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1938 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 9, SEPTEMBER 2001

Hydrogen-Sensitive Characteristics of a Novel Pd/InPMOS Schottky Diode Hydrogen Sensor

Wen-Chau Liu, Member, IEEE, Hsi-Jen Pan, Huey-Ing Chen, Kun-Wei Lin, Shiou-Ying Cheng, Member, IEEE, andKuo-Hui Yu

Abstract—Steady-state and transient hydrogen-sensing char-acteristics of a novel Pd/InP metal-oxide-semiconductor (MOS)Schottky diode under atmospheric conditions are presented andstudied. In presence of oxide layer, the significant increase ofbarrier height improves the hydrogen sensitivity even at loweroperating temperatures. Even at a very low hydrogen concentra-tion environment, e.g., 15 ppm H2 in air, a significant responseis obtained. Two effects, i.e., the removal of Fermi-level pinningcaused by the donor level in the oxide and the reduction of Pdmetal work function dominate the hydrogen sensing mechanism.Furthermore, the reaction kinetics incorporating the water for-mation upon hydrogen adsorption is investigated. The initial heatof adsorption for the Pd/oxide interface is estimated to be 0.42eV/hydrogen atom. The coverage dependent heat of adsorptionplays an important role in hydrogen response under steady-stateconditions. In accordance with the Temkin isotherm behavior, thetheoretical prediction of interface coverage agrees well with theexperimental results over more than three decades of hydrogenpartial pressure.

Index Terms—Barrier height, Fermi-level pinning, hydrogen re-sponse, hydrogen sensors, Schottky diode.

I. INTRODUCTION

I N the field of solid-state sensors, highly sensitive devicesbased on the principles of flow pressure or chemical reaction

are widely developed in detecting and monitoring the environ-ment in industrial processes and medical installations [1], [2].Among these, metal-oxide-semiconductor (MOS) structureswith the catalytic Pd metal have attracted extensive interestsin the implementation of hydrogen sensors and investigationof hydrogen adsorption properties [3]–[7]. For different typesof familiar Pd/SiO/Si MOS devices, e.g., capacitors, fieldeffect devices, or Schottky diodes, the hydrogen detectionmainly depends on the modification in electric propertiesresulting from the presence of charged hydrogen atoms at thePd/SiO interface. However, as the oxide thickness is reduced,especially for Schottky diode applications, the possibility of

Manuscript received November 27, 2000; revised March 13, 2001. This workwas supported in part by the National Science Council, Taiwan, R.O.C., underContract NSC 89-2215-E-006-029. The review of this paper was arranged byEditor K. Najafi.

W.-C. Liu, H.-J. Pan, and K.-H. Yu are with the Institute of Microelectronics,Department of Electrical Engineering, National Cheng-Kung University,Tainan, Taiwan 70101, R.O.C. (e-mail: [email protected]).

H.-I. Chen is with the Department of Chemical Engineer, NationalCheng-Kung University, Tainan, Taiwan 70101, R.O.C.

K.-W. Lin is with the Department of Electrical Engineering, Chien Kuo In-stitute of Technology, Changhua, Taiwan, R.O.C.

S.-Y. Cheng is with the Department of Electrical Engineering, Oriental Insti-tute of Technology, Taipei Hsien, Taiwan, R.O.C.

Publisher Item Identifier S 0018-9383(01)06910-6.

directed reaction between Pd and Si is apparently increased.This leads to a loss of hydrogen response due to the significantFermi-level pinning caused by the interfacial formation ofPd-silicide [4]. In addition, during the oxidization process, thesurface is easily contaminated to degrade the quality of oxidelayer. To achieve more hydrogen-sensitive Schottky junctions,the Schottky diodes based on III-V compound semiconductorswith metal work function dependent barrier heights are widelystudied [8]–[11]. Yousufet al. demonstrated essentially highhydrogen response in the current–voltage (– ) characteristicsof Pd/InP Schottky diode [12]. Although the enormous currentvariation is observed, the low barrier height associated withthe high defect state density at the Pd/InP interface severelyrestricts the allowable variation in barrier height. In particular,the device performances as well as the sensing applications athigher temperature are therefore deteriorated by consideringthe high leakage current density.

In this work, we present the planar fabrication and charac-terization of a highly hydrogen-sensitive Pd/InP MOS Schottkydiode. In presence of the shallow donor level at the Pd/oxideinterface by introducing a thin thermal oxide layer, the exper-imental results indicate an enhanced barrier height as high as0.63 eV is achieved [13]. Upon exposure to hydrogen, the hy-drogen responses of current variation with different tempera-tures and hydrogen concentrations are measured under steadystate and transient conditions. In addition, the reaction kineticsof the hydrogen response is established to study the initial heatof adsorption by the correlation between hydrogen pressure andinterface coverage [14].

II. EXPERIMENTS

The studied Pd/InP MOS Schottky diodes were grown on(100)-oriented semi-insulating (SI) InP substrates by a metalorganic chemical vapor deposition (MOCVD) system. Theepitaxial structure consisted of a 5000undoped InP bufferlayer and a 3000 n-type InP active layer with a Si-dopedconcentration of cm . After the epitaxy process,conventional photolithography, thermal evaporation, and lift-offtechniques were employed in the device fabrication process.The ohmic contacts were made by using AuGe alloy on thesurface of n-InP active layer with a following annealing in Ngas at 450 C for 2 min. Subsequently, the thermal oxidationprocess was carried out in flowing dry Oat 360 C for 25 min.For comparison, some substrates without the thermally oxidizedsurfaces were chemically etched with a solution consisting ofH SO :H O :H 5:1:1 and then a solution HF:H 1:1

0018–9383/01$10.00 © 2001 IEEE

LIU et al.: HYDROGEN-SENSITIVE CHARACTERISTICS 1939

Fig. 1. AES depth profiles of the studied Pd/InP MOS Schottky diode.

and immediately put into the vacuumed chamber prior toSchottky contact formation. The chemically etched substratesprepared by removing the native oxide layers were utilized toform the referred Pd/InP MS junctions. Finally, the Schottkycontacts were made on the surface of InP active layer by using Pdmetal. The sensitive area of Schottky contact was cmand the whole device was defined by a mesa etching process witha diluted HCl solution. The steady-state and transient responsemeasurements were performed in a stainless steel reactionchamber connected to a gas flow tube with a regulating valve.Different concentrations of hydrogen gas in air of 15, 48, 97,202, 537, and 1010 ppm were used in this study, respectively.The chamber was maintained under atmospheric conditions andthe continuous and stable flowing hydrogen/air mixture gas of500 sccm was introduced into the chamber.

III. RESULTS AND DISCUSSION

Fig. 1 shows the sputter Auger electron spectroscopy (AES)depth profiles of a studied Pd/InP MOS Schottky diode. Clearly,an abrupt interface with a uniform composition is observed.Also, an oxygen signal is detected at the Pd metal surface and in-terface. Based on the work proposed by Wager and Wilmsen, thechemical composition of this thermally grown oxide is approx-imately 70–75% InO and 25–30% PO [15]. The estimatedrange of 45–55 of oxide thickness from the sputtering rate inthe AES measurements is in good consistence of the predictedresult of 56.8 from the empirical equation. Fig. 2(a) and (b)illustrate the current–voltage – curves of the Pd/InP MOSand MS Schottky diodes measured under atmospheric condi-tions with different hydrogen concentrations at 20C, respec-tively. With increasing hydrogen concentration, the forward andreverse currents in both devices are substantially raised and thechange exhibits a highly sensitive linearity. To our knowledge,the sensitivity limit at least 15 ppm from a direct observationis the lowest value at the low temperature of 20C under at-mospheric conditions. For the Pd/InP MOS Schottky diode, asshown in Fig. 2(a), the sensitivity in the forward and reverse cur-rents from air (the reference) to 1010 ppm hydrogen/air environ-

(a)

(b)

Fig. 2. Measured(I–V ) characteristics of the (a) Pd/InP MOS and (b) Pd/InPMS Schottky diodes atT = 20 C under atmospheric condition with differenthydrogen concentrations.

ment can reach four orders of magnitude at the applied biasesof 0.5 and V. The ideality factor in air indicating a typicalvalue of 1.28 supports a slight tunneling effect in presence of aninterfacial oxide layer. However, a sensitivity smaller by 2 or-ders of magnitude is found for the Pd/InP MS Schottky diode asshown in Fig. 2(b). This difference can be mainly attributed toa remarkable reduction of leakage current in air resulting fromthe improved interface properties for the Pd/InP MOS Schottkydiode. The improvement is interpreted in Fig. 3 which presentsthe barrier height as a function of hydrogen concentrationfor the Pd/InP MOS and MS Schottky diodes. In both cases, thethermionic emission transport plays a dominant role in the cur-rent conduction mechanism. On the basis of thermionic emis-sion (TE) theory, the barrier height for the MOS Schottky diode


Fig. 3. Barrier height as a function of hydrogen concentration in air for thePd/InP MOS and MS Schottky diodes. The inset shows the plot ofln(I =T )versus1=T for both devices in air.

in air is estimated to be 0.63 eV from the saturation current inthe forward bias region and a much lower value of 0.42 eV isobtained for the MS junction. These values are consistent withthe estimated results from the Richardson plots as shown in theinset of Fig. 3. Within the hydrogen concentration range mea-sured, the relatively substantial reduction in barrier height ofthe Pd/InP MOS Schottky diode is found as compared with thePd/InP MS Schottky diode with the increase of hydrogen con-centration. The enhancement of 0.21 eV in barrier height for theMOS Schottky diode provides a wider regime of barrier heightmodulation at lower hydrogen concentrations. Based on pre-vious experimental evidence, it has been proposed that the am-photeric native defects including the donor and accept levels areresponsible for the interface Fermi-level pinning at the Pd-InPMS interface [15]. The pinning position of Fermi-level falling inthe range of 0.3–0.55 eV below the conduction band minimum(CBM) limits the barrier height modulation and hence the hy-drogen sensitivity. Fig. 4(a) and (b) depict the schematic energyband diagrams for the Pd/InP MOS structure in air and upon ex-posure to hydrogen, respectively. It is well known that the dis-tinct pinning by oxygen on InP surface can be induced by a highdensity shallow donor states in the oxide instead of that by theamphoteric native defects [16], [17]. The ideal barrier height of0.74 eV in Schottky limit case can be determined by the differ-ence between the work function of Pd (5.12 eV) and the electronaffinity of InP (4.38 eV) [18].

From the experimental results, the barrier height is asso-ciated with the movement of the Fermi-level position inducedby the donor level in the oxide. As shown in Fig. 4(a), the posi-tion of the donor level is supposed to locate at 0.1 eV below theCBM, which coincides with the observation of pinning behavioraccording to the unified defect model [16], [17]. Upon hydrogenadsorption, the hydrogen molecules are dissolved into hydrogenatoms by the catalytic property of Pd metal. The hydrogen atomspenetrate the Pd metal with a high diffusion coefficient and form

a dipole layer at the Pd-oxide interface. In Fig. 4(b), the polar-ization of the dipole layer either neutralizes the donor level torelease the pinning effect or to cause the reversible reductionof barrier height in the H environment. Meanwhile, the de-crease of Pd effective work function is due to the field acrossthe Pd/oxide interface. Although the interface coverage proper-ties are not well understood, it is supposed that a high coverage,i.e., the strong capability of filling the adsorption sites on theinterface, is crucial for the significant hydrogen sensitivity.

Fig. 5 shows the influence of hydrogen concentration on thechange in the current at the temperaturesof50 Cand80 Cunderseveral forwardbiasconditions. and

are measured currents under hydrogen environment and air,respectively. At 50C, the rapid increase in at low hydrogenconcentrations followed by a saturation trend at high concentra-tions is observed similar to the results reported for Si, SiC, andGaAs based hydrogen sensors [11], [19]. At the higher tempera-ture of 80 C, however, a smaller change in the forward currentanomalous to those reported devices can be explained as follows.At the large forward bias, the barrier-related transport processesincluding both the TE transport and the thermionic field emission(TFE) transport completely dominate the forward current changewith temperature.Asthe temperature iselevated, theTEtransportis increasingly dominant and the effect of oxide-limited barrierheight modulation becomes weakened as compared to the TFEtransport. On the contrary, at lower temperatures, the enhancedeffect of barrier height modulation of TFE transport can lead toa larger change in the forward current. Fig. 6 shows the transientresponse curves upon the introduction and removal of 202 ppmhydrogen/air gas at several temperatures under the forward biasof 0.3 V. The adsorption time constant, defined as the time toreach of thesaturationvalueof , isan important indicationof response rate upon adsorption. As shown in the inset table ofFig.6, thevalueof isdecreasedfrom34to6sasthetemperatureis elevated from 50C to 110 C. The high initial rate of changein current around 133 A/s for such small amount ofhydrogen detection is obtained even at 50C while increased to253 A/s at 100 C. The positive temperature dependence of ad-sorption reaction rate can be attributed to the increased hydrogendissociation and diffusion coefficients. In addition, it is believedthat thereductionofhydrogenadsorptionsitesblockedbyoxygengives a significant contribution. During the hydrogen desorptionprocess, theadditionalhydroxylandwaterproduction is involvedin the reaction when upon exposure to air. The reaction of waterformation appears to be the rate-limiting factor and further accel-erates the recovery response especially at high temperature.

In general, since hydrogen sensors are usually employedunder atmospheric conditions, the influence of water formationresulting from hydrogen-oxygen reaction must be consideredin the kinetic studies. In addition to the qualitative discussionabove, the hydrogen adsorption properties related to the presenceof oxygen is also required to be quantitatively investigated forthe Pd/InP MOS interface. According to the Langmuir form ofresponse proposedbyLundströmet al., the coverage ofhydrogenat the interface can be put in the form [20], [21]

(1)


Fig. 4. Schematic energy band diagram for the Pd/InP MOS structure (a) in air and (b) upon exposure to hydrogen.

Fig. 5. Change in the current�I as a function of hydrogen concentration inair atT = 50 and 80 C under various forward biases of 0.1, 0.2, 0.3, and 0.4V.

where is a rate constant and and are the partial pres-sure of hydrogen and oxygen, respectively. The reaction orderapproaches unity for temperatures above 75C and decreases toone-half for lower temperatures. Fig. 7 shows as afunction of under atmospheric conditions at the threetemperatures of 50, 80, and 110C. The linear dependence inthis plot clarifies that the results coincide with the reported ki-netic reaction of Pd supporting SiOinterface [22]. Therefore,under steady-state conditions, the change in barrier height

Fig. 6. Transient response curves upon the introduction and removal of 202ppm H /air gas at various temperatures under the fixed forward bias of 0.3 V.

induced by hydrogen adsorption can be assumed to be propor-tional to as [11]

(2)

where is themaximumchange inbarrierheight.Substi-tuting (2) into (1) and applying the formula of saturation currentgive [11], [19], [23]

(3)


Fig. 7. The � =(1 � � ) as a function of(P ) under atmosphericconditions atT = 50; 80, and 110 C. The inset illustrates the plot of1= ln(I =I ) versus(P ) at three temperatures.

where represents the saturation current in air and andthe corresponding and maximum saturation currents in

hydrogen environment, respectively. The plot ofagainst at the three temperatures are shown in theinset of Fig. 7. From the intercept of , the value of

can be calculated by

(4)

where is the thermal voltage. For all these cases, the consis-tent value of eV is obtained as referred to thesaturated interface with adsorption sites fully occupied by the hy-drogenatoms.Furthermore, the rate constantcan be computedfromtheslopeof thecurves in the insetofFig.7andthe initialheatof adsorption per hydrogen atom of 0.42 eV/atom can beapproximately estimated via [11]

(5)

where Torr . This value of is smallerthan that of 0.86 eV/atom for the Pd/SiOinterface but largerthan the adsorption energy of Pd bulk eV [14], [24].Fig. 8 shows the experimental and modeled hydrogen adsorp-tion isotherms for the Pd/InP MOS structure at high temperaturesof 80 C and 110 C. The temperature effect is apparently ob-served from the logarithmic dependence of the experimental dataof coverage on hydrogen partial pressure. As the temperature isincreased to 110C, is shifted toward higher hydrogen pres-sures. Fogelberget al.presented the model well describing thebehavior of interface coverage for the Pd-MOS device using therate equations of H-O reaction under steady-state conditions[25]. According to Temkin isotherm of hydrogen adsorption, the

Fig. 8. Experimental and modeled interface coverage as a function ofhydrogen pressure for the Pd/InP MOS structure atT = 80 and 110 C. Theinset shows the schematic potential energy diagram of the Pd/InP MOS system.

heat of adsorption varies linearly with the coverageat thePd/oxide interface and can be described as [14]

(6)

where is a proportionality constant. From the schematic po-tential energy diagram in the inset, the hydrogen interface stateseems to approach equilibrium with the hydrogen surface statewith respect to the strong dependence of hydrogen coverage atthe interface and the surface. Oncebecomes high enough sothat is decreased to , a substantial accumulation of hy-drogeninthePdbulk takesplace.Thisresults inanupper interfacecoverage limit due to the nonsensitive formation of Pd-hydride.Therefore, the maximum variation in heat of adsorption over thesensitiverangecanbeobtainedfromthedifferencebetweenand values ( eV) which coincides with the extracted

value of 0.31 eV. By using the value ofeV/atom in thismodel, the theoreticalprediction is ingoodagree-ment with the experimental data. The saturation of sensitivity tohydrogen takes place as the value ofof 0.75 corresponding tothe heat of adsorption at the interface of 0.1 eV is achieved. Thus,the hydrogen sensing range where the value ofvaries under theatmospheric conditions can extend over at least three decades ofhydrogen partial pressure. From the prediction, furthermore, thesensitivity limit of the hydrogen pressure can be brought down toless than , respectively.

IV. CONCLUSION

We have fabricated and presented a novel Pd/InP MOSSchottky diode with the high hydrogen sensitivity and linearhydrogen response. It is found that at the Pd/oxide interface,the specific donor level replaces the amphoteric native defectsto cause the interface Fermi-level pinning. This leads to an im-proved barrier height as high as 0.63 eV that provides the wider


range of barrier height lowing and hence the high hydrogensensitivity as compared with the conventional metal-semicon-ductor interface. Even at a very low hydrogen concentration(15 ppm H in air) environment, a substantial response to Hisobserved. At 20 C, the sensitivity in both forward and reversecurrents can approach four orders of magnitude for 1010 ppmhydrogen under atmospheric conditions. In addition, even atthe lower temperatures, the fast response rate is evaluated bythe adsorption time constant and the initial rate of changein . Through kinetic studies, the initial heat of adsorptionat the interface of 0.42 eV/atom is supported by the maximumvariation in barrier height of 0.31 eV upon hydrogen adsorption.The good agreement between the experimental results and thetheoretical prediction confirms the isotherms of Temkin type.From the prediction, the wide hydrogen sensing range overmore than three decades of hydrogen partial pressure can beattributed to the correlation between the interface coverage andthe heat of adsorption at the interface.

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[6] Y. K. Fang, S. B. Hwang, C. Y. Lin, and C. C. Lee, “Trench Pd/Simetal-oxide-semiconductor Schottky barrier diode for a high sensitivityhydrogen gas sensor,”Appl. Phys. Lett., vol. 57, pp. 2686–2688, 1990.

[7] Y. Morita, K. I. Nakamura, and C. Kim, “Langmuir analysis on hydrogengas response of palladium-gate FET,”Sens. Actuators B, vol. 33, pp.96–99, 1996.

[8] T. L. Poteat, B. Lalevic, B. Kuliyev, M. Yousuf, and M. Chen, “MOSand Schottky diode gas sensors using transition metal electrodes,”J.Electron. Mater., vol. 12, pp. 181–214, 1983.

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[13] A. A. Iliadis, “Nearly ideal enhanced barrier height Schottky contacts ton-InP for MESFET applications,”Electron. Lett., vol. 25, pp. 572–574,1989.

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[18] R. L. Van Meirhaeghe, W. H. Laflere, and F. Cardon, “Influence of defectpassivation by hydrogen on the Schottky barrier height of GaAs and InPcontacts,”J. Appl. Phys., vol. 76, pp. 403–406, 1994.

[19] C. K. Kim, J. H. Lee, Y. H. Lee, N. I. Cho, D. J. Kim, and W. P. Kang,“Hydrogen sensing characteristics of Pd-SiC Schottky diode operatingat high temperature,”J. Electron. Mater., vol. 28, pp. 202–205, 1999.

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[25] J. Fogelberg and L. G. Petersson, “Kinetic modeling of the H-O re-action on Pd and its influence on the hydrogen response of a hydrogensensitive Pd metal-oxide-semiconductor device,”Surf. Sci., vol. 350, pp.91–102, 1996.

Wen-Chau Liu (A’91–M’93) was born in Yurn-LinHsien, Taiwan, R.O.C., on June, 1957. He receivedthe B.S.E., M.S.E., and Ph.D. degrees from NationalCheng-Kung University, Tainan, Taiwan, in 1979,1981, and 1986, respectively, all in electricalengineering. He has passed the Higher Civil Serviceexaminations and has obtained the technical expertlicenses of R.O.C. in the electrical and electronicfields, in 1979 and 1982, respectively.

He joined the faculty at National Cheng-KungUniversity as an Instructor and an Associate

Professor in the Department of Electrical Engineering, in 1983 and 1986,respectively. Since 1992, he has been a Professor in the same department.His research and teaching concern semiconductor device physics, analysis,and modeling. His research presently focuses on III-V heterostructure andsuperlattice devices including induced base transistor (IBT), superlattice-gateand heterostructure buffer layer FET’s, camel structure gate FET, saw-tooth-doping-superlatticed (SDS) devices, heterostructure-emitter bipolartransistor (HEBT), superlattice-emitter resonant-tunneling bipolar transistor(SE-RTBT), heterostructure-emitter and heterostructure-base transistor(HEHBT), superlatticed negative-differential-resistance (NDR) device,quantum-well�-doped NDR devices, metal-insulator-semiconductor (MIS)like multiple switching devices, low-dimensional quantum electron devices,and deep sub-micron meter devices and technologies. He has authored andcoauthored more than 160 journal papers. He holds over 23 patents in thesemiconductor field.

Dr. Liu is a member of Phi Tau Phi.

Hsi-Jen Panwas born in Taipei, Taiwan, R.O.C. onNovember 11, 1975. He received the B.S. degree inelectrical engineering from Tam-Kang University,Taipei, Taiwan, in 1997 and is currently pursuing thePh.D. degree in electrical engineering at the NationalCheng-Kung University, Tainan, Taiwan.

His research concentrates on MOCVD growthtechnology, III-V high-speed and microwave semi-conductor devices, resonant-tunneling devices, andhydrogen sensors.


Huey-Ing Chenwas born in Tainan, Taiwan, R.O.C.,in 1957. She received the B.S., M.S., and Ph.D. de-grees from Cheng Kung University (NCKU), Tainan,Taiwan, in 1979, 1981, and 1994, respectively, all inchemical engineering.

She joined the faculty at NCKU as an Instructorin the Department of Chemical Engineering, in1981. She is currently an Associate Professor in thesame department. Her research presently focuses onhydrogen permselective Pd-based membranes, hy-drogen sensors, gas separations, and nanoparticles.

Kun-Wei Lin was born in Yurn-Lin, Taiwan,R.O.C., on May 21, 1971. He received the B.S.degree in electrical engineering from the Fu-JenUniversity, Taipei, Taiwan, R.O.C., in 1995 andthe M.S. degree in electrical engineering fromCheng-Kung University, Tainan, Taiwan, in 1997,where he is currently pursuing the Ph.D. degree.

His research interests are in the field of III-V semi-conductor devices, such as graded field-effect tran-sistor and heterojunction bipolar transistor.

Shiou-Ying Cheng(S’96–M’99) was born in TaipeiHsien, Taiwan, R.O.C., on February 8, 1969. Hereceived the B.S. degree from Fong-Chia University,Taichung, Taiwan in 1991, the M.S. degree fromNational Taiwan-Ocean University, Keelung,Taiwan, in 1996, and the Ph.D. degree from NationalCheng-Kung University, Tainan, Taiwan, in 1999,all in electrical engineering.

He joined the faculty at Oriental Institute of Tech-nology as an Assistant Professor in the Departmentof Electrical Engineering in 1999. His research areas

focus on compound semiconductor device.Dr. Cheng is a member of Phi Tau Phi.

Kuo-Hui Yu was bron in Chiayi, Taiwan, R.O.C.,on January 9, 1976. He received the B.S. degree inelectrical engineering from the National Cheng-KungUniversity, Tainan, Taiwan, in 1998 whre he is cur-rently pursuing the Ph.D. degree in the Electrical En-gineering Department.

His research has focused on the field of III-V het-erostructure field-effect transistors.

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