Comprehensive study on hydrogen sensing properties of a Pd–AlGaN-based Schottky diode

ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y

33 (2008) 2986– 2992

Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/he

Comprehensive study on hydrogen sensing properties of a Pd–AlGaN-based Schottky diode Tsung-Han Tsaia, Huey-Ing Chenb, Kun-Wei Linc, Ching-Wen Hunga, Chia-Hao Hsua, LiYang Chena, Kuei-Yi Chua, Wen-Chau Liua, a

Department of Electrical Engineering, Institute of Microelectronics, National Cheng-Kung University, 1 University Road, Tainan 70101, Taiwan, ROC b Department of Chemical Engineering, National Cheng-Kung University, 1 University Road, Tainan 70101, Taiwan, ROC c Department of Computer Science and Information Engineering, Chaoyang University of Technology, Taichung County, Taiwan, ROC

ar t ic l e i n f o

abs tra ct

Article history:

In this work, the temperature dependences of a Pd/AlGaN Schottky diode-type hydrogen

Received 22 February 2008

sensor are investigated. The effects of temperature on parameters such as breakdown

Received in revised form

voltage, response time, and series resistance are presented. Experimentally, under a fixed

26 March 2008

current bias of 2  105 A a reverse voltage response as high as 6 V is observed. The

Accepted 26 March 2008

hydrogen adsorption effect also exhibits influences on the series resistance which is

Available online 27 May 2008

decreased by 18 O upon exposing to hydrogen gas at 200 1C. Besides, the ideality factor n

Keywords: Pd

shows a decreasing trend with the introduction of hydrogen gas. The voltage dependence on sensor performance is also studied. By increasing the voltage from 0.35 to 1 V, the response time is decreased by 15 s under the 1010 ppm H2/air gas. Furthermore, based on

AlGaN Temperature dependence Response time

the kinetic adsorption analysis the rate constant kr increases from 6.22  101 to 1.54 s1 at 300 1C with exposing to 99.4 and 9660 ppm H2/air gases, respectively. Therefore, on the basis of the compatibility with AlGaN-based microwave devices, the studied Pd/AlGaN hydrogen sensor shows the promise for fabricating the on-chip wireless senor systems. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

In recent years, the limited sources of fossil oil have become national and international issues. In order to alleviate this situation, some possible solutions including solar energy, wind energy, water energy, nuclear, and hydrogen energies have been studied and used. Among them, hydrogen energy shows the most significant potential for the replacement of fossil oil. Hydrogen comes from renewable sources without the undesired air pollution problems. Furthermore, hydrogen has also been used in the fields of semiconductor process and medical applications [1,2]. However, hydrogen energy still has shortcoming of being explosive at high concentration [3,4]. To

protect human beings from suffering the dangerous situation, it is necessary to build up a pre-alarm system when using high concentration of hydrogen gases. Thus, a vast amount of researches about hydrogen sensors have been made [5–11]. With the progress of the semiconductor process and technology, the solid-state types of hydrogen sensors (e.g., capacitor, diode, and HEMT) have been fabricated. Over the past few decades, Si-based hydrogen sensors have been widely investigated [12,13]. Recently, there is a great interest in semiconductor materials with high electron mobility (e.g., GaAs and InP) for fabricating solid-state hydrogen sensors [6,14]. The III–V compound semiconductor-type hydrogen sensors exhibit higher sensitivity, shorter response time,

Corresponding author. Tel.: +886 6 275 7575; fax: +886 6 209 4786.

E-mail address: [email protected] (W.-C. Liu). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.03.055

ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y

and wide operating temperature regime as compared with Sibased hydrogen sensors [5–9]. Nowadays, III–N materials have become very popular to the researchers due to their inherent properties of the wider bandgap, higher saturation velocity, and higher resistance to radiators. Moreover, the wavelength of III–N materials is tunable while being fabricated into optical devices [15]. GaN-based materials have also been used for fabricating the hydrogen sensors. The robust temperature durability and high sensor response of GaN-based sensors are demonstrated [16,17]. For the GaN-based material systems, AlGaN/GaN heterostructure devices have potentially been integrated with monolithic microwave integrated circuit (MMIC) on the same chip. In this work, an interesting Pd–AlGaN/GaN Schottky diodetype hydrogen sensor is fabricated and studied. Pd metal is well known for its affinity to hydrogen gas and high diffusivity of hydrogen atoms. Thus, Pd metal is very suitable for hydrogen leak detection. A comprehensive analysis of steady- and transient-state responses with the hydrogen concentration as a parameter is presented in this work. In addition, the temperature dependences of the breakdown characteristics are also studied. The rate constant of hydrogen adsorption can be obtained by kinetic adsorption analysis.

2.

Experimental

Fig. 1 shows the epitaxial structure of the studied device. The epitaxial structure was grown by a metal organic chemical vapor deposition (MOCVD) system. From the 2-in c-plane substrate up, a 2 mm-thick GaN buffer layer was grown. Above the buffer layer, the active structure consisted of a 1 mm-thick undoped GaN layer and a 30 nm-thick n-AlGaN layer with a carrier concentration of 2  1018 cm3. To separate each sensing device and decrease leakage current, the physical dry etching was employed to form the mesa. Subsequently, a solution of HF:H2O ¼ 1:1 was used to remove the native oxide on the AlGaN surface. Ti (20 nm) and Al (100 nm) metals were evaporated in turn onto the AlGaN layer. Samples were placed into the rapid thermal annealing (RTA) system at 900 1C for 300 s in N2 ambiance to form Ohmic contact behavior. Schottky contacts were carried out by evaporating a Pd thin

T i/Al

Pd

30 nm-n+ AlGaN, n = 2×1018 cm-3 1 μm-undoped-GaN 2 μm-GaN-Buffer Sapphire Sub. Fig. 1 – Schematic cross-section of the studied Pd/AlGaN hydrogen sensor.

33 (2008) 2986 – 2992

2987

film of 30 nm. The area of Schottky contact was 2.05  103 cm2. In order to facilitate the process of the hydrogen testing experiments, the studied device was bonded onto the TO-8 metal can, and subsequently put into a sealed chamber. Hydrogen concentrations of 15.2, 99.4, 494, 1010, and 9660 ppm with synthetic air were used for the hydrogen sensing experiments. The electrical characteristics of the studied device were measured by an HP 4155 semiconductor parameter analyzer. The current variations as a function of time in response to different hydrogen concentrations were measured by a CHI 627A electrochemical analyzer.

3.

Results and discussion

Figs. 2(a)–(c) illustrate the current–voltage (I– V) characteristics of the studied sensor device under different hydrogen gases, at 30, 100, and 200 1C, respectively. Both forward and reverse currents exhibit systematic dependences on hydrogen concentration. Generally, hydrogen sensing mechanism can be briefly interpreted as follows. With the catalytic property of Pd metal, hydrogen molecules can be dissociated into hydrogen atoms. Due to the high diffusivity of hydrogen atoms in the Pd bulk, the hydrogen atoms diffuse quickly to the Pd/AlGaN interface. The hydrogen atoms are adsorbed at the Pd/AlGaN interface. The adsorbates are then polarized by the built-in electric field to form a dipolar layer. This dipole layer can decrease the effective Schottky barrier height [18]. Fig. 2 also shows the breakdown voltage BVd as a function of hydrogen concentration. The BVd is measured under a fixed current of 20 mA. Obviously, the BVd is also affected by the hydrogen adsorption effect. Fig. 3 illustrates the breakdown voltage BVd as a function of temperature. Clearly, the magnitude of BVd is decreased with the increase of hydrogen concentration as shown in Fig. 3. The BVd magnitude in air is slightly decreased before 150 1C; however, it is significantly increased with further increasing the working temperature. So far, the abnormal temperature dependences of BVd are not clear [19]. Under exposing to lowconcentration hydrogen gas, the similar tendencies are also observed. While the hydrogen concentration approaches 9660 ppm H2/air, the increase in BVd magnitude at higher temperature becomes insignificant. The BVd magnitude is almost independent with working temperature under higher hydrogen concentration. However, the variation of diode breakdown voltage jBVdH2  BVdair j tends to increase with temperature. The increase in variation of BVd results from the increase of the baseline voltage (BVdair). Therefore, the hydrogen sensing signal can be enhanced by increasing the temperature to an appropriate value. Fig. 4 shows the reverse voltage response versus temperature in different hydrogen gases. The bias current is kept at 2  105 A. Here, we define the voltage response as jVH2  Vair j. Obviously, a larger voltage variation can be acquired under a higher temperature operation. If the applied current is kept at 2  105 A, a forward voltage response can also be obtained. However, the forward voltage response is always below 0.5 V, which is much lower than the reverse one. Hence, the studied Schottky diode hydrogen sensor under reverse current bias exhibits a more usable sensing signal in

ARTICLE IN PRESS 2988

I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y

20

Breakdown Voltage BVd (V)

30°C

Current Id (μA)

-2

air 15.2 ppm H2/air 99.4 ppm H2/air

10

494 ppm H2/air 1010 ppm H2/air 9660 ppm H2/air

0

33 (2008) 2986 – 2992

-10

-4

-6 air -8

15.2 ppm H2/air 99.4 ppm H2/air

-10

50

-20 -5

-4

-3 -2 -1 Applied Voltage Vappl (V)

0

1010 ppm H2/air

494 ppm H2/air 100

9660 ppm H2/air

200 250 150 Temperature (°C)

1

300

350

Fig. 3 – Relationship between the breakdown voltage BVd and temperature.

T = 100°C

Current Id (μA)

10

0

air 15.2 ppm H2/air 99.4 ppm H2/air 494 ppm H2/air 1010 ppm H2/air 9660 ppm H2/air

-10

-20 -5

-4

-3 -2 -1 Applied Voltage Vappl (V)

0

1

Reverse Voltage Response |VH2-Vair| (V)

20

Bias Current = -2X10-5A

15.2 ppm H2/air 99.4 ppm H2/air

8

9660 ppm H2/air

494 ppm H2/air 6

1010 ppm H2/air

4

2

0 0

20 air 15.2 ppm H2/air

T = 200°C

Current Id (μA)

100

150

200

300

Fig. 4 – Reverse voltage response versus temperature in different hydrogen gases. The bias current is kept at 2  105 A.

9660 ppm H2/air

0

Generally, the current–voltage (I– V) of a Schottky diode can be expressed as [20]

-10

I ¼ A AT2 expðfB =kTÞ exp fqðV  Rs IÞ=nkTg

-20

250

Temperature (°C)

99.4 ppm H2/air 494 ppm H2/air 1010 ppm H2/air

10

50

-5

-4

-1 -3 -2 Applied Voltage Vappl (V)

0

1

Fig. 2 – Current–voltage (I– V) characteristics of the studied sensor upon exposing to different H2/air gases at (a) 30 1C, (b) 100 1C, and (c) 200 1C, respectively.

practical applications. If the Schottky diode is operated under the thermionic emission region, it will exhibit a large current sensitivity ðIH2 =Iair  1Þ [6] value on the order of 104 or more. Furthermore, if the Schottky diode is operated under the series resistance region, a large current variation (or current response) on the order of several mA is observed.

(1)

where A is the area of Schottky contact, A* the Richardson’s constant, T the absolute temperature, fB the Schottky barrier height, k the Boltzmann’s constant, V the applied voltage, n the ideality factor, and Rs the series resistance. At high temperature operation, one should take into account the effect of series resistance due to the increased reverse saturation current [21]. Moreover, the conventional method is not reliable to extract Schottky barrier height at high temperature condition. Based on the Werner method a more accurate and reliable series resistance can be calculated [22]. It is well known that for a diode type hydrogen sensor the Schottky barrier height fB is decreased with increasing the hydrogen concentration due to the formation of hydrogen dipole layer [16]. However, the hydrogen dipole effect on the series resistance of the Schottky diode has seldom been

ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y

14 99.4 ppm H2/air

12

6 4

200 30

300 11

Vappl = 0.35 V

: H2 off Current (μA)

Current (μA)

8

100 50.4

T(°C) a (s)

: H2 on

10

0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00

2

300°C

100°C

0

0

500 1000 1500 2000 2500 Time (s) 200°C

0

500

1000

1500 Time (s)

2000

100 50.4

300 11

2500

3000

90 1010 ppm H2/air

80

T(°C) a (s)

: H2 on

70 Current (μA)

studied. Fig. 5 shows the series resistance Rs of the studied device versus hydrogen concentration at 30, 150, and 200 1C. The Rs decreases after the exposure of hydrogen gas. Under the introduction of a 1010 ppm H2/air gas, the Rs decreases from 88 to 76 O. The decrease in Rs indicates that the electron in the neutral region of AlGaN is increased due to the hydrogen adsorption effect [23]. A possible explanation to the result is that the donor-like hydrogen atoms at the Pd/ AlGaN interface donate the electrons to the bulk of AlGaN. With further increasing the hydrogen concentration, however, the series resistance Rs appears to be less affected by the hydrogen adsorption effect. Also, a saturation trend is observed. As seen from Fig. 2, the I– V characteristics of the studied device show systematic dependences on hydrogen concentration when exposing to the hydrogen gases. This also confirms that the mechanisms for the hydrogen adsorption effect on the Schottky barrier height and series resistance are different. Table 1 lists the ideality factors of the studied device at room temperature. The ideality factor n decreases drastically with increasing hydrogen concentration. Moreover, the corresponding n values are 2.53 and 1.67 under the hydrogen concentrations of 15.2 and 9960 ppm H2/air, respectively. The larger value of ideality factor (n42) means that current transport is not completely dominated by the diffusion, drift and recombination mechanisms in space charge region [24]. As the hydrogen concentration is increased to higher than 494 ppm H2/air, the ideality factor n is smaller than 2 (between 1 and 2). It is speculated that the dipole effect changes the distribution of space charge region

2989

33 (2008) 2986 – 2992

200 30

300°C

: H2 off

60

Vappl = 0.35V

50 200°C

40 30 20

100°C

10 0 0

2000 Time (s)

1000

3000

0.35 9660 ppm H2/air

0.30

90 30°C

200°C 80

75

100°C

0.20

Current (mA)

Current (mA)

150°C

85 Series Resistance RS (Ω)

0.25

0.15 0.10 0.05

T(°C) a (s)

100 9.6

200 8.4

300 4

Vappl = 0.35 V 300°C

200°C 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

: H2 on 0

100 Time (s)

1000

1500 Time (s)

200

: H2 off

0.00

70

0

500

65 air

600 800 400 200 Hydrogen Concentration (ppm H2/air)

1000

Fig. 5 – Series resistance as a function of hydrogen concentration at 30, 150, 200 1C.

2000

2500

3000

Fig. 6 – Transient response curves at 100, 200, 300 1C upon exposing to (a) 99.4 ppm H2/air gases, (b) 1010 ppm H2/air gases, and (c) 9660 ppm H2/air gases. The inset reveals the corresponding response time. The voltage bias is kept at 0.35 V.

Table 1 – The ideality factor as a function of hydrogen concentration at room temperature Hydrogen concentration (ppm)

Air

15.2

99.4

494

1010

9660

Ideality factor, n

2.53

2.41

2.26

2

1.92

1.67

ARTICLE IN PRESS 2990

I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y

larger magnitude of the applied voltage exhibits a shorter response time. It is speculated that a larger voltage bias can provide the additional energy which hydrogen atoms are needed to overcome the potential energy to be adsorbed at Pd/AlGaN interface. Consequently, a shorter response time is observed under a larger absolute value of voltage bias. In this

ln [1-ln (Iog/Io)/ln (Iog, eq/Io)]

0.0 -0.2 -0.4 -0.6 -0.8

T = 100°C 99.4 ppm H2/air

-1.0

1010 ppm H2/air

-1.2

9660 ppm H2/air

-1.4 0

1

2

3

Time (s) 0.0 ln [1-ln (Iog/Io)/ln (Iog, eq/Io)]

and affects the statuses of diffusion, drift, and recombination behaviors. Figs. 6(a)–(c), show the transient response curves of the studied Pd/AlGaN Schottky diode at 100, 200, and 300 1C. The corresponding response time (ta) upon exposing to the 99.4, 1010, and 9660 ppm H2/air gases are shown in insets. The voltage bias is kept at 0.35 V. Obviously, the current variation increases with increasing the hydrogen concentration at these three temperatures. Here, the response time constant is defined as the time required for reaching the inverse exponential (e1) value of the saturation current from the baseline level. The response time decreases with the increase of hydrogen concentration and working temperature. The hydrogen concentration dependence of ta can be attributed to the increase of hydrogen dissociation, diffusion, and adsorption coefficients [25]. Moreover, the applied voltages also have great influences on the response time. Fig. 7 shows the response time ta versus temperature under the introduction of 1010 ppm H2/air gas at different applied voltages. The bias voltages are 1, 0.35, and 1 V which represents that the Schottky diode is operated in the series resistance, thermionic emission, and reverse saturation regions, respectively. Experimentally, the response time ta decreases to a saturated value on increasing the temperature due to the increased hydrogen dissociation coefficient. The sensor operated in series resistance region exhibits the shortest response time. It is worth noting that at the same working temperature, the response time ta can be improved by different biases. This suggests that bias voltage may affect the hydrogen dissociation, adsorption, or diffusion coefficients. However, under such a high temperature (T4100 1C), the transit time of hydrogen in the Pd metal thin film is on the order of about 105 s [26]. The applied voltage appears to less likely affect the hydrogen dissociation. Therefore, we attribute the decrease in response time to the adsorption step at the Pd/AlGaN interface. From the experimental results, the

33 (2008) 2986 – 2992

-0.2 -0.4 T = 200°C

-0.6

99.4 ppm H2/air 1010 ppm H2/air

-0.8

9660 ppm/H2air -1.0 0.0

25 1010 ppm H2/air Vappl

-1V

10

5

0 100

150 200 Temperature (°C)

1.2 Time (s)

1.6

2.0

0.0

1V 15

0.8

0.35 V

ln [1-ln (Iog/Io)/ln (Iog, eq/Io)]

ResponseTime τa (s)

20

0.4

-0.2 -0.4 T = 300°C -0.6

99.4 ppm H2/air 1010 ppm H2/air

-0.8

250

Fig. 7 – Transient response time sa as a function of temperature under the applied voltages of 0.35, 1, and 1 V. These applied voltages represent thermionic emission, reverse saturation, and series resistance operating regions, respectively.

9660 ppm H2/air -1.0 0

1 Time (s)

2

Fig. 8 – Plots between the ln[1ln(Iog/Io)/ln(Iog,eq/Io)] and time at (a) 100 1C, (b) 200 1C, and (c) 300 1C. The bias voltage is kept at 0.35 V.

ARTICLE IN PRESS I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y

Table 2 – The rate constant kr under different hydrogen gases and temperatures kr (s1)

Temperature (1C)

Hydrogen concentration (ppm H2/air)

100 200 300

99.4

1010

9660

4.4  102 2.13  101 6.22  101

5.9  102 2.23  101 7.4  101

0.108 2.4  101 1.54

work, the applied voltage shows a great influence on the response time ta. For further study of the transient responses, the kinetic adsorption analyses are performed. The Pd metal is known for its catalytic capability on hydrogen molecules and a fast diffusivity for hydrogen atoms [27]. Therefore, the related reaction equation can be considered as H  Ss þ Si 2H  Si þ Ss

(2)

where Ss is the surface adsorption sites, Si the adsorption sites at Pd/AlGaN interface. The response rate can be expressed by the equation: dyi dt ¼ kr ðyi;eq  yi Þ

(3)

33 (2008) 2986 – 2992

on the hydrogen concentration. From experimental results, the BVd is strongly dependent on the working temperature. Furthermore, a large sensing signal which is suitable for practical applications can be obtained under the bias current of 2  105 A. The decrease in Rs as exposed to hydrogen gas indicates that the electron concentration in the AlGaN bulk is increased. With the increase in hydrogen concentration, the ideality factor n is decreased. The response time ta is decreased from 50(9.6) to 11(4) s at 100(300) 1C with exposing to 99.4 and 9660 ppm H2/air gases, respectively. Besides, the applied voltage can affect the sensor performance. At 100 1C, the ta is improved by 15 s by increasing the applied voltage Vappl from 0.35 to 1 V under a 1010 ppm H2/air gas. Experimental results reveal that the studied Schottky diode-type hydrogen sensor exhibits a short response time and a large current variation in the series resistance region, a high sensitivity value in the thermionic emission region, and a significant voltage response in the reverse saturation region. The reaction rate constant kr derived from the kinetic adsorption analyses are 0.108 and 1.54 at 100 and 300 1C, respectively, upon exposing to a 9660 ppm H2/air gas. A large kr indicates a faster adsorption rate at the Pd/AlGaN interface. Consequently, the studied Pd/AlGaN Schottky diode hydrogen sensor is suitable for high temperature operation. Due to the improved performance including large voltage response, fast response time can be acquired by increasing the working temperature to an appropriate value.

where yi is the hydrogen coverage at Pd/AlGaN interface, yi,eq the equilibrium hydrogen coverage at Pd/AlGaN interface, kr the pressure-dependent rate constant. By integrating Eq. (3): ! ! yi Df ln 1  (4) ¼ ln 1  ¼ kr t Dfeq yi;eq

Acknowledgment

where t is the measuring time. Furthermore, Df can be replaced by the response current. Thus, Eq. (4) can be expressed as   lnðIog =Io Þ ¼ kr t (5) ln 1  lnðIog;eq =Io Þ

R E F E R E N C E S

where Io is the current in air, Iog the hydrogen response current, Iog,eq the steady-state response current. Figs. 8(a)–(c), illustrate the relationships between ln[1ln(Iog/Io)/ln(Iog,eq/Io)] and measuring time t at 100, 200, and 300 1C, respectively. These figures reveal the observed linear relationships. It means that the hydrogen adsorption rate is predominantly controlled by the hydrogen adsorption step [28]. The pressure-dependent rate constant kr can be extracted from the slopes of the linear plots. The rate constants kr are listed in Table 2. The kr increases from 6.22  101 to 1.54 s1 at 300 1C with exposing to 99.4 and 9660 ppm H2/air gases, respectively. Obviously the rate constant kr is increased with the hydrogen concentration and working temperature. This result is in agreement with the response time behaviors.

4.

Conclusions

In summary, the properties of a Pd/AlGaN Schottky diodetype hydrogen sensor are comprehensively investigated. The diode breakdown voltage BVd shows a systematic dependence

2991

Part of this work was supported by the National Science Council of the Republic of China under contract no. NSC-952221-E-434-MY2.

[1] Fukata N, Sato S, Fukuda S, Ishioka K, Kitajima M, Hishita S, et al. Passivation and reactivation of carriers in B- and Pdoped Si treated with atomic hydrogen. Physica B 2007;401–402:175–8. [2] Adlhart OJ, Rohonyi P, Modroukas P, Driller J. A small portable proton exchange membrane fuel cell and hydrogen generator for medical applications. Asaio J 1997;43:214–9. [3] Takeno K, Okabayashi K, Kouchi A, Nonaka T, Hashiguchi K. Dispersion and explosion field tests for 40 MPa pressurized hydrogen. Int J Energy Res 2007;32:2144–53. [4] Rosyid OA, Jablonski D, Hauptmanns U. Risk analysis for the infrastructure of a hydrogen economy. Int J Hydrogen Energy 2007;32:3194–200. [5] Cheng CC, Tsai YY, Lin KW, Chen HI, Hsu WH, Chuang HM, et al. Hydrogen sensing characteristics of Pd- and Pt–Al0.3Ga0.7As metal–semiconductor (MS) Schottky diodes. Semicond Sci Technol 2004;19:778–82. [6] Chou YI, Chen CM, Liu WC, Chen HI. A new Pd–InP Schottky hydrogen sensor fabricated by electrophoretic deposition with Pd nanoparticles. IEEE Electron Device Lett 2005;26:62–5. [7] Huang JR, Hsu WC, Chen YJ, Wang TB, Lin KW, Chen HI, et al. Comparison of hydrogen sensing characteristics for Pd/GaN and Pd/Al0.3Ga0.7As Schottky diodes. Sens Actuators B 2006;117:151–8.

ARTICLE IN PRESS 2992

I N T E R N AT I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y

[8] Hung CW, Cheng SY, Lin KW, Tsai YY, Lai PH, Fu SI, et al. Hydrogen detection by a GaAs-based transistor with a palladium (Pd) thin film gate structure. Adv Mater Res 2007;15–17:275–80. [9] Tsai YY, Hung CW, Fu SI, Lai PH, Chang HC, Chen HI, et al. Comprehensive investigation of hydrogen-sensing properties of Pt/InAlP-based Schottky diodes. Sens Actuators B 2007;124:535–41. [10] Satyajit S, Peng Z, Hyoung JC, Lawrence L, Sudipta S. Significance of electrode-spacing in hydrogen detection for tin oxide-based MEM sensor. Int J Hydrogen Energy 2008;33:470–5. [11] Adamyan AZ, Adamyan ZN, Aroutiounian VM, Arakelyan AH, Touryanb KJ, Turner JA. Sol–gel derived thin-film semiconductor hydrogen gas sensor. Int J Hydrogen Energy 2007;32:4101–8. [12] Sekhar PK, Sine A, Bhansali S. Effect of varying the nanostructured porous-Si process parameters on the performance of Pd-doped hydrogen sensor. Sens Actuators B 2007;127:74–81. [13] Filippov VI, Moritz W, Terentjev AA, Vasiliev AA, Yakimov SS. Hydrocarbon and fluorocarbon monitoring by MIS sensors using an Ni catalytic thermodestructor. IEEE Sens J 2007;7:192–6. [14] Liu WC, Pan HJ, Chen HI, Lin KW, Lin CK. Comparative hydrogen-sensing study of Pd/GaAs and Pd/InP metal-oxidesemiconductor Schottky diodes. Jpn J Appl Phys 2001;40:6254–9. [15] Mayes K, Yasan A, McClintock R, Shiell D, Darvish SR, Kung P, et al. High-power 280 nm AlGaN light-emitting diodes based on an asymmetric single-quantum well. Appl Phys Lett 2004;84:1046–8. [16] Ali M, Cimalla V, Lebedev V, Tilak V, Sandvik PM, Merfeld DW, et al. A study of hydrogen sensing performance of Pt–GaN Schottky diodes. IEEE Sens J 2006;6:1115–9.

33 (2008) 2986 – 2992

[17] Song J, Lu W, Flynn JS, Brandes GR. Pt–AlGaN/GaN Schottky diodes operated at 800 1C for hydrogen sensing. Appl Phys Lett 2005;87:133501-1–133501-3. [18] Hung CW, Chen HI, Tsai TH, Chang CF, Chen TP, Chen LY, et al. J Electrochem Soc 2008;155:H243–6. [19] Lai PH, Fu SI, Tsai YY, Yen CH, Chuang HM, Cheng SY, et al. IEEE Trans Device Mater Reliab 2006;6:52. [20] Sze SM. Physics of semiconductor devices. 2nd ed. New York: Wiley; 1981. [21] Khan SA, Elder AV, Hidekazu U, Teruaki K. Gas response and modeling of NO-sensitive thin-Pt SiC Schottky diodes. Sens Actuators B 2003;92:181–5. [22] Werner JH. Schottky barrier and pn-junction I/V plots-small signal evaluation. Appl Phys A 1988;47:291–300. [23] Neaman D. Semiconductor physics and devices basic principles. New York: McGraw-Hill; 2003. p. 295. [24] Verschraegen J, Burgelman M, Penndorf J. Temperature dependence of the diode ideality factor in CuInS2-on-Cutape solar cells. Thin Solid Films 2005;480:307–11. [25] Lin KW, Chen HI, Cheng CC, Chuang HM, Lu CT, Liu WC. Characteristics of a new Pt/oxide/In0.49Ga0.51P hydrogensensing Schottky diode. Sens Actuators B 2003;94:145–51. [26] Jewett DN, Makrides AC. Diffusion of hydrogen through palladium and palladium–silver alloy. J Chem Soc Faraday Trans 1965;61:932–9. [27] Lechuga LM, Calle A, Golmayo D, Briones F. Different catalytic metals (Pt, Pd, and Ir) for GaAs Schottky barrier sensors. Sens Actuators B 1992;7:614–8. [28] Chen HI, Chou YI, Hsiung CK. Comprehensive study of adsorption kinetics for hydrogen sensing with an electroless-plated Pd–InP Schottky diode. Sens Actuators B 2003;92:6–16.