Eok Su Kim

The influence of SiOx and SiNx passivation on the negative bias stability of Hf-In-Zn-O thin film transistors under illumination

The stability of hafnium indium zinc oxide thin film transistors under negative bias stress with simultaneous exposure to white light was evaluated. Two different inverted staggered bottom gate devices, each with a silicon oxide and a silicon nitride passivation, were compared. The latter exhibits higher field effect mobility but inferior subthreshold swing, and undergoes more severe shifts in threshold voltage (VT) during negative bias illumination stress. The time evolution of VT fits the stretched exponential equation, which implies that hydrogen incorporation during the nitride growth has generated bulk defects within the semiconductor and/or at the semiconductor/gate dielectric interface.

Effect of surface morphology on friction of graphene on various substrates

The friction of graphene on various substrates, such as SiO2, h-BN, bulk-like graphene, and mica, was investigated to characterize the adhesion level between graphene and the underlying surface. The friction of graphene on SiO2 decreased with increasing thickness and converged around the penta-layers due to incomplete contact between the two surfaces. However, the friction of graphene on an atomically flat substrate, such as h-BN or bulk-like graphene, was low and comparable to that of bulk-like graphene. In contrast, the friction of graphene folded onto bulk-like graphene was indistinguishable from that of mono-layer graphene on SiO2 despite the ultra-smoothness of bulk-like graphene. The characterization of the graphenes roughness before and after folding showed that the corrugation of graphene induced by SiO2 morphology was preserved even after it was folded onto an atomically flat substrate. In addition, graphene deposited on mica, when folded, preserved the same corrugation level as before the folding event. Our friction measurements revealed that graphene, once exfoliated from the bulk crystal, tends to maintain its corrugation level even after it is folded onto an atomically flat substrate and that ultra-flatness in both graphene and the substrate is required to achieve the intimate contact necessary for strong adhesion.

Piezoelectric touch-sensitive flexible hybrid energy harvesting nanoarchitectures

In this work, we report a flexible hybrid nanoarchitecture that can be utilized as both an energy harvester and a touch sensor on a single platform without any cross-talk problems. Based on the electron transport and piezoelectric properties of a zinc oxide (ZnO) nanostructured thin film, a hybrid cell was designed and the total thickness was below 500 nm on a plastic substrate. Piezoelectric touch signals were demonstrated under independent and simultaneous operations with respect to photo-induced charges. Different levels of piezoelectric output signals from different magnitudes of touching pressures suggest new user-interface functions from our hybrid cell. From a signal controller, the decoupled performance of a hybrid cell as an energy harvester and a touch sensor was confirmed. Our hybrid approach does not require additional assembly processes for such multiplex systems of an energy harvester and a touch sensor since we utilize the coupled material properties of ZnO and output signal processing. Furthermore, the hybrid cell can provide a multi-type energy harvester by both solar and mechanical touching energies.

Suspended single-layer MoS2 devices

We have fabricated and characterized suspended single-layer MoS2 devices to investigate the substrate effect on the electrical properties of MoS2. The MoS2 devices were fabricated on Si/SiO2 first by using e-beam lithography and were suspended by etching away half of the SiO2 layer with buffered oxide etchant and drying them with critical point dryer. Compared with SiO2 substrate-supported devices, the suspended devices show 2-10 times of mobility and on/off ratio improvement. While measuring the electronic properties, we observed that the suspended devices were annealed by joule heating and showed the performance improvement, whereas the supported devices did not. Our observations reveal that MoS2 devices are substrate-sensitive in their electrical properties and that proper substrates and cleaning is necessary for the optimal device performance.

The influence of sputtering power and O2/Ar flow ratio on the performance and stability of Hf–In–Zn–O thin film transistors under illumination

The performance and stability of amorphous HfInZnO thin film transistors under visible light illumination were studied. The extent of device degradation upon negative bias stress with the presence of visible light is found to be strongly sensitive to the extent of photoelectric effect in the oxide semiconductor. Highly stable devices were fabricated by optimizing the deposition conditions of HfInZnO films, where the combination of high sputtering power and high O2/Ar gas flow ratio was found to result in the highest stability under bias stress experiments.

Ti/Cu bilayer electrodes for SiNx-passivated Hf–In–Zn–O thin film transistors: Device performance and contact resistance

In this study, we examine the possibility of using Ti/Cu bilayer as source/drain electrodes for SiNx-passivated Hf–In–Zn–O (HIZO) thin film transistors by comparing their electrical properties with devices that use Mo electrodes. The Mo devices operate in depletion mode with a higher field effect mobility, while the Ti/Cu devices exhibit an improved subthreshold swing and operate in enhancement mode. Transmission electron microscopy characterization reveals the formation of an amorphous TiOx layer at the Ti/HIZO interface, which is suggested to be responsible for the disparate device characteristics in terms of contact resistance and threshold delay.

High Performance and Stability of Double-Gate Hf–In–Zn–O Thin-Film Transistors Under Illumination

Hafnium indium zinc oxide thin-film transistors (TFTs) with a double-gate structure were evaluated for the first time. Compared with devices with a single bottom gate, TFTs with an additional top gate exhibit improved subthreshold swing, threshold voltage, and field-effect mobility, as well as smaller subthreshold currents upon exposure to visible light. This phenomenon is attributed to the more effective suppression of excess photocurrents by the application of a double-gate structure. Negative-bias stress experiments under illumination indicate that the double-gate TFT exhibits very high stability compared with the device with a single-gate configuration.

High-Performance and Stable Transparent Hf–In–Zn–O Thin-Film Transistors With a Double-Etch-Stopper Layer

Transparent hafnium indium zinc oxide thin-film transistors adopting single- and double-etch-stopper layers were evaluated. Compared to devices with a single SiOx etch stopper (ES) grown at 150°C, a double ES with a second SiOx film grown at 350°C provides a superior device performance such as improved subthreshold swing, threshold voltage, field effect mobility, and higher stability under a negative bias stress. The stretched-exponential analyses of the bias stress results indicate that the denser high-temperature SiOx protects more effectively the underlying semiconductor during the source/drain etch process and suppresses the generation of defect states therein.

Defect Control in Zinc Oxynitride Semiconductor for High-Performance and High-Stability Thin-Film Transistors

The fabrication of thin-film transistor devices incorporating active semiconductors based on zinc oxynitride (ZnON) compound is presented. It is demonstrated that the addition of appropriate dopant, gallium, in ZnON, suppresses the formation of shallow donor, nitrogen vacancies, and significantly improves electrical characteristics of the resulting TFT. The Ga:ZnON devices with field-effect mobility values exceeding 50 cm2/Vs are achieved, which makes them suitable as switching or driving elements in next-generation flat-panel displays.

Multilayer transition-metal dichalcogenide channel Thin-Film Transistors

We show that multilayered transition-metal dichalcogenides such as multilayer MoS2 present a compelling case for Thin-Film Transistors (TFTs) for large-area display technology. Through a combined structural, optical, and electronic characterization of multilayer MoS2 TFTs, supported by density-functional theory based bandstructure calculations, we show the inherently attractive properties of these materials for such applications. We find that the current modulation of such devices is high, the current saturation is robust, normally-off operation is feasible, effective field-effect mobility at RT exceeds 100 cm2/V.s, and the channel can be operated in both accumulation and inversion modes. These properties make multilayer MoS2 more feasible than single layer versions to maintain processing robustness.