It has long been speculated that Earth accreted prebiotic organic molecules important for the origins of life from impacts of carbonaceous asteroids and comets during the period of heavy bombardment 4.5 x 10(9) to 3.8 x 10(9) years ago. A comprehensive treatment of comet-asteroid interaction with the atmosphere, surface impact, and resulting organic pyrolysis demonstrates that organics will not survive impacts at velocities greater than about 10 kilometers per second and that even comets and asteroids as small as 100 meters in radius cannot be aerobraked to below this velocity in 1-bar atmospheres. However, for plausible dense (10-bar carbon dioxide) early atmospheres, we find that 4.5 x 10(9) years ago Earth was accreting intact cometary organics at a rate of at least approximately 10(6) to 10(7) kilograms per year, a flux that thereafter declined with a half-life of approximately 10(8) years. These results may be put in context by comparison with terrestrial oceanic and total biomasses, approximately 3 x 10(12) kilograms and approximately 6 x 10(14) kilograms, respectively.

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InGaN multiple-quantum-well structures grown by metal–organic chemical-vapor deposition on GaN and capped by p-type GaN are found to contain inverted pyramids of indium-free GaN. High-resolution structural and chemical analyses of these “V-defects” by a number of complementary transmission electron microscopy techniques show that the InGaN quantum wells end abruptly at the V-defect interfaces, which lie on {10–11} planes. Each V-defect has at its center a threading edge dislocation, indicating that the defects are initiated at edge dislocation cores in the presence of indium. The lower temperatures of InGaN/GaN quantum-well growth (790 °C/950 °C) assist the formation of V-pits, which are subsequently filled in during the growth at higher temperature (1045 °C) of the p-type capping layer.

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