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 (ΔEV) greater than 2.5 eV, following the relation ΔEV(111) > ΔEV(110) > ΔEV(100). Similarly, utilizing the empirically determined a-Nb2O5 band gap, conduction band offsets (ΔEC) greater than 0.75 eV were found, likewise following the relation ΔEC(110) > ΔEC(100) > ΔEC(111). Leveraging the reduced ΔEC 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.