Interfacial Hole Accumulation Enables Selective Free-Surface Formation in Ge-on-Si

Abstract

Electrochemically formed two-dimensional free surfaces (2DFS) decouple Ge epilayers from the misfit-dislocation network at the Ge/Si interface, but the origin of their selectivity has remained unclear. We combine finite-volume current-distribution modelling, including charge-transfer kinetics, with a one-dimensional Poisson analysis of the p-Ge/p+-Si heterojunction. With realistic conductivities, an ohmic description would drive 77% of the anodic current through Ge, whereas hole-limited charge transfer reduces this fraction to 0.07%, independently of the electrolyte conductivity. The valence-band offset accumulates holes on the Ge side up to ~1019 cm--3 within a self-limited nanometric layer, which localizes dissolution at the interface. The model yields design rules for the doping contrast (NSi/NGe ___ 60) and the mesa width (___ 3.6$λ$, with $λ$ $\approx$ 1.7 $μ$m).