Semitransparent Field-Effect Transistors Based on ZnO ... - IEEE Xplore

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Mar 23, 2011 - Shi-Ming Peng, Yan-Kuin Su, Fellow, IEEE, Liang-Wen Ji, Member, IEEE, ... Wan-Chun Chao, Zong-Syun Chen, and Cheng-Zhi Wu.
IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 4, APRIL 2011

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Semitransparent Field-Effect Transistors Based on ZnO Nanowire Networks Shi-Ming Peng, Yan-Kuin Su, Fellow, IEEE, Liang-Wen Ji, Member, IEEE, Sheng-Joue Young, Chi-Nan Tsai, Wan-Chun Chao, Zong-Syun Chen, and Cheng-Zhi Wu

Abstract—This investigation demonstrates the fabrication of semitransparent field-effect transistors with self-assembling ordered ZnO nanowire (NW) networks, using a high-k HfO2 gate. The devices exhibit excellent optical transparency and transistor performance at on/off ratios of > 105 , a mobility of ∼7.59 cm2 · V−1 · s−1 , and threshold voltages of ∼4 V. Under UV illumination (3.65 eV), the devices exhibit the highest relative photoconductivity (∼2.08 × 105 ), corresponding to a photoresponsivity of 3.96 A/W at low operating voltages (VGS = 0 V and VDS = 1 V). The result suggests that the NW-based devices have low power consumption and high photosensivity when used in photodetection. Index Terms—Field-effect transistors (FETs), photoconductivity, ZnO nanowire (NW).

I. I NTRODUCTION

T

HE DEVELOPMENT of active-matrix liquid crystal displays and active matrix organic light-emitting displays is very important. Recently, the transistor channel length has been reduced, or high-k gate insulator materials (Al2 O3 , HfO2 , SiN, and Y2 O3 ) used to realize such displays that are cost effective consume little power and are highly transparent in the UV-to-midinfrared range. In particular, the wide-bandgap 1-D nanostructures are considered to be one of the strongest candidates for transistors, owing to their transparency, small Manuscript received December 25, 2010; accepted December 29, 2010. Date of publication February 14, 2011; date of current version March 23, 2011. This work was supported in part by the Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan, under projects from the Ministry of Education, the Bureau of Energy, the Ministry of Economic Affairs (No. 98-D0204-6), and the National Science Council of Taiwan (NSC 96-2221-E-006-079-MY3); by the Center for Micro/Nano Science and Technology, National Cheng Kung University; by TDPA “Lamp Development of White Light-Emitting Diode for Local Lighting” program; and by the National Science Council of Taiwan under Contracts TDPA 97-EC-17-A-07S1-105, NSC 97-2623-E-168-001-IT, and NSC-98-2221-E-150-005-MY3. The review of this letter was arranged by Editor A. Nathan. S.-M. Peng is with the Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan (e-mail: [email protected]). Y.-K. Su is with the Institute of Microelectronics, Department of Electrical Engineering, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan, and also with the Department of Electrical Engineering, Kun Shan University, Tainan 710, Taiwan (e-mail: [email protected]). L.-W. Ji, C.-N. Tsai, W.-C. Chao, Z.-S. Chen, and C.-Z. Wu are with the Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632, Taiwan (e-mail: [email protected]; 21705@yahoo. com.tw; [email protected]; [email protected]; jimy0704@ hotmail.com). S.-J. Young is with the Department of Electronic Engineering, National Formosa University, Yunlin 632, Taiwan (e-mail: [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2011.2104410

Fig. 1. Schematic of the fabricated bottom-contact-type S-FETs with ZnO NW network.

scale, and low power consumption, and increase the aperture ratio efficiency in active matrix arrays [1]. ZnO has attracted considerable interest because of its direct wide bandgap of 3.37 eV, high transparency to visible light, and multifunctional building blocks for short-wavelength UVregion applications [2]–[5]. In recent years, transistors based on nanowire (NW) networks have attracted much attention [6]–[9], owing to the fact that the NW networks create multiple channel paths in the active area. Thus, electrical carriers could be transported efficiently. In particular, ZnO NW transistors have great potential to replace existing poly-Si, amorphous-Si, or organic transistors, owing to their high transparency, mobility, and effective carrier confinement in one dimension. This investigation demonstrates the fabrication of semitransparent fieldeffect transistors (S-FETs) with self-assembling ordered ZnO NW networks, using a high-k HfO2 gate, all-transparent indium tin oxide (ITO) gate, and source/drain electrodes on a glass substrate. The fabrication strategy combines conventional photolithography with a bottom–up wet chemical method. The photosensitivity of ZnO NW network S-FETs is also demonstrated. II. E XPERIMENT First, a 300-nm-thick HfO2 (εr ∼ 25) was sputtered onto the ITO to act as a gate dielectric by radio frequency (RF) magnetron sputtering method. The RF power and chamber working pressure of mixed Ar/O2 = 18/2 are maintained to 100 W and ∼2 × 10−2 torr, respectively. The ZnO nucleus (200 nm) was covered with a layer of ITO and defined by photolithography. Drain–source electrodes of transparent ITO (120 nm) are deposited on ZnO film by RF-magnetron sputtering. Finally, ZnO NW networks were synthesized by hydrothermal decomposition method. The details of the seed layer and ZnO NW growth processes were reported by the authors in a previous study [10]. The channel width and length are 40 and 8 μm, respectively. A schematic representation of the bottom-contact-type S-FETs with NW network channel layer is shown in Fig. 1. Spectral responsivity measurements were

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IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 4, APRIL 2011

Fig. 2. (a) FE-SEM image with 45◦ tilt angle for the ZnO NW network S-FETs. (b) Transmission spectrum of the devices with the glass substrate. Average transmittance of the device is over 60%. The inset of (b) shows the photograph of a glass substrate with devices on top of a color image.

Fig. 3. (a) IDS –VDS characteristics with various gate bias voltages. (b) Transfer and gate leakage current characteristics of devices. (c) Transfer characteristics (VDS = 1 V) for constant power density with varying light wavelengths. (d) Relative photoconductivity and photoresponsivity relative to the energy of incident photons at a constant VDS = 1 V and VGS = 0 V (depletion state).

obtained using the TRAIX 180 system with a 300-W xenon arc lamp light source and standard synchronous detection scheme. An HP-4156C semiconductor parameter analyzer was used to measure the current–voltage characteristics of the proposed ZnO devices. III. R ESULTS AND D ISCUSSION Fig. 2(a) shows the FE-SEM images of the ZnO NW network S-FETs. The laterally aligned ZnO NW networks were selectively grown across the gap between the drain and source electrodes. The mutually parallel NWs ensured multiple transmission routes and increased the drain currents. Fig. 2(b) shows the optical transmission spectra of the entire devices in the wavelength range of 300–800 nm. The average optical transmission of the entire network S-FETs structure in the visible range (400–700 nm) of the spectrum is around 64.58%, while for 550 nm (to which wavelength human eyes are most sensitive), it is 67.7%. The inset in Fig. 2(b) shows devices on a glass substrate. The color image is hardly noticeable to human eyes, indicating the high transparency of the devices. Fig. 3(a) shows the typical output characteristics of the devices with gate voltage (VG ) from 0 to 6 V. The devices ex-

hibit a behavior associated with n-channel enhancement mode, because more electrons were produced as positive voltage is applied on the gate. These electrons then appeared at the NW-network–dielectric interface, increasing current through the channel. Fig. 3(b) shows the transfer curve of the devices with a constant drain–source voltage VDS of 1 V. They exhibit the characteristics of an enhancement mode with a threshold voltage (Vt ) at ∼4 V. The maximum gate leakage current as a function of VG is lower than 0.36 nA, indicating that the gate leakage current can be suppressed sufficiently by the high-k HfO2 dielectric. The current on/off ratio was approximately 1.69 × 105 , and the subthreshold swing (SS) of SS = (dVgs )/[d(log Ids )] was about 300 mV/decade. The effective field-effect mobility (μFE ) can also be calculated using the MOSFET model    dIDS Leff −1 (1) μFE = Ci−1 VDS dVg Weff where Leff and Weff are the effective length and width, respectively. Ci is the capacitance per unit area of the gate insulator. Calculations of mobility in network devices, which were made because of the effective network channel (Weff /Leff ), are difficult to advance. Accordingly, the channel in this study was treated as a fully covered channel for simplicity. The extracted value of μFE was 7.59 cm2 · V−1 · s−1 , which was comparable to that in the dense layer of interconnected in-plane ZnO nanorod transistors [7]. Unlike previous investigations [6]–[9], this investigation has more effective modulation of the channel conductance by the applied gate voltage and low power consumption. The fabricated FET devices (15 samples) exhibit a broad range of electrical performance [threshold voltage (3.7–4.7 V), transconductance (1.5–3 μS), SS (100–700 mV/ decade), mobility (4.07–8.14 cm2 · V−1 · s−1 ), and on/off current ratio (1.15 × 104 −2.33 × 106 )] because of the variation in contact quality of the metal/NW, the dielectric/NW, and the NW/NW interfaces. The spectral responses of the devices [Fig. 3(c)] were also obtained using monochromatic photon irradiation at red (700 nm), green (543 nm), blue (435 nm), and UV (340 nm) wavelengths, respectively. In all cases, the incident optical power was around 8.92 mW/cm2 . To further characterize ZnO NW photosensitivity, the relative photoconductivity (γ) and photoresponsivity (R) of the devices can be expressed as [11], [12] γ=

=

R(A/W ) =

Iph Signal ≈ N oise IDS(dark) IDS(photo) − IDS(dark) IDS(dark)

(2)

Iph Popt

(3)

where Iph is the drain photocurrent, IDS(photo) is the drain current under photon illumination, IDS(dark) is the drain current in darkness, and Popt is the incident optical power. Fig. 3(d) shows the relative photoconductivity (γ) and photoresponsivity (R) of the devices as a function of the energy of incident

PENG et al.: SEMITRANSPARENT FIELD-EFFECT TRANSISTORS BASED ON ZnO NANOWIRE NETWORKS

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used in photodetection exhibit low power consumption and high photosensivity. IV. C ONCLUSION In summary, this investigation has demonstrated the fabrication of self-assembling ordered ZnO NW network S-FETs with a high-k HfO2 gate dielectric on a glass substrate. The devices have their functioning at low temperature, are cost effective, and have excellent optical transparency and transistor performance. The photoelectrical characteristics of the devices used in photodetection exhibit low power consumption and high photosensivity. The proposed devices are expected to have potential extensive applications in transparent electronics and flexible devices. R EFERENCES

Fig. 4. Energy band diagram illustrates the photo-induced change in the UV photoconductivity of ZnO NW network S-FETs.

photons at a constant VDS = 1 V and VGS = 0 V (depletion state). When the light [1.77 eV (700 nm), 2.27 eV (546 nm), and 2.84 eV (436 nm)] had an energy of less than the bandgap, the drain currents were one to four orders of magnitude higher than the dark current. The photo-induced photoconductivity in ZnO is more complex than the generation of electron–hole pairs across the bandgap, owing to the large surface-to-volume ratio and the charge trapping effect of impurities and vacancy states in ZnO. Fig. 4 shows the energy band diagrams of the devices. ZnO has deep-level (DL) traps close to 2–3 eV, which is caused by the intrinsic defects or oxygen vacancies in the ZnO [13]. Under photon illumination with energy of less than the bandgap (hυ < Eg ), the carriers were injected from the DL traps of NW to the conduction band, and the formed holes were trapped at the localized trap sites at the interface between the channel and dielectric, significantly affecting the transfer characteristics. Accordingly, the ZnO NW network S-FETs were more sensitive to green and blue illumination than red illumination. The results show similar effects in the previous paper [14]. When the photon energy of illumination exceeded the energy bandgap (3.65 eV, hυ > Eg ), electron–hole pairs in ZnO were created, greatly increasing the carrier density. The relative photoconductivity (Iph /I(DS)dark ) was highest when the devices were operated in the depleted state, because the IDS(dark) depletion state had almost no charge carriers. The devices exhibited their highest relative photoconductivity (∼2.08 × 105 ), corresponding to a photoresponsivity of 3.96 A/W at low operating voltages (VGS = 0 V and VDS = 1 V), indicating that ZnO NW network S-FETs

[1] S. Ju, A. Facchetti, Y. Xuan, J. Liu, F. Ishikawa, P. Ye, C. Zhou, T. J. Marks, and D. B. Janes, “Fabrication of fully transparent nanowire transistors for transparent and flexible electronics,” Nat. Nanotech., vol. 2, pp. 378–384, Jun. 2007. [2] X. M. Zhang, M. Y. Lu, Y. Zhang, L. J. Chen, and Z. L. Wang, “Fabrication of a high-brightness blue-light-emitting diode using a ZnO-nanowire array grown on p-GaN thin film,” Adv. Mater., vol. 21, pp. 1–4, 2009. [3] M. H. Huang, S. Mao, H. Feick, H. Q. Yan, Y. Y. Wu, H. Kind, E. Weber, R. Russo, and P. D. Yang, “Room-temperature ultraviolet nanowire nanolasers,” Science, vol. 292, no. 5523, pp. 1897–1899, 2001. [4] L. W. Ji, S. M. Peng, Y. K. Su, S. J. Young, C. Z. Wu, and W. B. Cheng, “Ultraviolet photodetectors based on selectively grown ZnO nanorod arrays,” Appl. Phys. Lett., vol. 94, no. 20, p. 203 106, May 2009. [5] W. Kim and K. S. Chu, “ZnO nanowire field-effect transistor as a UV photodetector; optimization for maximum sensitivity,” Phys. Status Solidi (A), vol. 206, no. 1, pp. 179–182, Jan. 2009. [6] B. Sun and H. Sirringhaus, “Solution-processed zinc oxide field-effect transistors based on self-assembly of colloidal nanorods,” Nano Lett., vol. 5, no. 12, pp. 2408–2413, Dec. 2005. [7] B. Sun, R. L. Peterson, H. Sirringhaus, and K. Mori, “Low-temperature sintering of in-plane self-assembled ZnO nanorods for solution-processed high-performance thin film transistos,” J. Phys. Chem. C, vol. 111, no. 51, pp. 18 831–18 835, Dec. 2007. [8] S. H. Ko, I. Park, H. Pan, N. Misra, M. S. Rogers, C. P. Grigoropoulos, and A. P. Pisano, “ZnO nanowire network transistor fabrication on a polymer substrate by low-temperature, all-inorganic nanoparticle solution process,” Appl. Phys. Lett., vol. 92, no. 15, p. 154 102, Apr. 2008. [9] H. E. Unalan, Y. Zhang, P. Hiralal, S. Dalal, D. Chu, G. Eda, K. B. K. Teo, M. Chhowalla, W. I. Milne, and G. A. J. Amaratung, “Zinc oxide nanowire networks for macroelectronic devices,” Appl. Phys. Lett., vol. 94, no. 16, p. 163 501, Apr. 2009. [10] L. W. Ji, S. M. Peng, J. S. Wu, W. S. Shih, C. Z. Wu, and I. T. Tang, “Effect of seed layer on the growth of well-aligned ZnO nanowires,” J. Phys. Chem. Solids, vol. 70, no. 10, pp. 1359–1362, Oct. 2009. [11] Z. M. Liao, Y. Lu, J. Xu, J. M. Zhang, and D. P. Yu, “Temperature dependence of photoconductivity and persistent photoconductivity of single ZnO nanowires,” Appl Phys A, vol. 95, no. 2, pp. 363–366, May 2009. [12] K. W. Lee, K. Y. Heo, and H. J. Kim, “Photosensitivity of solution-based indium gallium zinc oxide single-walled carbon nanotubes blend thin film transistors,” Appl. Phys. Lett., vol. 94, no. 10, p. 102 112, Mar. 2009. [13] K. H. Tam, C. K. Cheung, Y. H. Leung, A. B. Djurisic, C. C. Ling, C. D. Beling, S. Fung, W. M. Kwok, W. K. Chan, D. L. Phillips, L. Ding, and W. K. Ge, “Defects in ZnO nanorods prepared by a hydrothermal method,” J. Phys. Chem. B, vol. 110, no. 42, pp. 20 865–20 871, Oct. 2006. [14] H. S. Bae and S. Im, “Ultraviolet detecting properties of ZnO-based thin film transistors,” Thin Solid Films, vol. 469/470, pp. 75–79, Dec. 2004.