Comprehensive investigation on planar type of Pd–GaN hydrogen sensors

international journal of hydrogen energy 34 (2009) 5604–5615

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Review

Comprehensive investigation on planar type of Pd–GaN hydrogen sensors Shao-Yen Chiua, Hsuan-Wei Huanga, Tze-Hsuan Huanga, Kun-Chieh Lianga, Kang-Ping Liua, Jung-Hui Tsaib, Wen-Shiung Loura,* a

Department of Electrical Engineering, National Taiwan Ocean University, 2 Peining Road, Keelung, Tainan, Taiwan, ROC Department of Electronic Engineering, National Kaohsiung Nomal University, 62 Shenjhong Road, Yanchao Township, Kaohsiung, Tainan, Taiwan, ROC

b

article info

abstract

Article history:

This paper reviews both static and dynamic characteristics of a planar-type Pd–GaN metal–

Received 7 April 2009

semiconductor–metal (MSM) hydrogen sensor. The sensing mechanism of a metal–

Received in revised form

semiconductor (MS) hydrogen sensor was firstly reviewed to realize the sensing mecha-

28 April 2009

nism of the proposed sensor. Symmetrically bi-directional current–voltage characteristics

Accepted 28 April 2009

associated with our sensor were indicative of easily integrating with other electrical/optical

Available online 29 May 2009

devices. In addition to the sensing current, the sensing voltage was also used as detecting signals in this work. With regard to sensing currents (sensing voltages), the proposed

Keywords:

sensor was biased at a constant voltage (current) in a wide range of hydrogen concentra-

GaN

tion from 2.13 to 10,100 ppm H2/N2. Experimental results reveal that the proposed sensor

Hydrogen sensor

exhibits effective barrier height variations (sensing responses) of 134 (173) and 20 mV (1) at

Metal–semiconductor

10,100 and 2.13 ppm H2/N2, respectively. A sensing voltage variation as large as 18 V was

Pd

obtained at 10,100 ppm H2/N2, which is the highest value ever reported. If an accepted

Dipole layer

sensing voltage variation is larger than 3 (5) V, the detecting limit is 49.1 (98.9) ppm. Moreover, voltage transient response and current transient response to various hydrogencontaining gases were experimentally studied. The new finding is that the former response time is shorter than the latter one. Other dynamic measurements by switching voltage polarity and/or continuously changing hydrogen concentration were addressed, showing the proposed sensor is a good candidate for commonly used MS sensors. Crown Copyright ª 2009 Published by Elsevier Ltd on behalf of International Association for Hydrogen Energy. All rights reserved.

1.

Introduction

In addition to solar energy, wind energy, and water energy, hydrogen energy that is clean, sustainable, and abundant is another possible solution to alleviate energy crisis resulting from the limited sources of fossil oil. Hydrogen can come from

renewable sources and prevent undesired air pollution problems. Hydrogen has also been employed in the fields of electronic manufacture, industrial and medical applications. So, hydrogen has been attracting much attention for several decades in view of international energy issues and scientific development [1]. However, hydrogen is very volatile and

* Corresponding author. Tel.: þ886 2 24622192; fax: þ886 2 24635408. E-mail address: [email protected] (W.-S. Lour). 0360-3199/$ – see front matter Crown Copyright ª 2009 Published by Elsevier Ltd on behalf of International Association for Hydrogen Energy. All rights reserved. doi:10.1016/j.ijhydene.2009.04.073

international journal of hydrogen energy 34 (2009) 5604–5615

extremely flammable. A small leakage of high concentration of hydrogen-containing gases could eventually lead to a devastating state. In order to protect human from being in such dangerous situation, it is absolutely necessary to organize an effective pre-alarm system when using hydrogen-containing gases. Thus, hydrogen sensors with high response to hydrogen-containing gases are demanded. In particular, it is highly desirable to develop a sensitive and reliable sensor, which is able to detect the leakage instantaneously [2–10]. In fact, a lot of researches about various hydrogen sensors have been reported over past two decades [11–21]. Among them, hydrogen sensors fabricated with semiconductors as sensing platforms and catalytic metals as sensing metals are of particular potential. The catalytic metals can absorb hydrogen spontaneously. This absorption alters the electrical and optical properties of these metals. These alternations together with semiconductors have thus been exploited for measuring hydrogen-containing gases. With regard to the group VIII transition metals, palladium (Pd) and platinum (Pt) are the two of most popular catalytic metals that have a remarkable property of absorbing hydrogen [12]. Concerning sensing platforms, almost all present semiconductors widely used are suitable for fabricating hydrogen sensors. They include at least Si-based [13], GaAs-based [14,15], InP-based [16,17], and GaNbased [18–20] semiconductors. Si-based hydrogen sensors are fabricated to take advantages of low cost and highly matured technology. Nevertheless, compound–semiconductor-based hydrogen sensors generally show higher sensing response, shorter response time, and wider range of operating temperature. This also explains why so many hydrogen sensors using Pd-related metals [21–23] (or Pt-related metals) [24,25] and compound semiconductors have been addressed. In addition to choosing appropriate sensing platforms and sensing metals, another consideration in designing hydrogen sensors is to determine device configuration. Metal– semiconductor (MS) Schottky diodes [26–28], metal–insulator– semiconductor (MIS) diodes [29,30], and field-effect transistors (FETs) [4,31,32] are the three of promising device configurations for detecting hydrogen-containing moieties. The former two configurations exhibit advantages of simple layer structure and easy fabrication. The FET-type hydrogen sensors show an advantage of down-size and promise for microelectronics-based smart sensing systems and micro electric and mechanical system (MEMS) applications. The FET-type sensors can also have various formations including suspended gate FET (SGFET) [33,34], hybrid SGFET [35], and capacitively-coupled FET (CCFET) [36,37]. Unfortunately, they generally suffer from the relatively complicated manufacturing processes. On the other hand, commonly accepted sensing sequences for the three kinds of hydrogen sensors are as follows. The hydrogen molecules are adsorbed and dissociated into hydrogen atoms at a catalytic metal surface. Rapid diffusion of hydrogen atoms to the metal– semiconductor (or metal–insulator) interface where the dipole layer is formed alters the surface charge by depletion. A dipole-induced current is increased by the lowering of Schottky barrier height. The current–voltage characteristics belong to rectifying behaviors for MS and MIS sensors in N2/air and in hydrogen-containing ambiences. In spite of functional applications, an undesirable conducting current brings about

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unnecessary power consumption when these hydrogen sensors are in forward-operation mode. Thus, MS and MIS hydrogen sensors operated in forward mode are not suitable for in situ long-term detection. In this review paper, another planar type of metal– semiconductor–metal (MSM) hydrogen sensor was comprehensively investigated. We were concerned about practicability of such a planar type of MSM hydrogen sensor. The catalytic metal and compound semiconductor used are not of particularity. Thus, Pd and GaN were employed in this work. Compared to commonly used MS and MIS hydrogen sensors, MSM hydrogen sensors exhibit key features as follows. Reduced processing steps are expected in the fabrication of MSM hydrogen sensors. The noble metals used for Ohmic electrodes can be eliminated. Instead of rectifying behaviors, symmetrically bi-directional sensing performance is available. Finally, a single lightly doped layer is required only. Hence cost down and integration with other electrical/optical devices can be carried out using MSM hydrogen sensors. Hydrogen-sensing mechanisms between MS and MSM hydrogen sensors were firstly reviewed in the next section. Experimental including the sensor structure, fabrication, and measurement was introduced in Section 3. The sensing characteristics and all possible applications were thoroughly investigated and comprehensively discussed in the following sections. Finally, a conclusion was made.

2. Sensing mechanism of MS/MSM hydrogen sensors 2.1.

Review sensing mechanism of an MS diode

Based on the metal–semiconductor contact theory, the band diagram and the charge distribution at equilibrium are shown in Fig. 1(a). The catalytic metal is assumed to be a perfect conductor. The charge transferred to it from the semiconductor exists in a very narrow region at the metal surface. The extent of the space charge in the semiconductor is denoted as W. Thus the maximum electric field (xm) and builtin voltage (Vbi) are given by xm ¼

qND W 3s

(1)

Vbi ¼

qND W2 23s

(2)

where 3s and ND are the dielectric constant and the carrier concentration, respectively, of the semiconductor. Fig. 1(b) is the cross-section of an MS hydrogen sensor subjected to a hydrogen-containing gas, showing the sensing processes. H2 stands for hydrogen molecule and H for hydrogen atom. Then the sensing mechanism can be briefly described as follows. In step 1, hydrogen molecules are dissociated into hydrogen atoms when the hydrogen-containing gas adsorbs at the surface of Pd catalytic metal. Some hydrogen atoms rapidly diffuse through the metal layer to the metal/semiconductor interface in step 2. Then, these hydrogen atoms adsorbed at the interface are polarized to form a dipole layer by the impact of the electric field across the metal/semiconductor interface

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international journal of hydrogen energy 34 (2009) 5604–5615

on the hydrogen atoms. The dipole layer at the interface leads to an opposite potential drop (DV) in respect to the built-in voltage, and hence reduction of the Schottky barrier height (DfB) in final step. DfB is directly dependent on the DV which is determined by [19]. DV ¼

mNi qi 3s

(3)

where m is the effective dipole moment, Ni is the number of sites per area at the interface, qi is the coverage of hydrogen at the interface. The corresponding band diagram and the charge distribution after forming the dipole layer are also illustrated in Fig. 1(b). Fig. 2(a) and (b) are the band diagrams of a conventional MS diode in N2 (solid line) and in an H2/N2 (dashed line) ambience under forward- and reverse-bias conditions, respectively. According to thermionic emission model [38], the current density of the MS diode biased at Vbias in N2 (JN2 ) is expressed by          Vbias fBn Vbias  1 ¼ A T2 exp exp 1 JN2 ¼ Js exp hVT VT hVT (4) Fig. 1 – Schematic band diagram and charge distribution for a metal–semiconductor hydrogen sensor at equilibrium (a) without hydrogen showing the charge transferred to the metal from the semiconductor exists in a very narrow region at the metal surface; and (b) with hydrogen indicating the sensing mechanism by steps 1–3 and the appearance of the dipole layer which can cause a voltage shift DV.

where Js is the reverse saturation current density, A* is the Richardson’s constant (y24 A cm2 K2 for GaN), T is the absolute temperature, VT is the thermal voltage, fBn is the effective barrier height (EBH) and h is the ideality factor. Thus the sensing current density (JH2 ) of the MS diode in an H2/N2 ambience is appropriately expressed by    DfB JH2 ¼ JN2 exp VT

(5)

Obviously, the JN2 and JH2 of an MS hydrogen sensor in N2 and in an H2/N2 ambience exhibit rectifying behaviors. In particular, the JH2 of the MS diode under a forward-bias mode

Fig. 2 – Schematic band diagrams of an MS hydrogen sensor under (a) forward- and (b) reverse-bias conditions, respectively, in N2 and in a hydrogen-containing ambience.

international journal of hydrogen energy 34 (2009) 5604–5615

in an H2/N2 ambience usually accompanies an additional large JN2 . Such an additional forward current is undesirable when the MS hydrogen sensor is applied to in situ long-term detection with low-power consumption. On the other hand, the increased current due to DfB is taken as the response signal of the sensor to the hydrogen-containing gas. One of the important parameters used to evaluate the performance of a gas sensor is the sensing response (S ), which is commonly defined as: S¼

JH2  JN2 JN2

(6)

According to Eqs. (4)–(6), the forward sensing response (SF) and reverse sensing response (SR) are then re-expressed by   DfBF 1 SF ¼ exp VT

(7)

  DfBR SR ¼ exp 1 VT

(8)

The DfBF and DfBR are the changes in EBH of an MS diode under a forward- and reverse-bias voltage of VF and VR, respectively. The larger the DfBF (DfBR) is the higher the SF (SR) is. The electric field within the depletion region in the semiconductor is reduced when the MS diode is under a forwardbias voltage of VF. This reduction in electric field would be more substantial as the applied VF is larger than 0.8 V. Once the electric field is not large enough, the dipoles formed at the metal/semiconductor interface are reduced. However, the electric field within the depletion region in the semiconductor is increased when the MS diode is under a reverse-bias condition. Therefore the DfBF is usually smaller than DfBR. The smaller DfBF as compared to DfBR explained why the forward-bias MS sensor generally has an SF smaller than SR. In other words, SF shows a narrow spread voltage-operating regime while SR shows a widespread voltage-operating regime. With regard to an MS hydrogen sensor, it is thus concluded that (1) perhaps rectifying characteristics are functional for circuit applications; (2) power consumption should be taken into consideration when it operates in the

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forward-bias mode; and (3) SR is generally higher as compared to SF.

2.2.

Develop sensing mechanism of an MSM diode

Fig. 3 shows a schematic band diagram of an MSM hydrogen sensor at equilibrium (solid line) and biased at a positive voltage of Vbias (dotted lines) in N2 and in an H2/N2 ambience. Basically, two MS diodes (MSF and MSR) coupled back to back are in a single MSM hydrogen sensor. The band diagram is symmetrical and both MSF and MSR diodes have the same EBH of fBn. Note also that MSF and MSR are connected to the Vbias and ground, respectively. In this situation, the MSF diode is under a forward-bias condition while the MSR is under reverse-bias condition. The EBH is considered to be independent of Vbias. As a result, the magnitude of the current density flowing through the MSM diode at Vbias in N2 should satisfy relationship expressed by     fBn VNR 1  exp VT hVT      f V NF Bn exp 1 ¼ A T2 exp VT hVT

JN2 ¼ A T2 exp

(9)

where VNF and VNR are the voltages across the MSF and MSR diodes, respectively. That is, VNF and VNR should satisfy the relationship expressed by VNR þ VNF ¼ Vbias

(10)

Due to the current continuity flowing through the MSF and MSR diodes in series connection, VNF is very small and VNR is close to Vbias. Thus, the magnitude of the current density obtained from the MSM diode in N2 is dominated by the MSR diode and is simplified as JN2 zA T2 exp

  fBn VT

(11)

When the MSM diode is in an H2/N2 ambience, sensing processes including dissociation of hydrogen molecules, diffusion of hydrogen atoms, and formation of dipole layers

Fig. 3 – Schematic band diagram of an MSM hydrogen sensor biased at Vbias in N2 and in a hydrogen-containing ambience.

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international journal of hydrogen energy 34 (2009) 5604–5615

occur at both MSF and MSR diodes. The dipole layer at the MSR interface leads to DfBR which is larger than DfBF induced by the dipole layer at the MSF interface. So the magnitude of the current density of an MSM diode at Vbias in an H2/N2 ambience could be expressed by       fBn VHR DfBR 1  exp exp VT hVT VT        f V Df HF Bn BF exp  1 exp ¼ A T2 exp VT hVT VT

JH2 ¼ A T2 exp

(12)

where VHF and VHR are the voltages across the MSF and MSR diodes, respectively. The current continuity should be valid and the VHR is close to Vbias. Thus, the magnitude of JH2 is still determined by the MSR diode and is finally obtained by JH2 zA T2 exp

    fBn DfBR exp VT VT

(13)

According to the theoretical results analyzed above, the final sensing current density depends only on the change in EBH of the MSR diode under a reverse-bias condition. When the Vbias is applied to MSR, the magnitude of the final sensing current is also determined by the MSF which is now operating in reverse-bias mode. That is, the sensing current density in an MSM hydrogen sensor is independent of the voltage polarity applied between two Schottky electrodes. Since the change in EBH of the reverse-bias MS diode in an H2-containing gas is not affected by an applied voltage, high S[¼exp (DfBR/VT)  1] values with both widespread forward- and reverse-operating-voltage regimes are expected in an MSM hydrogen sensor. Symmetrically bi-directional sensing, instead of rectifying sensing, is also promising for circuit design. In addition, the MSM hydrogen sensor is suitable for in situ long-term detection with low-power consumption because of a small JN2 in standby situation.

3.

Experimental

The GaN-based hetero-bipolar-transistor (HBT) epitaxial structure was employed to fabricate the MSM hydrogen sensor. The epitaxial structure was grown on a c-plane (0001) sapphire substrate by a metal organic chemical vapor deposition (MOCVD) system. The goal of employing HBT structure is to evaluate MSM’s practicability. The original epitaxial layers consisted of a 1.2 mm GaN transient layer, a 1.8 mm nþ-GaN subcollector doped to 5  1018 cm3, a 0.6 mm n-GaN collector layer doped to 3–5  1016 cm3, a 0.18 mm pþ-Al0.2Ga0.8N/GaN superlatticed base layer doped to 2  1018 cm3 (carrier concentration of w2  1017 cm3), a 0.06 mm n-Al0.2Ga0.8N emitter layer and finally a 0.11 mm nþ-GaN cap layer. Fig. 4 shows the cross-section of a Pd–GaN MSM sensor fabricated with two multi-finger Schottky electrodes. The device fabrication started with completely removing the cap, emitter, and base layers by a high-density plasma etching system. Both a GaN transient layer and an nþ-GaN sub-collector are denoted as the buffer. After a device mesa process the native oxide on the exposed n-GaN layer was removed using an HF:H2O ¼ 1:1 etching solution. Two Schottky electrodes with multiple fingers were formed by thermally depositing a 35 nm Pd metal. The Pd thickness was in situ monitored by a quartz crystal

Fig. 4 – Schematic view of a planar-type Pd–GaN MSM hydrogen sensor formed by depositing two multi-fingers Schottky electrodes.

head (MAXTEK TM-350). The effective sensing area of the Schottky electrode was 1.8  103 cm2. Hydrogen detection was carried out at room temperature using a lab-made gas sensing chamber of 235-ml volume. Hydrogen-containing gases with various hydrogen concentrations were employed in this experiment. The use of a wide range of 2.13–10,100 ppm H2 in N2 gas was aimed at calculating the coverage of hydrogen atoms at the interface. Response measurements were carried out by mounting the sensors on a test fixture with bonding wires contacting to both of the Pd Schottky electrodes. Two outputs, sensing current and sensing voltage, were used to evaluate static-state characteristics. The sensing current (sensing voltage) was obtained when testing gases were introduced to the MSM hydrogen sensor biased at a fixed voltage (current). Concerning the dynamic-state characteristics, the sensing current (sensing voltage) was measured as a function of time. Moreover, other dynamic measurements by switching voltage polarity and/or continuously changing hydrogen concentration were also included to assess reproducibility and durability of the MSM hydrogen sensor.

4.

Static-state analysis

Room-temperature current–voltage characteristics for the planar type of MSM hydrogen sensor are shown in Fig. 5. The studied planar-type sensor produced static characteristics at 2.13–10,100 ppm H2 in N2 after a several-hour exposure. Unlike those obtained from commonly used MS sensors, the measured currents are almost invariably symmetrical; indicating bi-directional detection in wide range of hydrogen

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international journal of hydrogen energy 34 (2009) 5604–5615

10 Pd/GaN MSM Sensor @300K

6

2.13

4

10.09 ppm

2

ppm

49.1

ppm

98.9

ppm

region I N2

0 -2

Area = 1.8 x 10-3 cm2

-4

503.1 ppm

-6

1080 ppm 4890 ppm region II

-8 -10 -50

-40

-30

-20

-10

10100 ppm 0

10

20

30

40

50

Voltage (V)

4.1.1.

Fig. 5 – Current–voltage characteristics of a planar-type Pd–GaN MSM hydrogen sensor subjected to various hydrogen-containing gases.

concentration. Two outputs are typically employed to monitor the leak of hydrogen-containing gases. One is the sensing current variation at a fixed bias (region I) and the other is the sensing voltage variation at a fixed current (region II). These static responses will be described as follows.

4.1.

with good detecting capability. A commonly used MS hydrogen sensor produces rectifying behaviors in N2 and in hydrogen-containing ambiences. Besides, MS hydrogen sensors usually fail to detect when it is under a forward bias larger than 1 V (i.e., a narrow spread forward-operatingvoltage regime). However, our planar-type MSM sensor has symmetrically bi-directional behaviors in N2 and in hydrogencontaining ambiences. Sensing current densities measured are 1.14  107 (1.07  107), 7.78  108 (7.65  108), and 3.90  108 (3.86  108) A/cm2 at Vbias ¼ þ1.5 (1.5) V when the MSM sensor is subjected to 10,100, 4890, and 1080 ppm H2/ N2, respectively. Even if the MSM sensor is at 2.13 (10.09) ppm H2/N2, a two (ten) times JN2 is still obtained. Clearly, such a voltage-polarity independent characteristic is beneficial in designing new sensing amplifiers.

Sensing current characteristics

Fig. 6 shows the sensing current densities (JH2 ) as a function of Vbias, which is an enlarged view of region I marked in Fig. 5. The lowest line represents JN2 for the MSM sensor operated in N2. It is found that JN2 is smaller than 5  109 A/cm2 at a Vbias of þ1.5 or 1.5 V. Thus, the MSM sensor’s EBH in N2 is calculated to be at least 0.88 V, indicating that good planar Schottky contacts. Concerning sensing characteristics of the MSM sensor, current densities are substantially enhanced with introduction of hydrogen-containing gases. This means that the planar-type MSM configuration with two multi-figure electrodes is really suitable for applying to hydrogen sensors

140 10

Pd/GaN MSM Sensor @300K

10-6 10-7 10-8 10-9

N2

10-10

98.9 ppm 49.1 ppm 10.09 ppm 2.13 ppm

10-11 10-12

-3

-2

10100 ppm 4890 ppm 1080 ppm 503.1 ppm -1

0

1

5V

0.14

100 80 60

2

3

4

0.12 0.10 0.08 0.06 10100 ppm 503.1 ppm 10.09 ppm

0.04

40

4890 ppm 98.9 ppm 2.13 ppm

1080 ppm 49.1 ppm

0.02

20

-5

-4

-3

-2

-1

1

2

3

4

5

Applied Voltage, Vbias (V)

0 -4

3V

120

EBH Variation (mV)

Sensing Current Density (A/cm2)

Pd/GaN MSM Sensor @300 K Vbias = 1 V

-5

10-13 -5

EBH variation and coverage of hydrogen atoms

In view of Eqs. (11) and (13), the EBH variation (DfBR) of the MSR diode under a reverse-bias condition is directly calculated by DfBR ¼ VT $lnðJH2 =JN2 Þ. Fig. 7 shows DfBR as a function of hydrogen concentration ([H2]). The used parameter is Vbias. At a fixed Vbias, the EBH variation is increased with increasing the [H2]. It is because more hydrogen molecules coming to front Pd surface can be dissociated into hydrogen atoms and then more hydrogen atoms can diffuse and arrive at the Pd–GaN interface where an enhanced dipole layer is formed. This enhanced dipole layer causes a more significant barrierlowering effect on the MSR diode. The calculated EBH variations (DfBR) are 134, 122, and 104 mV for the planar-type sensor at 10,100, 4890, and 1080 ppm H2/N2, respectively. When the [H2] is reduced to 2.13 (10.09) ppm, DfBR is 20 (65) mV. The calculated results also reveal that the Vbias does not influence DfBR when the [H2] is fixed. Detailed data about DfBR as a function of Vbias are shown as an inset in Fig. 7 with symmetrical behaviors found. The symmetrical DfBR versus Vbias characteristics explain that current densities flowing through the MSM hydrogen sensor are dominated by the reverse-bias MSR diode. Note also that the built-in electric field

EBH Variation (V)

Current (µA)

8

5

Applied Voltage, Vbias (V) Fig. 6 – Sensing current density as a function of applied voltage which is an enlarged view of region I indicated in Fig. 5.

0

2000

4000

6000

8000

10000

Hydrogen Concentration, [H2] (ppm) Fig. 7 – Effective barrier height (EBH) variations as a function of hydrogen concentration obtained from Fig. 6. The used parameter is the applied voltage and EBH variation versus applied voltage is also inset.

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international journal of hydrogen energy 34 (2009) 5604–5615

of the reverse-bias MSR diode is enhanced by increasing Vbias. However, the EBH variation is not improved. Thus, we concluded that the dipoles formed at the Pd–GaN interface are retained at some Vbias and the built-in electric field within the MS depletion region at equilibrium is high enough to saturate the dipole formation. The thermodynamics of hydrogen adsorption was also investigated using static sensing current characteristics. The Langmuir isotherm describing the dependence of the adsorbed gas coverage at solid surface on the gas partial pressure is employed. The coverage (qi) of hydrogen at the Pd–GaN interface is expressed as [39] pffiffiffiffiffiffiffiffi PH2 ffi pffiffiffiffiffiffiffi qi ¼ 1 þ K  PH2 K

(14)

where K is the effective equilibrium rate constant and PH2 is the partial pressure of H2. Accordingly, DfBR being proportional to qi is expressed as (15)

DfBR ¼ DfBR;max  qi

where DfBR,max is the maximum EBH variation by totally occupying the coverage sites. Combining with current derived from thermionic emission model, Eq. (14) can be rewritten as 1 1 1 1  pffiffiffiffiffiffiffiffi þ J  J  ¼ J H2 H2 ;max H2 ;max P H2 ln JN K  JN ln JN 2

2

(16)

2

where JH2 ;max is the maximum current density of the diode at DfBR ¼ DfBR,max. Then important parameters are extracted by typical plot of ½lnðJH2 =JN2 Þ1  as a function of ðPH2 Þ1=2 at a bias of 3 V. The K value is 4.61 Torr1/2 with DfBR,max ¼ 139.4 mV. The magnitude of qi is enhanced with increasing the [H2]. Roomtemperature qi values at 2.13, 49.1, 503.1, and 4890 ppm H2/N2 are 15.6, 47.1, 74.0, and 89.8%, respectively. Larger qi values responding to higher [H2] explain larger EBH variations.

Vbias-independent sensing response (S ), the responsive trend versus [H2] is the same as that in Fig. 7. It is reasonable since S is calculated by [S ¼ exp(DfBR/VT)  1] and now is shown in logarithmic scale. So, we find that the S is increased with increasing the [H2]. The calculated S is increased from 1 to 173 when the [H2] introduced to the MSM sensor is increased from 2.13 to 10,100 ppm H2/N2. Furthermore, the proposed MSM sensor subjected to various hydrogen concentrations has quite stable and flat curves according to the plot of sensing response as a function of applied bipolarity biases. These results together with large breakdown voltages being available in GaN material reveal that the planar-type Pd–GaN sensor produces hydrogen-sensing properties with wide positive- and negative-bias regimes. Current sensitivity (A/ ppm H2) defined as the ratio of current difference ðJH2  JN2 Þ to [H2] is also included as an inset [39,40]. Unlike the relationship between the S and [H2], the current sensitivity is decreased with increasing the [H2]. The number of hydrogen atoms dissociated in a low [H2] ambience is smaller than that did in a high [H2] ambience. However, hydrogen atoms dissociated in a low [H2] ambience can find more vacant sites at the Pd–GaN interface to efficiently form the dipoles. Experimental results about the current sensitivity are in agreement with those reported previously [39]. Conclusions based on sensing current characteristics experimentally finished are as follows: (1) symmetrical and bi-directional sensing current characteristics are really obtained in the planar-type sensor configuration, (2) voltage-independent EBH variations and hence sensing responses with widespread voltage-operating regimes are expected, and (3) the possible origin of failing to detect in a forward-bias MS sensor is due to the substantially reduced electric field and the sensing mechanism developed in the former section well describes those important results about both MS and MSM sensors.

4.2. 4.1.2.

Fig. 8 shows the logarithmic sensing response as a function of [H2] for the MSM sensor at room temperature. In addition to Pd/GaN MSM Sensor @300 K Vbias = 1 V

5V

3V

100 -7

10

Current Sensitivity (A/ppm)

Sensing Response, S (no unit)

Sensing voltage characteristics

Sensing response and current sensitivity

10

-8

Vbias = 1 V

10

3V

5V

-9

10

-10

10

-11

10

-12

10

1

0

2000

4000

6000

8000

10000

[H2] (ppm)

0

2000

4000

6000

8000

10000

Hydrogen Concentration, [H2] (ppm) Fig. 8 – Sensing response as a function of hydrogen concentration obtained from Fig. 6. The used parameter is the applied voltage and current sensitivity versus hydrogen concentration is also inset.

Fig. 9(a) illustrates the sensing voltage characteristics which is an enlarged view of region II marked in Fig. 5. The most left curve represents current-dependent sensing voltages of the MSM sensor in N2 and others at various [H2]. With regard to present measurements, the MSM hydrogen sensor is biased by a constant current (Ibias) and sensing voltages responding to hydrogen-containing gases are output as signals. As shown in Fig. 9(a) where the Ibias is 5 mA, the MSM sensor in N2 produces a sensing voltage ðVN2 Þ as high as 42.3 V. The magnitudes of sensing voltages reflecting hydrogen-containing gases ðjVH2 jÞ are reduced with increasing the [H2]. jVH2 j changes from 41.76 V at 2.13 ppm to 35.5 V at 98.9 ppm, to 28.1 V at 1080 ppm, and to 23.8 V at 10,100 ppm H2/N2. The reverse-bias MSR diode in the MSM sensor obviously dominates the breakdown-like behaviors. The avalanche multiplication process is employed here due to a lightly doped GaN layer. To derive the breakdown-like current, we assume that a current In0 is incident at the MSR interface. If the electric field in the GaN depletion region is high enough to possibly initiate the avalanche multiplication process, the electron current will increase with distance through the depletion region to reach a value of Mn  In0 at W, where Mn is the multiplication factor defined as

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international journal of hydrogen energy 34 (2009) 5604–5615

a Applied Current, Ibias (µA)

-1 -2

Pd/GaN MSM Sensor @300 K

-3 -4 Ibias= -5 µA

-5 -6

respectively, in 4890 and 10,100 ppm ambiences. The table inset lists the sensing voltage variations as a function of [H2] at a constant current of 1, 4, and 8 mA. At the same [H2], the sensing voltage variation is nearly retained as jIbiasj is larger than 4 mA. This is due to the breakdown-like behaviors dominated only by avalanche multiplication. If an acceptable value of the sensing voltage variation is 3 (5) V, the detection limit is 49.1 (98.9) ppm H2/N2.

-7 10100 ppm -8

N2

-9 -10 -50

-45

-40

-35

-30

-25

-20

-15

5.

Dynamic-state analysis

5.1.

Current transient response to gas

5.1.1.

Response between air and hydrogen-containing gas

Sensing Voltage (V)

b Sensing Voltage Variation (V)

20

Pd/GaN MSM Sensor @300 K

15 Ibias = 1 µA

10

4µA

Ibias (µA)

[H2] (ppm)

5

2.13 10.09 0.18 1.1 0.54 1.5 0.54 1.6

1 4 8

0 0

8µA

2000

4000

49.1 3.6 4 4.1

98.9 503.1 1080 4890 10100 7.2 13.3 15.1 17.5 19 6.7 12.2 14.2 16.6 18.4 7 12.4 14.2 16.7 18.5

6000

8000

10000

Hydrogen Concentration, [H2] (ppm) Fig. 9 – (a) The dependence of sensing voltages on applied current for a planar-type Pd–GaN MSM hydrogen sensor subjected to various hydrogen-containing gases. (b) Sensing voltage variations as a function of hydrogen concentration. The used parameter is the applied current.

Mn ¼

In In0

(17)

Fig. 10 shows the current transient response of the MSM sensor to the introduction and removal of hydrogencontaining gases at room temperature. The [H2] of hydrogencontaining gases introduced to the sensor are in wide range of 10.09–10,100 ppm H2/N2 at the rate of 500 sccm. After saturation in the sensing current density, air gas is used to completely remove the 10,100 ppm H2/N2 and the sensing current density returns to its baseline. Measurements have been repeatedly performed in ambiences cycled from air/N2 to other hydrogen-containing gases and then back to air/N2. On the other hand, such a planar-type MSM sensor has already addressed to provide both wide positive- and negativevoltage-operating regimes. So, our experiments in current transient responses were not focused on the effects of increasing temperature but on the effects of the Vbias applied between two Schottky electrodes on response time (ta). The ta is defined as the time that the current density reaches from baseline value ðJN2 Þ to Ja ¼ ð1  e1 Þ  ðJH2 ;sat  JN2 Þ þ JN2 . JH2 ;sat is the saturated current density in a hydrogen-containing ambience. At a constant Vbias, our MSM sensor responds well to various hydrogen-containing gases and produces transient currents with repeatable curves. The saturated current densities are reduced by introducing low [H2] gases. At the same [H2], Vbias plays few effects on the response time. The transient response curves are almost similar to each other,

The breakdown-like current, In, should satisfy Z

10-4

W

adx

(18)

0

where a is the ionization rate for both electron and hole, which is a function of electric field and hence voltage. Since the initial In0 of the MSM sensor in a higher [H2] ambience is larger, a smaller Mn is required for such an MSM sensor to reach a constant current of Ibias. So, a large voltage shift can be obtained under a constant current of Ibias. In practical circuit applications, sensing voltages output as signals are more acceptable in comparison to sensing currents. This is referred to Fig. 9(b) showing sensing voltage variations ðVH2  VN2 Þ as a function of [H2] for the proposed MSM sensor biased at various currents. Note that the trend of sensing voltage variation versus [H2] is similar to that in Fig. 7. The sensing voltage variation at a constant current of 4 mA was enhanced from 1.1 V in a 10.09 ppm ambience to 3.6 V in a 49.1 ppm ambience and to 7.2 V in a 98.9 ppm ambience, and then to 17.5 and 19 V,

Sensing Current Density (A/cm2)

In  In0 1 ¼1 ¼ In Mn

Vbias = 5 V 1V

Pd/GaN MSM Sensor @300 K

10-6 10-8

1080

4890

10100

1080

3V

503.1

503.1

-10

10

10-6

0

20

40

60

80

100

10-8 98.9 98.9 10-10

0

49.1 100

49.1 200

10.09

10.09 300

400

Time (min) Fig. 10 – Transient current response of the proposed hydrogen sensor to the introduction and removal of various hydrogen-containing gases.

500

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international journal of hydrogen energy 34 (2009) 5604–5615

but locate at different current levels. The saturated current density is improved from 8.07  108 A/cm2 at Vbias ¼ 1 V (dotted line) to 5.42  107 A/cm2 at Vbias ¼ 3 V (dashed line), and to 3.5  106 A/cm2 at Vbias ¼ 5 V (solid line) as the MSM sensor is subjected to 10,100 ppm H2/N2. In other words, the higher the Vbias is the larger the current difference is. Although a large current difference between in air/N2 and in H2/N2 ambiences is beneficial at designing sensing amplifiers, a sensor with a high Vbias generally suffers from large power consumption in standby mode. At a fixed hydrogen concentration of 1080 ppm H2/N2, the Vbias-independent response time is 150 s. When the hydrogen concentration in N2 is increased to 10,100 ppm, 2 min reduction in response time (30 s) is obtained. Response times as a function of hydrogen concentration (dashed line) at Vbias ¼ 3 V are shown in Fig. 11 and measured values are listed in table inset. Experimental results reveal that the response time strongly depends on the hydrogen concentration. The response time in a 2.13 (10.09) ppm ambience is even as long as w3 h (w1 h). Previous reports indicated similar results to our work and inferred that a shorter response time in a higher [H2] ambience is due to more hydrogen molecules being dissociated into hydrogen atoms. Calculated values of qi/[H2] defined as coverage sits per ppm also explain the inference. The value of qi/[H2] is reduced from 7.32%/ppm in a 2.13 ppm ambience to 0.96%/ppm in a 49.1 ppm ambience and to 0.018%/ppm in a 4890 ppm ambience. The smaller qi/[H2] means that a shorter time is required for hydrogen atoms’ occupation of coverage sites.

introducing the final hydrogen-containing gas. The initial [H2] is varied from 2.13 to 4890 ppm H2/N2 and the final [H2] is fixed at 10,100 ppm H2/N2 for Fig. 12(a). It is found that the response time is almost independent of the initial [H2] as the final [H2] is fixed at 10,100 ppm H2/N2. Note that the response time in this evaluation is defined the time that the current density reaches from JH2 ;1 to ð1  e1 Þ  ðJH2 ;2  JH2 ;1 Þ þ JH2 ;1 . JH2 ;1 and JH2 ;2 are the saturated current densities in the initial and final hydrogen ambiences, respectively. Obviously, the response time of 30 is only determined by the 10,100 ppm H2/N2, no matter what initial hydrogen-containing gases are. As shown in Fig. 12(b) and (c), the final [H2] is set to be 1080, 98.9 ppm H2/N2. Unlike the high final [H2] of 10,100 ppm, the response time obtained from Fig. 12(b) depends on the initial hydrogen gas. When the initial is close to the final [H2], the response time is reduced. This reduction in response time is more substantial as the final [H2] is decreased to 98.9 ppm. New findings from response between two hydrogen-containing gases are: (1) coverage sites occupied by hydrogen atoms in an initial hydrogen-containing ambience are useful to reduce the response time as the final [H2] is low and (2) more timedependent leak of a hydrogen-containing gas can be continuously detected with a shorter time. Furthermore, excellent responses to hydrogen-containing gases are confirmed through continually and monotonically increasing [H2] from 2.13 to 10,100 ppm H2/N2 and then continually and monotonically decreasing to 2.13 ppm H2/N2, which gives rise to the ‘‘ladder’’-like response shown in Fig. 13.

5.1.2.

5.1.3.

Response between two hydrogen-containing gases

Transient response between two hydrogen-containing gases is also shown to evaluate our MSM hydrogen sensor being used in a continuous pre-alarm system. The measurements started with introduction of 10,100 ppm H2/N2 to the sensor after 10 min exposure in N2. When the sensing current density is saturated, the initial hydrogen ambience is obtained by introducing 2.13 ppm H2/N2 to the sensor. Then transient response between two hydrogen ambiences is measured by

14

Bi-directional sensing

Another key feature of our MSM sensor is its bi-directional and symmetrical sensing characteristic. Fig. 14 shows transient responses of the MSM sensor measured by switching Vbias from a positive voltage of þ3.0 V to a negative voltage of 3.0 V, and then to a positive voltage of þ5 V. When Vbias ¼ þ3.0 V is applied to the sensor subjected to ambiences cycled from N2 to 10,100 (or 4890) ppm H2/N2 and then back to air/N2, clear sensing characteristics were observed at the time indicated by the H2-on marker. The hydrogen-containing gas was allowed to flow through the sensor. After saturation at the

Pd/GaN MSM Sensor @300K [H2] (ppm)

10 Vbias = 3 V 8 6 4 2

2.13 10.09 49.1 98.9 503.1 1080 4890 10100

Vbias

Ibias

ta (s)

tb (s)

10920 3340 1280 650 210 150 80 30

-2820 1200 580 200 120 50 30

Sensing Current Density (A/cm2)

Response Time (× ×103 s)

12

Ibias = 1 µA

0 1

10

100

1000

10000

Hydrogen Concentration, [H2] (ppm) Fig. 11 – Response times as a function of hydrogen concentration for the proposed hydrogen sensor biased at a constant voltage (dashed line) and a constant current (solid line).

10-4 10-5 10-6 10-7 10-8

(a)

(b)

10-7

110s

100s 110s110s

Vbias= 5 V

140s

150s

(c)

: Air 49.1

10.09

2.13

10-7

30s30s

49.1 98.9 503.1 10.09

2.13

10-8

98.9 503.1 1080 4890 30s 30s 30s

30s

30s

30s

10-6

10-6

49.1

10.09

2.13

330s

350s

650s

: N2

300s

10-8 0

50

200

250

300

350

400

450

Time (min) Fig. 12 – Transient current response between two hydrogen-containing gases with final hydrogen concentrations of (a) 10,100, (b) 1080, and (c) 98.9 ppm, respectively.

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international journal of hydrogen energy 34 (2009) 5604–5615

g

Pd/GaN MSM diode @300K 10-6

f

f

e

5.2.

h g e d

d c

c b

b 10-7

a 10-8

a

e. 503.1 ppm f. 1080 ppm g. 4890 ppm h. 10100 ppm

a. 2.13 ppm b.10.09 ppm c. 49.1 ppm d. 98.9 ppm

AIR

N2 0

350

400

450

500

550

750

Time (min) Fig. 13 – Current responses to hydrogen-containing gases by continually and monotonically increasing [H2] from 2.13 to 10,100 ppm H2/N2 and then continually and then decreasing to 2.13 ppm H2/N2, which gives rise to the ‘‘ladder’’-like performance.

time indicated by H2-off marker, air was flowed continuously through the sensor to remove the absorbed hydrogen. As a result, recovering characteristics were obtained and the sensing current is decreased to the baseline existing prior to the introduction of a hydrogen-containing gas. By switching the voltage polarity applied between two Schottky electrodes (Vbias ¼ þ3.0 to 3.0 V and 3 to þ5 V), clear sensing and recovering characteristics were also observed at the times indicated by the H2-on and H2-off markers, respectively. Measurements on different hydrogen sensors in the same chip indicate nearly the same performance. Repeated Measurements on the same devices were also performed after several days. No clear degradation in dynamic responses was found. The experimental results show that our MSM sensor is completely reversible and durable. Besides, the MSM sensors are more suitable for integrating with other electrical/ optical devices due to voltage-polarity independent sensing performance.

Voltage transient response to gas

Fig. 15 shows the voltage transient response of the MSM sensor to the introduction and removal of hydrogen-containing gases at room temperature. The [H2] of hydrogencontaining gases introduced to the sensor are in wide range of 10.09–10,100 ppm H2/N2 at the rate of 500 sccm. After saturation in the sensing voltage, air/N2 gas is used to completely remove the 10,100 ppm H2/N2 and the sensing voltage returns to its baseline. Measurements have been repeatedly performed in ambiences cycled from air/N2 to other hydrogencontaining gases and then back to air/N2. Similar to ta defined in current transient response, the response time (tb) in voltage transient response is defined as the time that the voltage reaches from VN2 to Vb ¼ ð1  e1 Þ  ðVH2 ;sat  VN2 Þ þ VN2 . VH2 ;sat is the saturated voltage in a hydrogen-containing ambience. At a constant Ibias ¼ 1 mA, the sensor in N2 has the maximum magnitude of output voltage of VN2 z37 V. We infer a conclusion that the current of the MSM sensor biased at a high voltage is totally dominated by avalanche multiplication. Besides, our MSM sensor responds well to various hydrogen-containing gases and produces transient voltages with repeatable curves. The magnitudes of saturated voltages are decreased by introducing higher [H2] gases, resulting in larger voltage variations. For example, the magnitude of the saturated voltage is reduced from 29.8 V at 98.9 ppm to 23.7 V at 503.1 ppm, to 21.7 V at 1080 ppm, and to 18 V at 10,100 ppm H2/N2. Therefore, the voltage variation is improved from 7.2 V at 98.9 ppm to 13.3 V at 503.1 ppm, to 15.1 V at 1080 ppm, and to 19 V at 10,100 ppm H2/N2. To our best knowledge, these voltage variations are the highest values ever reported. On the other hand, the tb in voltage transient curves also depends on the hydrogen concentration. At 1080 ppm H2/N2, the tb is 120 s. When the hydrogen concentration is increased to 10,100 ppm, 1.5 min reduction in tb is obtained. The tb as a function of hydrogen concentration in voltage transient curves at Ibias ¼ 1 mA is also included in Fig. 11. Compare tb to ta listed in table, the new finding is that the response times in transient voltage curves is generally shorter than those in transient

-15

: H2 ON 0.4

2

: H2 OFF

0.2 +3 V

0.0

10100 ppm 4890 ppm

+5 V

-0.2

0 -5V

-2

-0.4 -0.6 -0.8

-3 V 0

6

12 18 24 30 36

54 60 66 72 78 84

Time (min) Fig. 14 – Switching behaviors of the proposed MSM hydrogen sensor measured by changing the voltage polarity.

-4

-25

Sensing Voltage (V)

Sensing Current Density (uA/cm2)

Pd/GaN MSM Sensor @300 K

Sensing Current Density (uA/cm2)

0.6

Pd/GaN MSM Sensor @300 K

-20

4

0.8

-30 10100

-35 -40

0

4890

20

40

1080

60

80

1080 100

503.1 120

503.1 140

160

-30 Ibias = -1µA -35 98.9 98.9 -40

0

49.1 100

49.1 200

10.09 300

10.09 400

Time (min) Fig. 15 – Transient voltage response of the proposed hydrogen sensor to the introduction and removal of various hydrogen-containing gases.

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international journal of hydrogen energy 34 (2009) 5604–5615

-15

g f

-20

f d

d

Sensing Voltage (V)

Pd/GaN MSM diode @300 K

e

e

10,100 ppm H2/N2, which indicates high potential applications. If an accepted sensing voltage variation is larger than 3 (5) V, the detecting limit is 49.1 (98.9) ppm. Moreover, current transient response and voltage transient response to various hydrogen-containing gases were experimentally studied. The new finding is that the response time in voltage transient curves is shorter than that in current transient ones. Other dynamic measurements by switching voltage polarity and/or continuously changing hydrogen concentration were also included to assess reproducibility and durability of the MSM hydrogen sensor that is a good candidate for commonly used MS sensors.

g f d

e

-25 c

c

-30

c b

b

b

a

a

-35 d. 503.1 ppm

-40

a. 10.9 ppm

-45 -50

AIR

0

e. 1080 ppm

b. 49.1 ppm

f. 4890 ppm

c. 98.9 ppm

g. 10100 ppm

50

100

150

200

300

Ibias = -1µA 350

400

450

Time (min) Fig. 16 – Voltage responses to hydrogen-containing gases by continually and monotonically increasing and then decreasing [H2], which gives rise to the ‘‘ladder’’-like performance.

Acknowledgment This work was in part supported by the National Science Council of the Republic of China under contract no. NSC 952221-E-019-059-MY3.

references current curves. Our explanation about the shorter response time is based on the definitions in tb to ta. When the sensor is subjected to an [H2] ambience, the ta is required for the sensor’s current density to increase to Ja from JN2 . At the same time, the Ja will produce a breakdown-like current of Ibias ¼ 1 mA at some Vbias whose magnitude is smaller than that of Vb. In other words, the tb required for the sensor’s voltage to change from JN2 to Vb is shorter than ta. Besides, the reduction in response time is more substantial with a lower hydrogen concentration. This implies that the MSM sensor biased by a constant current is suitable for sensing a low hydrogen concentration when considering the demand of a short response time. Furthermore, good response to hydrogencontaining gases is confirmed through continually and cyclically changing [H2] from 10.9 to 10,100 ppm H2/N2, which gives rise to the ‘‘ladder’’-like response shown in Fig. 16.

6.

Conclusions

A planar type of Pd–GaN metal–semiconductor–semiconductor (MSM) hydrogen sensor has been comprehensively investigated to review its all possible applications. Symmetrically bi-directional current–voltage characteristics were indicative of easily integrating with other electrical/optical devices. Two outputs, i.e. sensing current and sensing voltage, were employed as detecting signals for our planar-type hydrogen sensor. Concerning sensing currents, the proposed sensor was biased at a constant voltage. Effective barrier height variations (EBH) and sensing responses (S ) were obtained for the proposed sensor in static state. Experimental results reveal that the MSM hydrogen sensor exhibits EBH (S ) of 134 (173), 122 (121), and 104 mV (54) at 10,100, 4890, and 1080 ppm H2/N2, respectively. When the [H2] is reduced to 2.13 (10.09) ppm, EBH is 20 (65) mV. In case of sensing voltages, the proposed hydrogen sensor was biased at a constant current. A sensing voltage variation as large as 18 V was obtained at

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