Byoungjun Park

Flexible TFTs based on solution-processed ZnO nanoparticles

Flexible electronic devices which are lightweight, thin and bendable have attracted increasing attention in recent years. In particular, solution processes have been spotlighted in the field of flexible electronics, since they provide the opportunity to fabricate flexible electronics using low-temperature processes at low-cost with high throughput. However, there are few reports which describe the characteristics of electronic devices on flexible substrates. In this study, we fabricated flexible thin-film transistors (TFTs) on plastic substrates with channel layers formed by the spin-coating of ZnO nanoparticles and investigated their electrical properties in the flat and bent states. To the best of our knowledge, this study is the first attempt to fabricate fully functional ZnO TFTs on flexible substrates through the solution process. The ZnO TFTs showed n-channel device characteristics and operated in enhancement mode. In the flat state, a representative ZnO TFT presented a very low field-effect mobility of 1.2 x 10(-5) cm(2) V(-1) s(-1), while its on/off ratio was as high as 1.5 x 10(3). When the TFT was in the bent state, some of the device parameters changed. The changes of the device parameters and the possible reasons for these changes will be described. The recovery characteristics of the TFTs after being subjected to cyclic bending will be discussed as well.

Capacitance characteristics of MOS capacitors embedded with colloidally synthesized gold nanoparticles

In this work, the capacitance characteristics of gold nanoparticle-embedded metal–oxide-semiconductor (MOS) capacitors with Al2O3 control oxide layers are investigated. The capacitance versus voltage (C–V) curves obtained for a representative MOS capacitor embedded with gold nanoparticles synthesized by the colloidal method exhibit large flat-band voltage shifts, which indicate the presence of charge storages in the gold nanoparticles. Their hysteresis characteristics are dependent on the voltage sweep range. The clockwise hysteresis and rightward shift of the flat band voltages observed from the C–V curves imply that electrons are trapped in a floating gate layer consisting of the gold nanoparticles present between SiO2 and Al2O3 layers in the MOS capacitor, and that these trapped electrons originate from the top electrode. In addition, the characteristics of the capacitance versus time curves for the gold nanoparticle-embedded MOS capacitor are discussed in this paper.

Resistance switching memory devices constructed on plastic with solution-processed titanium oxide

Resistance switching memory devices constructed on flexible plastic substrates via the spin-coating of titanium oxide solution were characterized in this study. The resistance switching memory device exhibited a ratio of the high resistance to low resistance states of more than 102, and this large resistance ratio was maintained even after 104 s. These memory characteristics are comparable to those of resistance switching memory devices based on titanium oxide films deposited on Si substrates. Moreover, the endurance of the flexible memory device investigated by means of a continuous substrate bending test revealed that the ratio of the high resistance to low resistance states was negligibly changed up to two hundred cycles. Its resistance switching characteristics were not degraded by the bending of the substrate, due to its short length channel and the high ductility of the electrode.

Photoluminescence characteristics of Mn- and Pr-doped ZnS nanoparticles optically annealed with UV illumination

Undoped, Mn-doped, Pr-doped, and Mn- and Pr-codoped ZnS nanoparticles were synthesized by a wet chemical method, and these nanoparticles were optically annealed in air or vacuum. In the photoluminescence (PL) spectra of the optically annealed nanoparticles, a sulfur–vacancy-related PL band at around 420 nm, a PL band associated with the 4T1→6A1 transition of Mn2+, and PL peaks associated with Pr-related complexes were investigated during optical annealing. The integrated PL intensities for the nanoparticles annealed in air increased more markedly with the optical annealing time than those for the nanoparticles annealed in vacuum. This increase may be explained by the photooxidation of the surface, the enhancement of the crystal quality, and the gradual diffusion of luminescent ions into the crystal lattice.

Nano-Floating Gate Memory Devices Composed of ZnO Thin-Film Transistors on Flexible Plastics

Nano-floating gate memory devices were fabricated on a flexible plastic substrate by a low-temperature fabrication process. The memory characteristics of ZnO-based thin-film transistors with Al nanoparticles embedded in the gate oxides were investigated in this study. Their electron mobility was found to be 0.18 cm2/V·s and their on/off ratio was in the range of 104–105. The threshold voltages of the programmed and erased states were negligibly changed up to 103 cycles. The flexibility, memory properties, and low-temperature fabrication of the nano-floating gate memory devices described herein suggest that they have potential applications for future flexible integrated electronics.

Capacitance-voltage characterization of Ge-nanocrystal-embedded MOS capacitors with a capping Al2O3 layer

Capacitance versus voltage (C–V) curves of Ge-nanocrystal (NC)-embedded MOS capacitors with and without a single capping Al2O3 layer are characterized in this work. C–V curves of the Ge-NC-embedded MOS capacitor with the Al2O3 layer are counterclockwise in the voltage sweeps, which indicates the presence of charge storages in the Ge NCs by the tunnelling of charge carriers between the Si substrate and the Ge NCs. In contrast, clockwise hysteresis of the C–V curves and leftward shifts of the flat band voltages are observed for the embedded MOS capacitor without the Al2O3 layer. It is suggested here that the observed characteristics of the C–V curves are due to the charge trapping at oxygen vacancies within a SiO2 layer. In addition, the illumination of the white light enhances the lower capacitance part of the C–V hysteresis. The origin for the enhancement is discussed in this paper.

Transparent nano-floating gate memory on glass

We construct fully transparent nano-floating gate memory devices on a glass substrate. These memory thin-film transistors consist of channel layers of ZnO films, electrodes of Al/ITO, and floating gate nodes of Al nanoparticles, exhibiting a transmittance of approximately 71% in the visible region. Their electron mobility, on/off ratio, and threshold voltage shift are estimated to be 0.92 cm(2) V( - 1) s( - 1), about 10(4), and 3.1 V, respectively. Moreover, their programming/erasing, endurance and retention are characterized in this study. Our study suggests that our memory devices have great potential for realizing transparent systems-on-glass.

Electrical Characteristics of Hybrid Nanoparticle–Nanowire Devices

Gold nanoparticles synthesized by a colloidal method were deposited in an Al2O3 dielectric layer of an omega-gated single ZnO nanowire FET. These gold nanoparticles were utilized as localized trap sites. The adsorption of the gold nanoparticles on an Al2O3-coated ZnO nanowire was confirmed by high-resolution transmission electron microscopy. In this study, a hybrid nanoparticle-nanowire device was fabricated by conventional Si processing. Its electrical characteristics indicated that electrons in the conduction band of the ZnO nanowire can be transported to the localized trap sites by gold nanoparticles for gate voltages greater than 1 V, through the 10-nm-thick Al 2O3 tunneling oxide layer.

Transport of Charge Carriers in HgTe/CdTe Core-Shell Nanoparticle Film

HgTe and HgTe/CdTe core-shell structured nanoparticles were synthesized by the colloidal method. The photoluminescence (PL) spectrum of a HgTe/CdTe nanoparticle film exhibited an exciton peak at 700 nm, which was blue-shifted by about 400 nm for the HgTe nanoparticle film. The current–voltage curves and photoresponse of the HgTe/CdTe nanoparticle film were compared with those of the HgTe nanoparticle film. When the film is exposed to light, the current of the HgTe/CdTe nanoparticle film decreases in the ambient air while increasing in vacuum. Finally, the temperature dependence of the photocurrent is discussed in this paper.

Capacitance-voltage Characteristics of MOS Capacitors with Ge Nanocrystals Embedded in HfO 2 Gate Material

Ge nanocrystals(NCs)-embedded MOS capacitors are characterized in this work using capacitance-voltage measurement. High-k dielectrics HfO₂ are employed for the gate material in the MOS capacitors, and the C-V curves obtained from O₂-and NH₃-annealed HfO₂ films are analyzed.