Keon-Hee Lim

UV–Visible Spectroscopic Analysis of Electrical Properties in Alkali Metal‐Doped Amorphous Zinc Tin Oxide Thin‐Film Transistors

Solution-processed and alkali metals, such as Li and Na, are introduced in doped amorphous zinc tin oxide (ZTO) semiconductor TFTs, which show better electrical performance, such as improved field effect mobility, than intrinsic amorphous ZTO semiconductor TFTs. Furthermore, by using spectroscopic UV-visible analysis we propose a comprehensive technique for monitoring the improved electrical performance induced by alkali metal doping in terms of the change in optical properties. The change in the optical bandgap supported by the Burstein-Moss theory could successfully show a mobility increase that is related to interstitial doping of alkali metal in ZTO semiconductors.

Solution-processed amorphous hafnium-lanthanum oxide gate insulator for oxide thin-film transistors

Solution-processed high-K dielectrics for oxide thin-film transistors (TFTs) have been widely studied with the objective of achieving high performance and low-cost TFTs for next-generation displays. In this study, we introduce an amorphous hafnium-lanthanum oxide (HfLaOx) gate insulator with high electrical permittivity which was fabricated by the simple spin-coating method. In particular, the solution-processed HfLaOx dielectric layer, which was achieved by a mixture of two Hf and La metal hydroxide precursors, showed amorphous properties, a low leakage current and a high dielectric constant. The solution-processed HfLaOx dielectric layers showed a breakdown voltage as high as 5 MV cm−1 in strength and a dielectric constant above 22. Based on their implementation as a gate insulator, the solution-processed ZnO/HfLaOx TFTs showed good and stable performances during operation at a low voltage. A mobility of μ = 1.6 cm2 V−1 s−1, an on/off current ratio of 106, and a threshold voltage of 0.0015 V were obtained under a 5 V gate bias. Our results show the possibility of the solution-processed amorphous HfLaOx dielectric layer as a gate insulator for oxide TFTs. We believe that this amorphous HfLaOx dielectric has good potential for next-generation high-performance TFT devices.

All‐Solution‐Processed Transparent Thin Film Transistor and Its Application to Liquid Crystals Driving

All-solution-processed transparent thin film transistors (TTFTs) are demonstrated with silver grid source/drain electrodes, which are fabricated by printing and subsequent silver nanoparticles solution coating, which allows continuous processing without using high vacuum systems. The silver grid electrode shows a reasonable transmittance in visible range, moderate electrical conductance and mechanical strength. The TTFTs are employed to drive liquid crystal cells and demonstrate a successful switching operation.

The structural, optical and electrical characterization of high-performance, low-temperature and solution-processed alkali metal-doped ZnO TFTs

The structural, electrical and optical properties of high-performance, low-temperature and solution-processed alkali metal-doped ZnO TFTs were studied using various analytic instruments, including HR-TEM, AFM, XPS, EDS, electrical bias stability test and UV-vis spectroscopy. Furthermore, we successfully demonstrated that a change in the optical bandgap energy of Li-doped ZnO semiconductor films supported by Burstein–Moss theory can show a trade-off relationship between the field effect mobility of Li-ZnO TFTs and the Li doping concentrations. The relative broadening of the Eopt values, which are strongly related to the amount of excited electrons from the Fermi level in the valance band to the conduction band, was observed from the undoped ZnO film to the Li-doped ZnO film (10 mol%). The increase in the electron donor concentration was the dominant reason for the enhancement in the electron mobility of the alkali metal-doped ZnO TFTs.

Aqueous zinc ammine complex for solution-processed ZnO semiconductors in thin film transistors

We fabricated zinc oxide (ZnO) TFTs using a zinc ammine complex with various zinc oxide sources such as ZnO, intrinsic Zn(OH)2, and precipitated Zn(OH)2. From the analyses of the reaction mechanism, surface morphology, crystal structure, and oxygen vacancy in the ZnO films, we confirmed the same intermediate in ZnO semiconductor films irrespective of the type of zinc oxide source in the zinc ammine complex precursor. The results showed the analogous value of the average field effect mobility, on/off current ratio, and turn-on voltage in all solution-processed ZnO TFTs. In conclusion, we confirmed that directly dissolving pristine ZnO into ammonia water is the most efficient method for preparing the ZnO semiconductor precursor, the zinc ammine complex, for low-temperature, solution-processed, and high performance ZnO TFTs.

Interface engineering for suppression of flat-band voltage shift in a solution-processed ZnO/polymer dielectric thin film transistor

Flexible and transparent thin film transistors (FTTFTs) can lead to next generation displays that involve large area, future-oriented flexible and transparent displays. In order to achieve stable FTTFTs, solution processes of organic and inorganic compounds have received significant attention. Above all, transparent oxide semiconductors such as ZnO have been studied to enhance flexibility with high electrical performance by integration with organic dielectrics. However, interfacial traps between inorganic and organic compounds are derived by interface dipole, which induce a considerable flat band shift. Herein, we have developed a self-assembled inorganic layer (SAIL) via the photo-induced transformation of a mono-poly(dimethylsiloxane) (PDMS) layer as interface engineering. Especially, the shifting of flat band voltage (VFB) was effectively suppressed by the SAIL process, which was analyzed with a single-piece analytical model for ZnO TFTs. In addition, flexible ZnO/SAIL/polymer dielectric TFTs with low process temperature as high as 200 °C exhibited a good field-effect mobility μ = 0.28 cm2 V−1 s−1, more than 106 on–off current ratio and excellent device operational stability and flexibility.

Strong Influence of Humidity on Low-Temperature Thin-Film Fabrication via Metal Aqua Complex for High Performance Oxide Semiconductor Thin-Film Transistors

Oxide semiconductors thin film transistors (OS TFTs) with good transparency and electrical performance have great potential for future display technology. In particular, solution-processed OS TFTs have been attracted much attention due to many advantages such as continuous, large scale, and low cost processability. Recently, OS TFTs fabricated with a metal aqua complex have been focused because they have low temperature processability for deposition on flexible substrate as well as high field-effect mobility for application of advanced display. However, despite some remarkable results, important factors to optimize their electrical performance with reproducibility and uniformity have not yet been achieved. Here, we newly introduce the strong effects of humidity to enhance the electrical performance of OS TFTs fabricated with the metal aqua complex. Through humidity control during the spin-coating process and annealing process, we successfully demonstrate solution-processed InOx/SiO2 TFTs with a good electrical uniformity of ∼5% standard deviation, showing high average field-effect mobility of 2.76 cm2V-1s-1 and 15.28 cm2V-1s-1 fabricated at 200 and 250 °C, respectively. Also, on the basis of the systematic analyses, we demonstrate the mechanism for the change in electrical properties of InOx TFTs depending on the humidity control. Finally, on the basis of the mechanism, we extended the humidity control to the fabrication of the AlOx insulator. Subsequently, we successfully achieved humidity-controlled InOx/AlOx TFTs fabricated at 200 °C showing high average field-effect mobility of 9.5 cm2V-1s-1.

Photosensitivity of InZnO thin-film transistors using a solution process

Oxide semiconductor devices play a role in both switches and photo-sensors in interactive displays. During the fabrication of oxide semiconductor devices, the sol-gel solution process that is used to form an oxide semiconductor has various merits, including its simplicity and low cost as well as its good composition controllability. Here, we present the photosensitivity characteristics of an oxide photo thin-film transistor (TFT) created using the InZnO (IZO) sol-gel process. Upon exposure to light, photocurrent (Iphoto) in the negative gate bias regime is significantly increased with a negligible threshold voltage shift. The photosensitivity is modulated by geometrical factors and by the IZO material composition. We observed a significant effect of the channel thickness and IZO composition on the photosensitivity, which was attributed to the screening effect of optically ionized oxygen vacancies (Vo++). In particular, the optimized bi-layered oxide photo-TFT presents a good Iphoto/Idark photosensitivity ...

A systematic study on effects of precursors and solvents for optimization of solution-processed oxide semiconductor thin-film transistors

Solution-processed oxide semiconductor thin-film transistors (OS TFTs) have attracted much attention as a future display technology, because they have intrinsic properties such as flexibility and transparency as well as fabrication process advantages. Accordingly, to realize solution-processed high performance OS TFTs, various solutions have been developed. However, since it has been focused on the development of the solution itself, there have been no systematic approaches to independently study the effects of precursors and solvents for understanding and optimizing solution-processed OS TFTs. Here, we report a systematic study on the effects of precursors and solvents in solution-processed OS TFTs. Preferentially, InZnxOy (IZO) TFTs fabricated with various specific precursors and solvents are analyzed. It is confirmed that the electrical properties of IZO TFTs including field-effect mobility and Von are strongly affected by the types of precursors and solvents. Through various analyses including the TFT model, and X-ray based analyses, we discover that changes in the electrical properties are related to changes in the physical and intrinsic film properties of IZO films depending on the types of precursors and solvents. With observation of trends in the changes, the effects of precursors and solvents were investigated to better understand and optimize solution-processed OS TFTs.

Pressure-assisted electrode fabrication using simply synthesized Cu3Sn alloy nanoparticles

Alloy nanoparticles have been widely investigated due to their unique properties and various applications. In particular, electrodes based on alloy nanomaterials can be good alternatives for advanced electrode fabrication due to their inherent, unique properties. Herein, we suggest a simple one-pot synthesis of Cu3Sn nanoparticles with high oxidation-resistivity, straightforward processability, and good electron conductivity. In addition, we introduce fabrication of pressure-assisted electrodes at room temperature with the synthesized Cu3Sn nanoparticles, which have good potential for various functional devices on a plastic substrate.