Shigeo Yoshii

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.

Electrostatic placement of single ferritin molecules

We electrostatically placed a single ferritin molecule on a nanometric 3-aminopropyltriethoxysilane (APTES) pattern that was on an oxidized Si substrate. The numerical analysis of the total interaction free energy for ferritin predicted that a quadrilateral array of 15nm diameter APTES nanodisks placed at intervals of 100nm would accommodate a single molecule of ferritin in each disk under a Debye length of 14nm. The experiments we conducted conformed to theoretical predictions and we successfully placed a single ferritin molecule on each ATPES disk without ferritin adsorbing on the SiO2 substrate surface.

High-Density and Highly Surface Selective Adsorption of Protein-Nanoparticle Complexes by Controlling Electrostatic Interaction

High-density cage-shaped proteins with inorganic cores were selectively adsorbed as a monolayer onto a 3-aminopropyl-triethoxysilane (APTES) layer on a Si substrate. The electrostatic interaction between the protein and substrate surface was studied and it was proven that protein adsorption density depends on the quantitative balance of surface charge on the substrate and protein. The combination of a highly positive APTES layer and moderately negative ferritin, Fer-4, achieved an adsorption density of 7.6×1011 cm-2 and the combination of the APTES layer and Listeria ferritin (Lis-fer) reached an adsorption density of 1.3×1012 cm-2. The adsorption process including the reduced charge of Lis-fer due to denaturation further enhanced the adsorption density up to 1.5×1012 cm-2, whereas no Lis-fer was adsorbed onto the SiO2 surface under the same conditions. This new technique makes it possible to produce a nanodot monolayer with a density higher than 1×1012 cm-2, which can be applied to floating nanodot gate memories.

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...

Making Monolayer of Inorganic Nanoparticles on Silicon Substrate

A monolayer of inorganic nanoparticles (NPs) was fabricated on a silicon wafer using a cage-shaped protein, ferritin, which can sequester several kinds of inorganic NP in their cavities. Ferritins were bound electrostatically in aqueous condition to the silicon wafer which was modified with aminosilane molecules. The obtained sample was heat-treated at 500°C under oxygen gas, and the protein moiety and aminosilane were completely eliminated. The obtained NP monolayer showed no aggregation or sintering. This new method can be used to produce a dispersed inorganic NP monolayer on a silicon substrate as designed, which could be used as a nanodot array in floating nanodot gate memories.

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.

Suppression of Inhomogeneous Segregation in Graphene Growth on Epitaxial Metal Films

Large-scale uniform graphene growth was achieved by suppressing inhomogeneous carbon segregation using a single domain Ru film epitaxially grown on a sapphire substrate. An investigation of how the metal thickness affected growth and a comparative study on metals with different crystal structures have revealed that locally enhanced carbon segregation at stacking domain boundaries of metal is the origin of inhomogeneous graphene growth. Single domain Ru film has no stacking domain boundary, and the graphene growth on it is mainly caused not by segregation but by a surface catalytic reaction. Suppression of local segregation is essential for uniform graphene growth on epitaxial metal films.

Non-volatile flash memory with discrete bionanodot floating gate assembled by protein template

We demonstrated non-volatile flash memory fabrication by utilizing uniformly sized cobalt oxide (Co(3)O(4)) bionanodot (Co-BND) architecture assembled by a cage-shaped supramolecular protein template. A fabricated high-density Co-BND array was buried in a metal-oxide-semiconductor field-effect-transistor (MOSFET) structure to use as the charge storage node of a floating nanodot gate memory. We observed a clockwise hysteresis in the drain current-gate voltage characteristics of fabricated BND-embedded MOSFETs. Observed hysteresis obviously indicates a memory operation of Co-BND-embedded MOSFETs due to the charge confinement in the embedded BND and successful functioning of embedded BNDs as the charge storage nodes of the non-volatile flash memory. Fabricated Co-BND-embedded MOSFETs showed good memory properties such as wide memory windows, long charge retention and high tolerance to repeated write/erase operations. A new pathway for device fabrication by utilizing the versatile functionality of biomolecules is presented.

Electrostatic self-aligned placement of single nanodots by protein supramolecules

Electrostatic self-aligned positioning of a single 7 nm nanoparticle in the cage-shaped protein ferritin onto an aminosilane disk pattern as large as next-generation photolithography can produce is demonstrated. Genetic modification of the ferritin increased its surface charge density and therefore improved its electrostatic interaction. Single molecules of the recombinant ferritin could achieve self-aligned placement on 32–45 nm disks under the optimal solution condition, which was calculated by numerical analysis. This biological self-aligned placement, incorporated into next-generation photolithography techniques, will be a useful wafer-scale nanofabrication tool.

Resistive random access memory utilizing ferritin protein with Pt nanoparticles

This study reports controlled single conductive paths found in resistive random access memory (ReRAM) formed by embedding Pt nanoparticles (Pt NPs) in NiO film. Homogeneous Pt NPs produced and placed by ferritin protein produce electric field convergence which leads to controlled conductive path formation. The ReRAM with Pt NPs shows stable switching behavior. A Pt NP density decrease results in an increase of OFF state resistance and decrease of forming voltage, whereas ON resistance was independent of the Pt NP density, which indicates that a single metal NP in a memory cell will achieve low power and stable operation.