A novel NHi-nitrided Schottky-diode hydrogen sensor has been successfully fabricated Measurements have been performed to investigate the sensitivity, stability and response speed of the sensor at different temperatures and hydrogen concentrations. It can respond to hydrogen variation very quickly and can give significant response ewn at low hydrogen concentration. The studied device exhibits high sensitivity of 350 % at 300 °C when 800 ppm IJ in N2 gas is introduced. The sensitivity is 15 times greater than that of the Pt-SiC sensor. The excellent hydrogen-sensing characteristics of this novel sensor make it very suitable for detecting hydrogen leakage in high-temperature environment. The effects of hydrogen adsorption on the barrier height and hydrogen reaction kinetics are also investigated. ©2004 IEEE.published_or_final_versio

Chan, CL

Lai, PT

Tang, WM

Xu, JP

HKU Scholars Hub

Title Sensing characteristics of a novel NH 3-nitrided schottky-diodehydrogen sensorAuthor(s) Tang, WM; Lai, PT; Xu, JP; Chan, CLCitation Ieee International Symposium On Industrial Electronics, 2004, v.1, p. 43-47Issued Date 2004URL http://hdl.handle.net/10722/46504Rights Creative Commons: Attribution 3.0 Hong Kong LicenseSensing Characteristics of a Novel NH3-Nitrided Schottky-Diode Hydrogen SensorW.M. TANG1, P.T. LAV, J.P. XU2, C.L. CHAN1Depaitment of Electrical and Electronic Engineering, the University of Hong Kong, Pokfulam Road,Hong Kong2 Department of Electronic Science & Teclnology, Huazhong University of Science and Technology,Wuhan, 430074, People 's Republic of ChinaFax: (852) 2559 8738 Email: laip(%teee.hku.hkAbstract - A novel NUt-nitrided Schottky-diode hydrogensensor has been successfully fabricated. Measurementshave been performrd to investigate the sensitivity,stability and response speed of the sensor at differenttemperatures and hydrogen concentrations. It ca nrespond to) hvdrogen variation very quickly and can givesignificant response ewn at low" hydrogen concentration.The studied device exhibits high sensitivity of 350 % at300 °C when 800 ppm Ij in N gas is introduced. Thesensitivity is 15 times greater than that of the Pt-SiCsensor. The excellent hydrogen-sensing characteristics ofthis novel sensor make it very suitable for detectinghydrogen leakage in high-temperature environment.The effects of hvdrogen adsorption on the barrier heightand hydrogen reaction kinetics are also investigated.I. INTRODUJCTIONHydrogen is a very dangerous gas because it cancause serious explosion if a spark is present. In order toprevent these accidents, hydrogen sensors have becomeincreasingly important in leakage detection for industrialfabrication processes and hydrogen-fueled vehicles,especially at high temperature. Gas sensors based on siliconcan operate in environments only below 250°C because of itsrelatively small band gap (1.12 eV). Compoundsemiconductors such as SiC, CdS, GaP, etc with wider bandgap (about 3 eV) can operate potentially up to 10000 'C. Thus,they are very suitable for making hydrogen sensors, whichcan function in harsh envirornents such as hot-engine andcar exhaust.A lot of research on hydrogen sensors has been done.Different materials and fabrication methods were used tomake the sensors [1 -4]. It has been reported that Schottkydiodes with palladium (Pd) directly deposited on SiC cansensitively detect hydrogen and hydrocarbon gases.However, after heating of this structure for a long time, thereare interfacial reactions between Pd and SiC formingpalladium silicides, which cause drift in sensor properties [5].In order to reduce these interfacial reactions, a Gate insulatoris placed between Pd and SiC. Several gate insulators such asSnO2 [6], SiO, [7-9] have been used. They are grown in 02ambient. Unfortunately, it has been demonstrated that there isstill intermixine of materials at the interfaces of thePd/Si02/SiC Schottky sensors after long- duration high-temperature annealing due to pooI interfacial diffusion barrier[6]. The sensitivity and stability of a sensor's responsedepend largely on the gate insulator. The interface betweenthe gate insulator and the substrate can greatly influence thesensor performance. Therefore, the development of high-quality gate insulator for the sensors is a very important issue.In order to simply the fabrication procedure as well as toprevent the formation of silicides at the Schottky interface, anovel NH3-nitrided Schottky-diode hydrogen sensor isproposed. This paper will first describe the fabricationprocedure of this hydrogen sensor and the measurementmethodology. Then, it will investigate the hydrogen sensingproperties of the device. Finally, the hydrogen reactionkinetics of the device are studied.II. EXPERIMENTSN-type (0001) Si-face 6H-SiC wafer, manufacturedby CREE Research, was used in this study. The SiC waferhad a 5-gm epitaxial layer grown on heavily doped substrate.The doping level of the epitaxial layer was 4 x 1015 cnt3. Thewafer was cleaned using the conventional RCA methodfollowed by a 20 sec. dipping in 1X, hydrofluoric acid toremove the native oxide. When the temperature of thefurnace reached 900 °C, the wafer was loaded into the furnacewith N, inside to prevent wafer from oxidation. Then N2 gaswas closed and NH3 with a flow rate 0.25 1/min was injectedinto the furnace to do nitridation for 5 min. This fornrd anitrogen-terminated passivation layer on the surface of theSiC wafer. This passivation layer introduced nitrogen to thewafer to help remove dangling bonds and fixed oxidecharges. Annealing was then done in-situ at the nitridationtemperature in N2 for 10 min. The purpose of doingannealing was to eliminate strains and defects and make the0-7803-8304-4/04/$20.00 ©2004 IEEE 43quality of the nitride better. Electrodes were then formed byDC-magnetron sputtering at a substrate temperature of 350'C. The front electrode of 10-nm TaSi, and 100-nm Pt in theform of dots with diameter of 0.5 mm was deposited on thenitride through a stainless steel mask. The back electrode of200-nlm TaSi, and 400-nm Pt was formed after etching off theoxide. The sample underwent metal annealing by loading itinto the furnace at 650 'C with N2 flowing inside(l1OOml/min) for 30 min. For the purpose of comparison, acontrol sample without any nitride or oxide layer was alsofabricated.After the fabrication of the hydrogen sensors, theirperformance was recorded. In order to facilitate taking data,a computer-controlled measurement system was used. Thismeasurement system mainly consisted of two parts: the gasmixing supply system and the parameter-testing system. Thesensor was placed in a copper reaction chamber enclosed im athermostat and the chamber was connected to digital gas flowcontrollers. Computer programs were used to investigate thecharacteristics of the hydrogen sensors by controlling thedigital gas flow controllers, the thermostat and the HP4145Bsemiconductor parameter analyzer.III. RESULTS AND DISCUISSIONFig.1 shows the I-V curves of the NH3-nitridedsample measured in air and in different hydrogenconcentrations (48 ppm, 238 ppm and 619 ppm) at 25 'C and300 'C. The IV curve shifts to the left as temperatureincreases. At 25 °C, the sensor shows little response uponexposure to different hydrogen concentrations, but thehydrogenation effect becomes more obvious at 300 'C. TheI-V curves shift to the left as hydrogen concentrationincreases. The shift toward a lower voltage is due to theformation of a polarized layer at the electrode-nitrideinterface. When the hydrogen-contain'ing molecules come tothe front electrode, they dissociate at the surface of theelectrode and form hydrogen atoms. These hydrogen atomswill diffuse through the electrode in the order of nano- tomicroseconds to the surface of the passivation layer and forma polarized layer at the electrode-nitride interface [lO]. Thispolarized layer causes a change im the number of mobilecarriers in the Schottky sensor and this provides an extraelectric field to lower the barrier. In study, reverse bias is notused to characterize the sensor response to different H0concentrations because no polarized layer is formed at thebottom electrode-nitride interface ulnder reverse bias.Therefore, not much shift in the I-V curve can be seen withexposure to hydrogen-containing gases.0.00080 00070.00060.0005C 0 0004aI)- 0.00030.00020 00010.00000.0 0.5 1.0 1.5Voltage (V)2.0 2.5Fig..1 I-V curves of the NH,,-nitrided sample measured in air andin different hydrogen concentrations (48ppll. 238ppm and619ppm) at 2>C anid 300°C.Based on the thermionic emission conductionmechanism of Schottky diode, for forward bias V> 3kT/q,the I-V characteristics of a Schottky diode are given byI=J0exp( v,)nkT'(1)where k is the Boltzmann constant, T the temperature in K, nthe ideality factor, and I,, the saturation current defined asIo = A**T2exp(--)OT(2)where A is the junction area, A** the effective Richardsonconstant, 4t the barrier height of the Schottky diode and OrkT/q. Plot of ln I vs V (in air and in 800 ppm H2 in N2 gas) at300 °C is shown in Fig.2. The graph shows good linearity,which indicates that the current conduction mechanism of thenovel NH;,-nitrided Schottky hydrogen sensor is controlled bythe thermionic emission mechanism.-7.0300cC-75--8.0--8.5--9.0-9.51.4 1.6 1.8 2.0 2.2 2.4 2.6Vottage (V)Fig.2 I-V characteristics of the NH3-nitrided sample at 300 °C in(1) air, and (2) 800 ppm H2 in N2.44Air II -48ppm Pt238ppm I-Glppm 3000f>'C:300..250-Presented in Fig.3i is the sensor response of the NH3-nitrided sample upon alternate exposure to air and 800 ppmH2 in N2 at 25 °C and 300 'C. The applied forward current is1 mA. The sensor can ive a larger and more rapid and stableresponse at higher temperature s. It is because at hightemperature, hydrogen molecules have more energy and candecompose faster into hydrogen atoms at the surface of theelectrode. More hydrogen atoms can diffuse through theelectrode faster and hence give a larger voltage shift andfaster response. At 30( OC, the recovery time of the sample islonger than the response time. This shows that time is moreneeded for hydrogen atoms to diffuse out of the electrodesurface and then recombine together to form hydrogen gas.Fig.4 shows the sensitivity of the NIH;-nitridedsample and the control sample at different temperatures whenthe test chamber environment is changed from air to 800 ppmH, in EH. The sensitivity is defined as (I ai-l, )/I'airwhere IH. and IaitJ are currents measured under hydrogenenvironment and air respectively. The applied voltage is 2 V.The sensitivity of the NH3-nitrided sample is much higherthan that of the control sample over the whole temperaturerange from 25 °C to 45(0 'C. The sensitivities of bothsamples first increase with temperature but then decrease astemperature further increases. This phenomenon can beexplained as follows. When temperature increases, hydrogenmolecules can decompose faster to hydrogen atoms on thesurface of the electrode. Thus, more hydrogen atoms candiffuse through the electrode and hence increase thesensitivity. On the other hand, further increasing temperaturecan activate the traps inside the nitride as well as the traps atthe metal/nitride interface, thus decreasing the sensitivity.4.54.03.53.00> 2.52.015.500 1000Time (s)1500 2000543)LU)00 100 200 300Temperature ('C)400 500Fig.4 SensitivityT of the samples at different teimiperatures whenithe test chamber environment is clhanged from air to 80(10ppm H. in N, under V= 2 VPresented in Fig.5 is the sensitivity of the NH3-nitrided sample upon exposure to different H2 concentrationsin N2 at 300 'C. It is found that the sensitivity increases withdecreasint applied voltage. It is because the smaller theforward bias, the smaller is the leakage current lair and hencethe higher is the sensitivity.3.53.02.5>1a)ce2.01.51.00.50.0-0.50 200 400 600 800H, concentration in N2 (ppm)1000Fig.5 Sensitivity of the NH,-nitrided sample upon exposuire todifferent Ho concentration in N2 at 300 'C.Fig.3 Response of NH,-nitrided sensor upon altemate exposureto air and 800 ppm H2 in N2 at 25 °C and 300°C.45H conc=800ppm, V=2V- C ontrol sample--* NH-nhtrided sample01=1 mA, H. conc=800ppm25-CH - 300Cr 2i, $'lU ,300°Cm V=2VV=2.lVA V=2.2V-V V=2.3VI~ ~IThe sensitivity of the NH3-nitrided sample uponexposure to different Ii concentratiorns in N1 is comparedwith that of the control sample in Fig.6. At 300 °C under aforward voltage of 2 V, sensitivities of both samples increasewith hydrogen concentration. The control sample has asensitivity of 6 % at 48 ppm Ij and gets saturated veryquickly at 429 ppm with a sensitivity of about 19 %.However, he NH3-nitrided sample can give a significantresponse (sensitivity of 200 %) at 48 ppm H, and itssensitivity continuously increases to 350 °/o at 800 ppm anddoes not get saturated even at 1000 ppm. The enhalncedsensitivity of the NH3-nitrided sensor is due to the effect ofNH3 nitridation which improves the interfacial properties ofthe sensor. Excellent response of this sensor to hydrogen isfurther confirmed through continually changing hydrogenconcentration, which gives rise to a rapid and clear "step"response as showni in Fig.7.a)C)3 5302.5201.51.00.50.0-0.S0 200 400 600 800H concentration in N (ppm)1 000Fig.8 shows the change in the barrier height of theNH3-nitrided and control samrple upon exposure to differentH2 concentrations. Based on equation (2), the barrier heightis calculated using, the following formula:O =- 0T inl (3)IU (in air and in Et) can be found from the corresponding y-intercepts of the graph (ln I vs V). When hydrogenconcentration increases, the barrier height of the sensordecreases and the barrier-height variation defined as 4t (air) -4t (H2) increases. It is because when more hydrogen-containing molecules come to the front electrode, morehydrogen atoms can be absorbed at the electrode-nitrideinterface to form a polarized layer. This polarized layercauses a sionificant barrier lowering effect. The wider rangeof barrier-height variation in the NH3-nitrided sampleindicates that the sensor is more sensitive.0.15V-c02)4)6--o11)C)acv0 100o050.000 200 400 600 800H2 concentration in N2 (ppm)1000Fig.6 Sensitivity of the NH3-nitrided and control samiiples (biasvoltage = 2 V) upon exposure to different H,concentrations in No at 3000C.0 00250.00200.00154a1), 0.0010C)0.00050.00000 500 1000 1500Time (s)2000 2500Fig.8 Barrier-height variation of the NHI-nitrided and controlsample upon exposure to different H2 concentrations in N,at 300 -C.According to the hydrogen reaction kinetics, thehydrogen coverage 0 at the interface under steady-statecondition can be written as0- =k (4)1-0 o ItPHwhere lj, is a rate constant and PH2 is the hydrogen partialpressure. The change in voltage across the hydrogen dipolelayer, AV is proportional to the hydrogen coverage 0 (i.e. AV-c 0) and the proportionality constant-is AVmax(maximumvoltage change at a fixed temperature). By substituting theformula AV= AVmaxO into equation (4),1 1Fig.7 Response characteristics of the NH3-nitrided sample whencontinually changing H21AV AVmax k Av. X,H(5)46300-'C, V=2V-U- Control sample|@ NH,-nitrided sample1~ ~~~~--- aI3OtfC| Control sampleX NH3-nitrided samplea300C, I=1 mABy usingl equation (1), equation (5) can be written as1 1 1(1+ ) (6ogI k0)h o Iwhere tg and .Ilmax are the corresponding and maximumsaturation currents in hydrogen environment respectively. Ascan be seeni in Fig.9g the plot of 1/In I vs (1/PH2)t 4ivesa straight line, confirming the hydrogen reaction kinetics inthe NH3-nitrided Schottky diode.0 .38300'C0.37-0.360.35-0.34-0.33-0.320.030 0.035 0.040 0.045 0.050 0.055 0.060 0.005 0.070Fig.9 Steady-state reaction kinetic anialysis for hydrogenabsorptioni of the NH--nitrided sample at 3100 'C.IV. StUMMARYA novel NH3-nitrided Schottky-diode hydrogensensor has been fabricated and studied. The influences ofhydrogen concentration and temperature on hydrogen sensingperformance of the device are investigated. Expefimentalresults show that the sensor demonstrates high sensitivity,high stability, and fast response especially at a high operatingtemperature of 300 °C. These properties provide the sensorpotential applications for detecting hydrogen leakage in harshenviron-ments. The study has shown that the barrier height ofthe device decreases upon hydrogen adsorption and this leadsto an increase in its forward current. It is also founid that thecurrent-conduction mechanism of this sensor is controlled bythe thermionic emission mechanism. The hydrogen reactionkinetics in the device is also confirmed.V. REFERENCES[1] I. Lundstrom, M. Aungarth, L.-G. Petersson. " Physics withcatalytic metal gate chemical sensors," CRC Cn't. Rev. Sol. StateMater. Sci. 1i (21pp.201-278. 1989[2] A.L. Spetz P.Tobias. A.Baranzahi. P. Martensson and I.Lundstrom. Current status of silicon carbide based high-temperature gas sensors." IEEE Trans. Electron Devices, vol.46(3). pp.561 -566, 1999[3] CC.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 gassensor." Sensors and Actuators B vol.66; pp.1 16-118. 2000[4] 1. Lundstrom, S. Shivaraman, C. Svensson. and L. LundkvNist. " Ahydrogen-sensitive MOS field effect transistor."' Appl. Phys. Len._vol.26, pp.5 19-57.l97[5.] G.W.Hunter. P.G.Neudeck. L.-Y.Chen, et al.. " SiC-basedSchottky diode gas sensor," in 76 Int. conf Silicon Carbide, III-Nitndes and Related Matenrals. Stockholm. Sweden. Trans.Tech..vol.2. pp. 109)3 1096. 1997[6] G.W.Hunter. P.G. Neudeck, M. Gray. et al.. " SiC-based gassensor development." Materials Science Forum. Nol. 318-342.pp. 143 9- 1442. 2000[7] L.-Y.Chen. G.W.Hunter. P.G. Neudeck. et al.. " Compan'son ofinterfacial and electronic properties of annealed Pd SiC andPdi0S2j'SiC Schottky diodes." J. Vac.Sci.Technol.A. vol.15.pp.1228-1234. 1997[8] S. Zangooie. H.Arwin. I. Lundstrom. et al., " Ozone treatment ofSiC for improx ed performance of gas sensitive Schottky diodes."Matenals Science Forum. vol. 338-342. pp.1085-1 088. 2000[9] P. Tobias. A.Baranzahi. A.L.Spetz. et al.. " Fast chemical sensingwith inetal-insulator siliconi carbide structures." IEEE ElectronDevice Lett.. vol. 18 (6), pp.287-289. 1997[10] L.-G. Ekedahl. M. Eniksson. I. Ltndstrom " Hydr-ogeni senisingemechanisms of metal-insulator inteifaces." Acc. Chem. Res. 31(5). pp.249 -256. 199847

Sensing characteristics of a novel NH 3-nitrided schottky-diode hydrogen sensor

W.M. Tang

P.T. Lai

J.P. Xu

C.L. Chan

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Sensing characteristics of a novel NH/sub 3/-nitrided Schottky-diode hydrogen sensor

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Sensing characteristics of a novel NH 3-nitrided schottky-diode hydrogen sensor

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