Waveguide infrared photodetectors on a silicon chip

A novel infrared (IR) photodetector structure is discussed. It represents a waveguide in which the core is a strained-layer GexSi1-x/Si superlattice (SLS) sandwiched between Si layers of a lower refractive index. Absorption of infrared radiation occurs in the core region due to interband electron transitions, and photogenerated carriers are collected in the Si cladding layers. Due to the recently discovered effect of bandgap narrowing by the strain in alloy layers the fundamental absorption threshold of the SLS is shifted to longer wavelengths, so that the detector can be operated in the range of silica-fiber transparency, 1.3-1.55 µm. The optimum SLS composition and thickness have been estimated from the known material properties and waveguide theory. The detector quantum efficiency grows with the optical path length, remaining consistent with the requirements of high-speed fiber-optical communications. A major advantage of the proposed structure is the possibility of obtaining avalanche gain in the silicon cladding. First experimental results have demonstrated the validity of the concept.