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-Nb₂O₅), Ge/a-Nb₂O₅ 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 Nb₂O₅ on crystallographically oriented (100), (110), and (111)Ge epilayers. The as-deposited Nb₂O₅ 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-Nb₂O₅ band gap, yielding a direct gap value of 4.30 eV. Moreover, analysis of the heterointerfacial energy band alignment between a-Nb₂O₅ and epitaxial Ge revealed valance band offsets (ΔE_V) greater than 2.5 eV, following the relation ΔE_V⁽¹¹¹⁾ > ΔE_V⁽¹¹⁰⁾ > ΔE_V⁽¹⁰⁰⁾. Similarly, utilizing the empirically determined a-Nb₂O₅ band gap, conduction band offsets (ΔE_C) greater than 0.75 eV were found, likewise following the relation ΔE_C⁽¹¹⁰⁾ > ΔE_C⁽¹⁰⁰⁾ > ΔE_C⁽¹¹¹⁾. Leveraging the reduced ΔE_C observed at the a-Nb₂O₅/Ge heterointerface, we also perform the first experimental investigation into Schottky barrier height reduction on n-Ge using a 2 nm a-Nb₂O₅ interlayer, resulting in a 20× increase in reverse-bias current density and improved Ohmic behavior.