Takashi Fuyuki

Floating Nanodot Gate Memory Devices Based on Biomineralized Inorganic Nanodot Array as a Storage Node

The memory effect in floating nanodot gate field-effect-transistor (FET) was investigated by fabricating biomineralized inorganic nanodot embedded metal–oxide–semiconductor (MOS) devices. Artificially biomineralized cobalt (Co) oxide cores accommodated in ferritins were utilized as a charge storage node of floating gate memory. Two dimensional array of Co oxide core accommodated ferritin were, after selective protein elimination, buried into the stacked dielectric layers of MOS capacitors and MOSFETs. Fabricated MOS capacitors and MOSFETs presented a clear hysteresis in capacitance–voltage (C–V) characteristics and drain current–gate voltage (ID–VG) characteristics, respectively. The observed hysteresis in C–V and ID–VG are attributed to the electron and hole confinement within the embedded ferritin cores. These results clearly support the biologically synthesized cores work as charge storage nodes. This work proved the feasibility of the biological path for fabrication of electronic device components.

Study of low-temperature crystallization of amorphous Si films obtained using ferritin with Ni nanoparticles

A polycrystalline silicon thin film with a high crystallinity was obtained using ferritin with a Ni core (7nm), which enabled us to precisely control the density and position of the nucleus for crystal growth. The core density of ferritin adsorbed on the amorphous silicon surface was controlled in the range from 109cm−2to1011cm−2. Crystal growth was performed at 550°C in N2. Crystallinity or grain size strongly depended on Ni core density. Polycrystalline silicon film with the average grain size of 3μm and a high crystallinity was obtained at a low Ni atom density of 1012cm−2.

Floating nanodot gate memory fabrication with biomineralized nanodot as charge storage node

We have demonstrated floating nanodot gate memory (FNGM) fabrication by utilizing uniform biomineralized cobalt oxide (Co3O4) nanodots (Co-BNDs) which are biochemically synthesized in the vacant cavity of supramolecular protein, ferritin. High-density Co-BND array (>6.5×1011cm−2) formed on Si substrate with 3-nm-thick tunnel SiO2 is embedded in metal-oxide-semiconductor (MOS) stacked structure and used as the floating gate of FNGM. Fabricated Co-BND MOS capacitors and metal-oxide-semiconductor field effect transistors show the hysteresis loop due to the electron and hole confinement in the embedded Co-BND. Fabricated MOS memories show wide memory window size of 3–4V under 10V operation, good charge retention characteristics until 104s after charge programming, and stress endurance until 105 write/erase operation. Observed charge injection thresholds suggest that charge injection through the direct tunneling from Si to the energy levels in the conduction and valence bands of Co3O4 and long charge retention...

Electron confinement in a metal nanodot monolayer embedded in silicon dioxide produced using ferritin protein

A metal-oxide-semiconductor (MOS) structure with a buried monolayer of ferritin cores in the SiO2 layer was fabricated and the electron confinement in the cores was confirmed. A monolayer of ferritin molecule was adsorbed on the thermal silicon oxide layer. After the protein of the monolayer was eliminated, the ferrihydrite cores were buried in the silicon dioxide layer. We reduced the cores to conductive iron metal nanodots by low-temperature annealing. X-ray photoelectron spectroscopy and electron-energy-loss spectroscopy measurements confirmed the reduction of the cores. The MOS capacitance with the iron nanodots showed hysteresis in the capacitance-voltage measurement, indicating the charging and discharging behavior in iron nanodots.

Hot carrier analysis in low-temperature poly-Si TFTs using picosecond emission microscope

We have investigated the degradation of n-channel thin-film transistors under dynamic stress. Degradation was examined for various pulse parameters such as rising time or frequency. A shortfall time led to a large degradation. This mechanism was analyzed by using a picosecond emission microscope and a device simulator to examine the transient current, experimentally and theoretically, respectively. We have successfully detected emission at the pulse fall edge for the first time. Emission intensity increased with the decrease in pulse fall time. By means of the transient device simulation, transient current corresponding to the gate pulse was obtained. From the comparison between internal field and transient current, hot carrier current generated in the pulse fall was detected for the first time. A reasonable agreement between the data obtained by the emission microscope and those obtained by the device simulator clearly indicates that hot electrons are the dominant cause of degradation under dynamic stress. On the basis of the comparison between experimental and theoretical results, we proposed a model which takes into consideration electron traps in poly-Si.

Novel Method for Making Nanodot Arrays Using a Cage-like Protein

We have developed a new method using protein supramolecules to build quantum devices. In this paper, the fabrication of an array of quantum nanodots using a cage-like protein is described. A hexagonal array of iron-loaded ferritin molecules (with a diameter of 6 nm) was made on a silicon wafer. This array was treated with ozone to remove the protein shell, leaving an array of ferritin cores with a density of about 1012 dots per square centimeter. The elimination of the protein shell was confirmed by Fourier transform IR spectrophotometry (FTIR) and X-ray photoelectron spectroscopy (XPS). Furthermore, the metal cores were made conductive with heat treatment under hydrogen. The order of the array was not disturbed throughout the treatment. This process named bio nano process is expected to lead to a new approach for the development of nanoscale devices.

Reliability of low temperature poly-silicon TFTs under inverter operation

We have studied the reliability of low-temperature polycrystalline-silicon thin-film-transistors (TFTs) under dynamic bias stress using a CMOS inverter circuit. A remarkable decrease in the mobility and the ON-current was observed in n-channel TFTs under dynamic stress. The degradation depends strongly on the falling edge of the voltage pulse and the number of pulses. Observation by emission microscopy revealed that hot electrons were generated around the edge of the drain region in the n-channel TFT. We also confirmed that TFTs with lightly doped drain were less degraded. Based on these experimental results, a new degradation model was proposed. The model suggests that upon the gate voltage drop, electrons move rapidly to the drain, thus, becoming hot and creating electron traps in the grain boundaries around the drain. Consequently, the ON-current is decreased.

Effects of Dot Density and Dot Size on Charge Injection Characteristics in Nanodot Array Produced by Protein Supramolecules

The charge injection characteristics of nanodot arrays for floating nanodot gate memories (FNGMs) were studied using a metal–oxide–semiconductor (MOS) capacitor having density-controlled arrays of homogenous nanodots in a SiO2 layer. Nanodot arrays were prepared using cage-shaped proteins, Listeria ferritin and ferritin with a nanodot core, the diameters of which are 4.5 and 7 nm, respectively. Dot densities are from 3.3×109 to 1.8×1012 cm-2 for Listeria ferritin and from 3.8×109 to 7.9×1011 cm-2 for ferritin. The capacitance–voltage (C–V) characteristics of the obtained MOS capacitors were measured at 1 MHz by applying a DC bias voltage from -10 to +10 V. The flat-band voltage shift was found to depend on both dot density and dot size, and to be numerically proportional to the sum of the upper hemisphere surface areas of nanodots. It is important to balance dot density and dot size in order to fabricate advanced FNGMs, and the appropriate design of the array is necessary.

Analysis of Anomalous Charge-Pumping Characteristics on 4H-SiC MOSFETs

Anomalous charge-pumping characteristics of 4H-silicon carbide (SiC) MOSFETs were analyzed. Charge-pumping measurements of n- and p-channel 4H-SiC MOSFETs with and without NO annealing were performed. Measurements using various pulse fall times revealed that the geometric component exists in the n-channel 4H-SiC MOSFETs and is particularly large in the unannealed n-channel 4H-SiC MOSFETs with low channel mobility. In addition, influence of interface states on the charge-pumping curves is significant in the unannealed 4H-SiC MOSFETs. The charge-pumping curves are distorted by these two nonideal effects, making the analysis of the charge-pumping curves difficult. A sufficiently long pulse fall time, which is on the order of 1-10 mus for the n-channel 4H-SiC MOSFETs with a 10-mum gate length, is required to minimize the effect of the geometric component.

Crystallization of Double-Layered Silicon Thin Films by Solid Green Laser Annealing for High-Performance Thin-Film Transistors

We report a new laser crystallization method employing double-layered amorphous-Si ( a-Si) thin films for solid green laser annealing (GLA) crystallization that is called GLA double-layered xtallization (GLADLAX). Crystallization of the upper and lower a-Si layers of the double-layered substrate at a single laser scanning was achieved, with the upper a-Si becoming poly-Si with very large crystal grains and the lower a-Si layer becoming microcrystalline Si. Thin-film transistors using the upper layer of poly-Si that is crystallized by the method as their active channels had excellent switching performance, with their mobility exceeding 350 cm2/Vmiddots, demonstrating promising applicability of GLADLAX to thin-film electronics