Low-Temperature Photochemically Activated Amorphous Indium

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Low-Temperature Photochemically Activated Amorphous IndiumGallium-Zinc-Oxide for Highly Stable Room-Temperature Gas Sensors Rawat Jaisutti, Jaeyoung Kim, Sung Kyu Park, and Yong-Hoon Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05724 • Publication Date (Web): 19 Jul 2016 Downloaded from http://pubs.acs.org on July 20, 2016

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ACS Applied Materials & Interfaces

Low-Temperature Photochemically Activated Amorphous IndiumGallium-Zinc-Oxide for Highly Stable Room-Temperature Gas Sensors

Rawat Jaisutti,1,2 Jaeyoung Kim,3 Sung Kyu Park, 4,* and Yong-Hoon Kim2,3,*

1

Department of Physics, Faculty of Science and Technology, Thammasat University,

Pathum Thani, Thailand 2

School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon,

Korea 3

SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon,

Korea 4

School of Electrical and Electronic Engineering, Chung-Ang University, Seoul, Korea

ABSTRACT: We report on highly stable amorphous indium-gallium-zinc oxide (IGZO) gas sensors for ultraviolet (UV)-activated room-temperature detection of volatile organic compounds (VOCs). The IGZO sensors fabricated by a low-temperature photochemical activation process and exhibiting two orders higher photocurrent compared to conventional zinc oxide sensors, allowed high gas sensitivity against various VOCs even at room-temperature. From a systematic analysis it was found that by increasing the UV intensity, the gas sensitivity, response time, and

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recovery behavior of an IGZO sensor were strongly enhanced. In particular, under an UV intensity of 30 mW cm-2, the IGZO sensor exhibited gas sensitivity, response time and recovery time of 37%, 37 s and 53 s, respectively, against 750 ppm concentration of acetone gas. Moreover, the IGZO gas sensor had an excellent long-term stability showing around 6% variation in gas sensitivity over 70 days. These results strongly support a conclusion that a lowtemperature solution-processed amorphous IGZO film can serve as a good candidate for room temperature VOCs sensors for emerging wearable electronics.

KEYWORDS: Amorphous IGZO, Gas sensor, Room temperature, Light intensity, Sensitivity


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1. INTRODUCTION Monitoring of volatile organic compounds (VOCs) has been of particular interest in the field of industrial safety controls to prevent industrial accidents. Now, it is extending to fields in medical/healthcare sectors for new applications in personal healthcare systems and self-diagnosis devices. For instance, flexible or wearable electronic devices integrated with molecular sensors are now possible, including those that can monitor the concentration of ethanol,1 NO2,2 or CO3 species in the atmosphere, or even biological fluids in a non-invasive and low-cost point-of-care system.4 For detection of various VOCs, metal-oxide semiconductor-based gas sensors have been quite attractive for their fast response and high sensitivity to a wide range of target gases.5-7 However, despite these numerous advantages, the high temperature operation (200~400oC) required for these metal-oxide gas sensors, necessary to enhance the surface redox reactions and to ensure high gas sensitivity,8-10 has limited their usability in body-wearable gas monitoring systems due to the possibility of thermal burns to the skin when integrated in wearable devices. To operate metal oxide gas sensors at a low temperature, several methods have been developed, including: modified porous nano-structured materials,11 utilization of nano-sized particles,12-13 hybridization with an organic sensing layer,14 and ultraviolet-visible (UV-VIS) light-induced activation during sensor operation.15-19 Among these approaches, the photo-activation approach appears particularly promising, as the sensor can be operated at room temperature and a lowpower single UV light-emitting diode (LED) can be used as a light source.20-23 In previous studies on photo-activated gas sensors, however, conventional sensing materials such as nanocrystalline or polycrystalline SnO2, TiO2, ZnO (or their complexes) have been commonly used. However, these may not show a sufficiently high sensitivity, owing to their low conductivity (determined by their intrinsic carrier mobility and grain boundary effects), and


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insufficient photocurrent generation by UV irradiation. Therefore, to enhance the sensitivity of photo-activated metal-oxide gas sensors, a new sensing material with high conductivity and large photocurrent generation is preferable. Previously, nanostructures or noble metal decorated materials (to minimize the grain growth) have been reported,24-25 based on the fact that conductivity and gas sensitivity tend to sharply increase with decreasing grain size.26-27 However, when a nanocrystalline or polycrystalline material is used as a sensing layer, the formation of Schottky barriers at grain boundaries is unavoidable, seriously inhibiting efficient charge transport in the sensing layer. From this point of view, an amorphous structured metal-oxide semiconductor without any grain boundaries could be beneficial in achieving high conductivity and sensitivity, bearing a partial reduction in surface area. Recently, amorphous metal oxide semiconductors such as indium-gallium-zinc oxide (IGZO) have been extensively studied as an active material in thin-film transistors. The IGZO film exhibits high carrier mobility and good performance uniformity, and facile preparation by a low temperature solution process is possible.28-29 Due to its amorphous nature, IGZO can reduce the effects of grain boundaries, while the IGZO also has high UV absorption and large photoconductivity properties favorable for UV-activated gas sensors.29 Particularly, sputter-deposited IGZO films have been studied for the detection of ozone gas under UV illumination.30 In addition, the photochemical activated IGZO devices have shown more stable operation as well as fast recovery characteristics under light illumination condition comparable to high-temperature annealed devices.31 These approaches offer a simple and low temperature processing for obtaining high stability room temperature metal oxide sensing devices. In this study, an UVLED-activated room-temperature-operated IGZO gas sensor is realized by a low-temperature solution processing, for monitoring various VOCs including: acetone, ethanol, methanol,


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isopropanol, toluene, and chlorobenzene gases. In particular, we systematically investigated the effects of UV intensity on the gas sensitivity, response time, and recovery behavior of IGZO gas sensors. Additionally, the gas sensing mechanism of UV-activated IGZO sensors under a continuous UV irradiation was explained in terms of photo-generated electron-hole pairs and a surface gas adsorption-desorption process.

2. EXPERIMENTAL SECTION 2.1 Fabrication of IGZO Gas Sensors. For the fabrication of IGZO gas sensors, indium-tin oxide (ITO) interdigitated electrodes with an inter-spacing of 50 µm were patterned on a glass substrate using photolithography and wet etching. Then, an amorphous IGZO film was coated on the ITO electrodes using a solution process, as shown in Figure 1a. For the IGZO coating, an IGZO precursor solution was first prepared by dissolving powders of 0.085 M indium nitrate hydrate, 0.0125 M gallium nitrate hydrate, and 0.0275 M zinc acetate dihydrate in 2methoxyethanol. The solution was used after stirring at 75oC for 12 h. After spin-coating the IGZO precursor solution, deep UV photochemical activation in N2 atmosphere followed for 2 h. The atomic composition ratio and the thickness of the photochemically activated IGZO film can be found in our previous work.29 Then, the fabricated sensor was bonded on a print circuit board and connected to an electronic connector for analysis of electrical and gas sensing properties (Figure 1b). For characterization of the thin film properties, we investigated the surface morphology of IGZO films by using a scanning electron microscope (SEM; JSM-7600F, JEOL) and an atomic force microscope (AFM; NX10, Park Systems). Also, we measured the optical transmittance by using an UV-VIS-NIR spectrophotometer (UV-3600, Shimadzu) within a wavelength range of 200–900 nm.


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2.2 Gas Sensing Measurements. The gas sensing measurements were carried out using a home-built automated sensor test system, as shown in Figure S1 (Supporting Information). Initially, the fabricated IGZO sensor was placed in a glass chamber having an internal volume of 200 mL. A single UV LED having a peak emission wavelength of 390±2.5 nm was placed ~1 cm above the IGZO sensor to irradiate UV onto the sensor. The light intensity (IL) of the LED was modulated by adjusting the LED current (Ibias), using the following relations: IL=P/A and P=IbiasV, where P, A, and V were LED power, active area, and applied voltage, respectively. The light intensity was controlled by a computer and calibrated using an UV intensity meter (Model1000, Karl Suss). For the electrical characterization of the sensor, a source/measurement unit (Model 2400, Keithley) was used to supply the input voltage through the sensor at a constant voltage of 5 V and monitor the output currents during the measurement. Also, the concentration of acetone gas was calculated by the saturated vapor pressure of the organic vapor under a standard atmospheric pressure at a specific operating temperature. The lower concentration of the target gases was prepared by diluting the vapors with dry air (relative humidity of ~25±5%) before feeding into the test chamber. The total gas flow rate was fixed at 500 sccm, controlled by a mass flow controller unit. For the cleaning process, the target gases inside the test chamber were removed by flowing fresh dry air into the chamber. In addition, the humidity and temperature inside the test chamber were monitored throughout the measurement using a commercial sensor (Model SHT15, Sensirion AG).

3. RESULTS AND DISCUSSION 3.1 Characterization of IGZO Sensors. Figure 2a shows the optical transmittance spectra of an IGZO gas sensor. In the visible light region, the IGZO sensor (glass-ITO-IGZO) exhibits an


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optical transmittance of ~80%, compatible with the ITO-coated glass substrate (glass-ITO). The high optical transmittance of the IGZO sensor was attributed to the large band gap of IGZO (~3.19 eV), as shown in the inset of Figure 2a, where α is the absorption coefficient. In addition, the surface morphology of the IGZO sensor was examined to verify the conformal coating of the IGZO film on patterned ITO electrodes. Figure 2b shows the SEM images of an IGZO gas sensor indicating good surface coverage and uniform coating on both ITO electrodes and the glass substrate. Furthermore, as shown in the AFM images (Figure 2c), the IGZO film had a smooth surface morphology in the channel region with root-mean-squared (RMS) roughness of 0.45 nm. However, due to the polycrystalline nature of the ITO electrodes, the IGZO film on the ITO electrodes showed a rather rough surface with RMS roughness of 2.15 nm. Prior to the gas sensing measurement, we examined the contact property between the IGZO sensing layer and the ITO electrodes. Figure S2a shows the I-V characteristics of the IGZO sensor in dark and under UV irradiation conditions. Here, the UV intensity varied from 1 to 30 mW cm-2 and the I-V characteristics were measured after 180 s of UV irradiation. For all irradiation conditions (including the dark-state), linear and symmetrical I-V characteristics were observed, indicating a good Ohmic contact between the IGZO and ITO electrodes. In addition to the contact property, we also investigated the excess charge carrier generation by UV irradiation, since it is crucial to have a high photocurrent to achieve high sensitivity. Under dark-state, the output current (Iout) was approximately ~1.3×10-8 A at a fixed bias of 5 V (inset of Figure S2), increasing up to 1.9×10-4 A when the UV intensity increased to 30 mW cm-2. Considering the IGZO thickness of ~10 nm, the corresponding channel resistance values were 4.12×102 Ω-cm in dark conditions and 2.82×10-2 Ω-cm at 30 mW cm-2, representing about four orders of magnitude decrease by the UV irradiation. These results showed that a single UV LED chip can induce a


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sufficiently high photocurrent in the IGZO film for high gas sensitivity at room temperature. In fact, the photocurrent generation was largely dependent on the sensing material. Figure 3a compares the photocurrents generated in ZnO and IGZO films by UV illumination (30 mW cm2

). Here, the ZnO and IGZO films were coated on Si/SiO2 wafers using a solution process and

deep UV photochemical activation, and aluminum as electrodes (sensing channel width and length were 1000 µm and 60 µm, respectively). Although ZnO and IGZO have similar band gaps, the IGZO film exhibited much higher photocurrent (~2.2×10-6 A) compared to ZnO (~3.0×10-8 A), while the photo-to-dark current ratio (Iph/Idark) of the IGZO film was comparably higher than that of the ZnO film. 3.2 UV-activated Gas Sensing. The overall current response behavior of an UV-activated IGZO sensor (glass-ITO-IGZO structure) is shown in Figure 3b when exposed to acetone gas with concentration of 750 ppm. Initially, clean dry air was flowed into the chamber for purging (60 s). Next, to activate the IGZO sensor, a continuous UV was irradiated onto the sensor with an intensity of 30 mW cm-2. Upon UV irradiation, the current level increased due to the photogenerated charge carriers in the IGZO film and nearly saturated after 600 s of irradiation. Afterwards, acetone gas was introduced into the test chamber for 120 s, and then, the sensor was again purged with clean dry air and after 240 s the UV was turned off. During the entire measurement, temperature and humidity variations inside the chamber were monitored. The underlying mechanism for the UV-activated gas sensing of metal oxide semiconductors has been well described in previous reports.15,32-33 Basically, the change in electrical conductivity can be explained by the well-established adsorption-desorption model,33 in which the surface reaction between the gas molecules and the surface-adsorbed oxygen molecules is attributed to the change in electrical conductivity. In air ambient, oxygen gas molecules are physically


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adsorbed on the IGZO surface, becoming tightly bonded or chemically adsorbed onto the surface.34-35 These oxygen gas molecules then trap electrons from the IGZO film, forming a space charge region and slightly decreasing electrical conductivity of bulk film. When a reducing gas is introduced (e.g., acetone gas) at room temperature and in dark condition, the sensor showed no specific response to acetone gas as shown in Figure 3b, due to low electrical conductivity of the IGZO film and high activation energy of the chemisorbed oxygen species.35 Upon UV irradiation, a substantial number of electron-hole pairs (EHPs) are generated in the IGZO film (reaction 1), increasing its conductivity. Particularly, the output current increased more than three orders of magnitude after the UV irradiation (Figure 3b and Figure S3). In addition, the UV irradiation can induce photocatalytic adsorption of oxygen molecules at the IGZO surface as described by reaction 2.36 hv → h+ + e− O2( gas ) + e − → O2− ( hv )

(1) (2)

The adsorption of oxygen molecules and trapping of electrons at the surface form a space charge region and upward band bending with a built-in energy barrier of qVb, where q is the unit charge and Vb is the built-in potential, occur in the IGZO surface as modeled in the left panel of Figure 3c. It was claimed that compared to chemisorbed oxygen species, which are tightly bonded to the IGZO surface, the photo-catalytically adsorbed oxygen species are rather weakly bonded to surface,37 providing more active surface to target gas molecules. When the acetone gas is introduced to the test gas chamber, acetone molecules can react with the surface adsorbed oxygen species and release electrons to the IGZO film by the following reaction38:

CH3COCH3 + 4O2−( ads) → 3CO2 + 3H 2O + 4e−

(3)


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Therefore, upon exposure to acetone gas, a large amount of oxygen molecules are desorbed from the IGZO surface which induced a great thinning of space charge layer as depicted in the right panel of Figure 3c leading to high increase of carrier concentration and conductivity of an IGZO film. With a continuous UV illumination, the IGZO film is more active to acetone gas and has higher electrical conductivity compared to that in dark conditions, enhancing the sensitivity of the IGZO sensor and allowing outstanding room temperature operation. 3.3 Effects of UV Intensity on Sensing Properties. To elucidate the effects of UV intensity on gas sensing properties, we analyzed the gas response characteristics by varying the UV intensity from 1 to 30 mW cm-2 (acetone gas concentration was fixed at 750 ppm). Figure 4a shows the output current change (∆I) of an IGZO sensor when exposed to acetone gas under different UV intensities (initial photo-stabilization period not shown). Here, the ∆I was defined as ∆I = Ig – I0, where I0 and Ig were the output currents before and after introducing the target gas, respectively (shown in Figure 3b). As shown here, the gas response characteristics of an IGZO sensor were largely influenced by the UV intensity. Particularly, in dark conditions, the IGZO sensor exhibited a negligible response to acetone gas (∆I ~0.02 µA). However, with UV irradiation, the ∆I could be increased up to ~11 µA and ~96 µA at UV intensities of 1 mW cm-2 and 30 mW cm-2, respectively. For a more quantitative analysis of the sensing performances, we obtained the dependencies of the maximum output current change, ∆Imax, and the sensitivity, S = (∆Imax/I0)×100%, as shown in Figure 4b. As illustrated, both the ∆Imax and the sensitivity exhibited a linear relationship with the UV intensity. In particular, the ∆Imax and S were ~11 µA and 13%, respectively, at 1 mW/cm2, and ~96 µA and 37%, respectively, at 30 mW cm-2. This monotonous increase of ∆Imax and S can be understood by the following reasoning. First, by increasing the UV intensity, the number of photo-generated charge carriers increased within the


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IGZO film, contributing to an enhanced conductivity of the IGZO film. At the same time, due to photocatalytic oxidation, the number of weakly bonded oxygen species on the IGZO surface increased, enhancing the reaction with the acetone gas molecules. Therefore, under higher UV intensity, higher gas sensitivity could be obtained, due to the enhanced surface reaction and high conductivity of the IGZO film. In addition, it should be noted that the maximum UV intensity was set at 30 mW cm-2 in our study due to limited UV power available from a single LED chip. However, we believe this UV intensity and power level is sufficiently high for inducing a large photocurrent and sensitivity in IGZO sensors. Figure 4c shows the response time (T90; time required for the current to reach 90% of the saturation value, Ig,max) and recovery characteristics (%Recovery; the current decay ratio after 240 s of exposure) of an IGZO sensor under different UV intensities. Higher intensity allowed rapid gas detection, as well as a fast recovery. Particularly, the T90 decreased from 80 s to 37 s when the UV intensity was increased from 1 to 30 mW cm-2. Also, the %Recovery increased from 10% to 53% under the same intensity variation. The fast recovery observed at high UV intensity can be attributed to the increased number of photo-generated electrons, which induced a higher re-adsorption rate of oxygen molecules on the IGZO surface. Furthermore, temperature and relative humidity (%RH) changes inside the testing chamber may affect the sensing current. However, we observed that temperature and humidity variations were relatively small during the measurement as shown in Figure 4d and Figure S4. Also, from a controlled experiment by introducing dry air into the sensor chamber without any analyte, a relatively small variation of current was observed (∆I ~ 0.7 µA, UV intensity of 30 mW cm-2) compared to that when an analyte gas such as acetone was introduced (Figure S5).


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To investigate the concentration-dependent sensing properties of an IGZO sensor, we plotted the maximum output current change (∆Imax) at different acetone gas concentrations. Figure 5a shows the ∆Imax vs. acetone gas concentration in a range of 50–1000 ppm, observing that the ∆Imax showed linear dependency on the acetone gas concentration with a slight slope change at around ~500 ppm. At low gas concentrations, the coverage of acetone gas molecules on the IGZO surface is low, therefore, an efficient surface reaction between the acetone gas molecules and the surface-adsorbed oxygen molecules can be possible (corresponding to a slope of 0.145 as shown in Figure 5a). However, as the acetone concentration increased, above 500 ppm, the surface coverage of acetone gas molecules on the IGZO surface became high, which may have led to a reduction in the surface reaction (corresponding to a slope of 0.037). 3.4 Gas Sensor Stability and Selectivity. Figure 5b shows the response characteristics of an IGZO sensor under repeated exposure to acetone gas (100 ppm) at an operating UV intensity of 30 mW cm-2. The response current variation of the sensor was reproducible for repeated testing cycles, showing that the initial and amplitude responses of the first cycle had no obvious change compared to other test cycles. For 100 ppm acetone gas, the maximum sensitivity and current change reached ~14% and ~35 µA, respectively. These results exhibit that the IGZO sensor had a good response and reproducibility. Furthermore, the long-term stability of the IGZO sensor was also evaluated which is one of the most significant parameters for the practical application of sensor devices. For the long-term stability test, the IGZO sensor sample was kept under an openair ambient condition over a period of 70 days and the gas sensing properties were measured at specific time intervals (acetone gas concentrations of 100 ppm and 750 ppm). As shown in Figure 5c, the sensitivity of the IGZO sensor decreased only ~6% after 70 days in comparison to the original performance. In addition, the I0 value was also stable up to 70 days showing a high


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response to UV illumination (Figure 5c). These finding suggest the reliable long-term stability of the amorphous IGZO materials for room temperature gas sensor operations. Figure 5d represents the gas sensitivity of an IGZO sensor exposed to various VOCs gases including: methanol, ethanol, isopropanol, acetone, toluene, and chlorobenzene. Here, the gas concentrations were fixed at 100 ppm, while the UV intensity was set at 30 mW cm-2. As shown in the graph, the IGZO sensor exhibited much higher sensitivity to methanol and ethanol gases compared to chlorobenzene gas. In particular, the sensitivity ratio of methanol-to-chlorobenzene was 43.2. Although further investigation is needed, the IGZO sensors appear to be capable of performing a reasonable selectivity between a group of alcohols (methanol and ethanol) and a group of organic solvents (toluene and chlorobenzene).

4. CONCLUSIONS In this study, we investigated room-temperature-operable gas sensors using a solutionprocessed amorphous IGZO film. We found that the gas sensitivity, response time, and recovery rate of the IGZO sensors were largely enhanced by irradiating high intensity UV light onto the sensor. The enhanced gas sensitivity under UV irradiation was explained by the enhanced conductivity of the IGZO film, as well as by adsorption of weakly bonded oxygen species on the IGZO surface by photo-catalytic oxidation. Based on these results, we believe that amorphous IGZO-based sensors are a good candidate for room-temperature-operable VOC sensors, and further studies should be carried out to clearly identify the roles of each metallic component on the sensing properties of an IGZO sensor and to obtain an optimal metallic component ratio for highly sensitive IGZO gas sensors.


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ASSOCIATED CONTENT Supporting Information Schematic diagram of an automated gas measurement system, I-V characteristics of the IGZO sensor and the extracted current response as a function of power intensity, the overall percentage relative humidity (%RH) and temperature inside the measurement chamber. This information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Authors *Yong-Hoon Kim ([email protected]) *Sung Kyu Park ([email protected]) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource support from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20154030200870), by Basic Science Research program through the National Research


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Foundation of Korea funded by the Ministry of Science, ICT and Future Planning (NRF2014R1A4A1008474), and also supported by the Technology Innovation Program (No. 10047756, Development of tetra-pyrrole type for Color, light-emitting, detecting Devices) funded by the Ministry of Trade, industry & Energy (MI, Korea).


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Figure 1. (a) Schematic illustration of the patterning of ITO interdigitated electrodes, solution processing of the IGZO film on the electrodes, and sensing operation under UV-LED irradiation and acetone exposure; (b) optical images of a transparent IGZO sensor before and after bonding on a print circuit board.


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Figure 2. (a) Optical transmittance spectra (inset shows the Tauc plot exhibited the band gap of the IGZO film), (b) SEM images, and (c) AFM images of the IGZO film on glass and ITO substrates (scan size : 10 µm× 10 µm).


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Figure 3. (a) Comparison of the photocurrents generated by UV illumination (30 mW cm-2) in ZnO and IGZO films, in which the ZnO and IGZO films were coated on Si/SiO2 wafers using a solution process and aluminum as electrodes (sensing channel width and length were 1000 µm and 60 µm, respectively); (b) the overall response current behavior (I vs. t) of an IGZO gas sensor operated under UV intensity of 30 mW cm-2 and acetone gas concentration of 750 ppm; (c) Schematic illustrations showing the energy band blending under photo-induced oxygen ions without (left) and with (right) interaction of acetone gas molecules at IGZO surface.


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acetone gas

90

∆Ι (µΑ)

30 20 10 5 1 0

UV Intensity -2 (mW cm )

60 30 0 0

60

120

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-2

Power Intensity (mW cm )

4 6 3 o

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∆Humidity (% RH)

75

5

15

d

% Recovery

60

0

10


90

30

5

-2

Time (s)

c

∆Imax (µA)

b

4

2

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∆Temp. ( C)

120

Sensitivity (%)

a

Response Time (s)

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1

0

5

10

15

20

25

30

0

-2


Figure 4. Gas sensing properties of UV-activated IGZO gas sensors operated under different UV intensities (1 to 30 mW cm-2, with acetone gas concentration fixed at 750 ppm), (a) current change (∆I), (b) gas sensitivity and maximum current change (∆Imax), (c) response time (T90) and %Recovery; and (d) the percentage relative humidity (%RH) and temperature variations during acetone gas exposure at different UV intensities.


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b

∆Imax (µA)

100

Sensing current (µA)

a y = 0.035 x + 68.7

80 60

y = 0.141x + 19.9

40 20 0

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d

320 S (100 ppm) S (750 ppm) -2 I0 (30 mW cm )

240

0

10 20 30 40 50 60 70

Time (day)

200

Sensitivity (%)

70 60 50

I0 (µA)

c

180 360 540 720 900 1080

Time (s)

Concentration (ppm)

Sensitivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

UV Intensity : 30 mW cm-2 Gas concentration : 100 ppm

20 10 0

ne ne ne nol nol nol tha etha propa aceto tolue benze e m lo iso cho

Figure 5. (a) Maximum current change (∆Imax) variation of IGZO gas sensors as a function of acetone gas concentration (50 to 1000 ppm), (b) sensing response (output current) to a repeated exposure of acetone gas (each 100 ppm), (c) stability test data for IGZO gas sensors over a period of 70 days in which the sensitivity and I0 values were obtained for both 100 ppm and 750 ppm of acetone gas, and (d) gas sensitivity to 100 ppm of methanol, ethanol, isopropanol, acetone, toluene, and chlorobenzene gases.


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Table of Contents (TOC) graphic


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Low-Temperature Photochemically Activated Amorphous Indium

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