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1. Introduction

sing ad envce is inconsumieve g

diode (LED) technology has enabled the production of differentwavelengths and powers of LEDs [12]. Also, this development hasenabled a wide usage and study of LEDs in gas sensors.Ultraviolet (UV) light irradiation has been proposed for operatingsome metal oxide gas sensors at room temperature [1315]. Our

get (atomic ratioering protained, anre (25 C).

diffraction analysis revealed that the IGZO lm was amoin nature. No crystalline peaks appear in the XRD patternof the lms deposited at temperatures up to 600 C. Thuslm deposited below 600 C reveals amorphous [20].

The amorphous IGZO (a-IGZO) lms were placed in a home-made chamber with dimensions of 13.0 cm 6.5 cm 5.5 cm.The thin lms were continuously irradiated using a 370-nm UVLED at room temperature (25 C). The LED power followsP = Iapp V, where P, Iapp, and V are LED power, applied current,and voltage, respectively. Thus, LED power can be modulated by

Corresponding author at: Department of Physics, National Chung HsingUniversity, 250 Kuo Kuang Rd., Taichung 402, Taiwan. Tel.: +886 4 22840427;fax: +886 4 22862534.

E-mail address: chwu@phys.nchu.edu.tw (C.-H. Wu).

Analytical Chemistry Research 4 (2015) 812

Contents lists availab

Analytical Chem

.esumption and low selectivity. To enhance the sensitivity of MOSsensors, the sensors have typically been heated to high tempera-tures (200500 C) [911]; however, this strategy is inconvenientfor long-term operation. The recent progress in light emitting

RF-sputtering system with an IGZO ceramic tarIn:Ga:Zn = 1:1:1) [11,20,21]. During the sputt200-mtorr pure argon atmosphere was mainsubstrate was maintained at room temperatuhttp://dx.doi.org/10.1016/j.ancr.2015.03.0012214-1812/ 2015 Published by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).cess, ad theX-ray

rphousof any, IGZOmance, several methods [6] (optical, gas chromatographic, andacoustic methods) and materials (metal oxide semiconductors[3], carbon nanotubes [7], and polymers [8]) have been developed.

Metal oxide semiconductors (MOS) are the most commonsensing materials. The advantages of MOS are their low cost, fastresponse, wide range of target gases, and long lifetimes.However, they also have the disadvantages of high energy con-

[11,1719]. The sensitivity, response time, and time constant of thesensors irradiated by different LED power are discussed.

2. Experimental

N-type transparent IGZO Thin lms with thicknesses of 6070-nm were deposited on 5 mm 5 mm glass substrates using an[15]. A gas sensors good performanas sensitivity, response time, energytoring, and reproducibility [4]. To achGas sensing is receiving increaproduction, medical applications, ancreativecommons.org/licenses/by-nc-nd/4.0/).

ttention in industrialironmental monitoringdicated by factors suchption, long termmoni-ood gas sensing perfor-

previous experimental results suggest that continuous UV irradia-tion by a single UV LED can replace conventional heating, and UVlamps to enhance the sensing properties of SnO2 gas sensors[16]. However, a detailed experiment about the relation betweenLED power (intensity) and gas sensor sensitivity is still lacking. Inthis study, indiumgalliumzincoxide (IGZO) lm sensors wereirradiated with different LED intensities under ozone environmenttime. Similar results were also observed in other kinds of materials such as TiO2. According to the results,the sensing properties of gas sensors can be modulated by controlling the light intensity.Gas sensing properties of indiumgalliumin different light intensity

Kuen-Lin Chen a, Guo-Jhen Jiang b, Kai-Wei Chang b,aDepartment of Physics, National Chung Hsing University, Taichung 402, Taiwanb Institute of Nanoscience, National Chung Hsing University, Taichung 402, TaiwancDepartment of Materials Science and Engineering, Da-Yeh University, Changhua 515, T

a r t i c l e i n f o

Keywords:Gas sensorGas sensitivityLight intensity

a b s t r a c t

We have successfully obseby controlling the light emsity dependence of sensorsensor sensitivity, which dgas sensor sensitivity but arate per second. Low inten

journal homepage: wwwzincoxide gas sensors

-Han Chen c, Chiu-Hsien Wu a,b,

an

d the change in indiumgalliumzincoxide (IGZO) gas sensor sensitivityng diode (LED) power under the same gas concentrations. The light inten-perties is discussed. Different LED intensities obviously affected the gasys with increasing LED intensity. High LED intensity decreases not onlythe response time (T90), response time constant (sres) and the absorptionirradiated to sensor causes high sensitivity, but it needs larger response

le at ScienceDirect

istry Research

lsevier .com/locate /ancr

the applied current. The distance between the IGZO thin lm andthe LED is approximately 0.5 cm. The light intensity (I) follows asI PA ; where A is the area. The LED power was measured by apower meter. The intensity depends on light power at a xed dis-tance between LED and lm. Ozone was generated from an ozonegenerator and passed through a ow meter. The ozone gas wasmixed with dry air (RH 25 2%) from an air pump before chan-neling it into the homemade test chamber. Ozone was rstdetected by the thin lm sensor, and the O3 concentration insidethe chamber was also monitored using an O3 monitor (2BTechnologies 106-L) [16].

In this work, the lm sensor was continuously irradiated withthe UV LED. The IGZO lms were rst exposed to a UV LED at axed power. The generation of electronhole pairs caused the sen-sor resistance to decrease until a stable state (the lowest resis-tance) was reached. The resistance increased as a result of

K.-L. Chen et al. / Analytical Chemiabsorption process and reached equilibrium with ozone channel-ing into the chamber. Subsequently, when the absorption processreached equilibrium under ozone environment, ozone was turnedoff, and the LED power was changed. The resistance of the lm dur-ing cyclic exposure to trace gas concentrations was measuredusing a multimeter, and the data was collected by a computerusing data acquisition software.

3. Results and discussion

Fig. 1 shows the resistancetime (RT) curve of an IGZO thinlm under LED exposure with a constant intensity of 13.93 mW/cm2 while maintaining constant O3 concentrations of 0.5 and1 ppm. The lowest equilibrium resistance (Ra), resistance change(DR), sensitivity (S = (Rg Ra)/Ra), and sensor response time variedslightly with the same LED power (Fig. 1) [11]. The response of thesensor was almost constant for three cycles, demonstrating thegood reproducibility of the sensor as well as the good performanceof our IGZO lms. Fig. 2 shows the rates of ozone desorption underdifferent LED intensities. First, 0.5 ppm of ozone was absorbed bythe IGZO lm under irradiation at LED intensity of 13.93 mW/cm2. The intensity of LED was changed, and the ozone was turnedoff when saturation resistance was reached, as shown in the top ofFig. 2. The bottom of Fig. 2 shows the RT curves under differentlight intensities during the desorption processes. The saturationresistances were almost the same under the same environment(intensity and ozone concentration).

When LED irradiates the lm, the following response equationsapply.

hm! h e photo excitation; 1Fig. 1. The resistance-time curves of IGZO sensor at 0.5 and 1 ppm ozone.h e ! hm electron hole recombination; 2When measurements are performed, the following response

equations of absorption and desorption apply.

O3 e ! O3 adsorption; 3

2O3 hv ! 3O2g 2e desorption; 4The peaks in the bottom of Fig. 2 were induced by electronhole

combination; at low intensity, the generation of electronholeswas reduced, while the recombination of electronholes increased(Eqs. (1) and (2)). However, the decreased rate of resistancedepends on light intensity after the gas is shut off; greater irradia-tion intensity resulted in a faster ozone desorption rate (Eqs. (3)and (4)). The RT curves also revealed that Ra varied with LEDpower. The lowest resistance decreased as the light intensityincreased, which may be explained on the basis of the number ofexcited molecules. As more intense light irradiates the IGZO mole-cules, a larger number of molecules get excited, which may gener-ate more electronhole pairs (Eq. (1)). Thus, the lowest equilibriumresistance was found to decrease with increasing light intensity.

Fig. 3 shows the RT curves under different LED powers whilemaintaining a constant O3 concentration of 2 ppm. The LED inten-sity was varied from 1.41 to 13.93 mW/cm2, as shown by the blueline in Fig. 3. The inset shows the enlarged graph of RT curve. Thelowest resistances are obvious different under different intensities.Fig. 4 shows that sensors Ra and resistance of sensor in gas (Rg)depend on the light intensity. A larger intensity resulted in a sig-nicantly lower Ra and Rg, which both exhibited an exponentialresponse to LED intensity. The Rg 900 MX was very high underan intensity of 1.41 mW/cm2; however, the change in Rg becamesmall when the intensity was larger than 9.25 mW/cm2. The sameresults were observed in different samples. The results can be

explained by the following relationship: dndt dNabsdt dNdesdt , wheredn/dt is the net rate of absorption of O3 molecules, and dNabs/dtand dNdes/dt are the rates of absorption and desorption of O3 mole-cules, respectively (Eqs. (3) and (4)). The adsorptiondesorptionactivity at any time causes the adsorptiondesorption processesto reach equilibrium under continuous irradiation. The resistanceof sensor is proportional to dndt .

dndt and resistance will increase when

dNabsdt >

dNdesdt (ozone channels into test box). It will reach equilibrium

if dNabsdt dNdesdt , and dndt will be negative when the ozone stops channel-ing into the test box (dndt 0 dNdesdt : Therefore, the different Rgobserved in Fig. 3 is caused by different net desorption rates atthe different intensities. Rg will be reduced with the same gas con-centrations at high intensity because the rate of desorption (dNdes/dt) is increased.

Fig. 5 shows the sensitivity (S) and resistance change(DR = Rg Ra) under different intensities. The sensitivity to ozonegas, S = (Rg Ra)/Ra, is a function of light intensity. The sensorsensitivity is found to decrease with increasing intensity; DR andS are approximately 843 MX and 13.7, respectively, at 1.42 mW/cm2 and approximately 93 MX and 8.29, respectively, at13.92 mW/cm2. With increasing intensity and with more electronsreleased from the trap state into the conduction band, the surfacedesorption reaction proceeds more actively (Eq. (4)). Different LEDpowers obviously affected resistance change, which exponentiallydecays with increasing LED power. No obvious regular tendency isobserved for the decay of S in our experiments. The change of resis-tance is considerably large at low intensity; this large DR will bebenecial for detecting stably low gas concentrations.

Fig. 6 shows the response time (T90: R reaches to the 90% of

stry Research 4 (2015) 812 9equilibrium value) and response time constant (sres) underdifferent intensities [11,22]. The RT curves were tted by a simplefunction of the form R A Bet=sres , where A and B correspond to

em10 K.-L. Chen et al. / Analytical ChRg and DR. The sres decreased with increasing intensity; sdecreased from 259.3 to 80.3 s when the intensity increased from

Fig. 2. The rates of ozone desorption

Fig. 3. The RT curves under different LED powers while maintaining a constant O3concentration of 2 ppm.

Fig. 4. Sensors resistances in air and in gas (Rg) depend on the light intensity.istry Research 4 (2015) 8121.42 to 13.92 mW/cm2. The results can be tted with one exponen-tial decay function. A similar decay tendency is observed for T90;s decreased from 600 to 205 s when the intensity increased from1.42 to 13.92 mW/cm2. High intensity causes a short responsetime.

Fig. 7 shows the rst-order differential (dR/dt) for the RT curveof Fig. 3 [23]. It indicates the rate of resistance variation caused bynet absorption (the numbers of absorbed molecules minus des-orbed ones) of O3 molecules per second. The curves for differentintensities have different raising rates, implying that the rate ofchange in resistance is also different. The value of dR/dt is positiveand dependent on the intensities with the same ozone concentra-tion during absorption. Moreover, different peaks are observed fordifferent intensities in the rst-order derivative curves (Fig. 7). Atlower intensity, the instantaneous adsorption of ozone moleculesis higher, causing a high rate of change. The maximum rate ofchange of adsorption as a function of intensity is shown in Fig. 8.The relation between the maximum rate of change ((dR/dt)max)and light intensity is approximately exponential. These resultscan also be explained by the competition between adsorptionand desorption rates at different intensities.

under different LED intensities.

Fig. 5. The sensitivity (S) and resistance change (DR = Rg Ra) of IGZO sensor underdifferent intensities.

hemiK.-L. Chen et al. / Analytical CFig. 9 shows the resistancetime curve of an TiO2 thin lmunder different LED powers while maintaining a constant O3 con-centration of 1.7 ppm. Ra, Rg and DR also depend on the light inten-sity. The results of TiO2 lm are consistent with those of IGZO lm.A larger intensity resulted in a signicantly lower Ra and Rg.

Fig. 6. The response time (T90) and response time constant (sres) under differentintensities.

Fig. 7. The rst-order difference for the RT curve of Fig. 3

Fig. 8. The maximum rate of change of adsorption as a function of intensity.stry Research 4 (2015) 812 11Fig. 9(b) shows the S, DR and dR/dt under different intensities. S,DR and (dR/dt)max to ozone gas are a function of light intensity,which all exhibited an exponential response to LED intensity.Fig. 9(c) shows the T90 and sres under different intensities. A similardecay tendency is observed for T90 and sres when the intensityincreased from 1.42 to 13.92 mW/cm2. High intensity causes ashort response time.

4. Conclusion

RT curves for an IGZO gas sensor were successfully observedwith different LED intensities. At lower intensity, greater sensitiv-ity and DR of the sensor were obtained. However, the sres and T90are larger at lower intensity. According to the results, it possiblyprovides a good method for rapidly determining the target gasconcentration by controlling light intensity; sensors can achieve

Fig. 9. The sensing properties of TiO2 sensor at 1.7 ppm ozone under differentintensities. (a) The resistance-time curves and rst-order difference for the RTcurve. (b) The sensitivity the maximum rate of change of resistance and resistancechange of TiO2 sensor under different intensities. (c) The response time (T90) andresponse time constant (sres).

a fast response time using high light intensity. They can also showbetter sensitivity by using low light intensity. Similar results werealso observed in other kinds of materials (TiO2). Results show thatthe sensitivity of gas sensor of oxide semiconductor can be modu-lated by controlling the light intensity under LED continuousirradiation.

Acknowledgement

The authors thank the nancial supports of the Ministry ofScience and Technology of Taiwan (MOST 103-2112-M-005-002).

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Gas sensing properties of indiumgalliumzincoxide gas sensors in different light intensity1 Introduction2 Experimental3 Results and discussion4 ConclusionAcknowledgementReferences