Heterostructure modeling considerations for Ge-on-Si waveguide photodetectors

The computer-aided design and optimization of Ge-on-Si pin waveguide photodetectors is usually performed by a multiphysics analysis, where carrier transport is described with the drift-diffusion model. This paper addresses the shortcomings of this approach when applied to the study of the n-on-p detector configuration, ascribes them to the overly simplified model of the Si/Ge heterojunction, and proposes an enhanced description of the interfacial region. Starting from technological considerations, a thin buffer layer is introduced between Si and Ge, with a graded composition profile that mimics the initial low-temperature epitaxial growth process and mitigates the sharp discontinuity of the abrupt Si/Ge heterojunction in the band diagram. The benefits of this more realistic model on the determination of the electrooptic response are demonstrated and discussed.