Byoung-Ho Cheong

Carbon-nanotube-based nonvolatile memory with oxide–nitride–oxide film and nanoscale channel

We have fabricated a single-wall carbon-nanotube (CNT)-based nonvolatile memory device using SiO2–Si3N4–SiO2 (ONO) layers as a storage node. The memory device is composed of a top gate structure with a channel width of a few nanometers and the ONO layer embedded between CNT and gate electrode. When the bias voltage between the CNT and gate electrode increases to 4 V, charges are tunneled out from CNT surfaces and captured to the traps in the ONO layers. Stored charges on the trap sites make the threshold voltage shift of 60 mV and is independent of charging time, suggesting that the ONO has traps with a quasiquantized energy state. The quantized state is related to the localized high electric field associated with CNT channel. The CNT-field-effect transistor with an ONO storage node could be used for an ultrahigh-density nonvolatile memory.We have fabricated a single-wall carbon-nanotube (CNT)-based nonvolatile memory device using SiO2–Si3N4–SiO2 (ONO) layers as a storage node. The memory device is composed of a top gate structure with a channel width of a few nanometers and the ONO layer embedded between CNT and gate electrode. When the bias voltage between the CNT and gate electrode increases to 4 V, charges are tunneled out from CNT surfaces and captured to the traps in the ONO layers. Stored charges on the trap sites make the threshold voltage shift of 60 mV and is independent of charging time, suggesting that the ONO has traps with a quasiquantized energy state. The quantized state is related to the localized high electric field associated with CNT channel. The CNT-field-effect transistor with an ONO storage node could be used for an ultrahigh-density nonvolatile memory.

Two-dimensional photonic crystal color filter development

Reflective color filters using two-dimensional photonic crystals based on sub-wavelength gratings were proposed and constructed. Using low-cost nanoimprint lithography, an amorphous silicon layer was deposited through the low-temperature PECVD process and patterned into two-dimensional structures. The isolated amorphous silicon patterns were readily crystallized using a multi-shot excimer laser annealing at low energy. A study of the close relationship between color filter reflectance and silicon pattern crystallinity is introduced. Theoretical and experimental results show that the proposed color filters have high reflectance and, moreover, decrease the dependence on incident angle compared to one-dimensional photonic crystal color filters.

High angular tolerant color filter using subwavelength grating

A reflective color filter based on a two-dimensional subwavelength grating is proposed. The filter has been designed by performing numerical simulations and has been fabricated on 5×5 mm2 quartz glass using electron beam lithography. The grating color filter shows reflectance of 74% and spectral width of about 80 nm. Furthermore, it has good angular tolerance, up to ±45°, for unpolarized incident light. By analyzing the reflectance spectral distributions in the band diagram, we found that high angular tolerance is achieved due to the high refractive index contrast of grating structures.

Separated carrier injection control in carbon nanotube field-effect transistors

The ambipolar behavior limits the performance of carbon nanotube field-effect transistors. A double-gate device is proposed to suppress this behavior. In this device, the first gate controls carrier injection at the source contact and the second one controls carrier injection at the drain contact, which can be used to suppress parasitic carrier injection. The effect of the second gate voltage on the performance of the device has been investigated. Our results indicate that by applying a proper voltage range to the second gate, improved device characteristics can be achieved.

Switching of liquid-crystal devices between reflective and transmissive modes using long-pitch cholesteric liquid crystals

We propose liquid-crystal (LC) devices capable of switching between reflective and transmissive modes using the scattering and transparent states of long-pitch cholesteric LCs (CLCs). Two different device configurations can be realized by changing the location of a CLC layer. Low-power operation without the parallax problem can be achieved using the bistable switching of CLCs. We believe that the proposed devices are potential candidates for highly efficient transflective displays.

Improving the ambipolar behavior of Schottky barrier carbon nanotube field effect transistors

Due to the capability of ballistic transport, carbon nanotube field-effect transistors (CNTFETs) have been studied in recent years as a potential alternative to CMOS devices. CNTFETs can be fabricated with ohmic or Schottky type contacts. We focus here on Schottky barrier CNTFETs which operate by modulating the transmission coefficient of Schottky barriers at the contact between the metal and the carbon nanotube (CNT). The ambipolar behavior of Schottky barrier CNTFETs limits the performance of these devices. We show that a double gate design can suppress the ambipolar behavior considerably. In this structure, for an n-type device, the first gate which is near the source controls electron injection and the second gate which is near the drain suppresses hole injection. The voltage of the second gate can be set to a constant voltage or to the drain voltage. We investigated the effect of the second gate voltage on the performance of the device and finally discuss the advantages and disadvantages of these designs.

Pretilt angle control and multidomain alignment of liquid crystals by using polyimide mixed with liquid crystalline prepolymer

We propose a method to control the pretilt angle of liquid crystals by employing a mixture of the vertical alignment polyimide and the liquid crystalline prepolymer (LCP). The pretilt angle between 10° and 90° can be controlled continuously by changing the mixing ratio of the LCP. Multidomains with different pretilt angles can be formed because the pretilt angle between 50° and 90° can be obtained by varying the UV exposure time region by region under a fixed mixing ratio. We confirmed experimentally that proposed alignment layers are thermally stable.

Optimization of single-gate carbon-nanotube field-effect transistors

The performance of Schottky-barrier carbon-nanotube field-effect transistors (CNTFETs) critically depends on the device geometry. Asymmetric gate contacts, the drain and source contact thickness, and inhomogenous dielectrics above and below the nanotube influence the device operation. An optimizer has been used to extract geometries with steep subthreshold slope and high I/sub on//I/sub off/ ratio. It is found that the best performance improvements can be achieved using asymmetric gates centered above the source contact, where the optimum position and length of the gate contact varies with the oxide thickness. The main advantages of geometries with asymmetric gate contacts are the increased I/sub on//I/sub off/ ratio and the fact that the gate voltage required to attain minimum drain current is shifted toward zero, whereas symmetric geometries require V/sub g/=V/sub d//2. Our results suggest that the subthreshold slope of single-gate CNTFETs scales linearly with the gate-oxide thickness and can be reduced by a factor of two reaching a value below 100 mV/dec for devices with oxide thicknesses smaller than 5 nm by geometry optimization.

Simulation of carrier transport in carbon nanotube field effect transistors

We discuss models to describe carrier transport in axial and lateral type carbon nanotube field-effect transistors (CNT-FET). Operation is controlled by the electric field from the gate contact which can lead to strong band bending, allowing carriers to tunnel through the interface barrier. We find that the difference between lateral and axial CNT-FETs is that in devices with axially aligned carbon nanotubes tunneling becomes negligible and transport can be modeled by means of thermionic emission. In lateral CNT-FETs tunneling dominates, for which we present a model for the transmission coefficient using the WKB method and a non-parabolic dispersion relation. The simulated output and transfer characteristics show reasonable agreement with experimental data for both lateral and axial CNT-FET devices.

The effect of device geometry on the static and dynamic response of carbon nanotube field effect transistors

A numerical study of ohmic contact carbon nanotube field effect transistors is presented. The effect of the gate-source and gate-drain spacers on the static and dynamic response of the device was studied. Simulation results suggest that by appropriately selecting the gate-source and gate-drain spacers both the dynamic and static characteristics of the device are improved.