Hollow Defects Research Articles

Surface step model for micropipe formation in SiC

Silicon carbide (SiC) single crystals, a promising material for high power and high temperature semiconductor devices, have microscopic hollow defects, so-called “micropipes.” Their formation mechanism is not satisfactorily clarified yet. In this paper, a surface step model for micropipe formation in SiC single crystals is proposed, where the strong repulsive interaction between surface steps on the SiC(0 0 0 1) surface is a major driving force for coalescing unit cell size screw dislocations.

Hollow defect elimination during solution growth of SiC

ABSTRACTUsing pure and alloyed silicon melt saturated with carbon we investigated systematically hollow defect elimination during SiC solution growth over a wide temperature range from 1500°C to 2100°C. In the process of solution growth all hollow defects present in a substrate demonstrate an evident tendency to act as growth centers and after an adequate period of treatment they were overgrown. Growth morphologies observed in the vicinity of hollow defects are rather different. A new visualization method is proposed, which reveals crystalline defects associated with hollow cores and allows to detect the quantity and the distribution of HD in a whole SiC wafer. Classification of hollow defects based on these observations is presented and the corresponding closing mechanisms are discussed.

Mg-doped GaN: Similar defects in bulk crystals and layers grown on Al2O3 by metal–organic chemical-vapor deposition

Defects were observed in GaN:Mg grown on sapphire substrates using metal–organic chemical-vapor deposition (MOCVD) with Mg-delta doping similar to those previously observed in bulk GaN:Mg grown from Ga solution under high hydrostatic pressure of nitrogen. Pyramidal defects (pinholes) with (11̄00) hexagonal facets on the (0001) base plane and six {112̄2} side facets, and defects with a rectangular shape also delineated by planar facets on the basal (0001) planes, were observed for growth with Ga polarity for both of these very different growth methods. The Mg dopant is apparently responsible for their formation since the oxygen concentration in the MOCVD-grown samples was orders of magnitude lower than in the bulk samples. Mg segregation on these planes apparently does not allow uniform continuous growth on these planes leading to these hollow defects. Some defects in the heterolayers also develop into longer nanotubes elongated along the c axis. Change of polarity from Ga to N followed by a change back to Ga polarity also resulted in formation of planar defects previously observed in bulk samples for growth with N polarity.

THICKNESS MEASUREMENTS AND FLAW DETECTION IN CONCRETE USING ULTRASONIC ECHO METHOD

This article considers the problem of thickness measurements and flaw detection in concrete with one side access. It can be solved by ultrasonic echo methods based on the application of multiple-unit antenna arrays, with further data processing in accordance with tomographic algorithms. The data processing algorithm and criteria of the selection of antenna arrays parameters, based on experimental investigations, are described in the article. The results of concrete inner structure inspection obtained by the proposed method are presented. Practically, it is possible to receive an image of concrete cross section at depths of up to one meter and to detect hollow defects with diameters of more than 50 mm.

Method of predictive calculation of the high-temperature strength of carbon-carbon composite materials

A method of the predictive calculation of the strength of carbon-carbon materials, which is based on consideration of the properties of its structure, and also its high-temperature variation, is presented. It is shown that local stress concentration near hollow structural defects occur within the material in conformity with their shape and size. The parameters indicated are associated with the structure of the carbon fabrics and the method of their arrangement in the composite materials, as well as with the thermal and force regimes of the production process of pyrolysis during the manufacture of the compositions. These indicators are united into a single computational model that takes into account the interaction between stresses and structural defects, which play the role of stress raisers and which change at high temperatures.

Scanning electron microscopy of pulmonary vascular endothelium in rats with hypoxia-induced hypertension.

Scanning electron microscopy was used to study the endothelial surface of the pulmonary trunk, artery, and vein in normobaric control rats as well as in rats exposed to hypobaric hypoxia for 7 and 21 days. The individual endothelial cells of the normobaric pulmonary trunk and hilar artery were flat and slightly elongated with elevated nuclear regions, and those of the intermediate-sized artery were more elongated and had more microvilli than the large arteries studied. Their endothelial cell boundaries were outlined by beaded cytoplasmic projections. The surfaces of the normobaric hilar and intermediate-sized veins were smooth and demonstrated numerous longitudinal streaks. These venous endothelial cells were elongated and their cell boundaries were outlined by low discontinuous marginal folds. Exposure to hypobaric hypoxia caused the following changes on the arterial surface: elevation of the endothelial cells; formation of microvilli-rich cell clusters; formation of hollow defects; and the attachment of leukocytes. Hypobaric hypoxia also caused the disappearance of the longitudinal streaks and the occurrence of microvilli-rich cells in the hilar veins. The endothelial surface modifications in the hypobaric rats could be related to thickening of the endothelium, intimal edema, increased intimal connective tissue, luminal invasion of leukocytes, and increased endothelial cell proliferation, known to occur in systemic arteries of hypertensive animals.