| Summary: | In recent years, Germanium (Ge) photodiodes have established a widespread utilization in photonic-integrated circuits (PICs). However, the devices commonly exhibit a prominent dark current due to the substantial defects at the Ge/Si heteroepitaxial interface. Herein, we demonstrate normal-incidence, buffer-less Ge vertical p-i-n photodiodes with remarkably low dark current density (Jdark, 0.78 mA/cm2 at -1 V), on a high-quality 200-mm Ge-on-insulator (GOI) substrate. The high-quality GOI was achieved by the removal of the highly dislocated Ge/Si interfacial region, sequentially via wafer bonding, layer transfer and oxygen (O2) furnace annealing. Compared to un-annealed GOI, the threading dislocation density (TDD) in Ge was reduced by more than two orders of magnitude to 1.2×106 cm-2. Correspondingly, the device Jdark and bulk leakage (Jbulk) were reduced by ~70× and ~145×. On the other hand, the photodiodes present a reasonable responsivity of 0.29 A/W at 1,550 nm and a nearly 100% internal quantum efficiency without external bias. The specific detectivity (D*, 2.17×1010 cm⸱Hz1/2⸱W-1 at 1,550 nm and -0.1 V) is comparable with that of commercial bulk Ge photodiodes. In addition, the low temperature bonding and layer transfer can enable a compact integration at the back-end-of-line for PIC applications. This work paves the way for GOI photodiodes towards advanced high-resolution imaging and sensing applications on PICs at the near infrared and short-wave infrared wavelength.
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