Fabrication of the GaAs based terahertz photoconductors and the photometer for Tera-GATE

摘要

The present status of the development of an extrinsic photoconductor based on a high-purity GaAs is reported. Thisphotoconductor utilizing the shallow donor levels in GaAs and is highly sensitive for incident terahertz photons in thewavelength range 150 to 300 micron. The n-type GaAs crystal has been growth by liquid phase epitaxial (LPE) method,which is suitable to obtain thick and high-purity GaAs. The impurity concentration in GaAs layer has been decreased tothe order of 10~(13)atoms/cm~(-3). By doping the donors lightly in LPE growth process, C/Si, (background doped) Se andTe doped GaAs layers has been fabricated. The GaAs photoconductors using these crystals are sensitive in longerwavelength region than Ge:Ga photoconductors used in the past far-infrared astronomical observations. The most sensitive detector is obtained with C or Si background doped GaAs, of which NEP is reached to 3x10~(-16)W/Hz~(0.5)at thetemperature of 1.5 K, at 290 micron, the peak of its responsivity spectrum. A balloon-borne telescope utilizing our GaAs photoconductors, Tera-GATE (THz observation with GaAsphotoconductors and a balloon-borne Telescope) is now under development. The Tera-GATE is a 69 cm diametertelescope. On its focal plane, a photoconductor array with Winston cone has 2-mm entrance aperture and leads theincident photons to a cavity where 0.5-mm size photoconductor is installed. Measured optical efficiency of thecone/cavity system is in an acceptable range —40 percent.