectroscopyanalysis of fast responding activated metal oxide silicon carbide gas sen-sors,” Thin Solid Films, vol. 299, pp. 183–189, 1997.[6] E. Danielsson, C. I. Harris, C.-M. Zetterling, and M. 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Improved hydrogen-sensitive properties of MISiC Schottky sensor with thin NO-grown oxynitride as gate insulator

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Improved hydrogen-sensitive properties of MISiC Schottky sensor with thin NO-grown oxynitride as gate insulator

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