Papers by Keyword: Defect

Abstract: Hydrogen- and corrosion -related defects of pure iron and AISI 304 and 316 stainless steels were investigated by measuring Doppler broadening energy spectra of positron annihilation. The significant increase in S-parameter after corrosion in iron was interpreted as due to the formation of defects and voids during corrosion processes. However, a decrease of the S-parameter in AISI 304 and 316 stainless steels after corrosion was interpreted as dissolution of passivity oxide film. Furthermore, similar results in iron and stainless steels after hydrogen-charging were observed. Hydrogen damage between the surface and the bulk has a significant variation with depth. The larger increase in S-parameter near the surface after hydrogen-charging mainly comes from the formation of large-size defects (e.g. voids); however the increase in S-parameter in the bulk after hydrogen-charging mainly comes from the production of small-size defects.

Abstract: Recovery behavior of 20% plastically deformation of casting AlSi11.35Mg0.23 in various stages of isochronal annealing has been investigated by positron Lifetime (LT). The experimental results show that the positron mean lifetime is a function of annealing temperature. Lifetime of the positron annihilating in perfect lattice is 187.3ps and in 20% deformed is 229.8 ps. There are two regions in the isochronal annealing, one of them relating to the point defect and the other to the dislocation. The activation enthalpy for the dislocation is calculated from the isothermal study in the dislocation region from (575-675) K by slow and fast cooling as 0.16±0.02 and 0.53±0.06 eV respectively.

Abstract: The mechanism of slow positron annihilation in ion-implanted Si has been discussed in terms of the Diffusion-Trapping model (DTM). The trapping of positron has been considered in native vacancies (monovacancies) and ion induced vacancies i.e. vacancy clusters. The model has been used to calculate the Doppler broadening line shape parameter (S-parameter) as a function of incident positron energy for different ion-implanted Si. It has been found that at lower energies the monovacancies and vacancy clusters both contribute to the S-parameter while, with the increase in positron energy the vacancy clusters are reduced. The S-parameter is found to be dependent on the fluency of the implanted ions.

Abstract: Multiple-energy nitrogen ions (energies:1 to 100 keV and a net concentration:2.24 x 1020 cm-3) are implanted into ZnO bulk single crystals grown by the hydrothermal method. Rutherford backscattering-channeling studies show the presence of displaced Zn atoms (Zni) of about 4 % in as-implanted samples. An A-emission band related to the interstitial oxygen (Oi) is observed at 580 nm in 600 oC-annealed samples, and a new emission appears at 515 nm in 800 oC-annealed samples. It is proposed that the new emission band consists of the superposition of the green band (~525 nm) observed in unimplanted ZnO and the residual A-emission band. In 800 oC-annealed samples, a band to acceptor transition at 3.26 eV is also observed in addition to a donor to acceptor pair transition, suggesting that nitrogen acceptor is located at about 180 meV above the valence band. A thermally stimulated current peak, P1 (165 meV), which has been attributed to a native point defect, observed in unimplanted samples almost disappears in nitrogen-implanted samples annealed at 800 oC.

Abstract: In this work, we observed and investigated electro-luminescence (EL) from defects in 4H-SiC avalanche photodiodes. The EL irradiance originated from parallel lines oriented along the [11-20] crystallographic direction. Optical microscopy imaging was employed to analyze the intensity distribution of luminescencing lines at different current densities. Electron beam induced current (EBIC) methodology was employed to find correlation between the luminescencing defects and dislocations in the epi-layers. TEM analysis of the substrate region having the brightest luminescencing line was performed. There were a few defects at the depth of about 3 μm from the sample surface where EL intensity had the highest value.

Abstract: In this article, the correlation of surface morphological defects and barrier-height inhomogeneities with the electrical characteristics of defective 4H-SiC Schottky barrier diodes (SBDs) before and after chemical-mechanical polishing (CMP) is investigated. The forward characteristics, an ideality factor and a single barrier height of a SBD, remain the same after CMP, so that CMP does not affect SBD characteristics. Most barrier-height inhomogeneities are eliminated or improved after CMP. Therefore, leakage current induced by barrier-height inhomogeneities are improved by CMP as well. In addition, about 40% of SBDs with carrots inside the active areas exhibits double barriers before CMP. This excludes that carrots are a cause of barrier-height inhomogeneities. In reverse-bias mode, CMP reduces reverse leakage current at low bias and increases breakdown voltage due to the reduction of thermionic field emission and elimination of local enhanced electric fields.

Abstract: We have identified the crystal planes of 4H-SiC, interfacing with gate oxide, which will lead to minimum defect density and lowest interface corrugations. The atomic surface density, surface energy, and surface roughness of various crystal planes of 4H-SiC have been computationally characterized using Molecular Dynamics simulations. We have also investigated the screening distances of defects in SiO2 and SiC using a multiscale approach.

Abstract: The defects produced by irradiation with 7 MeV C+ induce a change in the electrical properties of 4H-SiC Schottky diodes. Capacitance-voltage and Current-voltage characteristics of the diodes fabricated in epilayers doped with different nitrogen concentrations were monitored before and after irradiation with different fluences. The Capacitance-voltage curves show free carrier compensation after low fluence irradiation and it was found that the reduction of carriers per ion induced vacancy increases with nitrogen content. The forward current-voltage characteristics of the diodes show an increase in the series resistance after irradiation. This change is mainly related to the high compensation occurring around the end of the ion range.

Abstract: Defect formation during the ion-implantation/annealing process in 4H-SiC epilayers is investigated by X-ray topography, KOH etching analysis and transmission electron microscopy. Nitrogen and phosphorus ions are implanted in the 4H-SiC epilayers and then activation annealing is performed at 1670 °C. Linearly arrayed or clustered extended defects are found to be formed during the implantation/annealing process by comparing X-ray topography images taken before and after the process. It is confirmed that the defect arrays are formed underneath a shallow groove on the surface and consist of a high density of basal-plane Shockley-type stacking faults.

Abstract: Crystalline recovery mechanism in the activation annealing process of Al implanted 4H-SiC crystals were experimentally investigated. Annealing temperature and annealing time dependence of acceptor activation and activated hole’s behavior were examined. Poly-type recovery from the implantation induced lattice disordering during the annealing was investigated. The existence of meta-stable crystalline states for acceptor activation, and related scattering centers due to annealing is reported To achieve 100% acceptor activation and to reduce strain after ion implantation, annealing at 2000°C for 10 min. was required.