Spin-dependent charge transport in crystalline BaTiO₃-germanium tunnel junctions

The epitaxial integration of functional oxides on conventional semiconductors using advanced molecular beam epitaxy opens new opportunities for coupling their unique properties with semiconductors for post-CMOS computing paradigms. Here we measure spin-dependent transport in Permalloy-BaTiO₃-Ge tunnel junctions using the three-terminal Hanle technique. We show that the aligned conduction bands at the epitaxial BaTiO₃-Ge interface allow controllable charge transport between quantum tunneling and trap-assisted conduction through an external bias. In the quantum tunneling region, we observe a negative magnetoresistance with a full width at half maximum of ~100 mT when an external magnetic field is applied perpendicular to the BaTiO₃-Ge interface. The negative magnetoresistance evolves into a superposition of two peaks with opposite polarity and distinct linewidth when the trap-assisted conduction becomes dominant. Possible mechanisms include a spin-dependent transport model involving defect states in the heterostructure. The correlation between charge transport and magneto-resistive response is the basis of a sensitive technique to measure defects in oxide semiconductor heterostructures.