First tests of a 1 megapixel near-infrared avalanche photodiode array for ultra-low background space astronomy

Spectroscopy of Earth-like exoplanets and ultra-faint galaxies are priority science cases for the coming decades. Here, broadband source flux rates are measured in photons per square meter per hour, imposing extreme demands on detector performance, including dark currents lower than 1 e-/pixel/kilosecond, read noise less than 1 e-/pixel/frame, and large formats. There are currently no infrared detectors that meet these requirements. The University of Hawaii and industrial partners are developing one promising technology, linear mode avalanche photodiodes (LmAPDs), using fine control over the HgCdTe bandgap structure to enable noise-free charge amplification and minimal glow. Here we report first results of a prototype megapixel format LmAPD operated in our cryogenic testbed. At 50 Kelvin, we measure a dark current of about 3 e-/pixel/kilosecond, which is due to an intrinsic dark current consistent with zero (best estimate of 0.1 e-/pixel/kilosecond) and a ROIC glow of 0.08 e-/pixel/frame. The read noise of these devices is about 10 e-/pixel/frame at 3 volts, and decreases by 30% with each additional volt of bias, reaching 2 e- at 8 volts. Upcoming science-grade devices are expected to substantially improve upon these figures, and address other issues uncovered during testing.