Germanium (Ge)-Silicon (Si) structures, optoelectronic devices and method for forming same. A structure comprises a Si substrate, a Ge seed layer and a Ge epitaxial layer separated by respective interfaces that share a common plane normal, wherein the Si substrate and the Ge seed layer have a same first doping type with a first doping level, and a locally doped region formed in the Si layer adjacent to the Ge seed layer and having a second doping type with a second doping level, wherein the locally doped region is designed to reduce leakage currents between the Si substrate and the Ge epitaxial layer when an electrical bias is applied to the structure.