Slow-light effect in a silicon photonic crystal waveguide as a sub-bandgap photodiode

Abstract

We demonstrate a Si sub-bandgap photodiode in a photonic crystal slow-light waveguide that operates at telecom wavelengths and can be fabricated using a Ge-free, standard Si-photonics CMOS process. In photodiodes based on absorption via mid-bandgap states, the slow-light enhancement enables performance that is well balanced among high responsivity, low dark current, high speed, wide working spectrum, and CMOS-process compatibility, all of which are otherwise difficult to achieve simultaneously. Owing to the slow-light effect and supplemental gain at a high reverse bias, the photodiode shows a responsivity of $0.15\mathrm{A}/\mathrm{W}$ with a low dark current of 40 nA, which is attributed to no particular processes such as ion implantation and excess exposure of the Si surface. The maximum responsivity was $0.36\mathrm{A}/\mathrm{W}$. The modest gain allows for sufficient frequency bandwidth to observe an eye opening at up to $30\mathrm{Gb}/\mathrm{s}$.