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We present a brief summary of asphericity effects in thermonuclear and core collapse supernovae (SN), and how to distinguish the underlying physics by their observable signatures. Electron scattering is the dominant process to produce polarization which is one of the main diagnostical tools. Asphericities result in a directional dependence of the luminosity which has direct implications for the use of SNe in cosmology. For core collapse SNe, the current observations and their interpretations suggest that the explosion mechanism itself is highly aspherical with a well defined axis and, typically, axis ratios of 2 to 3. Asymmetric density/chemical distributions and off-center energy depositions have been identified as crucial for the interpretation of the polarization P. For thermonuclear SNe, polarization turned out to be an order of magnitude smaller strongly supporting rather spherical, radially stratified envelopes. Nevertheless, asymmetries have been recognized as important signatures to probe A) for the signatures of the progenitor system, B) the global asymmetry with well defined axis, likely to be caused by rotation of an accreting white dwarf or merging WDs, and C) possible remains of the deflagration pattern.
During the last decade, advances in observational, theoretical and computational astronomy have provided new insights into the nature and physics of SNe and gamma-ray bursts. Due to the extreme brightness of these events, they are expected to continue to play important role in cosmology. SNe Ia allowed good measurements of the Hubble constant both by statistical methods and theoretical models.
Near-infrared (NIR) spectroscopy of several stripped-envelope core-collapse supernovae (SNe) are presented. NIR spectra of these objects are quite rich, exhibiting a large number of emission features. Particularly important are strong lines of He I and C I, which probe the outermost ejecta and constrain the pre-collapse mass-loss. Interestingly, the SN 1998bw-like broad-line Type Ic SN 2002ap does not exhibit the strong C I features seen in other Type Ic SNe. NIR spectra also exhibit strong, relatively isolated lines of Mg I, Si I, Ca II, and O I that provide clues into the kinematics and mixing in the ejecta. Finally, late-time NIR spectra of two Type Ic events: SN 2000ew and SN 2002ap show strong first-overtone carbon monoxide (CO) emission, providing the first observational evidence that molecule formation may not only be common in Type II SNe, but perhaps in all core-collapse events.
Near-infrared (NIR) spectroscopy is a powerful tool for the study of supernovae (SNe), offering new insights into the kinematic, chemical, and evolutionary properties of these events. Here we present applications of NIR spectroscopy for the study of three stripped-envelope supernovae, the Type Ib SN 2001B, the Type Ic SN 2000ew and the broad-line Type Ic SN 2002ap. All of the data presented here were obtained using TIFKAM on the 2.4 m Hiltner telescope at MDM Observatory, except for the SN 2002ap data set which also includes spectra obtained at Lick Observatory, IRTF, and Subaru. The reduced spectra are presented in Figures 6.1–6.3.
Nanoscale structures have been recently proposed as charge storage nodes due to their potential applications for future nanoscale memory devices. Our approach is based on the idea of using Si nanodots as discrete floating gates. To experimentally investigate such potential, we have fabricated MOS structures with Si nanocrystals. The dots have been deposited onto an ultra-thin tunnel oxide by chemical vapour deposition, and then annealed at 1000 °C for 40 s, to crystallize all the dots. After deposition the dots have been covered by a CVD SiO2 layer, thus resulting in dots completely embedded in stoichiometric silicon oxide. The nanocrystal density and size have been studied by energy filtered TEM (EFTEM) analysis. An electrostatic force microscope has been used to locally inject the charge. By applying a relatively large tip voltage a few dots have been charged, and the shift in the tip phase has been monitored. The shift in the phase is attributed to the presence of the charge in the sample. A comparison between n and p type samples is also shown.
To form crystalline Si dots embedded in SiO2, we have deposited thin films of silicon rich oxide (SRO) by plasma-enhanced chemical vapor deposition of SiH4 and O2. Then the materials wereannealed in N2 ambient at temperatures between 950 and 1100 °C. Under such processing, the supersaturation of Si in the amorphous SRO film produces the formation of crystalline Si dots embedded in SiO2. The narrow dot size distributions, analyzed by transmission electron microscopy, are characterized by average grain radii and standard deviations down to about 1 nm. The memory function of such structures has been investigated in metal-oxidesemiconductor (MOS) capacitors with a SRO film sandwiched between two thin SiO2 layers as insulator and with an n+ polycrystalline silicon gate. The operations of write and storage are clearly detected by measurements of hysteresis in capacitance-voltage characteristics and they have been studied as a function of bias.
We have studied the effects of nitridation on the leakage current of thin (7-8 nm) gate or tunnel oxides. A polarity dependence of the tunneling current has been found this behavior is related to the presence of a thin silicon oxynitride layer at the SiO2/Si-substrate interface. The oxynitride layer lowers the tunneling current when electrons are injected from the interface where the oxynitride is located (substrate injection). The current flowing across the oxide when electrons are injected from the opposite interface (gate injection) is not influenced by the oxynitride. The increase of nitrogen concentration leads to a decrease of the tunneling current for substrate electron injection.
The evolution of radiation damage and of dopant profiles in Si samples subjected to self-annealing implantation with 160 keV As+ ions, under various transient heating conditions, depending on the beam current density, has been investigated. For temperatures in excess of 880 °C, the formation of voids is evidenced by Transmission Electron Microscopy observations. They are located in a layer extending from the surface over a depth of about 0.8 of the ion projected range, Rp (≃110 nm), while extended interstitial-type defects are observed in the region below. With increasing temperature (due to increasing beam power density and/or irradiation time), voids tend to anneal in the near surface region, while they survive and grow in a region about 40 nm thick, centered at a depth of about 50 nm, where an anomalous peak in the dopant profile is developed. The annealing of the extended interstitial-type defects at depths ≥Rp, which becomes appreciable for T ≥ 1050 °C, is coupled to a large enhancement of dopant diffusivity in the same region. It is argued that the local vacancy supersaturation, which leads to void formation, and the corresponding interstitial excess in the deeper region, which leads to extended defect formation, are a natural consequence of the collision kinetics of the displacement process, as suggested by Monte Carlo simulations reported in the literature. The evolution of damage and the anomalies in the redistribution of dopant, observed by increasing the temperature, are described as the result of the increase in the population of mobile point defects and of their influence on the mechanism of As diffusion in the different regions of the implanted layer.
We find a significant alteration of the surface properties of SI- GaAs as a result of a thermal treatment with SiO under vacuum. Low temperature photoluminescence measurements reveal a tenfold increase in emissions attributed to free or donor bound excitons and the exciton bound to a silicon acceptor. A paramagnetic center is also generated as a result of this treatment. The EPR signal has a g-value of 2.0017 and a linewidth of 0.1 mT. The enhanced photoluminescence and the EPR signal are both quenched by a short exposure to hydrogen plasma at room temperature. Chemical and spectroscopic evidence indicates that the resonance is due to a silicon related center near the GaAs surface. The surface stabilization is attributed to a reaction or incorporation of SiO with the arsenic depleted GaAs surface.
We have detected two dominant paramagnetic centers in porous silicon by electron paramagnetic resonance (EPR). One of them is isotropic, assigned to a defect in amorphous silicon oxide in the porous silicon layer. The other is anisotropic, and is very much like a Pb center at a planar Si/SiO2 interface. This EPR center is unambiguously identified as an •Si≡Si3 moiety, a silicon with dangling orbital, back-bonded to three silicon atoms, by 29 Si hyperfine structure (HFS) associated with the dangling orbital, and 29 Si superHFS from three neighboring silicon atoms, as similarly observed in the usual planar surface Pb structure. The dangling orbitals are highly localized and heavily p character. The disposition of dangling orbitals is evidence that the skeletal structure of luminescent porous silicon is crystalline and has a lattice which is aligned and continuous with the wafer substrate. The possibility that these centers are the major photoluminescent killers or quenchers is not supported by our hydrogen annealing experiments.
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