Hyun-Jun Jeong

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.

\mathrm{cm}^{2}

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.

/Vs

ABSTRACT This paper describes the recent advances in flexible oxide thin-film transistors (TFTs), one of the rapidly emerging technologies for the next-generation display applications. First, the paper focuses on the effect of the buffer layer over the plastic substrate, which significantly influences the electrical performance and stability of oxide TFTs. Then oxide semiconductor TFTs fabricated through atomic layer deposition among the various oxide semiconductor fabrication methods were reviewed due to their potential as high-performance flexible TFTs. Finally, the mechanical fatigue behaviors of the TFTs were investigated, including the various mechanical factors, such as the bending radius, cycles, and stress directions. Structural solutions for the TFT were also introduced, such as TFT design modification and the use of the neutral plane concept, to improve the mechanical durability.

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

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.

Review of recent advances in flexible oxide semiconductor thin-film transistors

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.

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

Zinc tin oxide (Zn-Sn-O, or ZTO) semiconductor layers were synthesized based on solution processes, of which one type involves the conventional spin coating method and the other is grown by mist chemical vapor deposition (mist-CVD). Liquid precursor solutions are used in each case, with tin chloride and zinc chloride (1:1) as solutes in solvent mixtures of acetone and deionized water. Mist-CVD ZTO films are mostly polycrystalline, while those synthesized by spin-coating are amorphous. Thin-film transistors based on mist-CVD ZTO active layers exhibit excellent electron transport properties with a saturation mobility of 14.6 cm2/(V s), which is superior to that of their spin-coated counterparts (6.88 cm2/(V s)). X-ray photoelectron spectroscopy (XPS) analyses suggest that the mist-CVD ZTO films contain relatively small amounts of oxygen vacancies and, hence, lower free-carrier concentrations. The enhanced electron mobility of mist-CVD ZTO is therefore anticipated to be associated with the electronic band structure, which is examined by X-ray absorption near-edge structure (XANES) analyses, rather than the density of electron carriers.

Effect of mechanical stress on the stability of flexible InGaZnO thin-film transistors

Zinc oxynitride (ZnON) semiconductors are suitable for high performance thin-film transistors (TFTs) with excellent device stability under negative bias illumination stress (NBIS). The present work provides a first approach on the optimization of electrical performance and stability of the TFTs via studying the resonant interaction between anions or vacancies in ZnON. It is found that the incorporation of nitrogen increases the concentration of nitrogen vacancies (VN+s), which generate larger concentrations of free electrons with increased mobility. However, a critical amount of nitrogen exists, above which electrically inactive divacancy (VN-VN)0 forms, thus reducing the number of carriers and their mobility. The presence of nitrogen anions also reduces the relative content of oxygen anions, therefore diminishing the probability of forming O-O dimers (peroxides). The latter is well known to accelerate device degradation under NBIS. Calculations indicate that a balance between device performance and NBIS stability may be achieved by optimizing the nitrogen to oxygen anion ratio. Experimental results confirm that the degradation of the TFTs with respect to NBIS becomes less severe as the nitrogen content in the film increases, while the device performance reaches an intermediate peak, with field effect mobility exceeding 50 cm2/Vs.

High-Performance Zinc Tin Oxide Semiconductor Grown by Atmospheric-Pressure Mist-CVD and the Associated Thin-Film Transistor Properties

Zinc oxynitride (ZnON) is a relatively novel class of material, often regarded as a promising alternative to oxide semiconductors, owing to its relatively high electron mobility and low concentration of oxygen-related defects that affect the device reliability. In the present study, thermal annealing of ZnON for thin film transistor (TFT) applications is performed in conjunction with a source of ultraviolet (UV) radiation, as an attempt to lower the heat treatment temperature. The oxygen radicals and ozone produced in this process appear to oxidize the ZnON surface. As the annealing temperature increases in the presence of UV light, chemically stable ZnO and non-stoichiometric ZnxNy bonds are formed without significant change in the oxygen/nitrogen ratio within the film. Such a phenomenon is accompanied by a slight reduction in the field effect mobility and device stability under positive bias stress, however under optimized photo-thermal annealing conditions, ZnON TFTs fabricated at a relatively low annealing temperature (150 °C) exhibit high field effect mobility values exceeding 50 cm2 V−1 s−1 and reasonable reliability, as examined under positive bias stress conditions.

The resonant interaction between anions or vacancies in ZnON semiconductors and their effects on thin film device properties

Photochemical reactions in inorganic films, which can be promoted by the addition of thermal energy, enable significant changes in the properties of films. Metaphase films depend significantly on introducing external energy, even at low temperatures. We performed thermal-induced, deep ultraviolet-based, thermal-photochemical activation of metaphase ZnOxNy films at low temperature, and we observed peculiar variations in the nanostructures with phase transformation and densification. The separated Zn3N2 and ZnO nanocrystalline lattice in amorphous ZnOxNy was stabilized remarkably by the reduction of oxygen defects and by the interfacial atomic rearrangement without breaking the N-bonding. On the basis of these approaches, we successfully demonstrated highly flexible, nanocrystalline-ZnOxNy thin-film transistors on polyethylene naphthalate films, and the saturation mobility showed more than 60 cm2 V-1 s-1.

Supreme performance of zinc oxynitride thin film transistors via systematic control of the photo-thermal activation process

Amorphous oxide semiconductors have attracted attention in electronic device applications because of their high electrical uniformity over large areas, high mobility, and low-temperature process. However, photonic applications of oxide semiconductors are highly limited because of their larger band gap (over 3.0 eV). Here, we propose low band gap zinc oxynitride semiconductors not only because of their high electrical performance but also their high photoresponsivity in the vis-NIR regions. The optical band gap of zinc oxynitride films, which is in the range of 0.95-1.24 eV, could be controlled easily by changing oxygen and nitrogen ratios during reactive sputtering. Band gap tuned zinc oxynitride-based phototransistors showed significantly different photoresponse following both threshold voltage and drain current changes due to variation in nitrogen-related defect sites.