IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 62, NO. 4, APRIL 2015
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Improvement in Field-Effect Mobility of Indium Zinc Oxide Transistor by Titanium Metal Reaction Method Hyuk Ji, Ah Young Hwang, Chang Kyu Lee, Pil Sang Yun, Jong Uk Bae, Kwon-Shik Park, and Jae Kyeong Jeong
Abstract— This paper examined the effects of postdeposition annealing on the electrical properties of titanium-capped (TC) indium–zinc oxide (IZO) films and their IZO thin-film transistors. The TC IZO transistor oxidized at the temperature of 300 °C exhibited a high field-effect mobility of 61.0 cm2 /Vs, low subthreshold gate swing of 110 mV/decade, Vth of −0.4 V, and high ION/ OFF ratio of 2.3 × 108 . In addition, the positive gate bias stress-induced stability of the TC IZO transistor was better than that of the control device without metal capping treatment. This was attributed to the scavenging effect of the loosely bonded oxygen species in the IZO semiconductor by titanium thermal oxidation. Index Terms— Indium zinc oxide (IZO), metal capping, mobility, oxygen-related defect, thin-film transistors (TFTs).
I. I NTRODUCTION
Z
INC-OXIDE-BASED oxide thin-film transistors (TFTs) have attracted considerable attention because of their intriguing properties, such as high mobility (>10 cm2 /Vs), low-temperature processing, excellent uniformity, and good transparency to visible light [1], [2]. Extensive research efforts, such as semiconducting metal oxide composition, integration processes, and electrical reliability, have enabled the successful implementation of IGZO TFTs to commercial products, such as high-resolution 4.5-in mobile liquid crystal display [3] and 55-in organic light-emitting diode televisions [4]. Despite this, the field-effect mobility of the oxide TFTs needs to be improved up to 80 cm2 /Vs to compete with low-temperature polycrystalline silicon TFTs. For this purpose, interesting approaches for high mobility have been introduced including Manuscript received December 14, 2014; revised February 4, 2015; accepted February 18, 2015. Date of publication March 6, 2015; date of current version March 20, 2015. This work was supported in part by the Industrial Strategic Technology Development Program through the Ministry of Knowledge Economy/Korea Evaluation Institute of Industrial Technology under Grant 10041041 and in part by the National Research Foundation of Korea within the Korean Government through the Ministry of Environment, Science and Technology under Grant 2012 R1A2A2A0 2005854. The review of this paper was arranged by Editor H. Shang. (Corresponding author: Jae Kyeong Jeong.) H. Ji, A. Y. Hwang, C. K. Lee, and J. K. Jeong are with the Department of Materials Science and Engineering, Inha University, Incheon 402-751, Korea (e-mail:
[email protected]). P. S. Yun, J. U. Bae, and K.-S. Park is with the Research and Development Center, LG Display Company, Seoul 150-721, Korea. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TED.2015.2406331
In–Zn–Sn–O [5], [6] and double-channel structure [7], [8]. These approaches are based on an enhancement of the efficient intercalation of the ns orbital of In3+ or Sn4+ cation. As a different approach, Zan et al. [9] reported an exceptional high mobility of ∼100 cm2 /Vs by capping the Ca/Al layer on IGZO channel layer with subsequent stabilization in air. In particular, this metal capping method did not compromise any of the other important parameters, such as the OFF-state drain current (IOFF ) and subthreshold gate swing (SS) factor. However, it is accepted that the thermal oxidation of metal film, such as Al or titanium (Ti) on the IGZO semiconductor caused the oxygen deficiency of underlying IGZO surface layer, leading to the highly conductive film [10]–[12]. Indeed, this kind of chemical reaction was used to obtain the metallic access region in the self-aligned IGZO TFTs [10], [11]. In that case, the effective channel length of the resulting oxide TFTs would be drastically reduced by the length of metal capping layer, which can result in the misleading overestimation of the field-effect mobility. Furthermore, the IGZO TFTs fabricated in the previous report was stabilized after 40 days in air ambient [9]. Therefore, a processing route to the fast stabilization of metal capped device is needed. In addition, an extension of this concept to other metal oxide composition channel layers should be investigated. This paper examined the influence of postdeposition annealing (PDA) on the performance of Ti-capped (TC) oxide TFTs, where the sputtered indium–zinc oxide (IZO) film after PDA was used as the channel layer. The reason for choosing the Ti instead of reactive Ca as the capping layer is that the Ti film is available in the production line of the present TFT backplane [13], [14]. The field-effect mobility of TC IZO TFTs was enhanced from 35.8 cm2 /Vs (control device) to 61.0 cm2 /Vs without any accompanying loss in the other device parameters. This improvement was discussed based on the gathering effect of loosely bonded oxygen species in the IZO channel layer by the oxidation of the Ti film. II. E XPERIMENTAL P ROCEDURE A heavily doped p-type Si wafer and 100-nm-thick thermal SiO2 were used as the bottom-gate electrode and gate insulator, respectively. The IZO channel layer was formed on an SiO2 /Si substrate by dc magnetron sputtering, which was patterned through a shadow mask. The working pressure
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Fig. 1. Variations of the (a) free electron density (Ne ) and (b) Hall mobility (μHall ) for the TC IZO films oxidized at different PDA temperatures.
was 2 mtorr and the relative O2 flow rate of [O2 ]/(Ar + O2 ) was maintained at 0.1. The dc power was fixed to 100 W and the channel thickness was 35 nm. The ITO films, as the source/drain electrode, were deposited on the IZO/SiO2 /Si by dc magnetron sputtering. During the sputtering of the ITO (90 wt.% In), the dc power was fixed to 50 W and the working pressure was 5 mtorr under an Ar atmosphere. The fabricated IZO TFTs were annealed at 250 °C for 1 h in air ambient (referred to herein as control device, which has no metal capping layer). The fabricated TFTs had a bottom gate structure and their channel width (W ) and length (L) were 1500 and 300 μm, respectively. The Ti capping layer (CL) was sputtered selectively on the IZO channel region through a shadow mask with dimension of W/L = 2300/150 μm. The physical thickness of the Ti film was split into 5, 10, 20, and 40 nm. The PDA was performed at different temperatures ranging from 200 °C to 400 °C for 1 h in air. The transfer characteristics of the IZO TFTs were measured at room temperature using a Keithley 2636 source meter. III. R ESULTS AND D ISCUSSION The effect of PDA temperature on the electrical properties of the TC IZO films was examined, where the thickness of Ti films was fixed to 40 nm. Fig. 1 shows the variations of the free electron density (Ne ) and the Hall mobility (μHall ) for the TC IZO films oxidized at different PDA temperatures. The Hall-effect measurements for the IZO and TC IZO films were carried out using the van der Pauw configuration. In the case of TC IZO films, the square Ti film with a dimension of 8 mm × 8 mm was deposited on the IZO/SiO2 /Si substrate (10 mm × 10 mm), which was patterned through a shadow mask. After the PDA process, the sputtered ITO electrode with a diameter of 150 μm was formed at the corners of the IZO/SiO2 /Si substrate, which was followed by the contact annealing at 200 °C for 1 h in air. The Ne and μHall values of the TC IZO films increased with the increasing PDA temperatures. The simultaneous increase in Ne and μHall values can be explained by the well-known percolation conduction mechanic [1]. The Ne value (6.8 × 1017 cm−3 ) of the TC IZO films at the PDA temperature of ≤300 °C is still acceptable in use for the channel region. It can be understandable that the thermal oxidation of Ti during PDA process will reduce the underlying IZO film, leading to the creation of oxygen vacancy (VO ) defect and free electron
Fig. 2. Representative transfer characteristics of the (a) control and TC IZO TFTs at the PDA temperatures of (b) 200 °C, (c) 300 °C, and (d) 400 °C. Output characteristics of (e) control and (f) TC IZO TFTs at 300 °C.
carriers because Ti has a stronger oxidation power in air than In2 O3 and ZnO. The Gibbs free energies of formation (G f ) for In2 O3 , ZnO, and TiO2 are −506.2, −599.6, and −845.9 kJ/mol, respectively, at the 250 °C [15]. However, the lower PDA temperature (≤300 °C) in a given process time will kinetically hinder the reduction reaction because the breaking of cations (In- or Zn)-to-oxygen bonding requires a high activation energy (>1 eV). In this case, the weak bond oxygen species such as interstitial oxygen in the IZO film will preferentially be eliminated and then consumed in the form of TiO2 . The removal of the weak bond oxygen, which is a carrier scattering center, can result in the enhanced Hall mobility, as shown in Fig. 1(b): the Hall mobility of the TC IZO film at 300 °C had the high mobility of 78.8 ± 12.2 cm2 /Vs. The modest change in Ne and the Hall mobility of the TC IZO film at 200 °C can be also explained by this rationale. On the other hand, the high PDA temperature of 400 °C caused the metallic conducting state (Ne = 1.1 × 1019 cm−3 ) of TC IZO film, which cannot be used as the channel layer. Fig. 2 shows the transfer characteristics of the control and TC IZO TFTs, respectively. The field-effect mobility (μFE ) was determined by the maximum transconductance at a drain voltage of VDS of 0.1 V. The threshold voltage (Vth ) was determined by the gate voltage VGS , which induces a drain current of L/W × 10 nA at VDS of 5.1 V. The SS factor (SS = d VGS /dlogIDS [V/decade]) was extracted from the average inverse slope (in the IDS range from 10−10 to 10−8 A) in the linear part of the log(IDS ) versus VGS plot. The control device exhibited good performance: μFE , SS, Vth , and ION/OFF ratio of 35.8 cm2 /Vs, 180 mV/decade, −0.3 V, and 1.6 × 108, respectively, as shown in Fig. 2(a). The transporting properties were improved significantly for the TC IZO TFTs with an increasing PDA temperature of up to 300 °C. The μFE and SS values for the TC IZO TFTs at the PDA temperature of 300 °C were enhanced to 61.0 cm2 /Vs and 110 mV/decade, respectively, without any deterioration in the ION/OFF ratio (2.3×108 ) [Fig. 2(c)]. The superior transporting property of this TC IZO device compared with that of the control
JI et al.: IMPROVEMENT IN FIELD-EFFECT MOBILITY OF IZO
Fig. 3. Schematic cross-sectional structure of the (a) control and (b) TC IZO TFTs at the PDA temperature of 300 °C. During Ar plasma treatment, the exposed IZO channel region became a metallic state. Transfer characteristics of (c) control and (d) TC IZO TFTs before and after the Ar plasma treatment.
device [Fig. 2(e)] was reflected in the excellent output characteristics [Fig. 2(f)]. On the other hand, the TC IZO TFTs at PDA temperature of 400 °C severely suffered from the high OFF state IDS and low I ON/ OFF ratio, which stems from the high Ne value (1.1 × 1019 cm−3 ) of the TC IZO film. To double check the conductivity of the TC IZO layer, we investigated the effect of the Ar plasma treatment on the transfer characteristics of the control and Ti (40 nm)-capped IZO TFTs at the PDA temperature of 300 °C, as shown in Fig. 3(a) and (b). The Ar plasma treatment was performed for 60 s under the following condition: the Ar flow rate of 10 sccm, working pressure of 5 mtorr, and the plasma power of 25 W. In the case of the control IZO device, the transfer characteristics showed the simple metallic behavior, as shown in Fig. 3(c). Because the Ar treatment on the exposed IZO channel region results in the high free electron density due to the creation of the oxygen deficiency, this result can be easily understandable [16], [17]. In contrast, the TC IZO device still exhibited the normal transistor operation [Fig. 3(d)]. This result indicates that the high mobility for the TC IZO TFTs indeed comes from the reduction of the tailing trap state in the TC IZO region. The chemical states and microstructure of the TC IZO stack were characterized by X-ray photoelectron (XP) spectroscopy and transmission electron microscopy (TEM). Fig. 4(a) and (b) shows the Ti 2p and O 1s XP spectra of the TiOx film in the TiOx /IZO stack formed at the PDA temperature of 300 °C. The binding energy difference between the O 1s (530.1 eV) and Ti 2p3/2 (458.5 eV) peaks was ∼71.6 eV, indicating that the Ti film had been converted thermally to a TiO2 film (71.2–71.6 eV) rather than Ti2 O3 film (72.9 eV) [18], [19]. Ionic bonding between Ti and O was also confirmed by the metal-oxygen lattice peak (∼530.1 eV) in the O 1s XP spectra. Fig. 4(c) and (d) shows the O 1s XP spectra of the center IZO regions in the control IZO and 300 °C-annealed TiO2 /IZO stack,
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Fig. 4. (a) Ti 2p and (b) O 1s XP spectra of the TiO2 film. O 1s XP spectra of (c) control IZO film and (d) TC IZO film at the PDA temperature of 300 °C.
Fig. 5. (a) Cross-sectional TEM image of the TiO2 /IZO/SiO2 stack structure after PDA at 300 °C. Selective area diffraction patterns of (b) TiO2 and (c) IZO channel region.
respectively, which were obtained from depth profiling x-ray photoelectron spectroscopy analysis. The subpeaks at 530.9 ± 0.3 and 531.9 ± 0.3 eV were assigned to the oxygen-deficient bond and hydroxyl-group-related bond, respectively. The lattice oxygen-related portion of the TC IZO film increased from 80.3% (control IZO film) to 92.3%, whereas the oxygen-deficient and hydroxyl-group-related portions were diminished from 15.5% and 4.2% (control IZO film) to 5.1% and 2.6%, respectively. Fig. 5(a) shows a cross-sectional TEM image of the TiO2 /IZO/SiO2 stack structure at the PDA temperature of 300 °C. The interface between TiO2 and IZO was atomically smooth without segregation or secondary inclusion. The TiO2 film was partially crystallized during PDA at 300 °C, whereas the IZO channel was still in an amorphous phase, as shown in selective area diffraction patterns of Fig. 5(b) and (c). From the XP spectra and TEM analysis, the PDA process for the TC IZO stack caused the conversion of a Ti film to a partially crystalline TiO2 state. The loosely bonded oxygen species in the IZO channel would be consumed
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Fig. 6. Ti CL thickness-dependent transfer characteristics of the control and TC IZO TFTs. TABLE I VARIATIONS OF M OBILITY, SS, Vth , AND ION /OFF VALUES FOR THE VARIOUS IZO AND TC IZO TFTs
during the thermal conversion of TiO2 . As a result, the metal-lattice oxygen peak grew and the OH impurity-related peak intensity decreased in the TC IZO channel region. Indeed, it was recently calculated that some loosely bonded oxygen species can exist in the form of an OH impurity in metal–oxide–semiconductor, which is responsible for the gate bias instability of the oxide TFTs [20]. Because these oxygen interstitial defects can act as acceptor-like trap states, they will impede the carrier transport along the channel direction and reduce the carrier mobility in the field-effect device. Therefore, the improvement in the carrier mobility for the TC IZO device can be attributed to the elimination of defects centers such as interstitial oxygen via the Ti thermal oxidation. The thickness effect of the Ti CL on the enhancement of mobility was examined, as shown in Fig. 6. As the Ti CL thickness increased from 5 to 40 nm, the mobility of the resulting TFTs increased monotonously from 36.0 to 61.0 cm2 /Vs. This can be understandable because the thicker Ti film will have the strong gathering power of the interstitial oxygen species in the amorphous IZO channel region. Interestingly, the SS, Vth and ION/OFF values of the TC IZO TFTs were barely affected by the PDA process, as summarized in Table I. This paper investigated the positive gate bias stress (PBS) instability of the IZO TFTs. The devices were stressed under the following conditions: the VGS value of +20 V was applied, and the VDS was fixed to 5.1 V for 3600 s. The control device suffered from the positive Vth shift of ∼3.3 V during the application of PBS [Fig. 7(a)]. In contrast, the TC IZO TFTs exhibited a smaller Vth shift of ∼1.1 V under
Fig. 7. PBS instability of the (a) control and (b) 40-nm-thick Ti CL IZO device. CCS instability of the (c) control and (d) 40-nm-thick Ti CL IZO device.
identical PBS conditions [Fig. 7(b)]. The driving transistor for a given frame is stressed under the constant drain current in the operation of an active-matrix organic light-emitting diode (AMOLED) panel. Therefore, the constant drain current stress (CCS) instability of IZO TFTs was also compared, as shown in Fig. 7(c) and (d). IDS was set to 100 μA and VDS was fixed to 5.1 V for 3600 s. Because the typical IDS of ∼1 μA is needed to embody the full white color in the AMOLED devices, the applied CCS of 100 μA corresponds to very severe test conditions [21]. The superior stability of the TC IZO TFTs was also confirmed: the Vth shifts of the control and the TC IZO TFTs were 3.1 and 0.98 V, respectively. The better electrical stability including the PBS and CCS of the TC IZO device can be explained based on the carrier trapping model. The trapping event of accumulated free electron carriers under PBS or CCS conditions, which is responsible for the positive Vth shift, will be proportional to the summation of the overall acceptor-like trap states [22]. It is interesting to discuss the role of excessive oxygen species on the bias thermal stress (BTS) instability of the IZO TFTs. Although the effect of oxygen vacancy defect on the BTS-induced Vth instability has been intensively investigated with a viewpoint of the experimental and theoretical calculation, the studies regarding excessive oxygen or loosely bonded oxygen are limited [23]. Ide et al. [24] reported the bistability of excessive oxygen species on the performance of the IGZO TFTs. According to the recent electronic structure calculation, the interstitial oxygen (Oi ) prefers to accept the electron from the abundant hydrogen or the conduction band (in case of PBS) and can be stabilized to the isolated O2− i when the Fermi energy level (E F ) is near the conduction band edge [20]. This isolated O2− can be converted i to the oxygen dimer (O0i ) and a free electron when the quasi-E F level near the channel/dielectric interface moves downward due to the application of negative gate bias stress. This oxygen-related bistability can give rise to the BTS-dependent Vth instability. Because the oxide channel layer is prepared at the oxygen rich condition to reduce the oxygen deficiency, the excessive oxygen species can be
JI et al.: IMPROVEMENT IN FIELD-EFFECT MOBILITY OF IZO
introduced during the sputtering process. Thus, the reduction of this excessive oxygen via the metal capping reaction can result in the strengthening of the resistance to external PBS and CCS applications. The natural passivation effect (TiO2 ) of the back channel region for TC IZO devices cannot be excluded as the origin of their superior electrical stability [23]. IV. C ONCLUSION In summary, this paper examined the effect of PDA temperature on the electrical properties of the TC IZO films. The Ne and μHall values of the TC IZO films increased with increasing PDA temperature. Adequately controlling the PDA temperature (≤300 °C) allowed the TC IZO films to have the acceptable Ne (≤7 × 1017 cm−3 ) and enhanced μHall values (∼78.8 cm2 /Vs). Thus, the resulting TC IZO TFTs at the PDA temperature of 300 °C exhibited a high μFE of 61.0 cm2 /Vs, low SS of 110 mV/decade, Vth of −0.4 V, and high ION/OFF ratio of 2.3 × 108 , which were better than those of the control IZO TFTs. Furthermore, the PBS and CCS stability of the IZO TFTs were improved simultaneously by the PDA process of Ti CL. These results can be explained by the reduction of the acceptor-like trap states related to the loosely bonded oxygen in an amorphous IZO semiconductor. Therefore, the oxygen gathering technique using a Ti metal reaction can be a good approach for improving the mobility of the oxide TFTs. R EFERENCES [1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, “Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors,” Nature, vol. 432, no. 7016, pp. 488–492, Nov. 2004. [2] J. K. Jeong, “The status and perspectives of metal oxide thin-film transistors for active matrix flexible displays,” Semicond. Sci. Technol., vol. 26, no. 3, p. 034008, Feb. 2011. [3] Welcome to IGZO. [Online]. Available: http://www.sharpusa.com/ ForHome/HomeEntertainment/LCDTV/igzo.aspx, accessed Dec. 13, 2014. [4] OLED. [Online]. Available: http://en.wikipedia.org/wiki/OLED, accessed Dec. 13, 2014. [5] M. K. Ryu, S. Yang, S.-H. K. Park, C.-S. Hwang, and J. K. Jeong, “Impact of Sn/Zn ratio on the gate bias and temperature-induced instability of Zn-In-Sn-O thin film transistors,” Appl. Phys. Lett., vol. 95, no. 17, pp. 173508-1–173508-3, Oct. 2009. [6] J.-Y. Noh et al., “Cation composition effects on electronic structures of In-Sn-Zn-O amorphous semiconductors,” J. Appl. Phys., vol. 113, no. 18, pp. 183706-1–183706-7, May 2013. [7] H.-S. Kim et al., “Density of states-based design of metal oxide thinfilm transistors for high mobility and superior photostability,” ACS Appl. Mater. Inter., vol. 4, no. 10, pp. 5416–5421, Sep. 2012. [8] H. Y. Jung et al., “Origin of the improved mobility and photo-bias stability in a double-channel metal oxide transistor,” Sci. Rep., vol. 4, Jan. 2014, Art. ID 3765. [9] H.-W. Zan, C.-C. Yeh, H.-F. Meng, C.-C. Tsai, and L.-H. Chen, “Achieving high field-effect mobility in amorphous indium-gallium-zinc oxide by capping a strong reduction layer,” Adv. Mater., vol. 24, no. 26, pp. 3509–3514, Jul. 2012. [10] N. Morosawa, Y. Ohshima, M. Morooka, T. Ara, and T. Sasaoka, “A novel self-aligned top-gate oxide TFT for AM-OLED displays,” in SID Symp. Dig. Tech. Papers, Jun. 2011, vol. 42, no. 1, pp. 479–482. [11] N. Morosawa, Y. Ohshima, M. Morooka, T. Ara, and T. Sasaoka, “Selfaligned top-gate oxide thin-film transistor formed by aluminum reaction method,” Jpn. J. Appl. Phys., vol. 50, no. 9R, pp. 096502-1–096502-4, Sep. 2011. [12] K.-H. Choi and H.-K. Kim, “Correlation between Ti source/drain contact and performance of InGaZnO-based thin film transistors,” Appl. Phys. Lett., vol. 102, no. 5, pp. 052103-1–052103-5, Feb. 2013. [13] H.-S. Seo et al., “Development of highly stable a-IGZO TFT with TiOx as a passivation layer for active-matrix display,” in SID Symp. Dig. Tech. Papers, May 2010, vol. 41, no. 1, pp. 1132–1135.
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[14] C.-K. Lee et al., “Suppression in negative bias illumination stress instability of zinc tin oxide transistor by insertion of thermal TiOx films,” IEEE Electron Device Lett., vol. 34, no. 2, pp. 253–255, Feb. 2013. [15] P. S. Yun and J. Koike, “Metal reaction doping and ohmic contact with Cu–Mn electrode on amorphous In–Ga–Zn–O semiconductor,” J. Electrochem. Soc., vol. 150, no. 10, pp. H1023–H1040, Aug. 2011. [16] J.-S. Park, J. K. Jeong, Y.-G. Mo, H. D. Kim, and S.-I. Kim, “Improvements in the device characteristics of amorphous indium gallium zinc oxide thin-film transistors by Ar plasma treatment,” Appl. Phys. Lett., vol. 90, no. 26, pp. 262106-1–262106-3, Jun. 2007. [17] B. D. Ahn, H. S. Shin, H. J. Kim, J.-S. Park, and J. K. Jeong, “Comparison of the effects of Ar and H2 plasmas on the performance of homojunctioned amorphous indium gallium zinc oxide thin film transistors,” Appl. Phys. Lett., vol. 93, no. 20, pp. 203506-1–203506-3, Nov. 2008. [18] R. Sanjinés, H. Tang, H. Berger, F. Gozzo, G. Margaritondo, and F. Lévy, “Electronic structure of anatase TiO2 oxide,” J. Appl. Phys., vol. 75, no. 6, p. 2945, Oct. 1994. [19] Q. Guo, W. S. Oh, and D. W. Goodman, “Titanium oxide films grown on Mo(110),” Surf. Sci., vol. 437, nos. 1–2, pp. 49–60, Aug. 1999. [20] J. Robertson and Y. Guo, “Light induced instability mechanism in amorphous InGaZn oxide semiconductors,” Appl. Phys. Lett., vol. 104, no. 16, pp. 162102-1–162102-5, Apr. 2014. [21] J. K. Jeong et al., “Flexible full-color AMOLED on ultrathin metal foil,” IEEE Electron Device Lett., vol. 28, no. 5, pp. 389–391, May 2007. [22] R. B. M. Cross and M. M. De Souza, “Investigating the stability of zinc oxide thin film transistors,” Appl. Phys. Lett., vol. 89, no. 26, pp. 263513-1–263513-3, Dec. 2006. [23] J. K. Jeong, “Photo-bias instability of metal oxide thin film transistors for advanced active matrix displays,” J. Mater. Res., vol. 28, no. 16, pp. 2071–2084, Jul. 2013. [24] K. Ide, Y. Kikuchi, K. Nomura, M. Kimura, T. Kamiya, and H. Hosono, “Effects of excess oxygen on operation characteristics of amorphous In-Ga-Zn-O thin-film transistors,” Appl. Phys. Lett., vol. 99, no. 9, pp. 093507-1–093507-3, Sep. 2011.
Hyuk Ji received the B.S. degree from Chonbuk National University, Jeonju, Korea, in 2014. He is currently pursuing the M.S. degree with Inha University, Incheon, Korea. Ah Young Hwang received the B.S. degree from Inha University, Incheon, Korea, in 2013, where she is currently pursuing the M.S. degree.
Chang Kyu Lee received the B.S. degree in electronics engineering from Suwon University, Hwaseong, Korea, in 2012. He is currently pursuing the M.S. degree with Inha University, Incheon, Korea.
Pil Sang Yun received the Ph.D. degree in materials science and engineering from Tohoku University, Sendai, Japan. He is currently a Senior Researcher of oxide thin-film transistor with LG Display Company, Seoul, Korea.
Jong Uk Bae received the Ph.D. degree from the State University of New York, Buffalo, NY, USA, and the M.S. degree in electrical engineering from the University of California at Santa Barbara, Santa Barbara, CA, USA. He is currently a Leader of the oxide thin-film transistor team with LG Display Company, Seoul, Korea.
Kwon-Shik Park received the Ph.D. degree in materials science and engineering from the Korea Advanced Institute of Science and Technology, Daejeon, Korea. He is currently a Technology Fellow of oxide thin-film transistor with the LG Display Company, Seoul, Korea.
Jae Kyeong Jeong received the Ph.D. degree in material science and engineering from Seoul National University, Seoul, Korea, in 2002. He is currently an Associate Professor with the Department of Materials Science and Engineering, Inha University, Incheon, Korea.
