Transport Across Heterointerfaces of Amorphous Niobium Oxide and Crystallographically Oriented Epitaxial Germanium

Because of the high carrier mobility of germanium (Ge) and high dielectric permittivity of amorphous niobium pentoxide (a-Nb 2 O 5 ), Ge/a-Nb 2 O 5 heterostructures offer several advantages for the rapidly developing field of oxide-semiconductor-based multifunctional devices. To this end, we investigate the growth, structural, band alignment, and metal–insulator–semiconductor (MIS) electrical properties of physical vapor-deposited Nb 2 O 5 on crystallographically oriented (100), (110), and (111)Ge epilayers. The as-deposited Nb 2 O 5 dielectrics were found to be in the amorphous state, demonstrating an abrupt oxide/semiconductor heterointerface with respect to Ge, when examined via low- and high-magnification cross-sectional transmission electron microscopy. Additionally, variable-angle spectroscopic ellipsometry and X-ray photoelectron spectroscopy (XPS) were used to independently determine the a-Nb 2 O 5 band gap, yielding a direct gap value of 4.30 eV. Moreover, analysis of the heterointerfacial energy band alignment between a-Nb 2 O 5 and epitaxial Ge revealed valance band offsets (Δ E V ) greater than 2.5 eV, following the relation Δ E V (111) > Δ E V (110) > Δ E V (100) . Similarly, utilizing the empirically determined a-Nb 2 O 5 band gap, conduction band offsets (Δ E C ) greater than 0.75 eV were found, likewise following the relation Δ E C (110) > Δ E C (100) > Δ E C (111) . Leveraging the reduced Δ E C observed at the a-Nb 2 O 5 /Ge heterointerface, we also perform the first experimental investigation into Schottky barrier height reduction on n-Ge using a 2 nm a-Nb 2 O 5 interlayer, resulting in a 20× increase in reverse-bias current density and improved Ohmic behavior.