Jun Hee Choi

Effect of Ti thickness on contact resistance between GaN nanowiresand Ti∕Au electrodes

We demonstrate the effect of Ti thickness on the contact resistance between GaN nanowires and Ti∕Au electrodes. We have carried out systematic characterization of many GaN nanowires contacted by various Ti∕Au electrodes. We conclude that the average resistance is reduced by almost six orders of magnitude as Ti thickness increases from 0 to 20nm, and the resistance value then saturates when the Ti thickness further increases. Our observation can be explained by the formation of TiOx through the reaction of surface oxide and the Ti layer. Scanning Auger microscopy of the Au∕Ti∕SiO2 interface also supports this explanation.

Evaluation of power generated by thermoelectric modules comprising a p-type and n-type single walled carbon nanotube composite paper

We report on p-type and n-type thermoelectric (TE) materials made of single-walled carbon nanotube (SWCNT) networks incorporated into the cellulose fiber structure of a common packaging paper. This leads the paper to possess both mechanical flexibility from the cellulose fibers as a supporting matrix and the high electrical conductivity originating from the SWCNTs. Thermoelectric power of up to ±50 μV K−1 was successfully obtained as well, depending on their electronic type. Further, to demonstrate its thermoelectric voltage (VTEP) and generating power, a couple of thermoelectric modules composed of both p-type and n-type composite layers were assembled in series. The produced VTEP shows a quasi-linearity with respect to the number of p–n couples and the temperature difference ΔT. Our testing module enables the provision of VTEP and power generation as large as ≈16.8 mV and ≈75.5 nW upon inducing a 50 K temperature difference. The feasibility of commercial TE modules consisting of 10, 100 and 1000 p–n SWCNT couples was numerically calculated, taking into account our experimental results.

Conductive carbon nanotube paper by recycling waste paper

This paper reports on an easy process of fabricating conductive carbon nanotube (CNT) paper by recycling waste paper. In the absence of a dispersion agent, we just mixed and ground multi-walled carbon nanotubes (MWCNT) and shredded A4 paper together in water. CNT paper was obtained after filtering and rolling the mixture of MWCNT and waste paper. The thickness of the CNT paper was controllable by using different volumes of the mixture in the fabrication. Scanning electron microscope images showed that MWCNT was evenly stuck to the surface of the cellulose. Raman spectra indicated that the structure of a sidewall of MWCNT was influenced by cellulose through the mixing process. In addition, the thermal stability of CNT paper was enhanced in proportion to the amount of MWCNT used. The presence of MWCNT also contributed to the enhancement in the elastic properties of CNT paper. Depending on the amount of MWCNT and the thickness of CNT paper, the sheet resistance of CNT paper varied from 49.1 to 3365.6 Ω sq−1. This CNT paper can be utilized in the fields of electronic devices and energy storage devices, leading to easy mass-production at low cost.

Schottky nanocontact of one-dimensional semiconductor nanostructures probed by using conductive atomic force microscopy.

To develop the advanced electronic devices, the surface/interface of each component must be carefully considered. Here, we investigate the electrical properties of metal-semiconductor nanoscale junction using conductive atomic force microscopy (C-AFM). Single-crystalline CdS, CdSe, and ZnO one-dimensional nanostructures are synthesized via chemical vapor transport, and individual nanobelts (or nanowires) are used to fabricate nanojunction electrodes. The current-voltage (I -V) curves are obtained by placing a C-AFM metal (PtIr) tip as a movable contact on the nanobelt (or nanowire), and often exhibit a resistive switching behavior that is rationalized by the Schottky (high resistance state) and ohmic (low resistance state) contacts between the metal and semiconductor. We obtain the Schottky barrier height and the ideality factor through fitting analysis of the I-V curves. The present nanojunction devices exhibit a lower Schottky barrier height and a higher ideality factor than those of the bulk materials, which is consistent with the findings of previous works on nanostructures. It is shown that C-AFM is a powerful tool for characterization of the Schottky contact of conducting channels between semiconductor nanostructures and metal electrodes.

Ambipolar behavior in MoS2 field effect transistors by using catalytic oxidation

Modulation of electrical properties in MoS2 flakes is an attractive issue from the point of view of device applications. In this work, we demonstrate that an ambipolar behavior in MoS2 field effect transistors (FETs) can be easily obtained by heating MoS2 flakes under air atmosphere in the presence of cobalt oxide catalyst (MoS2 + O2 → MoOx + SOx). The catalytic oxidation of MoS2 flakes between source-drain electrodes resulted in lots of MoOx nanoparticles (NPs) on MoS2 flakes with thickness reduction from 64 nm to 17 nm. Consequently, N-type behavior of MoS2 FETs was converted into ambipolar transport characteristics by MoOx NPs which inject hole carriers to MoS2 flakes.

A Comparative Study of Fuzzy PID Control Algorithm for Position Control Performance Enhancement in a Real-time OS Based Laparoscopic Surgery Robot

Position control in joint space is a basic problem in robot control where the goal is to make the manipulator joint track a desired trajectory. A number of globally asymptotically stable position control algorithms are available in the literature. Among them, an application of intelligent fuzzy PID controller to position control of robot system is studied in this paper. The robot system used is the laparoscopic surgery robot of the National Cancer Center, Korea. The intelligent fuzzy control algorithms consist of rule-based fuzzy PID and learning fuzzy schemes. The results of the experiments for the rulebased fuzzy PID controller and the learning fuzzy controller are compared with results using conventional PID controller. Various performance indices like the RMS error, IAE, ISE and etc. are used for comparison. It is observed that the learning fuzzy controller gives the best performance. Further refinement of the proposed algorithm for the control performance enhancement is under way.

Soft-type trap-induced degradation of MoS2 field effect transistors

The practical applicability of electronic devices is largely determined by the reliability of field effect transistors (FETs), necessitating constant searches for new and better-performing semiconductors. We investigated the stress-induced degradation of MoS2 multilayer FETs, revealing a steady decrease of drain current by 56% from the initial value after 30 min. The drain current recovers to the initial state when the transistor is completely turned off, indicating the roles of soft-traps in the apparent degradation. The noise current power spectrum follows the model of carrier number fluctuation-correlated mobility fluctuation (CNF-CMF) regardless of stress time. However, the reduction of the drain current was well fitted to the increase of the trap density based on the CNF-CMF model, attributing the presence of the soft-type traps of dielectric oxides to the degradation of the MoS2 FETs.

Surface Modulation of Graphene Field Effect Transistors on Periodic Trench Structure

In this work, graphene field effect transistors (FETs) were fabricated on a trench structure made by carbonized poly(methylmethacrylate) to modify the graphene surface. The trench-structured devices showed different characteristics depending on the channel orientation and the pitch size of the trenches as well as channel area in the FETs. Periodic corrugations and barriers of suspended graphene on the trench structure were measured by atomic force microscopy and electrostatic force microscopy. Regular barriers of 160 mV were observed for the trench structure with graphene. To confirm the transfer mechanism in the FETs depending on the channel orientation, the ratio of experimental mobility (3.6-3.74) was extracted from the current-voltage characteristics using equivalent circuit simulation. It is shown that the number of barriers increases as the pitch size decreases because the number of corrugations increases from different trench pitches. The noise for the 140 nm pitch trench is 1 order of magnitude higher than that for the 200 nm pitch trench.

Induction heating effect on the performance of flexible MoS2 field-effect transistors

We investigated the induction heating effect on device characteristics of flexible molybdenum disulfide (MoS2) field-effect transistors (FETs). A polyimide film was employed as a flexible substrate, and poly(3,4-ethylenedioxythiophene):polystyrene sulfonate was coated on the flexible substrate as a bottom gate. After the annealing process on the flexible MoS2 FETs by induction heating, the field effect mobility was enhanced from 2.13 to 5.58 cm2/V·s with the slight increase of the on-off ratio from 5.17 × 102 to 1.98 × 103. Moreover, the low field mobility was almost unchanged from 7.75 to 7.33 cm2/V·s, indicating that the induction heating mainly contributed to the enhancement of the device performances by contact improvement between electrodes and MoS2. With the simple model of the diode and resistor connected in series, it was confirmed that the Schottky diode disappeared with contact enhancement. Our findings can contribute to the contact improvement with minimum damage when low dimensional nanomateri...