Dependence of process characteristics on atomic-step density in catalyst-referred etching of 4H-SiC(0001) surface

Catalyst-referred etching (CARE) is a novel abrasive-free planarization method. CARE-processed 4H-SiC(0001) surfaces are extremely flat and undamaged over the whole wafer. They consist of single-bilayer-height atomic steps and atomically flat terraces. This suggests that the etching properties depend principally on the atomic-step density of the substrate surface. We used on-axis and 8 degrees off-axis substrates to investigate the processing characteristics that affect the atomic-step density of these substrates. We found a strong correlation between the removal rate and the atomic-step density of the two substrates. For the on-axis substrate, the removal rate increased with increasing surface roughness, which increases with an increasing atomic-step density. The removal rate ratio is approximately the same as the atomic-step density ratio of the two substrates.     

Efficient wet etching of GaN (0001) substrate with subsurface damage layer

Photoenhanced chemical (PEC) etching is applicable for processing an n-GaN (0001) surface rapidly. In this process, the surface oxidation is enhanced by photo-generated holes and the resulting oxide can dissolve into solutions. In current work, we conduct bias-assisted PEC etching in a KOH solution with a positively biased wafer, to remove the crystallographically highly damaged layer. The employed substrate was mechanically polished with diamond slurry of sub-micrometer particle size. Without the positive bias, the rate of PEC etching was quite low because the photogenerated holes were quickly depleted by the recombination process at the crystallographic defects and they could not contribute to the oxidation. On the other hand, in the case where the bias was applied, the photogenerated holes and electrons are separated forcibly in the band-bended surface, which effectively contributed to surface oxidation. As a result, a high removal rate was realized even on the damaged surface.     

Interfacial phase-change memory

Phase-change memory technology relies on the electrical and optical properties of certain materials changing substantially when the atomic structure of the material is altered by heating or some other excitation process. For example, switching the composite Ge(2)Sb(2)Te(5) (GST) alloy from its covalently bonded amorphous phase to its resonantly bonded metastable cubic crystalline phase decreases the resistivity by three orders of magnitude, and also increases reflectivity across the visible spectrum. Moreover, phase-change memory based on GST is scalable, and is therefore a candidate to replace Flash memory for non-volatile data storage applications. The energy needed to switch between the two phases depends on the intrinsic properties of the phase-change material and the device architecture; this energy is usually supplied by laser or electrical pulses. The switching energy for GST can be reduced by limiting the movement of the atoms to a single dimension, thus substantially reducing the entropic losses associated with the phase-change process. In particular, aligning the c-axis of a hexagonal Sb(2)Te(3) layer and the 〈111〉 direction of a cubic GeTe layer in a superlattice structure creates a material in which Ge atoms can switch between octahedral sites and lower-coordination sites at the interface of the superlattice layers. Here we demonstrate GeTe/Sb(2)Te(3) interfacial phase-change memory (IPCM) data storage devices with reduced switching energies, improved write-erase cycle lifetimes and faster switching speeds.     

Anomalous microstructure formed at the interface between copper ribbon and tin-deposited copper plate by ultrasonic bonding

The present paper reports an anomalous microstructure formed at interfaces between Cu ribbons and Sn-deposited Cu plates by ultrasonic bonding. The interface consists of Cu-to-Cu directly bonded part and Sn-dispersed part. In the latter part, Sn is dispersed in the vicinity of the bond interfaces being retained as Sn. The formation process of the interfacial microstructure is discussed on the basis of detailed experimental analysis and theoretical analysis on the solid-state reaction at Cu/Sn interface. The theoretical analysis reveals three important points of the reaction. (1) The formation of Cu₆Sn₅ precedes that of Cu₃Sn. (2) The incubation time for the formation of Cu₆Sn₅ changes discontinuously at the η/η′ transition temperature. (3) The incubation time for the formation of η′Cu₆Sn₅ is longer than that for ηCu₆Sn₅.     

A crossed photon-atom beam method for absolute measurement of total photoionization cross sections on isolated metal atoms: Measurements on Ba and Eu atoms

A crossed photon-atom beam apparatus has been constructed for absolute measurement of total photoionization cross sections of isolated and neutral metallic atoms. The main purpose is to establish the technique which can be used as widely as possible for the metallic elements. Using this apparatus, measurements on Ba and Eu atoms have been made at selected energies in their 4d giant resonance regions 110-140 eV and 140-180 eV, respectively. A monochromatized synchrotron radiation was used as a light source. The target-atom density in the interaction section was determined with accuracy of 9% using the accumulation rate of metallic atoms on a quartz crystal sensor and the average velocity of the atoms obtained by a time-of-flight method combined with a pulsed electron gun. The number of photons was determined with use of a double-ion chamber preferably. The entire systematic errors have been estimated to be 20% for Ba and 27% for Eu. The comparison of the measured cross-section values with previous experimental and theoretical results is reasonable, indicating that the crossed photon-atom beam method is fairly promising technique.     

Preparation and characterization of carbonaceous materials containing nitrogen as electrochemical capacitor

Carbonaceous materials containing nitrogen (C/N materials) were prepared by a pyrolysis of 2,3,6,7-tetracyano-1,4,5,8-tetraazanaphthalene (CAN). A C/N material prepared by the pyrolysis of CAN at 1070 K (CAN-1070 K) had a C/N atomic ratio of 3.0 and a non-crystalline carbonaceous structure with a BET surface area of 880 m² g⁻¹. The material CAN-1070 K showed large capacitances of 160–180 F g⁻¹ and 110–120 F cm⁻³ in case of current density of 10 mA cm⁻² (2 A g⁻¹) by using three-electrode cell in 1 M H₂SO₄ aqueous solution, in comparison with that of activated carbon (160 F g⁻¹ and 55 F cm⁻³) having BET surface area of 2300 m² g⁻¹. ESCA study indicated that pyridinic and quarternary nitrogen atoms existed in the C/N materials, which could result in producing a pseudo-capacitance in addition to the electric double layer capacitance. Also introduction of nitrogen into the carbonaceous material could enhance the wettability of material, which might also improve the capacitance.     

Bacterial communities in petroleum oil in stockpiles

Bacterial communities in crude-oil samples from Japanese oil stockpiles were investigated by 16S rRNA gene cloning, followed by denaturing gradient gel electrophoresis (DGGE) analysis. 16S rRNA genes were successfully amplified by PCR after isooctane treatment from three kinds of crude-oil sample collected at four oil stockpiles in Japan. DGGE profiles showed that bacteria related to Ochrobactrum anthropi, Burkholderia cepacia, Stenotrophomonas maltophilia, Propionibacterium acnes, and Brevundimonas diminuta were frequently detected in most crude-oil samples. The bacterial communities differed in the sampling time and layer. Among the predominant bacteria detected in the crude oil, only three species were found for bacteria isolated on agar plates and were related to Burkholderia, Stenotrophomonas, and Propionibacterium, while Ochrobactrum sp. could not be isolated although this species seemed to be the most abundant bacterium in crude oil from the DGGE profiles. Using an archaea-specific primer set, methanogens were found in crude-oil sludge but not in crude-oil samples, indicating that methanogens might be involved in sludge formation in oil stockpiles.     

Molecular cloning, identification and characterization of 2-methyl-3-hydroxypyridine-5-carboxylic-acid-dioxygenase-coding gene from the nitrogen-fixing symbiotic bacterium Mesorhizobium loti

The gene (mlr6788) of a nitrogen-fixing symbiotic bacterium Mesorhizobium loti MAFF303099 has been identified as a gene coding for 2-methyl-3-hydroxypyridine-5-carboxylic acid dioxygenase (MHPCO), the seventh enzyme in degradation pathway I for pyridoxine, a free form of vitamin B(6). The gene was cloned and overexpressed in Escherichia coli cells co-transformed with chaperonin genes. The homogeneous recombinant enzyme showed similar enzymatic properties to the enzyme from Pseudomonas sp. MA-1. MHPCO was essential for the assimilation of pyridoxine in M. loti, but not for its growth in a nutrient-rich medium. From the infection experiment of a symbiotic plant Lotus japonicus with an M. loti mlr6788 gene disruptant, MHPCO was demonstrated to be dispensable for at least nodule formation on roots of seedlings in symbiosis.     

Fast-scanning atomic force microscopy reveals the molecular mechanism of DNA cleavage by ApaI endonuclease

Newly developed fast-scanning atomic force microscopy (AFM) allows the dissection of molecular events such as DNA-enzyme reactions at the single-molecule level. With this novel technology, a model is proposed of the DNA cleavage reaction by a type IIP restriction endonuclease ApaI. Detailed analyses revealed that ApaI bound to DNA as a dimer and slid along DNA in a one-dimensional diffusion manner. When it encountered a specific DNA sequence, the enzyme halted for a moment to digest the DNA. Immediately after digestion, the ApaI dimer separated into two monomers, each of which remained on the DNA end and then dissociated from the DNA end. Thus, fast-scanning AFM is a powerful tool to aid the understanding of protein structures and dynamics in biological reactions at the single-molecule level in sub-seconds.     

An OTFT‐driven rollable OLED display

Abstract— An 80‐μm‐thick rollable AMOLED display driven by an OTFT is reported. The display was developed so as to be rollable in one direction with an integrated OTFT gate driver circuit. It was successfully operated by an originally developed organic semiconductor, a peri‐xanthenoxanthene derivative. The display retained its initial electrical properties and picture quality even after being subjected to 1000 cycles of a roll‐up‐and‐release test with a radius of 4 mm.