HgCdTe e-APD detector arrays with single photon sensitivity for space lidar applications

A multi-element HgCdTe electron initiated avalanche photodiode (e-APD) array has been developed for space lidar applications. The detector array was fabricated with 4.3-μm cutoff HgCdTe which covered a spectral response from 0.4 to 4.3 μm. We have characterized a 4x4 detector array with 80 μm square elements and an integrated custom cryogenic silicon read-out integrated circuit (ROIC). The device operated at 77K inside a small closed-cycle Dewar. Measurements showed a unity gain quantum efficiency of about 90% at 1.55 μm. The bulk dark current of the HgCdTe e-APD at 77K was less than 50,000 input referred electrons/s at 12 V APD bias where the APD gain was 620 and the measured noise equivalent power (NEP) was 0.4 fW/Hz. The electrical bandwidth of the ROIC was about 6 MHz, which was chosen to match the laser pulse width of our CO2 lidar. Even with the relatively low bandwidth, the high APD gain and low dark current enabled the device to detect single photon events. Because the APD was biased below the break-down voltage, the detector output was linear with the input optical signal and there was no dead-time and afterpulsing. A new series of HgCdTe e-APDs are being developed with a much wider bandwidth ROIC and higher gain HgCdTe e-APD array, which is expected to give a much better performance in linear mode photon counting applications.