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

Abstract

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.

Keywords: X-ray photoelectron spectroscopy; band offset; germanium; metal−insulator−semiconductor; niobium oxide.