Kazuhito Kamei

AFM Studies on the Difference in Wear Behavior Between Si and SiO2 in KOH Solution

Wear behavior between a Si tip and a SiO 2 film in KOH solution at various pH values has been examined by using an atomic force microscope We found that the Si tip removal amount strongly depended on the solution pH value and was at a maximum at pH 10.2-12.5 This result indicates that wear behavior of the Si tip is similar to that of actual chemical mechanical polishing of a Si wafer It was also found that the Si removal volume in moles was approximately equal to that of SiO 2 irrespective of the solution pH value This equality implies that a Si-O-Si bridge is formed between one Si atom and one SiO 2 molecule at the wear interface, followed by the oxidation of the Si tip, and finally the bond rupture by the tip movement and the silica species including the Si-O-Si bridge is dissolved in the KOH solution.

Nano-tube-like surface structure in graphite particles and its formation mechanism: A role in anodes of lithium-ion secondary batteries

Nano-structures on the surface of graphite based carbon particles have been investigated by means of high resolution transmission electron microscopy. The surfaces consist of “closed-edge” structures in a similar manner as carbon nano-tube. That is, they are composed of coaxial carbon tubes consisting of adequate coupling of graphite layer edges. These graphite particles are chemically stable and, therefore, applicable for lithium-ion secondary battery anodes. Molecular dynamics simulations based on the Tersoff potential reveal that the vibrations of the graphite layers at the free edges play an important role in the formation of the closed-edge structures. In lithium-ion secondary batteries, Li ions can intrude into bulk carbon anodes through these closed-edge structures. In order to clarify this intrusion mechanism, we have studied the barrier potentials of Li intrusion through these closed edges using the first-principles cluster calculations. From electrochemical measurements, the carbon anodes compos...

Microstructure of porous silicon and its correlation with photoluminescence

Microstructure of porous silicon (PS) was observed through high‐resolution transmission electron microscope to study a relationship between the microstructure and a photoluminescence (PL) from PS. Three PS samples were made through anodization with different current densities. The samples had different PL spectra and were supposed to have different microstructures. Threadlike structures of Si and Si crystallites in the PS layer were observed in all the samples. The threadlike structure has the same density in all samples and has very little structural change, in spite of a significant change of PL spectra. The density of the Si crystallites varies for the samples and has a strong correlation with the PL intensity. No particular change of crystallites shape was observed. The size of the crystallites ranges from a few nanometers to a few tens of nanometers. Although no positive evidence to a quantum confinement is indicated, the Si crystallites are responsible for a light emission from PS.

High-Speed Growth of 4H-SiC Single Crystal Using Si-Cr Based Melt

In this study, we have investigated the rate-limiting process of 4H-SiC solution growth using Si-Cr based melt, and have tried high-speed growth. It is revealed that the rate-limiting process of SiC growth under our experimental condition is interface kinetics, which can be controlled by such factors as temperature and supersaturation of carbon. By enhancing the interface kinetics, SiC crystal has been grown at a high rate of 2 mm/h. The FWHM values of X-ray rocking curves and threading dislocation density of the grown crystals are almost the same as those of seed crystal. Possibility of high-speed and high-quality growth of 4H-SiC has been indicated.

Solution growth of SiC crystal with high growth rate using accelerated crucible rotation technique

We performed solution growth of SiC single crystals from Si-Ti-C ternary solution using the accelerated crucible rotation technique (ACRT). It was confirmed that the growth rate exceeding 200 μm/hr was achievable by several ACRT conditions. This high growth rate might be due to the enhancement of the carbon transport from the graphite crucible to the growth interface using the ACRT. Moreover, the incorporation of inclusions of the Si-Ti solvent in the grown crystal was significantly suppressed by using the ACRT. It was thought that the intensive convection near the growth interface resulted in not only the marked increase of SiC growth rate but also the superior homogeneity in the surface morphology. It was concluded that faster stable growth can be accomplished in the SiC solution growth using the ACRT.

Top-Seeded Solution Growth of 4H-SiC Bulk Crystal Using Si-Cr Based Melt

We have grown high-quality long cylindrical (12 mm thick) 4H-SiC bulk crystals by the meniscus formation technique, which was first applied for the solution growth of bulk SiC. It enabled long-term growth by suppressing parasitic reactions such as polycrystal precipitation around the seed crystal. In addition, we could control the growth angle from −22° to 61° by adjusting the meniscus height. The thickness of the grown cylindrical crystals was 12 mm, which is the largest reported until now, and corresponded to a growth rate of 0.6 mm/h. Smooth morphology growth was maintained on the (000-1) C-face. In cross-sectional transmission optical microscopy images, few solvent inclusions and voids were observed. XRD measurements revealed that the FWHM values of the grown crystals were almost the same as those of the seed crystal.

Solution Growth of Self-Standing 6H-SiC Single Crystal Using Metal Solvent

Silicon carbide (SiC) crystal growth from ternary solutions Si-C-X where X is a transition metal was studied. In order to select the desirable transition element and to determine the solution composition, we have conducted the calculations of ternary phase diagrams by means of CALPHAD (CALculation of PHase Diagrams) method. Preliminary growth experiments without a seed crystal were also performed. Among various Si-based solutions, Si-C-Ti was one of the most effective solutions to increase crystal growth rate compared with Si-C. Optical microscopic observation of the obtained SiC etched by molten KOH showed no micropipe defects in the crystals. We have also performed the growth experiments with 6H-SiC seed crystal under temperature gradient. As a result, we have successfully obtained a 12mm×12mm self standing SiC crystal. Introduction At present, commercially available silicon carbide (SiC) wafers are produced by the Physical Vapor Transport (PVT) technique. Although a large number of attempts have been made on the reduction of structural defects, PVT crystals still exhibit remaining defects, such as micropipes and dislocations. Especially, micropipe is a primary defect degrading electronic devise performance and thought to be inevitable using PVT technique. Thus it is very important to establish new growth method of the low defect SiC bulk crystal. Solution growth technique is a promising answer to this problem. Growth from liquid phase has expected to improve the quality of the crystals because the growth proceeds under thermal equilibrium. In the case of SiC, although the congruent melt cannot be obtained, SiC can be precipitated from Si-C based solutions[1,2,3]. Si solvent (self flux) has only a small C solubility at moderate temperature, which reduces the SiC growth rate two orders of magnitude lower than that of PVT technique. On the other hand, ternary Si-C-X solution, where X is a carefully selected additive metal might exhibit a large C solubility at relatively low temperature. However lack of well-established ternary phase diagram, Si-C-X makes it difficult to select the desirable X element and to design the solution. In this study we report the solution growth of 6H-SiC from ternary Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 123-126 doi:10.4028/www.scientific.net/MSF.457-460.123

Epitaxial Growth of a Low-Density Framework Form of Crystalline Silicon

Crystal growth processes of low-density framework forms of crystalline silicon, named Si clathrates ( Si34 and Si46), during solid phase epitaxy (SPE) have been successfully observed in molecular-dynamics simulations using the Tersoff potential. The activation energy of SPE for Si34 has been found to correspond with the experimental value ( approximately 2.7 eV) for the cubic diamond phase, while the SPE rates of Si46 are much lower than that of Si34. The structural transition from Si46 to Si34 can be also observed during the Si46-[001] SPE. The present results suggest that new wide-gap Si semiconductors with clathrate structures can be prepared using epitaxial growth techniques.

Crystallinity Evaluation of 4H-SiC Single Crystal Grown by Solution Growth Technique Using Si-Ti-C Solution

Crystallinity of 4H-SiC bulk crystal obtained by solution growth technique was characterized mainly by KOH etching of the off-ground and serially ground specimen. Marked reduction of basal plane dislocation, threading edge and screw dislocations during the growth of on-axis crystal was confirmed. Cross-sectional TEM observation revealed the rapid reduction behavior of threading dislocations microscopically. AFM observation of as-grown morphology showed that screw dislocation dipoles is related to the reduction of threading screw dislocations and single domain formation, which is essential for establishing the high crystallinity.

Analysis of Local Lattice Strain Around Oxygen Precipitates in Czochralski-Grown Silicon Wafers Using Convergent Beam Electron Diffraction

The local lattice strain field around oxygen precipitates in Czochralski-grown silicon (CZ-Si) wafers has been measured quantitatively using convergent beam electron diffraction (CBED). As a result of the strain analysis from higher-order Laue zone patterns in the CBED disk, strain of the silicon lattices was found in the vicinity of oxygen precipitates, i.e., platelet type and polyhedral type. The strain along the normal direction to the precipitate is compressive, and the strain along the parallel direction to the precipitate is tensile. The lattice strain field around the precipitate decreases monotonically as a function of distance from the precipitate/matrix interface. Further, the morphological change in the growth process of the precipitate is important for the formation of the local lattice strain.