Chihiro Iwamoto

Nanostructure of wetting triple line in a Ag–Cu–Ti/Si3N4 reactive system

Abstract Nanometer scale structures around wetting triple lines were studied in a Ag–Cu–Ti/Si 3 N 4 reactive system. Changes in the contact angle and radius of the molten metal on the substrate as a function of time were also measured in the system as macroscopic wetting behaviors. The macroscopic wetting behaviors showed two wetting stages and double layered reaction products consisting of upper Ti 5 Si 3 and lower TiN layers were observed in both first and second stages. The reaction product always lay in front of the triple line defined as a triple junction of Ag–Cu–Ti alloy/Ti 5 Si 3 /atmosphere. At the front of the reaction product, a dominant phase changed from TiN in the first stage to Ti 5 Si 3 in the second stage. It is considered that the structural change is one of the reasons why the macroscopic wetting behavior changed, and that the structural change was caused by a decrease of Ti activity as the reactive wetting progressed.

Atomic morphology and chemical reactions of the reactive wetting front

Abstract Morphology at the reactive wetting front during spreading was first observed in situ at an atomic scale using a high resolution transmission electron microscope. Two typical nanomorphologies of an Ag–Cu–Ti molten alloy were observed on a SiC substrate depending on the nucleation position of the reaction product, TiC. In the case of the precursor atomic thin layer spreading ahead of the droplet of the molten alloy, the TiC nucleated in the thick region of the precursor, which was a few nm behind the tip of the precursor layer. In contrast, molten alloy spreading with TiC growing simultaneously at the tip was also observed. In this case, the precursor layer on SiC did not exist. Nucleation and growth rate of the reaction product were related to the tip morphology.

Sublattice Reversal in GaAs/Si/GaAs (100) Heterostructures by Molecular Beam Epitaxy

Sublattice reversal in III-V compound semiconductors grown on group-IV epitaxial layers on III-V substrates has been proposed for fabricating nonlinear optical devices with domain-inverted compound semiconductor structures. Sublattice reversal epitaxy is demonstrated in the GaAs/Si/GaAs (100) system and confirmed by reflection high energy electron diffraction, cross-sectional transmission electron microscopy, anisotropic etching, and optical second-harmonic generation measurements. The present sublattice reversal seems to be assisted by self annihilation of antiphase domains generated at GaAs/Si interfaces.

High-quality cBN thin films prepared by plasma chemical vapor deposition with time-dependent biasing technique

Cubic boron nitride thin films were deposited on silicon (111) wafers by inductively coupled plasma-enhanced chemical vapor deposition. The influences of Ar flow rate and the time-dependent substrate biasing condition on the composition and the transition layers thickness of cBN films were systematically investigated. By using the time-dependent biasing technique, with decreasing the substrate bias voltage gradually from sputtering mode to a final appropriate value for cBN deposition mode, 600 nm thick high quality and stoichiometric cBN films consisting of more than 98% cubic phase were successfully deposited with the deposition rate of 1.5 nm/s. The transition layer consisted of an amorphous layer and a turbostratic boron nitride layer that could be reduced less than 10 nm, which proved that, by using proper deposition technique, high-quality cBN films similar to those prepared by physical vapor deposition methods can be prepared even by chemical vapor deposition.

Interface engineering of cBN films deposited on silicon substrates

We could determine the substrate pretreatment conditions required for the deposition of cubic boron nitride films without an interfacial amorphous layer by investigating nanostructures, chemical composition, and bonding states of the amorphous layer, which inevitably grew prior to the growth of cubic BN in ion-assisted chemical vapor deposition. The amorphous layer was composed of a native Si oxide layer and a complex oxide layer consisting of B, N, Si, and 10–20 at. % oxygen. However, by the substrate pretreatment, 1200 K heating in 20 mTorr H2 atmosphere for 120 min, the crystallinity of the Si substrate surface could be retained throughout the removal of the native oxide layer, and turbostratic BN was revealed to grow directly on the Si substrate. In addition, we could reduce the thickness of the turbostratic BN layer to less than 3 nm, and increase the adhesion strength markedly. The formation of the amorphous layer was found to be caused by two factors: the native oxide layer and the ion bombardment ...

Nanostructures of the turbostratic BN transition layer in cubic BN thin films deposited by low-pressure inductively coupled plasma-enhanced chemical vapor deposition

Nanostructures of the turbo-stratic boron nitride (tBN) transition layer in cubic BN (cBN) thin films deposited on silicon substrates by low-pressure inductively coupled plasma-enhanced chemical vapor deposition have been studied by means of cross-sectional high-resolution transmission electron microscopy. The variation of tBN (0002) basal plane orientation at different growth stages, from random to texture and then random again, was observed. Quantitative analysis of tBN orientation revealed that cBN nucleation occurred when approximately 64% of the tBN c axis oriented parallel to the substrate surface in the angle ±10° and the distribution function drawn in a polar diagram was approximated by the function: cos6 (θ). At the interface between tBN and cBN, both hexagonal and rhombohedral configurations were observed, and both were found to act as structural precursors and/or deposition sites for cubic boron nitride nucleation. In addition, the compressed tBN configuration accompanied by d spacing contracti...

Direct growth of c-BN on a mono-structured transition layer by plasma-enhanced chemical vapor deposition

Inductively coupled plasma-enhanced chemical vapor deposition with a time-dependent biasing technique was applied for cubic boron nitride (c-BN) deposition on a Si substrate, and the growth mode of the transition layer was investigated by high-resolution transmission electron microscopy. It was found that elimination of either an amorphous layer or a turbostratic BN (t-BN) layer was possible, and the two-layered structure, namely, direct growth of a c-BN layer on an amorphous layer or a t-BN layer on a Si substrate, was realized by pre-treatment conditions of positive bias.

Nanometric inversion domains in conventional molecular-beam-epitaxy GaN thin films observed by atomic-resolution high-voltage electron microscopy

GaN films grown on sapphire substrates by conventional molecular-beam epitaxy were investigated by means of atomic-resolution high-voltage electron microscopy (ARHVEM). The atomic positions of Ga and N could be directly discriminated by ARHVEM to determine the polarity in GaN. It was revealed that N polarity GaN films possessed a high density of nanometric inversion domains (IDs) with Ga polarity. The ID boundary was constructed by an inversion and a c/2 translation, and formed fourfold and eightfold coordination along the boundary.

Atomic observations at the reactive wetting front on SiC

Abstract Reactive wetting of a Ag-Cu-Ti molten alloy on a SiC substrate was observed in situ using a high-resolution transmission electron microscope. SiC dissolution and TiC nucleation and growth were repeated at the spreading front. The formation process of the interface between SiC and TiC was classified into two types depending on the nucleation site of the TiC. When the TiC nucleated on SiC. the interface between SiC and TiC was coherent and two crystallographic orientational relationships were observed. The stability of these relationships is discussed using a geometrical model. In contrast, when the TiC nucleated on preformed TiC, the TiC grew continuously with SiC dissolution.

Wurtzite-type ZnS nanoparticles by pulsed electric discharge

The synthesis of wurtzite-type ZnS nanoparticles by an electric discharge submerged in molten sulfur is reported. Using a pulsed plasma between two zinc electrodes of diameter 5 mm in molten sulfur, we have synthesized high-temperature phase (wurtzite-type) ZnS nanocrystals with an average size of about 20 nm. The refined lattice parameters of the synthesized wurtzite-type ZnS nanoparticles were found to be larger than those of the reported ZnS (JCPDS card no 36-1450). Synthesis of ZnMgS (solid solution of ZnS and MgS) was achieved by using ZnMg alloys as both cathode and anode electrodes. UV-visible absorption spectroscopy analysis showed that the absorption peak of the as-prepared ZnS sample (319 nm) displays a blue-shift compared to the bulk ZnS (335 nm). Photoluminescence spectra of the samples revealed peaks at 340, 397, 423, 455 and 471 nm, which were related to excitonic emission and stoichiometric defects.