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 transparentelectrode, an aluminum (Al)-doped Ge layer is formed at well-optimized dopingconcentration and layer thickness in terms of the three requirements: highfar-IR transmittance, low resistivity, and excellent ohmic contact. TheAl-doped Ge layer has the far-IR transmittance of >95 % within the wavelengthrange of 40--200 microns, while low resistivity (~5 ohm-cm) and ohmic contactare ensured at 4 K. We demonstrate the applicability of the MBE technology infabricating the far-IR transparent electrode satisfying the above requirements.
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