The National Aeronautics and Space Administration has awarded Rochester Institute of Technology $1.1 million to advance a new family of large format infrared detectors grown on silicon wafer substrates— Raytheon Visions Systems’ breakthrough technology in detector development. The RIT-Raytheon detectors someday could support future NASA missions to understand the nature of dark matter and dark energy, and to find Earth-like exoplanets.
RIT’s Center for Detectors will collaborate with Raytheon to design, fabricate and test the hybrid detectors grown on silicon wafer substrates. Raytheon is a leading provider of detectors and focal plane arrays for ground, airborne and space-based applications.
Raytheon’s process for depositing light sensitive material on silicon wafers departs from standard detector technology that, since the 1980s, has relied upon small, scarcely produced and expensive Cadmium Zinc Telluride wafers.
“Right now, infrared detectors are so expensive that there are only a few on the world’s biggest telescopes—Keck, Gemini and the Very Large Telescope,” says Donald Figer, director of the Center for Detectors at RIT. “Those are the only facilities that can afford them, and then they can only afford a few. They have big telescopes with big image planes and tiny detectors in the middle.”
The RIT-Raytheon device will have broad wavelength coverage that extends from the optical to the infrared in arrays of 1,024 by 1,024 pixels or 2,048 by 2,048 pixels—the standard size in use today. The detectors will scale upward to 14,000 by 14,000 pixels. This leap is in keeping with the Center for Detectors strategic goal to build and use advanced astronomical instrumentation, Figer says.
Last summer, RIT won a $1.2 million grant from the National Science Foundation for a similar collaboration between the Center for Detectors and Raytheon. The NSF award supports research advancing infrared detector technology for use on ground-based telescopes and in the fields of remote sensing and medical imaging.
Now, support from NASA extends the research to space applications requiring radiation-hardy instruments. Astronomical discoveries of dark energy, dark matter and Earth-like exoplanets become more attainable with the larger focal planes silicon wafers afford. The technology could also enhance NASA’s Planetary Science and Earth Science space missions, the latter for characterization of weather, climate and air pollution.
“The search for dark energy and dark matter are the major goals for space astronomy missions in the next decade according, to the National Research Council’s Decadal Survey,” Figer says. “High-performance detectors are essential for the Wide Field Infrared Survey Telescope, the mission envisioned to achieve this goal.”
The NASA-funded portion of the project will require the team to test and evaluate the detector in environments similar to conditions found in space. Figer and his engineers will also redesign the readout circuit to lower the device’s “noise” and signal interference.
Lowering the readout noise is essential to detector performance. Lower noise allows for fainter objects to be observed faster. This translates to instruments being able to do more science, faster and at lower cost.
“Reducing the cost of infrared detectors is only advantageous if the detector performance remains competitive with comparable detectors,” says Brandon Hanold, engineer in RIT’s Center for Detectors.
Introducing a new family of detectors based on silicon substrates will have a significant impact, Figer predicts. It will leverage the existing infrastructure built around the semiconductor industry, improve performance, and drive down the cost of building detectors. The wide commercial application makes silicon an attractive material for use in imaging detectors due to the high-volume production, large format and cost of silicon wafers compared to precious Cadmium Zinc Telluride wafers.
“Infrared detectors with lower cost focal planes and improved performance will push the boundaries of infrared astronomy and continue Raytheon’s 30-year service to the astronomy community,” says Elizabeth Corrales, program manager at Raytheon Vision Systems.
For the last 15 years, scientists have pursued the use of silicon substitutes in the quest for large infrared detectors. Until now, the crystal lattice mismatch between silicon and infrared materials has stymied advancement, causing defects that generate higher dark current, and thus higher noise, reduced quantum efficiency and increased image persistence.
Atoms in a silicon crystal are spaced closer together than in infrared light-sensitive materials, and the atomic spacing of the materials does not match up. The misalignment can cause photo-generated defects to masquerade as signals, which can get stuck, lost, or pop out of the lattice and present as false signals.
Raytheon’s technique uses a method of depositing light-sensitive material onto silicon substrates while maintaining high vacuum throughout the multi-step process. The material growth is done using a technique common to the semiconductor industry known as molecular beam epitaxy.
“Raytheon has come up with an innovation to combine the silicon wafer with the mercury cadmium telluride light-sensitive layer in a way that could end up dominating the field of infrared detectors for the next 20 years,” Figer says.