optoweleocr trlodnic
Low-temperature-grown body-cen-tered-cubic indium oxide (In2O3) nanoparticle thin films having a thickness of 15 nm were illuminated with light from a gallium indium ni-tride/gallium nitride (GaInN/GaN) blue LED operating at a wavelength of 400 nm. The sensor response toward ozone was determined from the changes in resistivity of the In2O3 layer, caused by a periodic switching between gas exposure and illumination. In the presence of different ozone concentrations, the ozone response increased as a function of LED optical power (see figure). These test results were sufficient to determine that the blue LED would efficiently activate the ozone sensing layer, and that an LED power of approximately 0.25 m W was necessary to activate the ozone-sensing layer.
Chip fabrication consisted of GaInN quantum-well blue LED arrays grown by low-pressure metal-organic chemical-vapor deposition (MOCVD) on a sapphire wafer substrate that was polished on both sides. The In2O3 films were grown on the back side of the wa-
fer in a horizontal MOCVD reactor. The finished package consisted of an integrated LED-array/sapphire-sub-strate/In2O3 chip that was exposed to various ozone concentrations through a 3 × 3 mm laser-cut window. The top side, with eight blue LEDs in a square array, was wire bonded to the top contacts of the chip.
“The combination of blue light-emitting GaN/GaInN LEDs and In2O3 nanolayers grown and processed on the two sides of a single sapphire substrate enables the realization of fast, cheap, and extremely selective ozone sensors,” says researcher Oliver Ambacher. “The possibility to measure a difference signal enabled by switching on and off the LED under ozone exposure enables an extremely high sensitivity. This compact chip is a demonstrator for a new generation of miniaturized gas and fluidic sensors combining GaN-based light-emitting devices with active metal oxide nanolayers.”
Gail Overton
REFERENCE
1. Ch. Y. Wang et al., Applied Phys. Lett. 91, 103509 (2007).
The highly energetic center of a quasar seems to be the source of a lot of cosmic dust—and even some microscopic jewels. 1 The dust was found by astronomers using NASA’s Spitzer Space telescope to examine a far-off quasar—a superactive and incredibly luminous young galaxy with a black hole at its center. The findings are a significant new clue in an old mystery: where did all the dust in the very early universe come from?
Hydrogen and helium were postulated to be the earliest ingredients in the universe, results of the Big Bang that clumped to form stars. Nuclear fusion inside the hot dense cores of the first stars formed the heavier elements, and the first dust particles would have formed as the first supernovas exploded long ago. But when astronomers look back to the earliest and farthest reach-
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