A large-format hybridized AlGaN photodiode array with an adjustable bandwidth features stray-light control, ultra-low dark-current noise to reduce cooling requirements, and much higher radiation tolerance than previous technologies. This technology reduces the size, mass, power, and cost of future ultraviolet (UV) detection instruments by using lightweight, low-voltage AlGaN detectors in a hybrid detector/multiplexer configuration. The solar-blind feature eliminates the need for additional visible light rejection and reduces the sensitivity of the system to stray light that can contaminate observations.
The AlGaN UV detector operating at 325 nm gives a 1,000× better extraterrestrial solar radiation rejection than silicon. This reduced need for blocking filters increases the quantum efficiency (QE) and simplifies the optical systems. The wide direct bandgap reduces the thermally generated dark current to levels that allow many observations at room temperature. Because of this, the AlGaN UV photodiode array doesn’t require the extensive cooling (and the associated cooling cost, complexity, and weight) that silicon does, significantly reducing system cost. Wide direct bandgap materials are naturally more radiation tolerant, which is crucial for instruments located outside of Earth’s atmosphere.
The device is most sensitive to UV radiation when operated in the photovoltaic mode at or near zero-reverse bias voltage. The effect of the bandgap is seen at the long wavelength cutoff of 365 nm, and shows a contrast ratio before and after the cutoff edge of better than 103. Between 355 and 365 nm, the QE is fairly flat, with a high of 50 percent at 360 nm at –0.5 V bias. The QE falls rapidly with decreasing wavelength reaching a minimum of 3 percent at 345 nm. The detector’s current responsivity at 360 nm and 0 V bias is 0.13 A/W. The spectral detectivity is 2.6 × 1015 cm Hz1/2W–1, corresponding to a detector noise equivalent power of 4.1 × 10–18 W/Hz1/2.
While the benefits for space-based UV detection are readily apparent, there are Earth-based applications that can benefit as well. These include plume measurements, flame sensing, UV lidar, biological agent detection, and measuring airborne particulate size and velocity.
This work was done by Shahid Aslam and David Franz of Goddard Space Flight Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. GSC-15673-1