Date of Graduation

2017

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

David Lederman

Committee Co-Chair

Cheng Cen

Committee Member

Fabien Goulay

Committee Member

Matthew Johnson

Committee Member

Mohindar Seehra

Abstract

The 4d transition metal oxide, NbO2, is a key material for a wide range of potential applications. It exhibits a thermally driven metal-to-insulator transition (MIT) with very high transition temperature (TMIT = 1083 K). High TMIT, electric field induced sharp threshold switching and current-controlled negative differential resistance (NDR) characteristics makes this material most suitable for current switching devices. Metal-to-insulator transition in NbO2 is accompanied by a structural transition from a rutile (high temperature) to a tetragonal (low temperature) structure with lowering temperature. In this work, pulsed laser deposition growth of NbO2 thin films and their structural and electrical characterization were investigated.;To study the switching mechanism in epitaxial NbO2 (110) film lateral devices, 20 nm thick films were grown by pulsed laser deposition on Al2O3 (0001) substrates. The Ar/O2 total pressure during growth was varied to demonstrate the gradual transformation between NbO2 and Nb2O5 phases, which was verified using x-ray diffraction, x-ray photoelectron spectroscopy, and optical absorption measurements. Electrical resistance threshold switching in current characteristics was studied in a lateral geometry using interdigitated Pt top electrodes to preserve the epitaxial crystalline quality of the films. Volatile and reversible transitions between high and low resistance states were observed in epitaxial NbO2 films, while irreversible transitions were found in the case of Nb2O5 phase. Electric field pulsed current measurements confirmed thermally-induced threshold switching.;Switching mechanism in the polycrystalline thin film of NbO2 was also investigated. Polycrystalline thin films were grown on the TiN coated SiO2/Si substrate using pulsed laser deposition. Thickness and contact size dependent study in threshold switching in current characteristics of NbO2 films grown using different growth pressures were performed. Threshold switching and current oscillations produced by the films having different defect densities were measured and compared. To increase defect density, such as oxygen vacancies, films grown in different growth pressures or using different oxygen mass flow ratios were used.;Finally, electrical properties of the polycrystalline thin film were compared with the epitaxial films grown using similar growth conditions. The epitaxial film was grown on Si-doped GaN coated substrate where Si:GaN layer acted as the bottom contact. Pulsed I-V as well as the input voltage dependent and time dependent frequency measurements were performed and compared with each other. Stability and switching rapidity of the thin film vertical devices were analyzed and compared in terms of the defect density presented in the film.

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