Kyoungwon Kim

Controllability of threshold voltage in Ag-doped ZnO nanowire field effect transistors by adjusting the diameter of active channel nanowire

Silver doped zinc oxide nanowires (NWs) were synthesized on (0001) sapphire substrate by hot-walled pulsed laser deposition. Both enhancement mode and depletion mode NW field effect transistors (FETs) were fabricated. The shift of threshold voltage of silver doped zinc oxide NW FET was observed from 2.45 to −3.2 V depending on the diameter of NWs without any significant changes of the subthreshold swing, carrier concentration, and on/off ratios. We demonstrate that the transfer characteristics of silver doped zinc oxide NW FETs were closely related with the size of NW diameter and control the shift of threshold voltage.

Effects of silver impurity on the structural, electrical, and optical properties of ZnO nanowires

Abstract1, 3, and 5 wt.% silver-doped ZnO (SZO) nanowires (NWs) are grown by hot-walled pulsed laser deposition. After silver-doping process, SZO NWs show some change behaviors, including structural, electrical, and optical properties. In case of structural property, the primary growth plane of SZO NWs is switched from (002) to (103) plane, and the electrical properties of SZO NWs are variously measured to be about 4.26 × 106, 1.34 × 106, and 3.04 × 105 Ω for 1, 3, and 5 SZO NWs, respectively. In other words, the electrical properties of SZO NWs depend on different Ag ratios resulting in controlling the carrier concentration. Finally, the optical properties of SZO NWs are investigated to confirm p-type semiconductor by observing the exciton bound to a neutral acceptor (A0X). Also, Ag presence in ZnO NWs is directly detected by both X-ray photoelectron spectroscopy and energy dispersive spectroscopy. These results imply that Ag doping facilitates the possibility of changing the properties in ZnO NWs by the atomic substitution of Ag with Zn in the lattice.

Temperature stress on pristine ZnO nanowire field effect transistor

We have investigated the effect of the temperature dependency on the device stability of pristine ZnO nanowires (NWs) field effect transistor (FET). Pristine ZnO NW FET shows a large threshold voltage (Vth) shift by 6.5 V after increasing the measured temperature from 323 to 363 K. This large shift in Vth is mainly due to thermally activated process. Thermally activated electrons from the deep level trap site can be free carriers which results in the shift in Vth in negative direction. Also, activation energy of ZnO NW FET is derived to be about 1.432 eV based on thermally activated Arrhenius model.

Effects of the thickness asymmetry of nanostructured exchange-coupled trilayers on their dynamic magnetization switching

The effects of thickness asymmetry (Δt) on the dynamic magnetization switching of exchange-coupled trilayers are investigated in this study. Elliptical thin films that have lateral dimensions of 200×100 nm2 are considered. A significant difference in the dynamic switching behavior is observed depending on Δt. Both the switching field and the switching time decrease as Δt increases. A Fourier analysis of the magnetization oscillation, which is followed by the initial coherent rotation, shows multiple oscillation frequencies. This tendency is more pronounced at a larger Δt. The main oscillation frequency tends to increase with an increase in Δt.

Effects of external field on the dynamic magnetization switching of a nanostructured exchange-coupled trilayer

The dynamic magnetization switching of a nanostructured exchange-coupled trilayer with a small thickness asymmetry is investigated by micromagnetic simulation. The magnetization switching initially occurs by coherent rotation, followed by magnetization oscillation. The switching time, which is about 9 ns at an applied field just above the switching field, is a factor of two longer than that observed for a single-layered thin film. A Fourier analysis of the magnetization oscillation shows multiple oscillation frequencies, the main frequency of which decreases as the applied magnetic field increases.

Magnetization reversal under nonuniform magnetic fields at conditions relevant to magnetic random access memory applications

Magnetization reversal behavior is examined under various nonuniform fields, the conditions of which are relevant to magnetic random access memory applications. During the magnetization reversal, the end domains play a key role at a uniform field, but they play a negligible role at a nonuniform field. Instead, a ripple pattern is initially formed in the interior and it progresses to form a vortex, resulting in a reversed domain. The switching field is found to be greater in the case of a nonuniform field, but, under a bias field, it is reduced greatly to a level similar to that for a uniform field. This result may indicate a wide window for the bit selectivity under a nonuniform field in magnetic random access memory applications.