Extrinsic germanium photoconductors for far-IR astronomy: Research results and works in progress

The authors report on Ge:Ga and Ge:Sb photoconductor materials and detectors that are under development. The best unstressed Ge:Ga devices exhibit dark currents lower than 200 electrons per second with a concurrent responsivity of 2 A/W and detective quantum efficiency (DQE) of 5%. For higher backgrounds an operating temperature of 3 K can be used. This increases the DQE to 7% and the responsivity to 4.5 A/W. The figures of merit are roughly the same for stressed Ge:Ga operated at 1.5 K and 2 K. Recently the authors began investigating n-type Ge:Sb as an alternative photoconductor material. Two crystals of Ge:Sb were grown and a number of test detectors were fabricated and evaluated. A t2 K the best device produced dark currents of less than 100 e{sup {minus}}/s with concurrent responsivity of 1 A/W and DQE of 4%. At 3 K the dark current increases to 10{sup 5} e{sup {minus}}/s, the DQE rises to 7% and responsivity to 4 A/W. Using p-type Ge:Ga crystals the authors are in the process of constructing 2-D monolithic photoconductor arrays. The monolithic approach should afford low cost array fabrication and sensitivity similar to cavity-mounted devices.