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 ( ${J}_{\text {dark}}$ , 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 $1.2\times 10^{{6}}$ cm−2. Correspondingly, the device ${J}_{\text {dark}}$ and bulk leakage ( ${J}_{\text {bulk}}$ ) were reduced by $\sim 70\times $ and $\sim 145\times $ . 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 ( ${D}^{\ast }$ , $2.17\times 10^{{10}}$ cm $\cdot $ Hz $^{{1}/{2}}\,\,\cdot \,\,\text{W}^{-{1}}$ 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.