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Sensors and Actuators B 160 (2011) 1481– 1484

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l h o mepage: www.elsev ier .com/ locate /snb

hort communication

n an indium–tin-oxide thin film based ammonia gas sensor

heng-Wei Lina, Huey-Ing Chenb, Tai-You Chena, Chien-Chang Huanga, Chi-Shiang Hsua,ong-Chau Liuc, Wen-Chau Liua,∗

Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, 1 University Road, Tainan 70101, Taiwan, ROCDepartment of Chemical Engineering, National Cheng-Kung University, 1 University Road, Tainan 70101, Taiwan, ROCChung Shan Institute of Science and Technology, Tao-Yuan, Taiwan, ROC

r t i c l e i n f o

rticle history:eceived 3 May 2011eceived in revised form 18 July 2011ccepted 22 July 2011vailable online 30 July 2011

a b s t r a c t

An ammonia sensor basing on the indium tin oxide (ITO) thin film on a quartz substrate which wasfabricated by RF sputtering with substrate thermal treatment, is studied and demonstrated. From theexperimental results, the good NH3 sensing performances including high response of 2312%, fast responseand recovery times of 73 and 104 s upon the introduction of a 1000 ppm NH3/air gas at 150 ◦C are observed.In comparison, the proposed sensor is superior to other previously reported ITO thin film based ammonia

eywords:TOmmonia sensorF sputteringrain size

sensors. Due to the advantages of simple structure, easy operation, low cost, and excellent performances,the studied device gives a promising use in high-performance ammonia sensor applications.

© 2011 Elsevier B.V. All rights reserved.

xygen deficiency

. Introduction

Due to the pervasive awareness of public safety and environ-ental protection, the continuous monitoring of air quality is one of

mportant and indispensable issues nowadays. Therefore, a varietyf gas sensors have been developed and used in laboratories, hospi-als, factories, etc. [1]. In order to detect the hazard gases such as H2,O, and NO2 [2], many different gas sensors have been fabricateduccessfully. Over the past years, numerous solid-state devices foretecting gaseous species were produced and reported [2–4]. Usu-lly, gas sensors could detect specific gases by chemical reactionsr intrinsic physical properties of the used materials [5]. AmmoniaNH3) is a colorless gas with the pungent odour properties. The

ain sources of ammonia are nitrification, ammonification, andombustion from chemical plants and motor vehicles [6]. Ammo-ia has been widely employed in the production of fertilizer andefrigeration systems. It could be dissolved in water to become themmonium hydroxide solution. It is known that, upon exposure toround 50 ppm ammonia gas, the human skin, eyes, and respiratoryystem would be irritated [6]. Therefore, it is necessary to fabri-

ate a reliable and sensitive ammonia gas sensor to continuouslyonitor NH3 concentration in environmental condition.Recently,

he indium tin oxide thin film has drawn a remarkable attention

∗ Corresponding author. Fax: +886 6 209 4786.E-mail address: wcliu@mail.ncku.edu.tw (W.-C. Liu).

925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2011.07.041

in gas sensors fabrication [7–12]. Generally, ITO is an n-type semi-conductor with a wider bandgap (3.5–4.06 eV) [13]. It also showsthe smooth surface morphology, superior electrical conductivity,and optical transparency [14]. In this work, an ITO thin film basedammonia sensor with good NH3 detection performance is fabri-cated and studied.

2. Experiments

The studied device was fabricated by sputtering ITO thin filmon a quartz substrate. Firstly, the quartz substrate was cleaned byimmersing in acetone and hydrochloric acid solutions, respectively,with ultrasonic cleansing for 10 min to remove surface particles,organic contaminants, and native oxides. Then, a pair of inter-digitized electrodes were produced using the thermal evaporatingCr (chromium) and Pt (platinum) metals, respectively, with thethickness of 100 A. The sensing area was defined by photolithog-raphy process. The ITO thin film was deposited on the defined areaby sputtering with a RF power of 100 W. The thickness of ITO films isabout 800 A. ITO thin films were deposited on the quartz substratesat different substrate temperature such as 100, 150, 200, and 250 ◦C.The used ITO target was composed of 95 wt.% In2O3 and 5 wt.%SnO2. For gas-sensing measurement, the device was placed in a

sealed stainless-steel chamber. The testing chamber with compo-nent parts of a stainless steel cell and a coil heater was hermeticallysealed to prevent from the interference of external atmosphere andhumidity. The pressure inside the chamber was maintained at an

1482 C.-W. Lin et al. / Sensors and Actuators B 160 (2011) 1481– 1484

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Fig. 2. (a) Response ratio (SR%) versus operating temperature of the studied device.(substrate temperature Ts of 100 ◦C) upon the introduction of 35, 200, and 1000 ppmNH3/air gases. (b) Response ratio (SR%) versus substrate temperature of the studied

ig. 1. Current–voltage (I–V) characteristics of the studied device (substrate tem-erature Ts of 100 ◦C) upon the introduction of 35, 200, and 1000 ppm NH3/air gasest 150 ◦C. The inset reveals the related schematic cross section of the studied device.

tmospheric condition. Air and NH3 gases (including 35, 200, and000 NH3 ppm/air) are obtained by BOCLH INDUSTRIAL GASES Co.td. The air is composed of 21% O2 and 79% N2. First, an air flowas introduced into a testing chamber. The flow rate was set at

00 cc/min. A stable NH3 flow was continuously introduced intohe chamber. NH3 gases with different concentrations were used foresting the sensing performance. Experimental results were mea-ured by a Keithley 4200 semiconductor characterization system.he schematic cross section of the studied device is illustrated inhe inset of Fig. 1.

. Results and discussion

Fig. 1 shows the current–voltage (I–V) characteristics of thetudied device, with a substrate temperature Ts of 100 ◦C, uponhe introduction of 35, 200, and 1000 ppm NH3/air gases at 150 ◦C.bviously, with the increase of ammonia concentration, the cur-

ent is increased. Experimentally, the electrical resistance of thetudied device is decreased from 35.3 to 1.6 k�, upon exposure to

1000 ppm NH3/air gas. This device indicates the n-type sensingehavior and large resistance variation.

Based on the metal oxide semiconductor sensing mechanism4,15], under air ambient, oxygen molecules could be adsorbedn the ITO surface. Then the adsorbed oxygen molecules capturelectrons from the ITO conduction band and are changed as O− or2

− oxygen ions [15]. The captured electrons cause the increase ofurface depletion layer of ITO and leads to the decrease of carrieroncentration. This certainly causes the increase of resistance ofhe ITO thin film. The adsorption reaction of oxygen species coulde presented as follows:

2(ambience) + e− → O−2(surface) (1)

−2(surface) + e− → 2O−

(surface) (2)

n which O2(ambience) is the oxygen molecule in a testing environ-ent, O−

2(surface) and O−(surface) are the adsorbed oxygen ions on the

TO surface, and e− the electronic charges.Once ITO is exposed to ammonia gas, the reaction between

mmonia molecules and oxygen ions causes the electrons to escaperom the valance band to conduction band [4]. This can be explainedsing the following reaction:

NH3 + 3O−(surface) → N2 + 3H2O + 3e− (3)

hus, the carrier concentration (or resistance) of ITO is increasedor decreased) as expected.

device (substrate temperature Ts of 100 ◦C) upon the introduction of a 1000 ppmNH3/air gas at 150 ◦C. The inset shows SEM images of the studied ITO thin film(substrate temperature of 100 ◦C).

Fig. 2a shows the ammonia detection response of the studieddevice, with a substrate temperature Ts of 100 ◦C at different tem-peratures. The relative response ratio (SR) is defined here as [16]:

SR(%) =(

Rair

Rgas− 1

)× 100% (4)

where Rair and Rgas are resistances measured in air and NH3/aircombination conditions, respectively. It is found that, the maxi-mum SR (%) of the studied device is 2312% at 150 ◦C. The relationshipbetween substrate temperature Ts and NH3 response ratio SR (%) isshown in Fig. 2b. It could be found that the studied device showsa best SR behavior at a substrate temperature Ts of 100 ◦C. Thedependence of SR on the substrate temperature Ts may be related tothe temperature-dependent reactivity of oxygen ions [4,15,16]. Theinset of Fig. 2b shows the SEM image of the studied ITO thin film ata substrate temperature Ts of 100 ◦C. The average grain size is about26.6 ± 2 nm. With increasing the substrate temperature, the largergrains are formed. This may be caused by the coalescence effect ofnuclei [17]. The formation of coalescence of nuclei is mainly due tothe sufficient activation energy in adatoms (adsorption atoms) athigher temperature [18]. Furthermore, when the substrate temper-ature is increased, it will result in the increase of nucleation densityby chemical reactions among the sputtered indium and tin atoms[19]. The larger grain size could cause less grain boundaries and

improve the mobility of charge carriers [20]. Another effect is theoxygen deficiency due to the increased substrate temperature Ts

[21,22]. The increase of oxygen deficiencies gives the decrease inresistance of the ITO film. So, a high substrate temperature Ts results

C.-W. Lin et al. / Sensors and Actuat

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ig. 3. Transient response curves of the studied device (substrate temperature Ts of00 ◦C) upon the introduction of 35, 200, and 1000 ppm NH3/air gases at 150 ◦C.

n a low response to NH3/air gas. Therefore, the best response undern optimal substrate temperature Ts of 100 ◦C could be attributedo the effects of more grain boundaries and presence of oxygeneficiencies.

Fig. 3 shows transient response curves of the studied device,ith a substrate temperature Ts of 100 ◦C, upon the introduc-

ion and removal of 35, 200, and 1000 ppm NH3/air gases at50 ◦C. The studied device shows the n-type sensing behavior andeversible sensing performance. When the ammonia concentra-ion is increased, the current is increased from 56 to 1267 �A. Theesponse time �a is defined here as the time needed for the currento reach 70% of the final steady-state current from original baselineurrent. The recovery time �b is defined here as the time neededor the current reaching 70% of the final steady-state current. Thisevice shows good �a and �b values of 103 (73) and 663 (104) s,espectively, upon exposure to a 35 (1000) ppm NH3/air gas.

Empirically, the response S of semiconducting oxide gas sensorould be represented as [23]:

= Ag(Pg)ˇ (5)

here Ag is a prefactor, Pg is the partial pressure of target gashich is proportion to the gas concentration, and the exponent

n Pg [23,24]. Usually, based on the surface interaction betweenhemisorbed oxygen species and reducing gas (e.g., NH3, CO, and2 etc.), the ideal value of is 0.5 or 1 [23,24]. In this study, the ˇalue is about 0.438. The little deviation from the ideal value of.5 may be caused by the agglomeration of nanostructures or lessensitive areas presented in the studied device [24]. As mentionedbove, when the grain size is smaller than that of double Debyeength (D � 2LD), the sensing mechanism will be mainly controlledy the grain boundary barriers [4].

. Conclusions

In this work, ITO thin film-based ammonia sensors on a quartzubstrate, made by RF sputtering with substrate thermal treatment,re fabricated and studied. The studied device with a substrateemperature Ts of 100 ◦C exhibits high responses of 530, 1125,nd 2312%, respectively, upon exposure to 35, 200, and 1000 ppmH3/air gases at 150 ◦C. The response performance of the studiedevice is superior to previously reported ITO thin film based NH3ensors. In addition, this device shows the fast response and recov-

ry times of 73 and 104 s. Therefore, due to its advantages of simpletructure, easy operation, low cost, and excellent behaviors, thetudied device gives a promise for high-performance NH3 sensingpplications.

ors B 160 (2011) 1481– 1484 1483

Acknowledgment

This work was supported in part by the National Science Councilof the Republic of China under contract no. NSC-97-2221-E-006-237-MY3.

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Biographies

Cheng-Wei Lin received the B.S. degree from the Department of Electronic Engi-neering, Feng-Chia University, Taichung, Taiwan in 2008. He is currently pursuing

ing at the National Cheng-Kung University. His research has focused on the field ofsemiconductor gas sensors.

Huey-Ing Chen received the B.S., M.S., and Ph.D. degrees from Cheng Kung Univer-sity (NCKU), Tainan, Taiwan, in 1979, 1981, and 1994, respectively, all in Chemical

1 Actuat

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484 C.-W. Lin et al. / Sensors and

ngineering. She joined the faculty at NCKU as an Instructor, an Associate Profes-or, and a Professor in the Department of Chemical Engineering in 1981, 1994, and003, respectively. She is currently a professor in the same department. Her researchresently focuses on hydrogen permselective Pd-based membranes, hydrogen sen-ors, gas separations, and nanoparticles.

ai-You Chen received the B.S. degree from the Department of Electronic Engineer-ng, Feng-Chia University, Taichung, Taiwan, in 2008. He is currently working towardhe Ph.D. degree in the Institute of Microelectronics, Department of Electrical Engi-eering, National Cheng-Kung University, Tainan, Taiwan. His research has focusedn the field of semiconductor gas sensors.

hien-Chang Huang received the B.S. degree from the Department of Electronicngineering, Feng-Chia University, Taichung, Taiwan, in 2008. He is currently work-ng toward the Ph.D. degree in the Institute of Microelectronics and the Departmentf Electrical Engineering, National Cheng Kung University, Tainan, Taiwan. Hisesearch has focused on III-V heterostructure field-effect transistors.

hi-Shiang Hsu received the B.S. degree in electronic engineering from Chienkuoechnology University, Changhua, Taiwan, in 2007, and the M.S. degree in com-uter science and information engineering from Chaoyang University of Technology,aichung, Taiwan, in 2009, where he is currently pursuing the Ph.D. degree inhe institute of microelectronics and department of electrical engineering at theational Cheng-Kung University. His research has focused on semiconductor gas

ensors.

ong-Chau Liu received the B.S. and M.S. degrees from National Cheng-Kung Uni-ersity in 1980 and 1982 respectively, and the Ph.D. degree from National Taiwanniversity in 1991, all in electrical engineering. He has been working in Chung Shan

ors B 160 (2011) 1481– 1484

Institute of Science and Technology (CSIST) since 1982, where he is in charge ofautomatic testing system design, reliability engineering, and quality assurance ofelectronic systems. Dr. Liu is promoted to be Senior Scientist of CSIST in 2003.He also served as part-time associate professor at National Cheng-Kung Univer-sity from 1992 to 1998. His primary interest is in performance evaluation, datacommunication and automatic testing system.

Wen-Chau Liu received the B.S., M.S., and Ph.D. degrees in electrical engineeringfrom the National Cheng-Kung University (NCKU), Tainan, Taiwan, in 1979, 1981,and 1986, respectively. He was with the faculty at National Cheng-Kung Univer-sity, as an Instructor and an Associate Professor with the Department of ElectricalEngineering in 1983, 1986, and 1992, respectively. Since 2002, he has been a Dis-tinguished Professor in the same department, and since 2005, he has been anAssociate Chair with the Department of Electrical Engineering and the Director ofthe Institute of Microelectronics, NCKU. His research and teaching concern semi-conductor device physics, analysis, and modeling. His research currently focuseson III–V heterostructure and superlattice devices including induced base transistor,superlattice-gate and heterostructure buffer layer FETs, camel structure gate FET,sawtooth-doping-superlatticed devices, heterostructure-emitter bipolar transistor,superlattice-emitter resonant-tunneling bipolar transistor, heterostructure-emitterand heterostructure-base transistor, superlatticed negative-differential-resistance(NDR) device, quantum-well-doped NDR devices, metal–insulator–semiconductorlike multiple switching devices, low-dimensional quantum electron devices, deepsubmicrometer meter devices and technologies, and high-sensitivity semiconduc-

tor gas sensors. He has published more than 300 journal papers. He is the holderof 54 patents in the semiconductor field. Dr. Liu has passed the Higher Civil Serviceexaminations and has received the technical expert licenses of ROC in the electricaland electronic fields in 1979 and 1982, respectively. He is a member of Phi Tau Phiand the IEEE Electron Devices Society.