Changjoon Yoon

Enhanced Performance of ZnO Nanowire Field Effect Transistors by H2 Annealing

The electrical properties of ZnO nanowires are significantly dependent on their surface states. The surface trap charges degrade the device performance of field effect transistors. These trap charges are reduced by H2 annealing. In this work, a back-gate ZnO nanowire field effect transistor (FET) was fabricated by a photolithographic process, and its electrical properties were characterized. This back-gate FET was subsequently annealed under a flow of H2/Ar gas for 20 min. The back-gate FET annealed for 20 min exhibited remarkably enhanced electrical characteristics, as compared with the as-fabricated back-gate FET; the peak transconductance was increased from 40 to 448 nS, the field effect mobility from 27 to 302 cm2 V-1 s-1, and the Imax/Imin ratio from 1.5 to 105.

Electrical Characteristics of GaAs Nanowire-Based MESFETs on Flexible Plastics

GaAs nanowire (NW)-based metal-semiconductor field-effect transistors (MESFETs) were constructed on flexible plastic substrates by a conventional top-down approach. The top-down approach utilized in this paper combines photolithography of high-quality GaAs bulk wafers with anisotropic chemical etching processes for preparation of GaAs NWs and photolithographic processes for formation of metal electrodes. For a representative GaAs NW-based MESFET, peak transconductance, the Ion/Ioff ratio, and the subthreshold slope are estimated to be approximately 19.7 S, ~107, and ~100 mV/dec, respectively. The electrical characteristics of the GaAs NW-based MESFETs were maintained during 3000 times of bending cycles under maximal tensile strains of 0.77% and 1.02%. These results demonstrate the possibility of using these devices in high-speed and high-performance flexible electronics.

Flexible logic gates composed of high performance GaAs-nanowire-based MESFETs with MHz-dynamic operations

High performance NOT, NAND and NOR logic gates composed of GaAs-nanowire (NW)-based metal-semiconductor field-effect transistors (MESFETs) were constructed on flexible plastics through a noble top-down route. The representative GaAs-NW-based MESFETs exhibited superior electrical characteristics such as a high mobility (∼3300 cm(2) V(-) s(-1)), large I(on)/I(off) ratio (∼10(8)) and small subthreshold swing (∼70 mV/dec). The NOT, NAND and NOR logic gates showed a maximum voltage gain of 108 and logic swings of 97-99%. All of the logic gates successfully retained their electrical characteristics during 2000 bending cycles. Furthermore, the logic gates were well operated by square-wave signals of up to 100 MHz under various strain conditions. The high performances demonstrated in this study open the way to the realization of high speed flexible logic devices.

Type conversion of n-type silicon nanowires to p-type by diffusion of gold ions

The simple type conversion of n-type silicon nanowires (SiNWs) to p-type by the diffusion of Au ions is demonstrated in this study. An Au thin film with a thickness of 10 nm was thermally deposited on an n-type SiNW and a rapid thermal annealing process was performed subsequently to diffuse the Au ions into the SiNW. The electrical characteristics of a back-gate field-effect transistor with a channel composed of the Au-diffused SiNW show that the Au-diffused SiNW acts as a p-type one. The type conversion phenomenon of the SiNW caused by the diffusion of Au ions is discussed in detail in this paper.

Ultraviolet Electroluminescence Emission from n-Type ZnO/p-Type Si Crossed Nanowire Light-Emitting Diodes

The optical characteristics of an n-type ZnO/p-type Si crossed nanowire (NW) light-emitting diode (LED) were investigated in this study. N-ZnO nanowires (NWs) were synthesized by thermal chemical vapor deposition, and p-Si NWs were fabricated by etching a single crystalline Si wafer. The p–n heterojunction LED formed by the cross of the n-ZnO and p-Si NWs selected from the NWs prepared in this work exhibited the current rectifying behavior with the turn-on voltage of 1.3 V. Our investigation of the photoluminescence spectrum of the as-grown n-ZnO NWs and electroluminescence spectrum of the n-ZnO/p-Si crossed NW LED reveals that both spectra have the same position of peaks at 390 nm. This result indicates that the UV emission from the crossed NW LED is mostly attributed to the band-to-band transition of electrons in the ZnO NW.

Nanocrystal-mediated charge screening effects in nanowire field-effect transistors

ZnO nanowire field-effect transistors having an omega-shaped floating gate (OSFG) have been successfully fabricated by directly coating CdTe nanocrystals (∼6±2.5 nm) at room temperature, and compared to simultaneously prepared control devices without nanocrystals. Herein, we demonstrate that channel punchthrough may occur when the depletion from the OSFG takes place due to the trapped charges in the nanocrystals. Electrical measurements on the OSFG nanowire devices showed static-induction transistorlike behavior in the drain output IDS-VDS characteristics and a hysteresis window as large as ∼3.1 V in the gate transfer IDS-VGS characteristics. This behavior is ascribed to the presence of the CdTe nanocrystals, and is indicative of the trapping and emission of electrons in the nanocrystals. The numerical simulations clearly show qualitatively the same characteristics as the experimental data and confirm the effect, showing that the change in the potential distribution across the channel, induced by both the...

Comparison of electrical characteristics of back- and top-gate Si nanowire field-effect transistors

Top-gate(TG) field effect transistors (FETs) with channels composed of Si nanowires were successfully fabricated in this study using photolithographic processes. In the TG FETs fabricated on oxidized Si substrates, the channels composed of Si nanowires with diameters of about 100 nm with natural SiO 2 . The surfaces of the Si nanowires with natural SiO2 were covered with the gate metal to form TG FETs.

Memory characteristics of top-gate ZnO nanowire field-effect transistors with floating gate nodes of Au nanoparticles

In this work, top-gate single ZnO nanowire-based FETs embedded with Au nanoparticles as the charge storage were fabricated and their memory effects were characterized.

Synthesis and Optoelectronic Characteristics of Single-crystalline Si Nanowires

Photocurrent of a single-crystalline Si nanowire is investigated in this paper. Single-crystalline Si nanowires with amorphous shells were first synthesized from ball-milled SiO powders by thermal chemical vapor deposition, and then the amorphous shells were etched out from the as-synthesized Si nanowires. For a single-crystalline Si nanowire, photocurrent-voltage curves taken in air at room temperature were non-linear, and rapid photoresponses were observed when the light was switched on and off. The photocurrent was not changed in intensity under the illumination. Photocurrent mechanism in the single-crystalline Si nanowire is discussed in this paper.