INFRARED DETECTORS: Far-IR array developed for space observatory

Researchers at the University of Arizona Steward Observatory (Tucson, AZ) have built an infrared detector array sensitive to wavelengths from 40 to 120 µm—a region previously inaccessible to infrared arrays. This far-IR germanium array is to be used in the Space Infrared Telescope Facility (SIRTF) that will launched by NASA in 2001. The previously launched Cosmic Background Explorer satellite used a single detector with a lower sensitivity. With the new array scientists hope to find galaxies and quasars at their birth, when they are still buried in cocoons of dust that absorb their light and emit it in the far-IR. They will also image debris clouds surrounding nearby stars that may spawn new planets.

The SIRTF array will reach the sensitivity limit set by the far-IR glow of the zodiacal dust cloud of concentrated meteoric particles around the sun. This cloud determines how faint a distant source can be seen in the far-IR because individual IR photons of interest become indistinguishable from those of the much more numerous dust particle emissions or "shot noise," according to team member George Rieke.

Existing silicon detectors cannot work at wavelengths longer than 40 µm because their excitation energy is too large. The germanium detectors were developed by researchers at the University of California at Berkeley. These detectors are combined with silicon-IC signal readouts in the array design. But conventional silicon-based electronics cannot operate at the low -465°F temperature required for far-IR detection because of an increase in low-frequency noise and a loss of dc stability. With NASA funding assistance, Hughes Aircraft (Los Angeles, CA) developed low-noise silicon readouts that can operate at the low temperatures. Key to the material`s use is a thin, clean, lightly doped epitaxial layer over a degenerate substrate that ensures a fully depleted active region.

The array is a 4 × 32 detector matrix and is built so additional arrays can be stacked to create a 32 × 32 array for the SIRTF (see figure). Because far-IR radiation does not penetrate the atmosphere to be tested on ground-based telescopes, University of Arizona researchers have been operating the array in laboratory Dewars that create an artificial space environment. Steward Observatory program manager Debra Wilson notes, "Unlike arrays for shorter wavelengths, there’s been no military development of detectors in the far-infrared. We had essentially to start from scratch, testing a number of concepts and moving on to new concepts."