High-performance waveguide-integrated Ge/Si avalanche photodetector with small contact angle between selectively epitaxial growth Ge and Si layers**Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFB0402502 and 2017YFF0104803), the National Natural Science Foundation of China (Grant Nos. 61674140 and 61505003), the Beijing Natural Science Foundation, China (Grant No. 4162063), Beijing Education Commission Project (Grant No.

Abstract

Step-coupler waveguide-integrated Ge/Si avalanche photodetector (APD) is based on the vertical multimode interference (MMI), enhancing light scattering towards the Ge active region and creating mirror images of optical modes close to the Ge layer. However, there are two ineluctable contact angels between selectively epitaxial growth Ge and Si layers and selectively epitaxial growth Si and Si substrate, which has an effect on the coupling efficiency and the absorption of the photodetector. Therefore, step-coupled Ge/Si avalanche photodetectors with different step lengths are designed and fabricated. It is found that responsivity of APDs with step-coupler-length of is 0.51 A/W at −6 V, 21% higher than that of , which matches well with simulation absorption. The multiplication gain factor is as high as 50, and the maximum gain-bandwidth product reaches up to 376 GHz.