Strain-balanced silicon-germanium materials for near IR photodetection in silicon-based optical interconnects

Abstract

Strain-balanced silicon-germanium superlattices grown on high quality compositionally graded buffers, or virtual substrates. make a complete range of alloy composition and biaxial strain combinations accessible. This structure is a unique way to achieve high quantum efficiency near IR photodetection for silicon-based optical interconnects. The growth of the strain-balanced superlattices by molecular beam epitaxy (MBE) and ultra high vacuum chemical vapor deposition (UHV-CVD) is presented and the role of adsorbed hydrogen during UHV-CVD growth is addressed. Hydrogen adsorption 0,1 the growth surface proved a useful technique to minimize coherent strain relaxation at the higher growth temperatures required for UHV-CVD silicon-germanium growth. The near IR absorption spectrum of the strained silicon-germanium materials possible using strain-balanced superlattices is critically required in the design of a photodetector. A model based on deformation potential theory and semiconductor absorption physics is used to predict the absorption coefficient as a function of strain and alloy composition. Photocurrent junction spectroscopy of strain-balanced silicon­germanium materials is used to confirm the results of the model. The effects of threading dislocations associated with the compositionally graded buffers on the bulk leakage current of photodiodes is determined using electron-beam induced current imaging techniques to measure dislocation density. The correlation between dislocation density and leakage current yielded a current per dislocation line length of 200 pA [mu]m·1. Coupling strategies for the integration of high dielectric contrast polycrystalline silicon/ Si02 strip waveguides and silicon-germanium photodetectors are presented. The high optical power densities possible with the polycrystalline silicon waveguides permits the miniaturization of photodetectors. The effects of integration and miniaturization on photodetector performance are discussed.