IEEE SENSORS JOURNAL, VOL. 14, NO. 3, MARCH 2014 651

Analysis of Nano-Structured In2O3 Thin Film NOxSensor by AC Impedance SpectroscopyChinnasamy Ramaraj Mariappan, Ethirajulu Prabhu, Kovilpillai Immanuel Gnanasekar,

Venkataraman Jayaraman, and Thiagarajan Gnanasekaran

Abstract— Nano-structured In2O3 thin film was made usinga pulsed laser deposition technique. The surface topographyand structural properties of the thin film were characterizedby atomic force microscopy and X-ray diffraction, respectively.Complex impedance spectroscopy of In2O3 thin film gas sensorwas investigated from 275 °C to 425 °C when exposed into cleanair and air containing a trace level of NOx. Significant NOxsensing characteristics of thin film were observed at 325 °C by acimpedance spectroscopic analysis. The resistance and capacitanceof indium oxide film increased when exposed into the trace level ofNOx. A mechanism for this increase of resistance and capacitanceis proposed.

Index Terms— In2O3 thin film, impedance spectroscopy, gassensor, space charge layer, selectivity.

I. INTRODUCTION

N ITROGEN oxides termed as NOx, comprises of NO andNO2, are hazardous gases with threshold limit values of

25 vppm and 2 vppm respectively. Hence the NOx gas sensorfor monitoring NOx in the ambient is important for environ-ment protection. Indium oxide (In2O3), n-type semiconductor,shows considerable change in resistance towards trace lev-els of mono-nitrogen oxides by dc resistance measurementtechniques [1]–[6]. Also the In2O3 has been investigated indifferent material configurations such as bulk, thin film, nano-wire for NOx gas sensing [1]–[4]. Since the NOx gas detectionmechanism of In2O3 is surface-controlled, both the magnitudeand response of the sensor, along with its long-term stabilityare strongly influenced by details of the electronic structure ofsolid-gas interfaces, leading to differences in the performancethat can be associated with differences in synthesis routes andmicrostructure. Consequently it is well known that chemicalsensing behavior is controlled by the transport properties ofgrain and grain boundary [3]. Therefore the grain and grainboundary resistance need to be investigated for understanding

Manuscript received June 6, 2013; revised October 5, 2013; acceptedOctober 7, 2013. Date of publication October 17, 2013; date of current versionJanuary 7, 2014. The associate editor coordinating the review of this paperand approving it for publication was Dr. Francis P. Hindle.

C. R. Mariappan was with the Chemistry Group, Indira Gandhi Centre forAtomic Research, Kalpakkam 603 102, India. He is now with the Departmentof Physics, National Institute of Technology, Kurukshetra 136 119, India(e-mail: crmari2005@yahoo.com).

E. Prabhu, K. I. Gnanasekar, V. Jayaraman, and T. Gnanasekaran arewith the Chemistry Group, Indira Gandhi Centre for Atomic Research,Kalpakkam 603 102, India (e-mail: eprabhu@igcar.gov.in; igsk@igcar.gov.in;vjram@igcar.gov.in; gnani.home@gmail.com).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2013.2286198

TABLE I

IN-SITU PULSED LASER DEPOSITION PARAMETERS

the mechanism of sensing by granular materials AC impedancespectroscopic method is a powerful tool to study the natureof conduction processes and the mechanism of gas/solidinteractions, which exploits the time constants of the differentprocesses by varying the frequency [7]–[9]. Most of the studiesfocused on the effect of hazardous gas on the dc resistanceof In2O3, and there is no report on the effect of nitrogenoxides on the other electrical quantities such as capacitanceand impedance.

In this paper, thin films of nano-structured In2O3 werefabricated using pulsed laser ablation method. AC impedanceand electrical relaxation behaviour of these thin films wereinvestigated at different temperatures (275–425 °C) in clean airand on exposure to air containing 5 volume parts per million(vppm) of NOx. The changes in complex impedance of thinfilm as a function of NOx concentration (2 to 20 vppm) at325 °C were also investigated. A mechanism for the changesin impedance and capacitance is proposed.

II. EXPERIMENT

Indium oxide powders were coaxially pressed into a pellet(thickness of 2 mm and diameter of 16 mm, typically) andsintered at 900 °C in air for 10 h. Sintered pellet was usedas target for depositing thin film by KrF pulsed laser ablationtechnique. The parameters used for the film deposition aregiven in Table I. Before the deposition of the In2O3 film,a twisted platinum heater pattern was screen printed on oneside of the alumina substrate along with necessary goldelectrical pads. On the other side of the substrate where, theIn2O3 film would be made, four gold pads were screen printedfor making electrical contacts with the sensing film and theheater respectively as shown in Fig. 1.

1530-437X © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

652 IEEE SENSORS JOURNAL, VOL. 14, NO. 3, MARCH 2014

Fig. 1. (a) Front view of In2O3 thin film sensor (b) rear view of the sensorwith platinum (Pt) heater pattern.

Crystallographic phase formation and surface morphologyof the thin film were characterized by X-ray diffraction (ModelD500, Siemens, Germany) and atomic force microscopy(Solver Pro, NT–MDT, Russia) respectively. The NOx gassensing behaviour of In2O3 thin film was investigated in thetrace level which obtained in the following ways. The NOxgas is diluted by taking a known concentration of gas using amicro-litre syringe and injected into a glass vessel of a knownlarge volume (sampler). The vessel is allowed to equilibratefor at least 10 min. Once the volume of the vessel housingthe sensor is known, a specific quantity of the NOx gasfor required trace level of NOx is calculated based on thesimple gas equation and injected into the vessel containingthe sensor using a microlitre syringe. For gas sensing studies,the film was housed in a glass container of 50 ml capacityand provided with sample injection port, air inlet and air outlet.The temperature of the thin film was controlled by applying dccurrent through the heater and the NOx sensing properties werestudied by two probe ac impedance spectroscopy technique(Model SI 1260, Solartron, UK) over the frequency range1.0 MHz–10 Hz, with an applied Vrms of 50 mV, in thetemperature range of 275–425 °C.

III. RESULTS AND DISCUSSION

A. Structural and Surface Analysis

Figures 2(a) and (b) show the XRD patterns of the bulk(target) and thin film of In2O3 deposited at 500 °C on poly-crystalline alumina substrate respectively. The XRD patternof target In2O3 pellet matched with the standard JCPDS cardno. 71-2194 (body centered cubic structure, space group: Ia-3#206) as shown in Fig 1(a). It reveals the polycrystalline natureof In2O3 target and their most intense reflection is (222).In figure 2(b) apart from the reflections of thin film In2O3lines corresponding to the XRD pattern of the polycrystallinealumina substrate. The maximum intensity of reflection (400)of thin film In2O3 on the Al2O3 substrate is observed. Thefilms are highly a-axis textured as evident from the highintense reflections (400) and (800). In textured films, thereflections corresponding to the specific orientation is seenwith high intensity and other reflections including the inten-sity of the most intense reflection of the randomly orientedpolycrystalline would appear as weak or sometimes wouldnot be even seen. Therefore the most intense reflection (222)of the randomly oriented polycrystalline film is not seen inFig. 2(b).

Fig. 2. X-ray diffraction pattern of (a) bulk In2O3 and (b) thin film of In2O3on polycrystalline Al2O3 substrate

Fig. 3. Atomic force microscope image of In2O3 thin film scanned over thearea of (a) 13 μm x 13 μm, (b) 1.6 μm x 1.6 μm, (c) 13 μm x 13 μm forbare polycrystalline Al2O3 substrate.

Surface topography of the textured thin film was carriedout by using atomic force microscopy (AFM). Figure 3(a)shows the AFM topography (13 μm x 13 μm) of the In2O3film deposited on mechanically polished polycrystalline Al2O3substrate. A magnified AFM image (1.6 μm x 1.6 μm) ofthe film is shown in Fig. 3(b). It reveals that the grains areregularly shaped and the size of the grains varies from 100to 300 nm. The image also reveals that individual grainsare well connected, which would facilitate the gas sensingcharacteristics of the film.

The thickness of the film was about 3000 Å. Morphologyof the plain substrate is also recorded to compare with thethin film morphology as shown in Fig. 3(c). The AFM imageshows that the substrate morphology is rough and granular(size ∼5 μm) containing long channels formed as a resultantmechanical polishing. Its morphological features are superim-posed onto the film.

MARIAPPAN et al.: ANALYSIS OF NANO-STRUCTURED In2O3 THIN FILM NOx SENSOR 653

Fig. 4. (a) Complex impedance spectra of In2O3 thin film at differenttemperatures. (b) A representative equivalent circuit for fitting the impedancedata.

B. Impedance Spectroscopy Studies

The real part Z′(ν) and imaginary part Z′′(ν) of the compleximpedance of In2O3 thin film at various temperatures werecalculated from the measured modulus |Z| and phase angle θby using the following expressions:

Z ′(ν) = |Z |cos(θ) (1)

Z ′′(ν) = |Z |sin(θ). (2)

Nyquist plot of ac impedance spectra for indium oxide thinfilm at different temperatures in air is shown in Fig. 4(a).A single semi-circle is observed in the temperature rangeof 275–425 °C. The Z"(ν) become positive value at highfrequencies due to the influence of inductance effect. Basedon the measured impedance data, the representative electricalequivalent circuit was established as shown in Fig. 4(b). Theequivalent circuit elements R is represented as resistanceof thin film (Ra) in the presence of air, C is denoted ascapacitance of thin film (Ca) in the presence of air, and L isrepresented as inductance. It is noteworthy that the Ca is calledas space charge layer capacitance. The chemisorbed oxygenmolecules are converted into oxide ions by capturing the freeelectrons at grain surface of In2O3 thin film as shown in thefollowing expression:

O2(air) + 4e−(surface) → 2O2−(adsorbed on the surface) (3)

It creates the negatively charged layer over the grain and alsothe positively charged grain interior [9]. This space chargelayer is a kind of capacitor (called as space charge capacitorCa), since the electrical charge exists locally at the grainboundary [11]–[13]. The measured impedance data were fittedto the equivalent circuit (Fig. 4(b)) using Zview�(version2.70) software. Fitted values of electrical components ofequivalent circuit are given in Table II. The Ra of the filmdecreases with increase in temperature due to increase of thenumber density and mobility of charge carrier, while the Caincreases with temperature. The relaxation time (τa = RaCa)of the film in air decreases with temperature as shown inTable II.

Assuming that the shape of In2O3 grain is spherical, theinfluence of width of the space charge layer on the capaci-tance is described by spherical capacitor. The net charge of

Fig. 5. Illustration of the cross-sectional view of oppositely charged core-shell spheres. ‘b’ is the radius of the outer spherical shell with charge –Q,‘a’ is the radius of the inner spherical shell with charge +Q and (b−a) is thethickness of space charge layer.

negatively charged chemisorbed oxide ions (O2−) at grainboundary is –Q which creates an equal and opposite positivecharge +Q at grain interior to hold charge neutrality. Thus itis treated as oppositely charged core-shell sphere as illustratedin Fig. 5.

The capacitance for the oppositely charged core-shell sphereis obtained from the ratio of charge Q to the potentialdifference �V between the spherical core-shells [14]:

Ca = Q

�V= 4πε

(ab

b-a

)(4)

where b is radius of the outer spherical shell i.e. radius ofthe grain, a is radius of the inner spherical shell i.e. radiusof the grain interior, (b−a) is the thickness or width of thespace charge layer and ε is the dielectric constant. The Ca isinversely proportional to the thickness of space charge layer(b−a). Probably a few chemisorbed oxygen ions at grainboundary of the film could be dissociated by lowering thedissociation energy of adsorbents at higher temperature region,which leads to shrinkage of the thickness of space charge layer(b−a). Therefore the shrinking of space charge layer thicknessis one of the main contributions for increase in Ca of thin filmwith temperature (see in Table II) in air.

C. Gas Sensing Studies

Figure 6(a) shows the typical Nyquist plot of thin film at325 °C in the presence of air and also in the presence ofa typical concentration of 5 vppm of NOx. Similar compleximpedance spectra were observed in the temperature range of275 °C to 425 °C. In order to analyze the influence of the NOxon the electrical properties of the thin film surface, we fittedthe impedance spectra to equivalent circuit which is same asin Fig. 4(b). Here the equivalent circuit elements R and Crepresented as the resistance (Rg) and the capacitance (Cg)of thin film respectively in the presence of analyste gas. BothRg and Cg increased in the presence of oxidizing NOx gasas shown in Table II. The resistance and capacitance arefirst increases and then decreases with temperature in thepresence of 5vppm of NOx. The relaxation time (τg = RgCg)of the film in the presence of NOx is higher than that of inair. The changes of resistance (�R=Rg–Ra) and capacitance(�C=Cg–Ca) for the In2O3 film were relatively higher at325 °C (see in Table II), where Ra and Ca are the resistanceand capacitance of thin film respectively in air. and Rg andCg are the resistance and capacitance of thin film respectively

654 IEEE SENSORS JOURNAL, VOL. 14, NO. 3, MARCH 2014

TABLE II

RESISTANCE IN AIR (Ra), CAPACITANCE IN AIR (Ca), RELAXATION TIME IN AIR (τa) RESISTANCE IN THE PRESENCE OF 5 VPPM OF NOx (Rg),

CAPACITANCE IN THE PRESENCE OF 5 VPPM OF NOx (Cg) AND RELAXATION TIME IN THE PRESENCE OF 5 VPPM OF ANALYTE GAS (τg) FOR

In2O3 THIN FILM AT DIFFERENT TEMPERATURE ARE OBTAINED USING COMPLEX IMPEDANCE SPECTROSCOPIC ANALYSIS

(b)

Fig. 6. (a) Nyquist plot of indium oxide thin film with and without analyte gasat 325 °C. (b) Typical real part of impedance (Z′) and real part of capacitance(C′) responses of In2O3 film at 100 kHz towards 5 vppm of NOx at 325 °C.

in air containing NOx gas. These investigations revealed thatIn2O3 film shows a significant NOx sensing characteristics at325 °C.

Usually the gas sensor mechanism depends on the worktemperature, because the mechanism is thermally activated.The sensor response coefficient increases with temperatureunder optimal conditions, such as Debye length (L), Knudsendiffusion co-efficient (Dk) and rate constant (k) and thereafterit goes down further increase of the temperature due todecreases of utility factors [15], [16].

Fig. 7. Nyquist plot of indium oxide thin film at 325 °C with differentconcentration of NOx.

Ling et al [7] studied the time resolved response of a WO3pellet sensor towards NOx gas by ac impedance and modulusspectroscopic analysis. They showed the overall impedanceand modulus changes in the presence of 1.5 vppm of NOxdue to the grain boundary resistance. The grain boundarycapacitance or space charge layer capacitance of WO3 wasunchanged throughout the response period in the presence ofNOx gas. Whereas in the present studies the capacitance andresistance of the In2O3 thin film were found increasing onexposure to air containing traces of NOx. In order to confirmthis increasing behavior of resistance and capacitance, wemeasured them as function of time at a fixed ac frequency,which was close to the relaxation frequency of the film(ν=1/2πRaCa) and the results are shown in Fig. 6(b). Thefigure shows that the change in capacitance of film is muchfaster than that of the resistance on its exposure to NOx. Therecovery time was almost same for both quantities (∼10 h) at325 °C. At higher temperature (>330 °C), the recovery timereduced but with low gas sensitivity. We discussed the gassensitivity in later.

Figure 7 depicts the impedance spectra of indium oxidethin film as a function of concentration of NOx from 2 to20 vppm at 325 °C. The resistance Rg and capacitance Cg ofthe thin film increased with trace levels of NOx concentration.The gas sensing mechanism of In2O3 belongs to the surfacecontrolled nature, in which the grain size, surface states andthe density of chemisorbed oxygen ions have significant roles

MARIAPPAN et al.: ANALYSIS OF NANO-STRUCTURED In2O3 THIN FILM NOx SENSOR 655

Fig. 8. Conductance (SG) and capacitance (SC) based response of In2O3thin film towards different concentration of NOx at 325 °C.

[1]–[4]. As deposited In2O3 film is n-type semi-conductordue to native defects. As discussed before, when the film isexposed to air at elevated temperature, chemisorbed oxygenmolecules can capture free electrons from indium oxide andcreate a space charge layer and thus, the resistance of thefilm increases. On the other hand, the space charge layer is akind of capacitor which is connected in parallel to resistanceas shown in the inset of Fig. 4(b). As soon as the film isexposed to the oxidizing nitrogen dioxide gas, these moleculesget adsorbed on surface of the film due to the higher electronaffinity of NO2 (220 kJ/mol) which is five times higher thanthe electron affinity of O2 (42 kJ/mol) [5]. It provides furthersurface traps and increases the resistance of the film (or reducethe number density of electron). Thus, proposed mechanismsfor gas sensing are:

NO(gas) + e−(bulk) ↔ NO−

(ads) (5)

NO2(gas) + e−(bulk) ↔ NO−

2 (ads) (6)

Simultaneously the thickness of space charge layer (b−a)could be further increased by reducing number density ofelectron n, which leads to lowering of the capacitancevalue (Cg = 4πε(ab/a−b) α (ne2/kBT)1/2. However, exper-imentally we observed that the Cg value increased in thepresence of NOx, although the thickness of space chargelayer was increased. One possible reason for the increaseof Cg at a given temperature is the existence of the anions(NO−/ NO−

2 ) with high dielectric constant ε of grain boundary.The high dielectric constant of grain boundary region mighthave overcome the increasing space charge layer thickness andthus resulting in enhanced capacitance values.

Since both resistance and capacitance of the In2O3 filmincreased on exposure to trace levels of NOx, two differentelectrical responses towards NOx were estimated as givenbelow:

SG = Gg

Ga(7)

SC = Cg

Ca(8)

where SG is the response of film based on the conductancechanges Ga is the conductance (1/Ra) of film in air,Gg is conductance (1/Rg) of film in analyte gas and SCis the response of the film based on capacitance changes.The estimated SR and SC values as a function of NOxconcentration are shown in Figure 8.

The electrical responses (resistance and capacitance) showthe linear behaviors from 2 to 20 vppm of NOx. In orderto find the cross sensitivity of thin film, the ac impedancewas measured towards reducing gas at 325 °C. Considerablechanges on resistance and capacitance were not observedon introduction of 5000 vppm of H2 to In2O3 thin film at325 °C. It is noteworthy that the SC is strongly dependenton adsorbants and the dielectric constant of the space chargelayer, while the SR depends on the number density of chargecarriers. Therefore, the capacitance based response of In2O3gas sensor is promising specific to NOx.

IV. CONCLUSION

The NOx gas sensing properties of In2O3 thin film wereinvestigated by using ac impedance spectroscopy at differenttemperatures. The significant NOx gas sensing characteristicsof thin film were observed at 325 °C. The changes on theresistance of the film in the presence of NOx were governedby the number density of charge carrier, while the changeson the capacitance were controlled by the adsorbants andalso the dielectric constant of space charge layer. The presentinvestigations clearly show that AC impedance/capacitance-type In2O3 thin film can be used as a NOx gas specific sensor.

ACKNOWLEDGMENT

We also thank profusely Dr. Scott E. Harpstrite andDr. S. Jothilingam, Washington University in St. Louis, USA,for critical reading of the manuscript.

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Chinnasamy Ramaraj Mariappan is an Assis-tant Professor of Physics with the Physics Depart-ment, National Institute of Technology, Kurukshetra.He received the M.Sc. degree in physics fromBharthiyar University, Coimbatore, in 1998, and thePh.D. degree in 2004 from Pondicherry University,Pondicherry. His main research interests are devel-opment of ion conducting solids, metal oxide semi-conductors for advanced battery systems, super-capacitors and gas sensors and bio-ceramics forbone replacements. He also worked as Postdoctoral

Research Fellow in France (2005), Alexander von Humboldt Fellow inGermany (2006-2008), and European Research Fellow in England (2007).

Ethirajulu Prabhu received the M.Sc. Degree inchemistry from with the University of Madras andstarted his research career with the Indira GandhiCentre for Atomic Research since 1995. Researchareas of his interests are chemical sensors for ambi-ent monitoring and materials processing for chemi-cal sensors.

Kovilpillai Immanuel Gnanasekar received theM.Sc. degree in chemistry from the Indian Instituteof Technology, Bombay, in 1994 and the Ph.D.degree in 1998. He has been a Scientist with theIndira Gandhi Centre for Atomic Research since1996. His main research interests are thin filmsuperconductors, chemical sensors and materials foradvanced battery systems.

Venkataraman Jayaraman received the M.Sc.degree in chemistry from the Indian Institute ofTechnology, Madras, in 1988 and the Ph.D. degree in1999 from the Indian Institute of Science, Bangalore.He started his research career with the Indira GandhiCentre for Atomic Research from 1988. Researchareas of his interest are solid electrolytes, chemicalsensors for ambient monitoring and soft chemicalsynthesis of multi-component oxides.

Thiagarajan Gnanasekaran received the M.Sc.degree in chemistry from the University Madras in1973 and the Ph.D. degree in 1987. He has been aScientist with the Indira Gandhi Centre for AtomicResearch since 1974. His main research interests arechemical sensors high temperature thermo-chemistryand alkali metals chemistry.