Germanium on double-SOI photodetectors for 1550 nm operation

We have fabricated and characterized the first resonant cavity enhanced (RCE) germanium photodetectors on double silicon-on-insulator substrates (Ge/DSOI) for operation around the 1550 nm communication wavelength. The Ge layer is grown through a novel two-step UHV/CVD process, while the underlying double-SOI substrate is formed through an ion-cut process. Absorption measurements of an undoped Ge-on-Si (Ge/Si) structure reveal a red-shift of the Ge absorption edge in the NIR, due primarily to a strain-induced bandgap narrowing within the Ge film. By using the strained-Ge absorption coefficients extracted from the absorption measurements, in conjunction with the known properties of the DSOI substrate, we were able to design strained-Ge/DSOI photodetectors optimized for 1550 nm operation. We predict a quantum efficiency of 76% at 1550 nm for a Ge layer thickness of only 860 nm as a result of both strain-induced and resonant cavity enhancement, compared to 2.3% for the same unstrained Ge thickness in a single-pass configuration. We also estimate a transit-time limited bandwidth of 28 GHz. Although the fabricated Ge/DSOI photodetectors were not optimized for 1550 nm operation, we were able to demonstrate an over four-fold improvement in the quantum efficiency, compared to its single-pass counterpart.