In 2016, Thach et al. reported figures of merit of the GeSn photodiodes with large mesa sizes of 500 and 250 μm to show the potential of the GeSn materials in short-wave infrared photonics²³. However, there are only a few discussions about high frequency capabilities of GeSn photodetectors. It is needed to understand the potential of GeSn detectors as high frequency devices. This paper discusses comprehensively about the performance of GeSn photodiodes with 6.44 and 9.24% Sn for high frequency applications including high speed measurements and simulations. With high Sn incorporation, the cutoff wavelength is extended up to 2.2 and 2.5 μm wavelengths for 6.44 and 9.24% Sn devices, respectively. The photodiodes' bandwidth is 1.78 GHz, and the simulation shows excellent agreement with measurement results. The reported GeSn photodetectors together with recently reported GeSn lasers and other GeSn microwave photonic components will be a potential candidate for integrated microwave photonics.

In recent years, Ge and Ge_1−xSn_x materials and devices have achieved rapid progress in integrated photonics. However, conventional heteroepitaxy of active photonic devices compromises the area on Si for CMOS electronics, limiting the scale of integration. Furthermore, it is not possible to grow GeSn epitaxially on amorphous and/or flexible substrates toward 3D photonic integration in mid-infrared (MIR) regime. Here, we present low-temperature crystallization of direct bandgap, high crystallinity Ge_1−xSn_x (0.08 < x < 0.26) on amorphous dielectrics insulators (GeSnOI) toward 3D and flexible MIR integrated photonics. Utilizing eutectically-enhanced crystallization (EEC), an extraordinarily large average grain size of ~100 μm has been achieved in blanket GeSn films crystallized on SiO₂ layers, flexible glass, and polyimide substrates alike. Furthermore, using Sn nanodot enhanced composition enhancement (NICE), we have achieved an average Sn composition as high as 26 at.% to further extend the optical response of GeSn toward λ = 3–5 μm. The achieved Sn composition of 8–26 at.% far exceeds that of the equilibrium solubility limit of <1 at.%, even though the crystallization temperature of 350–450°C far exceeds the typical epitaxial growth temperature of GeSn. This result indicates that crystallization from amorphous GeSn (a-GeSn) may offer better metastability compared to direct epitaxial growth of GeSn. Attesting to the high crystallinity, a peak optical gain of 2,900 cm⁻¹ with a lifetime approaching 0.1 ns is achieved at λ = 2,200–2,350 nm at 300 K. The gain lifetime is on the same order as epitaxial GeSn, and it is >100x longer than the direct gap transition in Ge, confirming the indirect-to-direct band gap transition in GeSn at ~9 at. Sn composition. Moreover, a prototype p-GeSn/n-Si photodiode from a-GeSn crystallization achieves 100 mA/W responsivity at λ = 2,050 nm and T = 300 K, approaching the level of some commercial PbS detectors. The device also demonstrates photovoltaic behavior and a low dark current density of 1 mA/cm² at −1 V reverse bias, comparable to epitaxial Ge/Si photodiodes. These results indicate that crystallization of GeSnOI offers a promising solution for active devices toward 3D MIR photonic integration and/or MIR photonics on flexible substrates.