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

One of the primary goals of HgCdTe linear-mode avalanche photodiode arrays is to provide a 1kx1k pixels format, @15 μm pitch, near-infrared (0.9 to 2.5 μm) detector suitable for ultra-low background astronomical applications and long integration times. Such science goals impose very strict detector requirements, namely a dark current <0.001 [e⁻/pix/sec] and a sub-electron read noise. The Institute for Astronomy (IfA), University of Hawaii has partnered with Leonardo Company to develop such devices, using fine control of the photodiode process to enable noise-free amplification of the charge carriers and a readout circuit optimized for minimal glow. We discuss the first results of the tests conducted at the IfA on this new device operated in our cryogenic testbed. We report the values of dark current, read noise and conversion gain, as well as its cosmetic qualities that we have measured at a temperature of 50K. The measured dark current of these devices at low bias voltages is of ∼3 [e-/pix/ksec] (ksec=1000 seconds). We show that this dark current is dominated by the glow emitted by the ROIC of the detector when it is being read out. The intrinsic dark current of these devices is consistent with zero, with a best estimate of ∼0.1 [e-/pix/ksec]. The glow coming from the ROIC is measured to be ∼0.08 [e-/pix/frame], or 1 [e-/pix] every ∼12 frames. The read noise of these devices starts around ∼10 [e-/pix/frame] at a bias voltage of 3V, and decreases by a factor of 1.3 with each +1V increment of the bias voltage, in agreement with theory. It is reduced to ∼2 [e-/pix/frame] at a bias voltage of 8V.