On an extremely high-speed dislocation-free Ge-on-Si image sensor

Innovative technologies are presented to develop Ge-on-Si SWIR extremely-high-speed mage sensors. In imaging with Ge-on-Si image sensors, a large dark current is generated by dislocations distributing along the interface of Ge and Si crystals due to a 4.2% difference between the lattice constants of a Ge crystal, 0.5658 nm, and of a Si crystal, 0.5431 nm. Two countermeasures are presented to eliminate and mitigate such dislocations: (1) Polka-dot coupling in which Ge and Si crystal layers are separated with a thin insulation layer with openings at a pitch of a sort of LCM of their lattice constants, 13.54 nm, that can be processed by the most advanced 2-nm process using tin plasma with the wavelength of 13.5 nm, and (2) an inverted-pyramid Ge photodiode with the surface much wider than the diffraction limit of about 750 nm for the longest sensitive wavelength of 1500 nm of the Ge crystal and with the bottom narrower than 500 nm to suppress threading dislocations. For ultra-high-speed imaging, the authors have presented an image sensor structure to achieve a frame rate of 10 Gfps. An image sensor concept is presented with the pixels with both the Ge-on-Si photoelectron conversion unit and the structure to achieve the ultra-high frame rate.