HgCdTe Arrays for the Far

The development of focal plane arrays (FPAs) has dramatically increased the sensitivity and efficiency of large optical and infrared telescopes. For the 1-2.5 mm band, the 1Kx1K HgCdTe “Hawaii” chip is the largest operational array in astronomy. Similar arrays of 2Kx2K are now in development, with the goal of assembling an 8Kx8K mosaic array for the Next Generation Space Telescope (NGST) project (Hall 1998). The spectral versatility of the HgCdTe alloy is well recognized for wavelengths as long as the Long Wavelength Infrared (LWIR) band from 8-20 mm. What is not so recognized, however, is that theoretically there is no long wavelength limit for appropriately composed HgCdTe. Only one serious effort appears to have been made to apply this material at far-infrared (FIR) wavelengths between 20-150 mm. Spears (1988) successfully demonstrated a photoconductive response from HgCdTe at 120 mm, but this pioneering effort was never followed up, in part because of the difficulty of controlling the HgCdTe alloy composition in bulk crystals. With the advent of precise molecular-beam-epitaxy (MBE) of HgCdTe in the early 1990s, it is now appropriate to reconsider this semiconductor for detector applications well longward of the 20 mm LWIR band cut-off. We wish to develop large FPAs for photometric imaging at high spatial resolution. The astrophysical subjects are molecular cloud complexes with temperatures of 30-100 K, and corresponding peak emission wavelengths between 50 and 150 mm. Because the Earth’s lower atmosphere absorbs strongly at these wavelengths, the observations must necessarily be done from airborne, balloon-borne, or space-borne platforms.