Sanghyun Ju

Indium selenide nanowire phase-change memory

Nonvolatile memory device using indium selenide nanowire as programmable resistive element was fabricated and its resistive switching property was studied as functions of electrical pulse width and voltage magnitude. The nanowire memory can be repeatedly switched between high-resistance (∼1011Ω) and low-resistance (∼6×105Ω) states which are attributed to amorphous and crystalline states, respectively. Once set to a specific state, the nanowire resistance is stable as measured at voltages up to 2V. This observation suggests that the nanowire can be programed into two distinct states with a large on-off resistance ratio of ∼105 with significant potential for nonvolatile information storage.

High performance ZnO nanowire field effect transistors with organic gate nanodielectrics: effects of metal contacts and ozone treatment

High performance ZnO nanowire field effect transistors (NW-FETs) were fabricated using a nanoscopic self-assembled organic gate insulator and characterized in terms of conventional device performance metrics. To optimize device performance and understand the effects of interface properties, devices were fabricated with both Al and Au/Ti source/drain contacts, and device electrical properties were characterized following annealing and ozone treatment. Ozone-treated single ZnO NW-FETs with Al contacts exhibited an on-current (Ion) of ~4 µA at 0.9 Vgs and 1.0 Vds, a threshold voltage (Vth) of 0.2 V, a subthreshold slope (S) of ~130 mV/decade, an on–off current ratio (Ion:Ioff) of ~107, and a field effect mobility (μeff) of ~1175 cm2 V−1 s−1. In addition, ozone-treated ZnO NW-FETs consistently retained the enhanced device performance metrics after SiO2 passivation. A 2D device simulation was performed to explain the enhanced device performance in terms of changes in interfacial trap and fixed charge densities.

Fully transparent pixel circuits driven by random network carbon nanotube transistor circuitry

Optically transparent and mechanically flexible thin-film transistors have recently attracted attention for next generation transparent display technologies. Driving and switching transistors for transparent displays have challenging requirements such as high optical transparency, large-scale integration, suitable drive current (I(on)) in the microampere range, high on/off current ratio (I(on)/I(off)), high field-effect mobility, and uniform threshold voltage (V(th)). In this study, we demonstrate fully transparent high-performance and high-yield thin-film transistors based on random growth of a single-walled carbon nanotube (SWNT) network that are easy to fabricate. High-performance SWNT-TFTs exhibit optical transmission of 80% in visible wavelength, I(on)/I(off) higher than 10(3), and a high yield with reproducible electrical characteristics.

Metamaterial Absorber for Electromagnetic Waves in Periodic Water Droplets.

Perfect metamaterial absorber (PMA) can intercept electromagnetic wave harmful for body in Wi-Fi, cell phones and home appliances that we are daily using and provide stealth function that military fighter, tank and warship can avoid radar detection. We reported new concept of water droplet-based PMA absorbing perfectly electromagnetic wave with water, an eco-friendly material which is very plentiful on the earth. If arranging water droplets with particular height and diameter on material surface through the wettability of material surface, meta-properties absorbing electromagnetic wave perfectly in GHz wide-band were shown. It was possible to control absorption ratio and absorption wavelength band of electromagnetic wave according to the shape of water droplet–height and diameter– and apply to various flexible and/or transparent substrates such as plastic, glass and paper. In addition, this research examined how electromagnetic wave can be well absorbed in water droplets with low electrical conductivity unlike metal-based metamaterials inquiring highly electrical conductivity. Those results are judged to lead broad applications to variously civilian and military products in the future by providing perfect absorber of broadband in all products including transparent and bendable materials.

Interface studies of ZnO nanowire transistors using low-frequency noise and temperature-dependent I-V measurements

Single ZnO nanowire (NW) transistors fabricated with self-assembled nanodielectric (SAND) and SiO2 gate insulators were characterized by low-frequency noise and variable temperature current-voltage (I-V) measurements. According to the gate dependence of the noise amplitude, the extracted Hooge’s constants (αH) are ∼3.3×10−2 for SAND-based devices and ∼3.5×10−1 for SiO2-based devices. Temperature-dependent I-V studies show that the hysteresis of the transfer curves and the threshold voltage shifts of SAND-based devices are significantly smaller than those of SiO2-based devices. These results demonstrate the improved SAND/ZnO NW interface quality (lower interface-trap states and defects) in comparison to those fabricated with SiO2.

Fully Transparent Thin-Film Transistors Based on Aligned Carbon Nanotube Arrays and Indium Tin Oxide Electrodes†

Adv. Mater. 2008, 20, 1–5 2008 WILEY-VCH Verlag Gmb The development of mechanically flexible and/or optically transparent electronics could enable next-generation electronics technologies, which would be easy-to-read, light-weight, unbreakable, transparent, and flexible. Potential applications could include transparent monitors, heads-up displays, and conformable products. Recent reports have demonstrated transparent thin film transistors (TFTs) using channels consisting of semiconductor nanowires (ZnO, SnO2, or In2O3) and random networks of single-walled carbon nanotubes (SWNTs). Transparent TFTs are attractive for the drive circuitry in transparent and/or flexible active matrix display devices. These devices could overcome the limitations of conventional polycrystalline silicon and amorphous silicon thin film transistors, such as low mobility, nontransparency, or high temperature processing. Among these nanowire and nanotube materials, SWNTs could be a strong candidate for integrating high performance transistor circuits while satisfying the requirements for high density nanoscale integration, including ballistic transport, low power consumption, mechanical flexibility, and optical transparency. SWNT field effect transistors (FETs) using Pd source/drain electrodes and high-k atomic layer deposition (ALD)-based ZrO2 and HfO2 thin films as gate dielectrics have exhibited high performance transistor characteristics, including near ballistic transport and near ideal values of subthreshold slope (S) of 60mV/decade in non-hydrogen ambient conditions. Even though SWNT-FETs provide excellent electrical properties, the integration of individual SWNTs has been impeded by uncontrolled variations of SWNT-TFTs, such as variation of chirality and diameter of SWNTs during thermal chemical vapor deposition (CVD) growth, and alignment for device integration. In order to overcome these limits, new approaches for realization of complex circuits have been demonstrated using well-aligned SWNTarrays on insulating substrates, where the average number of assembled nanotubes are uniform from device to device. Previous studies involving SWNT networks and other semiconductors have demonstrated highly bendable, transparent TFTs, but the network-based TFTs suffer from relatively low mobility and difficulties in scaling down the channel length, making it difficult to obtain high performance transistors. Here, we report the first demonstration of fully transparent TFTs based on well-aligned SWNTs arrays, with conduction from source to drain occurring through individual SWNTs, rather than through networks. A recently developed technique for realizing aligned SWNT arrays on quartz substrates is utilized to place SWNTs into a specific area for the active channel layer. Transparent indium tin oxide (ITO) source/drain and gate electrodes provide excellent contacts to the SWNTs, resulting in high performance transistor characteristics. Representative SWNT-TFTs exhibit high performance depletion-mode p-type transistor characteristics with 83% transparency over the visible wavelength range. The fully transparent SWNT-TFTs could be attractive candidates for future flexible and/or transparent electronics. Figure 1a shows a cross-sectional view of a SWNT-TFT, which employs an aligned array of SWNTs as the active channel, ITO gate, and source/drain electrodes, and a HfO2 gate dielectric. Prior to growth of SWNTs, a ST-cut quartz substrate is annealed for 8 h at 900 8C in air. SWNTs are synthesized on the annealed quartz substrates by thermal CVD of methane using an iron catalyst. Nearly perfect alignment of SWNTs is achieved with direct growth of nanotubes (Fig. 1b). The insert in Figure 1b is a higher-magnification field emission scanning electron microscopy (FESEM) image, showing the well-aligned SWNTs. The parallel arrays of grown SWNT have diameters of 1–5 nm, and an average density of 0.5 tubes mm . The quality of the array could potentially be improved by increasing the annealing time, which might increase the degree of the order in the crystal lattice near the surface. The separation of individual SWNTs in the aligned array avoids junctions within the array, as well as electrostatic screening of the gate field by adjacent SWNTs, resulting in conductance properties per SWNT comparable to studies involving single SWNTs per device. This is a significant improvement over previous reported results on random network SWNT transistors. After depositing ITO source/drain electrodes (100 nm) by ion-assisted deposition (IAD) at room temperature, the electrical burning described below is performed

Chalcogenide-Nanowire-Based Phase Change Memory

We report fabrication of phase change random access memory (PRAM) using nanowires (NWs) of GeTe and In2Se3. NWs were grown by a vapor-liquid-solid technique and ranged from 40 to 80 nm in diameter and several micrometers long. A dynamic switching ratio (on/off ratio) of 2200 and 2 times 105 was realized for GeTe and indium selenide devices, respectively. The programming power for the RESET operation is only tens of microwatts compared to the milliwatt power levels required by the conventional thin-film-based PRAM.

1∕f noise of SnO2 nanowire transistors

The low frequency (1∕f) noise in single SnO2 nanowire transistors was investigated to access semiconductor-dielectric interface quality. The amplitude of the current noise spectrum (SI) is found to be proportional to Id2 in the transistor operating regime. The extracted Hooge’s constants (αH) are 4.5×10−2 at Vds=0.1V and 5.1×10−2 at Vds=1V, which are in general agreement with our prior studies of nanowire/nanotube transistors characterized in ambient conditions. Furthermore, the effects of interface states and contacts on the noise are also discussed.

Effects of bias stress on ZnO nanowire field-effect transistors fabricated with organic gate nanodielectrics

The effects of bias stress (gate stress or drain stress) on nanowire field-effect transistor (NW-FET) stability were investigated as a function of stress bias and stress time. The n-channel NW-FETs used a nanoscopic self-assembled organic gate insulator, and each device contained a single ZnO nanowire. Before stress, the off current is limited by a leakage current in the 1nA range, which increases as the gate to source bias becomes increasingly negative. The devices also exhibited significant changes in threshold voltage (Vth) and off current over 500 repeated measurement sweeps. The leakage current was significantly reduced after gate stress, but not after drain stress. Vth variations observed upon successive bias sweeps for devices following gate stress or drain stress were smaller than the Vth variation of unstressed devices. These observations suggest that gate stress and drain stress modify the ZnO nanowire-gate insulator interface, which can reduce electron trapping at the surface and therefore redu...

Fully transparent quantum dot light-emitting diode integrated with graphene anode and cathode.

A fully transparent quantum dot light-emitting diode (QD-LED) was fabricated by incorporating two types (anode and cathode) of graphene-based electrodes, which were controlled in their work functions and sheet resistances. Either gold nanoparticles or silver nanowires were inserted between layers of graphene to control the work function, whereas the sheet resistance was determined by the number of graphene layers. The inserted gold nanoparticles or silver nanowires in graphene films caused a charge transfer and changed the work function to 4.9 and 4.3 eV, respectively, from the original work function (4.5 eV) of pristine graphene. Moreover the sheet resistance values for the anode and cathode electrodes were improved from ∼63,000 to ∼110 Ω/sq and from ∼100,000 to ∼741 Ω/sq as the number of graphene layers increased from 1 to 12 and from 1 to 8, respectively. The main peak wavelength, luminance, current efficiency, and optical transmittance of the fully transparent QD-LED integrated with graphene anode and cathode were 535 nm, ∼358 cd/m2, ∼0.45 cd/A, and 70-80%, respectively. The findings of the study are expected to lay a foundation for the production of high-efficiency, fully transparent, and flexible displays using graphene-based electrodes.