Iwao Ohdomari

Enhancing semiconductor device performance using ordered dopant arrays

As the size of semiconductor devices continues to shrink, the normally random distribution of the individual dopant atoms within the semiconductor becomes a critical factor in determining device performance—homogeneity can no longer be assumed. Here we report the fabrication of semiconductor devices in which both the number and position of the dopant atoms are precisely controlled. To achieve this, we make use of a recently developed single-ion implantation technique, which enables us to implant dopant ions one-by-one into a fine semiconductor region until the desired number is reached. Electrical measurements of the resulting transistors reveal that device-to-device fluctuations in the threshold voltage (Vth; the turn-on voltage of the device) are less for those structures with ordered dopant arrays than for those with conventional random doping. We also find that the devices with ordered dopant arrays exhibit a shift in Vth, relative to the undoped semiconductor, that is twice that for a random dopant distribution (- 0.4 V versus -0.2 V); we attribute this to the uniformity of electrostatic potential in the conducting channel region due to the ordered distribution of dopant atoms. Our results therefore serve to highlight the improvements in device performance that can be achieved through atomic-scale control of the doping process. Furthermore, ordered dopant arrays of this type may enhance the prospects for realizing silicon-based solid-state quantum computers.

Low Temperature Synthesis of Extremely Dense and Vertically Aligned Single-Walled Carbon Nanotubes

A novel point-arc microwave plasma chemical vapor deposition (CVD) apparatus was employed to grow single-walled carbon nanotubes (SWNTs) on Si substrates coated with a sandwich-like nano-layer structure of 0.7 nm Al2O3 (top)/0.5 nm Fe/5–70 nm Al2O3 by conventional high frequency sputtering. The growth of extremely dense and vertically aligned SWNTs with an almost constant growth rate of 270 µm/h within 40 min at a temperature as low as 600°C was demonstrated for the first time. The volume density of the as-grown SWNT films is as higher as 66 kg/m3.

Novel Interatomic Potential Energy Function for Si, O Mixed Systems

A novel interatomic potential energy function is proposed for condensed systems composed of silicon and oxygen atoms, from SiO2 to Si crystal. The potential function is an extension of the Stillinger-Weber potential, which was originally designed for pure Si systems. All parameters in the potential function were determined based on ab initio molecular orbital calculations of small clusters. Without any adjustment to empirical data, the order of stability of five silica polymorphs is correctly reproduced. This potential realizes a large-scale modeling of SiO2/Si interface structures on average workstation computers.

Real-time imaging of single-molecule fluorescence with a zero-mode waveguide for the analysis of protein-protein interaction

Real-time imaging of single-molecule fluorescence with a zero-mode waveguide (ZMW) was achieved. With modification of the ZMW geometry, the signal-to-background ratio is twice that obtainable with a conventional ZMW. The improved signal-to-background ratio makes it possible to visualize individual binding-release events between chaperonin GroEL and cochaperonin GroES at a concentration of 5 microM. Two rate constants representing two-timer kinetics in the release of GroES from GroEL were measured with the ZMW, and the measurements agreed well with those made with a total internal reflection fluorescence microscopy. These results indicate that the novel ZMW makes feasible the direct observation of protein-protein interaction at an intracellular concentration in real time.

Study of the interfacial structure between Si (100) and thermally grown SiO2 using a ball‐and‐spoke model

Structural models of the a‐SiO2/(100)Si interface have been constructed using plastic balls and spokes to study the atomic scale structure of the thermally grown a‐SiO2/(100)Si interface. Various properties of the models such as distortion energy, composition, and interface undulation have been estimated on the basis of the models. The results of the simulation indicate that the energetically favorable interface is not flat but undulated with (111) facets or has a transition region with partially oxidized Si atoms. High‐resolution transmission electron microscopy images have also been simulated for the models.

Investigation of thin‐film Ni/single‐crystal SiC interface reaction

Interface reaction between Ni thin film and bulk SiC during heat treatment was investigated by MeV ion backscattering spectrometry using resonance scattering of helium‐carbon, x‐ray diffraction, and Auger electron spectroscopy (AES). Polycrystalline nickel‐silicide, Ni2Si, was formed by heat treatment at 600 °C in forming gas. Carbon compounds were not detected in the reaction products. Carbon was distributed uniformly with a concentration of about 25 at. % in the reacted film, and the C KLL line shape of AES in the reaction products as similar to that of elementary carbon.

Elemental composition of β-Sic(001) surface phases studied by medium energy ion scattering

Abstract β-SiC surfaces have been investigated in terms of surface composition and reconstruction by medium energy ion scattering (MEIS), Auger electron spectroscopy (AES), and low energy electron diffraction (LEED). A (3 × 2) phase is produced by evaporating Si on a β-SiC surface. Heat treatment at 1065°C causes consecutive transformation into (5 × 2), c (4 × 2), (2 × 1), (1 × 1) and c (2 × 2) phases. Quantitative analysis of MEIS spectra shows that the c (4 × 2) surface has a full silicon topmost layer, whereas the c (2 × 2) surface has a full carbon topmost layer. The (3 × 2) and (5 × 2) phases are believed to originate from additional Si dimer rows on top of a Si terminated crystal.

The structural models of the Si/SiO2 interface

In order to get a comprehensive understanding of a microscopic structure of the Si/SiO2 interfaces, various interface models constructed by using plastic balls and spokes have been characterized in terms of distortion energy and composition of the interface region in the models. The distortion energy has been represented by Keating-type potential. The composition has been simulated with our hypothetical atom probe measurements. The magnitude of the distortion energy at the interface depends on the orientation of crystalline Si and decreases in the order of (100), (110) and (111). The distortion energy at the (100)Si/SiO2 interface delineated by (111) facets is smaller than the one at the flat and abrupt interface. These results are in good agreement with TEM observations that the (111)Si/SiO2, interface is flat while the (100)Si/SiO2 interface is usually rough. A transition region which apparently seems to be stoichiometric SiO is identified by the hypothetical atom probe measurement between crystalline Si and SiO2 in the model of the (100)Si/SiO2 interface delineated by {111} facets.

Low‐temperature redistribution of As in Si during Ni silicide formation

We have investigated the redistribution of implanted As during Ni2Si formation at 275 and 300 °C and NiSi formation at 400 to 700 °C with neutron activation analysis and Hall effect measurement. Some of the implanted As atoms were found to redistribute themselves near the silicide‐Si interface during both Ni2Si and NiSi formation. The depth of the redistribution extends about 100 A into Si and is affected slightly by the formation temperature of NiSi. A fraction of the redistributed As is electrically active and the fraction increases with the annealing temperature. The maximum electrical activity of redistributed As during NiSi formation at 700 °C is estimated to be 6.5%.

Strain distribution around SiO2/Si interface in si nanowires : A molecular dynamics study

We have performed three-dimensional molecular dynamics simulations to investigate strain and stress distributions in silicon nanostructures covered with thermal oxide films, by using our original molecular force field for Si, O mixed systems. We have modeled a wire-shaped nanostructure by carving a Si(001) substrate, and then an oxide film with a uniform thickness was formed by inserting oxygen atom into Si–Si bonds from the surface. The simulation results show that a compressive stress is concentrated on the oxide region in the vicinity of the side SiO2/Si interface of the nanowire. At the top interface, there is also a compressive stress in the [110] direction, whereas the [001] component of the normal stress tensor is almost relaxed. These results suggest that the oxidation is strongly suppressed at the side faces of the silicon nanowire.