Person:

Zhou, You

We report on growth and physical properties of vanadium dioxide ((VO_2)) films on model conducting oxide underlayers (Nb-doped (SrTiO_3) and (RuO_2) buffered (TiO_2) single crystals). The (VO_2) films, synthesized by rf sputtering, are highly textured as seen from X-ray diffraction. The (VO_2) film grown on Nb doped (SrTiO_3) shows over two orders of magnitude metal–insulator transition, while (VO_2) film on (RuO_2) buffered (TiO_2) shows a smaller resistance change but with an interesting two step transition. X-ray photoelectron spectroscopy has been performed as a function of depth on both sets of structures to provide mechanistic understanding of the transition characteristics. We then investigate voltage-driven transition in the (VO_2) films grown on Nb-doped (SrTiO_3) substrate as a function of temperature. The present study contributes to efforts towards correlated oxide electronics utilizing phase transitions.

In this work we study the metal-insulator transition in vanadium dioxide and samarium nickelate and the application of such transitions in electronic devices. Chapter 1 provides an introduction to the Mott metal-insulator transition mechanisms and an overview of the interplay between various degrees of freedom in correlated oxides. The phase transition in vanadium dioxide is presented as an example to emphasize the overarching electron-phonon and electron-electron interaction driven transition mechanisms. In Chapter 2, we describe the growth and structure-functionality relationship of thin film transition metal oxides. Chapter 3 goes on to examine the mechanism of voltage-triggered metal-insulator transition in vanadium dioxide two-terminal threshold switches through dynamic studies. Chapter 4 delves into the mechanism of conductance modulation in electrolyte-gated vanadium dioxide transistors, which reveals the importance of electrochemical effects versus electrostatic effects in these devices. Utilizing the idea of electrochemical doping, we designed and realized a strongly correlated insulating phase in samarium nickel oxide through electron doping with hydrogen and lithium interstitials in Chapter 5. Such techniques can be extended to other materials to achieve reversible and controllable carrier doping with high concentration to study the related physics.

Modulation is a cornerstone of optical communication, and as such, governs the overall speed of data transmission. Currently, the two main strategies for modulating light are direct modulation of the excited emitter population (for example, using semiconductor lasers) and external optical modulation (for example, using Mach–Zehnder interferometers or ring resonators). However, recent advances in nanophotonics offer an alternative approach to control spontaneous emission through modifications to the local density of optical states. Here, by leveraging the phase-change of a vanadium dioxide nanolayer, we demonstrate broadband all-optical direct modulation of 1.5 μm emission from trivalent erbium ions more than three orders of magnitude faster than their excited state lifetime. This proof-of-concept demonstration shows how integration with phase-change materials can transform widespread phosphorescent materials into high-speed optical sources that can be integrated in monolithic nanoscale devices for both free-space and on-chip communication.