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

Abstract

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 Hawai'i 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.

Leonardo UK high performance shortwave APDs for astronomy

“…At 15V bias there were only 36 pixels showing excess noise as shown in Figure 11. Preliminary results on the engineering samples 13 of 1kx1ks showed that at 50K the thermionic dark current was than 0.1 e/ks/pixel where a ks = 1000 s and the measurement was probably still residual glow. With the maximum usable avalanche gain, the read noise of two electrons was limited by a glow source during readout, traced to a sneak path through the metal layers.…”

Pixel source follower glow in HxRG detector

“…$$$$ {S}_{\mathrm{dark}}={I}_{\mathrm{dark}}+\frac{F}{dt}, $$$$ where $$$ F $$$ is the glow per frame expressed in electrons per read, $$$ {I}_{\mathrm{dark}} $$$ is the real dark current, $$$ {S}_{\mathrm{dark}} $$$ is the effective dark current, and $$$ dt $$$ is the sampling time, the time period between two consecutive reads of the same pixel. Recently, investigations were conducted on dark signal dependence with pixel readout frequency using $$$ 1024\times 1024 $$$ pixels array Linear‐mode Avalanche Photodiode (LmAPDs) Claveau et al (2022).…”