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-NbO), Ge/a-NbO 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 NbO on crystallographically oriented (100), (110), and (111)Ge epilayers. The as-deposited NbO 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-NbO band gap, yielding a direct gap value of 4.30 eV. Moreover, analysis of the heterointerfacial energy band alignment between a-NbO and epitaxial Ge revealed valance band offsets (ΔE) greater than 2.5 eV, following the relation ΔE > ΔE > ΔE. Similarly, utilizing the empirically determined a-NbO band gap, conduction band offsets (ΔE) greater than 0.75 eV were found, likewise following the relation ΔE > ΔE > ΔE. Leveraging the reduced ΔE observed at the a-NbO/Ge heterointerface, we also perform the first experimental investigation into Schottky barrier height reduction on n-Ge using a 2 nm a-NbO interlayer, resulting in a 20× increase in reverse-bias current density and improved Ohmic behavior.