Terahertz emission and detection using Ge-on-Si photoconductive antennas

Germanium-on-Silicon (Ge-on-Si) is a promising, CMOS-compatible platform for integrated terahertz (THz) photonics, offering a low-cost alternative to III-V semiconductors. A primary challenge for Ge-based photoconductive antennas (PCAs), however, has been the long carrier lifetime of bulk Ge, preventing its use as a detector. Here, we demonstrate that amorphous Ge (a-Ge) films overcome this limitation, possessing inherent ultrashort carrier lifetimes ~ 1.11-1.38 ps. We leverage this property to demonstrate, for the first time to our knowledge, coherent THz pulse detection using undoped a-Ge-on-Si PCAs. We present a comparative study of devices fabricated on a-Ge films grown by plasma-enhanced chemical vapor deposition (PECVD) and DC magnetron sputtering. The PECVD-Ge device, with better homogeneity and a smoother morphology in the films, demonstrates superior performance for both THz emission and detection. As an emitter, the PECVD-Ge PCA achieves a 40 dB signal-to-noise ratio (SNR) with a bandwidth of ~ 3 THz. As a detector, it achieves a 32 dB SNR and a ~ 2 THz bandwidth, representing a ~2.5-fold increase in detected signal amplitude over the sputtered-Ge device. These results establish amorphous Ge-on-Si as a viable and scalable platform for both THz generation and detection, paving the way for fully integrated Si-based THz time-domain systems.