Fabrication and Electrical Characterization of Correlated Oxide Field Effect Switching Devices for High Speed Electronics

The response of correlated oxides to strong electric fields and their dynamics is investigated using electrical transport measurements and electronic structure studies.

Air Force Research Laboratory, Arlington, Virginia

Metal insulator transitions (MITs) in oxides are an intriguing problem from both a fundamental materials physics and an applied technology perspective. Though the precise roles of electron correlations and lattice distortions on the phase transition remains an active area of research, many recent theoretical studies have suggested intimate interplay among the orbital splitting/polarization, correlation effects, and Peierls dimerization in the 3d¹ system. Occupied states have been probed by x ray photoelectron spectros-copy (XPS), and a rough structure of unoccupied 3d-like states have been deduced by O K-edge x-ray absorption measurements. NbO₂, a 4d¹ system, like VO₂ cry stallizes in a distorted rutile type structure with Nb dimers and undergoes a temperature induced MIT, albeit at a considerably higher temperature of ∼1083 K. It is commonly accepted that because 4d orbital valence states are more dispersed in both space and energy, Mott physics is less important in 4d transition metal oxides than in 3d ones. Along this line of reasoning, it is perhaps surprising that the insulating state of NbO₂ persists to higher temperatures than that of VO₂.

A proposed explanation for this difference is that the Peierls effect in NbO₂ is stronger due to larger Nb metal-metal overlap of 4d orbitals, leading to greater orbital splitting between occupied d|| states and the unoccupied eg-states; however, given the many attempts to revise and improve theoretical and computational studies of VO₂, the physical and electronic properties of NbO₂ also should be examined more thoroughly. Currently, there are few experimental studies that provide insight into the electronic structure of NbO₂.