High-stability oxygen sensor based on amorphous zinc tin oxide thin film transistor

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High-stability oxygen sensor based on amorphous zinc tin oxide thin film transistor

Yu-Chun Chen, Ting-Chang Chang, Hung-Wei Li, Wan-Fang Chung, Chang-Pei Wu, Shih-Ching Chen, Jin Lu, Yi-Hsien Chen, and Ya-Hsiang Tai

Citation: Applied Physics Letters 100, 262908 (2012); doi: 10.1063/1.4731773 View online: http://dx.doi.org/10.1063/1.4731773

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/100/26?ver=pdfcov Published by the AIP Publishing

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High-stability oxygen sensor based on amorphous zinc tin oxide

thin film transistor

Yu-Chun Chen,1Ting-Chang Chang,1,2,a)Hung-Wei Li,3Wan-Fang Chung,4 Chang-Pei Wu,1Shih-Ching Chen,1Jin Lu,1Yi-Hsien Chen,1and Ya-Hsiang Tai3

1

Department of Physics, National Sun Yat-Sen University, Kaohsiung, Taiwan

2

Advanced Optoelectronics Technology Center, National Cheng Kung University, Taiwan

3

Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan

4

Department of Electronics Engineering & Institute of Electronics, National Chiao Tung University, Hsinchu, Taiwan

(Received 22 May 2012; accepted 12 June 2012; published online 27 June 2012)

This research presents a sol-gel derived zinc tin oxide thin film transistor (TFT) as a high-stability oxygen sensor. Due to its high sensitivity, oxygen has been traditionally regarded as having a negative influence on the electrical characteristics of zinc-based TFTs; however, TFTs can also act as an oxygen sensor. After illumination with visible light in oxygen-rich ambient, a significant increase in drain current of nearly 104 times occurs with fixed gate and drain voltages. It is expected that an optimized method of illumination can help to reset the electrical characteristics or distinguish the on/off state of this reliable oxygen sensor.VC _{2012 American Institute of Physics.}

[http://dx.doi.org/10.1063/1.4731773]

During recent years, amorphous oxide semiconductors (AOSs) have shown great progress as materials crucial to thin film transistors (TFTs).1 In particular, one of the most fruitful areas of AOSs research has focused on zinc-based semiconductors owing to potential applications involving active matrix organic light-emitting diode products or other functions in innovative future displays.2,3Compared to con-ventional polycrystalline-silicon, the zinc-based AOSs attain low cost and good uniformity over a large area deposition by sputtering or by the sol-gel method.4Although the electrical characteristics are superior to amorphous hydrogenated-silicon, the sensitivity of zinc-based TFTs to environment factors still remains a disadvantage when considered as a switching element.5,6Several studies have suggested that the varying conductivity under illumination is associated with oxygen adsorption/desorption in the backchannel of the active layer.1,7In this Letter, the unique oxygen sensitivity of sol-gel derived amorphous zinc tin oxide (a-ZTO) TFTs is utilized to develop a room-temperature-operated oxygen sen-sor. Moreover, illumination with visible light is adopted as a resetting method for the oxygen sensor, and further exam-ined by varying the wavelength of light and operation time.

The detailed fabrication procedure of passivation-free a-ZTO TFTs with a bottom-gate bottom-contact configuration, as shown in the inset of Fig. 1, has been reported previ-ously.8,10 After forming a 300-nm-thick molybdenum tung-sten gate and silicon nitride dielectric, source and drain electrodes were deposited by sputtered indium tin oxide. Pre-cursor solution for fabricating spin-coated Zn-Sn-O thin films was synthesized by dissolving zinc acetate dihydrate [Zn(OAc)22H2O] and tin chloride dihydrate (SnCl22H2O)

in monoethanolamine and 2-methoxyethanol at 60C for 3 h, respectively.8 Next, the active channel of the a-ZTO film

was deposited by spin-coating with a thickness of 80 nm at room temperature and at atmospheric pressure. The channel layer was patterned by standard photolithography and wet etching. Finally, the device was subjected to thermal anneal-ing at 350C for 1 h under ambient oxygen in a furnace. The channel width and length are 50 and 8 lm, respectively. All current-voltage characteristics were measured at room tem-perature and in a vacuum chamber with gas-flow system and probe station using an Agilent B1500 precision semicon-ductor parameter analyzer. The threshold voltage (VT) is

determined by using the constant current method as the gate-to-source voltage (VG), which induces a drain current (ID) of

1 nA. The light illumination of 5000 lux in intensity in this work was obtained by a halogen lamp whose spectrum is shown in the inset of Fig.2(b).

FIG. 1. TransferID-VGcharacteristics of a-ZTO TFTs in different oxygen

partial pressures (10 torr, 100 torr, and 760 torr) and after illumination of visible light with an intensity of 5000 lux in oxygen ambient of 760 torr, respectively. The inset shows the schematic cross-sectional view of a fabri-cated bottom-gate a-ZTO TFT.

a)_{Author to whom correspondence should be addressed. Electronic mail:}

[email protected].

0003-6951/2012/100(26)/262908/3/$30.00 100, 262908-1 VC2012 American Institute of Physics

APPLIED PHYSICS LETTERS 100, 262908 (2012)

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Figure1shows the ID-VGelectrical characteristics of

a-ZTO TFTs at drain-to-source voltage (VD) of 1 V, linear

region, in oxygen ambient pressures of 10 torr, 100 torr, and 760 torr and after illumination of visible light in oxygen am-bient of 760 torr, respectively. Clearly, the VTvariation of

a-ZTO TFTs seems to be closely correlated with the amount of surrounding oxygen molecules. As the environmental oxy-gen increases, it exhibits a VTshift about 20 V with rare

var-iations in subthreshold slope. In general, literatures have described the surrounding oxygen molecules as capturing electrons from the conduction band, then causing the adsorp-tion of oxygen ions (O, O2) on the active layer, resulting in a depletion layer in the backchannel and an increase in the VT of ZTO TFTs.

6,7,9

Interestingly, after illumination with visible light for 120 s, the VTin oxygen ambient shows

dras-tic negative shifts. This phenomenon suggests that the photo-generation of holes discharges the negatively charged adsorbed oxygen ions. Meanwhile, electrons are released into active layer as a form of O2

2 (ads)1 h 1 _{ﬁ O} 2 (solid) 1 e 2 . This result reveals the important role of oxygen, which can affect the VTand the electrical characteristics in

passivation-free devices.

The purpose of this work is to logically utilize this oxy-gen adsorption/desorption process to develop an oxyoxy-gen sen-sor. Therefore, a large positive voltage is applied on the gate trying to repel holes from the front-channel to backchannel, which can therefore be expected to discharge the chemical adsorbed oxygen ions on the active layer. Figure2(a)shows the ID-VGcharacteristics after applying 60 V on the gate for

120 s with grounded drain and source in oxygen ambient, and subsequent relaxation of 120 s with all terminals grounded in the same oxygen ambient. After applying bias or a relaxation operation, the electrical characteristics dem-onstrate a persistent positive shift, which means that more chemical adsorption of oxygen occurs due to gate bias opera-tion.10Clearly, another efficient resetting method is required. Since the existing high-density electron traps above the va-lence band maximum with a large energy range, holes cannot drift to the backchannel of a-ZTO and desorb the adsorbed oxygen, unless under illumination.4 Accordingly, a reset method is undertaken by illumination of visible light. The ID-VGcharacteristics of a-ZTO TFTs after 120 s of

illumina-tion and relaxaillumina-tion for 120 s both in oxygen-rich ambient are shown in Fig.2(b), respectively. The transfer curve exhibits an obvious negative shift due to the oxygen desorption by photogeneration holes, a shift phenomenon that still occurs after experimental repetition. This suggests that the reset and

adsorbed operation of an oxygen sensor can be reproduced by visible light illumination.

To further verify the characteristics of an oxygen sensor, time evolution of IDbefore and after illumination for 120 s in

oxygen ambient is extracted with fixed gate voltage of 8 V after repeating the oxygen adsorption/desorption cycle are shown in Fig. 3. Thus, we can differentiate the IDbetween

high- or low-current state, defined as the on- and off-state of the oxygen sensor, respectively. The ratio of IDbetween

on-state (desorption of oxygen) and off-on-state (adsorption of oxy-gen) is as high as more than 104times, which is quite impor-tant in identification of the sensor state, resulting in a reduction of erroneous judgment. Moreover, the on/off states were stable and well reproduced throughout the repeated cy-cling. It should be noted that the fluctuation of IDin the

off-state during repeating the oxygen adsorption/desorption cycle can be attributed to the noise level of electrical mea-surement. However, after illumination in oxygen ambient, the oxygen can be expected to chemisorb back on the back-channel rapidly because of quick decrease in ID after light

turning off for 120 s. This result indicates that the operating condition of sensor can have more examination.

The different operating condition of oxygen sensor are examined in more detail in Fig. 4, which depicts the extracted IDratio of the on/off states while reducing the

illu-mination and relaxation time from 300 s to 120 or 60 s with varying wavelength of light in the same photon flux. The monochromatic light, red (660 nm), green (550 nm), or blue FIG. 2. (a) TransferID-VGcharacteristics of

a-ZTO TFTs after experimental repetition of a-60-V-gate-bais for 120 s and subse-quent relaxation for 120 s in oxygen ambi-ent. (b) ID-VG characteristics of a-ZTO

TFTs after 120 s illumination time and sub-sequent relaxation for 120 s in oxygen ambient.

FIG. 3. Time dependence of drain current which is extracted with fixed gate voltage of 8 V and drain voltage at 1 V for repeating the oxygen adsorption/ desorption cycle.

262908-2 Chen et al. Appl. Phys. Lett. 100, 262908 (2012)

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(490 nm), comes from the visible light passing through opti-cal fiber and transfers through fiber cable and probe station microscope to focus the illumination on the a-ZTO TFT channel, respectively. These results demonstrate that the monochromatic blue light is still reliable to reset the oxygen sensor to on-state. A comparison of the IDratio of the on/off

states after different monochromatic illuminations for the same time indicates that the desorption rate of chemisorbed oxygen is a function of illuminated wavelength for the reset operation.11,12 Since the binding energy between adsorbed oxygen species (O, O2) and Zn-based oxide surface is

more than 2 eV, the illumination of green and blue lights are expected to enhance the formation of photogeneration holes.13

In summary, the sol-gel derived a-ZTO TFT as an oxygen sensor with efficient operation is discussed herein. High-stable on/off state ratio about more than four orders of

difference in IDat room temperature can be reproduced and

be controlled by illumination with visible light for 120 s. Consequently, the sol-gel derived a-ZTO TFTs have the advantage of acceptable electrical characteristics with low fabrication cost as a switching device and of sensitivity to surrounding oxygen molecules for use as an oxygen sensor, which can be expected to be employed in all multifunctional AOSs-based devices, known as “system-on-glass.”

This work was performed at National Science Council Core Facilities Laboratory for Science and Nano-Technology in Kaohsiung-Pingtung area. The authors would like to acknowledge the financial support of the National Science Council of the Republic of China under Contract No. NSC-100-2120-M-110-003.

1

T. Kamiya, K. Nomura, and H. Hosono, Sci. Technol. Adv. Mater.11, 044305 (2010).

2_{K. M. Kim, C. W. Kim, J.-S. Heo, H. Na, and J. E. Lee,}_{Appl. Phys. Lett.}

99, 242109 (2011).

3

T.-C. Chang, F.-Y. Jian, S.-C. Chen, and Y.-T. Tsai,Mater. Today14(12), P608 (2011).

4_{T. Kamiya, K. Nomura, and H. Hosono,}_{J. Display Technol.}_{5(7), 273}

(2009).

5

W.-F. Chung, T.-C. Chang, H.-W. Li, and C.-W. Chen, Electrochem. Solid-State Lett.14, H114 (2010).

6_{W.-F. Chung, T.-C. Chang, H.-W. Li, S.-C. Chen, and Y.-C. Chen,}_Appl. Phys. Lett.98, 152109 (2011).

7

Y. Takahashi, M. Kanamori, A. Kondoh, and H. Minoura,Jpn. J. Appl. Phys.33, 6611 (1994).

8_{S.-C. Chiang, C.-C. Yu, M.-C. Wang, and T.-C. Chang,}_{Taiwan Display}

Conference (2008), p. B106.

9

J. Bao, I. Shalish, Z. Su, R. Gurwitz, F. Capasso, X. Wang, and Z. Ren,

Nanoscale Res. Lett.6, 404 (2011).

10_{Y.-C. Chen, T.-C. Chang, H.-W. Li, S.-C. Chen, J. Lu, and W.-F. Chung,} Appl. Phys. Lett.96, 262104 (2010).

11

P. Go¨rrn, M. Lehnhardt, T. Riedl, and W. Kowalsky,Appl. Phys. Lett.91, 193504 (2007).

12_{T.-C. Chen, T.-C. Chang, T.-Y. Hsieh, S.-C. Chen, C.-S. Lin, and M.-C.}

Hung,Appl. Phys. Lett.97, 192103 (2010).

13

J. Lagowski, E. S. Sproles, and H. C. Gatos, J. Appl. Phys. 48, 3566 (1977).

FIG. 4. Variation of extracted IDratio of on/off state as a function of reset

and adsorption time with different wavelengths (k¼ 660, 550, and 490 nm) of visible light.

262908-3 Chen et al. Appl. Phys. Lett. 100, 262908 (2012)