Low-Temperature Al-Migration Module for SACM Ge-on-Si Photodiodes

A low-temperature post-processing Al-migration module is used to achieve separate absorption, charge, and multiplication (SACM) functionality in vertical Ge-on-Si photodiodes without dedicated charge-layer formation by implantation or epitaxy. During a 400 °C alloying step, Al migrates from the contacts through perimeter openings of selectively grown Ge islands and modifies the Ge/Si interface, increasing the conduction-band potential barrier height. This mechanism is supported by I–V and C–V characterization and spatially resolved photoresponse under selective illumination of Ge and Si. At low reverse bias, the barrier suppresses carrier collection from Ge, resulting in uniformly low near-infrared (NIR) responsivity. Increasing reverse bias lowers the barrier and increases responsivity before avalanche. Room-temperature optical gain is demonstrated at 1310 and 1550 nm, reaching approximately 5 at 1310 nm. The photoresponse remains spatially uniform across the Ge island at all reverse biases. Light-emission mapping and wavelength-selective laser probing indicate that multiplication occurs predominantly in Si. Separate 1D and 2D TCAD simulations investigate the impact of interface charge and Ge-island geometry on electric-field redistribution, showing that moderate negative interface charge shifts the peak field into Si, while corner-induced field crowding, which will locally lower the interface barrier, facilitates the transport of photogenerated carriers from Ge into high-field regions in Si. The proposed module is potentially compatible with back-end-of-line processing and applicable to other vertical Ge-on-Si structures.