Flexible a-IGZO Thin-film Transistors Research Articles

Long-Term Aging of Al2O3 Passivated and Unpassivated Flexible a-IGZO TFTs

Amongst the new materials studied for the fabrication of high-performance flexible thin-film transistors (TFTs), amorphous indium-gallium-zinc-oxide (a-IGZO) exhibits a combination of advantages that enables its application in commercial electronics. Hence, it is crucial to understand the electrical stability of a-IGZO TFTs over long periods of time. In this work, we present the effects of long-term aging on Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> passivated and unpassivated flexible a-IGZO TFTs over a period of 80 months (≈ 6.5 years). It is found that although remaining functional, these devices are influenced by different instability effects. More specifically, positive gate bias stress experiments indicate that the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> passivation layer contributes to the degradation of the devices' performance. These results show that the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> passivation, although beneficial to the initial device stability, does not prevent the degradation of the passivated devices in comparison with their unpassivated counterparts after long periods of storage.

Double-stacked gate insulators SiOx/TaOx for flexible amorphous InGaZnO thin film transistors

The double-stacked gate insulators (DSGI), which consisted of SiOx (300 nm)/TaOx (300 nm), were used to improve the electrical performance and bias-stress stability of flexible amorphous InGaZnO (a-IGZO) thin film transistors (TFTs). Compared with the devices with single-layer gate insulators (SLGI, 600-nm-thick SiOx), the flexible a-IGZO TFTs with DSGI exhibited not only better electrical performance but also more stable properties during their positive bias-stress (PBS) tests. This was assumed to result from the fact that DSGI had both larger relative dielectric constant and better interface to a-IGZO. Especially, the flexible a-IGZO TFTs with DSGI were more prone to the bending treatment during their PBS tests. This was mostly due to the smaller bending-induced tensional stresses in the devices with DSGI, which arose from the much larger Young's modulus of TaOx films.

Improvement of contact resistance in flexible a-IGZO thin-film transistors by CF4/O2 plasma treatment

In this work, we analyze the effect of CF4/O2 plasma treatment on the contact interface between the amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) semiconductor and Titanium-Gold electrodes. First, the influence of CF4/O2 plasma treatment is evaluated using transmission line structures and compared to pure O2 and CF4 plasma, resulting in a reduction of the contact resistance RC by a factor of 24.2 compared to untreated interfaces. Subsequently, the CF4/O2 plasma treatment is integrated in the a-IGZO thin-film transistor (TFT) fabrication process flow. We achieve a reduction of the gate bias dependent RC by a factor up to 13.4, which results in an increased current drive capability. Combined with an associated channel length reduction, the effective linear field-effect mobility μlin,FE,eff is increased by up to 74.6% for the CF4/O2 plasma treated TFTs compared to untreated reference devices.

(Invited) Channel Length Dependence of Mechanical Strain for Flexible (a-IGZO) TFT

We investigated the dependence of mechanical strain upon channel length, in flexible amorphous-indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) on 20 μm thick Polyimide substrate by using different radii cylinder. The negative shift of the transfer characteristic associated with strain decreases for increasing channel length (L) and is practically suppressed in devices with L=100 μm. Amorphous oxide semiconductor (AOS) based devices are the future of transparent and flexible displays because of many advantages such as low deposition temperature and low manufacturing [1]-[2]. Recently, flexible a-IGZO TFTs on plastic substrates have been enthusiastically studied because they have the advantage of being thinner, lighter, bendable, rollable, and unbreakable [3]-[4]. Fig. 1 (a)-(c) shows the effects of vertical mechanical bending on the electrical performance of the flexible TFTs up to 0.80% strain having the TFT channel width (W) = 50 μm and length (L) = 10, 50, and 100 μm, respectively. A distinct robust TFT performance is observed by long channel TFT (L = 100 μm) as compared to short channel TFT (L = 10 μm) under the same strain condition. All the TFTs exhibited a negative shift of VTH (V) after mechanical strain. Negative VTH (V) shift under mechanical strain is thought to generation of donor-like defects (ΔNGD) at the bending part of the TFT channel [5], which might be at the center part of the channel. This highly strained channel region might generate more defects, resulting in an increase of carrier concentration. It has been reported that long channel length TFTs are less sensitive against the generation of deep donor-like defects [6] which also support our experimental results. Therefore, the TFT with different channel lengths can have a different change in transfer curve. We also performed TCAD simulation to fit the data shown in Fig.1. Circles represent measurement data and solid lines TCAD fitting. In the TCAD simulation, we adopt density of states (DOS) parameters from Mehedi et al.[5]. We also found that the increment of defect states after different strain. These defects are mainly oxygen vacancies [5]-[8]. Therefore, it is confirmed by experimental results and TCAD simulation that the long channel TFTs are more robust and generation of oxygen vacancy might be the origin of the instability under mechanical bending strain. References S. Lee, J. Shin, and J. Jang, Adv. Funct. Mater., 27, 1604921 (2017).S. Lee, D. Jeong, A. Takagi, M. Mativenga, and J. Jang, Adv. Funct. Mater., 27, 1700437 (2017) .H. Nishide, and K. Oyaizu, Science, 319, 5864, PP. 737-738, (2008).M. Koo, K. I. Park, S.-H. Lee, M. Suh, D. Y. Jeon, J. W. Choi, K. Kang, and K. J. Lee, “Nano lett., 12, 9,(2011).M. M. Hasan, M. M. Billah, M. N. Naik, J.G. um and J. Jang, IEEE Electronic Device Letter, 38, 8, (2017).J. G. Um, M. Mativenga, P. Migliorato and J. Jang, App. Phys. Lett., 117, 23, (2015).M. M. Billah, M. M. Hasan and J. Jang, IEEE Electronic Device Letter, 38, 7, (2017).M. M. Hasan, M. M. Billah, and J. Jang, IEEE Electronic Device Letter, 39, 2, (2018). Figure 1

Low-temperature fabrication of sputtered high-k HfO2 gate dielectric for flexible a-IGZO thin film transistors

In this work, low temperature fabrication of a sputtered high-k HfO2 gate dielectric for flexible a-IGZO thin film transistors (TFTs) on polyimide substrates was investigated. The effects of Ar-pressure during the sputtering process and then especially the post-annealing treatments at low temperature (≤200 °C) for HfO2 on reducing the density of defects in the bulk and on the surface were systematically studied. X-ray reflectivity, UV-vis and X-ray photoelectron spectroscopy, and micro-wave photoconductivity decay measurements were carried out and indicated that the high quality of optimized HfO2 film and its high dielectric properties contributed to the low concentration of structural defects and shallow localized defects such as oxygen vacancies. As a result, the well-structured HfO2 gate dielectric exhibited a high density of 9.7 g/cm3, a high dielectric constant of 28.5, a wide optical bandgap of 4.75 eV, and relatively low leakage current. The corresponding flexible a-IGZO TFT on polyimide exhibited an optimal device performance with a saturation mobility of 10.3 cm2 V−1 s−1, an Ion/Ioff ratio of 4.3 × 107, a SS value of 0.28 V dec−1, and a threshold voltage (Vth) of 1.1 V, as well as favorable stability under NBS/PBS gate bias and bending stress.

Development of extremely low temperature processed oxide thin film transistors via atmospheric steam reforming treatment: Interface, surface, film curing

We have developed an “atmospheric steam reforming treatment” technique to obtain high-performance flexible amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) for applications in wearable/imperceptible electronics. The oxide TFTs, using as-grown a-IGZO with low-temperature sputtering and atomic layer deposition of Al2O3, showed unstable electrical behavior and poor transfer characteristics, which originated from a highly defective channel, gate insulator, and channel/gate insulator interface. Surprisingly, stable a-IGZO TFTs were obtained by atmospheric steam reforming treatment at an extremely low temperature (∼90 °C), and they exhibited good TFT performance with high field-effect mobility (13.3 cm2/V·s), high on/off ratio (4.5 × 108), and small SS value (0.23 V/dec). Despite the extremely low thermal budget, this steam treatment has a positive effect on the TFT characteristics because of an effective oxygen diffusion source, thereby resulting in successful healing of oxygen-related defects in the bulk oxide channel, channel/gate insulator interface, and gate insulator. Finally, by carrying out the atmospheric steam treatment at an extremely low temperature, we successfully fabricated ultrathin flexible a-IGZO TFTs with good electrical performance from a vacuum deposition system at the maximum process temperatures of 100–150 °C.

Electro-Thermal Annealing Method for Recovery of Cyclic Bending Stress in Flexible a-IGZO TFTs

Amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) fabricated by low-temperature processes on a flexible substrate can easily be degraded by mechanical deformation. Furthermore, lower performance in terms of the initial characteristics and reliability levels compared to those fabricated on glass substrates with relatively high heat treatments is inevitable. To solve these problems, a local electro-thermal annealing (ETA) method was applied to flexible a-IGZO TFTs processed at low temperature to enhance the inferior initial characteristics and reliability under a bending state. The enhancement of the characteristics and reliability by ETA can be attributed to the reduction of defects related to the oxygen through a localized Joule heat treatment with an extremely short duration (~1 ms). In addition, the effectiveness of ETA to recovery from bending stress even under harsh cyclic bending operation (strain condition of 0.833%) is verified.

Effect of Tensile and Compressive Bending Stress on Electrical Performance of Flexible a-IGZO TFTs

We report the changes in device performance of flexible amorphous indium-gallium-zinc-oxide thin-film transistors (TFTs) by 1.65% tensile or compressive bending stress for 10 k times. The TFTs exhibit negative threshold voltage ( $\Delta {V}_{\sf Th}$ ) shift and enhanced drain current ( ${I}_{D}$ ). TFT performance under repetitive tensile bending stress exhibits comparatively large threshold voltage shift ( $\Delta {V}_{\sf Th} = - {\sf 2.3}$ V) than compressive bending stress ( $\Delta {V}_{\sf Th}= - 0.9$ V), which might be originated from both the generation of interface states ( ${N}_{\sf int})$ and gap trap density (dN $_{\sf gap}$ /dE) by $3.5\times 10^{11}$ /cm2 and $5.7\times 10^{12}$ /cm2 eV, respectively under tensile bending stress. These are much higher than those ( ${N}_{\sf int}$ : $1.2\times 10^{11}$ /cm2 and dN $_{\sf gap}$ /dE: $2.2\times 10^{12}$ /cm2eV) for compressive bending. The increase in the DOS appears after both types of the bending stress which is related to the generation of oxygen vacancies. According to technology computer aided design simulation, the 10 k times repetitive compressive bending stress generates donor like states ( $\Delta {N}_{\sf GD}) \sim 2\times 10^{16}$ cm $^{-3}$ and tensile bending stress generates $\Delta {N}_{\sf GD} \sim 9.5\times 10^{16}$ cm $^{-3}$ at $\sim {E} _{C} -0.35$ eV.

Impact of repeated uniaxial mechanical strain on flexible a-IGZO thin film transistors with symmetric and asymmetric structures

This letter investigates repeated uniaxial mechanical stress-induced degradation behavior in flexible amorphous In-Ga-Zn-O thin-film transistors (TFTs) of different geometric structures. Two types of via-contact structure TFTs are investigated: symmetrical and UI structure (TFTs with I- and U-shaped asymmetric electrodes). After repeated mechanical stress, I-V curves for the symmetrical structure show a significant negative threshold voltage (VT) shift, due to mechanical stress-induced oxygen vacancy generation. However, degradation in the UI structure TFTs after stress is a negative VT shift along with the parasitic transistor characteristic in the forward-operation mode, with this hump not evident in the reverse-operation mode. This asymmetrical degradation is clarified by the mechanical strain simulation of the UI TFTs.

Role of ultrathin Al2O3 layer in organic/inorganic hybrid gate dielectrics for flexibility improvement of InGaZnO thin film transistors

We investigated flexible amorphous InGaZnO (a-IGZO) thin film transistors (TFTs) on a polyimide (PI) substrate by using organic/inorganic hybrid gate dielectrics of poly-4vinyl phenol (PVP) and ultrathin Al2O3. IGZO TFTs were fabricated with hybrid PVP/Al2O3 gate dielectrics having Al2O3 layers of different nanoscale thicknesses, which were deposited by atomic layer deposition (ALD). The electrical characteristics of the TFTs with the organic/inorganic hybrid gate dielectrics were measured after cyclic bending up to 1,00,000 cycles at the bending radius of 10mm. The ultrathin Al2O3 layer in the hybrid gate dielectrics improved the mechanical flexibility and protected the organic gate dielectric against damage during the sputter deposition of the IGZO layer. Finite elements method (FEM) simulations along with the structural characterization of the cyclically bent device showed the importance of optimizing the thickness of the Al2O3 layer in the hybrid gate dielectrics to obtain mechanically stable and flexible a-IGZO TFTs.

Effects of Mechanical Strains on the Characteristics of Top-Gate Staggered a-IGZO Thin-Film Transistors Fabricated on Polyimide-Based Nanocomposite Substrates

In this paper, we had successfully implemented flexible top-gate staggered amorphous In-Ga-Zn-O (a-IGZO) thin- film transistors (TFTs) on colorless and transparent polyimide (PI)-based nanocomposite substrates using fully lithographic and etching processes that are compatible with existing TFT mass fabrication technologies. The use of the selectively coated release layer between the nanocomposite PI film and the glass carrier ensured smooth debonding of the plastic substrate after TFT fabrication. The TFTs showed decent performances (with mobility >; 10 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V · s) either as fabricated or as debonded from the carrier glass. By bending the devices to different radii of curvature (from a flat state to an outward bending radius of 5 mm), influences of mechanical strains on the characteristics of flexible a-IGZO TFTs were also investigated. In general, the mobility of the flexible a-IGZO TFT increased with the tensile strain, whereas the threshold voltage decreased with the tensile strain. The variation of the mobility in a-IGZO TFTs versus the strain appeared smaller than those observed for amorphous silicon TFTs.

High-Performance Flexible a-IGZO TFTs Adopting Stacked Electrodes and Transparent Polyimide-Based Nanocomposite Substrates

We demonstrated flexible amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) on fully transparent and high-temperature polyimide-based nanocomposite substrates. The flexible nanocomposite substrates were coated on the carrier glass substrates and were debonded after the TFT microfabrication. The adoption of the Ti/IZO stacked electrodes as source/drain/ gain electrodes significantly improved the etching compatibility with other material layers, enabling successful implementation of flexible a-IGZO TFTs onto the transparent nanocomposite substrates by conventional lithographic and etching processes. The flexible a-IGZO TFTs exhibited decent mobility and mechanical bending capability. Field-effect mobility of up to 15.9 cm2/V · s, a subthreshold swing of 0.4 V/dec, a threshold voltage of 0.8 V, and an on/off ratio of >; 108 were extracted from the TFT characteristics. The devices could be bent down to a radius of curvature of 3 mm and yet remained normally functional. Such successful demonstration of flexible oxide TFTs on transparent flexible substrates using fully lithographic and etching processes that are compatible with existing TFT fabrication technologies shall broaden their uses in flexible displays and electronics.