Nondestructive defect characterization of SiC substrates and epilayers

This study presents a nondestructive and in-depth defect characterization method, based on the principle of polarized light microscopy (PLM), which can be used to quickly evaluate SiC substrates and epilayers. The developed PLM system has the capability to map, on a wafer scale, micropipes, elementary screw dislocations, and domain boundaries in SiC wafers. One unique feature of the PLM system is the ability to characterize the wafer with and without an epilayer, providing a newly found opportunity to investigate threading defect propagation in the overgrown epilayer. The correlation between SiC substrate defects and epilayer defects will be established.     

Interimistic data dissemination

We present low-overhead protocols for disseminating streaming data in an interimistic environment in which the availability of computers, networks, and other resources is very unpredictable. An important example of such an environment is the state of computing and communication resources immediately following a natural disaster or a similar disruptive event. A key characteristic of such environments is the continual change in network membership and topology. Our protocols are based on using the intensional and extensional overlap among the data requirements of network nodes. We describe our testbed, CoDD, that implements these protocols and provide an experimental evaluation of the techniques. A preliminary version of this paper appears in the Proceedings of the third workshop on e-business, December 2004, Washington.     

Effect of trace additions of cadmium,silver and zirconium on the precipitation hardening behavior of aluminum 6061 alloy

Age hardening response was investigated in Aluminum 6061 alloy to determine the influence of trace additions of 0.01 and 0.1 % of cadmium, 0.01, 0.1 and 0.5% of silver, 0.05 and 0.1% of zirconium, as well as 0.1% each of Cd + Zr and Ag + Zr. Thermo-mechanical stabilization treatment followed by artificial aging of these alloys were carried out at 313 K, 353 K, 393 K and 453 K. Vickers hardness was used to monitor the progress of age hardening. The Precipitate morphology, distribution and size were studied by Scanning Electron Microscopy. Tensile tests on selected alloys after selected treatment conditions assisted in determining the UTS, YS and % elongation. Fractography of the tensile fractures by scanning electron microscopy was used to delineate the effect of trace element addition on the fracture type. The trace addition effects observed seem to be caused by the precipitate size refinement and distribution. The observed effects seem to corroborate results obtained by several other techniques. The present work strengthens, both qualitatively and quantitatively, the viewpoint that it is possible to choose an age hardenable aluminum base alloy and improve it significantly by making judicious additions of trace elements.     

Dynamic Indentation Response of Fine-Grained Boron Carbide

Boron carbide disks with three different grain sizes were consolidated from submicrometer-sized boron carbide powder using the plasma pressure compaction technique. Static and dynamic indentations were performed to determine their loading-rate dependence on mechanical properties. Dynamic indentations resulted in a decrease in hardness and fracture toughness, and induced more severe damage compared with static indentations. Using Raman spectroscopy, the mechanism responsible for loss of strength under dynamic loads was identified as the solid-state structural phase transformation in the dynamically loaded regions. The influence of processing conditions and the resulting microstructure on the observed rate dependency of mechanical properties are discussed.     

Surface morphology of 6H-SiC after thermal diffusion

Diffusion of aluminum into 6H-SiC has been carried out in the temperature range of 1800–2100°C. Aluminum carbide (Al₄C₃) was used for a p-type impurity source; the diffused surface exhibited good stoichiometry and surface morphology. A thin-layer graphite film was developed to protect the wafer surface from deterioration during the high-temperature diffusion process. A high-resolution optical microscope (HROM) and atomic force microscopy (AFM) were employed to evaluate the surface morphology of the diffused samples. The protective graphite layer significantly decreased the surface roughness. X-ray photoelectron spectroscopy (XPS) was used to identify the Si/C ratio near the surface regions. Very little surface graphitization occurred during diffusion. In addition, secondary ion-mass spectroscopy (SIMS) was used to investigate the influence of the thin graphite film on the diffusion properties in SiC. There were no significant differences in doping profiles in the samples with and without the graphite film.     

A fast algorithm for computing sparse visibility graphs

AnO(¦E¦log² n) algorithm is presented to construct the visibility graph for a collection ofn nonintersecting line segments, where ¦E¦ is the number of edges in the visibility graph. This algorithm is much faster than theO(n ²)-time andO(n ²)-space algorithms by Asanoet al., and by Welzl, on sparse visibility graphs. Thus we partially resolve an open problem raised by Welzl. Further, our algorithm uses onlyO(n) working storage.     

Effect of N-acetylcysteine on human diaphragm strength and fatigability

Free radical injury is believed to be important in diaphragm dysfunction. N-Acetylcysteine (NAC) is a potent free radical scavenger shown in animal models to attenuate diaphragm fatigue; however, its effects on human diaphragm function are unknown. We assessed diaphragm function by electrophrenic twitch stimulation (PdiT) and twitch occlusion (to yield Pdimax) in four healthy subjects 35 +/- 3 yr of age (mean +/- SD). We intravenously administered NAC (150 mg/kg in 250 ml D5W) or placebo (CON) (250 ml D5W) in a randomized manner after subjects were premedicated with antihistamines. There were no significant side effects with the infusion. After infusion, we measured baseline Pdimax and PdiT at FRC. Diaphragm fatigue was then induced by subjects breathing through an inspiratory resistive load. Pdimax and PdiT were then measured at 15 to 30 min and 1, 2, 3, 4, and 20-25 h after fatigue. Times to fatigue were 13 +/- 4 min (CON) and 21 +/- 6 min (NAC) (p = 0.04). At 15 min after fatigue, PdiT was reduced to 40% (CON) compared with 30% (NAC) initial PdiT value (p = 0.05). Other twitch characteristics (maximal rate of relaxation and maximal contraction rate) were reduced to a greater degree after placebo compared with NAC. There were no significant differences in the rate of recovery between CON and NAC. Pdimax at 30 min after fatigue was significantly greater with NAC; however, at 1 h after fatigue, Pdimax for CON and NAC were not different, suggesting similar rates of recovery in high-frequency fatigue. These data suggest that NAC may attenuate low-frequency human diaphragm fatigue.     

The influence of the semiconductor and dielectric properties onsurface flashover in silicon-dielectric systems

New experimental results on surface flashover are reported for high field silicon-dielectric systems. Different conditions of the lateral surface, contacts and ambient dielectrics have been studied. The strong influence of the semiconductor quality, and that of the dielectric properties, on the prebreakdown and breakdown response of the system, is demonstrated. All experimental results strongly support the conclusion that surface flashover in silicon systems is a physical process totally different from semiconductor surface breakdown. This conclusion has important practical application in the improvement of the performance of photoconductive power switches, severely limited by premature breakdown effects