| Summary: | Mechanical strain engineering has been promising for many integrated photonic
applications. However, for the engineering of material electronic bandgap, a trade-off exists
between the strain uniformity and the integration compatibility with photonic-integrated
circuits (PICs). Herein, we adopted a straightforward recess-type design of silicon nitride
(SiNx) stressor to achieve a uniform strain with enhanced magnitude in the material of interest
on PICs. Normal-incidence, uniformly 0.56%-tensile strained germanium (Ge)-on-insulator
(GOI) metal-semiconductor-metal photodiodes were demonstrated, using the recessed
stressor with 750-MPa tensile stress. The device exhibits a responsivity of 1.84±0.15 A/W at
1,550 nm. The extracted Ge absorption coefficient is enhanced by ~3.2× to 8,340 cm-1 at
1,612 nm and is superior to that of In0.53Ga0.47As up to 1,630 nm limited by measurement
spectrum. Compared to the non-recess strained device, additional absorption coefficient
improvement of 10‒20% in the C-band and 40‒60% in the L-band were observed. This work
facilitates the recess-strained GOI photodiodes for free-space PIC applications and paves the
way for various (e.g. Ge, GeSn or III-V based) uniformly strained photonic devices on PICs.
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