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We have fabricated vertical p-i-n silicon photodetectors that are monolithically integrated with compact silicon-oxynitride channel waveguides. By comparing the evanescent coupling from low index-contrast waveguides and compact, high index-contrast waveguides, the dependence of evanescent coupling behavior on the waveguide index and geometry designs was analyzed. The effects of fabrication variations in the coupling structure have been studied and it was found that an offsetting step in the waveguide can help compensate for the mode mismatch at the transition interface from the input bus waveguide to the waveguide on top of the photodetector. Finally, we present a design map, built by drawing the evanescent coupling rate contour lines in the waveguide design space, which well predicts the evanescent coupling trends.