Papers by Author: Takashi Sekiguchi

Abstract: Small angle grain boundaries (SA-GBs) are known as the most electrically active defects in multicrystalline silicon. These SA-GBs are classified in “general” and “special” by the normal and strong electrical activity at 300K, respectively. In this study, the origins of these electrical activities of SA-GBs were elucidated by using electron beam induced current (EBIC) and transmission electron microscopy (TEM). It was found that both general and special SA-GBs were composed of edge-type and 60 deg / screw dislocations. The fraction of edge dislocation in special SA-GB was higher than that of general one, which suggests that strong electrical activity is mainly originated in edge dislocations.

Abstract: We evaluated the properties of crystalline defects in silicon substrate, and clarified the origin of small-angle grain boundaries. In order to eliminate the effects of grain boundaries, the ingot was fabricated by unidirectional solidification technique with seed crystal. In single-crystalline region, Σ3 twin boundaries and SiC precipitates were observed near the seed crystal. No obvious relationship between twin boundaries and precipitates was observed. These defect decreased once and the precipitations appeared again. The density of precipitates increased through the crystal growth procedure. These precipitates were consisted of Si, C, and N. After the precipitation density increased, the small-angle grain boundaries appeared and some precipitates were observed at the boundaries. We considered the precipitations consisted of light element impurities such as C and N were one of the major origins of the small-angle grain boundary generation.

Abstract: In this work we report the enhancement of the 3C-SiC band edge luminescence induced by the SiO2 shell in SiC/SiO2 core/shell nanowires (NWs) system. We demonstrate that the shell enhances the SiC near band edge luminescence and we argue the formation of a type-I quantum well between the SiC core and the SiO2 shell, with the consequent injection of carriers from the larger band-gap shell to the narrower band-gap core.

Abstract: This study focus on the nature of different Si-based sensitizers for Er³⁺ ions in Silicon- Rich Silicon oxide thin films. The samples were first analyzed by Cathodoluminescence technique to probe all emitting centers in the films. Some of these centers were found to be potential sensitizers for Er³⁺ ions, such as Silicon Oxygen Deficient Center and Non-Bridging Oxygen Hole Center, in addition to the well-known Silicon-nanoclusters (Si-nc). The influence of the thickness was subsequently examined, revealing that the formation of Si-nc is inhibited for films thinner than 100 nm and this led to less sensitization of the Er³⁺ ions. We demonstrate that the introduction of a SiO₂ buffer layer can overcome this issue and increase the luminescence of Er³⁺ ions by a factor of five for films thinner than 50 nm that are usually used for electrically-driven photonic devices.

Abstract: We have characterized optical property of small-angle (SA) grain-boundaries (GBs) in high-pure multicrystalline Si by using cathodoluminescence (CL). Prior to CL measurement, the electrical activity of GBs were evaluated by using electron-beam-induced current (EBIC). The SA-GBs are categorized into two groups with room temperature (RT-) EBIC contrast. The SA-GBs with misorientation angle about 1º give weak RT-EBIC contrast and yield D3 and D4. The SA-GBs with 2.5º show strong EBIC contrast and yield D1 and D2. These correspondences reflect the dislocation density at the SA-GBs. We also found the curious distribution of D1 emission in some special GBs, which is now difficult to explain. It is noticed that large-angle GBs do not show any D-line emissions at all.

Abstract: We report a dynamic and microscopic investigation of electrical stress induced defects in metal-oxide-semiconductor (MOS) devices with high-k gate dielectric by using electron-beam induced current (EBIC) technique. The correlation between time-dependent dielectric breakdown (TDDB) characteristics and EBIC imaging of breakdown sites are found. A systematic study was performed on pre-existing and electrical stress induced defects. Stress-induced defects are related to the formation of electron trapping defects. The origin of pre-existing defects is also discussed in terms of oxygen vacancy model with comparing different gate electrodes.

Abstract: We report the electrical, structural and mechanical properties of grain boundaries (GBs) in multicrystalline Si (mc-Si) based on electron-beam-induced current (EBIC), transmission electron microscope (TEM), and scanning infrared polariscope (SIRP) characterizations. The recombination activities of GBs are clearly classified with respect to GB character and Fe contamination level. The decoration of Fe impurity at boundary has been approved by annular dark field (ADF) imaging in TEM. Finally, the distribution of residual strain around GBs, and the correlations between strain and electrical properties are discussed.

Abstract: The breakdown failure points in the 4H-SiC PiN diodes were analyzed by the electron beam induced current (EBIC). We focused on the failure, which showed the avalanche breakdown, and we determined the failure points by an emission microscopy. We observed the basal plane dislocation around the failure point and at measured temperatures below 200K we found the dark spots in the EBIC. However, in the X-ray topography image, no spots were found around the dislocations. We therefore think that these spots originated from the metal contamination. The electric field was multiplied due to a permittivity change, and this multiplication caused the avalanche breakdown.

Abstract: The effect of phonon confinement and impurity doping in silicon nanowires (SiNWs) synthesized by laser ablation were investigated. The diameter of SiNWs was controlled by the synthesis parameters during laser ablation and the subsequent thermal oxidation. Thermal oxidation increases the thickness of the SiNWs’ surface oxide layer, resulting in a decrease in their crystalline Si core diameter. This effect causes a downshift and asymmetric broadening of the Si optical phonon peak due to phonon confinement. Boron doping was also performed during the growth of SiNWs. Local vibrational modes of boron (B) in silicon nanowires (SiNWs) synthesized by laser ablation were observed at about 618 and 640 cm–1 by Raman scattering measurements. Fano broadening due to coupling between discrete optical phonons and the continuum of interband hole excitations was also observed in the Si optical phonon peak. These results prove that B atoms were doped in the SiNWs.

Abstract: We have succeeded in imaging the leakage sites of hafnium silicate gate dielectrics of metal-oxide-semiconductor field-effect transistors (MOSFETs) by using electron-beam-induced current (EBIC) method. Leakage sites of p-channel MOSFETs were identified as bright spots under appropriate reverse bias condition when the electron beam energy is high enough to generate carriers in the silicon substrate. Most of the leakage sites were observed in the peripheries of shallow trench isolation. These results suggest that some process induced defects are the cause of leakage in these MOSFETs. Our observation demonstrates the advantage of EBIC characterization for failure analysis of high-k MOSFETs.