Wontaek Park

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.

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.

Junction area dependence of breakdown characteristics in magnetic tunnel junctions

Breakdown characteristics of the magnetic tunnel junctions (MTJ) with different junction areas of S=200 μm2 and S=0.5 μm2 are investigated under constant voltage stress. The breakdown process is found to be quite different for the two junction areas. For the large junctions with S=200 μm2, magnetoresistance (MR) ratio decreases gradually with increasing time of constant voltage stress and lasts for 10–30 min. The low frequency 1/f noise power also increases with increasing stress time due to the Johnson and shot noises, caused by current flowing through ohmic shorts or pinholes across tunnel barrier. Meanwhile, the junctions with S=0.5 μm2 show abrupt junction breakdown after stress time of 1–7 h. There is no significant change in both MR ratio and its bias dependence during the voltage stress. In particular, soft-breakdown events are observed before total breakdown occurs. The junction breakdown of small junction area is of intrinsic nature of the junction while the one of large junction area is due to e...

Spatial variation of plasma parameters and ion acceleration in an inductive plasma system

Plasma parameters of inductively coupled plasma system with an annular plasma source have been studied experimentally. At low pressures (about 1mTorr), electron temperature inside the plasma source is rather high (8–13eV) and is much greater than in the diffusion (main) chamber (4–5eV). The plasma potential inside the source is also much higher than in the main chamber. There is a rapid drop of the electron temperature and plasma potential at the boundary between the plasma source and the main chamber. The drop of the plasma potential at the boundary (about 20V) means the existence of a strong axial electric field, which retards the electrons inside the plasma source and accelerates the ions from the source into the main chamber. Measurements of ion energy distributions in the main chamber volume reveal the existence of ions with kinetic energies about 15eV.

Effect of nitrogen plasma treatment at the Al2O3/Fe interface in magnetic tunnel junction

We investigated the effects of nitrogen plasma treatment on top surface of Fe pinned layer for short times (tex=0, 10, 30, and 60 s) in magnetic tunnel junctions and annealing of the junctions. The nitrogen-treated junctions show much reduced magnetoresistance (MR) ratio and significantly lower resistance-area (RA) products compared with the untreated junction, i.e., MR≈3%, RA≈30 kΩ μm2 for tex=10 s and MR≈10%, RA≈60 kΩ μm2 for tex=0 s. The untreated junction showed enhanced MR ratio up to about 17% and higher RA (≈70 kΩ μm2) upon thermal annealing at Ta=230 °C, as expected. For the nitrogen-treated junctions, while the MR ratio also increases up to about 16% upon annealing at Ta=230 °C, which is almost the same value as the one of the optimal reference junction, the RA values of the annealed junctions still keep as low as their initial values. We believe that the redistribution of nitrogen during the annealing process is responsible for the change of properties of nitrogen-treated junction. The bias depe...

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.

Improving DC and AC characteristics of ohmic contact carbon nanotube field effect transistors

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

Ponderomotive effects in an electromagnetic accelerator

An inductive coupled plasma (ICP) source of a new type for dry etching lias been developed. It is driven by a planar inductive coil, which is placed at the top of the channel formed by two coaxial dielectric cylinders. At low pressure, the radial component of the radiofrequency (RF) magnetic field causes a ponderomotive force acting on the electrons directed towards the bottom of the channel. Charge separation gives rise to an ambipolar electric field, which retards electrons and accelerates ions. The source represents an electrodeless electromagnetic accelerator (KMA) and can create an axially directed flux of ions. EMA has been modeled as an axisymmetric ICP discharge in argon gas based on experimental data of the plasma density and electron temperature profile. The magnetic field and RF current densities obtained have been used for calculation of the Lorentz force acting on the electrons and ambipolar plasma potential. Simulations of electron trajectories show that electrons are trapped in the EMA channel and make a number of oscillations before leaving the channel. The ponderomotive potential produces acceleration of the ions in the channel up to 13 -16 eV.

Geometry-dependence of the DC and AC Response of Ohmic Contact Carbon Nanotube Field Effect Transistors

In this work the DC and AC responses of ohmic contact carbon nanotube field effect transistors are investigated. To account for ballistic transport in these devices, the coupled system of Poisson and Schrödinger equations was solved. Good agreement between simulation and experimental results confirms the validity of this model. For AC analysis the quasi static approximation was assumed. Simulation results indicate the both the DC and AC response are effectively dependent on the device geometry. Therefore by careful device design, optimized device characteristics can be achieved.

Anisotropic etching in inductive plasma source with no rf biasing

An inductive plasma source driven with phase shifted antenna coils at 2 MHz has been developed to accelerate ions for semiconductor etching process. The experiment was carried out in SF6/O2/Ar gas mixtures in the pressure range between 0.3 and 0.9 mTorr and rf power between 0.6 and 1.5 kW. Measurement of the ion energy spectra behind the wafer has shown high energy ions (up to 70 eV). An anisotropic etching (without rf biasing) of a polysilicon film has been demonstrated in this experiment. The acceleration of the electrons was numerically studied based on the fluid theory. The numerical results show that electrons affected by Lorentz force and thermal pressure gradient make axial electron currents, which contribute to form axial electric fields and ion acceleration.