GaAs:Se and GaAs:Te Photoconductive Detectors in 300 µm Region for Astronomical Observations

The wavelength region λ= 200–300 µm is a little-known spectral window in observational astronomy. Recently, satellite-borne observatories, which are unaffected by atmospheric absorption and emission, have become available for astronomical research; however, the detector technology for this wavelength region is still undeveloped. N-type gallium arsenide is a good candidate for use in a high-sensitivity extrinsic photoconductor in this wavelength region. However, extrinsic photoconductors require very pure GaAs single crystals to achieve high performance. Liquid phase epitaxy is a suitable crystal growth method for realizing such pure GaAs crystals for use in fabricating the photoconductor because of the sufficient purity of the grown GaAs crystals and the considerable thickness were made possible by this method. We have GaAs doped with selenium and tellurium for fabricating the extrinsic photoconductors by the liquid phase epitaxy. By controlling the doping quantity, lightly doped GaAs:Te and GaAs:Se were successfully obtained (net donor concentrations of ND ≈1014 cm-3). The fabricated GaAs extrinsic photoconductors exhibited different spectral responses because of their different doping materials. The spectroscopic measurements of the GaAs photoconductors showed that they are sensitive over a wide wavelength range of 170–320 µm. The GaAs:Te and GaAs:Se that we developed have almost the same or higher sensitivities than GaAs photoconductors developed in the past or semiconductor-based bolometers operating at T = 1.6 K. Moreover, they are sensitive in a wider wavelength region than Ge:Ga photoconductors.