Mapping hidden space-charge distributions across crystalline metal oxide/group IV semiconductor interfaces*

Dopant profiles in semiconducting heterojunctions can have major effects on band energies which in turn drive transport properties. Here we use core-level photoelectron line shapes excited with hard x-rays to extract information about electric fields resulting from internal charge transfer in epitaxial La-doped Sr(Ti,Zr)O₃/Ge(001) heterostructures. Experiments were carried out using both n- and p-type Ge substrates. These heterojunctions were not amenable to electronic characterization of all regions by transport measurements because the doped substrates act as electrical shunts, precluding probing the more resistive films and masking interface conductivity. However, the core-level line shapes were found to be a rich source of information on built-in potentials that exist throughout the heterostructure and yielded valuable insight into the impact of band bending on band alignment at the buried interfaces. The electronic effects expected for Ge with uniform n- and p-type doping are eclipsed by those of unintended oxygen dopants in the Ge near the interface. This study illustrates the power of hard x-ray photoemission spectroscopy and related modeling to determine electronic structure in material systems for which insight from traditional transport measurements is limited.