High Uniform Size Research Articles

Three-dimensional islands of Si and Ge formed on SiO 2 through crystallization and agglomeration from amorphous thin films

The crystallization process was examined for amorphous thin films of silicon ( a-Si) and germanium ( a-Ge) on quartz glass (SiO 2) substrate. Three-dimensional crystalline islands were formed through crystallization and agglomeration. These islands indicated a bimodal size distribution. The mechanism of crystalline island ( c-Si, c-Ge) formation was discussed on the basis of thermodynamics. In studying the crystallization of the thin films, the influence of the film-substrate interfacial energy should be taken into consideration. It was found that the thickness of the as-deposited amorphous films is an essential factor in determining the crystallization behavior and in controlling island size. Above all, a high size uniformity of crystalline islands could be obtained under moderate thermal annealing conditions.

Structural studies of nanoporous carbon produced from silicon carbide

X-ray diffraction and small-angle scattering study of nanoporous carbon samples prepared from polycrystalline α SiC and single-crystal 6H SiC is reported. The distribution function of carbon nanoclusters in size was found. In α SiC samples, the small size (10–12 A) of nanoclusters is combined with their high size uniformity. Graphite-like nanoclusters 30–60 A in size were found in samples of both types. In 6H SiC samples, such clusters make up a notable fraction of the volume. The experimentally observed structural anisotropy of the samples is discussed.

Monitoring of the structure of siliceous mesoporous molecular sieves tailored using different synthesis conditions

The hexagonally ordered siliceous mesoporous molecular sieve MCM-41 and the disordered mesoporous molecular sieve designated as KIT-1 were studied by means of the X-ray diffraction (XRD), transmission electron microscopy and low temperature nitrogen adsorption over a wide pressure range. It was shown that the combination of these techniques allows for a thorough characterization of novel mesoporous materials. A repeated pH adjustment happens to be crucial in the synthesis of MCM-41 samples of high pore size uniformity. An additional influence of the salt effect (i.e. the addition of Na 4EDTA during the hydrothermal crystallization) allows one to obtain MCM-41 materials of excellent porous structures and remarkably high thermal and hydrothermal stability. The latter features may be a result of the presence of relatively thick pore walls for those mesoporous molecular sieves. A disordered character of KIT-1, which was synthesized with an addition of Na 4EDTA to the initial synthesis gel, was supported by the fact that the sample exhibits a high pore size uniformity but yet a rather featureless XRD pattern. It was shown that comparative methods, such as the α s-plot, are very useful for analysis of mesoporous molecular sieves, allowing for an accurate calculation of the specific surface area of the samples, which exhibit condensation in mesopores in the pressure range used in the standard BET method. A method to calculate the pore diameter for MCM-41 samples from XRD and adsorption data was shown to be convenient and useful for an accurate pore size assessment.

Room temperature luminescence from self-organized quantum dots with high size uniformity

Self-organized growth of In x Ga 1-x As/GaAs quantum dots (QD) for 0.5 2% is needed, from thermodynamic considerations, for the onset of 3D growth. The presence of fine excitonic features superimposed on the emission peaks confirms that the second peak observed in the PL spectra of quantum dots is from an excited state transition. We believe that the saturation of the ground-state emission observed under moderately high excitation photon densities is due to the long decay time constant (∼2.2 ns) as measured by time-resolved photoluminescence. Temperature variation of luminescence intensity shows a sharper decay for the ground-state emission than for the excited-state emission. Room-temperature luminescence is thus dominated by the excited state emission. Broad area lasers made from separate confinement heterostructure (SCH) laser structures with In 0.4 Ga 0.6 As quantum dots as the active region showed lasing from excited state, with J th = 650 A/cm 2 , and the single-mode ridge lasers gave a modulation bandwidth of 5 GHz at 2.51 th .

Epitaxial regrowth of Ge films on (001) GaAs by in situ thermal pulse annealing of evaporated amorphous germanium

Germanium thin films have been epitaxially regrown on (001) GaAs by in situ thermal pulse annealing of evaporated amorphous germanium under <102 Watt/cm2 broad-band irradiation in high vacuum. Epitaxial regrowth was found to occur only when the duration of the thermal pulse (te) was greater than a critical value tc (≃3.20 s). The crystal quality of the resultant film was examined by high resolution x-ray diffraction technique (HRXRD) and grazing-incidence x-ray diffraction technique (GIXRD). All rocking curves were found to have a full width at half-maximum of about 0.02°. Both HRXRD and GIXRD confirmed the Ge overlayer was grown epitaxially as well as pseudomorphically on the substrate. Scanning electron microscopy and atomic force microscopy revealed the very different surface morphologies resulting from different te. For te<tc, columnar germanium grains with a four-fold symmetry and a high uniformity in size were found, while for te⩾tc, epitaxial regrowth was observed. It is suggested that epitaxial regrowth takes place via a temporary formation of liquid phase Ge.

Cross‐species amplification of glutamine synthetase cDNA by polymerase chain reaction with degenerate primers

We have developed and optimized a consistent polymerase chain reaction (PCR)‐based strategy to quickly obtain specific sequence information on novel plant glutamine synthetase (GS, EC 6.3.1.2) cDNAs. Two sets of degenerate primer pairs were designed to discriminate regions conserved in either any kind of GS messenger or exclusively in those for the chloroplastic GS. Novel GS cDNA sequences were successfully amplified from total RNA obtained from 14 different monocotyledonous and dicotyledonous plants. The procedure, coupled with a further restriction analysis, allowed us to uncover the presence of GS cDNA polymorphism, which most likely stems from the different GS gene family members within a single species. Contrary to previously reported strategies in other systems, GS cDNA oligonucleotide primers were designed keeping the degeneracy level to a minimum, together with a high melting temperature. This approach proved to be particularly effective, generating high yields of the expected products without requiring extra nested amplification steps or time‐consuming optimization steps for each species GS cDNA amplification. Different clones containing sequence information from either the coding or the 3′‐untranslated regions were further sequenced and characterized, confirming the high sequence identity and size uniformity the of GS cDNAs across higher plant species. Therefore, this approach is proposed as a stand‐alone procedure to quickly determine the sequence of unknown GS cDNAs, as well as to speed up and complement classical molecular cloning methodologies.

水熱・焼成法によるＢａフェライト置換体微粒子の合成および磁性

Co-Ti substituted barium-ferrite fine particles (500 ∼1000A) with high dispersibility and high uniformity in size were fabricated by hydrothennal synthetic method accompanied by heat treatment, and their magnetic properties were studied. X-Ray diffraction, Mossbauer, magnetization measurements and electron microscopy were used for the characterization. Superior magnetic characteristics, which can cornpete with by other reported methods, were obtained by the present method. Mossbauer spectra with 16.4 kOe applied parallel to the γ-ray show the formation of magnetic easy cone for Co-Ti substituted bariumferrite, that can be the origin of an abrupt magneticsoftening of this system with a small amount of substitution. Increased saturation magnetization was obtained by lowering the Ti contents from equimolecular ratio with Co ions.