Prospects and challenges of silicon/germanium on-chip optoelectronics

Abstract

On-chip optoelectronics allows the integration of optoelectronic functions with microelectronics. Recent advances in silicon substrate fabrication (silicon-on-insulator (SOI)) and in heterostructure engineering (SiGe/Si) push this field to compact (chipsize) waveguide systems with high-speed response (50-GHz subsystems realized, potential with above 100 GHz). In this paper, the application and requirements, the future solutions, the components and the physical effects are discussed.
A very high refractive index contrast of the waveguide Si-core/SiO₂-cladding is responsible for the submicron line widths and strong bendings realized in chipsize waveguide lines and passive devices. The SiGe/Si heterostructure shifts the accessible wavelength into infrared up to telecommunication wavelengths 1.30–1.55 µm. Germanium, although also an indirect semiconductor as silicon, offers direct optical transitions which are only 140 meV above the dominant indirect one. This is the basic property for realizing high-speed devices for future above 10 GHz on-chip clocks and, eventually, a laser source monolithically integrated on the Si substrate.