GaAs extrinsic photoconductors for the terahertz astronomy

The terahertz waves are powerful tools for the astronomical research as for other applications. The emission from the cosmic dust particles, for example, enables us to investigate the formation of stars and planetary systems. However, detectors with high enough sensitivities to detect faint emission from the celestial bodies are not available yet. We have developed extrinsic photoconductors utilizing shallow donor levels in the GaAs for the astronomical applications. The high sensitivity detectors require very low impurity concentration in the GaAs crystal. We adopted the liquid-phase epitaxy to obtain the GaAs crystals which have high purity and enough thickness. The purest sample we have obtained to date has the carrier concentration 4x10 13 cm -3 , and high electron mobility 140,000cm 2 /Vs. The photoluminescence measurements showed the residual impurity elements are silicon and carbon. We have tried to fabricate terahertz detectors using three types of epitaxial layers, selenium- doped, tellurium-doped and un-doped (unintentional-silicon doped) layers. These photoconductors all have sensitivities in 1-2 THz at the operation temperature of 1.6 K. The photoconductors with the un-doped layers showed the highest responsivity 30 A/W and reached a good NEP as low as 3x10 -16 W/Hz 1/2 . We have also fabricated an eight-element linear array with feed horns to serve for the actual astronomical observations. This detector array is now under performance evaluation. The performance of the photoconductors and the linear array is described in this paper.