Seongwook Choi

Resistive switching mechanism of TiO2 thin films grown by atomic-layer deposition

The resistive switching mechanism of 20- to 57-nm-thick TiO2 thin films grown by atomic-layer deposition was studied by current-voltage measurements and conductive atomic force microscopy. Electric pulse-induced resistance switching was repetitively (> a few hundred times) observed with a resistance ratio ⪢102. Both the low- and high-resistance states showed linear log current versus log voltage graphs with a slope of 1 in the low-voltage region where switching did not occur. The thermal stability of both conduction states was also studied. Atomic force microscopy studies under atmosphere and high-vacuum conditions showed that resistance switching is closely related to the formation and elimination of conducting spots. The conducting spots of the low-resistance state have a few tens times higher conductivity than those of the high-resistance state and their density is also a few tens times higher which results in a ∼103 times larger overall conductivity. An interesting finding was that the area where the ...

Liquid Crystal Lens for Compensation of Spherical Aberration in Multilayer Optical Data Storage

In multilayer recording, it is strongly required to compensate the spherical aberration caused by a difference in substrate thickness. A novel liquid crystal (LC) lens is designed and fabricated to compensate the spherical aberration in this study. The new structure of the LC lens includes both concave and convex surfaces, which can compensate the spherical aberration with a relatively long range. Since a previously developed LC panel showed a very low tolerance to the shift of an objective lens, a new component has been proposed with a special LC lens structure to improve both the tolerance and compensation range. By optimized curvature control with a spherical LC lens, the aberration of a Blu-ray optical pickup can be maintained at 0.018λrms even for a thickness variation of ±25 µm. Finally, the LC lens has been fabricated using a standard one drop filling (ODF) process and evaluated by measuring the variation in focal length as a function of applied voltage.

Characteristics of gate-all-around silicon nanowire field effect transistors with asymmetric channel width and source/drain doping concentration

We performed 3D simulations to demonstrate structural effects in sub-20 nm gate-all-around silicon nanowire field effect transistors having asymmetric channel width along the channel direction. We analyzed the differences in the electrical and physical properties for various slopes of the channel width in asymmetric silicon nanowire field effect transistors (SNWFETs) and compared them to symmetrical SNWFETs with uniform channel width. In the same manner, the effects of the individual doping concentration at the source and drain also have been investigated. For various structural conditions, the current and switching characteristics are seriously affected. The differences attributed to the doping levels and geometric conditions are due to the electric field and electron density profile.

Fabrication of

A new concept of device structure that can selectively change the injection carrier type through a thin energy band engineering layer is proposed and demonstrated using the device simulation. As an example, the structure is applied to achieve the n-type field-effect transistor using carbon nanotube network (CNN). Tin oxide (SnO2) layer placed between an Au electrode and a CNN channel is used as an energy band engineering layer for enhancing an electron injection. By just adding the band engineering layer in the conventional p-type device, the n-type characteristics with -40 to +40 V bottom gate sweep is successfully demonstrated experimentally without other manipulations.

n

Non-Arrhenius behavior has been reported in a various temperature range for the retention time of CT Flash memories. In order to understand the physical origin of the multiple activation energy due to the non-Arrhenius behavior, we conduct a simulation study using a 3D self-consistent numerical simulator developed in-house. As a result, it is found that both vertical and lateral charge transport in the conduction band of nitride layer are responsible for the non-Arrhenius retention characteristic. Also, the tunneling current through the bottom oxide and a lifetime criteria are turned out to be the key parameters which determine the multiple activation energy.

-Type CNT Field-Effect Transistor Using Energy Band Engineering Layer Between CNT and Electrode

We exploit the potential of the pulsed tunneling bias condition in silicon junctions (the Zener junction and the drain surface junction in nMOSFETs) to detect near-infrared (NIR) photon signals. A combination of the Franz-Keldysh effect and avalanche multiplication with minimization of the thermal effect through a pulsed tunneling bias is found to provide infrared radiation sensitivities as high as 1.18 and 0.89 A/W at the wavelengths of 1310 and 1550 nm, respectively, in a Zener diode. The potential of the drain surface junction in MOSFETs to function as an NIR sensor is also investigated using a similar bias scheme.

Investigation on multiple activation energy of retention in charge trapping memory using self-consistent simulation

A new optical sensor system, called the pseudo-bipolar junction transistor (BJT) optical measurement system (PBOS), based on a pseudo-BVceo of the BJT is proposed by adding a back-to-back connection of a laser diode (LD) (or an LED) and a p-i-n photodiode (PD) in the conventional optical measurement system operated in the photoconductive mode. A back-to-back connection of two optoelectronic devices and illumination of the light from the LD to the PD generates an optical current gain in the PD. It is similar to the current flowing mechanism in the BJT under the base open condition, in which the forward emitter-base junction current generates an electrical current gain in the base-collector junction. Similar to the negative differential resistance (NDR) after BVceo of the BJT, the NDR is observed in the PBOS. Operating the PBOS in the NDR region, the system can provide much higher sensitivity and lower limit of detection compared with the conventional optical measurement system in the photoconductive mode with a p-i-n PD. We show a mathematical model of the sensitivity of the PBOS to the transmittance of the optical path and our initial data for glucose detection as a potential application of the system.

Near-Infrared Detection Using Pulsed Tunneling Junction in Silicon Devices

In this paper, the Si–H bond dissociation rate is calculated under a negative bias temperature instability (NBTI) condition that considers the quantum effect on the hole density in the inversion layer of a metal–oxide–semiconductor field-effect transistor (MOSFET). The physical model used in this study is composed of two terms: the number of holes in that Si–H bond, and the polarization of the Si–H bond under an external electric field. By adopting a density-gradient (DG) method with a penetration boundary condition and the Wentzel–Kramers–Brillouin (WKB) approximation, the penetrated hole density profile in the gate oxide and the tendency towards the hole amount in the Si–H bond according to the electric field have been identified and compared with other works. The results show that the NBTI field dependence and the lifetime of the devices under NBTI stress correlate to the power-law dependency.

A Pseudobipolar Junction Transistor for a Sensitive Optical Detection of Biomolecules

We propose a TDDB model which can predict the breakdown time distributions when the trap distribution in the oxide is “non-uniform”. This is an extension of the conventional cell-based model that has been limited to the case of uniform trap distribution. The verification of the proposed model is conducted by comparing it with the Monte Carlo simulation. It turns out that the proposed model successfully reproduce the MC simulation result for various trap profiles including the cases of a high-K gate stack and a BEOL oxide. Since the model can be coupled with more realistic trap generation models, more accurate predictions based on the rigorous physics may possible including the percolation theory and oxide degradation models.

An Analysis of the Field Dependence of Interface Trap Generation under Negative Bias Temperature Instability Stress using Wentzel–Kramers–Brillouin with Density Gradient Method

We present an experimental and simulation study about a desorption of albumin, a representative nonselective molecules in serum, on carbon nanotube (CNT) surface as an electrical bio sensing channel under the pulse train condition. The motivation of the study on binding kinetics between CNT surface and albumin is to suppress the adsorption of nonselective proteins in blood such as albumin, thereby enhancing the selectivity of the electrical biosensor. To theoretically model the behavior of molecules and ions under the step pulse bias, the physics on the reaction rate, mass transport, and the resulting surface pH-value are considered using the Poisson and drift-diffusion equations. For the simulation model, the phosphate buffered saline is considered as the electrolyte solution and albumin is considered as a representative charged molecule for nonspecific binding in serum. Both the transient simulation and experimental result indicate that the suppression of the nonspecific binding under the pulse train is...