Kyung-Chul Ok

The effects of buffer layers on the performance and stability of flexible InGaZnO thin film transistors on polyimide substrates

We demonstrated the fabrication of flexible amorphous indium gallium zinc oxide thin-film transistors (TFTs) on high-temperature polyimide (PI) substrates, which were debonded from the carrier glass after TFT fabrication. The application of appropriate buffer layers on the PI substrates affected the TFT performance and stability. The adoption of the SiNx/AlOx buffer layers as water and hydrogen diffusion barriers significantly improved the device performance and stability against the thermal annealing and negative bias stress, compared to single SiNx or SiOx buffer layers. The substrates could be bent down to a radius of curvature of 15 mm and the devices remained normally functional.

Highly Stable ZnON Thin-Film Transistors With High Field-Effect Mobility Exceeding 50

High-performance thin-film transistors (TFTs) based on ZnON channels were fabricated using a dc reactive sputtering method. To improve the photoinduced bias stability, a postannealing process was carried out at a low ambient pressure (~100 mTorr, air ambient) at 250 °C for various annealing times (1-5 h). The transfer characteristics of the postannealed ZnON TFTs exhibited an improved subthreshold swing ranging from 0.60 to 0.42 V/decade. Other transport properties remained similar including a high mobility (μsat) of 50 cm2/Vs, a threshold voltage (Vth) of -2.5 V, and an ON-OFF drain current ratio of 108. In addition, photoinduced bias reliability under a gate bias stress (VG = -20 V) was significantly improved from -10.88 V (1 h) to -2.28 V (5 h). These results can be explained by the enhancement of bonding properties between Zn metal and two different anions (O, N) as stable N-Zn-O states.

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Tantalum (Ta) is suggested to act as an electron donor and crystal phase stabilizer in titanium oxide (TiOx). A transition occurs from an amorphous state to a crystalline phase at an annealing temperature above 300 °C in a vacuum ambient. As the annealing temperature increases from 300 °C to 450 °C, the mobility increases drastically from 0.07 cm2/Vs to 0.61 cm2/Vs. The remarkable enhancement of thin film transistor performance is suggested to be due to the splitting of Ti 3d band orbitals as well as the increase in Ta5+ ions that can act as electron donors.

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Flexible top-gate amorphous InGaZnO thin-film transistors are fabricated on polyimide substrates. The effect of the alumina buffer layers on the device performance and stability is demonstrated using two types of atomic layer deposition reactant sources: 1) ozone and 2) water. Alumina buffers formed by water reactants have better barrier properties against the ambient than those formed by ozone. Furthermore, less charge trapping at sub-gap density-of-states occurs with higher film density of the buffer layer. Stability characteristics under negative bias temperature stress are enhanced by optimization of the buffer layer formation on flexible substrates.

The effect of Ta doping in polycrystalline TiOx and the associated thin film transistor properties

Electrical properties of Nb-doped titanium oxide films were evaluated with respect to annealing temperatures. Although an amorphous phase is preserved up to 450 °C, x-ray absorption spectroscopy analyses indicate that crystal field splitting in the conduction band begins to take place at this temperature. Such molecular orbital ordering effects induce a semiconducting behavior, which is manifested by working thin film transistor devices with field effect mobility values as high as 0.64 cm2/Vs. X-ray photoelectron spectroscopy studies disclose a drastic increase in Nb+5 states upon heat treatment, and these may be attributed to oxygen deficient states that generate free electrons.

Effect of Alumina Buffers on the Stability of Top-Gate Amorphous InGaZnO Thin-Film Transistors on Flexible Substrates

High-mobility zinc oxynitride (ZnON) semiconductors were grown by RF sputtering using a Zn metal target in a plasma mixture of Ar, N2, and O2 gas. The RF power and the O2 to N2 gas flow rates were systematically adjusted to prepare a set of ZnON films with different relative anion contents. The carrier density was found to be greatly affected by the anion composition, while the electron mobility is determined by a fairly complex mechanism. First-principles calculations indicate that excess vacant nitrogen sites (VN) in N-rich ZnON disrupt the local electron conduction paths, which may be restored by having oxygen anions inserted therein. The latter are anticipated to enhance the electron mobility, and the exact process parameters that induce such a phenomenon can only be found experimentally. Contour plots of the Hall mobility and carrier density with respect to the RF power and O2 to N2 gas flow rate ratio indicate the existence of an optimum region where maximum electron mobility is obtained. Using ZnON films grown under the optimum conditions, the fabrication of high-performance devices with field-effect mobility values exceeding 120 cm2/Vs is demonstrated based on simple reactive RF sputtering methods.

Semiconducting behavior of niobium-doped titanium oxide in the amorphous state

The effect of niobium (Nb) doping on the performance and stability of TiOx -based thin-film transistors (TFTs) was studied. While sputtered TiOx has an initial amorphous phase and begins to crystallize to anatase at an annealing temperature of 450??C, Nb-doped TiOx preserves the amorphous structure up to annealing temperatures as high as 550??C. TFT devices fabricated using Nb-doped TiOx as the active layer exhibit higher field-effect mobility and better stability upon negative and positive bias stress compared to pure TiOx devices. X-ray photoelectron spectroscopy analyses indicate that Nb doping induces higher levels of oxygen deficiency and a considerable amount of defect states near the valence band, which cannot account for the higher device stability. It is thus suggested that the grain boundaries in crystalline TiOx TiOx may act as the major charge traps, which induce larger shifts in threshold voltage (Vth) upon bias stress.

A study on the electron transport properties of ZnON semiconductors with respect to the relative anion content

Thin-film transistors (TFTs) based on In-Sn-Ga-O (ITGO) semiconductors were evaluated with respect to different post-annealing temperatures (200 °C ~ 350 °C). High-performance devices were obtained, exhibiting field-effect mobility values exceeding 25 cm2/Vs at all thermal treatments. However, the threshold voltage shift (AVth) under negative bias stress increased with increasing annealing temperature, which is opposite to what is generally observed in oxide semiconductor TFTs. It is suggested that annealing at elevated temperatures results in relatively large concentrations of oxygen deficient sites in ITGO. These defects act as sources of excess electron carriers, which induce large Vth shifts upon negative bias stress. Relatively low process temperatures are thus preferred in ITGO TFTs, which are anticipated to pave the way for the development of flexible displays.

The effect of Nb doping on the performance and stability of TiOx devices

This paper investigates the negative bias instability (NBS) and positive bias instability (PBS) of titanium oxide (TiOx) thin-film transistors (TFTs) with different annealing temperatures. Structural analyses suggested that TiOx films annealed at 450 and 550 °C had average grain sizes of 200 and 400 nm, respectively. A TiOx TFT annealed at 550 °C exhibited respective threshold voltage (Vth) shifts of only −1.4 and 10.2 V under NBS and PBS conditions. The origin of the instability was found to be a charge trapping mechanism caused by different grain sizes, boundaries, and changes in band edge states below the conduction band, which acted as electron and hole trap sites.

Stability Improvement of In-Sn-Ga-O Thin-Film Transistors at Low Annealing Temperatures

ABSTRACT Demonstrated herein is the effect of mechanical stress on the device performance and stability of amorphous indium–gallium–zinc oxide thin-film transistors (TFTs) on a flexible polyimide substrate. Flexible TFTs were placed on jigs with various bending radii to apply different degrees of mechanical strain on them. When the tensile strain on the TFTs was increased from 0.19% to 0.93%, the threshold voltage shifted after a 10,000 s increase in bias–temperature–stress (BTS), under vacuum conditions. The BTS instability was further exacerbated when the device was exposed to the air ambient at a 0.93% strain. The device reliability deteriorated due to the increase in the subgap density of states as well as the enhanced ambient effects via the strain-induced gas permeation paths.