Sub-mA/cm² Dark Current Density, Buffer-Less Germanium (Ge) Photodiodes on a 200-mm Ge-on-Insulator Substrate

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 (J _dark , 0.78 mA/cm ² 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 (O ₂ ) furnace annealing. Compared to un-annealed GOI, the threading dislocation density (TDD) in Ge was reduced by more than two orders of magnitude to 1.2×10 ⁶ cm ^-2 . Correspondingly, the device J _dark and bulk leakage (J _bulk ) 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*, 2.17×10 ¹⁰ cm · Hz ^1/2 · 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 toward advanced high-resolution imaging and sensing applications on PICs at the near-infrared and short-wave infrared wavelength.