The unusual interplay between dopants, surfaces, and interfacial structure in driving charge transfer across hybrid semiconductor-crystalline oxide heterojunctions

Dopants play a pivotal role in the transfer of charge and the formation of built-in electric fields across semiconducting heterojunctions. For hybrid heterojunctions comprised of a group IV semiconductor and a complex oxide, dopants exhibit an unusual interplay with the oxide surface and the structure at the interface in driving charge transfer. Using SrNb_xTi_1-xO₃/Si as a model system, we will discuss this interplay which begins when Si is unintentionally doped with oxygen impurities that diffuse from the oxide layer during epitaxial growth. The oxygen impurities act as n-type donors in the Si that provide itinerant carriers that are pulled toward the surface of the oxide due to surface depletion. This process of electron transfer from the Si to the oxide creates a built-in electric field that electrostatically modifies the structural dipole that exists at the epitaxial interface. Changes to the structural dipole consequently cause the band alignment to shift, which in turn promotes additional electron transfer. The coupling of charge transfer at the interface to the oxide surface as well as the interfacial dipole, provide degrees of freedom by which functional behavior can potentially be realized in hybrid heterojunctions.