GaN metal-oxide-semiconductor (MOS) Schottky diode

Accepted Manuscript Title: Hydrogen sensing performance of a Pd/HfO2 /GaN metal-oxide-semiconductor (MOS) Schottky diode Authors: Huey-Ing Chen, Ching-Hong Chang, Hsin-Hau Lu, I-Ping Liu, Wei-Cheng Chen, Bu-Yuan Ke, Wen-Chau Liu PII: DOI: Reference:

S0925-4005(18)30355-1 https://doi.org/10.1016/j.snb.2018.02.077 SNB 24172

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

4-10-2017 4-1-2018 9-2-2018

Please cite this article as: Huey-Ing Chen, Ching-Hong Chang, Hsin-Hau Lu, I-Ping Liu, Wei-Cheng Chen, Bu-Yuan Ke, Wen-Chau Liu, Hydrogen sensing performance of a Pd/HfO2/GaN metal-oxide-semiconductor (MOS) Schottky diode, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2018.02.077 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Hydrogen sensing performance of a Pd/HfO2/GaN metal-oxidesemiconductor (MOS) Schottky diode

Chen1, Bu-Yuan Ke1, and Wen-Chau Liu1, *

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Huey-Ing Chen, Ching-Hong Chang1, Hsin-Hau Lu1, I-Ping Liu, Wei-Cheng

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Department of Chemical Engineering, Nation al Cheng-Kung University, Tainan 70101, TAIWAN 70101, Republic of China

Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, 1 University Road, Tainan, TAIWAN 70101, Republic of China.

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*Corresponding author. E-mail: [email protected]

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Graphical abstract

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Metal-Oxide-Semiconductor-Type Schottky Diode

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Pd HfO2 GaN

160 Forward Voltage = 0.5 V Current (A)

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H2

Temp. = 300 K

120

1% H2/air

80 40

1000 ppm 100 ppm

0

Si

Time

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A high performance hydrogen gas sensor has been prepared based on a Pd/HfO2/GaN metal-oxide-semiconductor (MOS)-type Schottky diode.

Research Highlights of “Hydrogen sensing performance.0 of a Pd/HfO2/GaN metaloxide-semiconductor (MOS) Schottky diode” A Pd/HfO2/GaN-based hydrogen sensor with a catalyst Pd metal layer and a

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

sputtered HfO2 layer is successfully fabricated.

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Hydrogen sensing characteristics of this Pd/HfO2/GaN device are comprehensively

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studied.

Excellent hydrogen sensing response of 4.9× 105 is obtained under 1% H2/air gas at 300K. A lower detection limit of 5 ppm H2/air is obtained.

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Fast response(recovery) time of 5.3 s (2.5 s) is found under 1% H2/air gas at 300K.

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

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Abstract

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A hafnium oxide (HfO2) layer, prepared using a sputtering approach, is employed to produce a Pd/HfO2/GaN-based metal-oxide-semiconductor (MOS)-type Schottky

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diode. The hydrogen sensing characteristics of this MOS diode are comprehensively studied. Experimentally, upon exposure to 1% H2/air gas at 300 K, the studied device

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shows a higher sensing response of 4.9×105 (139) under an applied forward- (reverse-) voltage of 0.5 V(-2 V). A lower detection limit of 5 ppm H2/air is obtained. Reversible, high-speed sensing properties are found at higher operating temperatures. The response

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(recovery) time constant is decreased from 39 s (42 s) to 5.3 s (2.5 s) when the temperature is increased from 300 to 383 K. The humidity effect and hydrogen adsorption mechanism at the Pd/HfO2 interface are also studied in this work. The exothermic action of the hydrogen adsorption process leads to a decreased hydrogen sensing response at higher temperatures. Consequently, the studied Pd/HfO2/GaN MOS 2

diode is promising for high-performance hydrogen sensing applications and integration with other GaN-based high-speed devices on a chip. Keywords: HfO2, MOS, Schottky diode, GaN, hydrogen sensing

Introduction

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I.

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Based on continuous and enormous usage of fossil fuels, a serious increase in

greenhouse gases in the atmosphere and the related global climate change have become crucial issues over recent decades. Due to the fact that it is sustainable, renewable, and

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clean, hydrogen is a good candidate to replace petroleum fossils in order to develop

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eco-friendly environments [1-4]. Hydrogen has been widely used in chemical and

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petroleum industries, medical installations, laboratories, and hydrogen-fueled motor

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vehicles [5, 6]. Yet, due to its inherent combustibility (explosive nature) at concentrations over 4.65(18) vol%, hydrogen is a dangerous gas [7, 8]. Furthermore,

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based on the odorless and colorless properties of hydrogen, the fabrication of highperformance hydrogen sensors to accurately detect hydrogen concentrations and

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continuously monitor hydrogen leakages are indispensable for safety considerations [8].

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Over recent years, numerous metal-semiconductor (MS) Schottky diode type hydrogen gas sensors, based on different material systems such as GaAs [9, 10], InP [11, 12], AlGaAs [13, 14], InGaP [15, 16], InAlAs [17, 18], GaN [19-21], and AlGaN

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[22-24], accompanied with catalytic metals (Pd, Pt), have been reported to have advantage including fast response, wide operating temperature ranges, high sensing response, and small dimensions. Among these devices, the GaN(AlGaN)-based material system is suitable for use to fabricate stable, high-performance hydrogen sensors due to its inherent wide band gap and high exciton binding energy properties 3

[20-23]. On the other hand, the insertion of a thin insulating oxide layer between the catalytic metal and semiconductor, i.e., metal-oxide-semiconductor (MOS) structure, is also a good approach to produce high-performance hydrogen sensors [25, 26]. Pt/SiO2/GaN and Pd/SiO2/AlGaN MOS Schottky diode type hydrogen sensors have been reported to demonstrate good hydrogen responses of 4.5×104 and 3.3×105 under

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introduced ~1% H2/air gas at 300 K and 343 K, respectively [25, 26]. Recently, another dielectric material, i.e., hafnium oxide (HfO2), has been widely utilized to fabricate

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normally-off or enhancement-mode (E-mode) MOS-type AlGaN/GaN high electron mobility transistors (HEMTs) due to its inherent larger dielectric constant and wider

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band gap characteristics [27-31]. In this work, a new Pd/HfO2/GaN MOS-type

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hydrogen sensor is fabricated and reported. Good hydrogen sensing performance and

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lower detection limit are found. Studies on the humidity effect and hydrogen adsorption

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mechanism of the Pd/HfO2 interface are also included in this work.

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II. Experimental Procedure

The epitaxial structure was grown on a silicon substrate using a metal organic

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chemical vapor deposition (MOCVD) system. This structure consisted of a 1.5 μmthick buffer layer, a 1 μm-thick GaN layer, a 20 nm-thick Al0.2Ga0.8N layer, and a 20

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nm-thick GaN cap layer. After epitaxial growth, mesa isolation was achieved using an inductively-coupled-plasma reactive ion etching (ICP-RIE) system. The samples were then cleaned with acetone, HCl, and deionized water to remove fall-on particles, native

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oxide, and organic contaminants. Ohmic contacts were formed by depositing 10 nm/100 nm /10 nm/100 nm-thick Ti/Al/Ti/Au metals using thermal evaporation (TE). Thereafter, the samples were annealed with a rapid thermal annealing (RTA) system at 900oC in N2 ambience for 90 s. Subsequently, a 15 nm-thick HfO2 dielectric layer was deposited using an RF sputtering system with an RF power of 50 W and an Ar flow rate 4

of 15 sccm. A post annealing treatment was performed at 400oC for 15 min. Finally, Schottky contacts were conducted by evaporating a 40 nm-thick catalytic Pd metal with an effective area of 2.05×10-3 cm2. A schematic cross section diagram of the studied device is depicted in Fig. 1.

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The hydrogen gas sensing system and related measurement process used in this work were reported in detail elsewhere [32]. The experimental current-voltage (I-V)

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characteristics (hydrogen sensing behaviors) were measured with a Keithley 4200 semiconductor parameter analyzer.

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III. Results and Discussion

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The X-ray diffraction (XRD) pattern of the HfO2 thin film used in this work is

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shown in Fig. 2. All the diffraction peaks could be indexed as consistent with the HfO2 from JCPDS card no 43-1017. The relatively sharp diffraction peaks also indicate the

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high crystalline quality of the employed HfO2 thin film. The energy dispersive X-ray

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spectroscopy (EDS) analysis on the Schottky contact region of the studied device is shown in Fig. 3, where Ga, N, Pd, O, Hf, and Al peaks can be clearly observed. The Al

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peak is believed to be mainly from the Al0.2Ga0.8N layer underneath the GaN cap layer. This spectrum also indicates a clean, pure device structure based on the absence of other

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impurities. The experimental I-V characteristics of the studied Pd/HfO2/GaN MOStype sensor at 300 K are shown in Fig. 4. The related I-V characteristics of a compared

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Pd/GaN MS-type device, i.e., without the use of HfO2 thin film, are also included in Fig. 4. Clearly, similar I-V characteristics under applied forward voltage in the case of both devices are found. However, under applied reverse voltages, the studied MOS Schottky diode exhibits significantly lower leakage currents than the compared MS diode. For instance, under the applied reverse voltage of VR = -2 V, the leakage current 5

of the studied MOS device is 4.55×10-10 Å, which is about 40-fold lower than that of the compared MS device (1.81×10-8 A). This improved performance is certainly caused by the enhanced Schottky barrier height based on the insertion of the HfO2 dielectric layer between the Pd and GaN layer of the studied device [26].

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Figures 5(a) and 5(b) illustrate the measured I-V characteristics of the studied Pd/HfO2/GaN MOS diode upon exposure to various concentrations of hydrogen gases

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at 300 and 383 K, respectively. The corresponding energy band diagrams in air

ambience and under the introduced hydrogen gas are depicted in Fig. 5(c). Based on the hydrogen sensing mechanism [26, 33]. The conducting current increases with

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increases in the hydrogen concentration under a given voltage bias, as shown in Fig.

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5(a) and Fig. 5(b). Hydrogen molecules can be dissociated into hydrogen atoms by the

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Pd catalytic metal. Hydrogen atoms diffuse through Pd bulk to the Pd/HfO2 interface

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[26, 33]. In the current study, the adsorbed hydrogen atoms at the Pd/HfO2 interface are polarized to form hydrogen dipoles by the GaN internal electric field, as revealed in

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Fig. 5(c). Based on the opposed electric polarity between the hydrogen dipoles and GaN internal electric field, the Schottky barrier height in air (q𝜙𝑏,𝑎𝑖𝑟 ) is lowered to q𝜙𝑏,𝐻2

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under exposure to hydrogen gas. The lowered Schottky barrier height ( ∆q𝜙𝑏 =

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q𝜙𝑏,𝑎𝑖𝑟 − q𝜙𝑏,𝐻2 ) substantially increases the conducting current, as shown in Fig. 5(c) [26, 33]. Figures 6(a) and 6(b) illustrate the hydrogen sensing responses, SF and SR, versus the temperature of the studied Pd/HfO2/GaN MOS diode under the introduction

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of 5, 100, 1000 ppm H2/air and 1% H2/air gases at the applied forward voltage of VF = 0.5 V and reverse voltages of VR = -2 V, respectively. The sensing response SF (SR) is defined here as [26, 33]: 𝐼𝐻2

𝑆𝐹 (𝑆𝑅 ) = 𝐼 6

𝑎𝑖𝑟

− 1,

(1)

where 𝐼𝐻2 and Iair represent the currents in hydrogen-containing ambience and air, respectively. A maximum SF (SR) of 4.9×105 (139) is observed under the introduced 1% H2/air gas at 300 K. Generally, the sensing response increases with increases (decreases) in hydrogen concentration (temperature). For instance, at 300 K, the SF is increased from 1.6×102 to 4.9×105 when the hydrogen concentration is increased from

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5 ppm H2/air to 1% H2/air. Also, under the introduced 1% H2/air gas, the SF decreases

from 4.9×105 to 1.8×104 when the temperature is increased from 300 K to 383 K.

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Clearly, the studied device shows good hydrogen sensing ability over widespread

hydrogen concentration ranges at different temperatures. In addition, the studied MOS-

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type hydrogen sensor can be operated under applied forward- and reverse- bias voltages.

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Figure 7 shows the transient responses of the studied Pd/HfO2/GaN MOS diode

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under various introduced concentrations of hydrogen gases at 300 K. The applied

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voltage is fixed at VF = 0.5 V. The baseline current (in air) is 3.07×10-10 A. The corresponding performance under a lower introduced hydrogen concentration of 5 ppm

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H2/air is shown in the inset of Fig. 7. Obviously, the current is rapidly increased (decreased) once the hydrogen gas is introduced (removed). The response (recovery)

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time constant τa(τb) is defined as the time required to achieve a 63% (1-e-1) full response

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(recovery) action [25, 26, 33]. The relationships between τa, τb and temperature are shown in Figs. 8(a) and 8(b). τa and τb increase with decreases in hydrogen concentration and temperature. Experimentally, τa and τb are 39 s and 42 s, respectively,

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under 1% H2/air gas at 300 K. Moreover, at 383K, the corresponding values are 5.3 s and 2.5 s, respectively. The transient response of the studied Pd/HfO2/GaN MOS diode is similar and comparable to those reported in Schottky-diode type hydrogen sensors [4, 14, 17, 21, 26]. Figure 9 shows three repetitive dynamic responses upon the introduction and removal of 1% H2/air gas for the studied device at 383 K, where the 7

applied voltage is maintained at VF = 0.5 V. The average transient and steady-state ONstate currents, i.e., It(ave) and Is(ave), are 2.22×10-4 and 2.23×10-4 A, respectively. In addition, the corresponding very small standard deviations of δ(It)= 1.45×10-3% and δ(Is)= 1.05 ×10-3% are obtained, respectively. Clearly, the studied device exhibits

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reversible and repeatable hydrogen sensing performance. It is known that humidity is a crucial factor influencing gas sensing performance

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[8, 32]. The forward-biased I-V characteristics of the studied device under different

relative humidity RH (%) conditions at 300 K and 383 K upon exposure to 1% H2/air gas are shown in Fig. 10. It is apparent that the current is decreased with increases in

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the RH (%) value, especially at 300 K. This is mainly attributed to the partial occupation

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of the effective surface sites with water species chemisorption [8, 32, 34, 35]. Therefore,

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the chemisorption of hydrogen molecules at the device surface and the related hydrogen

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sensing performance are both decreased in a wet atmosphere, particularly at lower operating temperatures [8]. Figure 11 shows the sensing response SF versus relative

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humidity RH (%), under introduced 1000 ppm and 1% H2/air gases at 300 K and 383 K. The applied voltage is fixed at VF = 0.5 V. Clearly, the SF is substantially decreased

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from dry ambience (RH (%) = 0%) to different humidity environments at 300 K, as

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mentioned above. On the contrary, the deterioration of the hydrogen sensing response is negligible at 383 K. This is primarily caused by the nearly complete evaporation of water species at 383 K. Thus, the humidity effect on the hydrogen sensing performance

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can be effectively suppressed at higher operating temperatures ( ≥ 100oC). On the other hand, the appropriate surface passivation with poly-methyl methacrylate (PMMA), poly-perfluorobutenyl vinyl ether (Cytop), or fluoropolymer (Fluoropel) layers [36-38] could significantly block the chemisorption of water species on device surface. Therefore, the related humidity effect can be neglected based on this approach. 8

According to the Langmuir isotherm, under the steady-state conditions, the coverage of hydrogen at the Pd/HfO2 interface (𝜃𝑖 ) can be expressed as [25]: 𝜃𝑖 1−𝜃𝑖

=

𝐾𝑒 (𝑃𝐻2 )0.5 (𝑃𝑂2 )𝛽

,

(2)

where 𝜃𝑖 varies from 0 to 1, and 𝐾𝑒 is the temperature-dependent equilibrium

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constant that depends on the difference in hydrogen adsorption between the surface and interface [39]. 𝑃𝐻2 and 𝑃𝑂2 are the partial pressures of hydrogen and oxygen,

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respectively. 𝛽 is the reaction order [40, 41]. The variation in the height of the

Schottky barrier (∆𝜙𝐵 ) is caused by the adsorption of hydrogen atoms at the Pd/HfO2 interface. Thus, ∆𝜙𝐵 can be considered to be proportional to the hydrogen coverage

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𝜃𝑖 [25]:

∆𝜙𝑏 = ∆𝜙𝑏,𝑚𝑎𝑥 ∙ 𝜃𝑖 ,

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(3)

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where ∆𝜙𝑏,𝑚𝑎𝑥 is the maximum change in the Schottky barrier height. Based on the

expressed as [25]:

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combination of Eqs. (2) and (3), the related Langmuir isotherm equation can be

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=

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𝐼𝑜𝑔 ln( ) 𝐼𝑜

1

𝐼𝑜𝑔,𝑚𝑎𝑥 𝐾𝑒 ln( ) 𝐼𝑜

∙ ln(𝐶

1

0.5 𝐻2 )

−

1

,

𝐼𝑜𝑔,𝑚𝑎𝑥 ln( ) 𝐼𝑜

(4)

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where 𝐼𝑜𝑔 is the current corresponding to a given hydrogen concentration 𝐶𝐻2 , and the maximum current 𝐼𝑜𝑔,𝑚𝑎𝑥 is the current at saturated adsorption. 𝐼𝑜 is the current in air. Figure 12 shows the plots of (ln(Iog/Io))-1 versus ( 𝐶𝐻2 )-0.5 for the studied

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Pd/HfO2/GaN MOS diode in an air atmosphere at 300, 343, and 433 K. As can be seen in Fig. 12, linear relationships are found. From the slopes in Fig. 12, the corresponding 𝐾𝑒 values are 47.93, 7.18, and 0.82, respectively, at 300, 343, and 433 K. Clearly 𝐾𝑒 is decreased with increases in temperature. This implies that fewer hydrogen atoms are adsorbed at the Pd/HfO2 interface at higher temperatures. In other words, the coverage 9

of hydrogen at the Pd/HfO2 interface 𝜃𝑖 is decreased when the temperature is elevated [25]. The decrease in 𝜃𝑖 certainly leads to the relatively insensitive detection of hydrogen at higher temperatures [25]. Based on the adsorption thermodynamics, the dependence of temperature on 𝐾𝑒 can be deduced by a van’t Hoff relation as [25]: 1 d( ) 𝑇

=−

∆𝐻 𝑜 𝑅

,

(5)

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𝑑 ln(𝐾𝑒 )

where ∆𝐻 𝑜 is the enthalpy change in hydrogen adsorption, R is the ideal gas constant,

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and T is the absolute temperature. Figure 13 illustrates the logarithmic 𝐾𝑒 as a function

of the reciprocal temperature. From the slope, the ∆𝐻 𝑜 value of the studied

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Pd/HfO2/GaN MOS diode is -20 KJmole-1. The negative value of ∆𝐻 𝑜 indicates that

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the hydrogen adsorption process of the studied device is an exothermic action [25, 42].

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Therefore, the hydrogen sensing performance is decreased with increases in

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temperature.

A comparison of hydrogen sensing performance between this work and other

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reported MOS-type GaN-based Schottky diodes is provided in Tab. I. The studied device shows an excellent superior hydrogen sensing response of 4.9×105 (@ 1% H2/air,

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300 K) as compared to other properties. Hence, the studied Pd/HfO2/GaN MOS diode

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is a promising candidate for high-performance hydrogen sensors.

IV. Conclusions

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The hydrogen sensing characteristics of a Pd/HfO2/GaN-based MOS Schottky

diode under different concentrations of hydrogen gases are studied. Experimentally, upon exposure 1% H2/air gas at 300 K, the studied MOS diode shows a good hydrogen sensing response of 4.9×105 (139) under an applied forward-(reverse-) voltage of 0.5 V(-2 V). A lower detection limit of 5 ppm H2/air is obtained. The studied MOS diode 10

also exhibits reversible and high-speed responses when the temperature is elevated. The response (recovery) time constant τa(τb) is decreased from 39 s (42 s) to 5.3 s (2.5 s) once the temperature is increased from 300 K to 383 K. The humidity effect and hydrogen adsorption mechanism at the Pd/HfO2 interface are comprehensively studied. Similar to other Schottky diode-type sensors, the exothermic action of the hydrogen

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adsorption process causes a decreased sensing response with increases in temperature. Consequently, based on the good sensing properties indicated above, the studied

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Pd/HfO2/GaN MOS structure shows promise for the integration of high-performance E-mode HEMTs and hydrogen sensors on a chip.

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Acknowledgements

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Part of this work was supported by Ministry of Science and Technology of the

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Republic of China under Contract No. MOST-106-2221-E-006-224, the Advanced

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Optoelectronic Technology Center, National Cheng-Kung University. Technical

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assistance from Miss Hui-Jung Shih, Instrument Center, National Cheng-Kung

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University is appreciated.

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pp. 252109, 2007. [25] T. H. Tsai, J. R. Huang, K. W. Lin, W. C Hsu, H. I. Chen, W. C Liu, “Improved hydrogen sensing characteristics of a Pt/SiO2/GaN Schottky diode”, Sens. Actuators B Chem., vol. 129, pp. 292-302, 2008. [26] T. H. Tsai, H. I. Chen, K. W. Lin, Y. W Kuo, C. F Chang, C. W. Hung, L. Y. Chen, T. P. Chen, Y. C. Liu, W. C Liu, “SiO2 passivation effect on the hydrogen adsorption performance of a Pd/AlGaN-based Schottky diode”, Sens. Actuators B Chem., vol. 136, pp. 338-343, 2009. [27] C. Liu, E. F. Chor, L. S. Tan, “Investigation of HfO2/AlGaN/GaN metal-oxide-

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semiconductor high electron mobility transistors”, Appl. Phys, Lett., vol. 88, no. 17, pp. 173504, Apr. 2006. [28] Sugiura S, Kishimoto S, Mizutani T, Kuroda M, Ueda T, Tanaka T. “Normally-off AlGaN/GaN MOSHFETs with HfO2 gate oxide.,” Phys Status Solidi, vol. 5, pp.1923–5, 2008. [29] J. Shi, L. F. Eastman, X. Xin, M. Pophristic, "High performance AlGaN/GaN power switch with HfO2 insulation," Appl. Phys. Lett., vol. 95, no. 4, pp. 042103, 2009. 13

[30] S. Sugiura, Y. Hayashi, S. Kishimoto, T. Mizutani, M. Kuroda, T. Ueda, T. Tanaka,

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“Fabrication of normally-off mode GaN and AlGaN/GaN MOSFETs with HfO2 gate insulator”, Solid-State Electron., vol. 54, pp. 79-83, 2010. [31] O. Seok, W. Ahn, M.-K. Han, M.-W. Ha, “High on/off current ratio AlGaN/GaN MOS-HEMTs employing RF-sputtered HfO2 gate insulator”, Semicond. Sci. Technol., vol. 28, no. 2, pp. 025001, 2013. [32] C.C. Chen, H.I. Chen, I.P. Liu, H.Y. Liu, P.C. Chou, J.K. Liou, W.C. Liu, “Enhancement of hydrogen sensing performance of a GaN-based Schottky diode with a hydrogen peroxide surface treatment,” Sens. Actuators B Chem., vol. 211, pp. 303–309, 2015. [33] W.C. Liu, H.J. Pan, H.I. Chen, K.W. Lin, S.Y. Cheng, K.H. Yu, “Hydrogen

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sensitive characteristics of a novel Pd/InP MOS Schottky diode hydrogen sensor,” IEEE Trans. Electron Devices, vol. 48, pp. 1938–1944, 2001. [34] H. Miyazaki, T. Hyodo, Y. Shimizu, M. Egashira, “Hydrogen-sensing properties

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of anodically oxidized TiO2film sensors: effects of preparation and pretreatment conditions,” Sens. Actuators B: Chem., vol. 108, pp. 467–472, 2005. [35] Y. Shimizu, N. Kuwano, T. Hyodo, M. Egashira, “High H2 sensing performance of anodically oxidized TiO2 film contacted with Pd,” Sens. Actuators B: Chem., vol. 83, pp. 195–201, 2002. [36] A. Kumar, P. Zhang, A. Vincent, R. McCormack, R. Kalyanaraman, H.J. Cho, S. Seal, “Hydrogen selective gas sensor in humid environment based on polymer

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coated nanostructured-doped tin oxide”, Sens. Actuators B: Chem., vol. 155, pp. 884-892, 2011. [37] J. Hong, S. Lee, J. Seo, S. Pyo, J. Kim, T. Lee, “A highly sensitive hydrogen sensor with gas selectivity using a PMMA membrane-coated Pd nanoparticle/single-layer graphene hybrid”, ACS Appl. Mater. Interfaces, vol. 7, pp. 3554-3561, 2015. [38] S. Jung, K. H. Baik, F. Ren, S. J. Pearton, and S. Jang, “Pt-AlGaN/GaN Hydrogen Sensor With Water-Blocking PMMA Layer” IEEE Electron Device Lett., vol. 38, pp. 657-660, 2017. [39] C. Christofides, A. Mandelis, “Solid-state sensors for trace hydrogen gas

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detection,” J. Appl. Phys. vol. 68, pp. R1–R30, 1990. [40] I. Lundström, “Hydrogen sensitive MOS structures. Part 1. Principles and applications,” Sens. Actuators B: Chem., vol. 1, pp.403–426, 1981. [41] I. Lundström, “Hydrogen sensitive MOS structures. Part 2. Characterization”, Sens. Actuators B: Chem., vol. 2, pp. 105–138, 1981. [42] H.I. Chen, Y.I. Chou, C.K. Hsiung, “Comprehensive study of adsorption kinetics for hydrogen sensing with an electroless-plated Pd-InP Schottky diode,” Sens. Actuators B: Chem., vol. 92, pp. 6–16, 2003. 14

[43] H.I. Chen, K. C. Chuang, C. H. Chang, W. C.Chen, I-P. Liu, W. C Liu, “Hydrogen

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sensing characteristics of a Pd/AlGaOx/AlGaN-based Schottky diode”, Sens. Actuators B: Chem., vol. 246, pp. 408-414, 2017. [44] T. H. Tsai, J. R. Huang, K. W. Lin, W. C. Hsu, H. I. Chen, W. C. Liu, “Improved hydrogen sensing characteristics of a Pt/SiO2/GaN Schottky diode”, Sens. Actuators B: Chem., vol. 129, pp. 292-302, 2008. [45] T. H Tsai, H. I. Chen, K.W. Lin, Y. W. Kuo, C. F. Chang, C. W. Hung, L. Y. Chen, T. P. Chen, Y. C. Liu, W. C. Liu, “SiO2 passivation effect on the hydrogen adsorption performance of a Pd/AlGaN-based Schottky diode”, Sens. Actuators B: Chem., vol. 136, pp. 338-343, 2009. [46] C. C. Chou, H. I. Chen, I-P. Liu, C. C. Chen, J. K Liou, C. J. Lai, W. C. Liu,

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“Hydrogen sensing characteristics of Pd/SiO2-nanoparticles (NPs)/AlGaN metaloxide-semiconductor (MOS) diodes”, Int. J. Hydrogen Energy., vol. 39, pp.2031320318, 2014.

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Biographies for authors

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[47] T. H. Tsai, H.-I. Chen, K.-W. Lin, Y.-W. Kuo, C.-F. Chang, C.-W. Hung, L.-Y. Chen, T.-P. Chen, Y.-C. Liu, W.-C. Liu, “SiO2 passivation effect on the hydrogen adsorption performance of a Pd/AlGaN-based Schottky diode”, Sens. Actuators B: Chem., vol. 136, pp. 338-343, 2009.

A

Huey-Ing Chen received the B.S., M.S., and Ph.D. degrees from National Cheng Kung University (NCKU), Tainan, Taiwan, in 1979, 1981, and 1994, respectively, all in chemical engineering. She joined the faculty at NCKU as an Instructor, an Associate Professor, and a Professor in the Department of Chemical Engineering in 1981, 1994, and 2003, respectively. She is currently a Professor in the same department.

15

Ching-Hong Chang was born in Taipei, Taiwan, R.O.C., on October 9, 1991. He

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received the B.S. and M.S. degrees in the Department of Computer Science and Information Engineering, Chao yang University of Technology, Taichung, Taiwan, in

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2014 and 2015. He is currently working toward the Ph.D. degree in the Institute of

Microelectronics, Department of Electrical Engineering, National Cheng Kung

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MOS-field electron transistor and E-mode HEMT.

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University, Tainan, Taiwan. His research has focused on semiconductor sensors and

Hsin-Hau Lu received the M.S. degrees from Cheng Kung University (NCKU), Tainan,

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Taiwan, in 2015. His research has focused on Schottky type gas sensor.

I-Ping Liu received the B.S., M.S, and Ph.D. degrees in Department of Chemical Engineering, National Cheng-Kung University, Tainan, Taiwan in 2007, 2011, and 2017, respectively. His research has focused on quantum dot based solar cells and 16

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semiconductor gas sensors.

Wei-Cheng Chen was born in Taichung, Taiwan, R.O.C., on July 9, 1991. He received

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the B.S. and M.S. degrees in the Department of Electronic Engineering, National Ilan University, Ilan, Taiwan, in 2013 and 2015. He is currently working toward the Ph.D. degree in the Institute of Microelectronics, Department of Electrical Engineering,

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National Cheng Kung University, Tainan, Taiwan. His research has focused on

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semiconductor sensors and compound light-emitting diodes.

PT

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Bu-Yuan Ke was born in Taichung, Taiwan, R.O.C., on August 20, 1992. He received

A

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B.S. degree in the Department of Electronic engineering, Feng Chia University. Taichung, Taiwan, in 2016. He is currently studying in the Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan. His research has focused on Schottky type gas sensor.

17

Schematic cross section diagram of the studied Pd/HfO2/GaN MOS Schottky

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Fig. 1.

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Figure Captions

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Wen-Chau Liu (A’91-M’93-SM’02) received the B.S., M.S., and Ph.D. degrees in electrical engineering from the National Cheng-Kung University (NCKU), Tainan, Taiwan, in 1979, 1981, and 1986, respectively. He was with the faculty at National Cheng-Kung University, as an Instructor, an Associate Professor, and a professor with the Department of Electrical Engineering in 1983, 1986, and 1992, respectively. Since 2002, he has been a Distinguished Professor in the same department. His research currently focuses on III–V heterostructure high-speed and optical devices, and highsensitivity semiconductor sensors. Dr. Liu passed the Higher Civil Service examinations and received the technical expert licenses of ROC in the electrical and electronic fields in 1979 and 1982, respectively.

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diode hydrogen sensor.

XRD pattern of the HfO2 thin film used in the studied device.

Fig. 3.

EDS analysis of the Schottky contact region of the studied device.

Fig. 4.

I-V characteristics of the studied Pd/HfO2/GaN MOS-type and a compared

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Fig. 2.

Fig. 5.

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Pd/GaN MS-type devices.

I-V characteristics of the studied Pd/HfO2/GaN MOS diode in air and under

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introduced various hydrogen gases at (a) 300 K and (b) 383 K. The corresponding energy band diagrams are depicted in (c).

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Fig. 6.

Fig. 7.

Sensing response versus temperature under introduced different hydrogen gases at applied (a) forward voltage of VF= 0.5 V and (b) reverse voltage of VR = -2 V. Transient responses of the studied Pd/HfO2/GaN MOS diode under different concentrations of hydrogen gases. The corresponding performance under 5 ppm H2/air gas is shown in the inset. 18

Fig. 8.

(a) Response and (b) recovery time constants versus temperature of the studied device under introduced various concentrations of hydrogen gases.

Fig. 9.

Three repetitive dynamic responses upon the introduction and removal of 1% H2/air gas of the studied Pd/HfO2/GaN MOS diode at 383 K.

Fig. 10.

Forward-biased I-V characteristics, upon exposure to 1% H2/air gas, under

Fig. 11.

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different relative humidity RH (%) conditions at 300 K and 383 K.

Sensing response SR versus relative humidity RH (%) under 1000 ppm and

Fig. 12.

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1% H2/air gases, at 300 K and 383 K.

The reciprocal logarithmic value of current variation (ln(Iog/Io))-1 as a

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function of the square root of the reciprocal hydrogen concentration 𝐶𝐻−0.5 2 for the studied device at 300, 343, and 383 K.

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Logarithmic equilibrium constant ln(𝐾𝑒 ) as a function of reciprocal

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Fig. 13.

A

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temperature 1000/T of the studied device.

19

Pd HfO2

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Ohmic Contact Schottky Contact HfO2 (15nm) / Pd (40nm) Ti (10nm) / Al (100nm) / Ti (10nm) / Au (100nm) Area: 2.05 x 10–3 cm2 Pd

GaN (20 nm) GaN (1 μm) Buffer Layer (1.5 μm)

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Al0.2Ga0.8N (20 nm)

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HfO2

A

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Fig. 1

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A

Silicon Substrate

20

A ED

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CC E 30 40

Fig. 2

21 (112) (202)

(021) (211)

5

50

Scanning angle 2 (degrees) 60

(202) (013) (212) (222) (302) (320)

(213)

(220)

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(200)

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(022)

U

N

A

0 20 (002)

(011)

10

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Intensity (cps)

(111)

(100)

20

JCPDS 43-1017 HfO2

15

70

cps/eV

Al Ga

Hf

Pd

Hf

2

4

6

keV

Ga

8

10

AN

Series

unn. C [wt%]

norm. C [wt%]

Atom. C [at. %]

(1 Sigma) [wt%]

Ga

31

K

38.91

55.21

47.23

1.94

N

7

K

3.59

5.10

21.71

1.37

Pd

46

L

18.77

26.63

14.93

0.72

O

8

K

1.94

2.76

10.28

0.76

Hf

72

L

6.36

9.02

3.02

0.53

Al

13

K

0.90

1.28

2.83

0.10

Total:

68.85

100.00

100.00

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El

12

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w/i H2O2 treatment -Device A

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Hf O Pd N

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Fig. 3

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El: Element; AN: Atomic Number; norm.: normalization; wt.: weight percent; at.: atomic percent.

A

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

22

14

-1

10

Temp. = 300 K MS (Pd/GaN) MOS (Pd/HfO2/GaN)

-3

-5

10

-7

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10

-9

10

-11

10

-13

-2.0 -1.5 -1.0 -0.5

0.5

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Voltage (V)

A

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(a)

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Fig. 4

23

1.0

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10

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Current (A)

10

1.5

2.0

-2

10

Temp. = 300 K Pd/HfO2/GaN

-6

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10

-8

H2 content in air

10

1% 1000 ppm 100 ppm 5 ppm

-10

10

in normal air -12

10

-2.0 -1.5 -1.0 -0.5

0.5

1.0

1.5

2.0

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Voltage (V)

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Current (A)

-4

10

-6

10

ED

-8

H2 content in air

-10

1% 1000 ppm 100 ppm 5 ppm

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Current (A)

-4

10

Temp. = 383 K Pd/HfO2/GaN

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-2

10

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A

(b)

10

A

10

in normal air

-12

10

-2.0 -1.5 -1.0 -0.5

0.5

Voltage (V) 24

1.0

1.5

2.0

(c) In air atmosphere

qΦb,air qΦ EFM

,

+H+H+H+H+H+H+H+H-

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q∆Φb

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Upon exposing to hydrogen gas

HfO2

EF

GaN

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Pd

A

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EV

Fig. 5 25

H2 content in air

VF = 0.5 V

1% 1000 ppm

5

100 ppm 5 ppm

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10

3

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10

1

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10

-1

10

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7

10

300

320

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Sensing Response, SR (A/A)

(a)

340

360

PT

A

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(b)

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Temperature (K)

26

380

400

H2 content in air

VR = 2 V

100 ppm 5 ppm

1% 1000 ppm

2

10

1

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10

0

10

-1

10

300

320

340

360

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Sensing Response, SR (A/A)

3

10

380

Fig. 6

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Temperature (K)

400

200

PT

H2 content in air

A

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Current (A)

150 100

gas on gas off

Current (nA)

Temp. = 300 K 200 VF= 0.5 V 160 120

1%

H2 content in air Temp. = 300 K VF = 0.5 V 5 ppm H2/ air gas on

80

gas off

40 0 0

1000 2000 3000 4000 5000 Time (s)

1000 ppm

50

100 ppm 5 ppm

0 0

70

140 210 Time (min) 27

280

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Fig. 7

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5

A

10

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H2 content in air 4

5 ppm 100 ppm

3

2

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10 10

A

1000 ppm 1%

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10

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Response Time Constant, a (sec)

(a)

1

10

0

10

300

320

340

360

Temperature (K)

(b) 28

380

400

4

Recovery Time Constant, b (sec)

10

H2 content in air 5 ppm 100 ppm

3

10

1000 ppm 1%

2

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10

1

0

320

340

360

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300

380

400

Temperature (K)

A

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M

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10

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10

29

Fig. 8

300

Temp. = 383 K VF = 0.5 V 1% H2/air

200 Is It

It

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150

Is

Is I t

100 50 0 1000

2000

3000

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0

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Current (A)

250

Gas On Gas Off

4000

5000

M

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Time (sec)

A

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Fig. 9

30

-1

10

-1

Temp. = 300 K

10

Temp. = 383 K

-3

10

-5

10

-3 -5

10

1% H2/Air

1% H2/Air

in air RH=66.4% RH=36.2% RH=9.7% dry

-9

10

-11

10

in air RH=61.2% RH=39.1% RH=12.3% dry

-13

10

0.6 1.2 Voltage (V)

A

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0.6

31

-7

10

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-7

10

-9

10

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Current (A)

10

1.2

-11

10

-13

10

Fig. 10

7

Sensing Response, SF (A/A)

10

6

10

VF = 0.5 V

Temp. = 300 K 1% H2/air

5

1000 ppm H2/air

10

4

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10

3

10

Temp. = 383 K 1% H2/air 1000 ppm H2/air

1

10

0

10

20

30

40

50

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2

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10

60

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Relative Humidity (%)

A

Fig. 11

32

70

433 K 343 K 300 K

1.2 0.9

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1/ln(Iog/Io)

0.6

0.0 0.0

0.1

0.2

0.3

CH2

(ppm )

-0.5

-0.5

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0.3

0.4

0.5

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Reciprocal Logarithmic Current Variation

1.5

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A

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Square Root of the Reciprocal Hydrogen Concentration

A

Fig. 12

33

450 420 390 360 -1.0 -1.5

330

300

Pd/HfO2/GaN

-2.0 -3.0 1

o

-3.5 -4.0 -4.5 2.2

2.4

2.6

2.8

3.0

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H = 20 kJ mol

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-2.5

3.2

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ln K

Logarithmic Equilibrium Constant

Temperature (K)

-1

3.4

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A

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Reciprocal Temperature 1000 / T (K )

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Fig. 13

34

35

A ED

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N

A

M

I Temperature (K)

Sensing Response

Response Time (s)

Recovery Time (s)

Detection limit

Ref.

Pd/HfO2/GaN

10000 ppm H2/air

300

4.9  105

39

42

5 ppm H2/air

This work

Pd/AlGaOx/AlGaN

10000 ppm H2/air

300

3.9  105

33

17

1 ppm H2/air

[43]

10000 ppm H2/air

300

1.8  105

13.3

23.6

0.1 ppm H2/air

[18]

PT

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Concentration

9970 ppm H2/air

300

4.5  104

12

6

4.3 ppm H2/air

[45]

Pd/SiO2 nanoparticles/AlGaN

10000 ppm H2/air

300

1.6  104

12.4

6.3

<10 ppm H2/air

[46]

Pd/SiO2/AlGaN

10000 ppm H2/air

343

3.3 105

16

Pt/SiO2/GaN

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ED

Pd/GaOx/GaN

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Device (structure)

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Tab.I. Comparison of hydrogen sensing performance between this work and reported MOS-type Schottky diodes.

36

[47]