Molecular-Beam Epitaxial Growth of a Far-Infrared Transparent Electrode for Extrinsic Germanium Photoconductors

We have evaluated the optical and electrical properties of a far-infrared (IR) transparent electrode for extrinsic germanium (Ge) photoconductors at 4 K, which was fabricated by molecular beam epitaxy (MBE). As a far-IR transparent electrode, an aluminum (Al)-doped Ge layer is formed at well-optimized doping concentration and layer thickness in terms of the three requirements: high far-IR transmittance, low-resistivity, and excellent ohmic contact. The Al-doped Ge layer has the far-IR transmittance of >95% within the wavelength range of 40-200 μm, while low-resistivity (~5 Ω cm) and ohmic contact are ensured at 4 K. We demonstrate the applicability of the MBE technology in fabricating the far-IR transparent electrode satisfying the above requirements.